近陆深渊区海底沉积有机质源汇研究-以新不列颠海沟为例

It is generally accepted that the content and degradation rate of organic matter at the seafloor attenuate with increasing water depth and food scarcity and a dearth of benthic biomass thus prevail in hadal trenches. However, the funnel-like shape and relatively high-frequency fluid dynamics as well as frequent occurrence of mass wasting event (e.g. turbidites, earthquake deposits) within the hadal trenches favor the horizontal migration and the accumulation of sediment particles towards the trench axis. Therefore, hadal trenches may act as traps for the organic matter. Due to its closeness to the landmasses and frequent occurrence of earthquake induced by active tectonics, the New Britain Trench is regarded as a good place for investigating the particulate organic carbon (POC) transportation to the hadal zone. The proposed project will examine the contents of total organic carbon (TOC) and total nitrogen (TN), δ13C and δ15N values of organic matter, excess 210Pb activity, and concentrations of porewater species, combined with the early diagenetic modeling of organic matter in the sediments from both sides and inside of the New Britain Trench. Our aims are to trace the source of sedimentary organic matter and to calculate the accumulation and degradation rates of organic carbon in the New Britain Trench area. The proposed project has great implications for revealing the mechanism of transportation, degradation, and burial of particulate organic carbon and understanding the source and sink of organic matter in hadal zones close to the land.

通常来看，海底沉积物有机质含量和降解速率随着水深增加而降低，所以传统认为在水深大于6000米的深渊区是一个有机质和生物量均十分贫乏的区域。但是，海沟凭借其漏斗状的地形和较活跃的海流活动以及频发的地震和浊流，有利于促进颗粒物质的横向搬运，并汇聚在海沟轴部，故有学者提出深渊可能是有机质堆积和埋藏中心的假说。由于新不列颠海沟靠近陆地且地震活动频发，是研究颗粒有机碳向深渊输送的良好场所。本项目拟通过对从新不列颠海沟两侧延伸到海沟内和海沟轴部采集到的约20cm的浅层沉积物总有机碳和总氮含量、有机质碳氮稳定同位素、过剩210Pb活度、以及孔隙水组分含量的分析测试，并结合有机质早期成岩数值模拟的研究手段，确定不同水深沉积物有机质来源，计算颗粒有机碳的堆积和降解速率，以期揭示深海和深渊颗粒有机碳搬运、降解和埋藏规律。本项目对深入认识近陆深渊区有机质源汇格局和碳循环方面有重要的科学意义。

深渊区水深超过6000米，是人们难以企及并知之甚少的区域。最近研究发现，海沟轴部的生物呼吸速率明显高于深海平原，但是关于深渊区有机质降解速率的控制因素及有机质来源、搬运和埋藏过程还不太清楚。我们以近陆深渊区-新不列颠海沟为例，开展海底原位观测和沉积物碳氮含量，有机质碳氮同位素，放射性14C,210Pb，孔隙水地球化学研究，发现新不列颠海沟沉积有机质中陆源有机质的贡献比例可超过50%，同时，海沟海底的有机质降解速率不仅受控于表层初级生产力，同时与还与距离陆地远近、块体滑移等特殊沉积事件有关。海沟轴部较高的有机质降解速率可能是因为由于海沟特殊地形和沉积不稳定引起的较易降解的有机质优先在海沟轴部聚集，或者是由于在极大压力条件下某些微生物可以有效利用传统认为“老的”、“难降解的”陆源有机质。本项目在近陆深渊区海底沉积物有机质来源、搬运、降解和埋藏方面取得一些创新性认识，推动深渊海底碳循环的研究进程。