Mediterranean deoxygenation events: Clues from the deep past to understand contemporary and future environmental conditions

Globally, the open ocean has lost more than 2% of its oxygen, oxygen minimum zones are expanding, the volume of anoxic waters is increasing, and an increasing number of coastal sites are being classified as hypoxic. This is fundamentally caused by continuous greenhouse gas-driven global warming. Because oxygen plays a crucial role in the biological and biogeochemical processes within the ocean, it is fundamental for us to try to better constrain the mechanisms behind ocean deoxygenation on different time and spatial scales, especially if we aim to predict future scenarios. Interestingly, the Mediterranean Sea, a semi-enclosed marginal sea, has experienced past events of extreme, long-lasting, basin-wide bottom water anoxia punctuated throughout its geologic archive as dark, organic-rich sediment layers, known as sapropels. Although these events were caused by natural climate changes, the oceanographic mechanisms behind them are similar to those behind the current deoxygenation problem: increased precipitation, increased river runoff, increased thermal/density stratification, a decrease in deep water circulation, and increased productivity. Using multiproxy paleoecology (foraminfera, calcareous nannofossils, pollen, dinocysts) and geochemistry (foraminifera stable isotopes and Element/Calcium), this thesis presents four studies pertaining to sapropels deposited during different time periods (Pleistocene, Miocene), different climates (glacial, interglacial), and in different paleoenvironments (shallow, deep) with the goal of better understanding past deoxygenation events. The paleoecological results of this work reveal that each studied sapropel was deposited by different combinations of mechanisms depending on location and climate. We also find that diagenetic alteration plays an important role in foraminiferal geochemistry of the Mediterranean Sea. Lastly, we successfully applied a fairly new deoxygenation proxy to foraminifera collected within a sapropel, which is something that has never been tested before.