Biogeochemistry Of Settling Particles In The South China Sea And Its Significance For Paleo-environment Studies

In this thesis, the topic of "Biogeochemical process of particle settlement in theSouth China Sea and its significance for paleoenvironment studies" was discussedbased on results from time-series sediment trap experiments in the northern SouthChina Sea (SCS-N), central South China Sea (SCS-C), southwest South China Sea(SCS-SW1) and northeast South China Sea (SCS-NE2), which were carried out jointlybetween the Second Institute of Oceanography, SOA and the University of Hamburg,Germany. Two scientific questions are focused: (1) factors controlling the compositionof "biological pump" in the SCS; and (2) how modem processes affect geochemicalpaleo-proxies in the SCS. The results include the following three aspects:(1) The annual total flux was about 100 mg m^-2 d^-1 in the deep SCS, the spatialvariation order of the total flux was distinguished as SCS-SW1〉SCS-NE2〉SCS-N〉SCS-C. Higher fluxes appeared in winter or summer, suggesting the particleflux in the SCS is controlled by monsoons, while the E1 Nino event could reduce fluxto about 20%. There was decoupling of particle flux between upper and deep traps, andfurther more, some times deep trap collected more flux than shallower traps during thesame periods suggesting that advection was the main reason for this phenomenon.Biogenic material occupied the main part of bulk particles collected, up to 80%, andthe ratio of biogenic matter/lithogenic matter shows such an order: SCS-C〉SCS-NE2〉SCS-N〉SCS-SW1. Carbonate particles was equivalent to opal in the biogenicmatter which means that "carbonate pump" and "silica pump" are equally important inthe total "biological pump" in the SCS. There was no significant seasonal variations inthe "biological pump" structure. Although upwelling area was predominated by "silicapump" compared with non-upwelling areas, the "silica pump" could be suppressed inwarmer E1 Nifio years. As in an oligotrophic marginal sea, the sequence of monsoon-upwelling-nutrient supply (limitation)-dust trigger-biogeochemical response is thetypical "biological pump" process, which was worth further studies in the future.(2) Only about 1-2% of primary production (PP) sank into the deep SCS, and lessthan 0.26% of PP could ultimately be preserved as sediments. Remineralization anddissolution of biogenic matter as well as compositional alterations of organic mattermixtures appear to have taken place mainly in the upper layer of water column and atsediment/water interfaces, rather than in mid-waters between 1000m-3750m. Due tolower PP and faster turnover rate in deep waters, the biological pump efficiency in theSCS was about 1/3 of that in the Arabian Sea. The Martin curve of POC flux withdepth would be null because of low PP and advection of particulate matter. Selectivedecomposition of organic matter during settling and sedimentation of particles would affect reliability of some bulk paleo-proxies such asδ¹³C_org andδ¹⁵ N_org. The sedimentaccumulation rate of lithogenic matter was higher than water column lithogenic flux,suggesting that near bottom transportation such as nepheoid was quite frequent in theSCS, and this phenomenon may affect sediment organic carbon accumulation to beused as a paleo-production proxy (paleo-PP).(3) Comparisons between sedimentary organic carbon, chlorine and opal, and theupwelling area and upper layer PP indicate that organic carbon is a good paleo-PP, butchlorine and opal are not because sedimentary biogenic component is strongly affectedby carbonate dissolution and lithogenic dilution in this oligotrophic, low PP marginalsea. Exceptions may be found in areas with a relative shallow water depth, stablesedimentation rate and good organic mater preservation, where chlorine and opal couldalso be used as good paleo-PP proxies. In most cases, sea surface temperature (SST)measured by U^k37 in settling particulate matter differs from the upper layer remotesensing data, and there was also decoupling of particulate matter U^k37 signals betweenupper and deep traps during the same periods, all attributable to the advection ofparticles. On the other hand, the variation of coccolith bloom season as well as theirliving depth fluctuation in the euphotic layer could have also accounted for thisphenomenon. There was no significant change of U^k37 index during particle settling inthe water column. Although U^k37 temperature derived from settling particles disagreeswith remote sensing SST, a good correlation between the sediment U^k37 temperatureand the annual average temperature from the upper layer (30m) in the SCS confirmsthe empirical linear curve of U^k37 and SST, and suggests that a long term sedimentrecord (decadal to millennial) may smooth the short term fluctuations of environmentsignals. The increasing of C/N ratio from water column particles to sediment can beattributed to selective decomposition. The distribution ofδ¹³C_org in surface sediments,which indicates more organic matter in the shallow area than in the deep sea basin, wasaffected by terrigenous input, especially in areas near the Pearl River Estuary,north-east comer of the SCS, the Mekong River Delta and Sunda shelf. Theabnormally lightδ¹³C_org in these areas likely resulted from rapidly increasing pCO₂ dueto recent human activities, while the lowδ¹⁵N_org compared with the nitrate record inthe North Pacific Intermediate Water was due to incomplete nitrate utilization ataround 75m water depth in the SCS with selective degradation accounted for 2%0δ¹⁵N_org increase. The lowδ¹⁵N_org content recorded from some upwelling areas suggestsit can be used as a good indicator of upwelling in the SCS.