Description: In the accompanying Comment1, Spielhagen et al. respond to our recent Article2, raising several issues concerning the perceived impossibility of a glacial freshwater ocean, focusing on the Nordic Seas and the North Atlantic south of the Greenland–Scotland ridge (GSR). They argue that no basin-wide 230Thex minima were seen in the Nordic Seas at the times proposed, and that they were due to dilution by high mass fluxes; that 10Be records closely resemble those of 230Thex, which they assume to support only the dilution hypothesis; that the reportedly continuous foraminiferal δ18O records were incompatible with freshwater under an ice shelf north of the GSR; and that no corresponding meltwater signal was seen in North Atlantic foraminiferal δ18O records south of the GSR. We highlight the circular arguments that result from the reliance on just one overstretched proxy for constraining age, temperature and salinity. Our interpretation is in line with observations and with the rapid melting during glacial terminations.

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: A broad introduction regarding the properties of the Weddell Gyre, their relevance for biogeochemistry, including a discussion whether this system is typical for polar regions- or not.

Description: Rapid and profound climatic and environmental changes have been predicted for the Antarctic Peninsula with so far unknown impact on the biogeochemistry of the continental shelves. In this study, we investigate benthic carbon sedimentation, remineralization and iron cycling using sediment cores retrieved on a 400 mile transect with contrasting sea ice conditions along the eastern shelf of the Antarctic Peninsula. Sediments at comparable water depths of 330-450 m showed sedimentation and remineralization rates of organic carbon, ranging from 2.5-13 and 1.8-7.2 mmol C m-2 d-1, respectively. Both rates were positively correlated with the occurrence of marginal sea ice conditions (5-35% ice cover) along the transect, suggesting a favorable influence of the corresponding light regime and water column stratification on algae growth and sedimentation rates. From south to north, the burial efficiency of organic carbon decreased from 58% to 27%, while bottom water temperatures increased from -1.9 to -0.1 °C. Net iron reduction rates, as estimated from pore-water profiles of dissolved iron, were significantly correlated with carbon degradation rates and contributed 0.7-1.2% to the total organic carbon remineralization. Tightly coupled phosphate-iron recycling was indicated by significant covariation of dissolved iron and phosphate concentrations, which almost consistently exhibited P/Fe flux ratios of 0.26. Iron efflux into bottom waters of 0.6-4.5 µmol Fe m-2 d-1 was estimated from an empirical model. Despite the deep shelf waters, a clear bentho-pelagic coupling is indicated, shaped by the extent and duration of marginal sea ice conditions during summer, and likely to be affected by future climate change.

Description: The Polar Southern Ocean (PSO) provides an excess amount of macro-nutrients but productivity is largely limited by the availability of essential micro-nutrients, namely iron, manganese, zinc and others. Seasonal patches of increased productivity off major ice shelfs around Antarctica suggest that local sources of these deficient micro-nutrients must be present. With this session contribution we present a new study on marine ice from the Filchner-Ronne Ice Shelf (FRIS) as a potential source of iron and other limiting micro-nutrients for the Atlantic sector of the PSO. Marine ice is formed via partial melting of meteoric shelf ice near the grounding line of large ice shelves (e.g. FRIS). During this process small refrozen ice platelets accumulate in a layer of over 100 m thickness underneath the ice shelf to form marine ice containing high amounts of particulate material. In a project funded by the German Research Foundation (DFG) within the priority program SPP1158, we analyse 2 marine ice cores (B13: 62m, B15: 167m of marine ice) recovered in the 1990’s from the FRIS on their geochemical compositions. The coring location of B13 was about 40 km away from the shelf ice edge and B15 was drilled another 136 km further inland along the reconstructed flow line of B13. Due to shelf ice migration over the last 30 years, their locations have shifted about 30 km towards the shelf ice edge. First results show dissolved Fe (dFe) and Mn (dMn) concentrations ranging between 30 and 300 nMol and particulate Fe (pFe) of 20 to 120 µMol (0.2 to 1.4 µMol for pMn). These concentrations are orders of magnitude higher than the ones currently found in the PSO for those elements. Basal melting and ice-berg calving of marine ice with the accompanied release of these essential trace metals could therefore fuel local productivity in regions with large extent of shelf ice. With our study we aim to evaluate marine ice as potentially overlooked source for limiting micro-nutrients that could explain high productivity areas within an otherwise relatively low productive PSO.