Claus-Dieter Hillenbrand

Antarctic terrestrial life – challenging the history of the frozen continent?

Antarctica is a continent locked in ice, with almost 99.7% of current terrain covered by permanent ice and snow, and clear evidence that, as recently as the Last Glacial Maximum (LGM), ice sheets were both thicker and much more extensive than they are now. Ice sheet modelling of both the LGM and estimated previous ice maxima across the continent give broad support to the concept that most if not all currently ice‐free ground would have been overridden during previous glaciations. This has given rise to a widely held perception that all Mesozoic (pre‐glacial) terrestrial life of Antarctica was wiped out by successive and deepening glacial events. The implicit conclusion of such destruction is that most, possibly all, contemporary terrestrial life has colonised the continent during subsequent periods of glacial retreat. However, several recently emerged and complementary strands of biological and geological research cannot be reconciled comfortably with the current reconstruction of Antarctic glacial history, and therefore provide a fundamental challenge to the existing paradigms. Here, we summarise and synthesise evidence across these lines of research. The emerging fundamental insights corroborate substantial elements of the contemporary Antarctic terrestrial biota being continuously isolated in situ on a multi‐million year, even pre‐Gondwana break‐up timescale. This new and complex terrestrial Antarctic biogeography parallels recent work suggesting greater regionalisation and evolutionary isolation than previously suspected in the circum‐Antarctic marine fauna. These findings both require the adoption of a new biological paradigm within Antarctica and challenge current understanding of Antarctic glacial history. This has major implications for our understanding of the key role of Antarctica in the Earth System.

Drake Passage and Cenozoic climate: An open and shut case?

Drake Passage opening has often been viewed as a single, discrete event, possibly associated with abrupt changes in global circulation and climate at or near the Eocene-Oligocene boundary. A new plate tectonic model, based on recent reinterpretations of the opening history of basins in the Scotia Sea, suggests that an effective ocean gateway may have developed even earlier, during the middle Eocene. This is consistent with a growing body of evidence from sediment core proxy data for Eocene changes in Southern Ocean circulation and biological productivity. The period between earliest opening after ∼50 Ma and the latest Eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins. This shallow opening may have caused important changes in Southern Ocean circulation, contributing to Eocene cooling and the growth of Antarctic ice sheets.

LIFE HUNG BY A THREAD: ENDURANCE OF ANTARCTIC FAUNA IN GLACIAL PERIODS

Today, Antarctica exhibits some of the harshest environmental conditions for life on Earth. During the last glacial period, Antarctic terrestrial and marine life was challenged by even more extreme environmental conditions. During the present interglacial period, polar life in the Southern Ocean is sustained mainly by large-scale primary production. We argue that during the last glacial period, faunal populations in the Antarctic were limited to very few areas of local marine productivity (polynyas), because complete, multiannual sea-ice and ice shelf coverage shut down most of the Southern Ocean productivity within todays seasonal sea-ice zone. Both marine sediments containing significant numbers of planktonic and benthic foraminifera and fossil bird stomach oil deposits in the adjacent Antarctic hinterland provide indirect evidence for the existence of polynyas during the last glacial period. We advocate that the existence of productive oases in the form of polynyas during glacial periods was essential for the survival of marine and most higher-trophic terrestrial fauna. Reduced to such refuges, much of todays life in the high Antarctic realm might have hung by a thread during the last glacial period, because limited resources available to the food web restricted the abundance and productivity of both Antarctic terrestrial and marine life.

Subglacial bedforms reveal complex basal regime in a zone of paleo- ice stream convergence, Amundsen Sea embayment, West Antarctica

The flow of ice streams, which account for most discharge from large ice sheets, is controlled by processes operating at the ice stream bed. Data from modern ice stream beds are difficult to obtain, but where ice advanced onto continental shelves during glacial periods, extensive areas of the former bed can be imaged using modern swath sonar tools. We present new multibeam swath bathymetry data analyzed alongside sparse preexisting data from the Amundsen Sea embayment. The compilation is the most extensive, continuous area of multi-beam data coverage yet obtained on the inner continental shelf of Antarctica. The data reveal streamlined subglacial bedforms that define a zone of paleo–ice stream convergence, but, in contrast to previous models, do not show a simple downflow progression of bedform types along paleo–ice stream troughs. We interpret high spatial variability of bedforms as indicating a complex mechanical and hydrodynamic regime at the former ice stream beds, consistent with observations from some modern ice streams. We conclude that care must be taken when using bedforms to infer paleo–ice stream velocities.

Flow of the West Antarctic Ice Sheet on the continental margin of the Bellingshausen Sea at the Last Glacial Maximum

Geophysical data show that during the last glaciation the West Antarctic Ice Sheet (WAIS) drained to the continental shelf edge of the Bellingshausen Sea through a cross-shelf bathymetric trough (Belgica Trough) as a grounded, fast flowing, ice stream. The drainage basin feeding this ice stream probably encompassed southwestern Palmer Land, parts of southern Alexander Island, and the Bryan Coast of Ellsworth Land, with an area exceeding 200,000 km2. On the inner continental shelf, streamlined bedrock and drumlins mapped by swath bathymetry show that the ice stream was fed by convergent ice flow draining from Eltanin Bay and bays to the east, as well as by ice draining the southern part of the Antarctic Peninsula Ice Sheet through the Ronne Entrance. The presence of a paleoice stream in Belgica Trough is indicated by megascale glacial lineations formed in soft till and a trough mouth fan on the continental margin. Grounding zone wedges on the inner and midshelf record ice marginal stillstands during deglaciation and imply a staggered pattern of ice sheet retreat. These new data indicate an extensive WAIS at the Last Glacial Maximum (LGM) on the Bellingshausen Sea continental margin, which advanced to the shelf edge. In conjunction with ice sheet reconstructions from the Antarctic Peninsula and Pine Island Bay, this implies a regionally extensive ice sheet configuration during the LGM along the Antarctic Peninsula, Bellingshausen Sea, and Amundsen Sea margins, with fast flowing ice streams draining the WAIS and Antarctic Peninsula Ice Sheet to the continental shelf edge.

Grounding-line retreat of the West Antarctic Ice Sheet from inner Pine Island Bay

Ice loss from the marine-based, potentially unstable West Antarctic Ice Sheet (WAIS) contributes to current sea-level rise and may raise sea level by ≤3.3 m or even ≤5 m in the future. Over the past few decades, glaciers draining the WAIS into the Amundsen Sea Embayment (ASE) have shown accelerated ice flow, rapid thinning, and fast retreat of the grounding line (GL). However, the long-term context of this ice loss is poorly constrained, limiting our ability to accurately predict future WAIS behavior. Here we present a new chronology for WAIS retreat from the inner continental shelf of the eastern ASE, based on radiocarbon dates from three marine sediment cores. The ages document a retreat of the GL to within ∼100 km of its modern position before ca. 10,000 calibrated (cal.) yr B.P. This early deglaciation is consistent with ages for GL retreat from the western ASE. Our new data demonstrate that, in contrast to the Ross Sea, WAIS retreat from the ASE shelf was largely complete by the start of the Holocene. Our results further suggest either slow GL retreat from the inner ASE shelf throughout the Holocene, or that any episodes of fast GL retreat must have been short-lived. Thus, today’s rapid retreat may be exceptional during the Holocene and may originate in recent changes in regional climate, ocean circulation, or ice-sheet dynamics.

Evidence for elevated alkalinity in the glacial Southern Ocean

An increase in whole ocean alkalinity during glacial periods could account, in part, for the drawdown of atmospheric CO2 into the ocean. Such an increase was inevitable due to the near elimination of shelf area for the burial of coral reef alkalinity. We present evidence, based on downcore measurements of benthic foraminiferal B/Ca and Mg/Ca from a core in the Weddell Sea, that the deep ocean carbonate ion concentration, [CO32-], was elevated by similar to 25 mu mol/kg during each glacial period of the last 800 kyr. The heterogeneity of the preservation histories in the different ocean basins reflects control of the carbonate chemistry of the deep glacial ocean in the Atlantic and Pacific by the changing ventilation and chemistry of Weddell Sea waters. These waters are more corrosive than interglacial northern sourced waters but not as undersaturated as interglacial southern sourced waters. Our inferred increase in whole ocean alkalinity can be reconciled with reconstructions of glacial saturation horizon depth and the carbonate budget if carbonate burial rates also increased above the saturation horizon as a result of enhanced pelagic calcification. The Weddell records display low [CO32-] during deglaciations and peak interglacial warmth, coincident with maxima in percent CaCO3 in the Atlantic and Pacific oceans. Should the burial rate of alkalinity in the more alkaline glacial deep waters outstrip the rate of alkalinity supply, then pelagic carbonate production by the coccolithophores at the end of the glacial maximum could drive a decrease in ocean [CO32-] and act to trigger the deglacial rise in pCO(2).

The southern Weddell Sea: combined contourite-turbidite sedimentation at the southeastern margin of the Weddell Gyre

Abstract Sedimentary processes in the southeastern Weddell Sea are influenced by glacial-interglacial ice-shelf dynamics and the cyclonic circulation of the Weddell Gyre, which affects all water masses down to the sea floor. Significantly increased sedimentation rates occur during glacial stages, when ice sheets advance to the shelf edge and trigger gravitational sediment transport to the deep sea. Downslope transport on the Crary Fan and off Dronning Maud and Coats Land is channelized into three huge channel systems, which originate on the eastern, the central and the western Crary Fan. They gradually turn from a northerly direction eastward until they follow a course parallel to the continental slope. All channels show strongly asymmetric cross sections with well-developed levees on their northwestern sides, forming wedge-shaped sediment bodies. They level off very gently. Levees on the southeastern sides are small, if present at all. This characteristic morphology likely results from the process of combined turbidite-contourite deposition. Strong thermohaline currents of the Weddell Gyre entrain particles from turbidity-current suspensions, which flow down the channels, and carry them westward out of the channel where they settle on a surface gently dipping away from the channel. These sediments are intercalated with overbank deposits of high-energy and high-volume turbidity currents, which preferentially flood the left of the channels (looking downchannel) as a result of Coriolis force. In the distal setting of the easternmost channel-levee complex, where thermohaline currents are directed northeastward as a result of a recirculation of water masses from the Enderby Basin, the setting and the internal structures of a wedge-shaped sediment body indicate a contourite drift rather than a channel levee. Dating of the sediments reveals that the levees in their present form started to develop with a late Miocene cooling event, which caused an expansion of the East Antarctic Ice Sheet and an invigoration of thermohaline current activity.

West Antarctic ice sheet change since the Last Glacial Period

The potential for rapid deglaciation, or collapse, of the 2–million–square–kilometer West Antarctic Ice Sheet (WAIS) in response to climate change is one of the most serious environmental threats facing mankind. The WAIS is a marine ice sheet with large parts of its ice grounded below sea level. Complete collapse would result in a global sea level rise of approximately 5 meters, with immense social, economic, and ecological consequences.

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean

This chapter explores the Middle Miocene to Pliocene terrestrial and marine records of Antarctica and the Southern Ocean. The structure of the chapter makes a clear distinction between terrestrial and marine records as well as proximal (on or around Antarctica) and more distal records (Southern Ocean). Particular geographical regions are identified that reflect the areas for which the majority of palaeoenvironmental and palaeoclimatic information exist. Specifically, the chapter addresses the terrestrial sedimentary and fjordal environments of the Transantarctic Mountains and Lambert Glacier region, the terrestrial fossil record of Antarctic climate, terrestrial environments of West Antarctica, and the marine records of the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet (APIS), as well as the marine record of the Southern Ocean. Previous and current studies focusing on modelling Middle Miocene to Pliocene climate, environments and ice sheets are discussed.