Radiolarian Assemblages And Paleoceanographic Events Of The Nansha Area, Southern South China Sea Since The Last 1.34 Ma

For the first time in the South China Sea, a long sequence of deep sea sediments has beenanalyzed for radiolarians in two sedimentary cores 17957 and 17959, providing an opportunity toreconstruct the paleoceanographic evolution of the Nansha Area since the last 1.34 Ma. By themicropaleontological and geochemical analyses of 200 samples from the two cores, the radiolarianbiostratigraphic zones and framework of age have been established over the last 1.34 Ma; thesedimentary process of biogenic and nonbiogenic sediments and SiO₂ dissolution events havebeen revealed in the glacial cycles at a long time scale; the succession of radiolarian assemblagesand implied paleoceanographic events, history of the water mass change and response ofradiolarians and planktonic foraminifers to Mid-Pleistocene Revolution have been discussed.The paleomagnetic records from the Core 17957 show that the top and base of JaramilloEvent are at the depth of 966 cm and 1016 cm, respectively, and the boundary between Brunhesand Matuyama lies at the depth of 796—798 cm. Seven radiolarian marker species have beenfound for the first time for the South China Sea, and five of these have been used to set up the fivebiostratigraphic zones (NR 1—NR5), namely Buccinosphaera invaginata Zone, Collosphaeratuberosa Zone, Stylatractus universus Zone, Pterocorys campanula Zone and Anthocyrtidiumangulare Zone. The five radiolarian datums are directly correlated with the oxygen isotope andpaleomagnetic records of Core 17957, which allows an absolute age assignment of thebiostratigraphic radiolarian events in the South China Sea. The age determinations are in goodagreement to those from the equatorial Pacific. This study presents the first biostratigraphicradiolarian sequence from the China Sea covering the last 1.34 Ma, which provide an importantstratigraphic base for Quaternary paleoceanographic studies.Our on study of sediments from Core 17957 has been found opposite trends in downcorevariations between biogenic calcareous and siliceous components. Opal and radiolarians aredominant above the depth of 880—900 cm, whereas CaCO₃, coarse fraction and planktonicforaminifer predominate below 880—900 cm. Nonbiogenic component decreases in percentageabove 460—480 cm gradually, but reaches high percentages below that level due to the abundanceof volcanic ash. The sedimentary rate corresponds well to the accumulation rates of biogenic andnonbiogenic sediments in downcore variation in Core 17957 over the last 1.34 Ma, and 9 highvalue stages and 4 low value stages can be distinguished which might indicate the changes insurface productivity. Judging from abundance, accumulation and fragmentation rates ofradiolarians in Core 17957, 17 low value stages (R1—R17) of radiolarians have been recognized.Of these, R13 (0.87—0.89 Ma) and R15 (1.03—1.05 Ma) are caused by SiO₂ dissolution, whileR12 (0.74—0.81 Ma) and R 17 (1.28—1.32 Ma) by volcanic ash dilution. The SiO₂ dissolutionevents have been recorded not only in higher percentages of resistant species, but also in theloadings of Factor 3 from a Q-mode factor analysis. As indicated by the Q-mode factor analysis, Factor 1 represents the tropical assemblagewhose dominant species are Tetrapyle octacantha + Octopyle stenozona, presumably indicative ofproductivity. Factor 2 represents the equatorial assemblage with dominant species such asPterocorys zancleus group, Botryocyrtis scutum, Peromelissa phalacra, Hexapyle spp. andPyloniidae gen. et sp. indet.. Lophophaena hispida in the assemblage is an index species of theequatorial cold tongue and used to reflect the change of the equatorial cold water mass. Factor 3represents the dissolution resistant assemblage predominated by Acanthodesmia sp., Liriospyrisreticulata and Tholospyris sp., which suddenly increased in abundance at the last 0.87—0.89 Maand 1.03—1.05 Ma. Factor 4 represents the intermediate water assemblage with Dimelissathoracites and Dimelissa sp. as dominant species. The former is believed to be a deep waterspecies.The succession of radiolarian assemblages and the water mass story, which shows a gradualweakening of surface water and a gradual intensification of subsurface and intermediate waters,over the last 1.34 Ma are characterized by 7 turnover intervals, which are related to respectivepaleoceanographic changes: (1) at 0.88-0.90 Ma (δ¹⁸O 21/22 boundary, Mid-PleistoceneRevolution), the changes of radiolarian assemblages, the surface assemblage decreased inpercentage remarkably and intermediate and eurybathic assemblages enhanced may imply amarked drop of SST by the enlargement of glacial cap resulting from Mid-Pleistocene Revolutionand the beginning of intensification of intermediate water; (2) at 0.60—0.62 Ma (δ¹⁸O 14/15boundary), the decline of surface assemblage and rise of intermediate assemblage in percentagemay suggest a decrease of SST and an intensification of intermediate water; (3) at 0.40—0.42 Ma(δ¹⁸O 11 stage), the appearance of G. ruber (Pink), increase of G. rubescens in abundance, thelow percentage of surface assemblage, high percentage of intermediate assemblage and thereduction of subsurface assemblage may represent an increase of SST and strengthening ofintermediate water; (4) at 0.18—0.22 Ma, the decrease of subsurface assemblage and an increaseof intermediate assemblage may imply a decrease of SST and the intensification of intermediatewater; (5) at 0.18—0.12 Ma, the enhancement of subsurface assemblage and rapid reduction ofintermediate assemblage show a sharp weakening of the intermediate water and a remarkablyreinforcement of subsurface water; (6) at 0.12 Ma (δ¹⁸O 5e stage ) to 0.06—0.08 Ma, theappearance of G. conglomerata, predominance of sinistrial form in G. truncatulinoides (L) and thelast occurrence of G. ruber (Pink), the obvious decrease of subsurface assemblage and sharpreinforcement of intermediate assemblage with its maximum show a deep thermocline, intensemixture of upper water layer and a beginning of decrease of SST and a rapid strengthening ofintermediate water; (7) at 0.06—0.08 Ma (δ¹⁸O 4/5 boundary) to the present day, the lastoccurrence of G. conglomerata, the sharp reduction of intermediate assemblage and distinctenhancement of subsurface assemblage may imply a sharp weakening of intermediate water and aobvious consolidation of subsurface water.The Mid-Pleistocene Revolution event has been recorded in Core 17957 from the SouthChina Sea and is characterized by changes in abundance of radiolarian and planktonic foraminiferand their ratio. After the event, the average abundance of radiolarian increased 2 times andplanktonic foraminifer decreased in abundance by 0.6 time, resulting in a 4 time increase ofradiolarians/planktonic foraminifers ratio.