海底で記録された信号を分離する(Separating out signals recorded at the seafloor)

2023-11-23 ワシントン大学セントルイス校

＜関連情報＞

• https://source.wustl.edu/2023/11/separating-out-signals-recorded-at-the-seafloor/
• https://www.science.org/doi/10.1126/science.adg6103
• https://www.science.org/doi/10.1126/science.adh1215
• https://www.science.org/doi/10.1126/sciadv.abb7403

海洋堆積物の黄鉄鉱硫黄同位体比に対する微生物および環境制御の解明 Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios

R. N. Bryant,J. L. Houghton,C. Jones,V. Pasquier,I. Halevy,and D. A. Fike
Science  Published:23 Nov 2023
DOI:https://doi.org/10.1126/science.adg6103

Editor’s summary

The sulfur isotope composition of pyrite found in marine sediments and sedimentary rocks is often used to try to reconstruct the coupled cycles of carbon, oxygen, and sulfur. However, the resulting interpretations can be complicated by the competing effects of physical and biological processes. Halevy et al. show that inorganic reactions and transport in depositional environments, rather than microbial influence, leads to the wide range of sulfur isotopic values observed. The observed increase in sulfate–pyrite isotope fractionation over most of Earth’s history primarily reflects the effects of increasing marine sulfate concentration, except over the past 550 million years, when supercontinent breakup and assembly and variations in sea level were more important. Bryant et al. present a microanalytical method applied to individual pyrite grains that deconvolves the multiple signals influencing sulfur isotopes. They were able to determine both the microbial isotopic effects and inorganic fractionation produced by depositional conditions. Their approach will help in reassessing conflicting interpretations of the sulfur isotopic record. —H. Jesse Smith

Abstract

Reconstructions of past environmental conditions and biological activity are often based on bulk stable isotope proxies, which are inherently open to multiple interpretations. This is particularly true of the sulfur isotopic composition of sedimentary pyrite (δ³⁴S_pyr), which is used to reconstruct ocean-atmosphere oxidation state and track the evolution of several microbial metabolic pathways. We present a microanalytical approach to deconvolving the multiple signals that influence δ³⁴S_pyr, yielding both the unambiguous determination of microbial isotopic fractionation (ε_mic) and new information about depositional conditions. We applied this approach to recent glacial-interglacial sediments, which feature over 70‰ variations in bulk δ³⁴S_pyr across these environmental transitions. Despite profound environmental change, ε_mic remained essentially invariant throughout this interval and the observed range in δ³⁴S_pyr was instead driven by climate-induced variations in sedimentation.

海洋黄鉄鉱の硫黄同位体組成を制御する堆積パラメータ Sedimentary parameters control the sulfur isotope composition of marine pyrite

I. Halevy,D. A. Fike,V. Pasquier,R. N. Bryant,C. B. Wenk,A. V. Turchyn,D. T. Johnston,and G. E. Claypool
Science  Published:23 Nov 2023
DOI:https://doi.org/10.1126/science.adh1215

Editor’s summary

The sulfur isotope composition of pyrite found in marine sediments and sedimentary rocks is often used to try to reconstruct the coupled cycles of carbon, oxygen, and sulfur. However, the resulting interpretations can be complicated by the competing effects of physical and biological processes. Halevy et al. show that inorganic reactions and transport in depositional environments, rather than microbial influence, leads to the wide range of sulfur isotopic values observed. The observed increase in sulfate–pyrite isotope fractionation over most of Earth’s history primarily reflects the effects of increasing marine sulfate concentration, except over the past 550 million years, when supercontinent breakup and assembly and variations in sea level were more important. Bryant et al. present a microanalytical method applied to individual pyrite grains that deconvolves the multiple signals influencing sulfur isotopes. They were able to determine both the microbial isotopic effects and inorganic fractionation produced by depositional conditions. Their approach will help in reassessing conflicting interpretations of the sulfur isotopic record. —H. Jesse Smith

Abstract

Reconstructions of coupled carbon, oxygen, and sulfur cycles rely heavily on sedimentary pyrite sulfur isotope compositions (δ³⁴S_pyr). With a model of sediment diagenesis, paired with global datasets of sedimentary parameters, we show that the wide range of δ³⁴S_pyr (~100 per mil) in modern marine sediments arises from geographic patterns in the relative rates of diffusion, burial, and microbial reduction of sulfate. By contrast, the microbial sulfur isotope fractionation remains large and relatively uniform. Over Earth history, the effect of increasing seawater sulfate and oxygen concentrations on sulfate and sulfide transport and reaction may explain the corresponding increase observed in the δ³⁴S offset between sulfate and pyrite. More subtle variations may be related to changes in depositional environments associated with sea level fluctuations and supercontinent cycles.

海洋黄鉄鉱の硫黄同位体比は、地球規模ではなく局所的に強く支配されている。 Strong local, not global, controls on marine pyrite sulfur isotopes

V. Pasquier,R. N. Bryant,D. A. Fike,and I. Halevy
Science Advances  Published:26 Feb 2021
DOI:https://doi.org/10.1126/sciadv.abb7403

Abstract

Understanding variation in the sulfur isotopic composition of sedimentary pyrite (δ³⁴S_pyr) is motivated by the key role of sulfur biogeochemistry in regulating Earth’s surface oxidation state. Until recently, the impact of local depositional conditions on δ³⁴S_pyr has remained underappreciated, and stratigraphic variations in δ³⁴S_pyr were interpreted mostly to reflect global changes in biogeochemical cycling. We present two coeval δ³⁴S_pyr records from shelf and basin settings in a single sedimentary system. Despite their proximity and contemporaneous deposition, these two records preserve radically different geochemical signals. Swings of ~65‰ in shelf δ³⁴S_pyr track short-term variations in local sedimentation and are completely absent from the abyssal record. In contrast, a long-term ~30‰ decrease in abyssal δ³⁴S_pyr reflects regional changes in ocean circulation and/or sustained pyrite formation. These results highlight strong local controls on δ³⁴S_pyr, calling for reevaluation of the current practice of using δ³⁴S_pyr stratigraphic variations to infer global changes in Earth’s surface environment.