A large increase in the carbon inventory of the land biosphere since the Last Glacial Maximum: constraints from multi-proxy data

Abstract

Abstract. Atmospheric CO2 increased by about 90 ppm across the transition from the Last Glacial Maximum (LGM) to the end of the preindustrial (PI) period. The contribution of changes in land carbon stocks to this increase remains uncertain. Estimates of the PI-LGM difference in land biosphere carbon inventory (∆land) range from −400 to +1,500 GtC, based on upscaling of scarce paleo soil carbon or pollen data. A perhaps more reliable approach infers ∆land from reconstructions of the stable carbon isotope ratio in the ocean and atmosphere assuming isotopic mass balance with recent studies yielding ∆land values of about 300–400 GtC. Surprisingly, however, earlier studies considered a mass balance for the ocean–atmosphere–land biosphere system only. Thereby, these studies neglect carbon exchange with sediments, weathering-burial flux imbalances, and the influence of the deglacial reorganization on the isotopic budgets. We show this neglect to significantly bias low deglacial ∆land in simulations using the Bern3D Earth System Model of Intermediate Complexity v.2.0s. We constrain ∆land to ∼ 850 GtC (median estimate; 450 to 1250 GtC 1σ range) by using reconstructed changes in atmospheric δ13C, marine δ13C, deep Pacific carbonate ion concentration, and atmospheric CO2 as observational targets in a Monte Carlo ensemble with half a million members. Sensitivities of the target variables to changes in individual deglacial carbon cycle processes are established from factorial simulations over the past 21,000 years 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. Uncertainties in the estimate of ∆land remain considerable due to model and proxy data uncertainties. Yet, it is likely that ∆land is larger than 450 GtC and highly unlikely that the carbon inventory in the land biosphere was larger for the LGM than during the recent preindustrial period.

Evaluating the Extent of North Atlantic Deep Water and the Mean Atlantic δ13C From Statistical Reconstructions

“…If the system is assumed to be closed, given the low δ 13 C signature of terrestrial carbon (−22‰ to −25‰), and with the knowledge of the atmospheric CO 2 concentration, δ 13 CO 2 , and an estimate of global mean oceanic δ 13 C, total terrestrial carbon can theoretically be calculated using a simple mass balance approach. However, Jeltsch‐Thömmes et al () have shown that under the assumption of a closed system, estimates can be skewed because the accumulation of carbon in sediments, input fluxes from rock weathering, and the burial of consolidated sediments have been neglected. By considering these factors, Jeltsch‐Thömmes et al () suggest that the terrestrial carbon reservoir was ∼850 Gt C (median estimate) smaller during the LGM than preindustrial, compared to recent difference estimates of 300–400 Gt C (Menviel et al, ; Peterson et al, ).…”

Very intersting study on glacial oxygen

“…Several studies point to the potentially important role of the ocean/sediment/lithosphere fluxes of organic matter and how the associated burial-nutrient feedback modifies the magnitude and time scales of the response in CO2 and other tracers to changes in the marine biological cycles (Wallmann et al, 2016;Roth et al, 2014;Jeltsch-Thömmes et al, 2018). (Tschumi et al, 2011), for example, quantify the implication of oceansediment-lithosphere coupling for an experiment where the ocean P inventory is increased.…”