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High resolution δ13C measurements from the EPICA Dome C ice core
Abstract
Measurements of the isotopic composition of carbon dioxide were performed on EPICA Dome C ice on 76 different depth levels covering the last 40'000 years. The time resolution is in the order of 500 years for the last 18'000 years. For each depth level at least two determinations were obtained. The δ 13 C signals show different trends during the last 18000 years that are anti-parallel to the CO 2 concentration evolution as measured on the same ice core. However millennial scale deviations from these trends are observed for at least three time periods. The robustness and significance of these deviations are investigated by Monte Carlo simulations performed with different subsets of the measurements. The decreases of carbon isotopes could be connected with observed step-like increases of the CO 2 concentration. Furthermore, a similar evolution as for stable carbon isotopes is visible for detrended radiocarbon. We will discuss potential mechanisms responsible for the trends as well as for the millennial scale deviations in carbon-13, including changes in the thermohaline circulation as well as potential influences of a changing 17 O-18 O relationship.
A new mechanism is proposed in which climate, carbon cycle and icesheets interact with each other to produce a feedback that can produce quasi-100 ky glacial-interglacial cycles. A key process is the burial and preservation of organic carbon by icesheets. The switch from glacial maximum to deglaciation is triggered by the ejection of glacial burial carbon when icesheets grow to sufficiently large size and subglacial transport becomes significant. Glacial inception is initiated by CO<sub>2</sub> drawdown due to a ''rebound'' from a high but transient interglacial CO<sub>2</sub> value as the land-originated CO<sub>2</sub> invades into deep ocean via thermohaline circulation and CaCO<sub>3</sub> compensation. Also important for glacial inception is the CO<sub>2</sub> uptake by vegetation regrowth in the previously ice-covered boreal regions. When tested using a fully coupled Earth system model with comprehensive carbon cycle components and semi-empirical physical climate components, it produced self-sustaining glacial-interglacial cycles of duration about 93 ky, CO<sub>2</sub> change of 90 ppmv, temperature change of 6°C under certain parameter regimes. Since the 100 ky cycles can not be easily explained by the weak Milankovitch astronomical forcing alone, this carbon-climate mechanism provides a strong feedback that could interact with external forcings to produce the major observed Quaternary climatic variations.
A series of 14C measurements in Ocean Drilling Program cores from the tropical Cariaco Basin, which have been correlated to the annual-layer counted chronology for the Greenland Ice Sheet Project 2 (GISP2) ice core, provides a high-resolution calibration of the radiocarbon time scale back to 50,000 years before the present. Independent radiometric dating of events correlated to GISP2 suggests that the calibration is accurate. Reconstructed 14C activities varied substantially during the last glacial period, including sharp peaks synchronous with the Laschamp and Mono Lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments. Simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14C production rate together with plausible changes in deep-ocean ventilation and the global carbon cycle during glaciation.
Nitrous oxide (N2O) concentration records exist for the last 1000 years and for time periods of rapid climatic changes like the transition from the last glacial to today's interglacial and for one of the fast climate variations during the last ice age. Little is known, however, about possible N2O variations during the more stable climate of the present interglacial (Holocene) spanning the last 11 thousand years. Here we fill this gap with a high-resolution N2O record measured along the European Project for Ice Coring in Antarctica (EPICA) Dome C Antarctic ice core. On the same ice we obtained high-resolution methane and carbon dioxide records. This provides the unique opportunity to compare variations of the three most important greenhouse gases (after water vapor) without any uncertainty in their relative timing. The CO2 and CH4 records are in good agreement with previous measurements on other ice cores. The N2O concentration started to decrease in the early Holocene and reached minimum values around 8 ka (
The focus of this paper is the conversion of radiocarbon ages to calibrated (cal) ages for the interval 24,000–0 cal BP (Before Present, 0 cal BP = AD 1950), based upon a sample set of dendrochronologically dated tree rings, uranium-thorium dated corals, and varve-counted marine sediment. The 14 C age–cal age information, produced by many laboratories, is converted to Δ 14 C profiles and calibration curves, for the atmosphere as well as the oceans. We discuss offsets in measured l4 C ages and the errors therein, regional 14 C age differences, tree–coral 14 C age comparisons and the time dependence of marine reservoir ages, and evaluate decadal vs . single-year 14 C results. Changes in oceanic deepwater circulation, especially for the 16,000–11,000 cal BP interval, are reflected in the Δ 14 C values of INTCAL98.
Radiocarbon data from the Cariaco Basin provide calibration of the carbon-14 time scale across the period of deglaciation (15,000 to 10, 000 years ago) with resolution available previously only from Holocene tree rings. Reconstructed changes in atmospheric carbon-14 are larger than previously thought, with the largest change occurring simultaneously with the sudden climatic cooling of the Younger Dryas event. Carbon-14 and published beryllium-10 data together suggest that concurrent climate and carbon-14 changes were predominantly the result of abrupt shifts in deep ocean ventilation.
A record of atmospheric carbon dioxide (CO2) concentration during the transition from the Last Glacial Maximum to the Holocene, obtained from the Dome Concordia, Antarctica,
ice core, reveals that an increase of 76 parts per million by volume occurred over a period of 6000 years in four clearly
distinguishable intervals. The close correlation between CO2 concentration and Antarctic temperature indicates that the Southern Ocean played an important role in causing the CO2 increase. However, the similarity of changes in CO2 concentration and variations of atmospheric methane concentration suggests that processes in the tropics and in the Northern
Hemisphere, where the main sources for methane are located, also had substantial effects on atmospheric CO2 concentrations.
- M Stuiver
- P Reimer
- E Bard
- J Beck
- G Burr
- K Hughen
- B Kromer
- F Mccormac
- J V D Plicht
- M Spurk
M. Stuiver, P. Reimer, E. Bard, J. Beck, G. Burr, K. Hughen, B. Kromer, F. McCormac, J. v.d. Plicht, and
M. Spurk (1998), INTCAL98 radiocarbon age calibration, 24,000, Radiocarbon, 40, 1041-1083.