Paleo-records of histories of deoxygeneration and its ecosystem impact

Abstract

Deoxygenation including coastal hypoxia induced by eutrophication, open-ocean expansion of oxygen minimum zone by global warming, and their consequences are some of the most serious problems in the ocean. But, public awareness on these problems has only been raised recently. Coastal hypoxia has been addressed much more than open-ocean oxygen issues. Thus, instrumental or biological monitoring data are often only a few decades long or usually shorter. Although notable exceptions exist, they are seldom longer than a century. Only paleo-records (paleontological, sedimentological, and biogeochemical data from sediment cores) allow us to reconstruct multi-decadal, centennial, millennial, and even longer time-scale histories beyond the period of monitoring, and thus are essential to understanding long-term dynamics of dissolved oxygen levels and ecosystem impacts. In this presentation, we overview the methods to reconstruct past oxygen levels using sediment cores, and showcase prime examples of paleo-reconstruction of deoxygenation history. Then, we introduce ecosystem degradation caused by historical deoxygenation. Paleo-records are important not only to reconstruct physicochemical parameters (i.e., dissolved oxygen levels) themselves, but also to reconstruct their consequences and impacts on marine ecosystem. It will help us to overcome the shifting baseline syndrome and lead us to a better understanding of the long-term nature of human impact, natural baseline, and long-term natural variation, aiding better management of marine environments and ecosystems.