Identification Of Marine Strata And Indication Of Transgression Process Of The Late Quaternary In The Bays Of Northern Fujian

During the Quaternary,there has been periodic global sea level eustacy,which was archived in the coastal late Quaternary strata as transgression records.However,such records in eastern coast of China are not exactly consistent with global sea level eustacy due to regional tectonic movement.In order to further understand this inconsistency,it is essential to precisely identify the transgression periods in the late Quaternary strata.The previous work displayed that a lack of microfossils in the sediment confused the identification of marine strata of the late Quaternary in the Fujian coast,which greatly limits a further study on the process and mechanism of the late Quaternary transgression in this area.Therefore,the present study aims to promote the effective proxy system for marine strata identification in this region,and to study the transgression process during the late Quaternary in Fujian coastal area.Such study would provide reliable evidence for deeply understanding the transgression process,geomorphic development and interaction mechanism of land and ocean during the late Quaternary in the eastern coastal area of China.Core NDGK2 in the Sansha bay and core NDQK11 in the Funing bay of northern Fujian are taken as the research object in this issue.Through analysis of lithology,foraminifera fossils,geochemical elements of sedimentary origin and authigenic pyrite in the core,combined with the same proxies in surface sediment of the Sansha bay,the feasibility of indicating marine strata by geochemical elements of sedimentary origin is discussed.Based on this,together with indexes of carbon,nitrogen,stable carbon isotope and the existing chronological framework,the late Quaternary transgression process is discussed comprehensively.The knowledge obtained from this study is detailed as below:(1)There are three sets of dark grey layers in the Late Quaternary cores of northern Fujian bays(NDGK2 and NDQK11 cores),which are suspected of marine origin,characterized by high water content and uniform texture,rich in shell,snail shell and other debris,and more carbonized plant debris at the bottom.Above and below the three dark gray clay layers are layers of variegated mottled clay or sandy pebbles.There are great differences in color,structure,inclusions and sedimentary characteristics between the dark gray clay layers and the others.However,foraminifers and ostracodes are absent from the lower two clay layers in both boreholes.Especially in NDGK2 core of semi-enclosed Sansha bay,foraminifera fossils only appear in the top clay layer.This indicates that the foraminifera fossil index may be limited in identifying marine strata of the late Quaternary in the bays of northern Fujian.(2)The acid-soluble sediments in the three dark gray clay layers(???and?)of the two boreholes(NDGK2 and NDQK11 cores)are rich in Sr and low in Ba,and the Sr/Ba ratio is much higher than 1,which is similar to the modern tidal flat and bay system,implying typical marine origins.In the two boreholes,layers?and?are rich in Ba and poor in Sr.The Sr/Ba ratio of layer?is less than 1,indicating typical continental sedimentary characteristics.These are also supported by the occurrence of marine dinoflagellates and abundant authigenic pyrites in these layers(???and?).Therefore,the three dark grey clay layers in both boreholes are marine strata.Geochemical elements of sedimentary origin can be used as an effective proxy to identify the late Pleistocene transgression strata along the Fujian coast.Based on the existing chronology of the core(NDGK2),the three transgressive layers(???and?)are corresponding to the high-stands of MIS5,MIS3 and MIS1 from bottom to top,respectively.The reason for the lack of foraminiferal fossils in marine strata may be that their growth environment is restricted and the shell cannot be preserved after burial.(3)The sedimentary Sr/Ba and Mn/Fe of modern surface sediments in Sansha bay are in direct proportion to the water depth.The ratios of Sr/Ba and Mn/Fe in the cores increase from MIS5 to MIS1,and the organic geochemical indices(C/N and?¹³C)also show that the terrigenous organic matter contribution of MIS5 and MIS3 is significantly higher than that of MIS1,indicating that the influence of the transgression in this area has deepened significantly since the late Quaternary.(4)The marine strata of MIS5 in the northern Fujian revealed in this study is rarely reported,indicating that they are not widely distributed.The transgression record of the MIS5 period shows that its transgression intensity is significantly weaker than that of the MIS3 and MIS1,which is inconsistent with global sea level records and probably caused by the high paleo-topography in this area,indicating that the region has undergone subsidence since MIS5.