Central Ross Sea Research Articles

Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing

Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the Northern Hemisphere (NH) as northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however, despite the growing recognition that dynamic effects account for a sizeable proportion of mass-balance changes observed in modern ice sheets. Here we use a 5-km resolution whole-continent numerical ice-sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We first simulate the geometry and flow characteristics of an equilibrium LGM ice sheet, using pan-Antarctic terrestrial and marine geological data for constraint, then perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. Our results identify that fast-flowing glaciers in the eastern Weddell Sea, the Amundsen Sea, central Ross Sea, and in the Amery Trough respond most rapidly to ocean forcings, in agreement with empirical data. Most significantly, we find that although ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments-a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets.

Extent of the Ross Orogen in Antarctica: new data from DSDP 270 and Iselin Bank

AbstractThe Ross Sea is bordered by the Late Precambrian–Cambrian Ross–Delamerian Orogen of East Antarctica and the more Pacific-ward Ordovician–Silurian Lachlan–Tuhua–Robertson Bay–Swanson Orogen. A calcsilicate gneiss from Deep Sea Drilling Project 270 drill hole in the central Ross Sea, Antarctica, gives a U-Pb titanite age of 437 ± 6 Ma (2σ). This age of high-grade metamorphism is too young for typical Ross Orogen. Based on this age, and on lithology, we propose a provisional correlation with the Early Palaeozoic Lachlan–Tuhua–Robertson Bay–Swanson Orogen, and possibly the Bowers Terrane of northern Victoria Land. A metamorphosed porphyritic rhyolite dredged from the Iselin Bank, northern Ross Sea, gives a U-Pb zircon age of 545 ± 32 Ma (2σ). The U-Pb age, petrochemistry, Ar-Ar K-feldspar dating, and Sr and Nd isotopic ratios indicate a correlation with Late Proterozoic–Cambrian igneous protoliths of the Ross Orogen. If the Iselin Bank rhyolite is not ice-rafted debris, then it represents a further intriguing occurrence of Ross basement found outside the main Ross–Delamerian Orogen.

The areas and ice production of the western and central Ross Sea polynyas, 1992–2002, and their relation to the B-15 and C-19 iceberg events of 2000 and 2002

For 1992–2002, the paper investigates the heat loss and area of three polynyas in the western and central Ross Sea. These are the Ross Sea Polynya (RSP) and the much smaller Terra Nova Bay and McMurdo Sound polynyas. The importance of these polynyas is that their associated salt rejection contributes to the formation of the High Salinity Shelf Water (HSSW) that is crucial to the Antarctic Bottom Water formation. The study divides into two parts, 1992–1999, when there was negligible iceberg activity, and 2000–2002, when major icebergs calved and interacted with the polynyas. To retrieve the ice thicknesses and heat fluxes within the polynyas, the paper uses an algorithm based on the ratio of the vertically and horizontally polarized Special Sensor Microwave/Imager (SSM/I) 25-km resolution 37-GHz channels, combined with meteorological data. Because of sidelobe contamination, the ice shelf and icebergs are masked. Our results show that for the polynyas, and consistent with other observations, their mean winter area is about 30,000 km 2 and their combined ice production is about 500 km 3y − 1 . We also find that the polynya ice production approximately equals the ice export. This is in contrast to the Weddell Sea, where the polynya ice production equals about 6% of the ice export. For the years 2000 and 2002, the calving of large icebergs directly affect the ice production by inhibiting the ice production off the shelf due to piling up of first year ice upwind of the bergs and by generating new polynyas downwind of the bergs. The period 1992–2001 exhibits an upward trend in polynya productivity. The decadal increase in the ice production suggests that the observed HSSW salinity decrease in the western Ross Sea is not due to the polynyas, but is rather due to a change in the properties of the water flowing into the Ross.

Oligocene development of the West Antarctic Ice Sheet recorded in eastern Ross Sea strata

Seismic-reflection data from the easternmost Ross Sea image buried scour-and-fill troughs and flat-topped ridges interpreted as having formed by glacial erosion and deposition during the Oligo-cene. The NNW-SSE orientation of the troughs and lack of similar Oligocene glacial features within the central Ross Sea suggests that the ice issued from the highlands of Marie Byrd Land located 100 km away and that portions of the West Antarctic Ice Sheet formed earlier than previously accepted. Existing global climate models (GCMs) do not produce West Antarctic ice caps for the Oligocene, in part due to low elevations modeled for that time. Evidence for Oligocene ice beyond the paleocoast suggests a higher elevation for the early Cenozoic Marie Byrd Land and Ross Embayment than at present.

Isotopic constraints on the provenance of fine-grained sediment in LGM tills from the Ross Embayment, Antarctica

The major element, trace element and Nd, Sr, and Pb isotopic compositions were determined for the < 63 μm sediment fraction of fourteen onshore tills and twenty-one offshore tills in the Ross Sea and surrounding areas, Antarctica. The purpose was to determine the provenance of tills deposited on the continental shelf of the Ross Sea during the Last Glacial Maximum (LGM) and to assess the relative importance of the West and East Antarctic ice sheets in providing ice to the Ross Embayment during this time. Onshore tills exposed around the margins of the Ross Embayment associated with specific ice streams and outlet glaciers have Nd and Pb compositions that vary regularly with geographic position, reflecting changes in the age and isotopic compositions of basement rocks along the Ross Sea margin. Specifically, till ε Nd values range from − 5.6 to − 9.9 for the West Antarctic (Siple Coast) ice streams and the southern Transantarctic Mountains, while the ε Nd in the central portions of the mountains range from − 11.9 to − 14.9. Uranogenic Pb becomes less radiogenic in the same direction. The composition of Late Quaternary tills from the Ross Sea also varies geographically, with the < 63 μm fraction from both the western and eastern Ross Sea tills having higher ε Nd values (− 5.8 to − 7.5 and − 3.8 to − 6.9, respectively) than the till in the central Ross Sea (− 7.1 to − 12.5). Tills in the eastern Ross Sea (most proximal to West Antarctica) have isotopic and chemical compositions identical to onshore till from West Antarctica and the southern Transantarctic Mountains and were most likely derived from these areas. Tills in the western Ross Sea with the highest ε Nd values have low wt.% SiO 2 (46%), and high Ni, Nb, Ta and Sr contents requiring a large component of material eroded from extrusive igneous rocks from the adjacent Late Cenozoic McMurdo Volcanic Field. In contrast, tills in the central Ross Sea were most likely related to ice emanating from the central Transantarctic Mountains that flowed north across the Ross Embayment to the continental shelf margin. Overall, our data provide further direct evidence for the model described in a previously published companion paper [K.J. Licht, J.R. Lederer, J.R. Swope, Provenance of LGM glacial till (sand fraction) across the Ross Embayment, Antarctica, Quat. Sci. Rev. 24 (2005) 1499–1520.] that ice derived from the East Antarctic ice sheet was active and responsible for LGM till deposition throughout the western half of the Ross Sea (including the central Ross Sea). As a result, models for the past and future dynamics of the Antarctic ice sheets must account for a major East Antarctic contribution to the Ross ice sheet.

Radiocarbon constraint on relict organic carbon contributions to Ross Sea sediments

We estimate the relative contribution of relict organic matter to the acid‐insoluble organic carbon (AIOC) fraction of surface sediments from Ross Sea, Antarctica, on the basis of 14C abundance. The bulk isotopic characteristics of AIOC can largely be explained by simple two‐source models of modern and relict organic carbon, when samples are grouped according to two geographical regions, namely, southwestern and south central Ross Sea. This spatial variability in relict organic carbon could be controlled by proximity to the edge of the Ross Ice Shelf and ice drainage areas. Radiocarbon abundance in the AIOC is potentially an excellent tool to estimate the contribution of relict organic carbon in the Antarctic margin sediments.

Late Quaternary glacial marine to marine sedimentation in the Pennell Trough (Ross Sea, Antarctica)

Late Pleistocene and Holocene palaeoenvironmental changes were studied in four gravity cores up to 7.8 m long from the Pennell Trough, a NW–SE-trending basin 160 km long and 60 km wide in the central Ross Sea, Antarctica, with depths occasionally greater than 600 m. Differences in environments and depositional processes during the last glacial and postglacial epochs were investigated using X-rays and volume magnetic susceptibility (VMS). Further analyses included bulk and clay mineral composition, micropalaeontological studies (both benthic and planktic foraminifera) and radiometric dating. We compare our sedimentological, geochemical (organic carbon and nitrogen content), and geotechnical (shear strength and water content) results to those on cores previously taken from the region. These analyses suggest that prior to the Last Glacial Maximum (LGM), a glacial marine diamicton (< 37,000 yr BP uncorrected age) was deposited across the basin from beneath an expanded Ross ice shelf that was grounded on the basin flanks. Sediment gravity flow deposits (27,000–21,000 yr BP uncorrected ages) that overlie the diamicton in the deepest part of the southernmost area of the basin are interpreted to have been deposited during the Last Glacial Maximum (∼18,000 yr BP) as remobilized subglacial diamicton from the flanks of the basin, initiated by the movement of grounded ice. These sediments are followed by a period of non-deposition caused by basin starvation after retreat of the grounding line of Ross Sea ice far to the south. As a consequence, terrigenous supply was limited, and the persistence of floating shelf ice followed by multi-year sea-ice coverage inhibited the biogenic activity. During the Holocene, as climate became warmer, summer open-sea conditions began to dominate, leading to the deposition of a thin diatomaceous mud/ooze draping the basin.

Provenance of LGM glacial till (sand fraction) across the Ross embayment, Antarctica

The provenance of Late Quaternary Ross Embayment till was investigated by comparing the coarse sand composition of East and West Antarctic source area tills with till samples from across the Ross Sea. The West Antarctic samples from beneath the Whillans (B) and Kamb (C) ice streams are petrologically distinct from samples of lateral moraines flanking several East Antarctic outlet glaciers. The characteristic assemblage of four West Antarctic samples includes felsic intrusive and detrital sedimentary lithic fragments, plagioclase and abundant quartz. In contrast, most of the ten East Antarctic till samples contains abundant mafic intrusive and detrital sedimentary lithic fragments as well as less abundant quartz. The distinctive composition of these source areas can be linked to 33 samples from 20 cores of Last Glacial Maximum (LGM) age till distributed across the Ross Sea. Western Ross Sea till samples exhibit mineralogic and lithological similarities to East Antarctic till samples, although these western Ross Sea tills contain higher percentages of felsic intrusive and extrusive lithic fragments. Eastern Ross Sea till samples are compositionally similar to West Antarctic till, particularly in their abundance of quartz and dearth of mafic and extrusive lithic components. Central Ross Sea till exhibits compositional similarities to both East and West Antarctic source terranes including a mafic lithic component, and marks the confluence of ice draining from East and West Antarctica during the LGM, thus West Antarctic-derived ice streams did not advance into the western Ross Sea. This indicates that even if pre-LGM equivalents of the present Siple Coast ice streams existed, they did not simply expand allowing West Antarctic-derived ice to dominate the LGM Ross Ice Sheet.

Mixed diatom assemblages in glacigenic sediment from the central Ross Sea, Antarctica

We have investigated diatom assemblages and their distribution in piston and trigger cores from six sites in the central Ross Sea, including analysis of absolute abundance of diatom valves and diatom fragments, with the aim of characterising different glacigenic sediment facies and sediment packets. These data provide a new perspective on the current texturally based classification of diamicton, mud, and diatomaceous mud. All samples investigated include diatom taxa characteristic of the modern Ross Sea flora, thus all deposits evaluated are interpreted to be upper Quaternary in age. Reworked diatoms are ubiquitous, but they vary in terms of species content and concentration. Sediment units classified as diamicton are enriched in robust reworked valves of long-ranging taxa such as Paralia spp., Stephanopyxis spp., and Stellarima microtrias, but different diamicton packets can be distinguished based on variable concentrations of biostratigraphically constrained taxa including Denticulopsis spp., Thalassiosira spp., and Actinocyclus spp. Absolute abundance of diatoms and diatom fragments provide another useful criterion for distinguishing sedimentary processes. Together, these criteria permit interpretations of past glacial and glacial marine processes with greater certainty than allowed by textural analyses alone. We are able to distinguish between tills deposited beneath ice streams from those emplaced by slow-moving ice—information that is important in ice sheet reconstruction.

Decoding the dipstick: Thickness of Siple Dome, West Antarctica, at the Last Glacial Maximum

Ice thickness in West Antarctica at the Last Glacial Maximum (LGM) is poorly known, yet is key information for understanding ice streams and interpreting ice cores. Although trim lines, moraine limits, and exposure-age dating provide geologic constraints on ice thickness near the Transantarctic Mountains and in Marie Byrd Land, lack of exposed bedrock hampers traditional geologic methods in a central, ;2 3 10 6 km 2 region. Here we infer ice-sheet thickness changes in the central Ross Sea Embayment by using a tran- sient ice-flow model to find combinations of accumulation-rate and ice-sheet thickness histories that match the depth-age relationship and the measured layer-thickness pattern in the Siple Dome ice core. After we reject unreasonable accumulation-rate histories, the remaining history pairs indicate thinning of 200-400 m since the LGM. Our estimate is lower than previous reconstructions that were constrained by geologic evidence from the Transantarctic Mountains and by marine data from the Ross Sea floor, which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period. Low surface elevations in the central Ross Sea Embayment during the LGM do not preclude thicker ice along its boundaries. However, if this grounded ice sheet came over 1000 km from interior West Antarctica, as is usually assumed, then it had very low surface slope, requiring a very slippery bed. Alternatively, the grounded Ross Sea ice might have flowed from the Transantarctic Mountains and western Marie Byrd Land.

Effects of the physical properties of water masses on microbial activity during an Ice Shelf Water overflow in the central Ross Sea

During the 1997–98 Italian Expedition to Antarctica a five-day mesoscale experiment was carried out on the continental shelf-break in the central Ross Sea. This area is oceanographically characterized by shelf/slope interactions, through intense mixing processes, between the Circumpolar Deep Water (CDW) and the Ice Shelf Water (ISW), coming from beneath the Ross Ice Shelf and spilling over the shelf edge. The export of dense shelf waters is of crucial importance not only for the mass balance of the basin, but also for carbon export from the upper layers into the abyssal ones. The study investigated how the ISW interactions with the CDW may influence bacterial metabolism during an ISW downslope event. In particular, what effect does this have on the bacterial activities related to the utilization and transformation of the organic carbon substrate (ectoenzymatic activities, carbon production, growth rate) within the ISW and the CDW cores? Our data show that in the CDW the metabolic response was to increase the biomass and enzymes were less active due to a higher nutritional value for the substrate. In the ISW the bacterial metabolic activity shifted towards degradative processes. These results suggest differences in the quality of the organic carbon pool with a greater concentration of labile organic matter in the CDW and of low-degradable compounds in the ISW. The use of microbial parameters seems to be very promising in the evaluation of the carbon export during mixing processes, when the refractory fraction of the organic carbon pool might play a key role.

Evidence of dense water overflow on the Ross Sea shelf-break

This paper presents the results of the analysis of hydrological data of a 5-day mesoscale experiment (53 CTD casts) conducted during the XIIIth Italian Expedition to Antarctica (1997–98 cruise) in the framework of the CLIMA (Climatic Longterm Interaction for the Mass balance in Antarctica) Project of the Italian National Programme for Antarctic Research (PNRA). The experiment site was chosen for studying the dense water overflow in relation to the shelf-break in the central Ross Sea, after a large-scale synoptic survey, aimed to detect the general hydrological characteristics of the basin. A classical θ/S analysis was carried out for better understanding of the shelf-slope connection and the interactions between the water masses of this zone: the Circumpolar Deep Water (CDW) coming from the oceanic domain and the Ice Shelf Water (ISW) spreading from the Ross Ice Shelf (RIS) edge. Our results show the evidence of an overflow of dense water, originated on the continental shelf, on the shelf-break. This supercold water signal is found on the continental slope down to 1200 m depth. The shape of this tongue of modified ISW, whose thickness reaches up to 100 m, is very narrow, suggesting that the overflow occurs in very localized areas.

The 14 C Record of Late Pleistocene Ice Advance and Retreat in the Central Ross Sea, Antarctica

Twenty-three AMS radiocarbon dates obtained from central Ross Sea sediment cores constrain the timing of ice advance across the continental shelf and highlight the difficulties of determining a chronology of ice-sheet retreat in this region. Ages derived from till and postglacial sediments are bimodally distributed and the majority of dates are >20 14C ka. The dates obtained from the acid-insoluble organic fraction of till indicate that grounded ice advanced across the inner central Ross Sea continental shelf 17.8 ± 1 14C ka. Benthic foraminifera from till on the outer shelf indicate that grounded ice reached its maximum position after 13.8 14C ka indicating that the Ross Sea sector of the Antarctic ice sheet was advancing as eustatic sea level was on the rise. Postglacial sediment sequences in the central Ross Sea are up to 70 cm thick, and the ages are older than those from the western Ross Sea. These postglacial sediments produced ages >20 14C ka (uncorrected) that are apparently inconsistent with the timing of ice advance and preclude development of a high-resolution chronology of ice-sheet retreat. Surface sediments deposited under open-marine conditions in the central Ross Sea typically range from 4000 to 6000 14C yr BR Total organic carbon (TOC) measurements for the dated sediments in this study reveal no significant relationship between TOC content and reported 14C age.

The 14C Record of Late Pleistocene Ice Advance and Retreat in the Central Ross Sea, Antarctica

Twenty-three AMS radiocarbon dates obtained from central Ross Sea sediment cores constrain the timing of ice advance across the continental shelf and highlight the difficulties of determining a chronology of ice-sheet retreat in this region. Ages derived from till and postglacial sediments are bimodally distributed and the majority of dates are >20 14C ka. The dates obtained from the acid-insoluble organic fraction of till indicate that grounded ice advanced across the inner central Ross Sea continental shelf 17.8 ± 1 14C ka. Benthic foraminifera from till on the outer shelf indicate that grounded ice reached its maximum position after 13.8 14C ka indicating that the Ross Sea sector of the Antarctic ice sheet was advancing as eustatic sea level was on the rise. Postglacial sediment sequences in the central Ross Sea are up to 70 cm thick, and the ages are older than those from the western Ross Sea. These postglacial sediments produced ages >20 14C ka (uncorrected) that are apparently inconsistent with the timing of ice advance and preclude development of a high-resolution chronology of ice-sheet retreat. Surface sediments deposited under open-marine conditions in the central Ross Sea typically range from 4000 to 6000 14C yr BR Total organic carbon (TOC) measurements for the dated sediments in this study reveal no significant relationship between TOC content and reported 14C age.

Biological components of Ross Sea short-term particle fluxes in the austral summer of 1995–1996

The Ross Sea, a region of high seasonal production in the Southern Ocean, is characterized by blooms of the haptophyte Phaeocystis antarctica and of diatoms. The different morphology, structural composition and consumption of these two phytoplankton by grazing zooplankton may result in different carbon cycling dynamics and carbon flux from the euphotic zone. We sampled short-term (∼2 days) particle flux at 5 sites from 177.6°W to 165°E along a transect at 76.5°S with traps placed below the euphotic zone at 200 m during December 1995–January 1996. We estimated carbon flux of as many eucaryotic organisms and fecal pellets as possible using microscopy for counts and measurements and applying volume:carbon conversions from the literature. Eucaryotic organisms contributed about 20–40% of the total organic carbon flux in both the central Ross Sea polynya and in the western polynya, and groups of organisms differed in contribution to the carbon flux at the different sites. Algal carbon flux ranged from 4.5 to 21.1 mg C m −2 day −1 and consisted primarily of P. antarctica (cell plus mucus) and diatom carbon at all sites. Different diatom species dominated the diatom flux at different sites. Carbon fluxes of small pennate diatoms may have been enhanced by scavenging, by sinking senescent P. antarctica colonies. Heterotrophic carbon flux ranged from 9.2 to 37.6 mg C m −2 day −1 and was dominated by athecate heterotrophic dinoflagellate carbon in general and by carbon flux of a particular large athecate dinoflagellate at two sites. Fecal pellet carbon flux ranged from 4.6 to 54.5 mg C m −2 day −1 and was dominated by carbon from ovoid/angular pellets at most sites. Analysis of fecal pellet contents suggested that large protozoans identified by light microscopy contributed to ovoid/angular fecal pellet fluxes. Carbon flux as a percentage of daily primary production was lowest at sites where P. antarctica predominated in the water column and was highest at sites where fecal pellet flux was highest. This indicates the importance of grazers in carbon export.

Factors controlling the distribution of diatoms and Phaeocystis in the Ross Sea

The spatial and temporal distributions of phytoplankton pigments were investigated in the western and south central Ross Sea during austral spring 1994 and summer 1990. Large gradients in biomass and phytoplankton community composition were observed both in the east–west and south–north directions, in relation to differences in water column structure and stability, which themselves depend on the processes of ice retreat within the different areas. Important are melting in the western Ross Sea, which induces strong stratification, and ice breakup and wind stress in the south central Ross Sea, which result in deep mixing and weakly stratified waters. In the western Ross Sea, the highest chlorophyll a (chl. a) concentrations observed in this study (129–358 mg m −2 in the upper 100 m) were tightly coupled to the stratified region of meltwater influence and were dominated by diatoms, as indicated by elevated fucoxanthin concentrations (89–239 mg m −2 in the upper 100 m). In the diatom bloom area, high levels of phaeophorbides a (maximum value of 192 mg m −2 in the upper 100 m) indicated that the dominant grazers, identified as Limacina helecina and copepods [Hecq, J.H., Magazzù, G., Goffart, A., Catalano, G., Vanucci, S., Guglielmo, L., 1992. Distribution of planktonic components related to vertical structure of water masses in the Ross Sea and the Pacific sector of the Southern Ocean. In: Anonymous, Atti del 9° congresso A.I.O.L., Santa Margherita Ligure, 20–23 Novembre 1990, 665–678], transferred a sustained part of the diatom production to the herbivore trophic level. Synthesis of our data with published information suggests that the diatom bloom we observed in the western Ross Sea was dominated by the species Fragilariopsis curta, and occurs annually off the coast of Victoria Land from 72°30′S to 77°S within 100–250 km from the coastal ice edge. It is assumed to persist on the order of 2.5–3 months during summer. In the south central Ross Sea, which was characterized by a poorly or unstratified water column, moderate chl. a concentrations (55–186 mg m −2 in the upper 100 m) were found in the polynya and in the ice-edge area in early spring. The 19′-hexanoyloxyfucoxanthin (19′-HF), a biomarker for Phaeocystis, was the major contributor to the total carotenoid abundance (29–132 mg m −2 in the upper 100 m), corroborating the hypothesis that Phaeocystis are well adapted to develop in a relatively mixed water column. At the northern limit of the polynya, a quite narrow (ca. 30 km) diatom bloom (31–67 mg fucoxanthin m −2 in the upper 100 m) overlapped with the Phaeocystis bloom in the slightly stratified marginal ice zone. This was an area of increased grazing pressure by various types of herbivorous zooplankton, among which very large amounts of krill were observed.

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.

Estimation of bacterial respiration and growth efficiency in the Ross Sea, Antarctica

Seawater cultures were conducted in large volume (36 1) gas impermeable tri-laminate bags for the purpose of empirically deriving bacterial growth efficiency (BGE) and carbon conversion factors (CCF) in the south central Ross Sea. This experimental design allowed for concomitant measurements of metabolic reactants (loss of total and dissolved organic carbon [TOC and DOC]) and products (gain of total carbon dioxide [TCOz] and bacterial biomass) to be made from a single incubation vessel. Some previous studies have relied on proxy measurements (e.g. 02, 3H-thymidine incorporation and cell abundance) to determine BGE and CCF rather than direct carbon measurements. Our experimental design enabled a complete carbon budget to be constructed and eliminated variability associated with normally employed parallel bottle incubations. Uhlization of TOC was well balanced by the production of TC02, in 7 of 8 experiments, validating the use of tri-laminate bags for measuring microbial respiration. In 3 experiments, where TOC, DOC, TCOz and bacterial biovolume were directly measured, carbon mass balance yielded BGE estimates of 12, 32 and 38% and bacterial CCF of 77, 95 and 134 fg C pm-3. In experiments where independent DOC measurements were not made we used our empirically derived CCF values to determine bacterial carbon production and calculated DOC concentrations and BGE for these remaining experiments. The BGE derived from all the bag experiments conducted throughout the austral spring and summer 1995-1997 ranged from 9 to 38%. Our experimental design and carbon mass balance approach could be applied to other aquatic systems to empirically derive the BGE and CCF, factors essential for determining carbon flux through bacterioplankton.

Water column sediment fluxes in the Ross Sea, Antarctica: Atmospheric and sea ice forcing

We measured time series of the vertical particle flux at three locations in the Ross Sea, Antarctica, between January 1990 and February 1992 as part of an interdisciplinary project focusing on the accumulation and recycling of organic C and biogenic Si on a polar shelf. We estimate area‐wide annual average fluxes through the deep water column of 5 g organic C m−2 yr−l and 30 g biogenic Si m−2 yr−1, values similar to the highest annual average fluxes to the subsurface reported for other areas of the Antarctic continental shelf. Total particle and biogenic Si fluxes are highest during January and February in the southwestern Ross Sea, beneath a seasonally recurrent bloom of the diatom Fragilariopsis curta. Organic C fluxes are highest in the central Ross Sea, consistent with a surface water algal assemblage dominated by the prymnesiophyte Phaeocystis. While organic C flux decreases with depth at all three sites, the result of remineralization within the water column, biogenic opal fluxes are higher in near‐bottom traps than at 230 m at the two western Ross Sea sites. Some biogenic opal must be supplied to these deep traps via horizontal advection and possibly resuspension. Fecal pellets and large aggregates contributed between 4 and 70% of the vertical flux and settled at rates of 60 to &gt;400 m d−1. Maximum particle fluxes occur 2 to 10 weeks after surface waters become ice free. We discuss three hypotheses to explain lags between production and settling: (1) advection from surface waters with different ice cover characteristics, (2) lags in the development of a grazing Zooplankton community, and (3) early season windinduced inhibition of primary production. Interannual variability in surface wind stress is empirically linked to variability in biogenic fluxes. Windiness and relative phasing of the annual cycles of ice cover and air temperature may be responsible for the development of different algal communities in the central versus western Ross Sea.

The relationship between new production and vertical flux on the Ross Sea continental shelf

The relationship between new production, as assessed by short-term isotopic incubations, and vertical flux, quantified by both short- and long-term deployments of sediment traps, was investigated on the Ross Sea continental shelf in austral summer, 1990 and 1992. Three sites were analyzed, the first in the western Ross Sea where surface waters were dominated by pennate diatoms, the second in the south–central Ross Sea where the prymnesiophyte Phaeocystis antarctica dominated, and the third in the northern region which was dominated by diatoms. We found that the flux at the western site varied by a factor of 3.6 over a 1-month period (from 18.4 to 65.4 mg C m −2 day −1), whereas new production varied over nearly two orders of magnitude. At the Phaeocystis site, flux was much more variable. Although the temporally weighted flux averaged 7.34 mg C m −2 day −1, the flux at 570 m ranged from 0.5 to 74.8 mg C m −2 day −1, and the flux at 50 m in a floating trap reached 92.7 mg C m −2 day −1. New production ranged from 0.30 to 1.23 g C m −2 day −1. Primary production, new production and f-ratios at both sites decreased through time (mean February values of each were 42, 37 and 12% of January averages), but the seasonal decrease in vertical flux exhibited a significant time lag behind the productivity decrease. A carbon budget is constructed which suggests that heterotrophic mineralization within the water column is a significant removal term, and that substantial spatial variability in this term exists. The relationship between new production in the euphotic zone to that of export production is ambiguous in the Ross Sea, in part because isotopic methods quantify the material available for export on time scales that do not necessarily apply to processes which dominate export production. Integration of seasonal time scales, remineralization rates, and vertical flux rates is required to understand the quantitative relationships involved in export from the surface layer.