Ocean Sedimental Records And Their Responses To Climate Change In Prydz Bay, Antarctica

The biogenic material mainly with organic carbon in seawater, particulatematters, sediments and sediment trap samples were analyzed during severalCHINARE cruises in Prydz Bay, Antarctica. The variations of environmentalelements, primary productivity and phytoplankton community in the upper oceans aswell as the source, flux and annual changing tendency of sediment particles and theburial flux of organic carbon in sediments were discussed. Meanwhile, biologicalmarkers were utilized to reconstruct the changes of ocean primary productivity,phytoplankton community and sea surface temperatures, revealing the response ofmarine sediment records to the upper ocean environment and climate changes.The distribution tendency of nitrate, phosphate and silicate in the upper seawaterof Prydz Bay are consistent and their concentrations are low in shallow water with adepth less than50m, while dissolved oxygen is on the contrary. The contents ofparticulate organic carbon decreased from inner Prydz Bay to the outer with thecontent in inner bay is significant larger than that in outer bay. Particulate organiccarbon content also revealed significant interannual variation, which was positivelycorrelated to chlorophyll a, biogenic silicon, abundance of phytoplankton cells and soon, but negatively to nutrients. Particulate organic carbon is mainly fromphytoplankton bloomed in summer, whereas diatom community wherein is the maincontributor with its absolutely dominant position. Moreover there is no nutrient limiton phytoplankton growth in the study area.Over the years, the in situ measurement in Prydz Bay found that the inner shelfshowed the highest chlorophyll a, primary productivity and new productivity thatwere much higher than those in the slope and abyssal region outside the bay.Meanwhile the inner bay revealed relevantly high new productivity and regeneratedproductivity. Primary productivity had a relatively stable trend despite of significantinterannual differences and the actual-measured mean was0.91±0.52gC·m^-2·d^-1,which should be82±47gC·m^-2·a^-1based on90days annually, and the mean of newproductivity was35gC·m^-2·a^-1, showing a higher export productivity flux. Theabundance of phytoplankton cells was the highest in the inner-bay continental shelf where the primary productivity was mainly micro-plankton. Diatom assumed absolutedominance in phytoplankton community with degree of dominance more than80%.Particulate deposition flux showed obvious seasonal variation in inner Prydz Bay.The flux was higher in summer, but lower in all winter. The annual particulatedeposition flux acquired by sediment trap was74.31g·m^-2·a^-1, deposition fluxwherein around90days in summer accounted for66%. The annual organic carbondisposition flux was4.3gC·m^-2·a^-1, accounting for5.2%of primary productivity inthe upper ocean, indicating a higher organic carbon export flux. The biogeniccomponent deposition flux acquired by the trap was the highest in summer and almostnothing in winter, while the terrigenous component deposition flux was higher insummer and slightly lower in winter.The organic carbon and mud content in surface sediments shared the similardistribution trend in Prydz Bay, where revealed more mud sediment and a highercontent of organic carbon in the east inner bay and less contents of organic carbon andmud in the west area. The distribution of organic carbon and biogenic silicon insediments displayed a good consistency which was closely related to phytoplankton inthe upper ocean, and the dominant diatom community was the main contributor forthe sedimentation and burial of organic carbon. Organic carbon in the sediment wasclosely and linearly correlated to total nitrogen with C/N ratio ranging between5.5and8.2, reflecting that the main organic material was ocean biogenic sediment.Organic carbon content decreased with the increase of sediment depth and finallyreached stability at a certain depth. The study area had a high organic carbonpreservation rate and a high carbon sequestration.The sedimentation rates of5cores in Prydz Bay were estimated by210Pb datingmethods, and the mean was1.06mm·a^-1, which was equivalent to the sedimentationrate in the shelf ocean in Antarctic and Arctic oceans. The central inner bay areadisplayed a relatively faster sedimentation rate, and a relatively slow rate atcontinental margin and ice shelf. The sediment mass accumulation rate ranged from313to1254g·m^-2·a^-1and was influenced by terrigenous sand input and biogenicmatter sedimentation. The organic carbon accumulation rate was not only related tomass accumulation rates, but also influenced by the content of organic carbon in sediments. The average organic carbon accumulation rate was4gC·m^-2·a^-1,accounting for4.9%of the primary productivity in the upper ocean, indicating ahigher biogenic pump efficiency in the study area.The contents of biomarkers and biogenic matters such as organic carbon andbiogenic silicon in surface sediments displayed a good consistency. The surface watertemperature calculated by U37kin sediments ranged from-1.85℃to1.44℃in PrydzBay over the past100years. The sea surface temperature was clearly higher during ElNi o period and lower during La Ni a period, showing that it was apparentlyinfluenced by abnormal climate events. Phytoplankton markers were utilized forreconstruction of the changes of phytoplankton community structure which wasinfluenced by El Ni o and La Ni a events. Phytoplankton community changedsignificantly during El Ni o period with significantly increased content of diatom anddecrease of dinoflagellate and the opposite trends during La Ni a period. The contentof biomarker buried in sediments was highly consistent with diatom community,reflecting the response of ocean sediment to marine biogeochemistry process in theupper ocean.The biogenic barium flux in sedimental particulate matters was well consistentwith biogenic matter flux, and the export productivity estimated by flux modelreflected the primary productivity in the upper ocean. The content of biogenic bariumwas positively correlated to biogenic silicon and organic carbon in sediments, andalso closely related to the productivity and phytoplankton in the upper ocean. Theexport productivity in recent100years was estimated by flux model and the resultsindicated that in recent several decades such productivity displayed an increasingtrend which was the same with the sea surface temperature trend and corresponded toEl Ni o event, suggesting that the marine sedimentary records in polar regions can notonly reflect the environmental changes of the upper ocean, but also be closely relatedto global climate changes.