Impact of land-sea organic matter fluxes on the ocean biogeochemistry during the Last Deglaciation

The Last Deglaciation (21-10 ka) is the most recent transition from a glacial to interglacial state. It is characterized by a pronounced sea level change of 95 m resulting in flooding of land areas and changes of coastlines. This period is also marked by several millennial events like the Heinrich Event 1 with diverse effects on sea level, oceanic circulation, climate and carbon cycle. In case of flooding of land surfaces during periods of sea level rise, carbon and nutrients stored in terrestrial organic matter in vegetation and soils are transferred to the ocean, potentially impacting the global ocean biogeochemical cycle and the uptake/release of CO₂ once being remineralized. Changes in the ocean biogeochemical cycles are also indirectly related to the poorly constrained stoichiometry and remineralization time-scales of terrestrial organic matter, which both differ from the well-known parameters for marine organic matter.

We present here the first coupled transient simulation over the Last Deglaciation using the global ocean biogeochemical model HAMOCC (HAMburg Ocean Carbon Cycle) as part of the paleo-version of the MPI-ESM (Max Planck Institute Earth System Model) to study the impact of terrestrial organic matter input on the ocean biogeochemical cycle and oceanic CO₂ fluxes during large sea level variations. This model version combines (1) a fully interactive adaptation of the ocean bathymetry with corresponding changes of the land-sea distribution, (2) a transient river routing and (3) the land-sea terrestrial organic matter transfer after flooding. Our simulation provides new insights on the land carbon inputs to the ocean carbon inventory (water column and sediment) due to flooding, with 170 GtC between 21-10 ka, of which 21.1 GtC and 36.8 GtC are within two 1000 years large freshwater discharge events (between 15-14 ka and 12-11 ka). These inputs of carbon rich material to the ocean during flooding events have however only a local effect on ocean CO₂ outgassing, the global ocean remaining a sink of CO₂. To infer the response of CO₂ fluxes in this context, sensitivity experiments can be performed during the type of Heinrich event (15-14 ka) to evaluate and better constrain the terrestrial organic matter remineralization parameters.