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Late Quaternary sediments from deep-sea sediment drifts on the Antarctic Peninsula Pacific margin: Climatic control on provenance of minerals

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Late Quaternary sediments from deep-sea sediment drifts on the Antarctic Peninsula Pacific margin: Climatic control on provenance of minerals
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    1 Late Quaternary sediments from deep-sea sediment drifts on the Antarctic 1 Peninsula Pacific margin: chronostratigraphic framework and magnetic 2 mineralogy conundrum 34Alessandra Venuti (1), Fabio Florindo (1), Andrea Caburlotto (2), Mark W. Hounslow (5), Claus-5Dieter Hillenbrand (4), Eleonora Strada (1,3), Franco M. Talarico (3), Andrea Cavallo (1)67 (1) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy 8 (2) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/c, 34010 Sgonico (TS), 9 Italy 10 (3) Dipartimento di Scienze della Terra, Università di Siena, Via del Laterino 8, 53100 Siena, Italy 11 (4) British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, United Kingdom 12 (5) Lancaster Environmnent Centre, Lancaster University, Farrer Ave., Lancaster LA1 4YQ, United Kingdom 1314  Keywords: Antarctic Peninsula, Pacific margin, sediment drift, late Pleistocene, Mineral15magnetism, relative palaeointensity1617 Abstract We present results of detailed paleomagnetic investigations on deep-sea cores from18sediment drifts located along the Pacific continental margin of the Antarctic Peninsula. High-19resolution magnetic measurements on u-channel samples provide detailed age models for three of 20these cores collected from Drift 7, documenting an age of 122 ka for the oldest sediments recovered21near the drift crest at site SED-07 and a high sedimentation rate (11 cm/kyr) at site SED-12 located22close to the Alexander Channel system. Low- and high-temperature magnetic measurements in23conjunction with microscopic and mineralogic observations from drifts 4, 5 and 7 indicate that24 pseudo-single domain detrital titanomagnetite (partially oxidized and with limited Ti-substitution)25is the dominant magnetic mineral in the drift sediments. The titanomagnetite occurs in two26magnetic forms: i) a low coercivity form similar to laboratory-synthesized titanomagnetite, and ii) a27    2high coercivity form (B cr  > 60 mT), which is probably present as inclusions or lamellar 28intergrowths in composite silicate or Fe-Ti-oxide grains. These two forms vary in amount and29stratigraphic distribution across the drifts. We did not find evidence for the presence of diagenetic30magnetic iron sulfides as has been previously suggested for these drift deposits. The observed31change of magnetic mineralogy in sediments deposited during Heinrich-events on Drift 7 probably32relates to warming periods, which temporarily modified the normal glacial transport pathways of 33glaciogenic detritus to and along the continental rise and thus resulted in deposition of sediments34with a different provenance. Understanding this sediment provenance delivery signature at a wider 35spatial scale should provide information about ice-sheet dynamics in West Antarctica over the last36~100 kyr.3738 1. Introduction 39A system of twelve giant deep-sea sediment drifts that are separated by large channels, eroded by40turbidity currents, characterizes the continental rise west of the Antarctic Peninsula (Figure 1).41These drifts provide the most proximal continuous sedimentary records of late Neogene to42Quaternary ice-sheet dynamics in this part of West Antarctica [  Barker et al. , 1999b; 2002]. The43sediment drifts are up to 300 km long, 100 km wide and 1 km thick, with the main axis elongated44 perpendicular to the continental margin [e.g.,  Rebesco et al. , 1996; 1997; 2002; 2007;  Barker et al.,  451999a;  Amblas   et al  ., 2006; Uenzelmann-Neben , 2006]. The drifts are part of a complex glacial46sedimentary feeder-dispersal system composed of lobes and troughs on the outer shelf, a steep47continental slope and deep-sea channels that separate the drifts on the upper rise [e.g.,  Rebesco et  48 al  ., 1998;  Hernández-Molina et al., 2006].49During the 1990’s, these drifts were intensely investigated by the SEDANO program (SEdiment50Drift of the ANtarctic Offshore) of the Italian Programma Nazionale di Ricerche in Antartide51(PNRA), the British Antarctic Survey (BAS), and the Ocean Drilling Program (ODP) Leg 17852[  Barker et al. , 1999b; 2002]. During two SEDANO cruises with the R/V OGS-Explora (1995/1997-53    398), seismic surveys, gravity coring, and moorings with current meters and sediment traps yielded54information about the morphology and mechanisms of deposition along the Pacific continental55margin [e.g., Camerlenghi et al. , 1997a,b;  Rebesco et al  ., 1997, 2002, 2007;  Pudsey and  56 Camerlenghi , 1998;  Harland and Pudsey , 1999;  Lucchi et al  ., 2002; Giorgetti et al  ., 2003 ; Villa et  57 al., 2003; Lucchi and Rebesco , 2007]. The SEDANO program collected 17 gravity cores from Drift587 and an additional two cores at Drift 4 [  Lucchi et al., 2002], while BAS recovered 11 piston cores59from the drifts located to the NE of Drift 7 on cruise JR19 with RRS  James Clark Ross [  Pudsey,  602000].61Paleomagnetic and mineral magnetic studies were conducted by Sagnotti et al. [2001] and  Macrì   et  62 al. [2006] on seven gravity cores (SED-02, SED-04, SED-06, SED-14, SED-15, SED-16, and SED-6317) collected on Drift 7 (Figure 1). The ages of these sequences were determined using relative64 paleomagnetic intensity (RPI) [ Sagnotti et al., 2001;  Macrì   et al. , 2006], with biostratigraphic and65chemostratigraphic information providing additional age constraints [  Pudsey and Camerlenghi,  661998 ; Lucchi et al., 2002; Villa   et al. , 2003]. The RPI-based high-resolution age models established67 by Sagnotti et al. [2001] and  Macrì   et al. [2006] suggested a link between the occurrence of 68intervals with magnetic coercivity minima in Drift 7 and the simultaneous deposition of layers rich69in iceberg-rafted debris (IRD) in the North Atlantic [so-called ‘Heinrich layers’, e.g.,  Heinrich ,701988;  Hemming, 2004] during major rapid cooling events between ~12 kyr and ~60 kyr ago. In71cores from Drift 7, a variable mixture of detrital pseudo-single domain (PSD) magnetite (Fe 3 O 4 )72and fine-grained monoclinic pyrrhotite (Fe 7 S 8 ) was inferred by Sagnotti et al. [2001], with73magnetite being the main magnetic mineral [cf.  Macrì et al., 2006]. Contrary to this mineralogical74interpretation,  Hawkes et al. [2003] identified in sediment cores from drifts 3, 4, 4a and 5 only75magnetite. A full understanding of the magnetic mineral srcin of the remanence is important,76 because the diagenetic formation of an iron sulfide component may have delayed the magnetic field77recording process to some time after deposition [e.g.,  Florindo and Sagnotti, 1995 ; Roberts et al., 782005 ; Sagnotti et al., 2005 ; Florindo et al., 2007 ; Roberts et al., 2010a].79    4The main objectives of our study are to: (1) provide robust identification of the main magnetic80carriers in the drift sediments, which is important for understanding the paleomagnetic recording81 process, and (2) construct a robust chronostratigraphic framework for cores SED-7, SED-12, and82SED-13, which is which is important for reconstructing Late Pleistocene paleoenvironments of the83Antarctic Peninsula. The Antarctic Peninsula may represent the best natural laboratory to84investigate the interaction of atmosphere, hydrosphere and cryosphere and the response of Earth's85climate system to rapid regional warming [e.g.,  Florindo and Siegert, 2009]. Over the past 50 years,86the Antarctic Peninsula has experienced the greatest atmospheric temperature increase on Earth,87rising by nearly 3°C [e.g.,  King et al., 2003 ; Turner et al., 2005], which is approximately 10 times88the mean rate of present global warming [IPCC, 2007].89We have investigated the paleomagnetic properties of two gravity cores (SED-12 and -13) located90near the Alexander Channel system and of two cores (SED-07 and -14) located near the crest of 91Drift 7 (Figure 1). We also examined the spatial distribution of magnetic minerals in the Drift 792sediments, and compared these results to the magnetic mineralogy in sediments recovered from93drifts 4 and 5 located to the NE of Drift 7.9495 2. Oceanographic setting, site locations and core lithostratigraphy 96The Pacific continental margin of the Antarctic Peninsula is under the influence of the Southern97Boundary (SB) of the Antarctic Circumpolar Current (ACC), which is a massive eastward flowing98wind-driven current. The ACC plays an important role in the thermohaline circulation and heat99 budget of the world ocean (Figure 1) [e.g.,  Barker et al., 2007 ; Carter et al. , 2009]. Surface and100deep-water masses north of the SB are located within the Antarctic Zone, i.e. within the clockwise101flowing ACC. Nevertheless,  Hillenbrand et al. [2008a] showed that in the area of investigation the102ACC only affects ocean circulation above about 1000 m water depth, whereas today, and for most103of the late Neogene and Quaternary, a generally southwestward flowing bottom current affected the104deposition on the drifts. Surface water currents on the continental shelf to the south of our study105    5area are driven by the westward flowing Antarctic Coastal (or Polar) Current, where the direction of 106flow is mainly controlled by the orientation and shape of the Antarctic coast (Figure 1).107The large hemipelagic sediment drifts on the Pacific margin of the Antarctic Peninsula are108characterized by an asymmetric cross-section, with a steep southwestern side and a gently sloping109northeastern side (Figure 1). Based on both multi-channel seismic profiles [  Rebesco et al. , 1996;1101997; 2007] and core analysis [  Pudsey and Camerlenghi , 1998;  Pudsey , 2000;  Lucchi et al  ., 2002;111  Hillenbrand and Ehrmann , 2005], this geometry is interpreted to result from the interplay between112turbidity and bottom currents: fine-grained particles supplied by turbidity currents running through113the channels spill over the channel banks (in particular the western banks), and are entrained in the114SW-flowing bottom contour currents and so are redeposited over the continental rise, thereby115contributing to the growth of inter-channel sediment drifts [e.g.,  Rebesco et al  ., 2007]. The116sedimentary successions in cores from the gentle NE slope of Drift 7 facing the Alexander Channel117system (Figure 1) were therefore directly influenced by both turbidity and contour currents [  Lucchi 118 et al  ., 2002].119Up to nine lithostratigraphic units, named A to I, have been identified within the mid to late120Pleistocene succession of Drifts 1 to 7 [  Pudsey and Camerlenghi, 1998 ; Pudsey , 2000;  Lucchi et  121 al  ., 2002]. The lithostratigraphic units were correlated by magnetic susceptibility profiles, clay122mineral assemblages, sediment facies and textural characteristics, and were assigned to marine123isotope stages (MIS) 1 to 11 on the basis of bio- and chemostratigraphy and excess 230 Thorium124activity [  Pudsey and Camerlenghi , 1998;  Pudsey , 2000;  Lucchi et al  ., 2002; Villa et al., 2003].125Interglacial units consist of brown bioturbated hemipelagic muds with microfossils (diatoms,126foraminifera, radiolaria, calcareous nannofossils) and IRD, while glacial units consist of olive-grey127to grey, laminated terrigenous muds [  Pudsey and Camerlenghi , 1998;  Pudsey, 2000;  Lucchi et al  .,1282002; Villa et al., 2003]. Tephra layers with a trachytic composition   occur within lithostratigraphic129units D and C (latest MIS 6 and early MIS 5) [  Pudsey and Camerlenghi, 1998;  Pudsey , 2000;130
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