Philippe Ciais,1 Dan Zhu,1 Shushi Peng,1 Tao Wang,1 Gerhard Krinner,1 Sergei A. Zimov,2 Alessandro Tagliabue,1 Matthias Cuntz,3 Laurent Bopp,1 and Colin Prentice4
Quantifying the state of the global carbon cycle during the Last Glacial Maximum (LGM) is needed to understand how CO2 changed by some 80 ppm during the last deglaciation. Two isotopic tracers of the carbon cycle: 18O in O2 and 13C in the oceanic and atmospheric carbon reservoirs, were used by Ciais et al. (2011) to quantify the state of the carbon cycle during the Last Glacial Maximum (LGM) some 21,000 years ago.
Using δ13C measurements in the atmosphere from ice cores, and in the ocean from ben-thic foraminifera, we estimated the distribution of carbon into the land and ocean reservoirs during the LGM period. Our mass balance approach to bring a data driven constraint on the LGM carbon pools is derived from Bird et al. (1996). Namely, we assume that both the mass of both C and of its stable isotope 13C in the atmosphere-land-ocean carbon system is constant between the LGM and Holocene. Parameters entering into this calculation are: (i) atmospher-ic CO2 concentration and its δ13C composition, which were both constrained by new ice cores measurements, (ii) changes in δ13C of ocean dissolved carbon, that was diagnosed to 0.34 ± 0.05 ‰ using a new database of cibicides benthic foraminifera in 133 ocean cores, with 60 cores below 3 km depth (see http://motif.lsce.ipsl.fr ), or alternatively from an ensemble of 3D LGM ocean circulations simulations compatible with these data (Tagliabue et al. 2009) and (iii) the δ13C of the land biosphere during the LGM and pre-industrial periods, which was tested for values ranging between 0 ‰ and 2 ‰ higher than today, according to vegetation reconstructions (Bird et al. 1996) or to land carbon model calculations. Uncertainty in each parameter was propagated to the estimates of land and ocean carbon pools.
The main results of the Ciais et al. (2011) study indicated a low terrestrial gross primary productivity (40 ± 20 Pg C a–1) and a high marine productivity (60 ± 10 Pg C a–1) during the LGM as compared to the pre-industrial Holocene (PIH). Moreover, the terrestrial biosphere of the LGM contained “only” 370 PgC less than during the pre-industrial Holocene, which
1 Laboratoire des Sciences du Climat et de l’Environnement, CE Orme des Merisiers, 91191 Gif sur Yvette, France;
philippe.ciais@cea.fr.
2 Northeast Science Station, Pacific Institute for Geography, Russian Academy of Sciences, Cherskii 678830, Rus-sia; sazimov55@mail.ru.
3 Helmholtz Centre for Environmental Research – UFZ, Leipzig, Germany; mcuntz@bgc-jena.mpg.de.
4 AXA Chair of Biosphere and Climate Impacts, Department of Life Sciences, Grand Challenges in Ecosystems and the Environment (Silwood Park) and Grantham Institute for Climate Change, Imperial College, London; Colin.
Prentice@bristol.ac.uk.
P. Ciais, D. Zhu, S. Peng, T. Wang, G. Krinner, S. A. Zimov, A. Tagliabue, M. Cuntz, L. Bopp,and C. Prentice
56 Nova Acta Leopoldina NF 121, Nr. 408, 55 –57 (2015)
implies both a smaller than previously thought “biosphere regrowth” storage of terrestrial carbon between LGM and PIH and a smaller “ocean loss” of carbon to the atmosphere.
Combining the two global atmospheric constraints on gross primary productivity (GPP) and global land/ocean carbon storage, with a Bayesian inversion of the area of five mega-bi-omes, we infer during the LGM: (i) a small extent of tropical forests compared to PIH, (ii) a large extension of cold steppe and tundra biomes with carbon-rich soils, and (iii) the existence of a pool of inert carbon of 2300 Pg C matched with the typical 13 C signature of organic car-bon. This elusive inert organic carbon pool needed to close global mass balance of carbon and GPP, could be located in Yedoma sediments that had continuously and slowly been formed since 50,000 years BP over non-glaciated areas, in marine organic sediments, in wetlands that might have developed in the exposed continental shelves during the LGM and in peat.
Yet, reconstructions of LGM biomes from Pollen data suggest that peatland extent during the LGM should be much smaller than today.
To gain insights on the location of LGM inert pool (LIP), we will present results of the simulation of soil carbon in the Northern Hemisphere buried below the active layer by cryo-turbation, and of peatland potential distribution from flooded area during the LGM, using climate forcing anomalies from PMIP and the ORCHIDEE-MICTv4 land surface model that contains a specific parameterization of soil C formation and decomposition in the high-lat-itude frozen soils, coupled with a module simulating the effect on productivity and carbon storage of large herbivores mega-fauna present during the Pleistocene.
The LIP pool is defined to be stable with respect to the equilibrium between climate and the carbon cycle during the late glacial period, but it is not inert during the deglaciation and the Holocene when the carbon cycle was not in equilibrium with climate. In particular, the large shifts in climate and increase of atmospheric CO2 from 20 to 11 ka BP, together with the disappearance of ice sheets and sea-level rise, should have an impact on the stability of the LIP.
Thus, more insights are needed about the possible dynamics of this inert pool throughout the deglaciation history and the Holocene. An attempt to quantify boreal, temperate, and tropical forest storage change between LGM and PIH and the use of additional data from peat accumulation during the Holocene lead us to infer that during the deglaciation and evidence of Yedoma degradation during the Holocene warm period suggests that the LIP lost 700 Pg C to the atmosphere and ocean between LGM and PIH. This net loss is the sum of a gross loss of 1,400 Pg C and a gross gain of 600 PgC of carbon due to slow peat accumulation since the Early Holocene. The lack of evidence for large CO2 increases during the Holocene suggests that the 1,400 PgC loss should have occurred between 20,000 ka BP and the early Holocene period.
In absence of information about the timing of this large release of terrestrial carbon to the atmosphere and ocean we can only speculate on the signature of this process on observed changes in the δ13C of ocean dissolved carbon, changes in ∆14C of ocean dissolved carbon, in δ13C of atmospheric carbon during the last glacial transition.
An Attempt to Quantify Terrestrial Carbon Storage during the Last Glacial Maximum
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References
Bird, M. I., Llyod, J., and Farquhar, G. D.: Terrestrial carbon-storage from the last glacial maximum to the pres-ent. Chemosphere 33, 1675 –1685 (1996)
Ciais, P., Tagliabue, A., Cuntz, M., Bopp, L., Scholze, M., Hoffmann, G., Lourantou, A., Harrison, S. P., Prentice, I. C., Kelley, D. I., Koven, C., and Piao, S. L.: Large inert carbon pool in the terrestrial biosphere during the Last Glacial Maximum. Nature Geosci. 5/1, 74 –79 (2012)
Tagliabue, A. Bopp, L., Roche, D. M., Bouttes, N., Dutay, J.-C., Alkama, R., Kageyama, M., Michel, E., and Paillard, D.: Quantifying the roles of ocean circulation and biogeochemistry in governing ocean carbon-13 and atmospheric carbon dioxide at the last glacial maximum. Clim. Past 5, 695 –706 (2009)
Philippe Ciais, Ph.D.
Laboratoire des Sciences du Climat et de l’Environnement
CE Orme des Merisiers 91191 Gif sur Yvette France
Phone: +33 1 69089506 Fax: +33 1 69087716 E-Mail: philippe.ciais@cea.fr
Nova Acta Leopoldina NF 121, Nr. 408, 59 – 63 (2015)
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