Ocean carbon cycling since the middle Miocene: testing the metabolic hypothesis

Lead Research Organisation: Cardiff University
Department Name: Sch of Earth and Environmental Sciences

Abstract

Respiration - the process by which organic matter (food) is broken down to provide energy, releasing carbon dioxide - is strongly temperature-dependent. For every ten degrees increase in temperature, it occurs about 2 and a half times faster. We are respiring organisms but we don't notice this because our body temperatures are regulated, but cold-blooded creatures do, and so too do the most important respirers of all in terms of global processes - the bacteria and other microbes. This is why we put food in the fridge, and why a tropical swamp is a much more biologically active place than a temperate bog. Recently there has been a dawning realization among Earth System scientists that this marked temperature-dependency of microbial metabolism must be taken into account if we are to understand some of the big global feedbacks involved in climate change, and hence we should incorporate it into Earth System computer models.

One important process that helps regulate the amount of CO2 in the atmosphere occurs in the ocean, and is called the 'biological pump'. Algae photosynthesize in the photic zone at the surface, forming the base of the food chain. Most of this organic matter gets eaten up and respired in the surface layer and the CO2 is returned to the atmosphere, but a substantial proportion sinks to deeper water. Most of it does, eventually, also get broken down by bacteria, but here the CO2 released is isolated from the surface. Some of the organic matter can reach the sea floor where it can be incorporated into sediments, forming the hydrocarbon source rocks of the future. The rain of organic matter sinking to the deep sea and sediments produces a compensatory 'pump' of CO2 from the atmosphere to the ocean.

Now imagine we turn up the temperature in the water column as a result of climate change. This is good news for the bacteria which use up the sinking organic matter more efficiently. Less carbon gets removed from the surface ocean hence CO2 accumulates in the atmosphere until a new balance is restored. Because CO2 is an important greenhouse gas, contributing to global warming when it is in the atmosphere, this process could theoretically accentuate the warming process, or work the other way round on a cooling planet.

It is important that we understand how important this feedback is in the real world, and what knock-on effects it may have in other parts of the Earth System. We have devised a way of studying it in the Earth's past, using fossil sediments from the sea floor. We plan to take a series of sediment samples spanning the last 15 million years across the oceans to investigate the efficiency of the biological pump. The planet has cooled markedly over this period so we predict major changes to the functioning of ocean ecosystems and the biological pump. We will study the chemical composition of fossil shells of foraminifera (microscopic protists that occur in large numbers) that lived distributed through the water column. By using a combination of geochemical techniques we can establish the temperature profile, pH profile, and strength of the biological pump.

To explore the data we will use a specially modified version of a state-of-the-art Earth System Model that will take into account temperature-dependency of metabolic processes. We will then use the model to investigate its impact on a range of globally important factors such as patterns of organic carbon burial and atmospheric carbon dioxide, and investigate how important these factors are for future climate change.

We predict that global cooling over the last 15 million years has produced improved oxygenation and food supply in deep planktonic niches (the so-called 'twilight zone' of the ocean) and that this would have spurred evolutionary innovation at depth. We will test this idea by studying plankton abundance patterns at depth in time and space and investigating whether there has been enhanced evolution in this environment.

Planned Impact

This research will have positive impact in several areas:

1. Hydrocarbon Exploration Industry: Hydrocarbon exploration frequently involves modelling of spatial and temporal patterns of organic matter deposition, accumulation, and source rock formation. The proposed research has the potential of revolutionizing this field with the incorporation of metabolic rate effects on bacterial degradation in the ocean water column and, potentially, in the sediment. Our preliminary cGENIE model results already show very different patterns of organic matter accumulation when metabolic temperature-dependency is taken into account, and we will be greatly augmenting and improving this work. Because we do not propose to model shelf and slope environments, our approach would need to be extended to incorporate the most important areas of hydrocarbon accumulation. So realistically our research will have the greatest impact by raising awareness of the importance of metabolic rate effects and specific issues that may not be obvious, like the significance of sub-thermocline temperatures; moreover it is important that temperature-dependency of metabolic rates is appropriately quantified, and this project will provide real-world geological data from past warm climate states that could help with this aspect of the problem, and might be extended to warmer periods of accumulation such as the Jurassic. The project will also produce new data on the taxonomy and geographic distribution of planktonic foraminifera. These microfossils are widely used in industry for biostratigraphic correlation.

2. British and world Public: Public interest in climate change is at an all-time high but the issues surrounding ecological changes in the ocean, the feedbacks involved, and potential effects on the composition of plankton communities, are poorly known. Hence it is vital that innovative and striking ways of reaching the public are sought. The deep-dwelling plankton of the 'twilight zone' has an intrinsic fascination that may help us frame our science to maximize public interest (see below). Despite a political consensus on the need for action on reducing CO2 emissions, the public attitude is mixed and better public understanding is essential for creating the political will for concerted action.

3. Climate policy makers: Palaeoclimate forms part of the Intergovernmental Panel on Climate Change's Fifth Assessment report on the Science of Climate Change (IPCC 2013-14), which the principal investigator contributed to as co-author. Understanding the efficacy of models at capturing warm climate states is important for assessing how well they can predict future scenarios. However feedbacks on global climate change via metabolic rate effects are not covered in that report and are not currently incorporated into most predictions of future climate change. Although such feedbacks are likely to be strongest on long timescales (hundreds to thousands of years) because they require deep ocean warming, the issue is nevertheless important for the nearer future and partly impacts predictions of seawater oxygenation and acidifiction.

Tanzania science base: Tanzania is one of the world's poorest countries but has recently benefitted from a series of gas discoveries offshore, including one very large discovery of major economic importance. Our field results will be of benefit to understanding the geology of the onshore sedimentary basins. Just as important, our collaboration via Project partner Tanzania Petroleum Development Corporation (TPDC) helps develop scientific expertise within Tanzania.