Low terrestrial carbon storage at the Last Glacial Maximum: constraints from multi-proxy data

Past changes in the inventory of carbon stored in vegetation and soils remain uncertain. Earlier studies inferred the increase in the land carbon inventory (Delta land) between the Last Glacial Maximum (LGM) and the preindustrial period (PI) based on marine and atmospheric stable carbon isotope reconstructions, with recent estimates yielding 300-400 GtC. Surprisingly, however, earlier studies considered a mass balance for the ocean-atmosphere-land biosphere system only. Notably, these studies neglect carbon exchange with marine sediments, weathering-burial flux imbalances, and the influence of the transient deglacial reorganization on the isotopic budgets. We show this simplification to significantly reduce Delta land in simulations using the Bern3D Earth System Model of Intermediate Complexity v.2.0s. We constrain Delta land to similar to 850 GtC (median estimate; 450 to 1250 GtC +/- 1SD) by using reconstructed changes in atmospheric delta C-13, marine delta C-13, deep Pacific carbonate ion concentration, and atmospheric CO2 as observational targets in a Monte Carlo ensemble with half a million members. It is highly unlikely that the land carbon inventory was larger at LGM than PI. Sensitivities of the target variables to changes in individual deglacial carbon cycle processes are established from transient factorial simulations with the Bern3D model. These are used in the Monte Carlo ensemble and provide forcing-response relationships for future model-model and model-data comparisons. Our study demonstrates the importance of ocean-sediment interactions and burial as well as weathering fluxes involving marine organic matter to explain deglacial change and suggests a major upward revision of earlier isotope-based estimates of Delta land.