Abstract
Abstract. During the early part of the last glacial termination (17.2–15 ka) and coincident with a ∼35 ppm rise in atmospheric CO2, a sharp 0.3‰–0.4‰ decline in atmospheric δ13CO2 occurred, potentially constraining the key processes that account for the early deglacial CO2 rise. A comparable δ13C decline has also been documented in numerous marine proxy records from surface and thermocline-dwelling planktic foraminifera. The δ13C decline recorded in planktic foraminifera has previously been attributed to the release of respired carbon from the deep ocean that was subsequently transported within the upper ocean to sites where the signal was recorded (and then ultimately transferred to the atmosphere). Benthic δ13C records from the global upper ocean, including a new record presented here from the tropical Pacific, also document this distinct early deglacial δ13C decline. Here we present modeling evidence to show that rather than respired carbon from the deep ocean propagating directly to the upper ocean prior to reaching the atmosphere, the carbon would have first upwelled to the surface in the Southern Ocean where it would have entered the atmosphere. In this way the transmission of isotopically light carbon to the global upper ocean was analogous to the ongoing ocean invasion of fossil fuel CO2. The model results suggest that thermocline waters throughout the ocean and 500–2000 m water depths were affected by this atmospheric bridge during the early deglaciation.
Highlights
Atmospheric CO2 increased by 80–100 ppm between the Last Glacial Maximum (LGM) and the Holocene (Marcott et al, 2014; Monnin et al, 2001)
Decomposing the LOVECLIM δ13C signal into the δ13Csoft and δ13Cpref component, we find that the entire water column of the Southern Ocean is characterized by a strong positive δ13Csoft and a strong negative δ13Cpref (Fig. 3b, c, e, f, h, and g)
The δ13C patterns (Fig. 4a, d, and g) are qualitatively similar to that simulated by LOVECLIM (Fig. 3a, d, and g), the magnitude of positive δ13C in the deep Pacific and negative δ13C in the deep North Atlantic is larger in cGENIE. cGENIE does not simulate any large positive δ13Csoft or negative δ13Cpref in the Southern Ocean above depths of 3000 m (Fig. 4), in contrast to the apparent oxygen utilization (AOU)-based results from LOVECLIM (Fig. 3)
Summary
Atmospheric CO2 increased by 80–100 ppm between the Last Glacial Maximum (LGM) and the Holocene (Marcott et al, 2014; Monnin et al, 2001). During the initial ∼ 35 ppm rise in CO2 between 17.2 and 15 ka, ice core records document a 0.3 ‰ contemporaneous decline in atmospheric δ13C (Bauska et al, 2016; Schmitt et al, 2012) (Fig. 1a and b, interval highlighted in grey) This millennialscale trend was punctuated by an interval of even more rapid change, with a 12 ppm CO2 increase (Marcott et al, 2014) and a −0.2 ‰ decrease in δ13CO2 (Bauska et al, 2016) occurring in an interval of just ∼ 200 years between 16.3 and 16.1 ka (Fig. 1a and b, interval highlighted in red). During the early deglaciation, surface and thermocline-dwelling foraminifera around the global ocean recorded a distinct δ13C drop
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