Eugene W. Domack

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 1—Geophysical results

In the 1994, 1995, and 1998 austral field seasons, geophysical research efforts in the Ross Sea focused on acquiring a high-resolution database designed to permit (1) reconstruction of the maximum extent and configuration of the ice sheet during the Last Glacial Maximum (i.e., oxygen isotope stage 2); (2) reconstruction of conditions at the base of the ice sheet; and (3) assessment of the relative retreat history of the ice sheet following the Last Glacial Maximum. Five seismic facies are distinguished on the basis of external geometry, relationships among features, bounding surface amplitude, intensity of internal acoustic signature, and geometry of internal reflectors. Seismic facies 1 is a transparent draping unit correlated to diatomaceous mud, interpreted as being deposited under open-marine conditions. Seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations. These units are interpreted to be grounding-zone proximal deposits overridden by the expanded ice sheet. Seismic facies 2 may be a deforming-till unit. Seismic facies 4a is characterized by an internally massive to chaotic signature, an erosional, often flat, lower surface, and a hummocky upper surface displaying glacial lineations. This unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion (4a) and with an earlier (pre–Last Glacial Maximum) expansion (4b). Seismic facies 5 is an acoustically laminated, ponded, and draping deposit interpreted to contain proglacial and sub–ice-shelf materials deposited continuously since the last interglacial period. The associations of these units provide context for the interpretation of the ice-edge maximum position, conditions at the base of the ice sheet, and the relative retreat history of the region. There is compelling evidence for a much-expanded ice sheet in the Ross Sea during the Last Glacial Maximum. In the western Ross Sea, the maximum grounding position is marked by an isolated grounding-zone wedge, and placed in the vicinity of Coulman Island, approximately 150 km from the continental-shelf edge. In the central Ross Sea, the maximum grounding position is close to the continental-shelf break, based primarily on the presence of an extensive 60-m-thick sheet-like deposit with a fluted upper surface. Streaming ice may have occupied bathymetric lows on the continental shelf, as suggested by (1) the configuration of bathymetry; (2) the presence of glacially eroded troughs; (3) the concentration of sediment (interpreted to be deforming till) within the middle to outer shelf reaches of the troughs; and (4) the fluted nature of the upper surface. Absolute rates of streaming ice flow relative to inter-ice-stream areas are not implied. The western Ross Sea continental-shelf deposits record the retreat history of ice derived predominantly from the East Antarctic Ice Sheet and glaciers of the Transantarctic Mountains. Ice flow on the continental shelf is interpreted to have remained fixed in position, restricted by the walls of the troughs. On the inner shelf of the western Ross Sea, the ice flowed over and eroded lithified sedimentary strata. Slower-moving ice occupied flat-topped banks. A single grounding-zone wedge occurs on the western Ross Sea central shelf; no substantial deposits are observed on the inner continental shelf. This lack of grounding-zone features reflects a restricted sediment supply and a relatively steady, rapid retreat of the ice sheet. During retreat, ice flow acted independently in each major trough and on the bank tops. Ice retreated from Victoria Land Basin–Drygalski Trough before it retreated from JOIDES Basin. Ice remained on the bank tops, shedding material into the abandoned troughs. Grounded ice in the central Ross Sea was derived predominantly from an expanded West Antarctic Ice Sheet. The ice sheet remained grounded on the continental-shelf edge after retreat of ice from the western Ross Sea. Expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf. Ice-stream drainage shifted laterally, as recorded by rounded, laterally accreting ridges that separate bathymetric troughs. The central Ross Sea is the repository for larger volumes of sediment, derived from ice flow across basins of relatively thick, unlithified sedimentary deposits. Two till sheets mark grounding-zone positions in the central Ross Sea, reflecting the higher sediment supply and a stepped deglaciation. Ice remained grounded on the Pennell Bank to the west; a series of moraines marks the retreat of ice across the bank. Mega-scale glacial lineations and other streamlined, subglacial geomorphic features occur across the continental shelf, primarily within the troughs. The lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet. This deforming substrate may have contributed to the onset of deglaciation. Features associated with meltwater are rare or absent, suggesting that basal meltwater played a minor role in retreat of the ice sheet.

Marine Ecosystem Sensitivity to Climate Change

393 M ounting evidence suggests that the earth is experiencing a period of rapid climate change. Never before has it been so important to understand how environmental change influences the earth’s biota and to distinguish anthropogenic change from natural variability. Long-term studies in the western Antarctic Peninsula (WAP) region provide the opportunity to observe how changes in the physical environment are related to changes in the marine ecosystem. Analyses of paleoc limate records (MosleyThompson 1992, Peel 1992, Domack et al. 1993, Thompson et al. 1994, Dai et al. 1995, Domack and McClennen 1996, Leventer et al. 1996) have shown that the WAP region has moved from a relatively cold regime between approximately 2700 BP and 100 BP, to a relatively warm regime during the current century. Air temperature records from the last half-century show a dramatic warming trend, confirming the rapidity of change in the WAP area (Sansom 1989, Stark 1994, Rott et al. 1996, Smith et al. 1996). Significantly, polar ecosystem research over the last few decades (Fraser et al. 1992, Trivelpiece and Fraser 1996) and paleoecological records for the past 500 years (Emslie 1995, Emslie et al. 1998) reveal ecological transitions that have occurred in response to this climate change. In this article, we summarize the available data on climate variability and trends in the WAP region and discuss these data in the context of long-term climate variability during the last 8000 years of the Holocene. We then compare the available data on ecosystem change in the WAP region to the data on climate variability. Both historical and paleoenvironmental records indicate a climate gradient along the WAP that includes a dry, cold continental regime to the south and a wet, warm maritime regime to the north. The position of this climate gradient has shifted over time in response to the dominant climate regime, and it makes the WAP region a highly sensitive location for assessing ecological responses to climate variability. Our findings show that this century’s rapid climate warming has occurred concurrently with a shift in the population size and distribution of penguin species.

Stability of the Larsen B ice shelf on the Antarctic Peninsula during the Holocene epoch

The stability of the Antarctic ice shelves in a warming climate has long been discussed, and the recent collapse of a significant part, over 12,500 km2 in area, of the Larsen ice shelf off the Antarctic Peninsula has led to a refocus toward the implications of ice shelf decay for the stability of Antarcticas grounded ice. Some smaller Antarctic ice shelves have undergone periodic growth and decay over the past 11,000 yr (refs 7–11), but these ice shelves are at the climatic limit of ice shelf viability and are therefore expected to respond rapidly to natural climate variability at century to millennial scales. Here we use records of diatoms, detrital material and geochemical parameters from six marine sediment cores in the vicinity of the Larsen ice shelf to demonstrate that the recent collapse of the Larsen B ice shelf is unprecedented during the Holocene. We infer from our oxygen isotope measurements in planktonic foraminifera that the Larsen B ice shelf has been thinning throughout the Holocene, and we suggest that the recent prolonged period of warming in the Antarctic Peninsula region, in combination with the long-term thinning, has led to collapse of the ice shelf.

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 2—Sedimentologic and stratigraphic signature

Sedimentologic, geotechnical, geochemical, and accelerator mass spectrometer (AMS) radiocarbon data from two marine geologic cruises in the Ross Sea have allowed us to constrain facies relationships and temporal changes in the West Antarctic Ice Sheet. The selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality (CHIRP) subbottom reflection system. A complex but consistent succession of facies documents a number of environments from subglacial to open marine. Massive, mud-rich diamictons have low water contents, contain (in places) a calcareous microfossil assemblage, show minimal textural variation, and contain low and uniform total organic carbon values. This reflects a subglacial setting. This unit passes upward into a stratified, thin, granulated facies consisting of pelletized, sandy, muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble-sized clasts. This facies reflects the lift-off zone or thin water film between the basal debris and sea floor. Overlying this unit are silty clays that contain a well-sorted, very fine-grained sand component. There are no coarse grains within this unit. This facies is, in part, laminated and reflects deposition beneath an ice shelf, near and away from the grounding-line zone. The ice-shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open-marine setting. A sandy, volcaniclastic-rich subfacies marks the transition from ice shelf to open-marine environments found at the calving line. The 86 AMS radiocarbon dates on organic matter provide an accurate chronology for 19 cores. Ice-shelf conditions were established in the outer Drygalski Trough by 11 ± 0.25 ka and perhaps earlier. This transition took place in the JOIDES Basin by 10–8 ka. The calving front of the Ross Ice Shelf passed over the Drygalski Trough at 74°S by 9.5 ± 0.25 ka. The timing of deglaciation in the Ross Sea calls into question current models for the contribution of Antarctic glacial ice to Holocene sea-level rise and suggests that recession was relatively gradual and more closely aligned with Northern Hemisphere deglaciation and its associated eustatic pulse.

Chronology of the Palmer Deep site, Antarctic Peninsula: a Holocene palaeoenvironmental reference for the circum-Antarctic

Palmer Deep sediment cores are used to produce the first high-resolution, continuous late Pleistocene to Holocene time-series from the Antarctic marine system. The sedimentary record is dated using accelerator mass spectrometer radiocarbon methods on acid insoluble organic matter and foraminiferal calcite. Fifty-four radiocarbon analyses are utilized in the dating which provides a calibrated timescale back to 13 ka BP. Reliability of resultant ages on organic matter is assured because duplicates produce a standard deviation from the surface age of less than laboratory error (i.e., ±50 years). In addition, surface organic matter ages at the site are in excellent agreement with living calcite ages at the accepted reservoir age of 1260 years for the Antarctic Peninsula. Spectral analyses of the magnetic susceptibility record against the age model reveal unusually strong periodicity in the 400,–200 and 50-70 year frequency bands, similar to other high-resolution records from the Holocene but, so far, unique for the circum-Antarctic. Here we show that comparison to icecore records of specific climatic events (e.g., the ’Little Ice Age‘, Neoglacial, Hypsithermal, and the Bølling/Allerød to Younger Dryas transition) provides improved focus upon the relative timing of atmosphere/ocean changes between the northern anid southern high latitudes.

Productivity cycles of 200–300 years in the Antarctic Peninsula region: Understanding linkages among the sun, atmosphere, oceans, sea ice, and biota

Compared to the rest of the world9s oceans, high-resolution late Holocene paleoclimatic data from the Southern Ocean are still rare. We present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the Antarctic Peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr. Analyses completed include measurement of magnetic susceptibility and granulometry, bed thickness, particle size, percent organic carbon, bulk density, and microscopic evaluation of diatom and benthic foraminiferal assemblages and abundances. Downcore variability of these parameters demonstrates the significance of both short-term cycles, which recur approximately every 200 yr, and longer term events (≈2500 yr cycles) that are most likely related to global climatic fluctuations. In the upper 600 cm of the core, lower values of magnetic susceptibility (MS) are correlated with lower bulk density, the presence of thinly laminated units, specific diatom assemblages, and generally higher total organic carbon content. Below 600 cm, magnetic susceptibility is uniformly low, though variability in other parameters continues. The magnetic susceptibility signal is controlled primarily by dilution of ferromagnetic phases with biosiliceous material. This signal may be enhanced further by dissolution of magnetite in the magnetic susceptibility lows (high total organic carbon). The role of variable primary productivity and its relationship to paleoclimate is assessed through the diatom data. In particular, magnetic susceptibility lows are characterized by higher than normal abundances of Chaetoceros resting spores. Corethron criophilum and/or Rhizosolenia spp. also are found, as is a higher ratio of the most common species of Fragilariopsis versus species of Thalassiosira . These assemblages are indicative of periods of high primary productivity driven by the presence of a meltwater stabilized water column. The 200 yr cyclicity noted in other paleoclimatic records around the world suggests a global forcing mechanism, possibly solar variability. In addition to the cyclic changes in productivity, overall elevated productivity is noted below 600 cm, or prior to ca. 2500 yr B.P. This increased productivity may represent the tail end of a Holocene climatic optimum, which is widely recognized in other parts of the world, but as yet is poorly documented in Antarctica.

Marine sediment record from the East Antarctic margin reveals dynamics of ice sheet recession

The Antarctic shelf is traversed by large-scale troughs developed by glacial erosion. Swath bathymetric, lithologic, and chronologic data from jumbo piston cores from four sites along the East Antarctic margin (Iceberg Alley, the Nielsen Basin, the Svenner Channel, and the Mertz-Ninnis Trough) are used to demonstrate that these cross-shelf features controlled development of calving bay reentrants in the Antarctic ice sheet during deglaciation. At all sites except the Mertz-Ninnis Trough, the transition between the Last Glacial Maximum and the Holocene is characterized by varved couplets deposited during a short interval of extremely high primary productivity in a fjordlike setting. Nearly monospecific layers of the diatom Chaetoceros alternate with slightly more terrigenous layers containing a mixed diatom assemblage. We propose that springtime diatom blooms dominated by Chaetoceros were generated within well-stratified and restricted surface waters of calving bays that were influenced by the input of iron-rich meltwater. Intervening post-bloom summer-fall laminae were formed through the downward flux of terrigenous material sourced from melting glacial ice combined with mixed diatom assemblages. Radiocarbon-based chronologies that constrain the timing of deposition of the varved sediments within calving bay reentrants along the East Antarctic margin place deglaciation between ca. 10,500–11,500 cal yr B.P., post-dating Meltwater Pulse 1A (14,200 cal yr B.P.) and indicating that retreat of ice from the East Antarctic margin was not the major contributor to this pulse of meltwater.

Oceanographic and physiographic controls on modern sedimentation within Antarctic fjords

Physical oceanographic data and modern surface sediments were collected from eleven fjords along the western side of the Antarctic Peninsula and South Shetland Islands. Surface sediment samples (62) were analyzed for texture and total organic carbon content. The distribution of biogenic and terrigenous facies within the fjords is controlled by bay geometry and oceanographic regime. Climate plays a secondary role but, along with ice drainage basin size, controls the rate of terrigenous supply to the glacial marine environment. Specifically, fjords along the Danco Coast and Palmer Archipelago with a high length to width ratio tend to have bottom sediments that are arenaceous where ice-rafted sediment is released preferentially at the head of the fjord. Biogenic facies are favored where the bay geometry is complex. Where such complexity exists, separate oceanographic regimes develop that lead to separation of terrigenous and biogenic sediments. Processes of interflow (mid- and deep-water turbid cold tongues) and Coriolis deflection produce terrigenous facies along the inner fjord and western edges of a fjord system. Warm outer bay waters tend to develop a stable eddy circulation pattern that favors the productivity of phytoplankton in the surface layers. Outer bays are therefore floored with organic-rich siliceous muds and ice-rafted material. Only in the South Shetland Islands is melt-water input significant enough to generate estuarine circulation within the fjord, but here strong bottom currents result in arenaceous bottom sediments with no biogenic facies. Ice-rafted diamictons are produced proximal to the edges of small tide-water glaciers in the South Shetlands. The facies relationships established in this study provide a strong reference for paleoclimatic studies that utilize downcore measurements of texture and organic carbon.

Holocene Southern Ocean surface temperature variability west of the Antarctic Peninsula

The disintegration of ice shelves, reduced sea-ice and glacier extent, and shifting ecological zones observed around Antarctica highlight the impact of recent atmospheric and oceanic warming on the cryosphere. Observations and models suggest that oceanic and atmospheric temperature variations at Antarcticas margins affect global cryosphere stability, ocean circulation, sea levels and carbon cycling. In particular, recent climate changes on the Antarctic Peninsula have been dramatic, yet the Holocene climate variability of this region is largely unknown, limiting our ability to evaluate ongoing changes within the context of historical variability and underlying forcing mechanisms. Here we show that surface ocean temperatures at the continental margin of the western Antarctic Peninsula cooled by 3–4 °C over the past 12,000 years, tracking the Holocene decline of local (65° S) spring insolation. Our results, based on TEX86 sea surface temperature (SST) proxy evidence from a marine sediment core, indicate the importance of regional summer duration as a driver of Antarctic seasonal sea-ice fluctuations. On millennial timescales, abrupt SST fluctuations of 2–4 °C coincide with globally recognized climate variability. Similarities between our SSTs, Southern Hemisphere westerly wind reconstructions and El Niño/Southern Oscillation variability indicate that present climate teleconnections between the tropical Pacific Ocean and the western Antarctic Peninsula strengthened late in the Holocene epoch. We conclude that during the Holocene, Southern Ocean temperatures at the western Antarctic Peninsula margin were tied to changes in the position of the westerlies, which have a critical role in global carbon cycling.

Evidence for an earliest Oligocene ice sheet on the Antarctic Peninsula

There is growing consensus that development of a semipermanent ice sheet on Antarctica began at or near the Eocene-Oligocene (E-O) boundary. Beyond ice-rafted debris in oceanic settings, however, direct evidence for a substantial ice sheet at this time has been limited and thus far restricted to East Antarctica. It is unclear where glacier ice first accumulated and how extensive it was on the Antarctic continent in the earliest Oligocene. Sediments at the top of the Eocene marine shelf section on Seymour Island, Antarctic Peninsula, include glacial marine deposits and a lodgment till with clasts derived from a variety of rock units on the peninsula. Dinoflagellate biostratigraphy and strontium isotope stratigraphy indicate an age at or very close to the E-O boundary. Glacier ice extending to sea level in the northern peninsula at this time suggests the presence of a regionally extensive West Antarctica ice sheet, and thus an even more dramatic response to the forcing factors that facilitated high-latitude ice expansion in the earliest Oligocene.