Phosphorus concentrations and accumulation rates of ODP Leg 130 sites

Little is known about the fluxes to and from the ocean during the Cenozoic of phosphorus (P), a limiting nutrient for oceanic primary productivity and organic carbon burial on geologic timescales. Previous studies have concluded that dissolved river fluxes increased worldwide during the Cenozoic and that organic carbon burial decreased relative to calcium carbonate burial and perhaps in absolute terms as well. To examine the apparent contradiction between increased river fluxes of P (assuming P fluxes behave like the others) expected to drive increased organic carbon burial and observations indicating decreased organic carbon burial, we determined P accumulation rates for equatorial Pacific sediments from Ocean Drilling Program leg 138 sites in the eastern equatorial Pacific and leg 130 sites on the Ontong Java Plateau in the western equatorial Pacific. Although there are site specific and depth dependent effects on P accumulation rates, there are important features common to the records at all sites. P accumulation rates declined from 50 to 20 Ma, showed some variability from 20 to 10 Ma, and had a substantial peak from 9 to 3 Ma centered at 5-6 Ma. These changes in P accumulation rates for the equatorial Pacific are equivalent to substantial changes in the P mass balance. However, the pattern resembles neither that of weathering flux indicators (87Sr/86Sr and Ge/Si ratios) nor that of the carbon isotope record reflecting changes in organic carbon burial rates. Although these P accumulation rate patterns need confirmation from other regions with sediment burial significant in global mass balances (e.g., the North Pacific and Southern Ocean), it appears that P weathering inputs to the ocean are decoupled from those of other elements and that further exploration is needed of the relationship between P burial and net organic carbon burial.

关于新生代期间海洋磷（P）的收支通量——磷是地质时间尺度上海洋初级生产力与有机碳埋藏的限制性营养盐——相关研究迄今仍较为匮乏。既往研究指出，新生代时期全球范围内河流溶解态磷通量均呈上升趋势，且有机碳埋藏相较于碳酸钙埋藏有所减少，甚至可能在绝对量级上也出现下降。为厘清这一看似矛盾的现象：一方面，若磷通量与其他元素通量行为一致，则河流磷通量增加将推动有机碳埋藏提升；另一方面，观测结果显示有机碳埋藏反而有所下降——本研究对赤道太平洋沉积物的磷堆积速率进行了测定，样品分别取自东赤道太平洋大洋钻探计划（Ocean Drilling Program, ODP）138航次站位，以及西赤道太平洋翁通爪哇高原（Ontong Java Plateau）的130航次站位。尽管磷堆积速率存在站位特异性与深度依赖性，但所有站位的沉积记录均呈现出共通的重要特征：磷堆积速率在50~20 Ma期间持续下降，20~10 Ma期间存在一定波动，并在9~3 Ma期间出现显著峰值，峰值中心位于5~6 Ma。赤道太平洋地区的磷堆积速率变化，对应着磷物质平衡的显著改变。然而，这一变化模式既不同于风化通量指标（87Sr/86Sr比值与Ge/Si比值）的变化模式，也未体现出反映有机碳埋藏速率变化的碳同位素记录特征。尽管上述磷堆积速率模式尚需在全球物质平衡中占据重要地位的其他沉积区域（如北太平洋与南大洋）得到验证，但现有结果表明，海洋的磷风化输入通量与其他元素的风化输入通量并不耦合，且磷埋藏与净有机碳埋藏之间的关联仍有待进一步深入探索。