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

Peer reviewedPublisher PD

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

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

Late Quaternary glacial history, flow dynamics and sedimentation along the eastern margin of the Antarctic Peninsula Ice Sheet.

Geological and geophysical data from the NE Antarctic Peninsula continental shelf are used to reconstruct the glacial history, flow-dynamics and sedimentation of the Antarctic Peninsula Ice Sheet (APIS) along its eastern margin during the Late Quaternary. Ice advanced to the shelf edge during the Last Glacial Maximum (LGM) and deposited a stiff till across the shelf. The presence of highly attenuated bedforms indicates that fast-flowing outlets drained the APIS through cross-shelf troughs to the outer shelf after this ice advance. The bedforms are formed in deformation till in response to deforming bed processes. Deglaciation of Robertson Trough and the troughs of Prince Gustav Channel, Larsen-A and Larsen Inlet was continuous (and possibly rapid) on account of the absence of ice-margin recessional features. In contrast, grounding-zone wedges across the shelf of Northern Larsen-A and south of Prince Gustav Channel indicate that ice retreat was gradual and was punctuated by stillstands. The transition from a grounded ice sheet to ice shelf conditions was completed before 11–12 14C ka BP on the shelf south of Prince GustavChannel, and is marked across the shelf by a change from subglacial till to a transitional heterogeneous unit dominated by coarse-grained facies. Transitional sediments record mainly sub-ice shelf rain-out and restricted bottom current and sediment-laden plume activity, as well as localised debris flows. Meltwater-derived facies are largely absent indicating that release of meltwater was not significant beneath polar ice shelves, or during deglaciation of the APIS. In the broader context, the colder, eastern side of the APIS was extensive and was drained mainly through fast-flowing outlets (palaeo-ice streams). Therefore, the eastern APIS would have been an important contributor to sediment and iceberg flux to the Weddell Sea Embayment during the LGM and subsequent deglaciation

Bottom currents, contourites and deep-sea sediment drifts: current state-of-the-art

This paper provides both an introduction to and summary for the Atlas of Contourite Systems that has been compiled as part of the International Geological Correlation Project - IGCP 432. Following the seminal works of George Wust on the physical oceanography of bottom currents, and Charley Hollister on contourite sediments, a series of significant advances have been made over the past few decades. While accepting that ideas and terms must remain flexible as our knowledge base continues to increase, we present a consensus view on terminology and definitions of bottom currents, contourites and drifts. Both thermohaline and wind-driven circulation, influenced by Coriolis Force and molded by topography, contribute to the oceanic system of bottom currents. These semi-permanent currents show significant variability in time and space, marked by periodic benthic storm events in areas of high surface kinetic energy. Six different drift types are recognized in the ocean basins and margins at depths greater than about 300 m: (i) contourite sheet drifts; (ii) elongate mounded drifts; (iii) channel related drifts; (iv) confined drifts; (v) infill drifts; and (vi) modified drift-turbidite systems. In addition to this overall geometry, their chief seismic characteristics include: a uniform reflector pattern that reflects long-term stability, drift-wide erosional discontinuities caused by periodic changes in bottom current regime, and stacked broadly lenticular seismic depositional units showing oblique to downcurrent migration. At a smaller scale, a variety of seismic facies can be recognized that are here related to bottom current intensity. A model for seismic facies cyclicity (alternating transparent/reflector zones) is further elaborated, and linked to bottom current/climate change. Both erosional features and depositional bedforms are diagnostic of bottom current systems and velocities. Many different contourite facies are now known to exist, encompassing all compositional types. We propose here a Cl-5 notation for the standard contourite facies sequence, which can be interpreted in terms of fluctuation in bottom current velocity and/or sediment supply. Several proxies can be utilized to decode contourite successions in terms of current fluctuation. Gravel lag and shale chip contourites, as well as erosional discontinuities are indicative of still greater velocities. There are a small but growing number of land-based examples of fossil contourites, based on careful analysis using the recommended three-stage approach to interpretation. Debate still surrounds the recognition and interpretation of bottom current reworked turbidites

Flow dynamics and till genesis associated with a marine-based Antarctic palaeo-ice stream.

Geophysical and geological data indicate that during the last glacial cycle a palaeo-ice stream drained the Antarctic Peninsula Ice Sheet (APIS) through Marguerite Bay to the edge of the continental shelf via a bathymetric trough (Marguerite Trough). Mega-scale glacial lineations (MSGL) in outer Marguerite Trough are formed in an acoustically transparent sediment unit, consisting of a soft deformation till. The association of MSGL with deformation till indicates that bed deformation occurred beneath the ice stream, and the thickness of the subglacial deforming layer was on the order of several metres. MSGL within outer Marguerite Trough are related to a pre-existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove-ploughing. Morphologically, the MSGL differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform. An initial advance of the APIS across the Marguerite Bay continental shelf at the LGM deposited a stiff subglacial till. Following this advance, the ice margin within Marguerite Trough retreated 70–100 km before stabilising and depositing a grounding zone wedge. An ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till. Development of the ice stream may have been a glacio-dynamic response to regional deglaciation. Subsequent retreat of the Marguerite Trough ice stream was underway by 13,490 (uncorrected) 14C years before present. Based on the geophysical and core data, ice-stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo-ice streams from other locations in Antarctica. This implies marked regional variations in Antarctic ice-stream dynamics during Late Quaternary deglaciation