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

Abstract

Abstract. Past changes in the inventory of carbon stored in vegetation and soils remain uncertain. Earlier studies inferred the increase in the land carbon inventory (Δland) 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 Δland in simulations using the Bern3D Earth System Model of Intermediate Complexity v.2.0s. We constrain Δland to ∼850 GtC (median estimate; 450 to 1250 GtC ±1SD) 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. 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 Δland.

Peatland area and carbon over the past 21 000 years – a global process based model investigation

“…Peatlands contribute about 40% to the total land biosphere carbon increase of 892 GtC. The result for the total land carbon increase between LGM and PI is in good accordance with a recent estimate, integrating multiple proxy constraints (median: 850 GtC; 450 -1250 GtC ± 1 standard deviation ) (Jeltsch-Thömmes et al, 2019). The model also simulates the total change 20 https://doi.org/10.5194/bg-2020-110 Preprint.…”

The response to pulse-like perturbations in atmospheric carbon and carbon isotopes

“…The Bern3D consists of a three dimensional geostrophic ocean (Edwards et al, 1998;Müller et al, 2006) with an isopycnal diffusion scheme and Gent-Mc Williams parameterization for eddy-induced transport (Griffies, 1998), a single layer energy-moisture balance atmosphere to which a thermodynamic sea-ice component is coupled (Ritz et al, 2011). We use the ocean model coupled to a 10 layer sediment module (Tschumi et al, 2011;Heinze et al, 1999) with updated sediment parameters (Jeltsch-Thömmes et al, 2019) and to a global representation of rock weathering (Colbourn et al, 2013).…”

“…CC BY 4.0 License. 2018a, b;Jeltsch-Thömmes et al, 2019). Isotopic fractionation between model components is documented in Jeltsch-Thömmes et al (2019).…”

“…Denitrification is not considered in this model version, but O 2 is not consumed below a threshold, somewhat reflecting the process of denitrification without modeling NO − 3 . The respective reaction rate parameters for CaCO 3 dissolution and POC oxidation are global constants (see Roth et al, 2014;Jeltsch-Thömmes et al, 2019). The model assumes conservation of volume, i.e.…”

Benthic foraminiferal stable carbon isotope constraints on deglacial ocean circulation and carbon-cycle changes

“…During repeated glaciations of the past ~1 million years (Myr), about 100 ppm of CO 2 from the atmosphere was temporarily sequestered in the terrestrial biosphere and ocean. Although terrestrial biosphere carbon storage may have increased or decreased (see Jeltsch-Thömmes et al 2019 and references therein) between the Last Glacial Maximum (LGM, ~20,000 years (20 kyr) before present) and the preindustrial period, the vast, deep-ocean reservoir most likely controls glacial-interglacial carbon cycling and, hence, atmospheric CO 2 variations. On these timescales, ocean circulation and biological productivity influence carbon distribution in the deep ocean and regulate glacial carbon sequestration and deglacial CO 2 outgassing.…”