Marine Paleoproductivity And Organic Carbon Burial In The Northwest Pacific Over The Last 400ka With Its Environmental Implications

In this paper,based on the AMS¹⁴C dating and planktonic foraminiferal?¹⁸O records of the gravity core(KPR12,647 cm length)in the northern Kyushu-Palau Ridge in the Northwest Pacific Ocean,a reliable age model was established for over the last 400ka.This study focuses on the orbital-scale changes of organic carbon burial and oceanic paleoproductivity and their responses to the variation of terrestrial inputs and marine environment,and also discusses the influence factors and mechanism of the paleoproductivity and preliminarily establish a conceptual model of organic carbon burial and preservation and its environmental response in the glacial-interglacial cycles.The results show that the total organic carbon(TOC)content in the sediments of core KPR12 is relatively low(0.062%-0.443%,average of 0.216%).And the flux of TOC burial is between 0.82-11.74 mg/cm2/ka,which is equivalent to the open ocean but 1-2orders of magnitude lower to the marginal sea area.This may be related to the fact that there is almost no direct input of massive terrestrial organic carbon from large rivers and the relatively low marine productivity in the study area.The orbital cycle trends of TOC content and burial flux in the core are higher during the glacial periods and lower during the interglacial periods.Based on the ratio of C/N and?¹³C of organic carbon,it is estimated that the organic matter in this area is mainly derived from both marine biological source and terrestrial inputs,while the contribution of marine biological source is averaged?66%.Moreover,the terrestrial organic carbon in the study area is mainly transported by eolian dust input and rivers from the Kyushu Island in Japan.Meanwhile,under the common impact of the intensity of Kuroshio Current and sea level change,it shows a higher flux of terrestrial organic matter burial during the glacial periods.The silicon productivity in the study area is really low(the content of opal in core KPR12 is?1%),without obvious glacial cycle characteristics.However,the content of calcium carbonate(Ca CO₃)and biological barium(Bio-Ba)indicate that the viriation of paleoproductivity is characterized by cycles of higher during the interglacial periods,which is contrary to the change of TOC burial in this core and the eolian dust transport from the Asian continent,indicating that TOC as an index of paleoproductivity has limitations and the promotion effect of dust input on marine productivity in the study area may be limited.The flux of primary productivity of this area is reconstructed based on the Bio-Ba content,which is lower overall(105mg/m²/ka during the glacial periods and 128mg/m²/ka during the interglacial periods).This may be due to the low concentration of surface nutrients in this area.In addition,the formation of the North Pacific Intermediate Water(NPIW)during the glacial periods leads to the obvious stratification of water,hinders the transport of bottom nutrients to the surface,and thus inhibits the increase of marine productivity.The enrichment characteristics of sedimentary redox sensitive elements(RSEs)indicate that the sediment-water interface in this area is in a relative oxidation condition.The redox condition and the contents of fine particle matter in the sediment-water are different from the glacial to interglacial periods.Specifically,the degree of enrichment of RSEs(V,Th,Tl,Ga,etc.)is higher during the glacial periods,indicating that the dissolved oxygen in the sediment-water interface is relatively low,which is more conducive to the burial and preservation of organic matter.At the same time,fine particle matter adsorbs organic matter and accelerates its migration to the seabed subsidence,which protects the organic mattar from microbial degradation,then the TOC burial efficiency is higher in glacial periods.On the contrary,during the interglacial periods,the dissolved oxygen content in the sediment-water interface may be higher,the fine clay minerals input decreased,and the degradation of organic matter by microorganisms in the water column strengthened,thus the TOC burial decreased.Based on the study in this paper,TOC burial in the study area has a significant orbital scale sensitive response to paleoclimate change,with decoupled variation of paleoproductivity,which indicates that the paleoproductivity may not be the main controlling factor to affect the TOC burial.In addition,under the corporate drive of redox condition and the adsorption and deposition of fine minerals,the TOC burial in the study area shows an evolution pattern of that higher efficiency of TOC burial during glacial periods and more significant degradation during interglacial periods.