【作者】 陈建芳；

【导师】 汪品先；

【作者基本信息】 同济大学 ， 海洋地质， 2005， 博士

【摘要】 根据南海北部（SCS-N）、南海中部（SCS-C）、吕宋岸外（SCS-NE2）及越南岸外（SCS-SW1）时间系列的捕获器资料，对“南海沉降颗粒物的生物地球化学过程及其古环境意义”这一命题作了初步探索。主要包括两个方面的内容：（1）南海颗粒通量和“生物泵”结构的控制因素；（2）现代过程对部分地球化学古环境参数的改造。所取得的主要结果分以下三大部分。（1）在陆架以外的深海区，南海颗粒物年平均通量在100 mg m^-2 d^-1左右，其空间变化从大到小依此为SCS-SW1＞SCS-NE2＞SCS-N＞SCS-C。在季节变化上，颗粒通量受季风控制比较明显，高值主要出现在冬季或夏季；在年际变化上，E1 Ni（?）o暖事可以使通量减小20％。颗粒通量除了在少数时段上下变化比较一致外，大多数时侯上下变化并不一致，并经常出现下层通量反而比上层高的情况，这主要是由于水平流引起的颗粒物侧向运动比较频繁造成。沉降颗粒主要以生源物质为主，生源物质／岩屑的比值依此为SCS-C＞SCS-NE2＞SCS-N＞SCS-SW1。在南海中部，生源物质一般在80％以上。生源颗粒中，钙质生物和硅质生物大致相当，表明在南海“碳酸盐泵”和“硅质泵”具有同等重要的地位。“生物泵”组成结构的时空变化并没有明显的季节性变化，但上升流区比非上升流区可能更有利于硅质生物的生长。在E1 Ni（?）o影响年，碳酸钙／蛋白石比值有明显的升高，表明暖事件可以抑制硅质生物的生长。南海作为典型的热带寡营养海，季风—上升流—营养盐供应和限制—风尘物质的触发—生物地球化学响应是一个比较典型的颗粒通量和“生物泵”结构控制过程，值得进一步研究。（2）南海初级生产力中仅有1-2％进入深海水柱，而在深海沉积物中积累的有机碳仅占0.26％以下，颗粒有机物通量改变主要发生在表层到1000m以及底层海水与沉积物界面之间，而在中下水层（1000m-3750m）变化较小。南海“生物泵”效率大约是同为季风影响区的西阿拉伯海的1／3，沉积有机碳的保存效率低一个数量级，这可能与南海初级生产力与前者相比要低，同时南海深层水的更新速率较快（底部不容易形成低氧环境）有关。由于南海初级生产力较低，颗粒有机物通量随水深减小的关系式（Martin曲线）可能会因为颗粒沉降速率、生物泵组成结构、颗粒物侧向运动等因素而失效。在颗粒物沉降到沉积物过程中，有机质的选择性降解可能会影响有机地球化学的一些整体指标（如有机碳、氮同位素）示踪的有效性。从颗粒物到沉积物，碳酸钙、蛋白石通量损失72-95％，其变化主要发生于深层捕获器与沉降物之间。另外，无论南海中部还是北部，水柱岩源物质通量反而小于全新世的沉积物积累率，表明南海海洋底部雾状层在重力流、等深流作用下引起的沉积物异地搬运是比较常见的。这种异地搬运作用对于古环境信号（如有机碳古生产力指标）在沉积物中的保存可能会有一定影响。（3）通过考察表层沉积有机碳、绿素、生物硅与上层海洋上升流、生产力等的关系发现，沉积有机碳含量高值与三个传统意义上的上升流区有比较好的对应，表明其是比较好的古生产力指标，但对于寡营养、低生产力，而且碳酸盐溶解作用、陆地稀释作用影响明显的南海，绿素、生物硅并不是好的古生产力指标。但这也不排除在某些海域（如水深不大、沉积速率高且稳定、有机质保存较好）利用通一钻孔的相关参数反映古生产力的可能性。沉降颗粒物中的U₃₇^K信号与表层卫星估算的SST在大多数时段不一致，在同一时段，上、下层位间也有很大的不同，这主要是由于颗粒物质的侧向运动所引起；此外，颗石藻的生长季节、层位（水深）的不一致也能引起上述不一致；沉降过程中U₃₇^K指数并没有引起明显的改变，表明南海降解过程对U₃₇^K温度估算影响较小；虽然沉降颗粒物的U₃₇^K温度信号与实测或遥感SST不一致，但表层沉积物U^k₃₇的分析结果再次证实了U₃₇^K温度与上层多年平均的实测温度有较好的相关性，这表明沉积物（代表几十—几百年）实际上对短时间和空间尺度的差异进行了“平滑”，也就是说，局部的差异并不与全局的趋势相矛盾。在南海颗粒物沉降过程中，C／N比呈增大趋势，这主要是由于早期降解过程中由于有机质的选择性降解作用所致；沉积碳同位素分布表明，陆源有机碳的输入(δ¹³C轻值)主要出现在海盆周边的海域，尤其是珠江口、巽他陆架、湄公河口及南海东北部，而在海盆的中央，沉积有机质δ¹³C较重，表明有机质主要是海洋自身来源；颗粒物中的有机质δ¹³C“异常”可能是由于人类活动引起的pCO₂升高引起。南海沉降颗粒δ¹⁵N大约在1.4-3.2‰之间变化，比其次表层-中层的源水—北太平洋中层水（NPIW）低2‰左右，这很可能是由于浮游植物对75m层上下营养盐的不完全利用所致；沉降颗粒δ¹⁵N比沉积物的结果要低2‰左右，这是由有机质的选择性降解作用引起；表层沉积物δ¹⁵N在巽他陆架和湄公河口也有比较低的值，这与现代海洋这些地区常常发育上升流一致，表沉积物δ¹⁵N低值很可能可以作为南海上升流的一个指标。更多还原

【Abstract】 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.更多还原