Lucchi, Renata Giulia

A reconstruction of the last 2,000 years BP of environmental and oceanographic changes on the western margin of Spitsbergen was performed using a multidisciplinary approach including the fossil assemblages of diatoms, planktic and benthic foraminifera and calcareous nannofossils and the use of geochemistry (X-ray fluorescence spectroscopy, X-ray diffraction). We identified two warm periods (2,000–1,600 years BP and 1,300–700 years BP) that were associated with the Roman Warm Period and the Medieval Warm Period that alternate with colder oceanic conditions and sea ice coverage occurred during the Dark Ages (1,600–1,300 years BP) and the beginning of the Little Ice Age. During the Medieval Warm Period the occurrence of ice-rafted debris and Aulocoseira spp., a specific diatom genus commonly associated with continental freshwater, suggests significant runoff of meltwaters from local glaciers.

Reconstruction of geomagnetic field changes has a strong potential to complement geodynamo modeling and improve the understanding of Earth’s core dynamics. Recent works based on geomagnetic measure-ments pointed out that over the last two decades the position of the north magnetic pole has been largely determined by the influence of two competing flux lobes under Canada and Siberia. In order to understand if the waxing and waning of magnetic flux lobes have driven the path of geo-magnetic paleopoles in the past, we present an augmented and updated record of the chronology and paleosecular variation of geomagnetic field for the last 22 kyr derived from sedimentary cores collected along the north-western margin of Barents Sea and western margin of Spitsbergen (Arctic). The path of the virtual geomagnetic pole (VGP) has been reconstructed over this time period and compared with the maps of the radial component of the geomagnetic field at the core-mantle boundary, obtained from the most recent models. The VGP path includes centuries during which the VGP position is stable and cen-turies during which its motion accelerates. We recognize both clockwise and counterclockwise VGP paths, mostly developing inside the surface projection of the inner core tangent cylinder in the Arctic region. The VGP path seems to follow the appearance of Brpatches of normal magnetic flux, especially those located under Siberia and Canada areas, but also those that may cause peculiar paleomagnetic features such as the Levantine Iron Age Anomaly.

Water masses and depositional environments over the last 500 ka were reconstructed using absolute and relative abundances of lithogenous, biogenous and redox-sensitive elements in four sediment cores from two channel-levee systems of the Wilkes Land continental slope (East Antarctica). Sediments older than the Mid-Bruhnes event (MBE, 430 ka BP) show reduced glacial/interglacial variability in the abundance of elements associated to the terrigenous mineral phases (i.e. Al, Ti, Fe and partly Si). This suggests minor ice-sheet size changes occurred in the Antarctic margin during the pre-MBE “lukewarm” interval. Post-MBE sediments record instead a high variability between glacial and interglacial periods in the concentration of terrigenous and biogenous (i.e. Ca, Ba) elements suggesting larger amplitude changes in both ice-sheet size and ocean conditions toward the gradual establishment of last glacial cycle conditions. Moreover, a marked increase of Mn during the glacial to interglacial transitions, indicates a post-depositional migration of the redox front and re-oxidation of the surface sediment layers linked to major changes in bottom water oxygen conditions associated to Antarctic Bottom Water formation along the margin at the onset of deglaciations.

Reconstruction of Paleomagnetic Secular Variation (PSV) of the geomagnetic field is fundamental both to assess geodynamo models and to obtain age constraints for rocks, sediments and archaeological material. We present refined age-calibrated Holocene PSV and relative paleointensity (RPI) stack curves derived from Arctic marine sediments (Northwestern Barents Sea). The Holocene sections of four sedimentary cores were correlated on the basis of paleomagnetic trends and age models, and stacked. The resultant composite PSV and RPI Holocene records (NBS stack) and the reconstructed Holocene Virtual Geomag- netic Pole (VGP) path were evaluated in comparison with the most recent paleomagnetic stack curves and geomagnetic field models. The data indicate that during the Holocene time, the VGPs moved within the superficial projection of the inner core tangent cylinder, with the exception of short time intervals around 5600 and 3200 cal yr BP when VGPs extended to lower latitudes. These deviations might reflect regional geomagnetic features, such as persistent geomagnetic flux lobes at core-mantle boundary. Our data confirm that the large VGP shift observed around 5600 cal yr BP is the result of an increased radial magnetic field at the core-mantle boundary over North America, whilst the VGP shift around 3200 cal yr BP represents a major swing to middle latitudes toward the Middle East and might be associated to a regional high paleointensity peak, known as Levantine Iron Age Anomaly (LIAA).

We present a paleomagnetic and rock magnetic data set from two long sediment cores collected from Bellsund and Isfjorden contourite drifts located on the eastern side of the Fram Strait (western Spitsbergen margin). The data set gave the opportunity to define the behavior of the past geomagnetic field at high latitude and to constrain the palaeoclimatic events that occurred in a time framework spanning marine isotope stage 3 to the Holocene. A high‐resolution age model was reconstructed by coupling 26 radiocarbon ages and high‐resolution relative paleointensity and paleosecular variation of the geomagnetic field records for the last 60 kyr. We show the variation of the geomagnetic field at high latitudes, pointing out variability during periods of regular paleosecular variation (normal polarity) as well as during the most recent geomagnetic excursions, and we provide a high‐resolution record of the Laschamps excursion. Cross‐cores correlation allowed us to outline major, climate‐related, sedimentary changes in the analyzed stratigraphic sequence that includes the meltwater events MWP‐1a and MWP‐19ky, and the Heinrich‐like events H1, H2, H4, and H6. This contribution confirms that rock magnetic and paleomagnetic analysis can be successfully used as a correlation and dating tool for sedimentary successions at high latitudes, where accelerator mass spectrometry dates and oxygen isotope analyses are often difficult to obtain for the scarcity of calcareous microfossils and the uncertainties related to data calibration may be significant, as well as the complexity of water mass characteristics and dynamics through climate changes.

Paleomagnetic and rock magnetic data were measured on glaciomarine silty-clay successions along an E-W sediment-core transect across the continental shelf and slope of the Kveithola paleo-ice stream system (south of Svalbard, north-western Barents Sea), representing a stratigraphic interval spanning the last deglaciation and the Holocene. The records indicate that magnetite is the main magnetic mineral and that magnetic minerals are distinctly less abundant on the shelf than at the continental slope. The paleomagnetic properties allow for the reconstruction of a well-defined characteristic remanent magnetization (ChRM) throughout the sedimentary successions. The stratigraphic trends of rock magnetic and paleomagnetic parameters are used for a shelf-slope core correlation and sediment facies analysis is applied for depositional processes reconstruction. The new paleomagnetic records compare to the PSV and RPI variation predicted for the core sites by a simulation using the global geomagnetic field variation models SHA.DIF.14k and CALS7K.2 and closest PSV and RPI regional stack curves. The elaborated dataset, corroborated by available 14C ages, provides a fundamental chronological framework to constrain the coupling of shelf-slope sedimentary processes and environmental changes in the NW Barents Sea region during and after deglaciation.

The sedimentary sequence deposited during the deglaciation phase following the last glacial maximum in the Storfjorden trough, on the northwestern Barents Sea south of Svalbard, was sampled with 10 piston and gravity cores during the SVAIS and EGLACOM cruises. Three cores (SV-02, SV-03 and SV-05) collected on the upper continental slope are characterized by a thin (20–40 cm) Holocene interval and a thick (up to 4.5 m in core SV-03) late Pleistocene sequence of finely laminated fine-grained sediments that have been interpreted as plumites deposited during the Melt Water Pulse 1a (MWP-1a). Radiocarbon ages obtained at the top and bottom of this stratigraphic interval revealed that deposition occurred during less than two centuries at around 15 ka ago, with a very high sedimentary rate exceeding 3 cm a−1. We studied the palaeomagnetic and rock magnetic properties of this interval, by taking magnetic measurements at 1 cm spacing on u-channel samples collected from the three cores. The data show that this sequence is characterized by good palaeomagnetic properties and the palaeomagnetic and rock magnetic trends may be correlated at high resolution from core to core. The obtained palaeomagnetic data therefore offer the unique opportunity to investigate in detail the rate of geomagnetic palaeosecular variation (PSV) in the high northern latitudes at a decadal scale. Notwithstanding the palaeomagnetic trends of the three cores may be closely matched, the amplitude of directional PSV and the consequent virtual geomagnetic pole (VGP) scatter (S) is distinctly higher in one core (SV-05) than in the other two cores (SV-02 andSV-03). This might result from a variable proportion of two distinct populations of magnetic minerals in core SV-05, as suggested by the variable tendency to acquire a gyromagnetic remanent magnetization at high fields during the AF demagnetization treatment. For the plumite interval of cores SV-02 and SV-03, where the magnetic mineralogy is uniform and magnetite is the main magnetic carrier, a S value of about 9◦ is obtained. We consider this value as a reliable approximation of palaeomagnetic secular variation at a latitude of 75◦N over a time interval spanning a couple of centuries around 15 ka ago.

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.