Chapter 1 - Why Dissolved Organics Matter: DOC in Ancient Oceans and Past Climate Change

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Earth history is punctuated by a huge variety of transitions and perturbations in climate and global biogeochemical cycles. Some of these events exhibit evidence for greenhouse warming and CO2 release and hence potentially provide clues regarding future changes. Negative spikes and transients in the isotopic composition of carbon in sedimentary rocks bear the testimony of these carbon cycle perturbations and are generally taken to indicate the introduction of carbon with a relatively depleted isotopic signature into the ocean and atmosphere. One reservoir of isotopically depleted carbon is the dissolved organic carbon (DOC) present in the ocean. This has led to the idea that both the cycling of DOC and its reservoir size could have been fundamentally different in the past and that change in the oceanic DOC cycle may be mechanistically linked to major events in the geological record. This chapter provides an overview of how the proposed link between DOC and major global carbon cycle perturbation events in the geological record arises. We start by presenting a summary of the traditional view of how the marine carbon cycle operates. We then introduce how the geological record is interpreted, focusing on the ways in which the carbon isotopic signature of sedimentary rocks reflects past changes in global carbon cycling. We go on to critically assess recent thinking on the potential role of DOC as a driver for extreme climate events before touching on the future implications of a DOC cycle active on geological timescales.