Sedimentary Characters And Genesis Of Gravity Flow Deposits In The Southwestern Okinawa Trough Since 1.3ka B.P.

Taking a core HOBAB4-S1 from the southwestern Okinawa Trough as research subject,the sedimentary characters,genesis of gravity flow deposits and paleoclimatic significance of southwestern Okinawa were examined using laser particle size analysis,AMS14 C age dating,detrital mineral identification,organic carbon and nitrogen analysis and geochemical analysis.The results show that there are bathyal deposits and 17 segments of gravity flow deposits in the core.Bathyal deposits consist mainly of muddy silt.Massive bedding are present within the bathyal deposits The grain-size accumulation curve of bathyal deposits shows a 10-14?m centered unimodal distribution..The sample point distribution on C–M pattern is in the lower left corner,indicating still-water environment.Gravity flow deposits consist mainly of sandy silt.Parallel bedding,climbing ripple bedding and graded bedding are present within the gravity flow deposits The grain-size accumulation curve of gravity flow deposits shows a 70–130?m centered unimodal distribution..The sample point distribution on a C–M pattern is roughly parallel to the C=M baseline and below the PQ segment,indicating that the sediment was transported as gravity-flow suspensions.The results of identification of detrital minerals indicate that the main light minerals are quartz,plagioclase,mica and chlorite,and the heavy minerals are common hornblende,biotite,actinolite and limonite.Organic carbon and nitrogen analysis showed that the total organic carbon and total nitrogen content in the gravity flow deposits were significantly lower than that in the bathyal deposits,and the contents decreased with increasing depth because of natural degradation.Elemental geochemical analysis shows that the gravity flow deposits are relatively enriched with elements Si,Zr,Ca,and Sr,while elements Al,Fe,K and most trace elements are deficient.The results of element factor analysis show that the content of each element in the core is controlled by the three main factors of particle size composition,biogenic input and terrestrial material input.The rare earth distribution model shows that the gravity flow deposits and the bathyal deposits have the same rare earth distribution curve shape,similar to the sediments of the rivers in Taiwan,but is different from the sediments on the Yangtze River and the East China Sea shelf.According to the sedimentary structure characteristics of the gravity flow deposits,the gravity flow belongs to hyperpycnal flow,sediment is directly input from the estuary to the bottom of the trough.Two types of hyperpycnal flow deposit were recognized:(i)thick hyperpycnal flow deposits with basal erosional contacts,with pairs of basal coarsening-up units and top fining-up units,indicating a proximal sedimentary environment;and(ii)thin hyperpycnal flow deposits with only one pair of basal coarsening-up units and top fining-up units,indicating a distal sedimentary environment.There was an increase of hyperpycnal flow deposits between 800 and 1300 A.D.,which indicates that the climate at that time was marked by high temperature and high humidity,typhoons and heavy rainfall were frequent.This is an evidence of the Medieval Warm Period in East Asia.