An Evaluation Of Benthic Foraminiferal U/Ca And U/Mn Proxies For Deep Ocean Carbonate Chemistry And Redox Conditions

Glacia-Interglacial Cycles are the leading feature of Quaternary climate,but the driving mechanisms behind these multi-thousand-year oscillations remain elusive.The sun,our climate system's primary insolation source,is deemed to cause these major climate transitions.However,although variations in solar insolation are clearly important,they cannot fully explain the observed inception and pattern of ice ages.Many discrepancies between paleoclimate records and insolation forcing indicate the presence of feedbacks and amplification forcing mechanisms within the earth system.Carbon dioxide(CO2),an important greenhouse gas,is likely to trigger the positive and negative feedback processes of the earth system,and hence plays a crucial role in climate changes.Bubbles of ancient air trapped in ice cores indicate that atmospheric concentrations of CO2 were low during glacial periods(?180 ppm)and high during interglacial periods(-280 ppm).Hence,understanding mechanisms responsible for past atmospheric CO2 concentration variations is not only necessary for reconstructing paleoclimate changes,but also conducive to predictions of future climate changes.The ocean,especially the deep ocean,is the largest carbon reservoir of the atmosphere-land biosphere-ocean system,and marine processes that alter deep-water carbon content have exerted profound influences on atmospheric CO2 fluctuations on glacial-interglacial timescales.Reconstructions of the marine conditions,such as marine carbonate system and dissolved oxygen content,are therefore important to understand marine carbon cycling and the ocean's role in controlling atmospheric CO2 changes in the past.Foraminifera are unicellular organisms with a wide marine distribution.Many species secrete carbonate tests whose physical and chemical nature reflect the seawater conditions in which they grow.Thus,the chemical composition of foraminiferal carbonate from deep ocean sediments has been widely used to reconstruct physicochemical properties of seawaters from which shells precipitate.Some previous studies suggested that foraminiferal U/Ca could be used to infer seawater carbonate chemistry changes,but others showed complications from diagenesis and temperature.A recent downcore study suggested that foraminiferal U/Mn may be used for sedimentary redox-conditions.However,compared to planktonic studies,less data are available for benthic foraminifera,and the U/Ca-[CO32-]calibration is currently restricted to the South Atlantic with no downcore reconstruction.Also,given sparse downcore and core-top studies on U/Mn,its feasibility for sedimentary redox-condition reconstructions and potential controlling factors warrants further evaluation.In view of the above questions,we measured core-top and downcore U/Ca and U/Mn in two benthic species(C.wuellerstorfi and C.mundulus)collected from globally distributed samples to assess their controlling factors and feasibilities for marine condition reconstructions.In brief,for core-top data,we investigate possible influences from different cleaning procedures and various marine physical and chemical parameters.We then present new downcore U/Ca and U/Mn records from three Atlantic sediment cores,to test the feasibility of using these ratios for paleo-reconstructions(deep ocean carbonate chemistry and redox-conditions)by comparing with previous published records.The main contents and contributions of this study were summarized as follows:(1)For core-top samples,we investigated possible effects from cleaning,dissolution,and interspecies offsets on core-top U/Ca and U/Mn.First,we investigated influences of different cleaning methods on benthic U/Ca and U/Mn by comparing results from paired samples,whose shells were separately subject to the "Cd-cleaning"and "Mg-cleaning" methods.Compared to samples subject to the "Mg-cleaning"method,U/Ca and Mn/Ca ratios are significantly decreased when the reductive step was included.In contrast,U/Mn ratios show comparable values between the two cleaning methods.We suggest that most of U/Ca and Mn/Ca decreases during the reductive step associated with core-top samples are probably driven by preferential dissolution of foraminiferal carbonates enriched in uranium and manganese.Second,we compared U/Ca and U/Mn in Rose Bengal stained(recently living shells,subject to minimal dissolution)and coexisting unstained shells presumably subject to some degree of dissolution on the seafloor.Our results show little dissolution effect on U/Ca and U/Mn of core-top samples.We also compared U/Ca and U/Mn in paired C.wuellerstorfi and C.mundulus samples.The offsets between two species indicate that biological processes play an important role in the U incorporation into foraminiferal carbonates.(2)We explored potential influences of marine physical and chemical parameters(temperature,salinity,pH,[CO32-],?[CO32-],[O2])on benthic U/Ca.Our results show that,irrespective of cleaning methods used to clean our samples,benthic U/Ca is not significantly correlated with any parameters.We suggest that benthic U/Ca is controlled by multi-factors and complex processes,such as covariations of carbonate parameters,temperature,pore-water condition,U adsorption-desorption,carbonate surface structural feature,and biological processes,hence cannot directly affected by the deep-water carbonate chemistry.To further evaluate the feasibility of benthic foraminiferal U/Ca as a proxy for deep-water carbonate chemistry reconstruction,we applied the appropriate core-top U/Ca-[CO32-]relationships to convert benthic U/Ca from our cores into deep-water[CO32-],and compared these records to those in previous studies from the same cores.Deep-water[CO32-]derived from U/Ca are inconsistent with values derived from benthic B/Ca and past ocean circulation changes based on other proxies,suggesting that U/Ca from C.wuellerstorfi and C.mundulus should not be used for deep-water carbonate chemistry reconstructions.(3)We explored potential influences of marine physical and chemical parameters(temperature,salinity,pH,[CO32-],?[CO32-],[O2])on benthic U/Mn.Our results show no significant correlation of core-top U/Mn with deep-water[O2],suggesting that efficient cleaning procedures have removed U-Mn enriched authigenic coatings and/or core-top samples are barely subjected to post-depositional diagenetic processes.Our core-top U/Mn data also suggest that shell U/Mn is not affected by any other parameters.(4)At last,combined with previous studies,we explore the use of downcore U/Mn for sedimentary redox-condition reconstructions.Our results demonstrate that U/Mn,along with other proxies(such as paleo-productivity,nutrient content,ventilation,etc.),may be used as an auxiliary indicator for past sedimentary redox changes,and hence deep-water[O2],to infer past deep water conditions that are useful for paleoceanography studies.Our downcore benthic U/Mn records reveal enhanced stratification in the polar North Atlantic during the LGM.U/Mn in TNO57-21 shows systematic changes linked to atmospheric CO2 variations on millennial timescales,indicating tight coupling of the deep South Atlantic redox-conditions to carbon cycle during the last?100 ka.Furthermore,our U/Mn record at TNO57-21 indicates poorer ventilation of the deep South Atlantic during MIS 4 than during the LGM.