Deepwater carbonate ion concentrations in the western tropical Pacific since 250 ka: Evidence for oceanic carbon storage and global climate influence

Abstract

We present new “size‐normalized weight” (SNW)‐Δ[CO32−] core‐top calibrations for three planktonic foraminiferal species and assess their reliability as a paleo‐alkalinity proxy. SNWs of Globigerina sacculifer and Neogloboquadrina dutertrei can be used to reconstruct past deep Pacific [CO32−], whereas SNWs of Pulleniatina obliquiloculata are controlled by additional environmental factors. Based on this methodological advance, we reconstruct SNW‐based deepwater [CO32−] for core WP7 from the western tropical Pacific since 250 ka. Secular variation in the SNW proxy documents little change in deep Pacific [CO32−] between the Last Glacial Maximum and the Holocene. Further back in time, deepwater [CO32−] shows long‐term increases from marine isotope stage (MIS) 5e to MIS 3 and from early MIS 7 to late MIS 6, consistent with the “coral reef hypothesis” that the deep Pacific Ocean carbonate system responded to declining shelf carbonate production during these two intervals. During deglaciations, we have evidence of [CO32−] peaks coincident with Terminations 2 and 3, which suggests that a breakdown of oceanic vertical stratification drove a net transfer of CO2 from the ocean to the atmosphere, causing spikes in carbonate preservation (i.e., the “deglacial ventilation hypothesis”). During MIS 4, a transient decline in SNW‐based [CO32−], along with other reported [CO32−] and/or dissolution records, implies that increased deep‐ocean carbon storage resulted in a global carbonate dissolution event. These findings provide new insights into the role of the deep Pacific in the global carbon cycle during the late Quaternary.

Micro‐CT Scanning of Tests of Three Planktic Foraminiferal Species to Clarify DissolutionProcess and Progress

“…On the other hand, we found no significant effect of test size (long axis) on %Low‐CT‐number calcite volume, the suggestion being that %Low‐CT‐number calcite volume is independent of test size variation in sample sets. Although several factors that may affect the test dissolution sensitivity should be recognized and the quality of regression between the proxy and deep seawater Δ[CO 3 2− ] must be improved by analyzing samples from additional cores in future works, this proxy is already sufficiently robust to detect glacial‐interglacial change in deep seawater [CO 3 2− ], which has been estimated to only approximately ±10 to 20 μmol kg −1 (Qin et al, ) or ~28 μmol kg −1 (Fehrenbacher & Martin, ) variation in the equatorial Pacific.…”

“…2− ], which has been estimated to only approximately ±10 to 20 μmol kg −1 (Qin et al, 2017) or~28 μmol kg −1 (Fehrenbacher & Martin, 2011) variation in the equatorial Pacific.…”

“…In particular, the size-normalized weight (SNW) of foraminiferal tests is one of the most common proxies to estimate carbonate dissolution (e.g., Broecker & Clark, 2001;Lohman, 1995;Qin et al, 2017). Based on calibration between SNW in core-top sediments and bottom-water Δ[CO 3 2− ] in the equatorial Pacific, size-normalized test weights have served as a proxy for deep seawater [CO 3 2− ] (e.g., Broecker & Clark, 2001;Qin et al, 2017). In contrast, the test area density (μg μm −2 ) of foraminifera, which is the ratio of the weight of a test to its projected area, is a more rigorous size-standardized proxy than SNW, and it is also employed as a proxy of calcification intensity controlled by sea surface conditions (e.g., Moy et al, 2009;Weinkauf et al, 2013;Zarkogiannis et al, 2019).…”

Sequential changes in ocean circulation and biological export productivity during the last glacial cycle: a model-data study

“…The Pacific Ocean is thought to partially buffer the effects of ocean circulation on CO 2− 3 concentrations via changes in shallow (reef) and deep carbonate production and dissolution, and therefore displays less variation across the MIS (Yu et al, 2014b;Qin et al, 2017Qin et al, , 2018. The deep and abyssal Pacific-Indian ocean data shows a persistent trend of increasing CO 2− 3 through the glacial cycle, suggesting that it is influenced by variations in shallow and deep sea carbonate production and 10 dissolution, rather than by deep ocean circulation (Yu et al, 2014b;Qin et al, 2017Qin et al, , 2018 δ 13 C and atmospheric CO 2 (Fig. 5(B)), suggesting a possible common driver, and providing additional qualitative evidence for changes in either Pacific-Indian ocean circulation or biology, at this time.…”

Deep‐Water Carbonate Ion Concentrations in the Western Tropical Pacific Since the Mid‐Pleistocene: A Major Perturbation During the Mid‐Brunhes

“…Previous methods to investigate past changes in the Pacific carbonate system have made use mainly of foraminiferal δ 13 C (Bickert et al, ; Mix et al, ; Wang et al, ) and various qualitative carbonate dissolution proxies including sediment CaCO 3 content (Anderson et al, ; Farrell & Prell, ; Hodell et al, ), foraminiferal fragmentation (Le & Shackleton, ; Mix et al, ; Zhang et al, ), and coarse fraction percent (Lalicata & Lea, ; Wu et al, ). Recently, benthic foraminiferal B/Ca (Kerr et al, ; Yu et al, ; Yu & Elderfield, ) and planktonic foraminiferal SNW (Broecker & Clark, ; Qin et al, ) have been developed as quantitative deep‐water [CO 3 2− ] proxies. These studies have improved our understanding of the Pacific carbon cycle.…”