Carbon dioxide release from the North Pacific abyss during the last deglaciation

Abstract

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods, but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal. Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that--together with previously published results--show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000-15,000 years ago), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition approximately 14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration. We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic, which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir, but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.

Southern Ocean source of 14C-depleted carbon in the North Pacific Ocean during the last deglaciation

“…Early warming in the Southern Hemisphere and invigorated AAIW formation are well known 2,17,18 . In contrast, the deep and shallow Northern Pacific did not start responding to deglaciation until the beginning of the B-A period 19 (∼14.8 kyr bp), although localized intermediate water formation during HS1 in the western North Pacific has recently been reported 20 . Therefore, temporal differences in the deglacial history between the Northern and Southern hemispheres also imply that the Nd isotopic excursion at ∼18 kyr was controlled by intermediate water mass readjustment in the Southern Hemisphere.…”

Ventilation changes in the western North Pacific since the last glacial period

“…We suggest that the deepwater from the North Pacific may have helped produce the local mid-depth 14 C anomalies in the eastern equatorial Pacific by flushing a part of old deep Pacific water. From the abyssal North Pacific, old carbon release was suggested at the beginning of the Bølling-Allerød (Galbraith et al, 2007). Relatively low 14 C at core W8709A-13PC during the Bølling-Allerød might be caused by an influence of the aged abyssal water.…”

“…On the other hand, 14 C in core W8709A-13PC appears to be lower than that of core ODP 887 during the Bølling-Allerød (∼13.0-14.5 kyr BP). 14 C change in core MD01-2420 (this study), core ODP 887 in the Gulf of Alaska (3647 m, Galbraith et al, 2007) and core W8709A-13PC in the eastern North Pacific (2710 m, Mix et al, 1999;Lund et al, 2011) between 10 and 23 kyr BP along with Intcal09 and Marine09 curves (Reimer et al, 2009). variations suggest a reorganization of water-mass structure in the North Pacific during the deglacial period from a stratified glacial mode to an upwelling interglacial mode. The glacial Pacific Ocean had two water masses: well-ventilated and nutrient-depleted glacial North Pacific Intermediate Water (GNPIW) above ∼2000 m and less-ventilated and nutrientenriched deep water below ∼2000 m (Keigwin, 1998;Matsumoto et al, 2002).…”

“…Major reorganization of ocean circulation in the North Pacific during the glacial-deglacial period affected productivity through upwelling. During the last glacial maximum, primary productivity in the subarctic Pacific was low because of stratification (Narita et al, 2002;Jaccard et al, 2005;Galbraith et al, 2007;Brunelle et al, 2010). Thick GNPIW with low nutrient concentrations down to ∼2000 m suppressed biological productivity in the euphotic layer.…”

“…The western North Pacific 14 C records co-varied with atmospheric 14 C changes during the last glacial to deglacial periods, which is in clear contrast with data from the eastern Pacific showing that very old intermediate water masses observed at two sites off Baja California (Marchitto et al, 2007) and near the Galapagos Islands (Stott et al, 2009) during the last deglacial period. 3.3 Ventilation history and water mass structure change in the North Pacific Figure 5 shows a comparison of glacial-deglacial changes in 14 C records at 2101 m at core MD01-2420, at 3647 m water depth in the Gulf of Alaska at ODP Site 887 (Galbraith et al, 2007), and at 2710 m water depth in the eastern North Pacific off the Oregon coast at W8709A-13PC (Mix et al, 1999;Lund et al, 2011). During the last glacial maximum between 18 and 21 kyr BP, the difference in 14 C between MD01-2420 and ODP 887 was 76 ± 34 ‰ (weighed average).…”