Ventilation of Northern and Southern Sources of Aged Carbon in the Eastern Equatorial Pacific During the Younger Dryas Rise in Atmospheric CO₂

Abstract

Atmospheric carbon dioxide (CO2) levels rose by ~90 ppmv during the last deglaciation, but the source of this carbon remains unknown. One popular hypothesis suggests carbon accumulated in the deep Southern Ocean, becoming increasingly radiocarbon (14C) depleted during the last glacial period, and was released into Antarctic Intermediate Water (AAIW) and, subsequently, the atmosphere during deglaciation. Detection of extremely 14C depleted carbon in the intermediate‐depth tropical oceans during periods of atmospheric CO2 rise was initially considered the smoking gun for the hypothesis, but attempts to reproduce the anomalies closer to the Southern Ocean source have largely failed. Here we present new 14C records from four cores recovered at intermediate depths in the eastern equatorial Pacific (EEP). In the context of additional geochemical records, including benthic foraminiferal stable isotopes and sedimentary nitrogen isotopes, we demonstrate that the extreme 14C anomalies observed during the last deglaciation do not reflect the radiocarbon content of AAIW. We show that although AAIW likely transported modestly 14C depleted carbon to the EEP subsurface, the extreme 14C signatures might reflect a distinct source of aged carbon arriving from the north, suggesting the North Pacific helped transport deep ocean carbon to the atmosphere during the last deglaciation. In the EEP, additional local hydrothermal inputs of 14C‐free carbon near the Galapagos Islands magnified the already low‐14C signatures. Finally, regardless of arrival route (North/South Pacific or hydrothermal), 14C‐depleted carbon was released from the EEP subsurface during a late deglacial pulse of renewed upwelling, likely contributing to the Younger Dryas rise in atmospheric CO2.

Ventilation of the Deep Ocean Carbon Reservoir During the Last Deglaciation: Results From the Southeast Pacific

“…The homogenization of 14 C values in the water column in the SEP could be explained by deeper convection of AAIW in the SO during the deglaciation, as proposed by Ronge et al () in the SWP and Haddam () in the SEP. In fact, a similar change to least ventilated intermediate waters is observed in Δ 14 C in the East Pacific margin (Bova et al, ) and in the DP (Burke & Robinson, ) at depths bathed by southern sourced intermediate waters. Additionally, these changes are paralleled by a shift to δ 13 C enriched values between ~16.5 and 14 ka cal BP at ~1,500 m in the SEP (Siani et al, ) and at ~1,200 m in the SWP (Sikes et al, ) and in the East Pacific margin at ~1,000 m (Bova et al, ), which is consistent with increased presence of AAIW at these depths.…”

“…In fact, a similar change to least ventilated intermediate waters is observed in Δ 14 C in the East Pacific margin (Bova et al, ) and in the DP (Burke & Robinson, ) at depths bathed by southern sourced intermediate waters. Additionally, these changes are paralleled by a shift to δ 13 C enriched values between ~16.5 and 14 ka cal BP at ~1,500 m in the SEP (Siani et al, ) and at ~1,200 m in the SWP (Sikes et al, ) and in the East Pacific margin at ~1,000 m (Bova et al, ), which is consistent with increased presence of AAIW at these depths. Around this time, increasing ventilation in deepwater records is also observed in the SA (Skinner et al, ), SWP (Ronge et al, ; Sikes et al, ), EEP (De la Fuente et al, ; Umling & Thunell, ), and NP (Galbraith et al, ; Rae et al, ), though not all records show the same trend and timing (Figure ), maybe denoting local effects and/or inconsistencies in the chronologies of the records.…”

“…For depths where more than one planktic foraminifera specie was dated, the mean at each depth was used as references for the interpolation. R S estimates in (a) Galapagos platform: core TR163‐23 (0.41°S, 92.16°W, 2,730 m; Umling & Thunell, ), from tuning with Greenland Ice Core Chronology GICC05 and core GGC 43 (1°15.13′S, 89°41.07′S, 617 m; Bova et al, ); (b) the Southern Ocean, from tephrochronology in core MD07‐3088 in the South East Pacific sector (41°S, 74°W, 1,536 m; Siani et al, ; symbols are the measured R S while the black line corresponds to the interpolated values) and core MD97‐2121 (40°22.935′S, 177°59.68′E, 2,314 m; Skinner et al, ) the South West Pacific sector; (c) the East Pacific margin, at ~32°S obtained from paired mollusk shells and charcoal fragments from archeological sites near Los Vilos (Carré et al, ); in core CDH 26 (3°59.16′S, 81°18.52′W, 1,023 m; Bova et al, ) from tuning with EPICA Dome C and in core KNR176‐GGC17/JP30 (5.02°N, 77.63°W, 707 m; Zhao & Keigwin, ) from wood‐planktic foraminifera pairs. (d) G. bulloides δ 13 C from cores used in this study and core MD07‐3088 (Siani et al, ), the lines correspond to smoothed 3‐point moving averages of the original data (small symbols).…”

“…Meanwhile, in the Galapagos platform, R S estimated by tuning between different records (Bova et al, ; De la Fuente et al, ; Umling & Thunell, ) yield R S even higher than in the SO. In contrast, the EEP region along the south American margin, the R S obtained by tuning (Bova et al, ) and by wood‐planktic foraminifera pairs (Zhao & Keigwin, ) yield generally lower values. In conclusion, the exact mechanism(s) of carbon release remains highly debated, and more work is still needed to understand the details of oceanic circulation changes, from the LGM into the Holocene, that control this ventilation.…”

“…These records are consistent with the hypothesis of an isolated deep ocean reservoir during the Last Glacial Maximum (LGM) associated with increased sea ice extent around Antarctica (Adkins, ; Ferrari et al, ; Keeling & Stephens, ; Stephens & Keeling, ; Watson & Naveira Garabato, ). Additionally, extremely 14 C‐depleted values have been found in deepwater sediment cores dating from the LGM in the SWP (Ronge et al, ; Sikes et al, ; Skinner et al, ) and East Pacific Rise (EPR; Ronge et al, ) and during the deglaciation in intermediate water cores in the EEP (Bova et al, ; Stott et al, ), Baja California (Lindsay et al, ; Marchitto et al, ), and Arabian Sea (Bryant et al, ). It has been hypothesized that these extremely 14 C‐depleted waters are due to a breakdown in deep ocean stratification at the beginning of the deglaciation, and the subsequent transfer of a 14 C‐depleted water mass to intermediate waters, particularly to Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) as seen in 13 C records of planktonic foraminifera (Spero & Lea, ).…”