In Pursuit of the Mystery Reservoir: Marine Radiocarbon Evidence from the Eastern Tropical Pacific for a Deglacial CO2 Source

Glacial-interglacial cycles are responsible for the largest natural perturbations in atmospheric carbon dioxide (CO₂) over the past million years. Ice cores from Antarctica document a two-part rise in atmospheric CO₂ by nearly 80 ppm during the last glacial termination in phase with southern hemisphere warm events. Although the source and cause of the rise in atmospheric CO₂ remains a mystery, the rapid decrease in the ¹³C and radiocarbon signature of atmospheric carbon in sync with southern hemisphere warming provides key constraints [Monnin et al., 2001; Lourantau et al., 2010]. These data have led to the hypothesis that carbon accumulated in the abyssal Southern Ocean during glacial periods. Extensive sea ice and/or northward shifted westerly winds reduced upwelling and mixing of the carbon-rich deep waters to the surface ocean under glacial conditions, effectively isolating large volumes of carbon from the atmosphere. If correct, during glacial terminations we expect to find evidence for pulses of old carbon to the Southern Ocean surface and intermediate waters (SOIW) as the climate warmed.

Radiocarbon measurements on benthic foraminifera provide direct evidence for the presence or absence of old carbon in intermediate and deep water masses. We add to a growing dataset of benthic foraminiferal radiocarbon data from sites bathed by SOIWs during the last glacial termination by evaluating the radiocarbon age of coeval benthic and planktic foraminifera in the Eastern Equatorial Pacific (EEP). We find definitive evidence for a sustained input of old carbon to SOIWs during the Younger Dryas and pulses of old carbon during Heinrich Stadial 1. The magnitude and timing of the benthic radiocarbon excursions in our record are comparable to those measured previously at intermediate depths near Baja [Marchitto et al., 2007] but are significantly less depleted than measurements made at a neighboring site in the Galapagos Islands: we record a minimum Δ¹⁴C of -310‰ versus the nearly -600‰ measured by Stott et al. [2009]. Our record is strikingly similar in structure to a benthic ¹⁴C record from the Southeast Pacific [Bryan, 2010], just below the site of SOIW formation, which thus provides support for a Southern Ocean source of old carbon to the EEP and atmosphere during the last glacial termination.