Variation And Controlling Factors Of Shell Weights Of Planktonic Foraminifera In The Tropical Western Pacific During The Last250Kyr

Shell weights of planktonic foraminifera can be used as a proxy to reconstructsurface water [CO32-] in the past, and therefore provide clues on ocean carbon cycle.Shell weights of planktonic foraminifera are measured from tropical western Pacificsediment core WP7and MD06-3054. Drawbacks of previous cleaning procedure isanalysed and an improvement is proposed. The data also revealed the variations andcontrolling factors of the shell weights in the central of tropical western Pacific warmpool during the last250kyr and in the margin region of tropical western Pacific duringthe last27kyr.Sonification in shell weight proxy method is necessary for the cleaningprocedure to remove detritus trapped within the shell chambers. Previous procedurewith a long sonic cleaning time(40kHz,8s) in deionized water leads to shell breakup,biasing the ensemble of surviving shells toward the heavier end of the spectrum.Furthermore, the tendency toward breakup is greater for samples which haveundergone significant dissolution. As the sonic cleaning efficiency of sodiumhexametaphosphate solution is much greater than deionized water, the previouscleaning procedure is amended to sonification(40kHz,8s) in2%sodiumhexametaphosphate solution. The amended cleaning procedure provides a greaterdetritus cleaning efficiency and avoids shell breakup, making the shell weight data more reliable.Shell weights of surface dweller G. sacculifer measured from a tropical westernPacific sediment core WP7during the last250ka generally response to changes ofCO2concentrations in Antarctic Ice Core Vostok, which reveals that shell weights aresensitive to surface water [CO32-], except in MIS1and MIS4. By correlating with therecord of palaeoproductivity from the same core, shell weights of G. sacculifer areaffected, but not exclusively, by El Ni o-Southern Oscillation processes. Tropicalwestern Pacific carbonate dissolution events which may exist above the lysoclinehave imprinted in shell weights of G. sacculifer during the last250ka, which meansshell weights of G. sacculifer mainly controlled by surface water [CO32-] are alsoaffected by bottom water [CO32-]. Furthermore, shell weights of G. sacculifer can alsobe related to environmental and biological factors other than [CO32-]. All these abovereasons result in the exceptional shell weights of G. sacculifer in MIS1and MIS4.Shell weights of subsurface dwellers P. obliquiloculata and N. dutertrei aremeasured from a western Pacific warm pool sediment core WP7during the last250ka. Shell weights of N. dutertrei exhibit an excellent inverse relationship with CO2concentrations in Antarctic Ice Core Vostok，which reveals that calcification ofplanktonic foraminifer is sensitive to the concentration of atmospheric carbon dioxideand shell weights of N. dutertrei can be used as a proxy to reconstruct subsurfacewater [CO32-] in the past, and therefore provide clues about ocean carbon cycle andeven global climate change. Shell weight of P. obliquiloculata is a function ofmultiple and not atmospheric CO2exclusively. Furthermore, the significantly lowshell weights of G. sacculifer and N. dutertrei show the bottom water [CO32-] drops to60-70μmol/kg in the position of core WP7during MIS4, which may reflects a greaterpenetration of North Pacific Deep Water (NPDW) into the intermediate water ofwestern equatorial Pacific.Shell weights of surface dweller G. sacculifer measured from a tropical westernPacific sediment core MD06-3054during the last27ka were higher at times ofnorthern high latitude cooling, which means surface seawater pCO2reduced and theeffect of CO2rich cold water from tropical eastern Pacific decreased. This pattern implies shifts in the tropical Pacific to a persistent El Ni o–like conditions.