Evaluating the Biological Pump Efficiency of the Last Glacial Maximum Ocean using δ 13 C

Abstract. Although both physical and biological marine changes are required to explain the 100 ppm lower atmospheric pCO2 of the Last Glacial Maximum (LGM, ~ 21 ka) as compared to pre-industrial (PI) times, their exact contributions are debated. Proxies of past marine carbon cycling (such as δ13C) document these changes, and thus provide constraints for quantifying the drivers of long-term carbon cycle variability. This modelling study explores the relative roles of physical and biological changes in the ocean needed to simulate an LGM ocean in satisfactory agreement with proxy data, and here especially δ13C. We prepared a PI and LGM ocean model state (NorESM-OC) with full biogeochemistry (including the carbon isotopes δ13C and radiocarbon) and dynamic sea ice. The modelled LGM-PI differences are evaluated against a wide range of physical and biogeochemical proxy data, and show agreement for key aspects of the physical ocean state within the data uncertainties. However, the lack of a simulated increase of regenerated nutrients for the LGM indicates that additional biogeochemical changes are required to simulate an LGM ocean in agreement with proxy data. In order to examine these changes, we explore the theoretical effects of different global mean biological pump efficiencies on the simulated marine biogeochemical tracer distributions. We estimate that (besides changes in ocean circulation) an approximate doubling of the global mean biological pump efficiency from 38 % (PI) to 75 % (LGM) reduces model-proxy biases the most. The remaining absolute model-proxy error in δ13C (which is 0.07 ‰ larger than the 0.19 ‰ data uncertainty) indicates that additional changes in ocean dynamics are needed to simulate an LGM ocean in agreement with proxy data, such as increased aging or volume of Southern Source Waters. Besides that, our theoretical approach of increasing the biological pump efficiency may be too simplified to capture the vertical redistribution of regenerated nutrients – also suggested by a too weak chemocline. Our results underline that only those coupled climate models that contain the processes and/or components that realistically change both ocean circulation and biogeochemistry will be able to simulate an LGM ocean in satisfactory agreement with proxy data – and hence be reliable for use in climate projections. Therefore, future research should aim to identify the exact physical and biogeochemical processes that could have doubled the global mean biological pump efficiency (i.e., the interior regenerated signature) between the PI and LGM, with a likely central role for Southern Source Waters.