Tracking the history of Baltic Sea hypoxia with bivalve shells

海底缺氧（低于2毫克/升的溶解氧，DO）是沿海和近海水域日益严重的环境问题。严重和长期的低氧条件对海底生物来说可能是致命的，破坏底栖生物群落，造成生态系统紊乱，并最终形成死区。波罗的海是世界上最大的缺氧区，其缺氧的空间分布、强度和持续时间在上个世纪显著增加，无论是近岸浅海还是近海深水。由于缺乏资料，沿海缺氧的发生在时间和空间上的特征远不如较深水域。然而，主要的未解决的问题涉及波罗的海缺氧急剧扩大的原因：物理强迫（分层）和营养驱动的富营养化有多重要？强烈的盐斜阻止含氧地表水向下混合，它受气候控制，并由河流淡水流入和降水以及来自北海的盐水流入维持。相反，人为的养分输入通过分解有机物极大地刺激了初级生产和氧气消耗。为了获得对这些复杂过程的机械理解，需要长时间、高分辨率的记录。目前还没有这样的数据。事实上，目前缺乏DO水平的长期监测数据集，而且对波罗的海缺氧历史的了解（在仪器时代之前）几乎完全依赖于层状沉积物中存档的代用物。虽然后者可以提供长期DO趋势的独特见解，但它们的时间分辨率较差，无法从浅层设置中获得，并且无法量化DO水平。计划项目的主要目标是建立一个几乎未开发的缺氧和富营养化状态档案，即长寿命双壳类动物的壳。我们将开发一种多代理方法来重建DO (Mn/Ca, Fe/Ca, I/Ca, delta34S)，并应用该技术以前所未有的时间分辨率推断波罗的海选定地区的晚全新世缺氧历史。为此目的，将使用两种长寿双壳类动物的活壳和亚化石标本，即arctic islandica和Astarte borealis。将构建主年表，提供过去200年左右关于DO水平的不间断的季节性到年际解决信息。此外，放射性碳（AMS）定年的亚化石壳将用于打开环境/气候基线条件的窗口。为了阐明近几十年来物理强迫和人为富营养化对缺氧区急剧扩大的相对重要性，我们还将开发和应用富营养化状态的代用指标（delta15N, P/Ca）。计划研究的结果可以转移到其他海洋环境，从而将大大推进海洋生态学研究。

Bottom hypoxia (less than 2 mg/L dissolved oxygen, DO) is a growing environmental problem in coastal and offshore waters. Severe and prolonged low-oxygen conditions can be lethal for bottom dwellers, destroy benthic communities, cause ecosystem disturbances and eventually form dead zones. The spatial distribution, intensity and duration of hypoxia in the Baltic Sea, home of the largest oxygen-deficient zone, has increased strikingly during the last century, both in shallow nearshore and deeper offshore settings. Due to lacking data, the occurrence of coastal hypoxia in time and space is much less well characterized than in deeper waters. Yet, the main unresolved question concerns the causes for the drastic expansion of hypoxia in the Baltic Sea: How important are physical forcings (stratification) and nutrient-driven eutrophication? A strong halocline preventing downward-mixing of oxygenated surface waters is controlled by climate and maintained by riverine freshwater influx and precipitation as well as saline inflows from the North Sea. Conversely, anthropogenic nutrient input has greatly stimulated primary production and oxygen consumption by decomposing organic matter. To obtain a mechanistic understanding of these complex processes requires long, high-resolution records. Such data are currently not available. In fact, long-term monitoring data sets of DO levels are lacking, and knowledge of the history of Baltic Sea hypoxia (prior to the instrumental era) relies almost exclusively on proxies archived in laminated sediments. While the latter can provide unique insights into long-term DO trends of, they come with poor temporal resolution, are not available from shallow settings and do not permit to quantify DO levels.The main objective of the planned project is to establish a virtually untapped archive for hypoxia and eutrophic state, namely shells of long-lived bivalves. We will develop a multiproxy approach for the reconstruction of DO (Mn/Ca, Fe/Ca, I/Ca, delta34S) and apply this technique to infer the Late Holocene history of hypoxia in selected regions of the Baltic Sea with unprecedented temporal resolution. For this purpose, shells of live-collected and subfossil specimens of two long-lived bivalve species will be used, namely Arctica islandica and Astarte borealis. Master chronologies will be constructed that provide uninterrupted seasonally to inter-annually resolved information on DO levels during the last 200 years or so. In addition, radiocarbon (AMS) dated subfossil shells will be used to open windows into environmental/climate baseline conditions. In order to disentangle the relative importance of physical forcings and anthropogenic eutrophication on the drastic expansion of the hypoxic zone in recent decades, we will also develop and apply proxies for the eutrophic state (delta15N, P/Ca). Results of the planned study can be transferred to other oceanic settings and will thus significantly advance marine ecology research.