Fingerprinting the source of rising CO2 during the last deglaciation
During the last deglaciation, about 20,000 to 12,000 years ago, the concentration of CO2 in the atmosphere increased from about 190 ppm to 270 ppm. In this study the stable carbon isotope ratio (ratio of carbon-13 to carbon-12) in the atmosphere is used to fingerprint the source of the CO2 increase. Since plants preferentially absorb the lighter isotope carbon-12, the stable isotope ratio is less in plants than in the atmosphere. In this study the stable carbon isotope ratio is reconstructed over the past 24,000 years from Antarctic ice cores. The time series reveals that during the first period of increasing atmospheric CO2 from 17,000 to 15,000 years ago the stable isotope ratio dropped precipitously indicating that a source of the CO2 was a large pool of carbon-13 depleted carbon of organic origin. There is independent evidence that this old carbon resulted from upwelling of deep water in the Southern Ocean.
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.
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.
At the very end of the glacial, there is a sharp drop in delta-carbon-13 from 17,500 to 15,000 years before the present. This occurs at the same time that atmospheric CO2 begins to rise. The precipitous drop in delta-carbon-13 corresponds to an increase in atmosphere CO2 of 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
suggest that the rise in CO2 and the decline in atmospheric delta-carbon-13 and 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 atmospheric delta-carbon-14 data indicate that this release of isotopically depleted carbon from the deep ocean to the atmosphere to the occurred over about 2000 years from 17,000 to 15,000 years before the present.
Delta-carbon-13 was 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.
As the figure shows there is evidence from several sources that the start of the deglaciation occurred around 17,000 years before the present. The evidence also reveals that the rise in CO2, the drop in delta-cartbon-14, the intense deposition of biogenic opal indicating 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 delta-carbon-13 occurred within the first 2000 years after the start of the deglaciation. The new, high-resolution delta-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 delta-carbon-13 suggest that deep ocean ventilation only explains part of of the CO2 increase during deglaciation. Later in the deglaciation, the evolution of delta-carbon-13 became more complicated, but probably involved the regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation.
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