A high-resolution bathymetry map for the Marguerite Bay and adjacent west Antarctic Peninsula shelf for the Southern Ocean GLOBEC Program

One objective of the U.S. Southern Ocean Global Ocean Ecosystems Dynamics (SO GLOBEC) program is to gain a better understanding of the sea floor bathymetry in the program study area. Much of Marguerite Bay and the adjacent shelf west of the Antarctic Peninsula were poorly charted when the SO GLOBEC program started in 2000. Before the first SO GLOBEC cruise, an improved local area version (ETOPO8.2A) was created from the Smith and Sandwell (1997) topo_8.2.img 2-minute digital gridded bathymetry for the study area. The first SO GLOBEC mooring cruise on the R/V Lawrence M. Gould (March 2001) showed that the 2-minute spatial resolution of ETOPO8.2A did not resolve many of the canyons and abrupt changes in topography that characterize Marguerite Bay and the inner- to mid-shelf region. It also was not particularly accurate in the more uniform terrain regions. We then decided to collect as much multibeam bathymetry data as possible during the SO GLOBEC broad-scale survey cruises on the R/VIB Nathaniel B. Palmer and combine these data with all other available multibeam and trackline bathymetry data to construct a digital bathymetry database and map for the study area. The resulting database has high-resolution data over much of the shelf and parts of Marguerite Bay gridded at 2 seconds in latitude and 6 seconds in longitude spacing between 65° to 71° S and 65° to 78° W. This technical report describes the steps taken to assemble and construct this database and how to access the data via the Internet.Funding was provided by the Office of Naval Research under Contract No. N00014-99-1-0213

Long term geological record of a global deep subsurface microbial habitat in sand injection complexes

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Flow and retreat of the Late Quaternary Pine Island-Thwaites palaeo-ice stream, West Antarctica

Multibeam swath bathymetry and sub-bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo-ice stream that carried the combined discharge from the parts of the West Antarctic Ice Sheet now occupied by the Pine Island and Thwaites glacier basins. Sets of highly elongated bedforms show that, at the last glacial maximum, the route of the Pine Island-Thwaites palaeo-ice stream arced north-northeast following a prominent cross-shelf trough. In this area, the grounding line advanced to within similar to 68 km of, and probably reached, the shelf edge. Minimum ice thickness is estimated at 715 m on the outer shelf, and we estimate a minimum ice discharge of similar to 108 km(3) yr(-1) assuming velocities similar to today's Pine Island glacier (similar to 2.5 km yr(-1)). Additional bed forms observed in a trough northwest of Pine Island Bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior. The "style" of ice retreat is also evident in five grounding zone wedges, which suggest episodic deglaciation characterized by halts in grounding line migration up-trough. Stillstands occurred in association with changes in ice bed gradient, and phases of inferred rapid retreat correlate to higher bed slopes, supporting theoretical studies that show bed geometry as a control on ice margin recession. However, estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat. Our findings show that the ice stream channeled a substantial fraction of West Antarctica's discharge in the past, just as the Pine Island and Thwaites glaciers do today

Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula

The northern Antarctic Peninsula is currently undergoing rapid atmospheric warming. Increased glacier-surface melt during the twentieth century has contributed to ice-shelf collapse and the widespread acceleration, thinning and recession of glaciers. Therefore, glaciers peripheral to the Antarctic Ice Sheet currently make a large contribution to eustatic sea-level rise, but future melting may be offset by increased precipitation. Here we assess glacier-climate relationships both during the past and into the future, using ice-core and geological data and glacier and climate numerical model simulations. Focusing on Glacier IJR45 on James Ross Island, northeast Antarctic Peninsula, our modelling experiments show that this representative glacier is most sensitive to temperature change, not precipitation change. We determine that its most recent expansion occurred during the late Holocene a Little Ice Age' and not during the warmer mid-Holocene, as previously proposed. Simulations using a range of future Intergovernmental Panel on Climate Change climate scenarios indicate that future increases in precipitation are unlikely to offset atmospheric-warming-induced melt of peripheral Antarctic Peninsula glaciers

Calibration of a point-counting technique for estimation of biogenic silica in marine sediments

The simple technique of point counting is very quick and is suitable for shipboard use, or in any study where high accuracy is not necessary. Point-counting smear slides yields results different from measurements of weight-percent silica if there are systematic differences in shape and density of biogenic and terrigenous grains. In this note I present calibration curves for synthetic biogenic-terrigenous mixtures, to show how the accuracy of point counting may be improved

Introduction: studies in ancient historical demography

Book synopsis: Through a series of case studies this book demonstrates the wide-ranging impact of demographic dynamics on social, economic and political structures in the Graeco-Roman world. The individual case studies focus on fertility, mortality and migration and the roles they played in various aspects of ancient life. These studies – drawn from a range of populations in Athens and Attica, Rome and Italy, and Graeco-Roman Egypt – illustrate how new insights can be gained by applying demographic methods to familiar themes in ancient history. Methodological issues are addressed in a clear, straightforward manner with no assumption of prior technical knowledge, ensuring that the book is accessible to readers with no training in demography. The book marks an important step forward in ancient historical demography, affirming both the centrality of population studies in ancient history and the contribution that antiquity can make to population history in general

Antarctic Circumpolar Deep Water; a Quaternary paleoflow record from the northern Scotia Sea, South Atlantic Ocean

In the northern Scotia Sea, the main pathway of Circumpolar Deep Water (CPDW) flows north to pass through a deep gap in the North Scotia Ridge before turning east into the Falkland Trough. A sediment drift has developed on the seabed since the early-middle Miocene, coincident with the opening of Drake Passage and the inception of deep-water flow. Seismic and acoustic surveys show that the drift covers an area of 10,500 km 2 and forms a broadly asymmetrical mound up to 800 m thick. There is a zone of sediment thinning along the northwestern margin, the result of accentuated CPDW flow around rough ocean floor topography. Small debris flows originating around the margins of the drift suggest localized instability and high sediment supply. Four cores 3-9 m long have been recovered from the crest and margins of the drift in water depths of 3900-4300 m. Biostratigraphy and chemostratigraphy reveal that the longest core extends down to oxygen isotope stage 10 (approx. 370 ka). The sediments are predominantly fine-grained contourites and diatom-rich hemipelagites, capped by sandy-silty contourites rich in the planktonic foraminifer Neogloboquadrina pachyderma. Grain-size analysis of the fine fraction, finer than 4 phi (63 mm), combined with radiocarbon (AMS) dating and magnetic susceptibility, provide an indication of relative CPDW strength over the last 18 ka. Shortly after the last glacial maximum (LGM), at approximately 17 ka, silt modes fluctuated from 5.5 phi to up to 6.25 phi; this increased current winnowing is indicative of an unstable CPDW, with stormier glacial benthic conditions producing sporadic, high-energy currents across the drift crest and flanks. At approximately 12,280 ka, an increase in sediment sorting is noted, indicative of a strong flow of CPDW over the drift crest, suggesting an unstable and fluctuating deep-water flow. During deglaciation and into the Holocene, at approximately 10 ka, CPDW flow stabilized, becoming less vigorous across the drift crest and flanks with silt modes from 6 phi to 5.5 phi accompanied by increased sorting of the sediments. The gross average sedimentation rate from the crest of the drift is 11.2 cm/ky compared to 2.3 cm/ky on the southeastern flank. The unsteadiness of CPDW during glacials compared to interglacial periods may be the result of stronger wind forcing and a northward shift in the Polar Front. Older CPDW flow records from the cores suggest variable and cyclic bottom-current flow corresponding to glacial-interglacial episodes. Modern CPDW flow across the crest of the drift averages 11.6 cm s (super -1) but with intermittent benthic storm activity resuspending the fines

Recent benthonic foraminifera from the Western Antarctic Ocean

Approximately 60 species of benthonic foraminifera are recorded from 14 samples collected by the RRS Discovery from water depths of 200–4200 m on the Antarctic Peninsula Shelf and Slope and adjacent areas of the Western Antarctic Ocean. Discussion focuses firstly on benthonic foraminiferal abundance, diversity and assemblage composition and the effects of calcite dissolution and, secondly, on the characteristic assemblages of the study area and their relationships to water-masse