Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone.

Sulfur cycling is ubiquitous in sedimentary environments, where it mediates organic carbon remineralization, impacting both local and global redox budgets, and leaving an imprint in pyrite sulfur isotope ratios (δ34Spyr). It is unclear to what extent stratigraphic δ34Spyr variations reflect local aspects of the depositional environment or microbial activity versus global sulfur-cycle variations. Here, we couple carbon-nitrogen-sulfur concentrations and stable isotopes to identify clear influences on δ34Spyr of local environmental changes along the Peru margin. Stratigraphically coherent glacial-interglacial δ34Spyr fluctuations (>30‰) were mediated by Oxygen Minimum Zone intensification/expansion and local enhancement of organic matter deposition. The higher resulting microbial sulfate reduction rates led to more effective drawdown and 34S-enrichment of residual porewater sulfate and sulfide produced from it, some of which is preserved in pyrite. We identify organic carbon loading as a major influence on δ34Spyr, adding to the growing body of evidence highlighting the local controls on these records.

Acid digestion on river influenced shelf sediment organic matter: Carbon and nitrogen contents and isotopic ratios.

RATIONALE: Natural stable isotope ratios (δ13 Corg and δ15 N values) and associated elemental concentrations (i.e. total organic carbon and total nitrogen contents) preserved in marine sediments are frequently used for the determination of paleo-environmental processes such as the origin of organic matter. Previous studies highlighted biases in the determination of such geochemical proxies due to pre-analysis acid treatment methods. This study is the first systematic comparison of the effect of acid treatment methods on bulk organic matter using a unique sedimentary system, under two contrasting climatic contexts (i.e. glacial vs interglacial). METHODS: We used the most common method for pre-treatment analysis, which consists of the acidification of bulk sediments followed by several rinses with deionised water. We investigated the effect of acid type (hydrochloric and acetic acid), and also acid strength (from 0.2 to 10 mol.L-1 ) on the δ13 Corg , δ15 N, TOC, TN and C/N values of three samples from the Gulf of Lion. Two of them (S.304 and S.102) were deposited during glacial maxima (i.e. high sedimentation rate, low porosity and high terrestrial inputs) whereas S.157 characterizes interglacial conditions (high porosity, high foraminifera content, low sedimentation rates and low terrestrial inputs). RESULTS: For all three samples the δ13 Corg values are between -21.7 and -24.4‰ with the TOC varying from 0.56 to 0.84 %wt/wt. The δ15 N values are more stable with an average value of 3.0 ± 0.1‰ with a TN average of 0.08 ± 0.002 %wt/wt. CONCLUSIONS: We show that acid type did not significantly affect results. We also find that (i) glacial and interglacial samples do not react similarly to acid pre-treatment, (ii) high acid strength (>1.5 mol.L-1 ) induces significant bias on δ13 Corg and TOC values, and therefore on C/N ratios; (iii) 25% of an isotopically distinct pool of organic carbon was lost when using acid concentrations of 0.2-1.5 mol.L-1 , affecting δ13 Corg values by more than 1.5‰; and (iv) geochemical evidence indicates that the leachable organic carbon pool is preferentially composed of terrestrial organic matter. These findings call for precautions when using C/N ratios and associated δ13 Corg values for paleo-environmental and climate reconstructions.

Pyrite sulfur isotopes reveal glacial-interglacial environmental changes.

The sulfur biogeochemical cycle plays a key role in regulating Earth's surface redox through diverse abiotic and biological reactions that have distinctive stable isotopic fractionations. As such, variations in the sulfur isotopic composition (δ34S) of sedimentary sulfate and sulfide phases over Earth history can be used to infer substantive changes to the Earth's surface environment, including the rise of atmospheric oxygen. Such inferences assume that individual δ34S records reflect temporal changes in the global sulfur cycle; this assumption may be well grounded for sulfate-bearing minerals but is less well established for pyrite-based records. Here, we investigate alternative controls on the sedimentary sulfur isotopic composition of marine pyrite by examining a 300-m drill core of Mediterranean sediments deposited over the past 500,000 y and spanning the last five glacial-interglacial periods. Because this interval is far shorter than the residence time of marine sulfate, any change in the sulfur isotopic record preserved in pyrite (δ34Spyr) necessarily corresponds to local environmental changes. The stratigraphic variations (>76‰) in the isotopic data reported here are among the largest ever observed in pyrite, and are in phase with glacial-interglacial sea level and temperature changes. In this case, the dominant control appears to be glacial-interglacial variations in sedimentation rates. These results suggest that there exist important but previously overlooked depositional controls on sedimentary sulfur isotope records, especially associated with intervals of substantial sea level change. This work provides an important perspective on the origin of variability in such records and suggests meaningful paleoenvironmental information can be derived from pyrite δ34S records.