What were the effects of rapid ocean acidification events in the Earth’s past?

Global warming is not the only consequence of rising levels of carbon dioxide in the atmosphere. As CO₂ is an acidic gas, and a high proportion of the human emissions eventually get mixed into the ocean, the oceans are becoming more acidic. As humanity aims to limit emissions it is vital to predict what effect such acidification will have on marine life and chemical processes in the ocean. Experimental work suggests that plankton that construct their shells out of calcium carbonate ('calcify') may be particularly affected.

One way to predict the future is to study similar events in the geological past. The geological record contains a number of abrupt events where acidification increased rapidly in response to natural emissions (the largest being the Paleocene / Eocene thermal maximum event, 55 million years ago). So far the acidification during these events has not been measured adequately and details of the way marine life was affected have not been fully investigated.

The project will investigate a variety of geological records from the deep sea and the margins of the oceans. Unique among these is a newly recovered borehole through marine sediments in Tanzania. Preliminary work revealed the presence of a highly expanded (thick) record of the onset of the Paleocene / Eocene thermal maximum (PETM) event with very well-preserved planktonic microfossils that is suitable for new geochemical and biological studies.

Specific aims are:

• To produce new estimates of seawater acidity and atmospheric carbon dioxide levels for the period 65-40 million years ago
• To study the response of the carbon cycle during the rapid onset of the PETM using new data from Tanzania and elsewhere and new computer models
• To compare the PETM with other known smaller ocean acidification events in the Earth’s past
• To quantify the biotic response to ocean acidification events both directly (speciation, extinction, migration, malformation , etc.) and indirectly (ecosystem function, biogeochemical cycles)

The outputs of this programme will:

• Feed into the cross-government Climate Change Adaptation programme
• Make a significant contribution to the Living With Environmental Change programme
• Provide evidence to the IPCC 5th Assessment Report on Climate Change
• Provide information to marine bioresource managers, policy makers negotiating CO₂ emissions reduction and other stakeholders

This project will produce new data in the form of:

• Geochemical records (boron isotopes, oxygen isotopes, carbon isotopes, trace elements) through a variety of drill cores constraining the rate and amount of ocean acidification, temperature increase, and evolving atmospheric CO₂ levels
• High resolution quantitative biotic records of calcifying plankton and benthos during abrupt ocean acidification events (groups such as foraminifera, coccolithophorids and echinoderms)
• Carbon cycle models focusing on the functioning of the Earth System during past ocean acidification events of various magnitudes, constrained by the new data
• Identification of thresholds for observed biological responses and, as part of the broader thematic programme, comparison with experimental work on calcifying organisms and observations from the modern ocean