Shell Weight Of Planktonic Foraminifer Neogloboquadrina Pachyderma (Sinistral) In The Antarctic Zone Of The Southern Ocean:Controlling Factors And Paleooceanographic Applications

Neogloboquadrina pachyderma(sinistral)is the dominant planktonic foraminiferal species in the mid-high latitudes.However,the controlling factors on the calcification of this species and its role in the Southern Ocean carbon cycle,are poorly understood.In this study,the particle Size-normalized weight(SNW)measurement of the planktonic foraminifer N.pachyderma(sin.)in the surface sediments in the Antarctic Zone of the Southern Ocean reflects the calcification degree of this species,and the correlation analysis with modern marine environmental factors was carried out to explore the controlled factors of N.pachyderma(sin.)calcification process and its effect on carbon cycling in the Southern Ocean.The established SNW-subSST empirical formula of N.pachyderma(sin.)in the Antarctic Zone of modern Southern Ocean was applied to the sediments of the A3-05 core in the Amundsen Sea in the Antarctic Zone to explore the subsurface temperature variation and its paleooceanography application in this area.We opted for a shell cleaning procedure of 1-2 s sonication step in 2%Na6P6O18 solution for the SNW measurement,as this cleaning method ensures a detritus cleaning effect,minimizes the shell breakage,thereby bolstering the robustness of the SNW data.Then,the present study utilized the improved cleaning method to estimate the SNW of N.pachyderma(sin.)in surface sediments from the Amundsen Sea,the Ross Sea,and the Prydz Bay in the Antarctic Zone of the Southern Ocean.It was found that SNW of N.pachyderma(sin.)is not controlled by deep-water carbonate dissolution postmortem,and can be therefore,used to reflect the degree of calcification.The comparison between N.pachyderma(sin.)SNW and environmental parameters(temperature,salinity,nutrient concentration,and carbonate system parameters such as pH and DIC,etc.)in the calcification depth revealed that N.pachyderma(sin.)SNW is significantly and positively correlated with seawater temperature.The results showed that the SNW of N.pachyderma(sin.)with different particle size ranges(such as 200250 μm particle size range)was related to the annual average temperature,salinity,nitrate concentration,silicate concentration,phosphate concentration,DIC;pH,pCO2,and[CO32-]in 125 m water depth were 0.79,0.51,0.73,0.02,0.35,0.56,-0.54,0.58,and-0.26,respectively)according to the significance test(p value),after excluding autocorrelation Its SNW has the highest correlation with seawater temperature.Based on the relationship between the SNW and temperature of N.pachyderma(sin.),it is expected that the CaCO3 deposition flux in the Southern Ocean Antarctic will increase by～30%for every 1℃ increase in seawater temperature.The enhanced calcification of N.pachyderma(sin.)in the Southern Ocean Antarctic zone will weaken the ocean carbon sink,thereby generating a positive feedback for global warming.Finally,based on the SNW-temperature empirical formula of modern N.pachyderma(sin.)in the Antarctic Zone of the Southern Ocean,the subsurface seawater temperature changes in the Amundsen Sea since 70 ka were reconstructed.The results show that the subsurface seawater temperature in the Antarctic Zone of the Southern Ocean has changed from-0.4 to-2℃ since 70 ka.We preliminarily judge that the evolution of subsurface seawater temperature in the Amundsen Sea is mainly affected by the upwelling of the circumpolar deep water,that is,the westerly winds drive the ACC to carry warm CDW and MCDW upwelling and intrude into the ice shelf,causing the Amundsen Sea area of the Southern Ocean subsurface seawater temperature changes.Based on the SNW-subSST empirical formula of modern(sin.)in the Antarctic Zone of the Southern Ocean,the subsurface seawater temperature changes in the Amundsen Sea since 70 ka were reconstructed.The results show that the subsurface seawater temperature in the Amundsen Sea has changed from-0.4 to-2℃since 70 ka.On the orbital time scale,the subsurface seawater temperature showed a gradual upward trend from 73 to 32 ka,and decreased slightly since 32 ka;Peaks during Antarctic warm events(A1-A5)recorded in Byrd ice cores.Preliminary studies show that sea ice does not have an obvious effect on the subsurface seawater temperature in this area,while the upwelling of the circumpolar deep water dominates the subsurface seawater temperature evolution in this area:that is,the ACC driven by the westerly winds was brought to the study area after the warm water surged around the extremely deep water,and invaded the Getz Ice Shelf near the Amundsen Sea,causing the temperature of the subsurface seawater in the sea area to change.