Stable isotope constraints on Holocene carbon cycle changes from an Antarctic ice core

Abstract

Reconstructions of atmospheric CO 2 concentrations based on Antarctic ice cores 1,2 reveal significant changes during the Holocene epoch, but the processes responsible for these changes in CO 2 concentrations have not been unambiguously identified. Distinct characteristics in the carbon isotope signatures of the major carbon reservoirs (ocean, biosphere, sediments and atmosphere) constrain variations in the CO 2 fluxes between those reservoirs. Here we present a highly resolved atmospheric d 13C record for the past 11,000 years from measurements on atmospheric CO 2 trapped in an Antarctic ice core. From mass-balance inverse model calculations 3,4 performed with a simplified carbon cycle model, we show that the decrease in atmospheric CO 2 of about 5 parts per million by volume (p.p.m.v.). The increase in d 13C of about 0.25% during the early Holocene is most probably the result of a combination of carbon uptake of about 290 gigatonnes of carbon by the land biosphere and carbon release from the ocean in response to carbonate compensation of the terrestrial uptake during the termination of the last ice age. The 20 p.p.m.v. increase of atmospheric CO 2 and the small decrease in d 13C of about 0.05% during the later Holocene can mostly be explained by contributions from carbonate compensation of earlier land-biosphere uptake and coral reef formation, with only a minor contribution from a small decrease of the land-biosphere carbon inventory.The Holocene is the current interglacial period, starting about 11,000 years before present (11 kyr BP, where present is defined as AD 1950) following the Transition (here defined as 18-11 kyr BP) from the last glacial maximum. Variations in the atmospheric concentration of CO 2 during the Holocene were significant but small compared to glacial-interglacial changes of typically 100 p.p.m.v. (refs 5, 6). Yet a decrease of about 5 p.p.m.v. from 11-7.5 kyr BP could be observed, followed by an increase of about 20 p.p.m.v. to the pre-industrial level of about 280 p.p.m.v. (refs 1, 2, 7). Different explanations for these variations were discussed 7,8 , such as changes in the carbon inventories of vegetation, soils and peatlands 9 , in anthropogenic land use 10,11 , in sea surface temperature (SST) 7,12 , coral reef growth 13,14 or carbonate compensation 15 . The latter is a multi-millennial equilibration process of the atmosphere-ocean-sediment system and the weathering cycle. Moreover, model simulations of atmospheric CO 2 and d 13C during the Holocene have not provided an unambiguous quantitative explanation 7,8,16 . The major stumbling block has been the scarcity of reconstructions of d We focus on the evolution of the carbon isotopes on a timescale of a few thousand years. Therefore, we calculated a spline and its 1s uncertainty bands with a cut-off period of 5 kyr (Fig. 2). In a Monte Carlo simulation, standard deviations smaller than 0.07% were

A revised 1000 year atmospheric δ13C‐CO2 record from Law Dome and South Pole, Antarctica

“…Even though another high-resolution record of ı 13 C over the Industrial Period was produced by Kawamura et al [2000] from the Antarctic H15 ice core, unexplained differences from the F99 data set of, on average, 0.2-0.3 in the mean level and up to 0.5 around 1800 A.D. were found. Records of ı 13 C covering the last glacial period [Machida et al, 1996;Smith et al, 1999] or the Holocene period [Indermühle et al, 1999;Elsig et al, 2009] do not both span the LPIH and overlap firn measurements and have used the F99 record to link their longer records to direct atmospheric measurements. No other paleoarchives (speleothems, corals, and sediments) have been found to be comparably useful, especially in the Industrial Period.…”

Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation

“…Finally, decreased AMOC has been hypothesized to slow the delivery of low preformed nutrient water to the deep ocean and consequently drive a weakening of the biological pump. Experiments with the Model of Ocean Biogeochemistry and Isotopes/University of Victoria climate model of intermediate (10), Bern EDC data (orange circles) (11,45), sublimation measurements from EDC (blue triangles), and Talos Dome (purple squares) with an estimate of the 1-sigma uncertainty from a compilation of previous ice core δ 13 C-CO 2 data (11). (14) and discrete CO 2 (blue) (13) concentration data plotted with Taylor Glacier CO 2 and δ 13 C-CO 2 data (this study) (red markers, black line is a smoothing spline), the five-point running Keeling intercept with shading indicating the R 2 for each time interval.…”

Holocene peatland and ice-core data constraints on the timing and magnitude of CO2emissions from past land use

“…Positive values represent land C uptake. Shown is total global change in land C (∆Ctot) as inferred from the CO 2 and δ 13 C ice-core records (green bars) (10,17). Peatland C balance (∆Cpeat) is shown as derived from YML data (blue bars) and LPX simulations (black circles).…”

“…∆Ctot is inferred from a double-deconvolution analysis of ice-core atmospheric CO 2 and δ 13 C measurements at the millennial Holocene timescale from ref. 10 …”

“…In addition, the total terrestrial C balance, derived from the measured parallel evolution of atmospheric CO 2 and its isotopic signature (δ 13 C), suggests a small reduction of only 36 ± 37 PgC since 5 kyBP (10). Further, an ocean C-cycle modeling study (11) shows that the Holocene (since 11.7 kyBP) CO 2 and δ 13 C evolutions are quantitatively explained by the combination of the reconstructed terrestrial C balance (10) and ocean C-cycle changes after the Last Glacial Maximum (LGM) with delayed effects during the Holocene, associated with terrestrial carbon changes and the carbonate compensation mechanism (4,11,12), sea level rise, and coral reef buildup (13,14).…”