Evidence of CO2 outgassing from the Southern Ocean early in the last deglaciation

The last deglaciation, which stretched from about 19,000 to 11,000 years ago, was characterized by rising temperatures and pulses of increased atmospheric carbon dioxide, methane and other gases.  Carbon dioxide in the atmosphere originates from two sources, the oceans, which contain much more carbon dioxide than the atmosphere, and biofeedback.  In this study the radiocarbon (carbon-14) record for Southern Ocean deep and intermediate depths covering 35,000 to 10,000 years ago has been reconstructed from deep-sea corals. The record reveals that Southern Ocean deep water was radiocarbon-depleted throughout the last ice age, but the depletion and the deep stratification of the southern ocean disappeared between 16,600 and 14,600 years ago.  Since the oceans contain ten times as much carbon as the atmosphere, this is consistent with major Southern Ocean outgassing that contributed to the first pulse of increased atmospheric carbon dioxide between 17,000 and 14,500 years ago.

Background

Over the last deglaciation there was increase in atmospheric CO2 of about 80 parts per million. This increase occurred in pulses. During the first pulse, roughly a 2000 year period ending 14,500 years ago, there was a precipitous drop in the ratio of atmospheric carbon-13 to carbon-12 accompanied by a substantial reduction in the ratio of carbon-14 (radiocarbon) to carbon-12 in the atmosphere. Carbon-13 depletion is the hallmark of an organic carbon source because plants preferentially take up carbon-12 from the atmosphere. Radiocarbon depletion is characteristic of an organic source that has been isolated from the atmosphere for thousands if not tens of thousands of years.  Radiocarbon is generated in the atmosphere by cosmic rays but decays radioactively with a half-life of about 6,000 years.

Since the oceans contain ten times as much carbon as the atmosphere and ocean deep water contained radiocarbon-depleted old carbon during the last ice age, this atmospheric analysis suggested that the radiocarbon-depleted CO2 released into the atmosphere originated from the oceans. This study examined corals that lived at depths corresponding to southern ocean deep and intermediate waters to find evidence of deep water upwelling which would have resulted in the outgassing of radiocarbon-depleted CO2 into the atmosphere.

Observations

The Drake Passage is a known region of mixing and upwelling of major deep-water masses.  In this study a time series of radiocarbon measurements from deep-sea corals collected from sites in the Drake Passage in the Southern Ocean at depths corresponding to modern-day deep and intermediate waters, Antarctica intermediate water (AAIW), Upper circumpolar deep water (UUDW), and Lower circumpolar deep water (LCDW), were reported. Uranium-thorium (U-Th) dating was used to develop an independent calendar age.

Analysis

Atmospheric radiocarbon compared to deep sea radiocarbon during last deglaciation Science
(A) Atmospheric CO2 from Antarctica ice cores. (B) Temperature proxy (delta-18O) from Antarctica ice cores. (C) Atmospheric radiocarbon ratios. (D) Radiocarbon ratios from various Drake Passage sites: Antarctica intermediate water (blue), upper circumpolar deep water (red), and lower circumpolar deep water (brown). (E) Opal flux as a proxy for Southern Ocean nutrient upwelling

The Drake Passage deep sea radiocarbon records reveals that during the last glacial maximum deep water near Antarctica in the southern ocean was radiocarbon depleted, indicating that the deep water was more isolated from the atmosphere than it is today. The deep sea radiocarbon records captured at different locations and at different depths indicate that there was a stratification between different deep water masses that persisted during the ice age and reduced vertical mixing across water-mass boundaries and bound old carbon in deep water.

The most pronounced feature of the atmospheric radiocarbon record is the drop beginning at about 17,000 years ago and ending about 14.500 years ago. Over this period the deep sea record analyzed in this study reveals that at about 16,600 years ago the stratification and the radiocarbon depletion of deep water began to decline.  During the interval from 16,600 to 15,600 years ago, the radiocarbon ratio in deep water increased at the same rate that the radiocarbon ratio in the atmosphere decreased. This is consistent with the ventilation of radiocarbon-depleted carbon into the atmosphere from the deep ocean via the Southern Ocean.

Conclusion

Over the last deglaciation there was an increase in atmospheric CO2 in pulses. During the first pulse, 16,600 to 14,500 years ago, both atmospheric radiocarbon and carbon-13 ratios dropped. The carbon-13 drop indicates injection of CO2 from a carbon source of plant origin.   The drop in atmospheric radiocarbon indicates that the source contains old carbon, which is a characteristic of Southern Ocean deep water because during the ice age there was little mixing of the different strata.  This research concluded that the depletion of radiocarbon in the atmosphere is indicative of outgassing of carbon dioxide from Southern Ocean deep water resulting from increased mixing during this early period of the deglaciation.

The Southern Ocean’s role in carbon exchange during the last deglaciation, A. Burke and L. F. Robinson Science 335, 557 (2012). doi:10.1126/science.1208163