| The continental margin of the Northwest Atlantic is an important area for reconstructing paleoclimate and paleoocean changes.Since the last deglaciation,the Laurentide Ice Sheet(LIS)has experienced a millennium scale advance and retreat,resulting in the discharge of ice sheet meltwater and ice raft debris into the North Atlantic and promoting the deposition of the edge of the continental shelf.Sediments on the Northwest Atlantic continental shelf provide a record of ice sheet ocean atmosphere interactions.In recent years,scholars have focused on the study of LIS meltwater and sedimentary changes on the edge of Scotia in the Northwest Atlantic.At the same time,the edge of the Scotia continental shelf is affected by the Labrador cold current and the Gulf of Mexico warm current,which makes it an ideal area to study the impact of ocean current interaction on its sedimentary change and climate change.The study on the sedimentary dynamics of the Scotia continental shelf is helpful to fill the gap between the interaction of surface currents and Holocene climate change in this area.In this study,three sediment cores 96-20,93-07,and 87-07,collected on the Scotia Shelf and slope during three Hudson cruises,were used to reconstruct the sediment dynamics and interaction between cold and warm currents.Sediment physical properties including the X-radiographic images,magnetic susceptibility,density,sedimentation rates,and digital spectrophotometric reflectance were measured.After the cores were cut in half,core photographs were taken using a Canon Nikon D300 12.3 megapixel digital camera.Sediment bulk,geochemical data,i.e.,elemental concentrations,were obtained using a portable X-ray scanner.The sedimentary facies distribution of each core was identified by integrating the physical property,X-radiography,geochemical data,and core photographs.The calcium carbonate content of core 87-07 was calculated by measuring the total carbon and total organic carbon content.Combined with the total nitrogen data,the carbon to nitrogen ratio was also calculated.The fractions of glycerol dialkyl glycerol tetraether lipids(GDGTs)of core 87-07 were determined by HPLC-APCI-MS(high-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry)and various parameters of organic geochemical proxies were calculated.The 14C-AMS dates in mostly planktonic foraminifera Neogloboquadrina.pachyderma were used to construct the age models of three cores.The depth-age model of the cores was carried out by using the Undatable program in the MATLAB programming language in which CaliB8.1.0 and Marine20 calibration curve were applied.The main results of the study are as follows.:1.Source of sediments:the data show that 13.1-12.9 ka:thesediment facies is dominated by red mud indicating that the Laurentide Ice Sheet receded and the slope sediments were transported as plume and meltwater transported glacial sediments from the western Scotian slope and St.Lawrence Channel.12.9-11.6 ka:a*values,CaCO3(%)content decreases and the sedimentation rate decreases,the sediment facies during this period is dominated by red-brown mudstone and brown mudstones are mainly turbidite sediments,probably related to glacial discharge by the Gulf of St.Lawrence.11.6-8 ka;end of YD,large amounts of glacial meltwater and enhanced Labrador Current,resulting in increased Ca/Ti ratio and CaCO3(%)content.The olive-grey mud phase is semi-pelagic and derived from outer shelf glaciers.8-5 ka:Ca/Ti and K/Ti ratios show a fluctuating decreasing trend,matching the decreasing Ca/Ti ratio on the distant Labrador Shelf,mainly from sedimentary input by the LIS.5-1.8 ka:Ca/Ti and K/Ti ratios show an increasing trend,with an overall sandy texture and sediments derived from shelf glaciers,from the Labrador Current as freshwater plume or iceberg transported southward.1.8-0 ka:relatively low magnetic susceptibility during this period indicates little magnetic minerals in the sediment.Fluctuating decreasing trend in Ca/Ti ratio and slightly different variation in K/Ti ratio,sediment probably originated from nearby river banks of the Emerald Basin.2.Exploration of the applicability of GDGTs:By calculating iGDGT-0/Cren,iGDGT-2/Cren,%GDGT-2,and MI,the results indicate that core 87-07 iGDGTs mainly originated from marine chiguria.Based on BIT,(Ⅲa+Ⅲa’)/(Ⅱa+Ⅱa’)and,C/N,the main source of bGDGT was judged to be input from marine sources.Sea surface temperatures(SST)from the last ice age to the early Holocene on the Scotian Shelf in Atlantic Canada were reconstructed using the TEXL86 indicator for isopentadienyl glycerol dialkyl glycerol tetraether esters(iGDGTs).Due to the lack of specific calibration equations for the Northwest Atlantic region,four widely used calibration equations,namely the TEXL86-SST equations of Seki,Kim and Harning,and the TEX86-SST of Tierney and Tingley,were selected for the calculation of TEX86/TEXL86-SST in this study.The temperature calibration formula of Kim was finally selected for the analysis and comparison,and the SST reflected by core 87-07 is consistent with the UK’37 record and the Greenland ice core δ18O trend in the nearby region,demonstrating the applicability of TEX86 in the Northwest Atlantic region.3.13.1-12.9 ka:The strength and productivity of the bottom currents On the Scotian Shelf are relatively stable.12.9-11.6 ka:During this period,the core 87-07 temperature was low and freshwater generated a strong pressure gradient at the shelf edge,which promoted the enhancement of the Labrador Current.11.6-8 ka:The CaCO3(%)content also showed an increasing trend between 10.1 and 8.9 ka,with five anomalies at 10.04 ka,9.64 ka,9.45 ka,9.3 and 8.9 ka indicating that the ice meltwater event had some influenced on the study area.9.4-8.0 ka:Temperature,Ca/Ti and K/Ti ratios showed an increasing trend during this period,and the sortable silt((?))showed a decreasing trend,indicating a decrease in the Labrador Current intensity.8.0-5.0 ka:Ca/Ti and K/Ti ratios showed the temperature on the Scotian Sshelf differs from that recorded by the Greenland ice cores,with a decreasing trend of about 4℃ during this period,possibly due to a decrease in solar intensity,enhanced convection in the Labrador Sea,and a shift of the Gulf Stream path toward the equator that may have contributed to the cooling.5.0-1.8 ka:The Scotian shelf,the (?)indicates a gradual decrease in LC activity and a decreasing trend in temperature.1.8-0 ka:1.8-0.5 ka;this period shows a decreasing trend in temperature in the study area and a decreasing trend in Ca/Ti and K/Ti ratios,and an increasing trend in the SS,indicating an increase in the LC intensity. |
