Radiocarbon constraints on the glacial ocean circulation and its impact on atmospheric CO2

Abstract

While the ocean’s large-scale overturning circulation is thought to have been significantly different under the climatic conditions of the Last Glacial Maximum (LGM), the exact nature of the glacial circulation and its implications for global carbon cycling continue to be debated. Here we use a global array of ocean–atmosphere radiocarbon disequilibrium estimates to demonstrate a ∼689±53 14C-yr increase in the average residence time of carbon in the deep ocean at the LGM. A predominantly southern-sourced abyssal overturning limb that was more isolated from its shallower northern counterparts is interpreted to have extended from the Southern Ocean, producing a widespread radiocarbon age maximum at mid-depths and depriving the deep ocean of a fast escape route for accumulating respired carbon. While the exact magnitude of the resulting carbon cycle impacts remains to be confirmed, the radiocarbon data suggest an increase in the efficiency of the biological carbon pump that could have accounted for as much as half of the glacial–interglacial CO2 change.

A Synthesis of Deglacial Deep‐Sea Radiocarbon Records and Their (In)Consistency With Modern Ocean Ventilation

“…Recent studies have inferred the presence of 14 C age maxima at water depths of 3,500–4,500 m in the deep Atlantic during the LGM (Burke et al, ; Keigwin & Swift, ; Skinner et al, ). Age extrema at these depths cannot be captured by the ventilation model used in this study, as this model represents the entire Atlantic below 1,500 m with a single box.…”

Radiocarbon and Helium Isotope Constraints on Deep Ocean Ventilation and Mantle‐3He Sources

“…More recently, radiocarbon observations have been used to infer the ventilation age of deep waters using inverse models (Gebbie & Huybers, ; Khatiwala et al, ) and tracer‐only inversions (Holzer et al, , ) and to constrain rates of ocean circulation and ventilation in global ocean inverse models (DeVries & Primeau, ; Schlitzer, ). Measurements of the Δ 14 C of fossil foraminifera have also been used to constrain the circulation of the ocean during the last glacial maximum (Skinner et al, ) and the most recent deglaciation (Tschumi et al, ). (The abundance of 14 C is usually expressed relative to the abundance of the more common 12 C isotope and in turn normalized to the isotope ratio of the preindustrial atmosphere and corrected for biological fractionation effects.…”

Deglacial carbon cycle changes observed in a compilation of 127 benthic <i>δ</i><sup>13</sup>C time series (20–6 ka)

“…Processes that potentially explain atmospheric CO 2 change during glacial cycles include the efficiency of the biological pump (Martínez-Garcia et al, 2009;Galbraith and Jaccard, 2015), circulation changes (Ferrari et al, 2014;Schmittner and Lund, 2015;Lacerra et al, 2017;Menviel et al, 2017;Sikes et al, 2017;Wagner and Hendy, 2017), or a combination of multiple processes (Bauska et al, 2016;Skinner et al, 2017). Different processes could influence the carbon cycle on different timescales (Bauska et al, 2016;Kohfeld and Chase, 2017) and/or in different regions (e.g., Gu et al, 2017) and complicate interpretations of which processes are most responsible for atmospheric CO 2 change.…”