Rebesco, M.

The upper continental slope of the Storfjorden- Kveithola Trough Mouth Fans (NW Barents Sea) contains a several m-thick late Pleistocene sequence of plumites composed of laminated mud interbedded with sand/silt layers. Radiocarbon ages revealed that deposition occurred during about 130 years at a very high sedimentation rate of 3.4 cm a-1, at about 7 km from the present shelf break. Palaeomagnetic and rock magnetic analyses confirm the existence of a prominent, short-living sedimentary event. The plumites appear laterally continuous and were correlated with the sedimentary sequences described west of Svalbard and neighboring glacial depositional systems representing a major event at regional scale appointed to correspond to the deep-sea sedimentary record of Meltwater Pulse-1a. We also present new sedimentological and geochemical insights, and multi-beam data adding information on the palaeoenvironmental characteristics during MWP-1a and ice sheet decay in the NW Barents Sea.

The depositional history of the Storfjorden and Kveithola trough-mouth fans (TMFs) in the northwestern Barents Sea has been investigated within two coordinated Spanish and Italian projects in the framework of the International Polar Year (IPY) Activity 367, NICE STREAMS. The investigation has been conducted using a multidisciplinary approach to the study of sediment cores positioned on high-resolution multibeam bathymetry and TOPAS/CHIRP sub-bottom profiles. Core correlation and the age model were based on 27 AMS 14C samples, rock magnetic parameters, lithofacies sequences, and the presence of marker beds including two oxidized layers marking the post Last Glacial Maximum (LGM) inception of deglaciation (OX-2) and the Younger Dryas cold climatic event (OX-1). Sediment facies analysis allowed the distinction of a number of depositional processes whose onset appears closely related to ice stream dynamics and oceanographic patterns in response to climate change. The glacigenic diamicton with low water content, high density, and high shear strength, deposited during glacial maxima, indicates ice streams grounded at the shelf edge. Massive release of IRD occurred at the inception of deglaciation in response to increased calving rates with possible outer ice streams lift off and collapse. The presence of a several-meter-thick sequence of interlaminated sediments deposited by subglacial outbursts of turbid meltwater (plumites) indicates rapid ice streams' melting and retreat. Crudely-layered and heavily-bioturbated sediments were deposited by contour currents under climatic/environmental conditions favorable to bioproductivity. The extreme sedimentation rate of 3.4 cm a− 1 calculated for the plumites from the upper-slope area indicates a massive, nearly instantaneous (less than 150 years), terrigenous input corresponding to an outstanding meltwater event. We propose these interlaminated sediments to represent the high-latitude marine record of MeltWater Pulse 1a (MWP-1a). Different bathymetric and oceanographic conditions controlled locally the mode of glacial retreat, resulting in different thickness of plumites on the upper continental slope of the Storfjorden and Kveithola TMFs. It is possible that the southern part of Storfjorden TMF received additional sediments from the deglaciation of the neighboring Kveithola ice stream.

A high‐resolution paleomagnetic and rock magnetic study has been carried out on sediment cores collected in glaciomarine silty‐clay sequences from the continental shelf and slope of the southern Storfjorden trough‐mouth fan, on the northwestern Barents Sea continental margin. The Storfjorden sedimentary system was investigated during the SVAIS and EGLACOM cruises, when 10 gravity cores, with a variable length from 1.03 m to 6.41 m, were retrieved. Accelerator mass spectrometry (AMS) 14C analyses on 24 samples indicate that the cores span a time interval that includes the Holocene, the last deglaciation phase and in some cores the last glacial maximum. The sediments carry a well‐defined characteristic remanent magnetization and have a valuable potential to reconstruct the paleosecular variation (PSV) of the geomagnetic field, including relative paleointensity (RPI) variations. The paleomagnetic data allow reconstruction of past dynamics and amplitude of the geomagnetic field variations at high northern latitudes (75°–76° N). At the same time, the rock magnetic and paleomagnetic data allow a high‐resolution correlation of the sedimentary sequences and a refinement of their preliminary age models. The Holocene PSV and RPI records appear particularly sound, since they are consistent between cores and they can be correlated to the closest regional stacking curves (UK PSV, FENNOSTACK and FENNORPIS) and global geomagnetic model for the last 7 ka (CALS7k.2). The computed amplitude of secular variation is lower than that outlined by some geomagnetic field models, suggesting that it has been almost independent from latitude during the Holocene.

Paleomagnetic and rock magnetic investigations were carried out on four gravity cores recovered from the western continental rise of the Antarctic Peninsula during the SEDANO II cruise of RV OGS-Explora. The studied cores, each about 6.5 m-long, were collected at a depth of 3700–4100 m below the sea level, on the distal gentle side of sediment Drift 7, and consist of very fine-grained sediments spanning through various glacial–interglacial cycles. Detailed analysis of the paleomagnetic and rock magnetic data allowed to reconstruct relative paleointensity (RPI) records (NRM20 mT/ARM20 mT) for each core.We established a refined age model for the studied sequences by correlating individual SEDANO RPI curves to the global RPI stack SINT-800 [Y. Guyodo, J.-P. Valet, Global changes in intensity of the Earth's magnetic field during the past 800 kyr, Nature 399 (1999) 249–252]. The individual normalized SEDANO RPI records are in mutual close agreement; they were thus merged in a RPI stacking curve spanning the last 270 kyr and showing a low standard deviation. This study also points out that RPI records may provide a viable tool to date otherwise difficult-to-date sedimentary sequences, such as those deposited along peri-Antarctic margins. The new RPI chronology indicates that the sampled sedimentary sequence is younger than previously thought and allows a new high-resolution correlation to oxygen isotope stages. Furthermore, we recognized variations in the rock magnetic parameters that appear to be climatically-driven, with changes in the relative proportion of two magnetic mineral populations with distinct coercivities. Rock magnetic and lithological trends observed in the SEDANO cores indicate that during the climatic cycles of the Late Pleistocene this sector of the peri-Antarctic margin was subjected to subtle, yet identifiable, environmental changes, confirming a relatively higher instability of theWest Antarctic ice sheet with respect to the East Antarctic counterpart.

Recent developments in paleomagnetism and environmental magnetism provide new tools for the detailed correlation of climatically induced magnetic mineralogy changes in sedimentary sequences. Studies of these changes contribute to the reconstruction of climate history for the glacial^interglacial cycles of the Late Pleistocene and to the delineation of the range of natural variability for global climate during the past hundred thousands years. Here we show that sharp coercivity minima observed in fine-grained sediments from the continental rise of the western Antarctic Peninsula correlate to the major rapid cooling events of the northern Atlantic (Heinrich layers). We interpret such an environmental magnetic signal in terms of variations in deep sea diagenetic processes of sulfide formation, which reflect changes in the input of detrital organic matter controlled by sea-ice extent. With the inherent uncertainties in age controls, the sedimentary paleoclimatic markers of the two hemispheres are almost contemporaneous, but interhemispheric time lags or leads of the order of 1-2 kyr (such as those recently reported from the Greenland and Antarctic ice cores) are also compatible with the data.

Two ideal lithologic sections representing a tidal bar system and a fluvial complex were drawn in order to run seismic modeling programs developed by OGS on behalf of the European Community. The simulations allowed an accurate analysis of the seismic expressions of the two sections. The tidal bar system is formed by a number of sandstone lenses interlayered with siltstone and mudstone deposits. These lenses meet together on an erosion surface, while they thin and vanish in the other direction. The fluvial complex is fonned by a limestone basement overlain by coarse alluvial plain sediments which in turn are transgressed by finer flood plain sediments, including sandstone lenses stacking towards the top in a meandering belt. These lithofacies associations represent potential multi-pool reservoirs in which the mudstone layers constitute the plugs. As a function of the granulometric and depositional features of each lithological unit, together with fluid content, wave velocities and densities were evaluated. A 2D modeling for elastic plane wave propagation in these hypothesized geologic sections was run on a Cray supercomputer. The numerical scheme is based on solving the full wave equation by pseudospectral methods.