Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, Jan 28, 2016
Accumulations of sediment beneath the Antarctic Ice Sheet contain a range of physical and chemica... more Accumulations of sediment beneath the Antarctic Ice Sheet contain a range of physical and chemical proxies with the potential to document changes in ice sheet history and to identify and characterize life in subglacial settings. Retrieving subglacial sediments and sediment cores presents several unique challenges to existing technologies. This paper briefly reviews the history of sediment sampling in subglacial environments. It then outlines some of the technological challenges and constraints in developing the corers being used in sub-ice shelf settings (e.g. George VI Ice Shelf and Larsen Ice Shelf), under ice streams (e.g. Rutford Ice Stream), at or close to the grounding line (e.g. Whillans Ice Stream) and in subglacial lakes deep under the ice sheet (e.g. Lake Ellsworth). The key features of the corers designed to operate in each of these subglacial settings are described and illustrated together with comments on their deployment procedures.
Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 DC warmer than at... more Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 DC warmer than at present and atmospheric CO 2 concentrations were twice as high as today!, the Antarctic ice sheets may have been unstable 2 -7 I Institute ofGeological and Nuclear Sciences, PO Box 30368.
Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirme... more Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages 1 , fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles 2 . Furthermore, an understanding of the behaviour of the marinebased West Antarctic ice sheet (WAIS) during the 'warmerthan-present' early-Pliocene epoch ( 5-3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming 3 . Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to 3 6C warmer than today 4 and atmospheric CO 2 concentration was as high as 400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model 7 that simulates fluctuations in Antarctic ice volume of up to 17 m in equivalent sea level associated with the loss of the WAIS and up to 13 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt 8 under conditions of elevated CO 2 .
A clean hot-water drill was used to gain access to Subglacial Lake Whillans (SLW) in late January... more A clean hot-water drill was used to gain access to Subglacial Lake Whillans (SLW) in late January 2013 as part of the Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project. Over 3 days, we deployed an array of scientific tools through the SLW borehole: a downhole camera, a conductivity-temperature-depth (CTD) probe, a Niskin water sampler, an in situ filtration unit, three different sediment corers, a geothermal probe and a geophysical sensor string. Our observations confirm the existence of a subglacial water reservoir whose presence was previously inferred from satellite altimetry and surface geophysics. Subglacial water is about two orders of magnitude less saline than sea water (0.37-0.41 psu vs 35 psu) and two orders of magnitude more saline than pure drill meltwater (<0.002 psu). It reaches a minimum temperature of -0.558C, consistent with depression of the freezing point by 7.019 MPa of water pressure. Subglacial water was turbid and remained turbid following filtration through 0.45 mm filters. The recovered sediment cores, which sampled down to 0.8 m below the lake bottom, contained a macroscopically structureless diamicton with shear strength between 2 and 6 kPa. Our main operational recommendation for future subglacial access through water-filled boreholes is to supply enough heat to the top of the borehole to keep it from freezing.
Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a ... more Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise.
Leg 124 of the Ocean Drilling Project drilled Sites 767 and 770 in the northern Celebes Sea, reac... more Leg 124 of the Ocean Drilling Project drilled Sites 767 and 770 in the northern Celebes Sea, reaching late middle Eocene basaltic basement at both sites. Major shifts in sediment provenance record the changing tectonic setting of the basin. From late middle Eocene into early Miocene time pelagic sedimentation prevailed, with little influence from continental or volcanic arc sources. A major continental influence is first documented in middle Miocene time as a thick sequence of quartzose, mud-rich turbidites accumulated on the deeper basin floor, possibly in response to middle Miocene orogeny in northern Borneo. Terrigenous turbidite deposition waned during the late Miocene as active arc volcanism began to contribute significant amounts of hemipelagic sediment and ash layers, which have remained the dominant basinal sediment to the present. Although the Celebes Sea is now nearly surrounded by volcanic arc terranes, the absence of volcaniclastic sediment in the Eocene to early Miocene section suggests that the basin did not form by back-arc spreading.
Holes were drilled at three Sites in the Sulu Sea on Ocean Drilling Program Leg 124. Site 768 lie... more Holes were drilled at three Sites in the Sulu Sea on Ocean Drilling Program Leg 124. Site 768 lies in the deeper part of the SE sub-basin and Sites 769 and 771 lie on the flanks of the Cagayan ridge. The results indicate that the Sulu Basin originated in the late early Miocene (c.18.8 Ma) in a backarc setting. The Cagayan Ridge was a site of early to early middle Miocene arc volcanism with the deposition of a thick sequence of andesitic to basaltic volcaniclastic deposits. In the basin center an early Miocene pelagic sequence is interrupted by a thick unit of rhyolitic to dacitic pyroclastic flows. Middle to late Miocene sedimentation is more continental in character with thick quartz-rich turbidites in the basin center. Only the hemipelagic claystone related to these terrigenous turbidites were deposited on the Cagayan Ridge. A decrease in the supply of clastic detritus from arc and continental sources and a change in the level of the carbonate compensation depth in the upper Pliocene resulted in pelagic carbonate deposition throughout the late Pliocene and Pleistocene.
Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 128... more Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87 m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600 m and 40 Ar/ 39 Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well-dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre-existing strata. The combined age model implies relatively rapid (1 m/2-5 ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin-wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies: 1. The base of the cored succession (1275-1220 mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5 Ma by several reworked volcanic clasts; 2. A late-Miocene sub-polar orbitally controlled glacial-interglacial succession (1220-760 mbsf) bounded by two unconformities correlated with basin-wide reflectors associated with early development of the terror rift; 3. A late Miocene volcanigenic succession (760-596 mbsf) terminating with a~1 my hiatus at 596.35 mbsf which spans the Miocene-Pliocene boundary and is not recognised in regional seismic data; 4. An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial-interglacial succession (590-440 mbsf), separated from; 5. A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial-interglacial succession (440-150 mbsf) by a 750 ky unconformity interpreted to represent a major sequence boundary at other locations; 6. An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150-80 mbsf), and; 7. A late Pleistocene glacigene succession (80-0 mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment.
Prior to rotary coring, a range of soft-sediment coring tools were deployed to recover the sedime... more Prior to rotary coring, a range of soft-sediment coring tools were deployed to recover the sediment-water interface and the upper few metres of strata, whose integrity was threatened by embedment of the sea riser for drilling of the ANDRILL (AND)-1A/1B holes. These coring options ...
Here we present an initiative for the International Polar Year in 2007/08 to directly investigate... more Here we present an initiative for the International Polar Year in 2007/08 to directly investigate the subglacial environment and geology of Antarctica and Greenland, representing a new frontier in ice sheet research, and a proposal to develop the technology - a hydro-mechanical drill - necessary to obtain rock and sediment samples from beneath deep glacier ice. The Antarctic continent is
Some glacial sediment samples recovered from beneath the West Antarctic ice sheet at ice stream B... more Some glacial sediment samples recovered from beneath the West Antarctic ice sheet at ice stream B contain Quaternary diatoms and up to 10(8) atoms of beryllium-10 per gram. Other samples contain no Quaternary diatoms and only background levels of beryllium-10 (less than 10(6) atoms per gram). The occurrence of young diatoms and high concentrations of beryllium-10 beneath grounded ice indicates that the Ross Embayment was an open marine environment after a late Pleistocene collapse of the marine ice sheet.
Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first ... more Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clayrich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, subglacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.
Liquid water has been known to occur beneath the Antarctic ice sheet for more than 40 years1, but... more Liquid water has been known to occur beneath the Antarctic ice sheet for more than 40 years1, but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale2, 3, 4. Here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial Antarctic lake. Subglacial Lake Whillans (SLW) lies beneath approximately 800 m of ice on the lower portion of the Whillans Ice Stream (WIS) in West Antarctica and is part of an extensive and evolving subglacial drainage network5. The water column of SLW contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component. Heterotrophic and autotrophic production data together with small subunit ribosomal RNA gene sequencing and biogeochemical data indicate that SLW is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea. Our results confirm that aquatic environments beneath the Antarctic ice sheet support viable microbial ecosystems, corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes2, 4 that can mobilize elements from the lithosphere6 and influence Southern Ocean geochemical and biological systems7.
During ODP Leg 124, late middle Eocene to Quaternary sediment sequences were recovered from 13 ho... more During ODP Leg 124, late middle Eocene to Quaternary sediment sequences were recovered from 13 holes drilled at five sites in the Celebes and Sulu basins. Paleomagnetic measurements and biostratigraphic studies using calcareous nannofossils, planktonic and benthic foraminifers, radiolarians, and diatoms were completed and summarized here. Two Neogene sediment sections recovered in the Sulu Basin yielded excellent core recoveries and magnetic reversal records, allowing direct magnetobiostratigraphic correlations for the Pliocene and Quaternary at Site 768 and for the middle Miocene to Quaternary at Site 769. The interpolated ages of biohorizons are not consistent between sites and only a few of them are in good agreement with previous calibrations. The differences may be the results of redeposition by turbidity currents and selective dissolution of key fossils.
... 3. Material and methods. Trigger cores (TC) and piston cores (PC) from the polar research shi... more ... 3. Material and methods. Trigger cores (TC) and piston cores (PC) from the polar research shipNathaniel B. Palmer (NBP) cruises of 94-01 and 95-01 in the Ross Sea were selected for analysis (Fig. ... 12, Actinocyclus actinochilus (Ehrenb.) Simonsen, Pliocene–Recent. ...
The determination of absolute abundances of diatoms in paleolimnological and paleoceanographic st... more The determination of absolute abundances of diatoms in paleolimnological and paleoceanographic studies is now becoming a routine step in diatom paleoecological investigations. Several distinct methods have been described for establishing absolute diatom abundance. Each of these methods has its own strengths and weaknesses. The most common weakness in diatom preparation methods is the use of aliquot subsampling from an assumed 'wellmixed' diatom suspension. Described here is a simple, efficient and inexpensive method that utilizes settling of diatoms through a 10 cm water column, which yields an effectively random distribution, and uses no aliquot subsampling. Only a few milligrams of sample are needed, permitting microstratigraphic analysis of, for example, individual varves. The method is shown to be both reproducible and accurate for sediments ranging in diatom concentration from less than 10 million to more than one billion diatoms per gram. Multiple slides or SEM stubs can be simultaneously prepared. The method works well for the analysis of most silt-sized microfossils or other sedimentary particles.
In a laboratory experiment we investigated micro-and nanoscale changes in fossil diatom valves an... more In a laboratory experiment we investigated micro-and nanoscale changes in fossil diatom valves and in the texture of diatomaceous sediments that result from ice sheet overburden and subglacial shearing. Our experiment included compression and shearing of Antarctic diatom-rich sediments in a ring shear device and comparison of experimental samples with natural glacial sediments from the Antarctic continental shelf. The purpose of the experiment is to establish objective criteria for analyzing subglacial processes and interpreting the origin of glacial-geologic features on the Antarctic continental shelf. We find distinct changes resulting from different glacial settings, with respect to whole diatom frustules, diatom micromorphology, and microtextural properties of sedimentary units. By providing constraints on subglacial shearing, these observations of genetically controlled micro-and nanoscale diatom structures and architecture are contributing to the understanding of large-scale glacial processes, aiding the development of models of modern ice sheet processes, and guiding interpretation of past ice sheet configurations.
Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, Jan 28, 2016
Accumulations of sediment beneath the Antarctic Ice Sheet contain a range of physical and chemica... more Accumulations of sediment beneath the Antarctic Ice Sheet contain a range of physical and chemical proxies with the potential to document changes in ice sheet history and to identify and characterize life in subglacial settings. Retrieving subglacial sediments and sediment cores presents several unique challenges to existing technologies. This paper briefly reviews the history of sediment sampling in subglacial environments. It then outlines some of the technological challenges and constraints in developing the corers being used in sub-ice shelf settings (e.g. George VI Ice Shelf and Larsen Ice Shelf), under ice streams (e.g. Rutford Ice Stream), at or close to the grounding line (e.g. Whillans Ice Stream) and in subglacial lakes deep under the ice sheet (e.g. Lake Ellsworth). The key features of the corers designed to operate in each of these subglacial settings are described and illustrated together with comments on their deployment procedures.
Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 DC warmer than at... more Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 DC warmer than at present and atmospheric CO 2 concentrations were twice as high as today!, the Antarctic ice sheets may have been unstable 2 -7 I Institute ofGeological and Nuclear Sciences, PO Box 30368.
Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirme... more Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages 1 , fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles 2 . Furthermore, an understanding of the behaviour of the marinebased West Antarctic ice sheet (WAIS) during the 'warmerthan-present' early-Pliocene epoch ( 5-3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming 3 . Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to 3 6C warmer than today 4 and atmospheric CO 2 concentration was as high as 400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model 7 that simulates fluctuations in Antarctic ice volume of up to 17 m in equivalent sea level associated with the loss of the WAIS and up to 13 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt 8 under conditions of elevated CO 2 .
A clean hot-water drill was used to gain access to Subglacial Lake Whillans (SLW) in late January... more A clean hot-water drill was used to gain access to Subglacial Lake Whillans (SLW) in late January 2013 as part of the Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project. Over 3 days, we deployed an array of scientific tools through the SLW borehole: a downhole camera, a conductivity-temperature-depth (CTD) probe, a Niskin water sampler, an in situ filtration unit, three different sediment corers, a geothermal probe and a geophysical sensor string. Our observations confirm the existence of a subglacial water reservoir whose presence was previously inferred from satellite altimetry and surface geophysics. Subglacial water is about two orders of magnitude less saline than sea water (0.37-0.41 psu vs 35 psu) and two orders of magnitude more saline than pure drill meltwater (<0.002 psu). It reaches a minimum temperature of -0.558C, consistent with depression of the freezing point by 7.019 MPa of water pressure. Subglacial water was turbid and remained turbid following filtration through 0.45 mm filters. The recovered sediment cores, which sampled down to 0.8 m below the lake bottom, contained a macroscopically structureless diamicton with shear strength between 2 and 6 kPa. Our main operational recommendation for future subglacial access through water-filled boreholes is to supply enough heat to the top of the borehole to keep it from freezing.
Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a ... more Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise.
Leg 124 of the Ocean Drilling Project drilled Sites 767 and 770 in the northern Celebes Sea, reac... more Leg 124 of the Ocean Drilling Project drilled Sites 767 and 770 in the northern Celebes Sea, reaching late middle Eocene basaltic basement at both sites. Major shifts in sediment provenance record the changing tectonic setting of the basin. From late middle Eocene into early Miocene time pelagic sedimentation prevailed, with little influence from continental or volcanic arc sources. A major continental influence is first documented in middle Miocene time as a thick sequence of quartzose, mud-rich turbidites accumulated on the deeper basin floor, possibly in response to middle Miocene orogeny in northern Borneo. Terrigenous turbidite deposition waned during the late Miocene as active arc volcanism began to contribute significant amounts of hemipelagic sediment and ash layers, which have remained the dominant basinal sediment to the present. Although the Celebes Sea is now nearly surrounded by volcanic arc terranes, the absence of volcaniclastic sediment in the Eocene to early Miocene section suggests that the basin did not form by back-arc spreading.
Holes were drilled at three Sites in the Sulu Sea on Ocean Drilling Program Leg 124. Site 768 lie... more Holes were drilled at three Sites in the Sulu Sea on Ocean Drilling Program Leg 124. Site 768 lies in the deeper part of the SE sub-basin and Sites 769 and 771 lie on the flanks of the Cagayan ridge. The results indicate that the Sulu Basin originated in the late early Miocene (c.18.8 Ma) in a backarc setting. The Cagayan Ridge was a site of early to early middle Miocene arc volcanism with the deposition of a thick sequence of andesitic to basaltic volcaniclastic deposits. In the basin center an early Miocene pelagic sequence is interrupted by a thick unit of rhyolitic to dacitic pyroclastic flows. Middle to late Miocene sedimentation is more continental in character with thick quartz-rich turbidites in the basin center. Only the hemipelagic claystone related to these terrigenous turbidites were deposited on the Cagayan Ridge. A decrease in the supply of clastic detritus from arc and continental sources and a change in the level of the carbonate compensation depth in the upper Pliocene resulted in pelagic carbonate deposition throughout the late Pliocene and Pleistocene.
Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 128... more Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87 m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600 m and 40 Ar/ 39 Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well-dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre-existing strata. The combined age model implies relatively rapid (1 m/2-5 ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin-wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies: 1. The base of the cored succession (1275-1220 mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5 Ma by several reworked volcanic clasts; 2. A late-Miocene sub-polar orbitally controlled glacial-interglacial succession (1220-760 mbsf) bounded by two unconformities correlated with basin-wide reflectors associated with early development of the terror rift; 3. A late Miocene volcanigenic succession (760-596 mbsf) terminating with a~1 my hiatus at 596.35 mbsf which spans the Miocene-Pliocene boundary and is not recognised in regional seismic data; 4. An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial-interglacial succession (590-440 mbsf), separated from; 5. A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial-interglacial succession (440-150 mbsf) by a 750 ky unconformity interpreted to represent a major sequence boundary at other locations; 6. An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150-80 mbsf), and; 7. A late Pleistocene glacigene succession (80-0 mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment.
Prior to rotary coring, a range of soft-sediment coring tools were deployed to recover the sedime... more Prior to rotary coring, a range of soft-sediment coring tools were deployed to recover the sediment-water interface and the upper few metres of strata, whose integrity was threatened by embedment of the sea riser for drilling of the ANDRILL (AND)-1A/1B holes. These coring options ...
Here we present an initiative for the International Polar Year in 2007/08 to directly investigate... more Here we present an initiative for the International Polar Year in 2007/08 to directly investigate the subglacial environment and geology of Antarctica and Greenland, representing a new frontier in ice sheet research, and a proposal to develop the technology - a hydro-mechanical drill - necessary to obtain rock and sediment samples from beneath deep glacier ice. The Antarctic continent is
Some glacial sediment samples recovered from beneath the West Antarctic ice sheet at ice stream B... more Some glacial sediment samples recovered from beneath the West Antarctic ice sheet at ice stream B contain Quaternary diatoms and up to 10(8) atoms of beryllium-10 per gram. Other samples contain no Quaternary diatoms and only background levels of beryllium-10 (less than 10(6) atoms per gram). The occurrence of young diatoms and high concentrations of beryllium-10 beneath grounded ice indicates that the Ross Embayment was an open marine environment after a late Pleistocene collapse of the marine ice sheet.
Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first ... more Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clayrich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, subglacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.
Liquid water has been known to occur beneath the Antarctic ice sheet for more than 40 years1, but... more Liquid water has been known to occur beneath the Antarctic ice sheet for more than 40 years1, but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale2, 3, 4. Here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial Antarctic lake. Subglacial Lake Whillans (SLW) lies beneath approximately 800 m of ice on the lower portion of the Whillans Ice Stream (WIS) in West Antarctica and is part of an extensive and evolving subglacial drainage network5. The water column of SLW contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component. Heterotrophic and autotrophic production data together with small subunit ribosomal RNA gene sequencing and biogeochemical data indicate that SLW is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea. Our results confirm that aquatic environments beneath the Antarctic ice sheet support viable microbial ecosystems, corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes2, 4 that can mobilize elements from the lithosphere6 and influence Southern Ocean geochemical and biological systems7.
During ODP Leg 124, late middle Eocene to Quaternary sediment sequences were recovered from 13 ho... more During ODP Leg 124, late middle Eocene to Quaternary sediment sequences were recovered from 13 holes drilled at five sites in the Celebes and Sulu basins. Paleomagnetic measurements and biostratigraphic studies using calcareous nannofossils, planktonic and benthic foraminifers, radiolarians, and diatoms were completed and summarized here. Two Neogene sediment sections recovered in the Sulu Basin yielded excellent core recoveries and magnetic reversal records, allowing direct magnetobiostratigraphic correlations for the Pliocene and Quaternary at Site 768 and for the middle Miocene to Quaternary at Site 769. The interpolated ages of biohorizons are not consistent between sites and only a few of them are in good agreement with previous calibrations. The differences may be the results of redeposition by turbidity currents and selective dissolution of key fossils.
... 3. Material and methods. Trigger cores (TC) and piston cores (PC) from the polar research shi... more ... 3. Material and methods. Trigger cores (TC) and piston cores (PC) from the polar research shipNathaniel B. Palmer (NBP) cruises of 94-01 and 95-01 in the Ross Sea were selected for analysis (Fig. ... 12, Actinocyclus actinochilus (Ehrenb.) Simonsen, Pliocene–Recent. ...
The determination of absolute abundances of diatoms in paleolimnological and paleoceanographic st... more The determination of absolute abundances of diatoms in paleolimnological and paleoceanographic studies is now becoming a routine step in diatom paleoecological investigations. Several distinct methods have been described for establishing absolute diatom abundance. Each of these methods has its own strengths and weaknesses. The most common weakness in diatom preparation methods is the use of aliquot subsampling from an assumed 'wellmixed' diatom suspension. Described here is a simple, efficient and inexpensive method that utilizes settling of diatoms through a 10 cm water column, which yields an effectively random distribution, and uses no aliquot subsampling. Only a few milligrams of sample are needed, permitting microstratigraphic analysis of, for example, individual varves. The method is shown to be both reproducible and accurate for sediments ranging in diatom concentration from less than 10 million to more than one billion diatoms per gram. Multiple slides or SEM stubs can be simultaneously prepared. The method works well for the analysis of most silt-sized microfossils or other sedimentary particles.
In a laboratory experiment we investigated micro-and nanoscale changes in fossil diatom valves an... more In a laboratory experiment we investigated micro-and nanoscale changes in fossil diatom valves and in the texture of diatomaceous sediments that result from ice sheet overburden and subglacial shearing. Our experiment included compression and shearing of Antarctic diatom-rich sediments in a ring shear device and comparison of experimental samples with natural glacial sediments from the Antarctic continental shelf. The purpose of the experiment is to establish objective criteria for analyzing subglacial processes and interpreting the origin of glacial-geologic features on the Antarctic continental shelf. We find distinct changes resulting from different glacial settings, with respect to whole diatom frustules, diatom micromorphology, and microtextural properties of sedimentary units. By providing constraints on subglacial shearing, these observations of genetically controlled micro-and nanoscale diatom structures and architecture are contributing to the understanding of large-scale glacial processes, aiding the development of models of modern ice sheet processes, and guiding interpretation of past ice sheet configurations.
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Papers by Reed Scherer