CARBON CYCLE DURING THE LATE PALEOCENE THERMAL MAXIMUM: RECONSTRUCTION FROM A MARINE CARBON CYCLE MODEL

Late Paleocene thermal maximum (LPTM) is characterized by an abrupt climate warming, negative carbon isotope excursion and benthic foraminifera extinction. The hydrate dissociation hypothesis, in which dissociation of large quantities of methane hydrate occurred and CH4 and/or CO2 with a carbon isotopic composition of -60 per mil were released to the ocean-atmosphere system, may explain the climate warming and the carbon isotope excursion at the LPTM.

Here, we try to reconstruct changes of the marine carbon cycle during the LPTM based on the hydrate hypothesis by using an inversion model with the carbon isotope record. We developed a one-dimensional marine carbon cycle model, which includes the total dissolved inorganic carbon, the total dissolved 13C, nutrients, and dissolved oxygen as dissolved constituents in the ocean. The model includes diffusive and advective transports of dissolved constituents within the ocean, input fluxes due to weathering of crustal rocks and release of CH4 by dissociation of hydrate, bioproduction of particulate carbonate and organic matter in the surface ocean, dissolution of biogenic particles in the deep ocean, and carbon isotope fractionation for each process. The surface water carbon isotopic composition obtained from planktonic foraminifers and the deep water carbon isotopic composition obtained from benthic foraminifers during this event are given as forcing of the model. Variation of global mean upwelling rate, the bioproductivity and vertical profiles of dissolved constituents are obtained from the model.

We found that the productivity of organic carbon and the global mean upwelling rate rapidly increase at the carbon isotope excursion event. Nutrients should be supplied to the surface water at this time. So the increased productivity may be consistent with the result of the enhanced upwelling rate. Oxygen minimum zone expands vertically at this event because large amount of biogenic particles are decomposed. This might have been caused extinction of benthic foraminifera at this event.