A middle Eocene carbon cycle conundrum

1 The global carbon cycle The global carbon cycle encompasses the sum of processes that determine the amount of carbon within, and fluxes between, different carbon reservoirs on Earth (Fig. 1). These processes are quantitatively important on a range of timescales and can induce both short-term fluctuations and changes in steady-state conditions1–3. On timescales of up to centuries and thousands of years (kyr), such processes include photosynthesis, respiration and the short-term transfer between surface ocean, atmosphere, biosphere and soils. Annually, these processes transfer ~190 petagrams (1 Pg = 1015 g) of carbon between the surface reservoirs4. On timescales of 10–100 kyr, deep ocean circulation and orbitally paced climate changes (such as glacial–interglacial dynamics) affect global exogenic carbon cycling3. For example, ocean degassing and warming during the last glacial–interglacial transition (~20–10 kyr ago) probably caused a transfer of ~500 Pg of carbon from the ocean to the atmosphere and terrestrial biomass5. Traditionally, variations in the carbon inventory of the ocean– atmosphere system on timescales exceeding 100 kyr have been attributed to changes in the steady-state balance between sources and sinks into and out of the global exogenic carbon cycle (Fig. 1). The dominant inputs are volcanic degassing and weathering of carbonate and organic carbon, and two sinks primarily balance the input of carbon over such timescales6,7. First, carbon fixed in organic matter may be buried as organic carbon in marine and terrestrial sedimentary basins. Second, the weathering of silicates on land6 transfers atmospheric CO2 into dissolved the hydrogen carbonate ion (HCO3), which is transported to the oceans by rivers. In the ocean, HCO3 becomes part of the carbonate system and is used together with the carbonate ion (CO3) for calcification by marine biota, such as coccolithophores and foraminifers, producing solid calcium carbonate (CaCO3). Burial of CaCO3 in marine sediments completes the silicate-weathering process, resulting in a net sink for CO2. When biogenic CaCO3 is exported towards the sea floor, the calcite saturation state (Ω = [Ca][CO3]sea water / [Ca][CO3]saturation) becomes lower because of increasing pressure and acidity with depth. In open ocean settings, a fraction of the sinking calcite particles will A middle Eocene carbon cycle conundrum