Figures and Tables

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Existence and Characteristics of Asymptotes

In the oxic ocean, dissolved Mo (Mo_aq) is almost entirely MoO₄^2–. Particulate Mo (Mo_s) constitutes <0.001 % of ΣMo, except near terrigenous or hydrothermal particle sources (Ho et al., 2018

Ho, P., Lee, J.-M., Heller, M.I., Lam, P.J., Shiller, A.M. (2018) The distributions of dissolved and particulate Mo and V along the U.S. GEOTRACES East Pacific zonal transect (GP16): The roles of oxides and biogenic particles in their distribution in the oxygen deficient zone and the hydrothermal plume. Marine Chemistry 201, 242–255.

). However, where H₂S or HS^– replaces dissolved O₂, MoO₄^2– becomes thiolated, and Mo_aq concentrations fall sharply toward asymptotes while Mo_s in contiguous sediments rises. The resulting elevated Mo_s is interpreted as signifying sulfide’s presence during sedimentation.

A compilation in Table 1 shows that the span of Mo_aq asymptote concentrations in diverse sulfidic environments is remarkably narrow. Entries in Table 1 meet the criteria that (a) a water column or pore water becomes sulfidic at depth, (b) Mo_aq drops markedly (by >50 %) below where sulfide first appears, and (c) Mo_aq ultimately stabilises at asymptotes (designated Mo_aq,∞) that are distinctly above analytical detection limits. The median asymptote value is 7.8 nM, and four fifths of the values fall in the range 2.5–13.3 nM. Asymptotes are prevalent in nature, but not universal; beneath some lakes and coastal embayments, pore water Mo_aq concentrations fall to minima before rising at greater depths (see Morford et al., 2009

Morford, J.L., Martin, W.R., François, R., Carney, C.M. (2009) A model for uranium, rhenium and molybdenum diagenesis in marine sediments based on results from coastal locations. Geochimica et Cosmochimica Acta 73, 2938–2960.

; Dahl et al., 2010

Dahl, T.W., Anbar, A.D., Gordon, G.W., Rosing, M.T., Frei, R., Canfield, D.E. (2010) The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland. Geochimica et Cosmochimica Acta 74, 144–163.

; Havig et al., 2015

Havig, J.R., McCormick, M.L., Hamilton, T.L., Kump, L.R. (2015) The behavior of biologically important trace elements across the oxic/euxinic transition of meromictic Fayettville Green Lake, New York, USA. Geochimica et Cosmochimica Acta 165, 389–406.

; Sulu-Gambari et al., 2017

Sulu-Gambari, R., Roepert, A., Jilbert, T., Hagens, M., Meysman, F.J.R., Slomp, C.P. (2017) Molybdenum dynamics in sediments of a seasonally-hypoxic coastal marine basin. Chemical Geology 466, 627–640.

). The reason for this atypical pattern is unclear; infiltration of groundwater from surrounding uplands or very slow reductive dissolution of Mo-rich detrital phases sourced from nearby land may be contributing factors.

Removal within:	pH	S−II (mM)	Particulate POC* (mM)	Moaq,0 (nM)	Moaq,∞ (nM)	Moaq,∞/Moaq,0 (%)
Euxinic marine water columns
Black Sea (1991)		0.30–0.41	0.0025	36	2.6	7
Black Sea (2011)	7.6	0.30–0.37		37	7.8	21
Black Sea (2017		0.35–0.42	0.0025–0.005	41	5.9	14
Black Sea (2019)	7.34	0.31–0.36		36	6.9	19
Framvaren Fjord	7.1	0.30–7.20	0.0027 ± 0.0006	50	17.2	34
Kyllaren Fjord		4.23–5.02	0.003–0.008	71	7.5	11
Rogoznica Lake (Sept.)	7.2	0.42–2.34	0.24 ± 0.10	97	4.5	5
Sulfidic pore waters
Chesapeake Bay	7.2	1.15		55	7.8	14
Guaymas Basin (MUC9, >25 cm)		0.12–0.43	1580 ± 68	130	12.3	9
Hingham Bay (Oct. 2001, >7 cm)		0.49–1.01	1530 ± 79	112	9.3	8
Landsort Deep (LD 1)		1.73–2.09	2200 ± 900	26	2.1	8
Long Island Sound (FOAM, >20 cm)		2.5–3.3	680 ± 120	115	13.3	12
Storfjärden (Stn 3)		0.005–0.45	2100 ± 300	30	8.	27
Terrebonne Bay (9A)	7.0	0.09–1.61		33	7.3	22
Walker Lake (WL-2)	8.4	5.4–6.2	620–1100	5700	134.	2
York River groundwater	6.9	0.91–1.60		73	1.1	2
Euxinic lake water columns
Fayetteville Green Lake	6.9	0.44–1.86	0.066	121	13.2	11
Lago Cadagno (Aug 2007)	7.1	0.10	0.0068	14	2.5	18
Lake Fryxell	7.5	0.30–1.20	0.062 ± 0.023	26	3.5	13
Mahoney Lake	7.4	35.0		230	12.	5
Median					7.8	12

*Particulate POC concentrations are expressed per litre of solution. Where more than three POC measurements are available, mean ± standard deviation is reported; otherwise, the range is given.

Of particular importance in Table 1 is the million-fold range of particulate organic carbon (POC) concentrations (expressed per unit volume of solution). For instance, during diagenesis, solutes in Hingham Bay’s sulfidic pore waters are in diffusive contact with ∼1.5 moles POC/L (calculated for sediments with 3.2 % total organic carbon, porosity = 0.8, and 2.5 g/cm³ dry density; see Note II in the Supplementary Information). In contrast, measured POC concentrations in the Black Sea’s sulfidic deep waters are 2.5–5.0 × 10^–6 mol/L (Kaiser et al., 2017

Kaiser, D., Konovalov, S., Schulz-Bull, D.E., Waniek, J.J. (2017) Organic matter along longitudinal and vertical gradients in the Black Sea. Deep Sea Research Part I 129, 22–31.

). Despite the 10⁶ difference, similar Mo_aq,∞ asymptotes occur in both places. It could be argued that a mismatch in renewal rates of water and organic particles in the Black Sea makes this apparent POC disparity misleading. Below 1 km depth, the Black Sea’s water is renewed every ∼10³ yr whereas its POC is renewed every ∼10¹ yr (estimated from the POC accumulation rate in underlying sediments; Arthur et al., 1994

Arthur, M.A., Dean, W.E., Neff, E.D., Hay, B.J., King, J., Jones, G. (1994) Varve calibrated records of carbonate and organic carbon accumulation over the last 2000 years in the Black Sea. Global Biogeochemical Cycles 8, 195–217.

). Thus, deep water is cumulatively exposed to 10² times more POC than is present instantaneously. On this basis, the Black Sea’s POC concentration is only 10⁴, rather than 10⁶, times smaller than Hingham Bay’s, but the disparity is still huge.

Asymptotes are likewise insensitive to final S^–II concentrations (Table 1). For example, the maximum S^–II concentration in the Black Sea is 0.4 mM and that in Kyllaren Fjord is 5.0 mM, but both reach similar Mo_aq,∞ asymptotes. The extreme Mo_aq,∞ asymptotes in Table 1 (i.e. Walker Lake, 130 nM, and York River, 1.1 nM) mirror pH extremes, hinting that pH explains some of the asymptote variation, though the data are insufficient to establish this for certain. Any explanation for Mo_aq removal from sulfidic natural waters must account for why asymptotes are insensitive to ΣS^–II (= H₂S_aq + HS^–) and POC, but possibly sensitive to pH.

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Organic Scavenging

The principal argument for organic scavenging rests on correlations commonly, but not universally, observed between Mo_s and organic matter in sedimentary rocks. As reviewed by Helz and Vorlicek (2019)

Helz, G.R., Vorlicek, T.P. (2019) Precipitation of molybdenum from euxinic waters and the role of organic matter. Chemical Geology 509, 178–193.

, no satisfactory molecular mechanism to explain Mo_aq scavenging by organic matter under natural conditions has yet been found, and evidence is also lacking for significant biological scavenging. If organic scavenging exists, it is likely to be an abiotic process that can be described by a Langmuir isotherm as:

where Mo_s is non-lithogenic Mo adsorbed onto particulate organic carbon (mol Mo_s/L), x_s the mole fraction of a specific binding site (mol sites/mol C), POC the particulate organic carbon concentration (mol/L), K_n’ the conditional binding constant (L/mol) for the n^th MoO_4−nS_n^2– ion, and x_n the mole fraction of the n^th MoO_4−nS_n^2– ion in Mo_aq (see Fig. 1).

Full size image

For simplicity, Equation 1 assumes only one adsorbing MoO_4−nS_n^2– species and only one binding site. Multiple species or sites could be included by adding more terms. At the pH and Mo_aq concentrations of most sulfidic natural waters, the second term in the denominator is negligible, making isotherms linear. In the linear case, Equation 1 is tantamount to the empirical Mo_s/POC vs. Mo_aq relationship found for euxinic basin sediments by Algeo and Lyons (2006

Algeo, T.J., Lyons, T.W. (2006) Mo-total organic carbon covariation in modern marine environments: Implication for analysis of paleoredox and paleohydrographic conditions. Paleooceanography and Paleoclimatology 21, PA1016.

; see their Fig. 8a).

What does the linear form imply about aqueous asymptotes? Mass balance requires that:

where Mo_aq,0 is the initial total dissolved Mo and Mo_aq is the same during adsorption progress. Rearranging:

In diverse euxinic basins and sulfidic pore waters, asymptotes fix the ratio on the left hand side of this equation at a median value of 12 % (seventh column, Table 1). This requires that in nature the denominator’s second term has a restricted range centred near 8. Given the 10⁶ range of its POC factor, this requirement cannot possibly be met. Additionally, at any specific site, the ratio on the left is roughly constant through the asymptote zone, requiring that the second term in the denominator is also constant. However, H₂S_aq varies through asymptote zones (third column, Table 1), and x_n values strongly vary with H₂S_aq (Fig. 1). Only at high H₂S_aq concentrations, where x₄ becomes sulfide independent, can Equation 3 describe constant asymptotes at a constant POC concentration. (In this case, x₄ replaces x_n in Equation 3, implying that MoS₄^2– is the only adsorbed species.) However, H₂S_aq concentrations high enough for x₄ to approach constancy exceed the concentrations at which Mo_aq asymptotes commence in nature. For example, the Mo_aq asymptote in the Black Sea commences at H₂S_aq ≈ 50 μM (where ΣS^−II ≈ 300 μM), whereas Figure 1 shows that H₂S_aq must be much higher, near 200 μM, for x₄ to come within 5 % of its ultimate limiting value. Equation 3, which describes organic scavenging, therefore cannot explain Mo_aq asymptotes.

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FeMoS₄ Precipitation

Earlier workers rejected the possibility of Mo precipitation by sulfide. Emerson and Huested (1991)

Emerson, S.R., Huested, S.S. (1991) Ocean anoxia and the concentrations of molybdenum and vanadium in seawater. Marine Chemistry 34, 177–196.

pointed out that, unlike sulfide-precipitated metals, Mo_aq fails to decline continuously with increasing ΣS^–II. In agreement, Algeo and Lyons (2006)

Algeo, T.J., Lyons, T.W. (2006) Mo-total organic carbon covariation in modern marine environments: Implication for analysis of paleoredox and paleohydrographic conditions. Paleooceanography and Paleoclimatology 21, PA1016.

noted that Cariaco Basin sediments accumulate higher Mo_s concentrations than Black Sea sediments, even though Cariaco waters contain less sulfide.

Nevertheless, Vorlicek et al. (2018)

Vorlicek, T.P., Helz, G.R., Chappaz, A., Vue, P., Vezina, A., Hunter, W. (2018) Molybdenum burial mechanism in sulfidic sediments: iron-sulfide pathway. ACS Earth and Space Chemistry 2, 565–576.

found a colloidal precipitate, FeMoS₄, with a solubility that could explain Mo_aq asymptotes in the Black Sea. At equilibrium with this precipitate (see derivation in Note III, Supplementary Information):

where γ_MoS4 is the activity coefficient of MoS₄^2– and Q_FeS = {Fe²⁺}{H₂S_aq}/{H⁺}². Except for the x₃ term, the factors on the righthand side of Equation 4 display limited variability in asymptote zones. Q_FeS values are usually constrained by saturation with iron monosulfide phases and pH is constrained mainly by strong ΣCO₂/ΣALK buffering. On the other hand, x₃ varies considerably, passing through a maximum with increasing H₂S_aq activity (Fig. 1). In Table 2, values of H₂S_aq at x₃ maxima have been estimated over a moderate temperature range by the isocoulombic extrapolation method (Gu et al., 1994

Gu, Y., Gammons, C.H., Bloom, M.S. (1994) A one-term extrapolation method for estimating equilibrium constants of aqueous reactions at elevated temperatures. Geochimica et Cosmochimica Acta 58, 3545–3560.

). Equation 4 requires that at x₃ maxima, Mo_aq solubility reaches minima; this is illustrated by dashed curves in Figure 2. However, aging transforms FeMo^VIS₄ to an inert Mo^IV phase, preventing Mo redissolution when the solubility curves turn upward beyond the minima. As a result, the FeMoS₄ precipitation mechanism predicts that Mo_aq becomes fixed at asymptotes that are unresponsive to further H₂S_aq increases. The asymptotes are also independent of POC, despite the 10⁶ range in nature, because organic matter has no role in the precipitation and aging reactions, except as fuel for biological sulfate reduction (Helz and Vorlicek, 2019

Helz, G.R., Vorlicek, T.P. (2019) Precipitation of molybdenum from euxinic waters and the role of organic matter. Chemical Geology 509, 178–193.

). Thus FeMoS₄ precipitation meets the requirements listed above for a mechanism that can explain asymptotes.

Temperature (°C)	Switch point (MoO42– = MoS42–)	Solubility minimum (MoOS32– peak)	x4 = 0.95 (thiolation ≈ complete)
5	5	9	120
25	11	21	260
45	22	42	530

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Implications

Several implications of interest in the palaeoproxy field arise from this result.

Asymptotes block quantitative removal of Mo_aq from sulfidic waters, leaving dissolved residuals that ultimately escape the sulfidic environment. In the median case in Table 1, the residual (Mo_aq,∞/Mo_aq,0) is only 12 % but can be much higher. This finding undermines the common assumption that marine euxinic basins preserve the δ⁹⁸Mo signatures of global seawater because of quantitative precipitation. Residuals will be greatest in waters of lower salinity, where Mo_aq,0 will be lower, and in waters of higher pH, where Mo_aq,∞ will be higher. Both situations generate higher Mo_aq,∞/Mo_aq,0 ratios at asymptotes, permitting larger fractions of incoming Mo_aq to escape deposition.

The idea that Mo_s/POC ratios in sedimentary rocks can be used to estimate Mo_aq in ancient basins (Algeo and Lyons, 2006

Algeo, T.J., Lyons, T.W. (2006) Mo-total organic carbon covariation in modern marine environments: Implication for analysis of paleoredox and paleohydrographic conditions. Paleooceanography and Paleoclimatology 21, PA1016.

, and others) relies on the relationship in Equation 1, which is premised on the dominance of POC scavenging. Without scavenging, such Mo_aq estimates are ungrounded.

Table 1 shows that Mo_aq asymptotes in the pH range of modern environments all exceed 1 nM, the level thought to curb nitrogen fixation by Mo-nitrogenase (Glass et al., 2010

Glass, J.B., Wolfe-Simon, F., Elser, J.J., Anbar, A.D. (2010) Molybdenum-nitrogen co-limitation in freshwater and coastal heterocystous cyanobacteria. Limnology and Oceanography 55, 667–676.

). Therefore, past global euxinia would not have shut down Mo-nitrogenase activity, creating marine nitrogen crises, unless accompanied by substantial acidification, which lowers Mo_aq,∞ (Fig. 2).

Can FeMoS₄ precipitation explain why the Black Sea produces lower sedimentary Mo_s despite higher bottom water H₂S_aq concentrations compared to the Cariaco Basin? Consistent with the organic scavenging mechanism (Eq. 1), Algeo and Lyons (2006)

Algeo, T.J., Lyons, T.W. (2006) Mo-total organic carbon covariation in modern marine environments: Implication for analysis of paleoredox and paleohydrographic conditions. Paleooceanography and Paleoclimatology 21, PA1016.

regarded bottom water Mo_aq concentration, which is lower in the Black Sea, as the controlling variable. The Black Sea’s lower bottom water Mo_aq concentration, and consequent lower sediment Mo_s concentration, were attributed to a Mo_aq supply restriction at the narrow Bosporus. This explanation is probably incorrect. The Bosporus distributes 4–5 times more Mo_aq into the Black Sea than its sediments accumulate (Piper and Calvert, 2009

Piper, D.Z., Calvert, S.E. (2009 A marine biogeochemical perspective on black shale deposition. Earth-Science Reviews 95, 63–96.

), and the excess returns through the Bosporus. On the other hand, the Bosporus restricts the Black Sea’s river water throughput. Copious river inflow lowers surface salinity and intensifies density stratification, thus inhibiting introduction of oxidants into deep waters. Consequently, ΣS^–II accumulates sufficiently for the FeMoS₄ saturation point to ascend into the water column, causing Mo_aq drawdown there. Cariaco’s better ventilation, attested by its shorter water residence time, suppresses ΣS^–II despite opposition from a greater POC rain rate. This confines Cariaco’s saturation point to pore waters. Calculations based on ΣS^–II and pH data suggest that the Black Sea’s saturation point is near 150 m depth in the water column, whereas Cariaco’s is located a few mm below the sediment–water interface (Helz and Vorlicek, 2019

Helz, G.R., Vorlicek, T.P. (2019) Precipitation of molybdenum from euxinic waters and the role of organic matter. Chemical Geology 509, 178–193.

). Therefore, Cariaco’s bottom waters are pre-asymptote, whereas the Black Sea’s are syn-asymptote. Cariaco’s sediments accumulate more Mo_s by diffusion because of the very sharp, pre- to syn-asymptote Mo_aq gradient across the sediment–water interface.

To summarise, the narrow range of Mo_aq,∞ values that form in sulfidic water columns and pore waters characterised by enormous differences in POC concentrations preclude adsorption to organic matter as an important Mo fixation process in nature. In contrast, FeMoS₄ precipitation, which depends on the thermodynamic activities of H⁺, MoS₄^2–, and FeS rather than their concentrations, readily explains why Mo_aq asymptotes are independent of final sulfide and POC concentrations.

Editor: Liane G. Benning


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References

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Supplementary Information

The Supplementary Information includes:

Sources for Data Compiled in Table 1

Note on Calculation of POC Values for Pore Waters in Table 1

Derivation of Equation 4

Supplementary Information References

Download the Supplementary Information (PDF).