Volume 30

Abstract:
The ability of engineered enhanced weathering to impact atmospheric CO₂ has been challenging to demonstrate due to the many processes occurring in soils and the short time span of current projects. Here we report the carbon balance in an Icelandic Histic/Gleyic Andosol that has received large quantities of basaltic dust over 3300 years, providing opportunity to quantify the rates and long term consequences of enhanced weathering. The added basaltic dust has dissolved continuously since its deposition. The alkalinity of the soil waters is more than 10 times higher than in equivalent basalt dust-free soils. After accounting for oxidation and degassing when the soil waters are exposed to the atmosphere, the annual CO₂ drawdown due to alkalinity generation is 0.17 t C ha⁻¹ yr⁻¹. This study validates the ability of fine grained mafic mineral addition to soils to attenuate increasing atmospheric CO₂ by alkalinity export. Induced changes in soil organic carbon storage, however, likely dominate the net CO₂ drawdown of enhanced weathering efforts.

Abstract:
Exposure to volcanic SO₂ can have adverse effects on human health, with severe respiratory disorders documented on short- and long-term timescales. Here, we use melt inclusion and groundmass glass data to calculate potential syneruptive SO₂ emissions during medieval and recent (2021–2024) eruptions across the Reykjanes Peninsula, the most populated area of Iceland, which has recently undergone magmatic reactivation with the 2021–2024 eruptions at Fagradalsfjall and Svartsengi. We target 16 individual eruptions from the medieval volcanic cycle at the Reykjanes Peninsula, the 800–1240 AD Fires, along with the 2021–2023 Fagradalsfjall eruptions and the 2023–2024 eruptions at Sundhnúksgígar. We calculate potential SO₂ emissions across the RP for the individual eruptions to be in the range of 0.004–6.3 Mt. These estimates correspond to mean daily SO₂ emissions in the range of 1000–111,000 t/day, higher than the mean SO₂ measurements of 5240 ± 2700 t/day during the 2021 Fagradalsfjall eruption. By using pre-eruptive sulfur values preserved in undegassed melt inclusions, we develop an empirical approach to calculate best- and worst-case potential SO₂ emission scenarios of any past or ongoing Reykjanes Peninsula eruption of known effusion rate. We conclude that the potential sulfur emissions across the RP can be significantly higher than observed during the 2021 Fagradalsfjall eruption, mainly because of the more evolved nature and higher sulfur contents of magmas erupted during the medieval period. Based on dominant NW wind directions on the Reykjanes Peninsula, eruptions in Brennisteinsfjöll pose the greatest health hazard to the capital area. Sulfate aerosol produced during long-term eruptions may impact visibility and air quality in the Keflavík Airport area. Our findings enable assessment of SO₂ emission scenarios of future eruptions across the RP and can be used together with gas dispersal models to forecast SO₂ pollution at ground level, and its impact on human health.

Abstract:
Questions surrounding the biogenicity of ancient stromatolites have perplexed geobiologists for decades. Abiotic processes can produce superficially stromatolite-like structures; moreover, stromatolites frequently fail to preserve organic materials and cellular traces of their microbial architects. Using spatially correlated optical and electron microscopy coupled with Raman and FTIR microspectroscopy, we show that silicified stromatolites from the Tonian Skillogalee Dolomite (Flinders Ranges, South Australia) contain exceptionally well preserved microbial mat fragments and microbially induced sedimentary structures. These organic-rich layers exhibit mat-like laminations with low degrees of inheritance and reflect interactions between microbial communities and their environments, i.e. growth, sediment trapping and binding, and reactions to early diagenesis, and are inconsistent with abiotic formation. Although accounting for a minor proportion of the volume of the stromatolites, these kerogenous relics are demonstrably syngenetic and comprise aromatic and aliphatic organic materials, likely preserved due to early and rapid silicification. Constraining the origins of such lamination-scale features can elucidate relationships between morphogenesis and diagenesis and may assist in the resolution of controversies surrounding stromatolite biogenicity in deep time.

Abstract:
Planetary differentiation had a profound influence on the geochemical signature of the Earth’s silicate reservoirs. Some of the early created complementary reservoirs dissipated with time (e.g., Bennett et al., 2007

Bennett, V.C., Brandon, A.D., Nutman, A.P. (2007) Coupled ¹⁴²Nd-¹⁴³Nd Isotopic Evidence for Hadean Mantle Dynamics. Science 318, 1907–1910. https://doi.org/10.1126/science.1145928

) and only remnants can be observed. Here, we apply the short lived isotopic system ¹⁴⁶Sm-¹⁴²Nd to an alternative archive—marine chemical sediments—and show that alternating Fe- and Si-rich bands from the 2.7 billion-year-old Temagami banded iron formation (BIF), Canada, display significantly different ¹⁴²Nd isotopic compositions. The Fe-rich bands yield a depleted signature (expressed as deviation from the standard in μ notation) with an average μ¹⁴²Nd of +7.02 ± 0.71, while the Si-rich bands display modern mantle-like signatures (average μ¹⁴²Nd −2.83 ± 2.32) likely being the results of mixing between different sources. These complementary signatures reflect the dominant, locally derived source of Nd in the seawater at the time of deposition. Our results promote that layering in BIFs is a syn-depositional feature, and that BIFs are unique geochemical archives capable of recording silicate reservoirs that formed during the Hadean but were still extant during the Neoarchean.

Abstract:
The stable silicon isotopic composition of siliceous sponge skeletal elements, spicules, forms a potential proxy for past dissolved silicon (DSi) concentrations of bottom waters. Field-based studies have shown that there is a non-linear relationship between the concentration of ambient DSi and both the isotopic composition (denoted by δ³⁰Si) of spicules and apparent isotopic fractionation by sponges during growth. There is considerable scatter in the calibration, with some studies highlighting variation within an individual sponge, and between individuals, in both monospecific and more diverse communities. Furthermore, some derived sponge forms, including hypersilicified and carnivorous sponges, appear to have anomalous isotopic fractionation, deviating significantly from other sponges. When reconstructing past DSi, it is only possible to differentiate spicules by their morphology, which in many cases will not be taxonomically diagnostic. However, there has yet to be a systematic study of core top and downcore δ³⁰Si measurements from different spicule types. Here we address that gap using spicules extracted from two sediment cores taken at the summit of the Schulz Bank, a seamount located on the Arctic Mid-Ocean Ridge between the Norwegian and Greenland Seas. Mean isotopic compositions of downcore spicules of a given morphology were similar between nearby cores and mainly did not show any significant differences. Our results did reveal a systematic difference between spicule types extracted from a given sediment horizon, and a significant difference in the downcore mean compositions, between needle-like Oxea and other morphologies. These new findings imply that picking a single spicule type is best practice for palaeoceanographic applications of sponge archives, but the choice of Oxea spicules could bias these reconstructions towards high DSi concentrations.