Beschreibung: Radiogenic lead (Pb) and neodymium (Nd) isotope compositions extracted from authigenic phases in marine sediments are sensitive tracers to reconstruct past ocean circulation and water mass mixing. Chemical reductive leaching of hydrogenetic ferromanganese oxyhydroxides from bulk sediments is the most practical way to recover past seawater Pb and Nd isotope signatures in the Southern Ocean, due to the scarcity of alternative archives. However, the leached signal could be compromised if substantial quantities of Pb and Nd were released from non‐hydrogenetic sediment fractions during chemical extraction. Here we developed a very short 10‐seconds leaching method to extract reliable seawater Pb and Nd isotope signals from sediments in the Atlantic sector of Southern Ocean. The effect of a previously recommended MgCl2 pre‐wash, the role of chelate ligands in the leaching solution and length of leaching time were investigated. The results show that 10‐seconds exposure time of sediments to reductive leaching extracted sufficient and more reliable hydrogenetic Pb and Nd compared with the commonly used 30‐minute leaching approaches. The robustness of our improved leaching method was validated via direct comparison of Pb and Nd isotope signatures with actual seawater, porewater and corresponding sediment leachates from three stations in front of the Antarctic Filchner‐Rønne Ice Shelf. Our findings also indicate that in contrast previously studied sites on the West Antarctic continental shelf, the bottom seawater Nd concentration is less elevated through benthic fluxes in the area of the southern Weddell Sea shelf.

Beschreibung: Glacial/interglacial changes in Southern Ocean's air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate to deep water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943-2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

Beschreibung: Sedimentary architecture and morphogenetic evolution of a polar bay-mouth gravel-spit system are revealed based on topographic mapping, sedimentological data, radiocarbon dating and ground-penetrating radar investigations. Data document variable rates of spit progradation in reaction to atmospheric warming synchronous to the termination of the last glacial re-advance (LGR, 0.45–0.25 ka BP), the southern hemisphere equivalent of the Little Ice Age cooling period. Results show an interruption of spit progradation that coincides with the proposed onset of accelerated isostatic rebound in reaction to glacier retreat. Spit growth resumed in the late 19th century after the rate of isostatic rebound decreased, and continues until today. The direction of modern spit progradation, however, is rotated northwards compared with the growth axis of the early post-LGR spit. This is interpreted to reflect the shift and strengthening in the regional wind field during the last century. A new concept for the interplay of polar gravel-spit progradation and glacio-isostatic adjustment is presented, allowing for the prediction of future coastal evolution in comparable polar settings.

Beschreibung: The importance of macrobenthos in benthic‐pelagic coupling and early diagenesis of organic carbon (OC) has long been recognized but has not been quantified at a regional scale. By using the southern North Sea as an exemplary area we present a modelling attempt to quantify the budget of total organic carbon (TOC) reworked by macrobenthos in seafloor surface sediments. Vertical profiles in sediments collected in the field indicate a significant but nonlinear correlation between TOC and macrobenthic biomass. A mechanistic model is used to resolve the bi‐directional interaction between TOC and macrobenthos. A novelty of this model is that bioturbation is resolved dynamically depending on variations in local food resource and macrobenthic biomass. The model is coupled to 3D hydrodynamic‐biogeochemical simulations to hindcast the mutual dependence between sedimentary TOC and macrobenthos from 1948 to 2015. Agreement with field data reveals a satisfactory model performance. Our simulations show that the preservation of TOC in the North Sea sediments is not only determined by pelagic conditions (hydrodynamic regime and primary production) but also by the vertical distribution of TOC, bioturbation intensity, and the vertical positioning of macrobenthos. Macrobenthos annually ingest 20%–35% and in addition vertically diffuse 11%–22% of the total budget of TOC in the upper‐most 30 cm sediments in the southern North Sea. This result indicates a central role of benthic animals in modulating the OC cycling at the sediment‐water interface of continental margins.

Beschreibung: 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 1,100 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.

Beschreibung: Southern Ocean westerly wind intensity and position are thought to play a crucial role in controlling glacial/interglacial CO2 changes through their impact on Antarctic upwelling intensity and the delivery of iron-rich dust that stimulates biological production during glacial periods. Sediment-core grain size records can provide key insights into changes in wind strength and source-area characteristics over glacial-interglacial timescales. However, so far, little is known about G/IG grain size changes in Southern Ocean sediments. For this study, we analyzed the grain-size distributions of two subantarctic deep sea sediments cores from the Pacific (PS75/056-1) and Atlantic (ODP Site 1090) sectors of the Southern Ocean. Dust mean grain size shows opposing trends in the two Southern Ocean sectors. Coarser glacial grain sizes are observed in the Pacific sector, while finer glacial grain-sizes are observed in the Atlantic. Our results suggest that changes in the latitudinal position of the SWW had distinct impacts on grain size distribution in the Atlantic and Pacific sectors, also likely associated with shifts in the dust source areas. These findings indicate that more extensive studies of grain-size distribution in the Southern Ocean can provide important insights on the timing and latitudinal extent of the westerly winds changes during ice ages.

Beschreibung: The deep southern component water (SCW), comprising Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW), is a major component of the global oceanic circulation. It has been suggested that the deep Atlantic water mass structure changed significantly during the last glacial/interglacial cycle. However, deep SCW source-proximal records remain sparse. Here we present three coherent deep SCW paleocurrent records from the deep Argentine continental margin shedding light on deep water circulation and deep SCW flow strength in the Southwest Atlantic since the Last Glacial Maximum (LGM). Based on increased sortable silt values, we propose enhanced deep SCW flow strength from 14 to 10 cal ka B.P. relative to the early deglacial/LGM and the Holocene. We propose a direct influence of deep northern component water (NCW) on deep SCW flow strength due to vertical narrowing of deep SCW spreading, concurrent with a migration of the high-energetic LCDW/AABW interface occupying our core sites. We suggest a shoaled NCW until 13 cal ka B.P., thereby providing space for deep SCW spreading that resulted in reduced carbonate preservation at our core sites. Increased carbonate content from 13 cal ka B.P. indicates that the NCW expanded changing deep water properties at our core sites in the deep Southwest Atlantic. However, southern sourced terrigenous sediments continued to be deposited at our core sites, suggesting that deep SCW flow was uninterrupted along the Argentine continental margin since the LGM.

Beschreibung: The detailed Holocene inundation history of the Bermuda North Lagoon may be used as model for transgressive and highstand sequences in carbonate platforms. Sedimentation and facies development were controlled largely by sea‐level rise and antecedent topography. Four late Pleistocene to Holocene sequences may be identified in North Lagoon based on a combined analysis of 200 km shallow reflection seismics and 39 cores including 29 radiometric and U/Th‐ages. The sequences were deposited during sea‐level highstands and are separated by subaerial exposure horizons that formed during sea‐level lowstands. Sequence 1 (inferred MIS 7) consists of well‐cemented carbonate sands. Sequence 2 (MIS 5) is up to 20 m thick and consists of well‐sorted, inter‐reefal sands and reef sediments with mound‐like structures. Sequence 3 (inferred MIS 3) is up to ca 6 m thick and accumulated in topographic lows of the underlying sequences some 20 m below modern sea‐level. Sequence 4 (MIS 1, Holocene) includes lagoonal sediments up to 10 m thick, and reefs that accumulated on topographic highs of the MIS 5 sequences. Holocene sediments in topographic lows include peat, peaty sediment, freshwater mud, restricted marine carbonates, and open lagoonal carbonate sediments deposited in seagrass beds, shallow water, and deeper lagoon areas. Upward fining is an expression of deepening and the development of a reef‐protected lagoon environment. Holocene sedimentation on topographic highs usually lacks freshwater and transitional facies and starts with shallow marine mollusc shell accumulations overlain by carbonate sediments that show fining upward. Packstone (68%), wackestone (22%), grainstone (9%) and mudstone (1%) textures occur in cores, with Halimeda, molluscs, coralline algae and foraminifera being the most common constituent particles; coral fragments are rare. During the Holocene, an estimated volume of 1 km3 of carbonate sediments was deposited in North Lagoon. Average sedimentation rates are estimated to be 0.32 m/kyr.