Halogen enrichment on the continental surface: a perspective from loess
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Keywords: halogen elements, upper continental crust, loess, volatile cycling
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Abstract
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 Figure 1 Halogen (F, Cl, Br, and I) concentrations of loess samples. The blue dash-dotted and the red dashed lines represent the geometric and arithmetic means of the loess data set, respectively, along with uncertainties of 2 standard errors (s.e., light-shaded bars; see Fig. S-5 for more details on the calculation). Symbols above each histogram represent previous estimates of halogen abundances in the UCC. |  Figure 2 Fluorine versus (a) Al2O3 and (b) K2O contents of loess. (c) Normalised rare earth element (REE) and halogen concentrations of loess samples relative to the composition of the crystalline UCC (UCCXlt): REE estimates for UCC are from Rudnick and Gao (2003) and halogens are from Han et al. (2023). The cyan-shaded area shows the compositional field of glacial diamictite composites. |  Figure 3 Br/Cl versus I/Cl ratios of loess (symbols as in Fig. 2; this study) and glacial diamictite composites (cyan diamonds; Han et al., 2023). Note that (1) the leached loess residues are plotted as deep-red dashed symbols, and (2) five carbonate-rich and terrigenous sediment-rich glacial diamictites from Namibia have notably elevated I content and relatively low Br/I ratios and are shown with open diamonds (see Han et al., 2023 for discussion). Halogen ratios of seawater (grey triangle), bulk silicate Earth (BSE, grey plus sign), mid-ocean ridge and ocean island basalts (MORB/OIB, grey rectangle, Kendrick et al., 2017), arc/backarc lavas (grey outline, Kendrick et al., 2020), pelagic sediments (grey dashed outline, John et al., 2011), and marine pore fluids (green field, Fehn et al., 2006; Muramatsu et al., 2007) are plotted for comparison. Thin grey dashed lines are constant Br/I ratio contours, ranging from 0.01 to 10,000. The inset shows samples from the subduction zone setting: pelagic sediments, marine pore fluids, serpentinites (John et al., 2011; Kendrick et al., 2013), and blueschists (Pagé et al., 2016). |  Figure 4 (a) Mixing model for the source of halogens in loess. In the equations, CXlt, CHrl, and CLoess represent halogen concentrations of the UCC crystalline bedrocks, halogen-rich sedimentary layer, and loess, respectively; DOther represents other potential inputs from precipitation and/or evaporites that could contribute significant halogens without substantially affecting the system’s mass; mXlt and mHrl are assumed mass proportions of the UCC crystalline bedrocks and the sedimentary halogen-rich component in loess. (b) and (c) show the simulated relationship between mXlt and CHrl from Equation 3 in (a) when DOther = 0, for mixing of Br and I, respectively. The red and blue curves are calculated using either the geometric (blue) or arithmetic (red) means of the loess samples and the adjacent pale red/blue circles represent related errors (modelled from Monte Carlo re-sampling; 10,000 times). The vertical dashed lines are geometric (in blue, 2 s.e.) and arithmetic (in red, 2 s.e.) means of Japanese soils. The calculated mass proportion of crystalline rocks in loess is shown as horizontal dashed lines (with 2σ uncertainties), which are obtained when projecting the geometric or arithmetic mean of Japanese soils (Fig. S-10) to the red or blue curves. |
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Introduction
Halogens (F, Cl, Br, and I), an important group of lithophile and volatile elements, are primarily concentrated in Earth’s surface reservoirs (e.g., crust, seawater, and sediments) and play a critical role in modulating Earth’s habitability (Kendrick, 2024
Kendrick, M.A. (2024) Halogen Cycling in the Solid Earth. Annual Review of Earth and Planetary Sciences 52. https://doi.org/10.1146/annurev-earth-031621-111700
and references therein). To quantify the halogen cycle operating on Earth, it is necessary to have an accurate estimate of halogen distributions in different terrestrial reservoirs. However, such estimates are still lacking for much of the continental crust. This lack of knowledge regarding the geochemical behaviour of halogens during terrestrial geological processes, such as chemical weathering, metamorphic dehydration, and geobiological activity, limits our understanding of the role of the continental crust in the global halogen cycle, and also hinders our ability to utilise halogen concentrations and elemental ratios to trace geologic processes (Hanley and Koga, 2018Hanley, J.J., Koga, K.T. (2018) Halogens in terrestrial and cosmic geochemical systems: abundances, geochemical behaviors, and analytical methods. In: Harlov, D., Aranovich, L. (Eds.) The role of halogens in Terrestrial and Extraterrestrial Geochemical Processes. Springer, Cham. 21–121. https://doi.org/10.1007/978-3-319-61667-4_2
).To date, there are only five independent estimates of halogen concentrations in the upper continental crust (UCC), and these estimates can differ by up to a factor of ∼50 (Shaw et al., 1967
Shaw, D., Reilly, G., Muysson, J., Pattenden, G., Campbell, F. (1967) An Estimate of the Composition of the Canadian Precambrian shield. Canadian Journal of Earth Sciences 4, 829–853. https://doi.org/10.1139/e67-058
; Wedepohl, 1995Wedepohl, K.H. (1995) The composition of the continental crust. Geochimica et Cosmochimica Acta 59, 1217–1232. https://doi.org/10.1016/0016-7037(95)00038-2
; Gao et al., 1998Gao, S., Luo, T.C., Zhang, B.R., Zhang, H.F., Han, Y.W., Zhao, Z.D., Hu, Y.K. (1998) Chemical composition of the continental crust as revealed by studies in East China. Geochimica et Cosmochimica Acta 62, 1959–1975. http://dx.doi.org/10.1016/S0016-7037(98)00121-5
; Muramatsu and Wedepohl, 1998Muramatsu, Y., Wedepohl, K.H. (1998) The distribution of iodine in the earth’s crust. Chemical Geology 147, 201–216. https://doi.org/10.1016/S0009-2541(98)00013-8
; Han et al., 2023Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
). Poorly constrained halogen concentrations in continental rocks are partly due to heterogeneous halogen distributions, and partly due to the analytical challenges of obtaining precise and accurate halogen concentrations at low abundances, especially for Br and I. With recent advances in analytical techniques, Han et al. (2023)Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
reported a comprehensive set of high quality F-Cl-Br-I data for 24 composite samples of the fine grained matrix of ancient glacial diamictites, considered a proxy for upper continental crust (Gaschnig et al., 2016Gaschnig, R.M., Rudnick, R.L., McDonough, W.F., Kaufman, A.J., Valley, J.W., Hu, Z., Gao, S., Beck, M.L. (2016) Compositional of the upper continental crust through time, as constrained by ancient glacial diamictites. Geochimica et Cosmochimica Acta 186, 316–343. https://doi.org/10.1016/j.gca.2016.03.020
), and used these data to derive halogen estimates for the crystalline and weathered UCC. However, because the diamictites record chemical weathering in their provenance, the new estimates may represent minima for the crystalline UCC (Han et al., 2023Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
). Moreover, significant fractionation of Br/Cl and I/Cl ratios is observed in the glacial diamictites relative to pelagic sediments, and the reasons for this are unknown. These differences may reflect organic matter enrichment in the sea floor sediments relative to the UCC, chemical weathering of the UCC, or the effects of diagenesis/metamorphism on the diamictites (Han et al., 2023Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
). To better constrain the average halogen composition of the continental surface and to explore the mechanism(s) responsible for the fractionation of halogen ratios in sediments, we analysed 129 loess samples from worldwide localities.Loess is a silt-sized, terrestrial aeolian sediment that covers about 10 % of the continental surface (Pye, 1995
Pye, K. (1995) The nature, origin and accumulation of loess. Quaternary Science Reviews 14, 653–667. https://doi.org/10.1016/0277-3791(95)00047-X
). Previous studies have used loess as a proxy for the average composition of the UCC because of its wide scale sampling of the continental surface, limited chemical weathering (compared to shales) and minor mineralogical and elemental fractionation during sedimentary transportation and deposition (e.g., Taylor et al., 1983Taylor, S.R., Mclennan, S.M., McCulloch, M.T. (1983) Geochemistry of loess, continental crustal composition and crustal model ages. Geochimica et Cosmochimica Acta 47, 1897–1905. https://doi.org/10.1016/0016-7037(83)90206-5
; Gallet et al., 1998Gallet, S., Jahn, B.-m., Lanoë, B.V.V., Dia, A., Rossello, E. (1998) Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth and Planetary Science Letters 156, 157–172. https://doi.org/10.1016/S0012-821X(97)00218-5
; Chauvel et al., 2014Chauvel, C., Garçon, M., Bureau, S., Besnault, A., Jahn, B.-m., Ding, Z. (2014) Constraints from loess on the Hf–Nd isotopic composition of the upper continental crust. Earth and Planetary Science Letters 388, 48–58. https://doi.org/10.1016/j.epsl.2013.11.045
; Sauzéat et al., 2015Sauzéat, L., Rudnick, R.L., Chauvel, C., Garçon, M., Tang, M. (2015) New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature. Earth and Planetary Science Letters 428, 181–192. https://doi.org/10.1016/j.epsl.2015.07.032
). However, previous research on loess mainly focused on insoluble elements like rare earth elements, with little attention paid to halogens. Only two previous studies have reported halogen concentrations in loess, each focusing on just one or two halogens (F and Cl by Liu et al., 1981Liu, Y., Wen, Q., Chen, Q. (1981) F/Cl ratio in Luochuan loess section, Shaanxi province—A geochemical index for paleoclimatic fluctuation. Geochimica, 388–391
; I by Fan et al., 2021Fan, Y., Zhou, W., Hou, X., Kong, X., Chen, N., Burr, G.S. (2021) Indication of new climatic proxy by loess iodine variation. Quaternary Science Reviews 251, 106720. https://doi.org/10.1016/j.quascirev.2020.106720
) in loess-palaeosol profiles to track palaeoclimate. Thus, there have been no systematic studies of the behaviour of halogens in loess. Here, we report F-Cl-Br-I concentrations, along with major and rare earth element abundances, for 129 loess samples from Germany, Switzerland, the United States, Argentina, Kazakhstan, and China (Table S-1 and Fig. S-1), and discuss the behaviour of halogens on the continental surface and implications for global halogen recycling.top
Halogen Concentrations and Mineralogical Hosts in Loess
Halogen concentrations were analysed by two methods: (1) combustion ion chromatography analysis (C-IC) for F, and (2) NH4HF2 digestion and ICPMS analysis (N-ICPMS) for Cl, Br, and I (Supplementary Information). Halogen concentrations of loess are reported in Table S-1 and plotted in Figure 1. Each halogen has distinctive geochemical properties, making each likely to be controlled by different phases in loess. Loess is mainly composed of quartz, feldspar, and various amounts of phyllosilicates and carbonates (Pye, 1995
Pye, K. (1995) The nature, origin and accumulation of loess. Quaternary Science Reviews 14, 653–667. https://doi.org/10.1016/0277-3791(95)00047-X
). Fluorine, with the smallest anionic radius of the group, may readily replace the hydroxyl anion and be incorporated into phyllosilicates. This is supported by a good correlation between F and Al2O3 (R2 = 0.84) and K2O (R2 = 0.78) in loess, the latter two elements being enriched in phyllosilicates (Fig. 2a,b). Another mineral in loess capable of hosting F is carbonate, as two F− can substitute for CO32− (Feng et al., 2021Feng, X., Steiner, Z., Redfern, S.A. (2021) Fluorine incorporation into calcite, aragonite and vaterite CaCO3: Computational chemistry insights and geochemistry implications. Geochimica et Cosmochimica Acta 308, 384–392. https://doi.org/10.1016/j.gca.2021.05.029
). However, F and CaO show a slight negative correlation (Fig. S-3), suggesting that F in loess is mainly controlled by phyllosilicates, rather than carbonates. The anti-correlation between F and CaO contrasts with the relatively high F content observed in some marine carbonates (Rude and Aller, 1991Rude, P.D., Aller, R.C. (1991) Fluorine mobility during early diagenesis of carbonate sediment: An indicator of mineral transformations. Geochimica et Cosmochimica Acta 55, 2491–2509. https://doi.org/10.1016/0016-7037(91)90368-F
). The reason for a lack of carbonate control on F in the loess is unclear but may indicate that F substitution within carbonate occurs only in certain geological environments, which, in turn, might relate to the salinity, pCO2, and/or temperature of the water body from which the carbonates precipitate (e.g., Rude and Aller, 1991Rude, P.D., Aller, R.C. (1991) Fluorine mobility during early diagenesis of carbonate sediment: An indicator of mineral transformations. Geochimica et Cosmochimica Acta 55, 2491–2509. https://doi.org/10.1016/0016-7037(91)90368-F
; Ramos et al., 2005Ramos, A., Ohde, S., Hossain, M., Ozaki, H., Sirirattanachai, S., Apurado, J. (2005) Determination of fluorine in coral skeletons by instrumental neutron activation analysis. Journal of radioanalytical and nuclear chemistry 266, 19–29. https://doi.org/10.1007/s10967-005-0863-x
; Feng et al., 2021Feng, X., Steiner, Z., Redfern, S.A. (2021) Fluorine incorporation into calcite, aragonite and vaterite CaCO3: Computational chemistry insights and geochemistry implications. Geochimica et Cosmochimica Acta 308, 384–392. https://doi.org/10.1016/j.gca.2021.05.029
).Figure 1 Halogen (F, Cl, Br, and I) concentrations of loess samples. The blue dash-dotted and the red dashed lines represent the geometric and arithmetic means of the loess data set, respectively, along with uncertainties of 2 standard errors (s.e., light-shaded bars; see Fig. S-5 for more details on the calculation). Symbols above each histogram represent previous estimates of halogen abundances in the UCC.
Figure 2 Fluorine versus (a) Al2O3 and (b) K2O contents of loess. (c) Normalised rare earth element (REE) and halogen concentrations of loess samples relative to the composition of the crystalline UCC (UCCXlt): REE estimates for UCC are from Rudnick and Gao (2003)
Rudnick, R.L., Gao, S. (2003) Composition of the Continental Crust. Treatise on Geochemistry, 1–64. https://doi.org/10.1016/B0-08-043751-6/03016-4
and halogens are from Han et al. (2023)Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
. The cyan-shaded area shows the compositional field of glacial diamictite composites.The increasing anionic radii of Cl, Br, and I compared to F make them less likely to be incorporated into the crystal lattices of silicate minerals, particularly for Br and I. Instead, they may be absorbed on mineral surfaces or concentrated in fluid inclusions, evaporite minerals, and/or organic matter. We conducted a leaching experiment with MilliQ water on 14 samples to determine the location of these elements in loess (Table S-1 and Fig. S-4). The water soluble component of halogens in loess gradually decreases from Cl (as high as ∼85 %) to Br (∼50 %), to I (∼25 %) (Fig. S-4), suggesting that most of the Cl in loess exists in soluble form, while Br and I are mainly present in water-insoluble phases. The soluble halogens in loess (Cl and Br) may originate from a range of sources: (1) soluble inorganic halogens incorporated or absorbed onto the silicate phases within loess source materials, (2) soluble halogens in organic matter, (3) evaporite minerals like halite, and (4) post-depositional contamination from either anthropogenic sources and/or meteoric/ground water. The first three sources may reflect halogen distributions on the continental surface when loess deposits formed, while the fourth suggests potential disturbances of halogen contents after deposition. Since no significant regional differences are observed in the Cl and Br contents of samples from different areas (Fig. 1), and considering that their concentrations in meteoric water and groundwater are typically 10–100 times lower than those of loess (Worden, 2018
Worden, R.H. (2018) Halogen Elements in Sedimentary Systems and Their Evolution During Diagenesis. In: Harlov, D., Aranovich, L. (Eds.) The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes. Springer, Cham. 185–260. https://doi.org/10.1007/978-3-319-61667-4_4
), any enrichment from localised post-depositional processes, if they occurred, is likely minimal.The concentrations of residual Br and I (after leaching) in loess are significantly higher than their averages in glacial diamictites (Fig. S-4). Because both elements have a strongly biophilic affinity, they may primarily be hosted in organic matter. Loess contains a significantly greater amount of organic matter than crystalline bedrocks, which are the main source of glacial diamictites (Han et al., 2023
Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
). A possible reason for this difference might be the colder environment prevailing during the formation of glacial diamictites, leading to lower biomass on land surfaces and consequently less enrichment of halogens in the diamictites. This explanation also aligns with the proposition put forward by Fan et al. (2021)Fan, Y., Zhou, W., Hou, X., Kong, X., Chen, N., Burr, G.S. (2021) Indication of new climatic proxy by loess iodine variation. Quaternary Science Reviews 251, 106720. https://doi.org/10.1016/j.quascirev.2020.106720
regarding climate fluctuations and I content in loess: in warmer periods, more halogens will be transported from the ocean to land and retained by surface biota, while during colder periods, both halogen transport efficiency and retention on land will decrease. In other words, halogen concentrations in terrigenous sediments, particularly for the less soluble Br and I, may be used as a proxy to monitor past temperature changes and to trace patterns of halogen movement between land and sea.As a whole, the geometric and arithmetic means calculated from the data set are systematically higher than the recent estimates for crystalline UCC derived from data for ancient glacial diamictite composites (Han et al., 2023
Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
). Using a bootstrap approach, we estimate the degree of halogen enrichment in loess relative to crystalline UCC to be 1.3−0.4+0.7 times for F, 1.8−0.8+2.4 times for Cl, 3.8−1.0+1.3 times for Br, and 39−16+71 times for I (95 % confidence, see Fig. S-6), with an exponential increase in enrichment from F to I (i.e. F < Cl < Br << I; Fig. 2c and Fig. S-7). There are two non-mutually exclusive factors that could explain the higher halogen contents in loess relative to glacial diamictites: (1) a lower degree of chemical weathering in the provenance of loess compared to that of glacial diamictites, and (2) greater potential for incorporation and preservation of organic matter in loess than in glacial diamictites (Fig. S-8). On the other hand, the mean halogen concentrations in loess are comparable to previous estimates of halogens in the UCC based on either large scale sampling (Shaw et al., 1967Shaw, D., Reilly, G., Muysson, J., Pattenden, G., Campbell, F. (1967) An Estimate of the Composition of the Canadian Precambrian shield. Canadian Journal of Earth Sciences 4, 829–853. https://doi.org/10.1139/e67-058
; Gao et al., 1998Gao, S., Luo, T.C., Zhang, B.R., Zhang, H.F., Han, Y.W., Zhao, Z.D., Hu, Y.K. (1998) Chemical composition of the continental crust as revealed by studies in East China. Geochimica et Cosmochimica Acta 62, 1959–1975. http://dx.doi.org/10.1016/S0016-7037(98)00121-5
) or models based on lithologies (Wedepohl, 1995Wedepohl, K.H. (1995) The composition of the continental crust. Geochimica et Cosmochimica Acta 59, 1217–1232. https://doi.org/10.1016/0016-7037(95)00038-2
; Muramatsu and Wedepohl, 1998Muramatsu, Y., Wedepohl, K.H. (1998) The distribution of iodine in the earth’s crust. Chemical Geology 147, 201–216. https://doi.org/10.1016/S0009-2541(98)00013-8
) (Fig. 1), both of which incorporate data not only for crystalline bedrocks, but also for many halogen-rich lithologies such as shales, carbonates, and evaporites from the overlying sedimentary cover. The similarity between mean halogen concentrations in loess and these previous estimates suggests that halogens in loess may provide a useful average of halogen concentrations across various types of rocks and sediments at the continental surface. Halogen enrichment on the continental surface may derive from the oceans, a major halogen reservoir (Hanley and Koga, 2018Hanley, J.J., Koga, K.T. (2018) Halogens in terrestrial and cosmic geochemical systems: abundances, geochemical behaviors, and analytical methods. In: Harlov, D., Aranovich, L. (Eds.) The role of halogens in Terrestrial and Extraterrestrial Geochemical Processes. Springer, Cham. 21–121. https://doi.org/10.1007/978-3-319-61667-4_2
), with halogen fluxes from oceans to continents potentially regulated by climate fluctuations, as suggested by Fan et al. (2021)Fan, Y., Zhou, W., Hou, X., Kong, X., Chen, N., Burr, G.S. (2021) Indication of new climatic proxy by loess iodine variation. Quaternary Science Reviews 251, 106720. https://doi.org/10.1016/j.quascirev.2020.106720
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Halogen Ratios in Loess
The Br/Cl and I/Cl ratios of loess, as well as the leached residues of loess samples, are significantly fractionated relative to those in igneous rocks like MORB/OIB and arc/backarc lavas, closely tracking the compositional range seen in pelagic sediments, whose halogen concentrations are primarily controlled by marine organic matter (John et al., 2011
John, T., Scambelluri, M., Frische, M., Barnes, J.D., Bach, W. (2011) Dehydration of subducting serpentinite: implications for halogen mobility in subduction zones and the deep halogen cycle. Earth and Planetary Science Letters 308, 65–76. https://doi.org/10.1016/j.epsl.2011.05.038
) (Fig. 3). This supports the inference above, based on leaching experiments (Fig. S-4), that Br and I primarily reside in water-insoluble organic matter in loess. It also suggests that both terrigenous and marine organic matter have indistinguishable halogen ratios. This notion is further supported by the consistently narrow range of the Br/I ratio observed in organic-rich soils and sediments from Japan (Yamasaki et al., 2015Yamasaki, S.-i., Takeda, A., Watanabe, T., Tagami, K., Uchida, S., Takata, H., Maejima, Y., Kihou, N., Tsuchiya, N. (2015) Bromine and iodine in Japanese soils determined with polarizing energy dispersive X-ray fluorescence spectrometry. Soil Science and Plant Nutrition 61, 751–760. https://doi.org/10.1080/00380768.2015.1054773
) and China (He et al., 2018He, T., Xie, J., Hu, Z., Liu, T., Zhang, W., Chen, H., Liu, Y., Zong, K., Li, M. (2018) A rapid acid digestion technique for the simultaneous determination of bromine and iodine in fifty‐three Chinese soils and sediments by ICP‐MS. Geostandards and Geoanalytical Research 42, 309–318. https://doi.org/10.1111/ggr.12212
) (Fig. S-9). However, no correlation is observed between Br (or I) and TOC in either the loess or Japanese soil samples (Fig. S-10), which is somewhat counterintuitive and may result from varying Br/TOC (or I/TOC) ratios in different types of organic matter (Muramatsu and Wedepohl, 1998Muramatsu, Y., Wedepohl, K.H. (1998) The distribution of iodine in the earth’s crust. Chemical Geology 147, 201–216. https://doi.org/10.1016/S0009-2541(98)00013-8
; Mayer et al., 2007Mayer, L.M., Schick, L.L., Allison, M.A., Ruttenberg, K.C., Bentley, S.J. (2007) Marine vs. terrigenous organic matter in Louisiana coastal sediments: The uses of bromine: organic carbon ratios. Marine Chemistry 107, 244–254. https://doi.org/10.1016/j.marchem.2007.07.007
). Therefore, great caution should be exercised when using the correlation of Br and I with TOC to infer the presence of organohalogens in sediments.Figure 3 Br/Cl versus I/Cl ratios of loess (symbols as in Fig. 2; this study) and glacial diamictite composites (cyan diamonds; Han et al., 2023
Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
). Note that (1) the leached loess residues are plotted as deep-red dashed symbols, and (2) five carbonate-rich and terrigenous sediment-rich glacial diamictites from Namibia have notably elevated I content and relatively low Br/I ratios and are shown with open diamonds (see Han et al., 2023Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
for discussion). Halogen ratios of seawater (grey triangle), bulk silicate Earth (BSE, grey plus sign), mid-ocean ridge and ocean island basalts (MORB/OIB, grey rectangle, Kendrick et al., 2017Kendrick, M., Hémond, C., Kamenetsky, V., Danyushevsky, L., Devey, C.W., Rodemann, T., Jackson, M., Perfit, M. (2017) Seawater cycled throughout Earth’s mantle in partially serpentinized lithosphere. Nature Geoscience 10, 222–228. https://doi.org/10.1038/ngeo2902
), arc/backarc lavas (grey outline, Kendrick et al., 2020Kendrick, M.A., Danyushevsky, L.V., Falloon, T.J., Woodhead, J.D., Arculus, R.J., Ireland, T. (2020) SW Pacific arc and backarc lavas and the role of slab-bend serpentinites in the global halogen cycle. Earth and Planetary Science Letters 530, 115921. https://doi.org/10.1016/j.epsl.2019.115921
), pelagic sediments (grey dashed outline, John et al., 2011John, T., Scambelluri, M., Frische, M., Barnes, J.D., Bach, W. (2011) Dehydration of subducting serpentinite: implications for halogen mobility in subduction zones and the deep halogen cycle. Earth and Planetary Science Letters 308, 65–76. https://doi.org/10.1016/j.epsl.2011.05.038
), and marine pore fluids (green field, Fehn et al., 2006Fehn, U., Lu, Z., Tomaru, H. (2006) Data report: 129I/I ratios and halogen concentrations in pore water of Hydrate Ridge and their relevance for the origin of gas hydrates: A progress report. In: Tréhu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S. (Eds.) Proceedings of the Ocean Drilling Program: Scientific Results Volume 204. Texas A & M University, College Station, TX. 1–25. https://doi.org/10.2973/odp.proc.sr.204.107.2006
; Muramatsu et al., 2007Muramatsu, Y., Doi, T., Tomaru, H., Fehn, U., Takeuchi, R., Matsumoto, R. (2007) Halogen concentrations in pore waters and sediments of the Nankai Trough, Japan: Implications for the origin of gas hydrates. Applied Geochemistry 22, 534–556. https://doi.org/10.1016/j.apgeochem.2006.12.015
) are plotted for comparison. Thin grey dashed lines are constant Br/I ratio contours, ranging from 0.01 to 10,000. The inset shows samples from the subduction zone setting: pelagic sediments, marine pore fluids, serpentinites (John et al., 2011John, T., Scambelluri, M., Frische, M., Barnes, J.D., Bach, W. (2011) Dehydration of subducting serpentinite: implications for halogen mobility in subduction zones and the deep halogen cycle. Earth and Planetary Science Letters 308, 65–76. https://doi.org/10.1016/j.epsl.2011.05.038
; Kendrick et al., 2013Kendrick, M.A., Honda, M., Pettke, T., Scambelluri, M., Phillips, D., Giuliani, A. (2013) Subduction zone fluxes of halogens and noble gases in seafloor and forearc serpentinites. Earth and Planetary Science Letters 365, 86–96. https://doi.org/10.1016/j.epsl.2013.01.006
), and blueschists (Pagé et al., 2016Pagé, L., Hattori, K., de Hoog, J.C., Okay, A.I. (2016) Halogen (F, Cl, Br, I) behaviour in subducting slabs: A study of lawsonite blueschists in western Turkey. Earth and Planetary Science Letters 442, 133–142. https://doi.org/10.1016/j.epsl.2016.02.054
).The halogen ratios observed in loess are also similar to those found in blueschists and serpentinites from subduction zone settings (Fig. 3), indicating that the halogens in these metamorphic rocks may be overprinted by fluids derived from subducted terrigenous organic-bearing sediments, not just marine sediments (e.g., John et al., 2011
John, T., Scambelluri, M., Frische, M., Barnes, J.D., Bach, W. (2011) Dehydration of subducting serpentinite: implications for halogen mobility in subduction zones and the deep halogen cycle. Earth and Planetary Science Letters 308, 65–76. https://doi.org/10.1016/j.epsl.2011.05.038
; Kendrick et al., 2013Kendrick, M.A., Honda, M., Pettke, T., Scambelluri, M., Phillips, D., Giuliani, A. (2013) Subduction zone fluxes of halogens and noble gases in seafloor and forearc serpentinites. Earth and Planetary Science Letters 365, 86–96. https://doi.org/10.1016/j.epsl.2013.01.006
; Pagé et al., 2016Pagé, L., Hattori, K., de Hoog, J.C., Okay, A.I. (2016) Halogen (F, Cl, Br, I) behaviour in subducting slabs: A study of lawsonite blueschists in western Turkey. Earth and Planetary Science Letters 442, 133–142. https://doi.org/10.1016/j.epsl.2016.02.054
). Most glacial diamictite composites (excluding five carbonate and terrigenous sediment-rich samples from Namibia), however, have significantly higher Br/I ratios (7–74) compared to loess (0.25–6.3) (Fig. 3). This difference suggests that halogen ratios are significantly fractionated within the upper continental crust by an additional process, such as continental weathering and/or diagenesis/metamorphism. We note that the diamictites generally carry a stronger chemical weathering signature than loess (Fig. S-8), and most experienced greenschist facies metamorphism (Han et al., 2023Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
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Implications for Loess Provenance
Previous studies suggested that loess derives from rocks that have generally experienced moderate chemical weathering during multiple cycles of sedimentation (Gallet et al., 1998
Gallet, S., Jahn, B.-m., Lanoë, B.V.V., Dia, A., Rossello, E. (1998) Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth and Planetary Science Letters 156, 157–172. https://doi.org/10.1016/S0012-821X(97)00218-5
; Sauzéat et al., 2015Sauzéat, L., Rudnick, R.L., Chauvel, C., Garçon, M., Tang, M. (2015) New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature. Earth and Planetary Science Letters 428, 181–192. https://doi.org/10.1016/j.epsl.2015.07.032
). Based on the Li isotopic composition of loess, Sauzéat et al. (2015)Sauzéat, L., Rudnick, R.L., Chauvel, C., Garçon, M., Tang, M. (2015) New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature. Earth and Planetary Science Letters 428, 181–192. https://doi.org/10.1016/j.epsl.2015.07.032
estimated that the proportion of chemically weathered material at Earth’s surface was 37−10+17 %. However, because Li isotopes are not sensitive to the presence of organic matter, it remains unclear what proportion of material within loess is derived from pulverised unweathered crystalline bedrocks versus mature sediments that have extensively interacted with the biosphere. As Br and I are strongly enriched in organic-bearing sediments and their concentrations will be sensitive to the amount of such sediment in the loess provenance; we used a mixing model to calculate the proportions of the above sources in loess. In this calculation, the Br and I contents in loess are assumed to result from a mixture of (1) clastic sediments derived from glacially pulverised unweathered crystalline bedrocks, (2) an overlying halogen-rich sedimentary cover, and (3) inputs from other sources, such as precipitation (marine aerosols) and evaporite minerals (Fig. 4). Although the composition of the third end member is difficult to define and may vary between samples and locations, we assume that the first end member is well represented by the glacial diamictites (Han et al., 2023Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
), and the second by the Japanese soil samples (Yamasaki et al., 2015Yamasaki, S.-i., Takeda, A., Watanabe, T., Tagami, K., Uchida, S., Takata, H., Maejima, Y., Kihou, N., Tsuchiya, N. (2015) Bromine and iodine in Japanese soils determined with polarizing energy dispersive X-ray fluorescence spectrometry. Soil Science and Plant Nutrition 61, 751–760. https://doi.org/10.1080/00380768.2015.1054773
). When projecting the geometric or arithmetic mean of Japanese soils (Fig. S-11) onto the mixing line for either Br or I, assuming no other inputs, the calculated mass proportions of the UCC crystalline bedrocks in loess are consistently >80–90 % (Fig. 4). This proportion would be even higher if other inputs were considered. Overall, this suggests that the sediments that have extensively interacted with the biosphere in the loess provenance is <10–20 %, which is somewhat lower than the sum proportion of chemically weathered components, 37−10+17 %, at the continental surface as estimated by Sauzéat et al. (2015)Sauzéat, L., Rudnick, R.L., Chauvel, C., Garçon, M., Tang, M. (2015) New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature. Earth and Planetary Science Letters 428, 181–192. https://doi.org/10.1016/j.epsl.2015.07.032
. Thus, it can be concluded that the primary source of loess material is the crystalline bedrocks of the UCC, which validates its use to infer the average composition of the present day UCC (Taylor et al., 1983Taylor, S.R., Mclennan, S.M., McCulloch, M.T. (1983) Geochemistry of loess, continental crustal composition and crustal model ages. Geochimica et Cosmochimica Acta 47, 1897–1905. https://doi.org/10.1016/0016-7037(83)90206-5
; Gallet et al., 1998Gallet, S., Jahn, B.-m., Lanoë, B.V.V., Dia, A., Rossello, E. (1998) Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth and Planetary Science Letters 156, 157–172. https://doi.org/10.1016/S0012-821X(97)00218-5
; Chauvel et al., 2014Chauvel, C., Garçon, M., Bureau, S., Besnault, A., Jahn, B.-m., Ding, Z. (2014) Constraints from loess on the Hf–Nd isotopic composition of the upper continental crust. Earth and Planetary Science Letters 388, 48–58. https://doi.org/10.1016/j.epsl.2013.11.045
).Figure 4 (a) Mixing model for the source of halogens in loess. In the equations, CXlt, CHrl, and CLoess represent halogen concentrations of the UCC crystalline bedrocks, halogen-rich sedimentary layer, and loess, respectively; DOther represents other potential inputs from precipitation and/or evaporites that could contribute significant halogens without substantially affecting the system’s mass; mXlt and mHrl are assumed mass proportions of the UCC crystalline bedrocks and the sedimentary halogen-rich component in loess. (b) and (c) show the simulated relationship between mXlt and CHrl from Equation 3 in (a) when DOther = 0, for mixing of Br and I, respectively. The red and blue curves are calculated using either the geometric (blue) or arithmetic (red) means of the loess samples and the adjacent pale red/blue circles represent related errors (modelled from Monte Carlo re-sampling; 10,000 times). The vertical dashed lines are geometric (in blue, 2 s.e.) and arithmetic (in red, 2 s.e.) means of Japanese soils. The calculated mass proportion of crystalline rocks in loess is shown as horizontal dashed lines (with 2σ uncertainties), which are obtained when projecting the geometric or arithmetic mean of Japanese soils (Fig. S-10) to the red or blue curves.
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Acknowledgements
We dedicate this to the memory of Prof. Shan Gao (1962-2016), who influenced so much of our work on crust composition. We thank Hui Cao and Lang Wang for assistance with the N-ICPMS analyses, Gabriele Stoschek for help with the C-IC analyses, and Shin-ichi Yamasaki for providing the Japanese soil data. This work was supported by the State Key Laboratory of Geological Processes and Mineral Resources (GPMR202204), the University of California at Santa Barbara, and the U.S. National Science Foundation (EAR2321367). TH acknowledges the support from the National Natural Science Foundation of China (42103031). We are grateful to Romain Tartèse, Mark A. Kendrick, and two anonymous reviewers for constructive comments that helped us to improve the manuscript.
Editor: Romain Tartèse
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References
Chauvel, C., Garçon, M., Bureau, S., Besnault, A., Jahn, B.-m., Ding, Z. (2014) Constraints from loess on the Hf–Nd isotopic composition of the upper continental crust. Earth and Planetary Science Letters 388, 48–58. https://doi.org/10.1016/j.epsl.2013.11.045
Show in context Previous studies have used loess as a proxy for the average composition of the UCC because of its wide scale sampling of the continental surface, limited chemical weathering (compared to shales) and minor mineralogical and elemental fractionation during sedimentary transportation and deposition (e.g., Taylor et al., 1983; Gallet et al., 1998; Chauvel et al., 2014; Sauzéat et al., 2015).
View in article
Thus, it can be concluded that the primary source of loess material is the crystalline bedrocks of the UCC, which validates its use to infer the average composition of the present day UCC (Taylor et al., 1983; Gallet et al., 1998; Chauvel et al., 2014).
View in article
Fan, Y., Zhou, W., Hou, X., Kong, X., Chen, N., Burr, G.S. (2021) Indication of new climatic proxy by loess iodine variation. Quaternary Science Reviews 251, 106720. https://doi.org/10.1016/j.quascirev.2020.106720
Show in context However, previous research on loess mainly focused on insoluble elements like rare earth elements, with little attention paid to halogens. Only two previous studies have reported halogen concentrations in loess, each focusing on just one or two halogens (F and Cl by Liu et al., 1981; I by Fan et al., 2021) in loess-palaeosol profiles to track palaeoclimate.
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This explanation also aligns with the proposition put forward by Fan et al. (2021) regarding climate fluctuations and I content in loess: in warmer periods, more halogens will be transported from the ocean to land and retained by surface biota, while during colder periods, both halogen transport efficiency and retention on land will decrease.
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Halogen enrichment on the continental surface may derive from the oceans, a major halogen reservoir (Hanley and Koga, 2018), with halogen fluxes from oceans to continents potentially regulated by climate fluctuations, as suggested by Fan et al. (2021).
View in article
Fehn, U., Lu, Z., Tomaru, H. (2006) Data report: 129I/I ratios and halogen concentrations in pore water of Hydrate Ridge and their relevance for the origin of gas hydrates: A progress report. In: Tréhu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S. (Eds.) Proceedings of the Ocean Drilling Program: Scientific Results Volume 204. Texas A & M University, College Station, TX. 1–25. https://doi.org/10.2973/odp.proc.sr.204.107.2006
Show in context Halogen ratios of seawater (grey triangle), bulk silicate Earth (BSE, grey plus sign), mid-ocean ridge and ocean island basalts (MORB/OIB, grey rectangle, Kendrick et al., 2017), arc/backarc lavas (grey outline, Kendrick et al., 2020), pelagic sediments (grey dashed outline, John et al., 2011), and marine pore fluids (green field, Fehn et al., 2006; Muramatsu et al., 2007) are plotted for comparison.
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Feng, X., Steiner, Z., Redfern, S.A. (2021) Fluorine incorporation into calcite, aragonite and vaterite CaCO3: Computational chemistry insights and geochemistry implications. Geochimica et Cosmochimica Acta 308, 384–392. https://doi.org/10.1016/j.gca.2021.05.029
Show in context Another mineral in loess capable of hosting F is carbonate, as two F− can substitute for CO32− (Feng et al., 2021).
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The reason for a lack of carbonate control on F in the loess is unclear but may indicate that F substitution within carbonate occurs only in certain geological environments, which, in turn, might relate to the salinity, pCO2, and/or temperature of the water body from which the carbonates precipitate (e.g., Rude and Aller, 1991; Ramos et al., 2005; Feng et al., 2021).
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Gallet, S., Jahn, B.-m., Lanoë, B.V.V., Dia, A., Rossello, E. (1998) Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth and Planetary Science Letters 156, 157–172. https://doi.org/10.1016/S0012-821X(97)00218-5
Show in context Previous studies have used loess as a proxy for the average composition of the UCC because of its wide scale sampling of the continental surface, limited chemical weathering (compared to shales) and minor mineralogical and elemental fractionation during sedimentary transportation and deposition (e.g., Taylor et al., 1983; Gallet et al., 1998; Chauvel et al., 2014; Sauzéat et al., 2015).
View in article
Previous studies suggested that loess derives from rocks that have generally experienced moderate chemical weathering during multiple cycles of sedimentation (Gallet et al., 1998; Sauzéat et al., 2015).
View in article
Thus, it can be concluded that the primary source of loess material is the crystalline bedrocks of the UCC, which validates its use to infer the average composition of the present day UCC (Taylor et al., 1983; Gallet et al., 1998; Chauvel et al., 2014).
View in article
Gao, S., Luo, T.C., Zhang, B.R., Zhang, H.F., Han, Y.W., Zhao, Z.D., Hu, Y.K. (1998) Chemical composition of the continental crust as revealed by studies in East China. Geochimica et Cosmochimica Acta 62, 1959–1975. https://dx.doi.org/10.1016/S0016-7037(98)00121-5
Show in context
To date, there are only five independent estimates of halogen concentrations in the upper continental crust (UCC), and these estimates can differ by up to a factor of ∼50 (Shaw et al., 1967; Wedepohl, 1995; Gao et al., 1998; Muramatsu and Wedepohl, 1998; Han et al., 2023).
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On the other hand, the mean halogen concentrations in loess are comparable to previous estimates of halogens in the UCC based on either large scale sampling (Shaw et al., 1967; Gao et al., 1998) or models based on lithologies (Wedepohl, 1995; Muramatsu and Wedepohl, 1998) (Fig. 1), both of which incorporate data not only for crystalline bedrocks, but also for many halogen-rich lithologies such as shales, carbonates, and evaporites from the overlying sedimentary cover.
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Gaschnig, R.M., Rudnick, R.L., McDonough, W.F., Kaufman, A.J., Valley, J.W., Hu, Z., Gao, S., Beck, M.L. (2016) Compositional of the upper continental crust through time, as constrained by ancient glacial diamictites. Geochimica et Cosmochimica Acta 186, 316–343. https://doi.org/10.1016/j.gca.2016.03.020
Show in context With recent advances in analytical techniques, Han et al. (2023) reported a comprehensive set of high quality F-Cl-Br-I data for 24 composite samples of the fine grained matrix of ancient glacial diamictites, considered a proxy for upper continental crust (Gaschnig et al., 2016), and used these data to derive halogen estimates for the crystalline and weathered UCC.
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Han, P.-Y., Rudnick, R.L., He, T., Marks, M.A., Wang, S.-J., Gaschnig, R.M., Hu, Z.-C. (2023) Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites. Geochimica et Cosmochimica Acta 341, 28–45. https://doi.org/10.1016/j.gca.2022.11.012
Show in context To date, there are only five independent estimates of halogen concentrations in the upper continental crust (UCC), and these estimates can differ by up to a factor of ∼50 (Shaw et al., 1967; Wedepohl, 1995; Gao et al., 1998; Muramatsu and Wedepohl, 1998; Han et al., 2023).
View in article
With recent advances in analytical techniques, Han et al. (2023) reported a comprehensive set of high quality F-Cl-Br-I data for 24 composite samples of the fine grained matrix of ancient glacial diamictites, considered a proxy for upper continental crust (Gaschnig et al., 2016), and used these data to derive halogen estimates for the crystalline and weathered UCC.
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However, because the diamictites record chemical weathering in their provenance, the new estimates may represent minima for the crystalline UCC (Han et al., 2023).
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These differences may reflect organic matter enrichment in the sea floor sediments relative to the UCC, chemical weathering of the UCC, or the effects of diagenesis/metamorphism on the diamictites (Han et al., 2023).
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Fluorine versus (a) Al2O3 and (b) K2O contents of loess. (c) Normalised rare earth element (REE) and halogen concentrations of loess samples relative to the composition of the crystalline UCC (UCCXlt): REE estimates for UCC are from Rudnick and Gao (2003) and halogens are from Han et al. (2023).
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Loess contains a significantly greater amount of organic matter than crystalline bedrocks, which are the main source of glacial diamictites (Han et al., 2023).
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As a whole, the geometric and arithmetic means calculated from the data set are systematically higher than the recent estimates for crystalline UCC derived from data for ancient glacial diamictite composites (Han et al., 2023).
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Br/Cl versus I/Cl ratios of loess (symbols as in Fig. 2; this study) and glacial diamictite composites (cyan diamonds; Han et al., 2023).
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Note that (1) the leached loess residues are plotted as deep-red dashed symbols, and (2) five carbonate-rich and terrigenous sediment-rich glacial diamictites from Namibia have notably elevated I content and relatively low Br/I ratios and are shown with open diamonds (see Han et al., 2023 for discussion).
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We note that the diamictites generally carry a stronger chemical weathering signature than loess (Fig. S-8), and most experienced greenschist facies metamorphism (Han et al., 2023).
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Although the composition of the third end member is difficult to define and may vary between samples and locations, we assume that the first end member is well represented by the glacial diamictites (Han et al., 2023), and the second by the Japanese soil samples (Yamasaki et al., 2015).
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Hanley, J.J., Koga, K.T. (2018) Halogens in terrestrial and cosmic geochemical systems: abundances, geochemical behaviors, and analytical methods. In: Harlov, D., Aranovich, L. (Eds.) The role of halogens in Terrestrial and Extraterrestrial Geochemical Processes. Springer, Cham. 21–121. https://doi.org/10.1007/978-3-319-61667-4_2
Show in context This lack of knowledge regarding the geochemical behaviour of halogens during terrestrial geological processes, such as chemical weathering, metamorphic dehydration, and geobiological activity, limits our understanding of the role of the continental crust in the global halogen cycle, and also hinders our ability to utilise halogen concentrations and elemental ratios to trace geologic processes (Hanley and Koga, 2018).
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Halogen enrichment on the continental surface may derive from the oceans, a major halogen reservoir (Hanley and Koga, 2018), with halogen fluxes from oceans to continents potentially regulated by climate fluctuations, as suggested by Fan et al. (2021).
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He, T., Xie, J., Hu, Z., Liu, T., Zhang, W., Chen, H., Liu, Y., Zong, K., Li, M. (2018) A rapid acid digestion technique for the simultaneous determination of bromine and iodine in fifty‐three Chinese soils and sediments by ICP‐MS. Geostandards and Geoanalytical Research 42, 309–318. https://doi.org/10.1111/ggr.12212
Show in context This notion is further supported by the consistently narrow range of the Br/I ratio observed in organic-rich soils and sediments from Japan (Yamasaki et al., 2015) and China (He et al., 2018) (Fig. S-9).
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John, T., Scambelluri, M., Frische, M., Barnes, J.D., Bach, W. (2011) Dehydration of subducting serpentinite: implications for halogen mobility in subduction zones and the deep halogen cycle. Earth and Planetary Science Letters 308, 65–76. https://doi.org/10.1016/j.epsl.2011.05.038
Show in context The Br/Cl and I/Cl ratios of loess, as well as the leached residues of loess samples, are significantly fractionated relative to those in igneous rocks like MORB/OIB and arc/backarc lavas, closely tracking the compositional range seen in pelagic sediments, whose halogen concentrations are primarily controlled by marine organic matter (John et al., 2011) (Fig. 3).
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Halogen ratios of seawater (grey triangle), bulk silicate Earth (BSE, grey plus sign), mid-ocean ridge and ocean island basalts (MORB/OIB, grey rectangle, Kendrick et al., 2017), arc/backarc lavas (grey outline, Kendrick et al., 2020), pelagic sediments (grey dashed outline, John et al., 2011), and marine pore fluids (green field, Fehn et al., 2006; Muramatsu et al., 2007) are plotted for comparison.
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The inset shows samples from the subduction zone setting: pelagic sediments, marine pore fluids, serpentinites (John et al., 2011; Kendrick et al., 2013), and blueschists (Pagé et al., 2016).
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The halogen ratios observed in loess are also similar to those found in blueschists and serpentinites from subduction zone settings (Fig. 3), indicating that the halogens in these metamorphic rocks may be overprinted by fluids derived from subducted terrigenous organic-bearing sediments, not just marine sediments (e.g., John et al., 2011; Kendrick et al., 2013; Pagé et al., 2016).
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Kendrick, M.A. (2024) Halogen Cycling in the Solid Earth. Annual Review of Earth and Planetary Sciences 52. https://doi.org/10.1146/annurev-earth-031621-111700
Show in context Halogens (F, Cl, Br, and I), an important group of lithophile and volatile elements, are primarily concentrated in Earth’s surface reservoirs (e.g., crust, seawater, and sediments) and play a critical role in modulating Earth’s habitability (Kendrick, 2024 and references therein).
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Kendrick, M.A., Honda, M., Pettke, T., Scambelluri, M., Phillips, D., Giuliani, A. (2013) Subduction zone fluxes of halogens and noble gases in seafloor and forearc serpentinites. Earth and Planetary Science Letters 365, 86–96. https://doi.org/10.1016/j.epsl.2013.01.006
Show in context The inset shows samples from the subduction zone setting: pelagic sediments, marine pore fluids, serpentinites (John et al., 2011; Kendrick et al., 2013), and blueschists (Pagé et al., 2016).
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The halogen ratios observed in loess are also similar to those found in blueschists and serpentinites from subduction zone settings (Fig. 3), indicating that the halogens in these metamorphic rocks may be overprinted by fluids derived from subducted terrigenous organic-bearing sediments, not just marine sediments (e.g., John et al., 2011; Kendrick et al., 2013; Pagé et al., 2016).
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Kendrick, M., Hémond, C., Kamenetsky, V., Danyushevsky, L., Devey, C.W., Rodemann, T., Jackson, M., Perfit, M. (2017) Seawater cycled throughout Earth’s mantle in partially serpentinized lithosphere. Nature Geoscience 10, 222–228. https://doi.org/10.1038/ngeo2902
Show in context Halogen ratios of seawater (grey triangle), bulk silicate Earth (BSE, grey plus sign), mid-ocean ridge and ocean island basalts (MORB/OIB, grey rectangle, Kendrick et al., 2017), arc/backarc lavas (grey outline, Kendrick et al., 2020), pelagic sediments (grey dashed outline, John et al., 2011), and marine pore fluids (green field, Fehn et al., 2006; Muramatsu et al., 2007) are plotted for comparison.
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Kendrick, M.A., Danyushevsky, L.V., Falloon, T.J., Woodhead, J.D., Arculus, R.J., Ireland, T. (2020) SW Pacific arc and backarc lavas and the role of slab-bend serpentinites in the global halogen cycle. Earth and Planetary Science Letters 530, 115921. https://doi.org/10.1016/j.epsl.2019.115921
Show in context Halogen ratios of seawater (grey triangle), bulk silicate Earth (BSE, grey plus sign), mid-ocean ridge and ocean island basalts (MORB/OIB, grey rectangle, Kendrick et al., 2017), arc/backarc lavas (grey outline, Kendrick et al., 2020), pelagic sediments (grey dashed outline, John et al., 2011), and marine pore fluids (green field, Fehn et al., 2006; Muramatsu et al., 2007) are plotted for comparison.
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Liu, Y., Wen, Q., Chen, Q. (1981) F/Cl ratio in Luochuan loess section, Shaanxi province—A geochemical index for paleoclimatic fluctuation. Geochimica, 388–391.
Show in context However, previous research on loess mainly focused on insoluble elements like rare earth elements, with little attention paid to halogens. Only two previous studies have reported halogen concentrations in loess, each focusing on just one or two halogens (F and Cl by Liu et al., 1981; I by Fan et al., 2021) in loess-palaeosol profiles to track palaeoclimate.
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Mayer, L.M., Schick, L.L., Allison, M.A., Ruttenberg, K.C., Bentley, S.J. (2007) Marine vs. terrigenous organic matter in Louisiana coastal sediments: The uses of bromine: organic carbon ratios. Marine Chemistry 107, 244–254. https://doi.org/10.1016/j.marchem.2007.07.007
Show in context However, no correlation is observed between Br (or I) and TOC in either the loess or Japanese soil samples (Fig. S-10), which is somewhat counterintuitive and may result from varying Br/TOC (or I/TOC) ratios in different types of organic matter (Muramatsu and Wedepohl, 1998; Mayer et al., 2007).
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Muramatsu, Y., Wedepohl, K.H. (1998) The distribution of iodine in the earth’s crust. Chemical Geology 147, 201–216. https://doi.org/10.1016/S0009-2541(98)00013-8
Show in context To date, there are only five independent estimates of halogen concentrations in the upper continental crust (UCC), and these estimates can differ by up to a factor of ∼50 (Shaw et al., 1967; Wedepohl, 1995; Gao et al., 1998; Muramatsu and Wedepohl, 1998; Han et al., 2023).
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On the other hand, the mean halogen concentrations in loess are comparable to previous estimates of halogens in the UCC based on either large scale sampling (Shaw et al., 1967; Gao et al., 1998) or models based on lithologies (Wedepohl, 1995; Muramatsu and Wedepohl, 1998) (Fig. 1), both of which incorporate data not only for crystalline bedrocks, but also for many halogen-rich lithologies such as shales, carbonates, and evaporites from the overlying sedimentary cover.
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However, no correlation is observed between Br (or I) and TOC in either the loess or Japanese soil samples (Fig. S-10), which is somewhat counterintuitive and may result from varying Br/TOC (or I/TOC) ratios in different types of organic matter (Muramatsu and Wedepohl, 1998; Mayer et al., 2007).
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Muramatsu, Y., Doi, T., Tomaru, H., Fehn, U., Takeuchi, R., Matsumoto, R. (2007) Halogen concentrations in pore waters and sediments of the Nankai Trough, Japan: Implications for the origin of gas hydrates. Applied Geochemistry 22, 534–556. https://doi.org/10.1016/j.apgeochem.2006.12.015
Show in context Halogen ratios of seawater (grey triangle), bulk silicate Earth (BSE, grey plus sign), mid-ocean ridge and ocean island basalts (MORB/OIB, grey rectangle, Kendrick et al., 2017), arc/backarc lavas (grey outline, Kendrick et al., 2020), pelagic sediments (grey dashed outline, John et al., 2011), and marine pore fluids (green field, Fehn et al., 2006; Muramatsu et al., 2007) are plotted for comparison.
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Pagé, L., Hattori, K., de Hoog, J.C., Okay, A.I. (2016) Halogen (F, Cl, Br, I) behaviour in subducting slabs: A study of lawsonite blueschists in western Turkey. Earth and Planetary Science Letters 442, 133–142. https://doi.org/10.1016/j.epsl.2016.02.054
Show in context The inset shows samples from the subduction zone setting: pelagic sediments, marine pore fluids, serpentinites (John et al., 2011; Kendrick et al., 2013), and blueschists (Pagé et al., 2016).
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The halogen ratios observed in loess are also similar to those found in blueschists and serpentinites from subduction zone settings (Fig. 3), indicating that the halogens in these metamorphic rocks may be overprinted by fluids derived from subducted terrigenous organic-bearing sediments, not just marine sediments (e.g., John et al., 2011; Kendrick et al., 2013; Pagé et al., 2016).
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Pye, K. (1995) The nature, origin and accumulation of loess. Quaternary Science Reviews 14, 653–667. https://doi.org/10.1016/0277-3791(95)00047-X
Show in context Loess is a silt-sized, terrestrial aeolian sediment that covers about 10 % of the continental surface (Pye, 1995).
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Each halogen has distinctive geochemical properties, making each likely to be controlled by different phases in loess. Loess is mainly composed of quartz, feldspar, and various amounts of phyllosilicates and carbonates (Pye, 1995).
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Ramos, A., Ohde, S., Hossain, M., Ozaki, H., Sirirattanachai, S., Apurado, J. (2005) Determination of fluorine in coral skeletons by instrumental neutron activation analysis. Journal of radioanalytical and nuclear chemistry 266, 19–29. https://doi.org/10.1007/s10967-005-0863-x
Show in context The reason for a lack of carbonate control on F in the loess is unclear but may indicate that F substitution within carbonate occurs only in certain geological environments, which, in turn, might relate to the salinity, pCO2, and/or temperature of the water body from which the carbonates precipitate (e.g., Rude and Aller, 1991; Ramos et al., 2005; Feng et al., 2021).
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Rude, P.D., Aller, R.C. (1991) Fluorine mobility during early diagenesis of carbonate sediment: An indicator of mineral transformations. Geochimica et Cosmochimica Acta 55, 2491–2509. https://doi.org/10.1016/0016-7037(91)90368-F
Show in context The anti-correlation between F and CaO contrasts with the relatively high F content observed in some marine carbonates (Rude and Aller, 1991).
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The reason for a lack of carbonate control on F in the loess is unclear but may indicate that F substitution within carbonate occurs only in certain geological environments, which, in turn, might relate to the salinity, pCO2, and/or temperature of the water body from which the carbonates precipitate (e.g., Rude and Aller, 1991; Ramos et al., 2005; Feng et al., 2021).
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Rudnick, R.L., Gao, S. (2003) Composition of the Continental Crust. Treatise on Geochemistry, 1–64. https://doi.org/10.1016/B0-08-043751-6/03016-4
Show in context Fluorine versus (a) Al2O3 and (b) K2O contents of loess. (c) Normalised rare earth element (REE) and halogen concentrations of loess samples relative to the composition of the crystalline UCC (UCCXlt): REE estimates for UCC are from Rudnick and Gao (2003) and halogens are from Han et al. (2023).
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Sauzéat, L., Rudnick, R.L., Chauvel, C., Garçon, M., Tang, M. (2015) New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature. Earth and Planetary Science Letters 428, 181–192. https://doi.org/10.1016/j.epsl.2015.07.032
Show in context Previous studies have used loess as a proxy for the average composition of the UCC because of its wide scale sampling of the continental surface, limited chemical weathering (compared to shales) and minor mineralogical and elemental fractionation during sedimentary transportation and deposition (e.g., Taylor et al., 1983; Gallet et al., 1998; Chauvel et al., 2014; Sauzéat et al., 2015).
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Previous studies suggested that loess derives from rocks that have generally experienced moderate chemical weathering during multiple cycles of sedimentation (Gallet et al., 1998; Sauzéat et al., 2015).
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Based on the Li isotopic composition of loess, Sauzéat et al. (2015) estimated that the proportion of chemically weathered material at Earth’s surface was 37−10+17 %.
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Overall, this suggests that the sediments that have extensively interacted with the biosphere in the loess provenance is <10–20 %, which is somewhat lower than the sum proportion of chemically weathered components, 37−10+17 %, at the continental surface as estimated by Sauzéat et al. (2015).
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Shaw, D., Reilly, G., Muysson, J., Pattenden, G., Campbell, F. (1967) An Estimate of the Composition of the Canadian Precambrian shield. Canadian Journal of Earth Sciences 4, 829–853. https://doi.org/10.1139/e67-058
Show in context To date, there are only five independent estimates of halogen concentrations in the upper continental crust (UCC), and these estimates can differ by up to a factor of ∼50 (Shaw et al., 1967; Wedepohl, 1995; Gao et al., 1998; Muramatsu and Wedepohl, 1998; Han et al., 2023).
View in article
On the other hand, the mean halogen concentrations in loess are comparable to previous estimates of halogens in the UCC based on either large scale sampling (Shaw et al., 1967; Gao et al., 1998) or models based on lithologies (Wedepohl, 1995; Muramatsu and Wedepohl, 1998) (Fig. 1), both of which incorporate data not only for crystalline bedrocks, but also for many halogen-rich lithologies such as shales, carbonates, and evaporites from the overlying sedimentary cover.
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Taylor, S.R., Mclennan, S.M., McCulloch, M.T. (1983) Geochemistry of loess, continental crustal composition and crustal model ages. Geochimica et Cosmochimica Acta 47, 1897–1905. https://doi.org/10.1016/0016-7037(83)90206-5
Show in context Previous studies have used loess as a proxy for the average composition of the UCC because of its wide scale sampling of the continental surface, limited chemical weathering (compared to shales) and minor mineralogical and elemental fractionation during sedimentary transportation and deposition (e.g., Taylor et al., 1983; Gallet et al., 1998; Chauvel et al., 2014; Sauzéat et al., 2015).
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Thus, it can be concluded that the primary source of loess material is the crystalline bedrocks of the UCC, which validates its use to infer the average composition of the present day UCC (Taylor et al., 1983; Gallet et al., 1998; Chauvel et al., 2014).
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Wedepohl, K.H. (1995) The composition of the continental crust. Geochimica et Cosmochimica Acta 59, 1217–1232. https://doi.org/10.1016/0016-7037(95)00038-2
Show in context To date, there are only five independent estimates of halogen concentrations in the upper continental crust (UCC), and these estimates can differ by up to a factor of ∼50 (Shaw et al., 1967; Wedepohl, 1995; Gao et al., 1998; Muramatsu and Wedepohl, 1998; Han et al., 2023).
View in article
On the other hand, the mean halogen concentrations in loess are comparable to previous estimates of halogens in the UCC based on either large scale sampling (Shaw et al., 1967; Gao et al., 1998) or models based on lithologies (Wedepohl, 1995; Muramatsu and Wedepohl, 1998) (Fig. 1), both of which incorporate data not only for crystalline bedrocks, but also for many halogen-rich lithologies such as shales, carbonates, and evaporites from the overlying sedimentary cover.
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Worden, R.H. (2018) Halogen Elements in Sedimentary Systems and Their Evolution During Diagenesis. In: Harlov, D., Aranovich, L. (Eds.) The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes. Springer, Cham. 185–260. https://doi.org/10.1007/978-3-319-61667-4_4
Show in context Since no significant regional differences are observed in the Cl and Br contents of samples from different areas (Fig. 1), and considering that their concentrations in meteoric water and groundwater are typically 10–100 times lower than those of loess (Worden, 2018), any enrichment from localised post-depositional processes, if they occurred, is likely minimal.
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Yamasaki, S.-i., Takeda, A., Watanabe, T., Tagami, K., Uchida, S., Takata, H., Maejima, Y., Kihou, N., Tsuchiya, N. (2015) Bromine and iodine in Japanese soils determined with polarizing energy dispersive X-ray fluorescence spectrometry. Soil Science and Plant Nutrition 61, 751–760. https://doi.org/10.1080/00380768.2015.1054773
Show in context This notion is further supported by the consistently narrow range of the Br/I ratio observed in organic-rich soils and sediments from Japan (Yamasaki et al., 2015) and China (He et al., 2018) (Fig. S-9).
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Although the composition of the third end member is difficult to define and may vary between samples and locations, we assume that the first end member is well represented by the glacial diamictites (Han et al., 2023), and the second by the Japanese soil samples (Yamasaki et al., 2015).
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Supplementary Information
The Supplementary Information includes:
- Samples and Analytical Methods
- Table S-1
- Figures S-1 to S-11
- Supplementary Information References
Download Table S-1 (.xlsx)
Download the Supplementary Information (PDF)
