Feldspar Pb-isotope ratios for earliest Pleistocene sediments of the sub-polar North Atlantic Ocean

Relatively little is known in detail about the locations of the early Pleistocene ice-sheets responsible for ice-rafted debris (IRD) inputs to the sub-polar North Atlantic Ocean during intensification of northern hemisphere glaciation (iNHG). To shed new light on this problem, we present the first combined in-depth analysis of IRD flux and geochemical provenance of individual sand-sized IRD deposited in the sub-polar North Atlantic Ocean during the earliest large amplitude Pleistocene glacial, marine isotope stage (MIS) 100 (~2.52 Ma), arguably the key glacial during iNHG. IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single feldspar grains. We find that the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) of individual ice-rafted (>150 µm) feldspars deposited at DSDP Site 611A, ODP Site 981 and IODP Site U1308 during MIS 100 records a shift from predominantly Archaean-aged circum-North Atlantic Ocean continental sources during early glacial ice-rafting events to dominantly Palaeozoic and Proterozoic-aged sources during full glacial conditions. The distribution of feldspars in Pb-Pb space for full glacial MIS 100 more closely resembles that documented for feldspars deposited at the centre of the last glacial IRD belt (at IODP/DSDP Site U1308/609) during ambient (non-Heinrich-event) ice-rafting episodes of MIS 2 (~23.8 ka) than that documented for MIS 5d (~106 ka). Comparison of our early Pleistocene and last glacial cycle datasets suggests that MIS 100 was characterised by abundant iceberg calving from large ice-sheets on multiple continents in the high northern latitudes (not just on Greenland).

Sediment total reflectance measurements from the Cariaco Basin (MD03-2621) and northeastern Arabian Sea (SO130-289KL)

During the last glacial period, the North Atlantic region experienced pronounced, millennial-scale alternations between cold, stadial conditions and milder interstadial conditions-commonly referred to as Dansgaard-Oeschger oscillations-as well as periods of massive iceberg discharge known as Heinrich events. Changes in Northern Hemisphere temperature, as recorded in Greenland, are thought to have affected the location of the Atlantic intertropical convergence zone and the strength of the Indian summer monsoon. Here we use high-resolution records of sediment colour-a measure of terrigenous versus biogenic content-from the Cariaco Basin off the coast of Venezuela and the Arabian Sea to assess teleconnections with the North Atlantic climate system during the last glacial period. The Cariaco record indicates that the intertropical convergence zone migrated seasonally over the site during mild stadial conditions, but was permanently displaced south of the basin during peak stadials and Heinrich events. In the Arabian Sea, we find evidence of a weak Indian summer monsoon during the stadial events. The tropical records show a more variable response to North Atlantic cooling than the Greenland temperature records. We therefore suggest that Greenland climate is especially sensitive to variations in the North Atlantic system-in particular sea-ice extent-whereas the intertropical convergence zone and Indian monsoon system respond primarily to variations in mean Northern Hemisphere temperature.

Environmental analysis on sediment core CRP-2 from the Ross Sea, Antarctica

Cape Roberts Project drill core 2/2A was obtained from Roberts Ridge, a sea-floor high located at 77° S, 16 km offshore from Cape Roberts in western McMurdo Sound, Antarctica. The recovered core is about 624 m long and includes strata dated as being Quaternary, Pliocene, Miocene and Oligocene in age. The core includes twelve facies commonly occurring in associations that are repeated in particular sequences throughout the core and which are interpreted as representing different depositional environments through time. Depositional systems inferred to be represented in the succession include: outer shelf with minor iceberg influence, outer shelf-inner shelf-nearshore to shoreface under iceberg influence, deltaic and/or grounding-line fan, and ice proximal-ice marginal-subglacial (mass flow/rainout diamictite/subglacial till) singly or in combination. Changes in palaeoenvironmental interpretations up the core are used to estimate relative glacial proximity to the site through time. These inferred glacial fluctuations are then compared with the global eustatic sea level and d18O curves to evaluate the potential of glacial fluctuations on Antarctica influencing these records of global change. Although the comparisons are tentative at present, the records do have similarities, but there are also some differences especially in possible number (and perhaps magnitude) of glacial fluctuations that require further evaluation.

Estrategias para entrar y salir del mercado : La revista Bibliograma (1953-1957) y la renovación de la crítica argentina en los años '50

El estudio de la renovación crítica de los años '50 en Argentina ha tendido a hacer énfasis en unas pocas publicaciones emblemáticas-ante todo Contorno (1953-9)-, pero este conjunto es la punta de un iceberg proliferante de actividad crítica y reseñística. El análisis de la revista Bibliograma (1953-57) permite advertir, en un momento en que la rápida industrialización de la producción y distribución de libros obligaba a redefinir el estatuto y la legitimidad de las prácticas asociadas a la literatura, algunas de las principales esperanzas y temores que hicieron entonces de la crítica un espacio de militancia

Sediment properties of IODP Holes 303-U1302A and 303-U1305C

Episodes of ice-sheet disintegration and meltwater release over glacial-interglacial cycles are recorded by discrete layers of detrital sediment in the Labrador Sea. The most prominent layers reflect the release of iceberg armadas associated with cold Heinrich events, but the detrital sediment carried by glacial outburst floods from the melting Laurentide Ice Sheet is also preserved. Here we report an extensive layer of red detrital material in the Labrador Sea that was deposited during the early last interglacial period. We trace the layer through sediment cores collected along the Labrador and Greenland margins of the Labrador Sea. Biomarker data, Ca/Sr ratios and d18O measurements link the carbonate contained in the red layer to the Palaeozoic bedrock of the Hudson Bay. We conclude that the debris was carried to the Labrador Sea during a glacial outburst flood through the Hudson Strait, analogous to the final Lake Agassiz outburst flood about 8,400 years ago, probably around the time of a last interglacial cold event in the North Atlantic. We suggest that outburst floods associated with the final collapse of the Laurentide Ice Sheet may have been pervasive features during the early stages of Late Quaternary interglacial periods.

Estrategias para entrar y salir del mercado : La revista Bibliograma (1953-1957) y la renovación de la crítica argentina en los años '50

El estudio de la renovación crítica de los años '50 en Argentina ha tendido a hacer énfasis en unas pocas publicaciones emblemáticas-ante todo Contorno (1953-9)-, pero este conjunto es la punta de un iceberg proliferante de actividad crítica y reseñística. El análisis de la revista Bibliograma (1953-57) permite advertir, en un momento en que la rápida industrialización de la producción y distribución de libros obligaba a redefinir el estatuto y la legitimidad de las prácticas asociadas a la literatura, algunas de las principales esperanzas y temores que hicieron entonces de la crítica un espacio de militancia

Chemistry of Globigerinoides ruber shells from ODP Site 160-963

Here we present a high-resolution faunal, floral and geochemical (stable isotopes and trace elements) record from the sediments of Ocean Drilling Program Site 963 (central Mediterranean basin), which shows centennial/millennial-scale resemblance to the high-northern latitude rapid temperature fluctuations documented in the Greenland ice cores between 20 and 70 kyr BP. Oxygen and carbon isotopes, planktic foraminifera and calcareous nannofossil distributions suggest that Dansgaard-Oeschger (D/O) and Heinrich events (HE) are distinctly expressed in the Mediterranean climate record. Moreover, recurrent though subdued oscillations not previously identified in the Lateglacial Mediterranean sediments document a significant centennial-scale climate variability in the basin that is greater than previously thought. Alternations between climate regimes dominated by polar outbreaks during D/O stadials and warm D/O interstadials, with associated intensification of continental runoff, are well expressed in the ODP Site 963. These place the Mediterranean basin as an often overlooked recorder of the interplay between large- and regional- scale climate controls at intermediate latitudes, and of the possible interactions between different components of the climate system. Significant changes in Ba/Ca values measured in Globigerinoides ruber shells from a number of D/O stadials and interstadials suggest enhanced freshwater input from the north-eastern Mediterranean borderland during the D/O interstadials. However, the short duration of 3D stratification events never led to complete oxygen consumption along the water column, but clear effects of sluggish 3D circulation in the basin are testified to by negative excursions in d13C measured in selected species of planktic and benthic foraminifera. HEs are constantly associated with lightening in the d18O record of planktic foraminifera, possibly because of the impact of iceberg melting in the Iberian Margin on Mediterranean thermohaline circulation. Interestingly, in two cases in particular, HE2 and HE5, fresher water inputs also affected deeper horizons of intermediate waters, suggesting a basin-wide impact.

Estrategias para entrar y salir del mercado : La revista Bibliograma (1953-1957) y la renovación de la crítica argentina en los años '50

El estudio de la renovación crítica de los años '50 en Argentina ha tendido a hacer énfasis en unas pocas publicaciones emblemáticas-ante todo Contorno (1953-9)-, pero este conjunto es la punta de un iceberg proliferante de actividad crítica y reseñística. El análisis de la revista Bibliograma (1953-57) permite advertir, en un momento en que la rápida industrialización de la producción y distribución de libros obligaba a redefinir el estatuto y la legitimidad de las prácticas asociadas a la literatura, algunas de las principales esperanzas y temores que hicieron entonces de la crítica un espacio de militancia

Nd bulk isotopes and 40Ar/39Ar ages of individual minerals from Aurora and Wilkes sub-glacial basin sediments

The Wilkes and Aurora basins are large, low-lying sub-glacial basins that may cause areas of weakness in the overlying East Antarctic ice sheet. Previous work based on ice-rafted debris (IRD) provenance analyses found evidence for massive iceberg discharges from these areas during the late Miocene and Pliocene. Here we characterize the sediments shed from the inferred areas of weakness along this margin (94°E to 165°E) by measuring40Ar/39Ar ages of 292 individual detrital hornblende grains from eight marine sediment core locations off East Antarctica and Nd isotopic compositions of the bulk fine fraction from the same sediments. We further expand the toolbox for Antarctic IRD provenance analyses by exploring the application of 40Ar/39Ar ages of detrital biotites; biotite as an IRD tracer eliminates lithological biases imposed by only analyzing hornblendes and allows for characterization of samples with low IRD concentrations. Our data quadruples the number of detrital 40Ar/39Ar ages from this margin of East Antarctica and leads to the following conclusions: (1) Four main sectors between the Ross Sea and Prydz Bay, separated by ice drainage divides, are distinguishable based upon the combination of 40Ar/39Ar ages of detrital hornblende and biotite grains and the e-Nd of the bulk fine fraction; (2) 40Ar/39Ar biotite ages can be used as a robust provenance tracer for this part of East Antarctica; and (3) sediments shed from the coastal areas of the Aurora and Wilkes sub-glacial basins can be clearly distinguished from one another based upon their isotopic fingerprints.

Radiocarbon ages and clay minerals in sediment cores obtained during a 2009 Jan Mayen cruise to the Norwegian Sea

The origin of two acoustic sediment units has been studied based on lithological facies, chronology and benthic stable isotope values as well as on foraminifera and clay mineral assemblages in six marine sediment cores from Kveithola, a small trough west of Spitsbergenbanken on the western Barents Sea margin. We have identified four time slices with characteristic sedimentary environments. Before c. 14.2 cal. ka, rhythmically laminated muds indicate extensive sea ice cover in the area. From c. 13.9 to 14.2 cal. ka, muds rich in ice-rafted debris were deposited during the disintegration of grounded ice on Spitsbergenbanken. From c. 10.3 to 13.1 cal. ka, sediments with heterogeneous lithologies suggest a shifting influence of suspension settling and iceberg rafting, probably derived from a decaying Barents Sea Ice Sheet in the inner-fjord and land areas to the north of Kveithola. Holocene deposition was episodic and characterized by the deposition of calcareous sands and shell debris, indicative of strong bottom currents. We speculate that a marked erosional boundary at c. 8.2 cal. ka may have been caused by the Storegga tsunami. Whilst deposition was sparse during the Holocene, Kveithola acted as a sediment trap during the preceding deglaciation. Investigation of the deglacial sediments provides unprecedented details on the dynamics and timing of glacial retreat from Spitsbergenbanken.

Holocene sediment transport from the glaciated margin of East/Northeast Greenland to the N Iceland shelves

We examine variations in the ice-rafted sources for sediments in the Iceland/East Greenland offshore marine archives by utilizing a sediment unmixing model and link the results to a coupled iceberg-ocean model. Surface samples from around Iceland and along the E/NE Greenland shelf are used to define potential sediment sources, and these are examined within the context of the down-core variations in mineralogy in the <2 mm sediment fraction from a transect of cores across Denmark Strait. A sediment unmixing model is used to estimate the fraction of sediment <2 mm off NW and N Iceland exported across Denmark Strait; this averaged between 10 and 20%. Both the sediment unmixing model and the coupled iceberg-ocean model are consistent in finding that the fraction of "far-travelled" sediments in the Denmark Strait environs is overwhelmingly of local, mid-East Greenland, provenance, and therefore with a significant cross-channel component to their travel. The Holocene record of ice-rafted sediments denotes a three-part division of the Holocene in terms of iceberg sediment transport with a notable increase in the process starting ca 4000 cal yr BP. This latter increase may represent the re-advance during the Neoglacial period of land-terminating glaciers on the Geikie Plateau to become marine-terminating. The contrast in spectral signals between these cores and the 1500-yr cycle at VM28-14, just south of the Denmark Strait, combined with the coupled iceberg-model results, leads us to speculate that the signal at VM28-14 reflects pulses in overflow waters, rather than an ice-rafted signal.

Icebergs in the Amundsen Sea (Antarctica) determined from ENVISAT ASAR WSM data using object-based image analysis approach, link to ESRI GDB files

ENVISAT ASAR WSM images with pixel size 150 × 150 m, acquired in different meteorological, oceanographic and sea ice conditions were used to determined icebergs in the Amundsen Sea (Antarctica). An object-based method for automatic iceberg detection from SAR data has been developed and applied. The object identification is based on spectral and spatial parameters on 5 scale levels, and was verified with manual classification in four polygon areas, chosen to represent varying environmental conditions. The algorithm works comparatively well in freezing temperatures and strong wind conditions, prevailing in the Amundsen Sea during the year. The detection rate was 96% which corresponds to 94% of the area (counting icebergs larger than 0.03 km**2), for all seasons. The presented algorithm tends to generate errors in the form of false alarms, mainly caused by the presence of ice floes, rather than misses. This affects the reliability since false alarms were manually corrected post analysis.

Synthesized grounding line and ice shelf mask for Antarctica

Iceberg calving has been assumed to be the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000 gigatonnes per year. More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front. So far, however, no study has reliably quantified the calving flux and the basal mass balance (the balance between accretion and ablation at the ice-sheet base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases is therefore poorly constrained. Here we estimate the mass balance components for all ice shelves in Antarctica, using satellite measurements of calving flux and grounding-line flux, modelled ice-shelf snow accumulation rates and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321 ± 144 gigatonnes per year and a total basal mass balance of -1,454 ± 174 gigatonnes per year. This means that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves. We find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering and enhanced discharge. We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.

Ages and geochemical and geophysical characteristics of sediment core MD99-2294, Lofoten Contourite Drift

A sediment core from the Lofoten Contourite Drift on the continental slope off Northern Norway, proximal to the former Vestfjorden-Trsnadjupet Ice Stream, details the development, variability and decline of marine margins of the northwestern Fennoscandian Ice Sheet during the time interval 25.3-14 cal ka BP, including the Last Glacial Maximum and onset of the deglaciation based on high-resolution IRD records. From the core interval between 25.3 and 17.7 cal ka BP we report data points with a mean time step of 10 years, between 17.7 cal ka BP and the Holocene time steps are typically 50 years. The core is divided into 7 informal ice-rafted debris (IRD) zones based on the variations in IRD including 7 major IRD maxima (A-G), inferred to represent periods of high iceberg production. Petrological identification reveals dominance of crystalline IRD (monocrystalline, plutonic and metamorphic rock fragments) accounting for 75-80% of total IRD assemblages, while sedimentary fragments generally account for 15-20%. The crystalline fragments (including eclogite and mangerite from a nearby terrestrial source) increase across the IRD peaks while the sedimentary fragments remain constant. This points to the importance of erosional products from icebergs originating from fast-flowing paleo-ice streams including the Vestfjorden-Trsnadjupet Ice Stream draining from the Fennoscandian mainland during the IRD maxima periods. Increased temperature of the adjacent surface water masses was probably an important external forcing factor on the Fennoscandian Ice Sheet behavior because some IRD maxima and plumite deposition from meltwater plumes post-date periods of increased sea surface temperatures. The peak IRD depositions occur in centennial and millennial time cycles (~200, 1030 and 3900 year) indicating some external forcing by solar variation. Both mechanisms could explain the observed synchronous instability of the northwestern Fennoscandian Ice Sheet to other European Ice Sheets.