The Evolution of Deep Ocean Chemistry and Respired Carbon in the Eastern Equatorial Pacific Over the Last Deglaciation

Abstract

It has been shown that the deep Eastern Equatorial Pacific (EEP) region was poorly ventilated during the Last Glacial Maximum (LGM) relative to Holocene values. This finding suggests a more efficient biological pump, which indirectly supports the idea of increased carbon storage in the deep ocean contributing to lower atmospheric CO2 during the last glacial. However, proxies related to respired carbon are needed in order to directly test this proposition. Here we present Cibicides wuellerstorfi B/Ca ratios from Ocean Drilling Program Site 1240 measured by laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) as a proxy for deep water carbonate saturation state (Δ[CO32−], and therefore [CO32−]), along with δ13C measurements. In addition, the U/Ca ratio in foraminiferal coatings has been analyzed as an indicator of oxygenation changes. Our results show lower [CO32−], δ13C, and [O2] values during the LGM, which would be consistent with higher respired carbon levels in the deep EEP driven, at least in part, by reduced deep water ventilation. However, the difference between LGM and Holocene [CO32−] observed at our site is relatively small, in accordance with other records from across the Pacific, suggesting that a “counteracting” mechanism, such as seafloor carbonate dissolution, also played a role. If so, this mechanism would have increased average ocean alkalinity, allowing even more atmospheric CO2 to be “sequestered” by the ocean. Therefore, the deep Pacific Ocean very likely stored a significant amount of atmospheric CO2 during the LGM, specifically due to a more efficient biological carbon pump and also an increase in average ocean alkalinity.

Deep‐Sea Oxygen Depletion and Ocean Carbon Sequestration During the Last Ice Age

“…Extrapolating to the global ocean is tenuous with limited data, but to illustrate the potential scale of enhanced CO 2 storage during the LGP, we note that most of the deep Pacific (Figure 4a and Jaccard & Galbraith, 2012), Atlantic (Hoogakker et al, 2015) and Southern Oceans (Francois et al, 1997;Jaccard et al, 2016) held substantially more respiratory CO 2 (i.e., had lower DO) during the LGP (see also de la Fuente et al, 2017;Sarnthein et al, 2013;Skinner et al, 2015;Skinner et al, 2017). Partially offsetting this enhanced deep-sea CO 2 storage, the surface ocean, in equilibrium with atmospheric CO 2 at 180 to 200 ppm, would have held less dissolved carbon than today.…”

Past Carbonate Preservation Events in the Deep Southeast Atlantic Ocean (Cape Basin) and Their Implications for Atlantic Overturning Dynamics and Marine Carbon Cycling

“…We have emphasized a possible sea level control of carbonate compensation primarily in the Indo-Pacific Ocean that is qualitatively consistent with a varying output of alkalinity from the deep ocean due to variations in neritic carbonate deposition (Berger & Keir, 1984;Opdyke & Walker, 1992), but at the same time this does not rule out other drivers (Archer & Maier-Reimer, 1994;Boyle, 1988b;Broecker, 1982). Indeed, there is support for a longer-term (glacial-interglacial) contribution from varying respired carbon retention/release from the deep ocean, contributing to carbonate dissolution in the deep ocean during periods of CO 2 drop and carbonate preservation during CO 2 rise (Boyle, 1988a;de la Fuente et al, 2017;Kerr et al, 2017;Tschumi et al, 2011). Overall, the close agreement between changes in the carbonate burial in the deep Cape Basin and the rate of atmospheric CO 2 change within a few millennia (Figure 7) underlines the potential influence of marine carbonate chemistry (as modulated by ocean circulation and/or sea level change) on atmospheric CO 2 .…”

“…The "Pacifictype" carbonate preservation is believed to mainly reflect adjustments of the lysocline and the CCD to maintain alkalinity balance between riverine input and marine carbonate burial (Hodell et al, 2001;Kerr et al, 2017). This is supported by relatively small bottom water [CO 3 2À ] variations observed in the Pacific over glacial-interglacial transitions (~15 μmol/kg), suggesting an effective carbonate ion buffering (Anderson & Archer, 2002;de la Fuente et al, 2017;Kerr et al, 2017;Marchitto et al, 2005;Yu, Anderson, et al, 2013). The Atlantic-type carbonate preservation signal results mainly from changes in the formation and export of NADW Hodell et al, 2001;Howard & Prell, 1994).…”

Simulating stable carbon isotopes in the ocean component of the FAMOUS general circulation model with MOSES1 (XOAVI)

“…ncas.ac.uk/wiki/UmFamous (last access: 23 December 2019). The code detailing the advances described in this paper is available via the Research Data Leeds Repository (Dentith, 2019, https://doi.org/10.5518/621) under a Creative Commons Attribution 4.0 International (CCBY 4.0) license. These files are known as code modification ("mod") files and should be applied to the original model code, which can be viewed online at http://cms.…”