Introduction


During the last deglaciation there was increase in atmospheric CO2 of about 80 ppmv. This increase occurred in pulses. During the first pulse of CO2 increase, 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 (delta-carbon-13). This was accompanied by a substantial reduction in the ratio of radiocarbon (carbon-14) to carbon-12 (delta-carbon-14) in the atmosphere. Carbon-13 depletion is the hallmark for an organic 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 and is radioactive with a half-life of about 6,000 years).

Since the oceans contain ten times as much carbon as the atmosphere, these observations suggests that the carbon-13 and 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


In this study a time series of radiocarbon from deep-sea corals collected from sites in the Southern Ocean at depths corresponding to modern-day Upper Circumpolar Deep Water (UCDW) and Antarctica Intermediate Water (AAIW) are reported. Uranium-thorium (U-Th) dating was used to develop an independent calendar age. The Drake Passage where the samples were taken from is a known region of mixing and upwelling of major deep-water masses.

Radiocarbon depletion in the southern ocean - sampling locations
Radiocarbon depletion in the southern ocean - sampling locations
Figure Radiocarbon depletion in the southern ocean - sampling locations
(A) Locations of dredging sites (white circles) and sediment cores blue triangle, brown diamond, and and pink square.
(B) Map of the dredging sites (white circles). The Polar Front is indicated by a blue line (38). Depth-latitude sections of salinity (C) and oxygen (D) for the latitudinal transect indicated by the red box from (B). Corals (white circles) come from depths bathed today by Antarctic Intermediate Water (AAIW) and Upper Circumpolar Deep Water (UCDW). LCDW, Lower Circumpolar Deep Water. SFZ, Shackleton Fracture Zone.

Forty individual deep-sea corals were selected from corals that had been dredged from Burdwood Bank and seamounts in the Drake Passage. The chronology of coral samples was determined by U-Th dating. Delta-carbon-14 measurements of dissolved inorganic carbon (DIC) were made on the corals. Depending on where the corals lived, they reflect depths corresponding to Antarctic Intermediate Water (AAIW) and Upper Circumpolar Deep Water (UCDW).

Results


The most pronounced feature of the radiocarbon record is the drop beginning at about 17,000 years ago and ending at the beginning at about 14.500 years ago. The results corresponding to UCDW found in this study are consistent with reduced ocean ventilation of deep water during the last glacial maximum (LGM). They show that during the last glacial maximum carbon at these depths in the southern ocean was radiocarbon depleted, indicating that the deep water was more isolated from the atmosphere than it is today. The records reflecting UCDW, LCDW, and Antarctica Bottom Water (AABW) suggest that there was a geochemical gradient between deep Southern Ocean water masses at the LGM. This stratification reduced vertical mixing across water-mass boundaries and bound old carbon in deep water.

Radiocarbon time series from the southern ocean during the last deglacation
Radiocarbon time series from the southern ocean during the last deglacation
Figure Radiocarbon time series from the southern ocean during the last deglacation
(A) Atmospheric CO2 from EPICA Dome C (EDC) and from Vostok ice cores.
(B) Delta-oxygen-18 temperature proxy from the EPICA Dronning Maud Land Ice Core (EDML) ice core.
(C) Atmospheric delta-carbon-14.
(D) Difference (ΔΔ14C) of reconstructed delta-carbon-14 compared to that in the atmosphere for AAIW (blue), UCDW (red), and LCDW (brown).
(E) Opal flux in the Indian Ocean sector of the Southern Ocean as a proxy for Southern Ocean nutrient upwelling.
YD Younger Dryas, BA Bølling/Allerød, HS1 and HS2 Heinrich North Atlantic ice rafting events, LGM Last Glacial Maximum (22,000 to 18,000 years ago), AAIW Antarctica Intermediate Water, UCDW Upper Circumpolar Deep Water, LCDW Lower Circumpolar Deep Water.

It is suggested that because the atmospheric carbon reservoir is so small compared to that of the ocean, a direct outgassing of CO2 from the ocean to the atmosphere could have been responsible for the drop in atmospheric radiocarbon. During the interval from 16,600 to 15,600 years ago, the radiocarbon in the atmosphere decreased at a rate similar to the increase of radiocarbon in UCDW. This is consistent with the ventilation of radiocarbon-depleted carbon into the atmosphere from the deep ocean via the Southern Ocean.

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