Carbon-13 over last deglaciation

Southern Ocean outgassing initial source of rising CO2 during the last deglaciation

During last deglaciation stretching from about 19,000 to 11,000 years ago, the concentration of CO2 in the atmosphere increased from about 190 parts per million to 270 ppm. This occurred in pulses.  The first pulse of CO2 began about 17,000 and and ended about 14,500 years ago.   In this study the stable carbon isotope ratio (ratio of carbon-13 to carbon-12) in the atmosphere is used to qualify the source of the CO2 increase. Since plants preferentially absorb the lighter isotope carbon-12, the carbon-13 ratio is less in plants than in the atmosphere. In this study the carbon-13 ratio is reconstructed over the past 24,000 years from Antarctic ice cores. The time series reveals that during the first pulse of increasing atmospheric CO2 from 17,000 to 15,000 years ago the carbon-13 ratio dropped precipitously indicating that a source of the CO2 was a large pool of carbon of organic origin.  Comparison with other data including the atmospheric carbon-14 record point at outgassing from Southern Ocean deep water as the source of the CO2 increase in this early period of the deglaciation.

Background

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.

Analysis and conclusions

Comparison of C-13 with other variables over last deglaciation
(A) Atmospheric carbon-14 ratio. (B) Atmospheric carbon-13 ratio with sea surface temperature correction. (C) Opal flux in the Southern Ocean as a proxy for local upwelling. (D) Ice-rafted debris (IRD) in the North Atlantic associated with Heinrich events HS1 and HS2. (E) Greenland temperature proxy delta-18O. (F) Atmospheric CH4 concentration. (G) Antarctic temperature proxy delta-D. (H) Atmospheric CO2 concentration. Green bars indicate intervals with a strong net terrestrial carbon buildup; blue bars indicate intervals where sequestered deep ocean CO2 was released back to the atmosphere.
PB, Preboreal; YD, Younger Dryas; B/A, Bølling-Allerød warming; DO2, Dansgaard-Oeschger event 2; ACR, Antarctic Cold Reversal.

At the very end of the last ice age from 17,500 to 16,000 years before the present, there was a sharp drop in atmospheric carbon-13 ratio. This occurred at the same time as the first pulse of increasing atmospheric CO2. The increase in atmospheric CO2 in this period amounted to about 35 parts per million.

Comparison of these results with other evidence ( see figure) suggests that the rise in CO2 and the decline in atmospheric carbon-13 and radiocarbon (carbon-14) ratios 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.

Stable isotope ratios were stable from 24,000 to 17,000 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 outgassing of carbon-14 depleted carbon from the deep ocean to the atmosphere occurred during the period 17,400 to 15,000 years ago. During this interval atmospheric CO2 concentration rose from 190 ppm to 220 ppm, 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 carbon-13 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 carbon-13 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