渤海有机碳埋藏对近百年沉积汞的约束

Constraints of organic carbon burial on sedimentary mercury in the Bohai Sea over the past century

全球变暖背景下,近海陆架有机碳埋藏及其环境效应不仅是海洋碳循环的关键环节,而且对全面认识人类活动对近海生态系统的影响也具有重要意义。本研究基于2013年在渤海中部泥质区由箱式取样器采集的53 cm长的沉积岩芯,综合分析了近百年来总有机碳(TOC)及其稳定碳同位素(δ^(13)C)、生物标志物和沉积汞等指标的高分辨率沉积记录,探讨了不同来源有机碳的演变特征及其对沉积汞埋藏的约束作用。通过对有机碳δ^(13)C进行Suess效应校正,并利用双端元混合模型,估算得到海源有机碳贡献率约为59%~90%,且自1970年以来显著增加。海源有机碳与菜籽甾醇和甲藻甾醇的演化趋势基本一致,指示自1970年以来海洋初级生产力显著增加,可能与营养物质输入和气候变暖有关。沉积汞的埋藏记录与上述生产力参数的变化趋势总体一致,并与1970年后的海源有机碳等指标呈显著的正相关,表明近几十年浮游植物生产力的增加可能对沉积汞的累积具有重要作用,因此生物作用可能对近海汞的沉积和归宿具有不可忽视的影响。进一步对比了近期人为大气汞的排放量和沉积汞埋藏量的相对变化,发现大约2000s以后,沉积汞埋藏的比率明显下降(相对于排放量),这可能与同期营养盐和浮游植物群落结构改变(如甲藻显著增加)导致的生物对汞的清除作用效率降低有关。

In the global warming scheme,organic carbon burial and its environmental effects on coastal shelves are important in the marine carbon cycle,and a window for understanding the impact of modern human activities on coastal ecosystems.A 53-cm long core was acquired by box-coring in 2013 from the central mud deposition area of the Bohai Sea,North China.The high-resolution sedimentary records of total organic carbon(TOC),stable carbon isotopes(δ^(13)C),biomarkers,and sedimentary mercury over the past century were comprehensively analyzed,and the evolutionary characteristics of organic carbon from various sources and constraining effects on mercury burial were clarified.By correcting the δ^(13)C of organic carbon for the Suess effect and applying a two-endmember mixing model,we estimated that marine organic carbon contributes approximately 59% to 90%,with a marked increase since the 1970.The trends of marine organic carbon align closely with those of brassicasterol and dinosterol,indicating a significant increase in marine primary productivity since the 1970,due likely to increased nutrient input and climate warming.The burial records of sedimentary mercury are generally consistent with the trends of the aforementioned productivity parameters and exhibit a significant positive correlation with marine organic carbon and other indicators since the 1970.This suggests that the increase in phytoplankton productivity in recent decades may have played a crucial role in the accumulation of sedimentary mercury.Therefore,biological processes may have a significant impact on the sequestration and fate of mercury in coastal areas.A further comparison of the relative changes in recent anthropogenic atmospheric mercury emissions and sedimentary mercury burial rates revealed a significant decline in the burial efficiency relative to emissions after the 2000s.This decline may be related to concurrent changes in nutrient levels and phytoplankton community structure,such as a notable increase in dinoflagellates,which potentially reduced the efficiency of biological mercury scavenging.