全新世长江口低氧区形成与演化历史及控制机制

Coastal hypoxia has been significantly deteriorated by human activities in the recent decades,arising broad concerns of scientists and the public as a "dead zone". The occurrence of seasonal hypoxia off the estuaries is actually a natural phenomenon, and it may have occurred since the Holocene. Reconstruction of hypoxia history has therefore become a research hotspot. The Changjiang Estuary is one of the most widespread hypoxia zones in the world, and its modern and recent behaviors and their sedimentary records have been studied relatively in detail. This research proposes to set up multi-proxy methodology for hypoxia deposit study through systematic analyses of modern and recent sediments in the hypoxia region, involving foraminiferal identification, and measurements of redox sensitive elements and isotopes, organic elements, and so on. The multi-proxy method will be employed to study Holocene hypoxia deposits among three borehole cores from the Changjiang Subaqueous Delta. It will be further discussed the hypoxia development and evolution history, and its controlling factors and mechanism in the Holocene through time-series analyses of their occurrences in comparison with paleo-river floods, sea-level change, and activities of Taiwan Warm Current, China Coastal Current, and East Asia Winter Monsoon. It aims to improve our understanding of the development history and future trend of coastal hypoxia under the natural condition and increasing human activities, and contribute a new representative study case, some referential methods and hypothesis to other LOICZ-II, LDE, and RiOMars studies.

近几十年来人类活动导致河口低氧现象不断恶化，成为"死亡区域"而备受各界关注。但河口发生季节性低氧是一种自然过程，可能在全新世某一阶段便开始出现，因此对低氧发育历史过程的研究正在成为新的热点。长江口是全球河口低氧发育最显著的区域之一，其近现代过程和沉积记录已得到较详细的研究。申请课题拟通过近现代低氧沉积记录的系统研究，包括运用有孔虫鉴定、氧化还原敏感元素和同位素、有机元素分析等手段，建立低氧沉积记录的判别标志；将该方法运用于水下三角洲三个连续取芯的钻孔全新世地层的低氧事件判别，结合精确的AMS 14C测年，以及全新世古洪水、海平面变化、台湾暖流、沿岸流与东亚冬季风活动规律的研究，详细探讨全新世长江口低氧区形成、演化历史与规律，及其控制因素与机制，增进对河口缺氧的自然过程和人类活动影响态势的理解，为LOICZ-II、LDE和RiOMars等研究计划提供一个典型的实例、可借鉴的方法与理论。

近几十年来人类活动导致河口低氧现象不断恶化，成为“死亡区域”而备受各界关注。长江口近期低氧程度不断加剧、低氧面积不断扩大，已成为全球低氧面积最大的区域之一。已有研究多侧重现场水文观测或数值模拟分析，对低氧事件沉积记录研究较少，尤其是全新世长时间尺度低氧发育历史、演化规律和控制机制研究非常缺乏。课题通过参加基金委公共航次，或是组织海洋地质国重室公共航次，开展了长江口低氧区及邻近海域百余站位的水文环境观测与沉积物采样工作，室内完成80余个表层沉积物样、四个短柱沉积物样和两口数十米长钻孔岩芯的沉积学、年代学、微量元素地球化学、钼同位素、有机元素地球化学测试分析，和有孔虫鉴定等工作。得出以下主要结论：（1）夏季长江冲淡水与台湾暖流相互作用是控制长江口低氧发育的主要因素；（2）氧化-还原敏感元素中Mo与V、钼同位素、耐低氧底栖有孔虫组合（Bolivina sp.、Bulimina marginata、Epistominella naraensis）可作为长江口低氧沉积记录的主要替代指标；（3）近三十几年长江口低氧呈阶段性恶化，与长江营养盐输入和赤潮发生频次有良好关系，反映近期人类活动的显著影响；（4）全新世长江口低氧发育始于七、八千年前，当时处于全新世高海面、长江口高度层化是发育低氧的重要条件，之后呈阶段波动发展，主要受东亚夏季风强度变化引起的径流量变化和黑潮水入侵东海陆架强度变化等有关，当径流增强、黑潮次表层水侵入增强时，低氧也会增强；反之低氧减弱；（5）世界上其它近海环境低氧发育大多也是一种自然现象，但近期受人类活动影响显著，使得低氧程度加剧并更频繁发生。一些论文发表后已引起国内外同行的高度关注，另外，成果为国内首个“海底观测网科学大工程”中东海网建设提供了科学依据。