Description: Highlights: • Constraining sources of core and intact archaeal lipids with stable C isotopic ratios. • No evidence for sedimentary sources of IPL crenarchaeol. • Evidence of sedimentary production of IPL caldarchaeol and BDGT-0. • Higher organic matter content promotes higher activity of sedimentary archaea. • Archaeol is a sensitive indicator of sedimentary archaea. Archaea occupy an important niche in the global carbon cycle and their lipids are widely used as indicators of environmental conditions in both paleoenvironmental and modern biogeochemical studies. The principal sources of archaeal lipids in marine sediments are benthic archaea, fossil remnants of planktonic archaea, and allochthonous sources such as soils. However, the relative contributions of these sources to the sedimentary lipid pool have not been comprehensively constrained, complicating a mechanistic understanding of archaeal lipid proxies. In order to provide insights into the relative contributions of these sources and identify signals derived from sedimentary activity, we performed a systematic survey of stable carbon isotopic compositions (delta C-13) of both core and intact archaeal lipids via analyses of their phytanyl (Phy) and biphytanyl (BP) moieties in diverse marine sediments. The sample set consisted of 44 sediment horizons from the Mediterranean and adjacent basins and represented diverse sources of organic matter and depositional conditions. Complementary geochemical data enabled the comparison of lipid distributions and carbon isotopic signatures with prevailing redox conditions. The delta C-13 of tricyclic BP (BPcren) from the core and intact forms of crenarchaeol ranged from -19.1 to -28.6% and -18.1 to -27.4%, respectively. delta C-13 values of core and intact BPcren did not differ, suggesting that intact crenarchaeol is either a fossil relic from planktonic archaea or a product of lipid recycling by benthic archaea, as opposed to being synthesized de novo by sedimentary archaea. delta C-13 values of BP0 derived from core and intact forms of glycerol and butanetriol dibiphytanyl glycerol tetraethers (GDGTs and BGDTs, respectively), but predominantly from caldarchaeol (GDGT-0), ranged from -19.4 to -32.0% and -20.9 to -37.0%, respectively. In contrast to BPcren, intact-lipid derived BP0 was often C-13-depleted relative to its core counterpart, consistent with in situ production by sedimentary archaea. This relative depletion was most pronounced in sulfate reduction zones, likely due to heterotrophic activity. Core and intact archaeol exhibited the largest ranges in delta C-13 values, from -21.6 to -42.1% and -22.7 to -58.9%, respectively. This strong C-13-depletion relative to both total organic carbon and dissolved inorganic carbon is consistent with mixtures of functional sources of sedimentary chemolithoautotrophic, methanotrophic, methanogenic and heterotrophic archaea. Based on the substantial C-13-depletion of BPcren and BP0 in samples in the vicinity of the Rhone River delta relative to a distal marine reference site, we infer that the terrestrial soil contribution of archaeal lipids to these sediments is as high as 43%. Hence, terrestrial input of archaeal lipids, including their intact forms, can be substantial and suggests caution when using existing molecular proxies aimed at constraining riverine input. In summary, our comparative isotopic analysis of sedimentary core versus intact archaeal lipids improves the apportionment of their diverse sources and confidence in distinguishing in situ lipid production by sedimentary archaea.

Description: An understanding of how the coupled cycles of carbon, iron and sulfur in sediments respond to environmental change throughout Earth history requires the reconstruction of biogeochemical processes over a range of spatial and temporal scales. In this study, sediment cores from the southwestern Black Sea were analyzed to gain insight into past changes in biogeochemical processes with particular focus on the cycling of dissolved organic carbon (DOC). The sediment consists of Late Pleistocene deposits of iron oxide-rich and organic-poor lacustrine sediments, a Holocene sapropel layer deposited after the inflow of saline Mediterranean seawater about 9300 yr BP, and overlying recent marine sediments. The porewaters displayed high concentrations of DOC, acetate, dissolved iron and an extended depth interval over which sulfate and methane were both present. The historical fluctuations of the fluxes of carbon, sulfur and iron species at the seafloor that led to these present-day geochemical profiles, and which cannot be easily interpreted from the measured data alone, were hindcasted with a reaction-transport model. The model suggests that the inflow of Mediterranean seawater impacted the rain rate and reactivity of organic matter reaching the sediments, which shifted the sedimentary redox regimes throughout the Holocene that now are reflected on different lithology units. Organic matter in the sapropel layer is apparently the main source of modern-day accumulations of DOC and acetate, both of which probably sustained subsurface microbial activity throughout the post-glacial period. The ratio between DOC and dissolved inorganic carbon (DIC) flux to the bottom water decreased from ∼40% before the inflow of Mediterranean water to ∼2% at the present day. We suggest that the coexistence of methanogenesis and sulfate reduction was associated with sulfate-reducing bacteria and methanogens sharing common substrates of acetate and lactate and utilizing non-competitive substrates such as methylated compounds in the sapropel layer and in the bottom of modern marine deposits. Intense sulfur and iron cycling mainly took place in the organic-poor freshwater deposits, today characterized by high concentrations of dissolved iron and methane. In contrast to previous studies in similar environments, anaerobic oxidation of methane coupled to the reduction of ferric iron was negligible. The results have broad implications for coastal environments that are currently experiencing deoxygenation and seawater intrusion and also for understanding the role of DOC in the sedimentary carbon cycle.