Deoxygenation and organic carbon burial in past oceans

Abstract

Within the content of this thesis, I aim to qualitatively and quantitatively assess the driving mechanisms and consequences of marine deoxygenation and CORG burial in a greenhouse world (Fig. 1.5). The focus lies on instances of locally and globally enhanced deoxygenation (Fig. 1.6): the Holocene Baltic Sea (Chapter 2), three sapropels deposited in the Pliocene and Quaternary Mediterranean Sea (Chapter 3), the PETM (Chapters 3 and 4), OAE2 (Chapters 3 and 5) and Toarcian Oceanic Anoxic Event (Chapter 3). Key questions I aim to answer are the causes of deoxygenation on a local (Chapter 2) and global (Chapter 4) scale, the controls of P mineral formation on P recycling under anoxia (Chapter 3) and the global impact of enhanced redox-driven P recycling (Chapters 3 ‒ 5), and the effect of redox-driven CORG burial on the global carbon cycle (Chapters 4 and 5). In order to unravel the causes of extensive deoxygenation and its impact on CORG burial, I paired new geochemical and palynological sediment analyses, with the analysis of existing data sets and biogeochemical box modelling.

In summary, this thesis provides a framework for the causes and consequences of marine deoxygenation and CORG burial. Warming, both regional (Chapter 2) and global (Chapters 3 and 4), promotes the loss of dissolved O2 from ocean waters and the accumulation of CORG in marine sediments. Deoxygenation enhances P recycling relative to CORG. This recycling was more pronounced during greenhouse periods than at present (Chapters 2 – 4). During the PETM (Chapter 4) and OAE2 (Chapter 5), the geochemical cascade leading to the buildup of CORG in marine sediments was caused by increased CO2 emissions. The burial of CORG had a profound effect on the exogenic carbon cycle, resulting in the drawdown of CO2 from the atmosphere.

The findings presented in this thesis provide indications for the possible future evolution of marine biogeochemical cycles. The projected increase in temperature (IPCC, 2013) will lead to loss of O2 from open ocean waters, while it may also hinder the recovery of coastal areas (e.g. the Baltic Sea) from deoxygenation, upon nutrient input reductions (Chapter 2). Acidification will not only affect calcifying organisms and marine alkalinity, but may enhance P recycling and, hence, eutrophication (Chapter 3). As the oceans warm and CaCO3 dissolution spreads into more areas of the deep ocean (e.g. Sulpis et al., 2018), P recycling from deep marine sediments may increase. The impact of CORG burial on atmospheric CO2 (Chapters 4 and 5), even during a mild deoxygenation event such as the PETM (Chapter 4), underlines the importance of including this process and its causes in global biogeochemical and climate models when assessing the lifetime of our emissions in the atmosphere.