Causes and consequences of widespread ocean anoxia in the past

Oceanic oxygen concentrations vary widely in space and time. Since oxygen is essential for (most) life, oceanic oxygen concentrations influence the distribution and diversity of life in the marine realm. The surface ocean is rich in oxygen because it is in direct contact with the atmosphere. Deeper waters, however, lose oxygen with time, because it is consumed by (micro)organisms that feed on organic matter produced by plankton that sinks towards the sea floor. Oxygen-rich surface water has to be mixed downwards to supply the deep ocean with oxygen. If mixing is inhibited or the supply of organic matter to deeper waters intensifies (leading to increasing oxygen consumption), deeper water layers may become devoid of oxygen, hence development of anoxia. At present, oxygen concentrations are decreasing in many regions of the world’s oceans, due to human intervention. Much research aims to elucidate the mechanisms and time scales of this so-called deoxygenation, to better project future trends. Here, these issues are addressed by studying massive ocean anoxia during the greenhouse climate system in the mid-Cretaceous (Cenomanian-Turonian Oceanic Anoxic Event; OAE2; ~94 million years ago; with an approximate duration of 600.000 years) and a more recent anoxic period in the Eastern Mediterranean (Sapropel S1; 10.000-6.000 before present). More specifically, existing marine palynological (notably fossil remains of dinoflagellates, a group of unicellular plankton) and geochemical data were compiled and new data were generated for critical sediment cores to understand the key drivers responsible for ocean anoxia and the biosphere and climate feedbacks. The results of this study show that climate-driven changes in temperature and hydrology, have been of particular importance for the occurrence of widespread ocean anoxia during OAE2 and sapropel S1. This is illustrated by the strong influence of river discharge from the Nile on the formation of Eastern Mediterranean sapropel S1, which not only led to freshwater stratification-induced preservation of organic matter, but also contributed to enhanced production of organic matter by delivering nutrients for marine primary productivity, eventually leading to increased oxygen consumption. This mechanism was also shown to be responsible for organic matter deposition during OAE2 on flooded continental shelves. As a consequence of extreme warmth, the hydrological cycle accelerated. Enhanced precipitation and runoff ultimately led to freshwater stratification and enhanced marine primary productivity, culminating in widespread ocean anoxia and organic carbon sequestration. Interestingly, temporarily cooler and drier conditions during OAE2, correspond to a minimum in sedimentary organic carbon content. This minimum can be tracked throughout the, during OAE2, generally anoxic deep proto-North Atlantic basin, suggesting large-scale oxygenation of the water column. This again implies that temperature and hydrology were crucial factors in determining bottom water oxygen concentrations. This cooling event not only diminished organic carbon sequestration, but was also shown to drive equatorward migration of dinoflagellate species, showing that relatively small changes in temperature during the mid-Cretaceous super-greenhouse world had significant implications for biogeographical patterns