The shape of ice-shelf cavities are a major source of uncertainty in understanding ice-ocean interactions. This limits assessments of the response of the Antarctic ice sheets to climate change. Here we use vibroseis seismic reflection surveys to map the bathymetry beneath the Ekström Ice Shelf, Dronning Maud Land. The new bathymetry reveals an inland-sloping trough, reaching depths of 1100 m below sea level, near the current grounding line, which we attribute to erosion by palaeo-ice streams. The trough does not cross-cut the outer parts of the continental shelf. Conductivity-temperature-depth profiles within the ice-shelf cavity reveal the presence of cold water at shallower depths and tidal mixing at the ice-shelf margins. It is unknown if warm water can access the trough. The new bathymetry is thought to be representative of many ice shelves in Dronning Maud Land, which together regulate the ice loss from a substantial area of East Antarctica.

We traced diatom composition and diversity through time using diatom derived sedimentary ancient DNA (sedaDNA) from eastern continental slope sediments off Kamchatka (North Pacific) by applying a short, diatom-specific marker on 63 samples in a DNA metabarcoding approach. The sequences were assigned to diatoms that are common in the area and characteristic of cold water. SedaDNA allowed us to observe shifts of potential lineages from species of the genus Chaetoceros that can be related to different climatic phases, suggesting that pre-adapted ecotypes might have played a role in the long-term success of species in areas of changing environmental conditions. These sedaDNA results complement our understanding of the long-term history of diatom assemblages and their general relationship to environmental conditions of the past. Sea-ice diatoms (Pauliella taeniata (Grunow) Round & Basson, Attheya septentrionalis (Østrup) R.M.Crawford and Nitzschia frigida (Grunow)) detected during the late glacial and Younger Dryas are in agreement with previous sea-ice reconstructions. A positive correlation between pennate diatom richness and the sea-ice proxy IP25 suggests that sea ice fosters pennate diatom richness, whereas a negative correlation with June insolation and temperature points to unfavorable conditions during the Holocene. A sharp increase in proportions of freshwater diatoms at ~11.1 cal kyr BP implies the influence of terrestrial runoff and coincides with the loss of 42% of diatom sequence variants. We assume that reduced salinity at this time stabilized vertical stratification which limited the replenishment of nutrients in the euphotic zone.

Ocean sediment records document abrupt changes in glacial-to-deglacial circulation and mixing of the ocean, recorded as changes in 14C reservoir ages of surface waters. Here we present 14C-based high-resolution age records of four sediment cores derived by means of 14C plateau tuning. This provides a detailed and precise stratigraphic correlation between the western and eastern South Pacific and paleoclimate records of Antarctic ice cores as well as 14C reservoir ages. The accuracy and precision of plateau tuning are confirmed in two sediment cores off Chile; in one core by independent land-based age control of four tephra layers, in a second core by a suite of glacial sediment varves. During glacial times, high reservoir ages reaching up to ~1500 yr may reflect seasonal sea ice and/or a melt water lid at high latitudes both east of New Zealand and off southern Chile and may be linked to northward advection of upwelled old subsurface waters from the Polar Frontal Zone. Our results support the model of a bipolar seesaw at the onset of rapid deglacial Antarctic warming, moreover, they show that the Antarctic Cold Reversal immediately preceded the onset of the Younger Dryas cold spell.