| The ocean is the largest carbon reservoir on the earth's surface,which plays an important role in regulating the global carbon cycle.Traditionally,global ocean carbon sinks are concentrated in the mid-to high-latitude area,while the low-latitude oceans are mainly carbon sources.Recent studies such as modern observations and numerical simulations have shown that low-latitude oceans may also be potential carbon sinks,and that dust input is key factor contril the ocean biological carbon pump during the ice age.However studies foucs on low-latitude marine carbon reservoirs are limited.On the one hand,because the low-latitude oceans are more obviously affected by the solar radiation effect of global warming,but its impact on marine communities is not clear,which leads to the controversial results of the ancient temperature reconstruction of the geological greenhouse period based on the temperature proxies constructed by the marine organism.On the other hand,how the Aeolian dust input could influence the low-latitude marine carbon pool,and the related bio-geochemical process,and how to identify the aeolian dust signal in the modern ocean deserve worthy of further study.Therefore,this study we fouse on the middle and low latitude oceans to study,first of all,to assess the reliability of current marine temperature proxies and develop new temperature proxies for the high-temperature area,and secondly,by combining the lipid characteristics and its compound-specific isotopes and trace elements to trace the dust input to the marginal sea and its implication to low latitude carbon pool.Currently,several proxies based on lipid biomarkers(long-chain alkenones,U37K';isoprenoid glycerol dialkyl glycerol tetraethers,TEX86,and long chain diols,LDI)have been proposed for reconstructing sea surface temperature(SST)since the Late Cretaceous.However,evidence already showed that the U37K' might reach the threshold and cannot be used to reconstruct high-temperature during the greenhouse period.Therefore,one of the key points of this study is to test the reliability of the existing temperature indices in the high temperature ocean and whether there are new temperature proxies for the high temperature zone.We collected 65 surface sediments from the South China Sea(SCS)with temperature ranges from 24 to 29? and combined the published dataset to test the TEX86 proxy application.Our result show that the TEX86 is moderately correlated with the SST,and the SST is underestimate when applied the TEX86 global calibration in the northern and central SCS,and the residual temperature between estimated and measured SST is out of error in the northern SCS.Combined with the published results,we find the reasons for underestimate temperatures in different areas might be affected by multiple factors.The northern SCS isoGDGTs pool might be influenced by the fluvial input,while the central basin isoGDGTs pool recorded the winter growth season temperature.Meanwhile,we analyze another OH-GDGTs based temperature proxy(RI-OH)and find that the RI-OH also large underestimate the measured temperature,which might indicate that the RI-OH might not suitable for high-temperature SCS.By further study the fractional abundance of OH-GDGTs at different temperature ranges,we find the fractional abundance of OH-GDGT-0 and OH-GDGT-2 show opposite trend with the temperature increasing,which might indicate the different adaptation of the OH-GDGTs producer.Therefore,based on the linear relationship between OH-2/OHs and the measured SST,we proposed a new OH-GDGTs temperature proxy OH-2/OHs for higher temperature(>25?)ocean.Secondly,previous studies have been found that the LDI temperature proxy might not record the environment temperature change when measured temperature>27?.For this purpose,we analyzed 58 surface sediment samples from the tropical SCS to assess the performance of LDI proxy in high-temperature marine regimes.We find the LDI calibration yields temperature<27? for these samples which means the LDI-based temperature are not suitable for higher temperature reconstruction.However,we found the residual between the LDI-inferred temperature and the measured SST increased as SST increases.Furthermore,we analyzed the published global surface sediments and time-series sediment trap results and find the same linear relationship,which might indicate the adaptation of LCDs sources to temperature change.Therefore,we re-calibrate the LDI-inferred temperature to generate a new relationship that could be applied to higher temperature(>27?)or when other proxies like the U37 K' reach the maximum.Furthermore,bacterial 3-hydroxy fatty acid derived from Gram-negative bacteria has been developed and applied in terrestrial environment as temperature and pH proxy,while its source and distribution,as well as whether the 3-OH-FAs based proxies could use in the marine environment are still unknown.So we collected surface sediments from different latitudes of the Northern Pacific Ocean to analyze the 3-OH-FAs in marine environment.By statistical analysis and microbial genetic community analysis,comparing the distributions of 3-OH-FAs in soil and marine sediments,we find that 3-hydroxy fatty acids in the soil environment differed greatly from those in the marine environment,indicating that 3-hydroxy fatty acids in the marine environment were mainly derived from marine in-situ production and the soil 3-OH-FAs based proxies are not suitable for marine environment.By in-depth analysis of the relationship between 3-OH-FAs and SSTs,we find that the RAN13 proxy based on anteiso C13 and normal C13 3-OH-FA shows a strong exponential relationship with temperature,and its residual error is small in high-temperature sea areas.Therefore,we develop a new SST temperature proxy based on bacterial 3-OH-FAs,and it shows great potential for tropical temperature reconstruction.Finally,the South China Sea is used as an example to study the wind dust input signal in the modern low-latitude sea area,and the wind dust input signal in marine sediments is constrained by using higher plant lipids(n-alkanes)and their carbon/hydrogen isotopes and trace element Sr-Nd isotopes.By analyzing the Sr-Nd from this study and the results from potential sources area,we find the Sr-Nd isotopic compositions from the SCS central basin are consistent with inland area dust area and show large differences with the results from SCS surrounding island and river,indicating the existence of aeolian dust signal from inland Asia in the deep SCS basin.Meanwhile,we find the n-alkane carbon preference index(CPI)in the SCS central basin show an abnormal higher value,which represents the fresh terrestrial deposition.Similarly,the n-alkane hydrogen isotopes are more depleted in the central basin.We analyze the n-alkanes CPIs and hydrogen isotopes in latitudinal soils from northern China and find both are showed strong linear relationships with latitude,thereby could be used to trace the source of n-alkanes in the SCS.High CPIs and depleted hydrogen isotopes of n-alkanes indicate the source from mid-latitude Asia(40°N).Thus both Sr-Nd isotopes and leaf wax n-alkanes indicate that there are obvious dust input signals in the deep basin of the South China Sea,which may come from the inland desert of Asia.During the East Asian Winter Monsoon,a large amount of terrestrial aeolian material is imported into the SCS deep basin,and causes the phytoplankton bloom in the South China Sea deep-sea basin.The input of East Asian aeolian dust and the water mixture driven by the East Asian winter monsoon are the main reasons for the phytoplankton bloom in the oligotrophic SCS deep-sea basin in winter.In summary,this study analyzes the existing temperature proxies in high-temperature seas,and proposes new temperature calibrations OH-2/OHs and LDIM,and develop a new temperature proxy RAN13 based on bacterial 3-OH-FAs for temperature reconstruction in high-temperature seas.Finally,we identify the dust signal from the SCS central basin and find the potential source might from the Asia deserts.By combining published data to analyze the relationship between wind dust input and phytoplankton bloom in the SCS,we conclude wind dust input and wind-derived upwelling may be the main causes of winter phytoplankton bloom in the SCS. |
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