Carbonate accumulation rates of the Ninetyeast Ridge, Indian Ocean

More than 95% of the carbon lost from the "blue-ocean" reservoir to the sedimentary sink appears to be transferred as skeletal CaCO3, produced in the surface waters. This skeletal CaCO3 carries a productivity signal which is much better preserved in the underlying pelagic carbonate sediments than that of the refractory organic carbon accompanying it. Here, we develop a new method to quantify this signal in terms of organic carbon paleoproductivity, using the sedimentary mass accumulation rates of pelagic carbonate. These are converted into carbonate transit-paleofluxes, which are then translated into the corresponding transit-fluxes of organic carbon, via the carbonate to organic carbon ratios reported from deep-moored sediment trap experiments in modern blue-ocean environments. Paleoproductivity can then be estimated quantitatively by using published algorithms describing the relationship between the export production of particulate organic carbon at depth and primary productivity in the euphotic zone.
Although our approach seems rather straightforward, it contains several pitfalls, the effects of which are highlighted by an example comprising three Paleocene/Oligocene to Recent pelagic carbonate sequences drilled during ODP Leg 121 in the eastern Indian Ocean. Although some extreme values are likely due to errors, such as poorly constrained datum levels and dissolution peaks, the results for the Quaternary and Neogene correlate well from site to site and are within the productivity range of present-day low to medium latitude open oceans. Our method may provide an opportunity to actually quantify blue-ocean primary productivity in sedimentary carbonate environments, but requires validation by other, more established ones.

从“蓝洋储库（blue-ocean reservoir）”向沉积汇（sedimentary sink）输送过程中流失的碳，有超过95%似乎是以表层水体中生成的骨骼状碳酸钙（skeletal CaCO3）的形式完成转移的。这类骨骼状碳酸钙携带着生产力信号，相较于与之伴生的难降解有机碳，该信号在其下伏的远洋碳酸盐沉积物（pelagic carbonate sediments）中保存得更为完好。本研究提出一种新方法，基于远洋碳酸盐的沉积质量累积速率（sedimentary mass accumulation rates），对有机碳古生产力（organic carbon paleoproductivity）维度下的该信号进行定量表征。借助现代蓝洋环境中锚泊深水沉积物捕集器（deep-moored sediment trap）实验报道的碳酸盐与有机碳比值，可将上述累积速率转换为碳酸盐转运古通量（carbonate transit-paleofluxes），进而转化为对应的有机碳转运通量。随后可通过已发表的算法模型，基于深部颗粒有机碳输出生产力与透光带（euphotic zone）初级生产力之间的关联关系，定量估算古生产力水平。
尽管本方法看似较为直观，但仍存在若干潜在陷阱，以东印度洋大洋钻探计划（ODP）第121航次钻取的3套古新世/渐新世至现代远洋碳酸盐沉积序列为例，可清晰展现这些陷阱的影响。尽管部分极端值可能源于各类误差，例如基准面约束精度较差以及溶解峰（dissolution peaks）的影响，但第四纪（Quaternary）与新近纪（Neogene）的研究结果在不同站位间相关性良好，且处于当前中低纬度开阔大洋的生产力区间内。本方法或为定量估算沉积碳酸盐环境下的蓝洋初级生产力提供可行途径，但仍需借助其他更为成熟的方法进行验证。