Introduction


In this paper high-resolution stable carbon ratios for atmospheric CO2 are measured from Antarctic ice cores. On the time scales of thousands of years the stable carbon ratio of CO2 in the atmosphere is controlled by the exchange of dissolved inorganic carbon in the sea with the atmosphere and by climate changes in carbon storage in the terrestrial biosphere. For example, burning of fossil fuels from about 1850 has changed the stable isotope ratio of atmospheric CO2.

Observations


In this study stable carbon isotope ratios were measured from two Antarctic ice cores—EPICA (European Project for Ice Coring in Antarctica) Dome C and Talos Dome. To ensure the reproducibility of the measurements, three independent methods for determining the stable carbon isotope ratio were used in two different labs.

Carbon-13 ratios and CO2
Carbon-13 ratios and CO2
Figure Atmospheric stable isotope ratios and CO2 concentration from ice cores for the past 24,000 years. (A) Stable isotope ratios for atmospheric CO2. (B) CO2 concentration from EPICA Dome C (EDC). kyr BP is thousands of years before the present.

Results


At the very end of the last ice age from 17,500 to 15,000 years before the present, there was a sharp drop in stable isotope ratios. This occurred at the same time that atmospheric CO2 begins to rise for the first time. The increase in atmospheric CO2 in this period amounted to about 35 ppmv.

Comparison of these results with other evidence including
  • Delta-carbon-14 – a decrease in this ratio reflects the addition of old (carbon-14 depleted) carbon to the atmosphere
  • Biogenic opal deposition - proxy for local upwelling
  • Record of ice-rafted debris (IRD) in the North Atlantic associated with Heinrich events HS1 and HS2.
  • Delta-oxygen-18 - Greenland temperature proxy
  • Atmospheric CH4 concentration
  • Delta-deuterium from the EDC ice core - Antarctic temperature proxy
  • Atmospheric CO2
suggests that the rise in CO2 and the decline in atmospheric carbon-13 and radiocarbon (delta-carbon-14) between 17,400 and 15,000 years before the present resulted from bringing deep water old carbon into exchange with the atmosphere. This interpretation is supported by a study of deep sea corals that revealed that the deep glacial Southern Ocean ventilated its carbon-14-depleted reservoir during this interval. The new, high-resolution data indicate that the release of isotopically depleted carbon from the deep ocean to the atmosphere occurred over about 2000 years from 17,000 to 15,000 years before the present.

Comparison of Carbon-13 ratios to other time series for the past 24000 years
Comparison of Carbon-13 ratios to other time series for the past 24000 years
Figure Evolution of major components of the Earth climate system over the past 24,000 years. (A) Delta-carbon-14 ratios from various sources (B) Atmospheric delta- carbon-13 ratios from this study (C) Biogenic opal deposition as a proxy for local upwelling. (D) Record of ice-rafted debris (IRD) in the North Atlantic associated with Heinrich events HS1 and HS2. (E) Greenland temperature proxy delta-oxygen-18. (F) Atmospheric CH4 concentration. (G) Antarctic temperature proxy delta-deuterium from the EDC ice core. (H) Atmospheric CO2 - green bars indicate a strong net terrestrial carbon buildup; blue bars indicate release of sequestered deep ocean CO2 back into the atmosphere. PB, Preboreal; YD, Younger Dryas cooling; B/A, Bølling-Allerød warming; DO2, Dansgaard-Oeschger event 2; ACR, Antarctic Cold Reversal.

Stable isotope ratios were stable during the last ice age which indicates that the buildup of the carbon reservoir in the oceans must have occurred before 24,000 years before the present.

The start of the last deglaciation began around 17,000 years before the present. The evidence reveals that the rise in CO2, the drop in atmospheric radiocarbon, the intense deposition of biogenic opal which is a proxy for vigorous Southern Ocean upwelling, and the ice-rafting at the beginning of the first Heinrich event (HS-1) all occurred simultaneously.

In this context the ice core record reveals that the precipitous drop in carbon-13 occurred within the first 2000 years after the start of the deglaciation. The new, high-resolution carbon-13 record indicates that the release of isotopically depleted carbon from the deep ocean to the atmosphere occurred during the period 17,400 to 15,000 years before the present. During this interval atmospheric CO2 concentration rose from 190 ppmv to 220 ppmv, which is only about 35% of the rise to the pre-industrial average in the present warm period. The flattening and then increase in the stable isotope ratio suggest that deep ocean ventilation only explains part of of the CO2 increase during the deglaciation. Later in the deglaciation, the evolution of the stable isotope ratio became more complicated, but probably involved the regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation.

Source


Carbon Isotope Constraints on the Deglacial CO2 Rise from Ice Cores, Jochen Schmitt, Robert Schneider, Joachim Elsig, Daiana Leuenberger, Anna Lourantou, Jérôme Chappellaz, Peter Köhler, Fortunat Joos, Thomas F. Stocker, Markus Leuenberger, Hubertus Fischer, Science 11 May 2012:Vol. 336, Issue 6082, pp. 711-714 DOI: 10.1126/science.1217161