Description: Article Large positive sulphur isotope excursions, recorded in the wake of the Marinoan glaciation have previously been interpreted assuming stable ocean sulphate concentrations. Here, using multiple sulphur isotopes, the authors instead suggest significant ocean sulphate drawdown, driven by increased pyrite burial. Nature Communications doi: 10.1038/ncomms12192 Authors: Pierre Sansjofre, Pierre Cartigny, Ricardo I. F. Trindade, Afonso C. R. Nogueira, Pierre Agrinier, Magali Ader

Description: Natural stable isotopes ratios (d13Corg and d15N) and associated elemental concentrations (i.e. total organic carbon and total nitrogen contents) preserved in marine sediments are frequently used for the determination of paleoenvironmental processes such as the organic-matter origin. Previous studies highlighted bias 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 glacial and interglacial conditions. We used the common method for pre-treatment analysis, which consists of acidification of bulk sediment followed by several de-ionised water rinses. We investigated the effect of acid type (i.e. Hydrochloric acid and Acetic acid), but also strength (from 0.2 to 10 mol/l) on the d13Corg, d15N, TOC, TN and C/N ratio on three samples from the Gulf of Lion. Two (i.e. S.302 and S.102) were deposited during glacial maxima whereas S.157 characterizes interglacial conditions. Samples d13Corg values range between -21.7 and -24.4 per mil with TOC varying from 0.56 and 0.84 %wt/wt. d15N values is more stable with an average value of 3.0 ± 0.1 per mil with a TN average of 0.08 ± 0.002 %wt/wt. 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) induce significant bias on d13Corg values, TOC values and therefore on C/N ratio; (iii) 25% of an isotopically distinct pool of organic carbon was lost between the use of 0.2 mol/l and 1.5 mol/l affecting d13Corg values by more than 1.5 per mil; (iv) geochemical evidences indicate 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 d13Corgvalues for paleoenvironmental and climate reconstructions.

Description: In this study we use bulk sediment organic matter stable isotopes (d13Corg and d15N) to examine the nature and timing of preserved organic matter in borehole PRGL1-4 from Northwestern Mediterranean Sea. This region is known as a transitional zone between the North Atlantic atmospheric circulation and the North African monsoon system. In the Gulf of Lion (SE France), increased inputs of organic matter from sediment-laden rivers occurred during warm substages of the last 200 kyr (MIS 5e, 5c, and 5a; MIS 6d, and 6b). Increased terrestrial organic matter are inferred from organic carbon isotopic composition (d13Corg), and are interpreted as resulting from enhanced rainfall over the Rhone river catchment area. Such increase in terrestrial organic carbon induced enhancement of the primary productivity leading to the formation of local oxygen minimum zone as demonstrated by ~3‰ values in d15N. Comparison with regional paleohydrological records from the northern Mediterranean borderlands reveals the regional character of these pluvial events. Taking advantage of the location of PRGL1-4 borehole, out of Mediterranean cyclogenesis area, we suggest that the pluvial events recorded during warm substages of the last 200 kyr occurred in response to enhanced passage of North Atlantic atmospheric perturbation over the Gulf of Lion catchment area (especially over Rhone river watershed which represents 80% of the GoL catchment area), a scenario possibly similar to that encountered today during negative- NAO like conditions. At a regional scale, our data suggests that high rainfall events over the Gulf of Lion catchement area and the Rhone watershed occurred at the time of North African summer monsoon and the sapropel deposition in the Mediterranean basin, thus highlighting a close coupling between mid- (North Atlantic) and low-latitudes (monsoon) climate systems. Importantly, our geochemical evidence from the Gulf of Lion support an extra-Mediterranean source for the regional pluvial events described in many paleoclimatic records from the northern Mediterrean borderlands during warm substages of the last climate cycles. Consequently, we suggest that this region as a whole could provide, in addition to the river runoff from the North African sector, the necessary conditions for the sapropel deposition

Description: 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 (d34S) 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 d34S 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 (d34Spyr) necessarily corresponds to local environmental changes. The stratigraphic variations (〉76 per mil) 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 d34S records.

Description: Sedimentary hydrocarbon remnants of eukaryotic C26–C30 sterols can be used to reconstruct early algal evo- lution. Enhanced C29 sterol abundances provide algal cell membranes a density advantage in large temperature fluctuations. Here, we combined a literature review with new analyses to generate a comprehensive inventory of unambiguously syngenetic steranes in Neoproterozoic rocks. Our results show that the capacity for C29 24- ethyl-sterol biosynthesis emerged in the Cryogenian, that is, between 720 and 635 million years ago during the Neoproterozoic Snowball Earth glaciations, which were an evolutionary stimulant, not a bottleneck. This bio- chemical innovation heralded the rise of green algae to global dominance of marine ecosystems and highlights the environmental drivers for the evolution of sterol biosynthesis. The Cryogenian emergence of C29 sterol biosynthesis places a benchmark for verifying older sterane signatures and sets a new framework for our understanding of early algal evolution.