THE MARINE BIOLOGICAL PUMP IN THE EOCENE: LESSONS FOR THE FUTURE?

Temperature-dependency of metabolic rates including photosynthesis and respiration could mean that large ecosystem-scale processes operated quite differently in past warm climate states, and may do so again. In this contribution we focus on the marine 'biological pump' that transfers carbon from the surface mixed layer into the deep ocean and sea floor sediments. We test the hypothesis of Olivarez-Lyle and Lyle (2006, Paleoceanography v. 21, PA2007) that warm ocean temperatures through the water column would have caused faster (relative to modern) rates of bacterial degradation of sinking organic matter and hence carbon remineralization, reducing the efficacy of the biological pump. We show how depth-stratified planktonic foraminifera can be used to reconstruct vertical carbon isotope profiles of dissolved inorganic carbon for the Eocene. These profiles are much steeper than is typical of the modern ocean, which is consistent with the metabolic hypothesis. We use a 3D model of intermediate complexity to show how our reconstructed carbon isotope profiles can be modeled satisfactorily by incorporating moderate temperature-dependency to bacterial respiration rates. This result has many other implications for warm climate states that need exploring, for example regarding the depth stratification of marine plankton niches, the depth and intensity of oxygen minimum zones, nutrient cycling in the water column, ocean/atmosphere carbon exchange, and the locus of organic matter burial. As anthropogenic global warming takes us 'forward to the past' we might expect similar changes to occur once again. But because the biological pump is sensitive to temperatures over the top 1 km of the water column rather than simply the sea surface temperature, the response is likely to be slow.