Southwest Pacific deep water carbonate chemistry linked to high southern latitude climate and atmospheric CO2 during the Last Glacial Termination

Abstract

a b s t r a c tA greater amount of CO 2 was stored in the deep sea during glacial periods, likely via greater efficiency of the biologic pump and increased uptake by a more alkaline ocean. Reconstructing past variations in seawater carbonate ion concentration (a major component of alkalinity) enables quantification of the relative roles of different oceanic CO 2 storage mechanisms and also places constraints on the timing, magnitude, and location of subsequent deep ocean ventilation. Here, we present a record of deep-water inorganic carbon chemistry since the Last Glacial Maximum (LGM;~19e23 ka BP), derived from sediment core RR0503-83 raised from 1627 m in New Zealand's Bay of Plenty. The core site lies within the upper limit of southern-sourced Circumpolar Deep Water (CDW), just below the lower boundary of Antarctic Intermediate Water (AAIW). We reconstruct past changes in bottom water inorganic carbon chemistry from the trace element and stable isotopic composition of calcite shells of the epibenthic foraminifer Cibicidoides wuellerstorfi. A record of DCO 3 2À ðDCO 3 2À ¼ ½CO 3 2À in situ À ½CO 3 2À saturation Þ derived from the foraminiferal boron to calcium ratio (B/Ca) provides evidence for greater ice-age storage of respired CO 2 and reveals abrupt deglacial shifts in ½CO 3 2À in situ of up to 30 mmol/kg (5 times larger than the difference between average LGM and Holocene values). The rapidity of these changes suggests the influence of changing water mass structure and atmospheric circulation in addition to a decrease in CO 2 content of interior waters.

Calibration of the carbon isotope composition (δ13C) of benthic foraminifera

“…Our results have implications for glacial‐interglacial δ 13 C reconstructions. Carbonate ion concentrations were similar in the Pacific and Indian oceans but about 20 μmol kg −1 lower in the deep Atlantic and 20 to 30 μmol kg −1 higher in the upper Atlantic during the LGM compared to the preindustrial ocean [ Allen et al , ; Gottschalk et al , ; Yu et al , , , ]. Using equation with c = −2.6 × 10 −3 ‰/(μmol kg −1 ), this would produce a 0.05‰ reduction in δ 13 C DIC in the deep Atlantic and an increase of 0.05–0.08‰ in the upper Atlantic, thereby slightly enhancing the observed interglacial‐to‐glacial increase in the vertical gradient of ~1.0‰ in δ 13 C Cib [ Yu et al , , Figure 4].…”

Synchronous centennial abrupt events in the ocean and atmosphere during the last deglaciation

“…3C). Therefore, mechanisms other than sudden AMOC changes are likely to dominate this first centennial CO 2 increase, such as a rapid shift in ocean fronts driving the degassing of CO 2 from mid-depth waters of the Southern Ocean (36).…”

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

“…Numerous δ 13 C records from the well-characterized Atlantic Ocean demonstrate an enhanced vertical δ 13 C gradient between intermediate and deep water during the LGM (e.g., Curry and Lohmann, 1982a;Curry et al, 1988;Duplessy et al, 1988;Sarnthein et al, 1994;Hodell et al, 2003b;Curry and Oppo, 2005;Marchitto and Broecker, 2006;Herguera et al, 2010). The less well-sampled Pacific and Indian oceans also show signs of enhanced stratification at the LGM based on stronger vertical δ 13 C gradients and other nutrient and ventilation proxies (e.g., Kallel et al, 1988;Matsumoto and Lynch-Stieglitz, 1999;Matsumoto et al, 2002;Herguera et al, 2010;Lund et al, 2011b;Allen et al, 2015;Sikes et al, 2016).…”

“…Multiple causes have been proposed for stronger vertical δ 13 C gradients during the LGM, including increased surface productivity and export, increased ocean stratification, and changes in preformed δ 13 C in regions of deep water formation (e.g., Matsumoto et al, 2002;Curry and Oppo, 2005;Marchitto and Broecker, 2006;Lynch-Stieglitz et al, 2007;Marinov et al, 2008a, b;Herguera et al, 2010;Hesse et al, 2011;Lund et al, 2011a, b;Allen et al, 2015;Gebbie et al, 2015;Schmittner and Somes, 2016;Gloege et al, 2017;Menviel et al, 2017). Therefore, the large vertical δ 13 C gradient at the LGM could indicate a strong biological pump and/or weak vertical mixing, either of which would increase deep ocean carbon storage.…”