Sedimentary Diatoms Records And Their Paleoenvironmental Indications In The Southern Mariana Trench And Its Adjacent Areas

Diatom is a kind of photosynthetic eukaryotic unicellular algae.The sedimentary diatom assemblage carries abundant en\vironmental chronology information of geological period,which is very important for the research and reconstruction of Marine environment.The diatom mats deposits are formed by the vast bloom and rapid burial of large or"shade flora" diatoms,which are influenced by sea water temperature,salinity,nutrient content,the change of silicon content in sea water and the degree of stratification of water.They are ideal materials for Marine scientific research.The Philippine sea is the largest marginal basin in the western Pacific Ocean,and the mariana trench is well known for its depth.The complexity and uniqueness of its geographical location andsubmarine geomorphy make it more significant in the study of paleoceanography.In this paper,diatoms from the surface sediment samples of 113 stations in southern Mariana Trench of the study area and the P38 core sediment samples were idetified and quantitatively analyzed.Co1bining x,with the AMSt.C dating data and the physical and chemical parametersdata of sea surface water,comparing with prexvious studies in the Philippine sea,the main conclusions are follows:(1)A large scale of diatom mat deposits were found in the surface sediment of the study area,which were identified as Ethmodiscus rex.In the surface sediments of 113 stations,a total of 28 genera and 89 species of associated diatoms were found,most of which are warm water species and widespread species,indicating that the study area was a tropical oceanic environment.(2)The absolute abundance of diatoms ranged from 50 to 397,5000 frustules/g,and the average abundance of 316071 frustules/g.The abundance of the 6 stations in the north of the trench varied greatly;In the south of the trench,there were 7 high value regions with abundance more than 1 x 106 frustules/g.The highest diatom abundance in the study area occurred at station PI 04,with a depth of 6098m on the northern edge of the mariana trench.(3)Along the terrain,the relative abundance of Ethmodiscus rex is higher in the north of 9°N than that in the south,and is relati,e low in shallow-sea area of the Caroline seamount.which is in the southern study area.The correlation analysis shows that there is a weak positive correlation between the depth of the water and the relative abundance of Ethmodiscus rex.The associated diatoms are mainly distributed in the study area of 9 ° N,which is shallower.The Principal Component Analysis shows that their distributions are affected by the surface seawater temperature and salinity during their growth time,and the temperature is the main affect factor.(4)The study sea area was divided into 6 assemblage zones according to the distribution of diatom:zone i is located in the relatively shallow water area of the Caroline sea,and the relative abundance of Ethmodiscus rex is relatively low.Zone ii is located in the relatively deep water area in south of the Mariana Trench,which is in the central part of the study sea area,with higher relative abundance of Ethmodiscus rex.Zone iii is located in a v-shaped area between zone i and zone ii,where water depth is a little deeper than the surrounding area and the diatom distribution is complex,the surface temperature and salinity of sea water here are higher than the surrounding sea area,there may be a bottom ocean current.Zone iv only contains station P72 in the middle of the study sea area,the relative abundance of Ethmodiscus rex decreases suddenly,suggesting sedimentary anomaly existing here.Zone v is located near Guam of the northern study sea area,and the relative abundance of Ethmodiscus rex is lower.Zone vi is located at the northern edge of the Mariana Trench in the north of the study sea area,with higher relative abundance of Ethmodiscus rex.(5)P38 core is located in the southern study sea area,where the sediments are mainly pale white and light yellow ooze.It can be divided into three parts as a whole,the colors of sediments in the top and the bottom of the core are darker,and that in the middle 14-87cm part is lightert,containing a lot of diatoms,intersperse light yellow horizontal bedding layer.The core reflects a long-scale sedimentary process of the diatom mats from mass bloom to tailing off.A large amount of Ethmodiscus rex diatom mat deposition was also found on the P38 core,with 26 genera and 79 species of associated diatoms v,ere identified.The variation range of absolute abundance is 63000?198198 frustules/g,and the average abundance is 130330 frustules/g.Most of diatoms are warm water species and widespread species and the vertical variation is significant.(6)The AMS14C dating shows that the deposition occurred during the Last Glacial Period,and was in MIS3?MIS1.Diatom mats occurred around 4.5ka B.P.,it could be divided into four assemblage zones vertically:102?88cm(about>45ka B.P.).Under the affection of"silica leakage"of the southern ocean,the abundance of diatoms increased gradually.88?36cm(about 45?30ka B.P.),the climate fluctuated violently and the abundance of diatoms fluctuated rapidly.36-14cm(about 30-15ka B.P),the Last Glacial Maximum was a period of great prosperity of the bloom of diatom mats.The upper from 14cm(about<1 5ka B.P.),the absolute abundance of diatoms decreased and the relative abundance of associated diatoms increased,suggesting that the content of nutrients in surface seawater was continuously high during this period.