Description: We investigated sediments from three different depositional environments along the northern Argentine continental margin to assess the main processes controlling sediment deposition since the last glacial period. Further, we evaluated how different depositional conditions affect (bio)geochemical processes within sediments. Sediment cores were collected during expedition SO260 in 2018[1]. Two sites are located at ~1100 m water depth north and south of the Mar del Plata Canyon (N- and S-Middle Slope Site). Another site is situated at the lower continental slope at 3600 m water depth (Lower Slope Site). Reliable age constraints of sediments deposited during the last glaciation at the Argentine margin are difficult to obtain due limited amounts of carbonate. We overcame this issue by combining radio-isotope analyses (14C,230Thex) with sedimentological, geochemical and magnetic data demonstrating that all sites experienced distinct changes over time. Both, N- and S-Middle Slope Sites, record at least the last 30 ka. The S-Middle Slope Site is dominated by continuously organic carbon-starved and winnowed sandy deposits, which according to geochemical and magnetic data leads to insignificant sulfate reduction and sulfidation of iron (oxyhydr)oxides. Glacial sedimentation rates at the Middle Slope increase northwards suggesting a decrease in bottom-current strength. The N-Middle Slope Site records a transition from the last glacial period, dominated by organic carbon-starved sands, to the early deglacial period when mainly silty and organic carbon-rich sediments were deposited between 14-15 ka BP. Concurrently, glacial sedimentation rates of ~50 cm/ka significantly increased to 120 cm/ka. We propose that this high sedimentation rate relates to lateral sediment re-deposition by current-driven focusing as response to sea level rise. Towards the Holocene, sedimentation rates strongly decreased to 8 cm/ka. We propose that the distinct decrease in sedimentation rates and change in organic carbon contents observed at the N-Middle Slope Site caused the nonsteady-state pore-water conditions and deep sulfate-methane-transition (SMT) at 750 cm core depth. The Lower Slope Site records the last 19 ka. Continuously high terrigenous sediment input (~100 cm/ka) prevailed during the Deglacial, while sedimentation rates distinctly decreased to ~13 cm/ka in the Holocene. Here, pore-water data suggest current steady-state conditions with a pronounced SMT at 510 cm core depth. Our study confirms previous geochemical-modelling studies at the lower slope, which implied that the observed SMT fixation for ~9 ka at specific depth relates to a strong decrease in sedimentation rates at the Pleistocene/Holocene transition[2]. During the Holocene, total organic and inorganic carbon contents, inorganic carbon mass accumulation rates and XRF Si/Al ratios (preserved diatom flux) increase at our sites. We relate this to increased primary production in surface waters and less terrigenous input along the continental margin. Our multidisciplinary approach presents improved age constraints at the northern Argentine Margin and demonstrates that lateral/vertical sediment transport and deposition was strongly linked to Glacial/Interglacial variations in bottom currents, seafloor morphology, sea level and sediment supply. The dynamic depositional histories at the three sites still exert a significant control on modern sedimentary (bio)geochemical processes. [1]Kasten et al. (2019). Cruise No. SO260. Sonne-Berichte. [2]Riedinger et al. (2005). Geochim. Cosmochim. Acta. 69.

Description: Biogeochemical processes in subseafloor sediments can notably change over geological timescales due to variations in oceanographic, climatic and/or depositional conditions. To improve the understanding of changing biogeochemical processes on longer timescales, we investigated ~1.2 km deep and up to 120°C hot subseafloor sediments from the Nankai Trough offshore Japan (Site C0023), drilled during International Ocean Discovery Program Expedition 370 (Temperature Limit of the Deep Biosphere off Muroto)1. Over the past 15 Ma, the sediments have moved several hundreds of kilometers from the Shikoku Basin to the Nankai Trough due to tectonic motion of the Philippine Sea plate2. During this migration, the depositional, geochemical and thermal conditions have significantly changed. By combining geochemical data, sedimentation rates and reactive transport modeling, we reconstructed the evolution of biogeochemical processes in sediments at Site C0023. A distinctive feature at Site C0023 is an inverse sulfate-methane transition (SMT) at ~730 m depth with a broad sulfate-methane overlap zone of ~100 m, suggesting inefficient anaerobic oxidation of methane (AOM). This depth interval corresponds to a temperature of 80° to 85°C, which coincides with the known temperature limit of AOM-performing microbial communities3,4. Our model results demonstrate that the inverse SMT was formed at ~2.5 Ma after the onset of biogenic methanogenesis and AOM as a consequence of enhanced organic carbon burial. Depth-integrated AOM rates derived from the model markedly decrease since the beginning of trench-style deposition and the associated rapid heating of the sediments at ~0.4 Ma, indicating that the microbial activity of AOM-performing communities at the inverse SMT has already started to cease and the SMT is about to disappear. This successive fading of the SMT and, thus, a decrease in the efficiency of the microbial methane sink is ultimately related to the temperature increase beyond the threshold of being suitable for AOM-performing microbial communities. 1Heuer et al., (2017), In Proc. IODP Volume 370. 2Mahony et al., (2011), GSA Bulletin 123, 2201-2223. 3Holler et al., (2011), ISME J 5, 1946-1956. 4Biddle et al., (2012), ISME J 6, 1018-1031.