Ocean carbon cycling since the middle Miocene: Testing the metabolic hypothesis
This project focuses on marine carbon cycling since the Middle Miocene Climate Optimum (MMCO) 15 million years ago.
The ocean biological carbon pump, comprising photosynthesis, food web interactions and gravity, results in the removal of carbon from the surface ocean and its transport through the ocean to the deep (Fig. 1).
Only about 10% of the carbon exported from the surface ocean makes it to the deep and an even smaller fraction ends up on the sea floor. Biological processes such as photosynthesis and respiration in the euphotic zone, ingestion and respiration/remineralization of sinking organic matter in the mesopelagic zone (Fig. 2), control how much organic carbon reaches the deep ocean and eventually the ocean floor.
Ocean temperature appears to significantly affect the rate at which planktonic organisms process carbon (metabolism) with both photosynthesis and respiration occurring faster at warmer temperatures. However, heterotrophic respiration responds twice as fast to temperature changes than photosynthesis. This may potentially cause major changes to the carbon cycle, with more carbon being sequestered when ocean temperatures are cooler and vice versa when warming occurs.
Can this temperature dependency of metabolic rates in turn act as a global climate feedback? This intriguing idea is the 'Metabolic Hypothesis' as articulated by Olivarez Lyle and Lyle in 2006.
This project aims to answer this hypothesis by focusing on marine carbon and nutrient cycling since the Middle Miocene Climate Optimum (MMCO), that is, from about 15 million years ago (Ma) to the present – an interval of generally declining temperatures (Fig. 3).
We are using planktonic foraminifera (calcifying unicellular free-floating protists) as tracers for the biogeochemical cycling of carbon in the upper water column.
We are measuring oxygen and carbon stable isotopes on a range of (living and extinct) depth-stratified, size-constrained planktonic foraminiferal species (e.g. Fig. 4) to reconstruct past water column biogeochemistry and plankton ecology.
Foraminifera are picked from seven time slices spanning from the middle Miocene (15 Ma) up to the Pleistocene, and from ocean sediment samples from a range of latitudinally distributed Integrated Ocean Drilling Program (IODP), Ocean Drilling Project (ODP) and Deep Sea Drilling Project (DSDP) sites (Fig. 5).
Surface to depth δ13C and δ18O gradients will provide information about changes in the upper water column cycling of carbon and the evolution of deep niche habitats in planktonic foraminifera. Stable isotopes will be associated with planktonic foraminiferal assemblage data, to monitor changes in faunal diversity through time, and trace element ratios (in partnership with University College London).
We are using and developing an Earth system model incorporating temperature-dependencies in key ocean carbon cycle components to help interpret our data-based results and explore the relationship between water column temperature and carbon cycling.
By combining data and models, we will assess the consequences of temperature-dependent metabolism for past and future carbon cycling.
In order to model the Earth system, there is a balance to be had between the complexity of processes that are represented and the integration level of these processes. Simplified models and Earth system models of intermediate complexity (EMICs) are well suited to paleoclimate studies. We are able to run long, and transient simulations on reasonable timescales such that hypotheses for new processes can be tested.
CGENIE intermediate modelling
In this project we use the cGENIE intermediate complexity Earth system model, (Fig. 6), a 3D dynamic ocean coupled to a 2D atmosphere. The ocean includes a biogeochemical model of marine biota that simulates export production (“Biogem”). More recently, a plankton community model has been coupled to cGENIE (“Ecogem”), and we will use this to consider questions about plankton community changes.
To simulate the Miocene we are creating a new “Miocene World” for cGENIE. Locations of continents, ocean bathymetry and ocean circulation (that are different to the present day, Fig. 7) need to be validated before we can consider the carbon cycle.
Measurement data modelling
In the present-day, we have access to much ocean-based data, such as temperature, carbon-13 measurements, pH etc. This provides a means of validating our treatment of processes in the model for the present-day simulation. Paleo-data that reconstructs past climate conditions provides a second means of testing the model for paleo-simulations. At the same time, modelling studies can help to interpret changes seen in the data. Ultimately we can use the model to project possible future changes in the ocean carbon cycle, and to understand past changes.
We can add (and alter the treatment of) processes in the model. In this project, we are interested in metabolism and the system’s response to changes in temperature. The processes of interest include photosynthesis; respiration; remineralisation rates; and dissolved organic carbon cycling.
For the carbon cycle, we are interested in the flux of particulate organic carbon (POC flux) which tells us how much carbon is being transported to the deep from the surface. As an example, temperature dependent remineralisation and uptake results in more of the Particulate Organic Carbon (POC) that is exported at the surface reaching the deep (Fig. 8) in colder regions (and vice versa). In a globally warmer world this may mean that the ocean biological pump becomes less efficient.
U.S. DOE. 2008. Carbon Cycling and Biosequestration: Report from the March 2008 Workshop, DOE/SC-108, U.S. Department of Energy Office of Science. (p. 81) (website)
Zachos, J. C., Dickens, G. R., & Zeebe, R. E. (2008). An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature, 451(7176), 279-283.
Olivarez Lyle, A., & Lyle, M. W. (2006). Missing organic carbon in Eocene marine sediments: Is metabolism the biological feedback that maintains end‐member climates? Paleoceanography, 21(2).
This project is funded by the Natural Environment Research Council (NERC).