Sedimentary Characteristics And Paleoenvironmental Evolutions In The Northeastern Seychelles Islands Since The MIS9 Period

Based on 34 surface sediment materials and a gravity core from the northeastern area of Seychelles Islands in the equatorial western Indian Ocean,the geochemistry,clay mineralogy,organic carbon and nitrogen analyses were carried out in this paper.A reliable core age model was established based on the foraminiferal AMS¹⁴C dating and stable oxygen isotope records for about 340,000 years back to the MIS9.Based on the understanding of the temporal and spatial distribution patterns of sedimentology,geochemistry and mineralogy,the contribution of terrestrial materials and volcanic hydrothermal materials to the northeastern area of Seychelles Islands and the transport mechanism were identified using a multivariate index system,the applicability of productivity indicators established based on geochemistry was focused on,the paleoproductivity evolution pattern and its main controlling factors since the MIS9 phase were explored,and the glacial-interglacial cyclonic paleoenvironmental evolution of the study area was recovered.The surface sediments of the northeastern area of Seychelles Islands are mainly siliceous clay and foraminiferous clay.The clay minerals are mainly illite,palygorskite,smectite,kaolinite,chlorite 5 types.The high content of palygorskite reflects that the study area receives a wide range of wind and dust contributions from the surrounding continental deserts.The content of Ca is highest in major elements,followed by Si and Al,and P,Ti and Mn with an average content of less than 3%.The highest contents of trace elements are Ba and Sr,while the average contents of Cu and Ni are less than 70μg/g.The PAAS-normalised REE patterns shows weak positive Europium（δEu）,indicating aeolian dust input signal.The results of the cluster analysis divide the study area into two zones from north to south,the north part of the study shows a significant dust input signal,South Asian monsoon is the major driving force.While the southwest part receives the contribution of weathered material from the Seychelles Plateau,which controlled by the South Equatorial Counter Current.The overall low percentage of total organic carbon and nitrogen in the surface sediments may be related to the fact that the area is far from the continent,and it is difficult to obtain the huge amount of land-sourced detrital materials carried by rivers.The spatial distribution pattern of organic carbon and nitrogen is consistent with the transport path of wind and dust materials,the adsorption of organic matter by clay minerals imported from dust is an important factor affecting the distribution of total organic carbon（TOC）content in the study area.The application of TOC content as a productivity proxy is not well in this study area.The spatial distribution of calcium carbonate（Ca CO₃）,organic phosphorus(P_org),and Sr/Al content indicates a significantly higher productivity in the southern part of the study area than in the northern part at about 4°S.This reflects the contribution of strong upwelling activity along the Seychelles-Chagos Thermocline Ridge to the primary productivity of the sea surface,and also indicates that the contribution of wind and dust input to marine productivity in this area is limited.The overall U/Th ratio in the study area is less than 0.75,and U_authigenic<5μg/g indicates that the sediment-water interface in the study area is an oxide condition,indicating that Antarctic bottom water（AABW）near 3800 m water depth enhances deep-water ventilation in the study area,and the Ti-standardized ratios of U,Mn,and Cu and their enrichment factors(U_EF,Mn _EF,and Cu _EF)showed gradual rise to the south indicating relatively lower oxidation levels in the southern part of the study area,which we believe reflects the higher level of primary productivity in the south and the consumption of dissolved oxygen in the bottom water by organic matter degradation.Since MIS9,the higher relative percentages of palygorskite,Ti/Al,Mg/Al,and Rb/Al ratios indicating high dust input during the glacial period.The TOC and TN contents are very low and the variation pattern is similar to the dust input strength which further confirms that the variation of TOC content in this area should be mainly related to dust input.The low ratios of Ca CO₃,P_org,Ba/Al and Sr/Al indicate that the paleoproductivity of this study area is low during the glacial period,resulting from the Indonesian Throughflow weakened and the depth of the thermocline became shallow,the stratification of the upper water column intensified,preventing the transport of organic matter from the surface layer to the bottom layer;the increase of redox-sensitive elements（U,V,Mn）and their enrichment factors indicate that the dissolved oxygen content of bottom water in the study area was relatively higher in the glacial period.The relative percentages of palygorskite,Ti/Al,Mg/Al,Rb/Al ratios and other wind and dust material input indicators of CJ01-21 core in the interglacial period are low,and the paleoproductivity indicators as Ca CO₃,P_org,Ba/Al,Sr/Al,Cd/Al ratios increase,indicating that the strong summer monsoon and rising sea surface temperatures during the interglacial period prompted heat transfer from the upper water column to the subsurface,and the Indonesian Throughflow brought warm and highly saline seawater from the Pacific Ocean into the East Indian Ocean,which increased the depth of the thermocline ridge and promoted the surface primary productivity;the redox-sensitive elements（U,V,Mn）and their enrichment factors were relatively high,indicating that the primary productivity level increased during the interglacial period,and the dissolved oxygen consumption in bottom water intensified,leaving the bottom water in a relatively low oxygen state.