Description: The Antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers, conditioned by ocean temperature and bed topography. Glaciers retreating along retrograde slopes (that is, the bed elevation drops in the inland direction) are potentially unstable, while subglacial ridges slow down the glacial retreat. Despite major advances in the mapping of subglacial bed topography, significant sectors of Antarctica remain poorly resolved and critical spatial details are missing. Here we present a novel, high-resolution and physically based description of Antarctic bed topography using mass conservation. Our results reveal previously unknown basal features with major implications for glacier response to climate change. For example, glaciers flowing across the Transantarctic Mountains are protected by broad, stabilizing ridges. Conversely, in the marine basin of Wilkes Land, East Antarctica, we find retrograde slopes along Ninnis and Denman glaciers, with stabilizing slopes beneath Moscow University, Totten and Lambert glacier system, despite corrections in bed elevation of up to 1 km for the latter. This transformative description of bed topography redefines the high- and lower-risk sectors for rapid sea level rise from Antarctica; it will also significantly impact model projections of sea level rise from Antarctica in the coming centuries.

Description: In this contribution, high resolution multibeam swath bathymetry and PARASOUND sediment echosounder data are used to describe a region within the distal part of the central Mozambique Channel. The study area marks a transition from abyssal plain to abyssal hill type morphology within the sediment-rich Mozambique Fan and associated with a zone of extension in response to East African Rift System kinematics. Hosted within the abyssal hill lies an east-west orientated, elongate (80 km × 11 km) depression (relief of ca.175 m). Multibeam bathymetry and PARASOUND data show that the region surrounding the depression is variable in geomorphology including rugged irregular seafloor and sediment waves. Low gradient, smooth sea floor dominates the abyssal plain which returns several, distinct, sub-parallel sub bottom echoes. The flanks of the abyssal hill are marked by seafloor undulations likely evidence of bottom-current controlled geomorphology, and mass wasting deposits. The floor of the depression is characterised by hyperbolic echoes commonly associated with very rugged seafloor and basement outcrop with little sediment cover. The present-day geomorphology of the study area is the product of deep-seated ocean circulation and soft sediment deformation superimposed upon the antecedent geological framework, influenced by present-day kinematics of the East African Rift System. Faulting associated with these kinematics is manifest at the seafloor as the elongate steep-flanked depression; the result of an extensional regime expressed across the Mozambique channel from south-southwest to north-northeast. This contribution highlights the local, marine, ramification of a continental-scale largely terrestrial tectonic regime.

Description: Some of the oldest surviving oceanic basins in the world, the Mozambique and West Somali basins, were created during the breakup of Gondwana, starting around 180 Ma. Between the two basins, relative movements of West Gondwana and East Gondwana, including Madagascar, created a shear zone, the Davie Fracture Zone (DFZ) with a topographic elevation (Davie Ridge - DR) marking its centre. The crustal composition of the DFZ and DR is a subject of speculation and debate. In this study, we present seismic refraction data across the prominent topography of the southern DR. Ray tracing of the wide-angle data as well as additional seismic amplitude modelling and 2.5D density modelling constrain its crustal structure and architecture. The data indicate that in the Mozambique Channel the DR consists of fragments of continental crust with a thickness of 10 to 12 km. An oceanic crust indenter extends northward from the Mozambique Basin into the area between the DR and the East African margin at 16.5°S. Northeast of the DR, at 41.8°W/14.5°S, the Somali Basin is probably floored by 6 km thick oceanic crust. Hence, the continental DR separates oceanic crust of the Somali and Mozambique basins. The transitional crustal area at the central Mozambican margin is underlain by high velocity lower crust (HVLC). The HVLC has velocities up to 7.3 km/s and extents along the margin, vanishing northward between 16.5° and 14.5°S. At the Madagascan side of the DR, at 16.5°S, the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller HVLC of 2.9 km thickness and an E-W extent of 120 km. The oceanic crust at 14.5°S represents the oldest part the Somali Basin, which formed after the initial NW-SE rifting between East and West Gondwana.

Description: Changes in ocean gateway configuration are known to induce basin-scale rearrangements in ocean characteristics throughout the Cenozoic. However, there is large uncertainty in the relative timing of the subsidence histories of ocean gateways in the northern high latitudes. By using a fully coupled General Circulation Model we investigate the salinity and temperature changes in response to the subsidence of two key ocean gateways in the northern high latitudes during early to middle Miocene. Deepening of the Greenland-Scotland Ridge causes a salinity increase and warming in the Nordic Seas and the Arctic Ocean. While warming this realm, deep water formation takes place at lower temperatures due to a shift of the convection sites to north off Iceland. The associated deep ocean cooling and upwelling of deep waters to the Southern Ocean surface causes a cooling in the southern high latitudes. These characteristic impacts in response to the Greenland-Scotland Ridge deepening are independent of the Fram Strait state. Subsidence of the Fram Strait for a deep Greenland-Scotland Ridge causes less pronounced warming and salinity increase in the Nordic Seas. A stronger salinity increase is detected in the Arctic while temperatures remain unaltered, which further increases the density of the North Atlantic Deep Water. This causes an enhanced contribution of North Atlantic Deep Water to the abyssal ocean and on the expense of the colder southern source water component. These relative changes largely counteract each other and cause little warming in the upwelling regions of the Southern Ocean.

Description: Ongoing research aims to constrain the extent of grounded ice shelves around the Arctic Ocean during the last glacial periods. Here, the Chukchi region is of special interest because of its broad, shallow shelf. It is under debate if any ice sheets existed on the Chukchi Shelf, as well as their possible sources and areal extent. Bathymetric and sub bottom profiler studies of the last two decades recorded on the Chukchi Sea margins and the Arlis Plateau image complex patterns of glaciogenic erosion of the shallow sediments like Mega Scale Glacial Lineations (MSGL) at present-day water depths of more than 350 m. The different directions of those MSGL indicate the presence of several ice shelves and streams and point to an East Siberian Ice Sheet of unknown size. On the Chukchi Shelf, no evidences for the existence of a large ice shelf for water depths shallower than 350 m have been described yet. We re-processed 2D multi-channel seismic data acquired in 2011 from R/V Marcus G. Langseth to investigate glaciogenic features on the shallow shelf. Our presented data will reveal, along with sediment echosounder and bathymetric data, new insights into the glacial history of the outer Chukchi Shelf and Borderland. The first up to 300 ms TWT of the seismic data indicate eroded strata and reworked sediments separated from preglacial material by a high amplitude glacial base reflection. These layers are characterized by different seismic reflection characteristics indicating different erosion and deposition environments. Furthermore, the glacial-base reflection underlies a grounding zone wedge and recessional moraines within a bathymetric trough in modern water depths between 400 m and 600 m, indicating a stepwise glacial retreat towards the Chukchi Shelf. Moreover, one grounding zone wedge with a dimension of 48 km x 75 km as a product of several advance and retreat cycles built up on the Chukchi Rise. Our data indicate multiple glacial periods of this region as well as they document the presence of an ice shelf close to the present-day shelf edge during the Last Glacial Maximum, but provide no evidence that it extended onto the subaerial Chukchi Shelf.

Description: The Lomonosov Ridge (LR) and Fram Strait (FR) represent prominent morphologic features in the Arctic Ocean. Their tectonic evolution control ocean circulation, sedimentation environment, glacial processes and ecosystem through time. We present findings of a 300 km long seismic transect from the Gakkel Deep through the southeastern Amundsen Basin (AB), and onto the LR. The data image an up to 3 km thick sedimentary sequence that can be subdivided into six major seismic units. The two lower units AB-1 and AB-2 consist of syn-rift sediments of Paleocene to early Eocene age likely eroded off the Barents-Kara and Laptev Sea shelves, and the subsiding LR. AB-2 includes the time interval of the “Azolla event,” which is regarded as an era of a warm Arctic Ocean punctuated by episodic incursions of fresh water. The connection to North Atlantic waters via the Fram Strait was not yet established, and anoxic conditions prevailed in the young, still isolated Eurasian Basin. Also, the LR still was above or close to sea level and posed an obstacle for water exchange between the Eurasian and Amerasian basins. The top of AB-2 onlaps the acoustic basement at magnetic anomaly C21o (∼47.3 Ma). Its contact with unit AB-3 above is marked by a striking loss in reflection amplitudes. This prominent interface can be traced through the AB, indicating widespread changes in tectonic and deposition conditions in the Arctic Ocean since the middle Eocene. For younger crust the depth of acoustic basement rises significantly, as well as the deformation of the surface. Both are probably linked to a reorganization of tectonic plates accompanied by a significant decrease in spreading rates. Units AB-3 and AB-4 indicate the accumulation of sediments between the middle Eocene and the earliest Miocene. Erosional, channel-like interruptions indicate these layers to reflect the stage when Fram Strait opened and continuously deepened. Incursions of water masses from the North Atlantic probably led to first bottom currents and produced erosion, slumping, and subsequent mixing of deposits. The upper units AB-5 to AB-6 show reflection characteristics and thicknesses similar all over the Arctic Ocean indicating that basin-wide pelagic sedimentation prevailed at least since late Oligocene. Drift bodies, sediment waves, and erosional structures indicate the onset of a modern ocean circulation system and bottom current activity in the early Miocene in the Amundsen Basin. At that time, the FR was developed widely, and also the LR no longer posed an obstacle between the Amerasia and Eurasia Basins. Lastly, unit AB-6 indicates pronounced variations in the sedimentary layers, and is associated with the onset of glacio-marine deposition since the Pliocene (5.3 Ma).

Description: Acoustic and detailed swath bathymetry data revealed a systematic picture of submarine landslides on the Siberian part of Lomonosov Ridge. Whereas numerous studies on mass movement exist along the margin of the Arctic Ocean less is known from central Arctic. A regional survey comprising swath bathymetry, sediment echo sounder and multichannel seismic profiling was performed on the southeastern Lomonosov Ridge. The data provide constraints on the present-day morphology of the Siberian part of Lomonosov Ridge, between 81°–84°N and 140°–146°E. We mapped twelve crescent-shaped escarpments located on both flanks on the crest of Lomonosov Ridge. The escarpments are 2.1 to 10.2 km wide, 1.7 to 8.2 km long and 125 to 851 m high from which 58 to 207 m are occupied by crescent-shaped headscarps. Subbottom data show chaotic reflections within most of the escarpment areas. The unit is overlain by ~110–340 m of semi-coherent parallel reflections. At its bottom the chaotic reflections are limited by a partly eroded high-amplitude reflection sequence that is inclined with 〈1° basinwards. We find the escarpments to be remnants of submarine landslide events that mobilized 0.09 to 7.58 km3 of sediments between mid Pliocene and mid Miocene. The relatively small amounts of mobilized sediments seem to be typical for the Lomonosov Ridge. The epoch corresponds to the ongoing subsidence of the Lomonosov Ridge below sea level. During that time deposition and the load of sediments changed. We suggest that changes in sediment type preconditioned, and co-occurring earthquakes finally triggered the submarine landslides.

Description: Changes in ocean gateway configuration can induce basin‐scale rearrangements in ocean current characteristics. However, there is large uncertainty in the relative timing of the Oligocene/Miocene subsidence histories of the Greenland‐Scotland Ridge (GSR) and the Fram Strait (FS). By using a climate model, we investigate the temperature and salinity changes in response to the subsidence of these two key ocean gateways during early to middle Miocene. For a singular subsidence of the GSR, we detect warming and a salinity increase in the Nordic Seas and the Arctic Ocean. As convection sites shift to the north of Iceland, North Atlantic Deep Water (NADW) is formed at cooler temperatures. The associated deep ocean cooling and upwelling of deep waters to the Southern Ocean surface can cause a cooling in the southern high latitudes. These characteristic responses to the GSR deepening are independent of the FS being shallow or deep. An isolated subsidence of the FS gateway for a deep GSR shows less pronounced warming and salinity increase in the Nordic Seas. Arctic temperatures remain unaltered, but a stronger salinity increase is detected, which further increases the density of NADW. The increase in salinity enhances the contribution of NADW to the abyssal ocean at the expense of the colder southern source water component. These relative changes largely counteract each other and cause a negligible warming in the upwelling regions of the Southern Ocean.

Description: A regional seismic survey on the southeastern Lomonosov Ridge and adjacent basins provides constraints on the coupled evolution of ocean circulations, depositional regime and tectonic processes. First, Mesozoic strata on the Lomonosov Ridge, its faulted flanks and the initial Amundsen Basin were covered with syn-rift sediments of Paleocene to early Eocene age. Numerous vertical faults indicate differential compaction of possibly anoxic sediments deposited in the young, still isolated Eurasian Basin. The second stage, as indicated by a prominent high-amplitude-reflector sequence (HARS) covering the ridge, was a time of widespread changes in deposition conditions, likely controlled by the ongoing subsidence of the Lomonosov Ridge and gradual opening of the Fram Strait. Episodic incursions of water masses from the North Atlantic probably were the consequences, and led to the deposition of thin sedimentary layers of different lithology. The third stage is marked by continuous deposition since the early Miocene (20 Ma). At that time, the ridge no longer posed an obstacle between the Amerasia and Eurasia Basins and pelagic sedimentation was established. Drift bodies, sediment waves, and erosional structures indicate the onset of circulation. Faulting on the ridge slope has led to a series of terraces where sediment drifts have accumulated since the early Miocene. It is suggested that ongoing sagging of the ridge and currents may have shaped the steep sediment free flanks of the terraces. Lastly, a sequence of high-amplitude reflectors marks the transition to the early Pliocene large-scale Northern Hemisphere glaciations.

Description: Tristan da Cunha is assumed to be the youngest subaerial expression of the Walvis Ridge hot spot. Based on new hydroacoustic data, we propose that the most recent hot spot volcanic activity occurs west of the island. We surveyed relatively young intraplate volcanic fields and scattered, probably monogenetic, submarine volcanoes with multibeam echosounders and sub-bottom profilers. Structural and zonal GIS analysis of bathymetric and backscatter results, based on habitat mapping algorithms to discriminate seafloor features, revealed numerous previously-unknown volcanic structures. South of Tristan da Cunha, we discovered two large seamounts. One of them, Isolde Seamount, is most likely the source of a 2004 submarine eruption known from a pumice stranding event and seismological analysis. An oceanic core complex, identified at the intersection of the Tristan da Cunha Transform and Fracture Zone System with the Mid-Atlantic Ridge, might indicate reduced magma supply and, therefore, weak plume-ridge interaction at present times.