Geochemistry Of Surface Sediments From The Emperor Seamount Chain,North Pacific And Its Environmental Implications

The geochemical characteristics of marine sediments that contains abundant information of environmental and climate changes,reflect the contributions of various natural processes,such as terrigenous,biogenic,authigenic and volcanic sources.Investigating the composition and distribution of pelagic marine sediments is fundamental in the field of marine sedimentology.The composition,spatial distribution and material provenances of surface sediments from the Emperor Seamount Chain(ESC)in the North Pacific remain unclear due to the lack of sediment samples.A suite of geochemical proxies were analyzed for different sedimentary phases,such as the grain size,major and minor elements,rare earth elements and Ca CO₃ contents for bulk sediments(50 samples),and major and minor elements,rare earth elements and Pb-Nd isotopes for fine-grained(<63?m)silicate fractions(30 samples),and major and minor elements,Nd-Pb isotopes for Fe-Mn phase(14 samples).Based on these measurements above,we investigate spatial distribution characteristics of the sediment geochemistry,differentiate the provenance of fine-grained silicate fractions,and explore the main factors affecting the Nd-Pb isotope changes of Fe-Mn phase.These findings of this study can provide scientific support for understanding the modern sedimentary environment changes of the Emperor Seamount Chain in the North Pacific.On the basis of a variety of measurements for terrestrial fractions of bulk sediments,we divide them into three zones(?,?and ?)spatially with distinct features.Terrestrial fractions of sediments in Zones ?(?33°N-44°N)and ?(49.8°N-53°N)are dominated by clayey silt,and mainly consist of sand and silty sand in Zone II.The sortable silt mean grain size shows that the hydrodynamics in the study area are significantly stronger than those of surrounding abyssal plains,especially at water depth of 1000-2500 m.The CaCO₃contents in sediments above 4000 m range from 20% to 84% but decrease sharply to less than 1.5%below 4000 m,confirming that the water depth of 4000m is the carbonate compensation depth in the study area.Strong positive correlations between Al and Fe,Ti,Mg,K(R>0.9)in bulk sediments indicate pronounced contributions of terrigenous materials from surrounding continent mass to the study area.Furthermore,the eolian dust from the Asian inland srid regions contributes to the composition of bulk sediments as suggested by rare earth elements.There is no significant correlation between grain size and major and minor elements,which indicates that the sedimentary grainsize does not exert important effects on terrigenous components.There are significant negative Ce and positive Eu anomalies at all stations.The negative Ce anomaly mainly exists in carbonate-rich sediments,indicating the inheritance of the signal of seawater.The widespread occurrence of positive Eu anomaly indicates significant contributions of volcanism to the study area from active volcanic island arcs around the North Pacific.The relative contributions of terrestrial,volcanic,and biogenic materials vary with latitude and water depth in the study area.The measurements for geochemical elements and Nd-Pb isotopes of fine-grained silicate fractions(<63?m)indicate that the silicate fractions are a mixture of aeolian materials from the central Asian desert,and volcanic materials from surrounding volcanic island arcs,including Kuril Islands,Aleutian Islands,Kamchatka Peninsula and volcanic arcs of Japan.In Zone?(?33°N-44°N),there are more aeolian dust and few volcanic materials in sediments.Furthermore,more volcanic materials occur in Zones?and ?(?44°N-53°N).The method of“fast and soft reduction reagent”is applied to extract Pb and Nd isotopes from Fe-Mn coatings of sediments.It is found that the ratios of Al/Pb and Al/Nd in Fe-Mn phases are less than 100,which confirm the reliability of the extraction method.The?Nd values of Fe-Mn phases in surface sediments of the ESC range from-1.79 to-3.80,which is more radioactive than the?Nd values reported in seawater.This may be related to the timing of surface sediments,the early diagenetic processes,and the upwelling of Lower Circumpolar Deep Water.