Breakup of last glacial deep stratification in the South Pacific

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Age control and paleoceanographic evidence of marine sediment records might be challenged if authors solely build their stratigraphy on visual correlation to apparently well‐dated records from the same ocean basin, using, for example, highly resolved X‐ray fluorescence‐based element‐count records and correlation tools such as AnalySeries. While per se perfectly reasonable, this approach bears the risk of missing stratigraphic gaps in the sedimentary record and thus might result in imprecise and/or flawed interpretations. Here we present a unique series of 14 planktic 14C ages from a 7‐cm section of East Pacific Rise core PS75/059‐2. The ages suggest a 14‐ky‐long period of low‐to‐zero deposition during Last Glacial Maximum, mainly marked by continuous redistribution of winnowed foraminifers, probably the result of enhanced bottom currents, moreover, by some bioturbational mixing. On the basis of this data we demonstrate the impact of the hiatus on a South Pacific transect of apparent benthic ventilation ages (ΔΔ14C values) and their meaning for estimates of CO2 stored in Last Glacial Maximum Pacific deep waters.

Section: Main Textmentioning

confidence: 99%

East Pacific Rise Core PS75/059‐2: Glacial‐to‐Deglacial Stratigraphy Revisited

Williams

Sarnthein

Roberts

et al. 2019

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“…Enhanced CO 2 storage at depth during the LGM has been linked to changes in Southern Ocean circulation and structure, a colder, more saline and stratified deep ocean (Adkins et al, ), and expanded Antarctic sea‐ice extent (Benz et al, ; Ferry et al, ; Shemesh et al, ), all of which could have inhibited upwelled waters from equilibrating with the atmosphere (Adkins et al, ; Jansen, ; Kobayashi et al, ; Lund et al, ; Sikes, Cook, et al, ; Stephens & Keeling, ; Watson et al, ). Similarly, changes in the vertical structure of the Southern Ocean are one possible mechanism contributing to the release of CO 2 to the atmosphere during the deglaciation (e.g., Basak et al, ; Du et al, ; Roberts et al, ; Sikes et al, ; Skinner et al, ), though other mechanisms, such as changes in the efficiency of the biological pump (e.g., Bauska et al, ; Hertzberg et al, ; Martin, ), have also been invoked.…”

Section: Introductionmentioning

confidence: 99%

“…Sikes et al () and others (Bostock et al, ; Bova et al, ; Pahnke & Zahn, ; Sikes, Cook, et al, ; Sikes, Elmore, et al, ) interpret enriched intermediate water δ 13 C during HS1, relative to the LGM, as the release of CO 2 from these depths due to a poleward displacement of the SHW, causing the two‐step rise in atmospheric CO 2 across this interval. In contrast, although converging neodymium isotopes in deep South Pacific cores (>3,000 m) are interpreted as evidence of mixing and destratification—thereby releasing CO 2 from the deep ocean—the mechanism relies on deep ocean warming (Basak et al, ). Regional temperature reconstructions indicate that warming of the deep Pacific lagged the initial glacial termination and CO 2 rise (Elderfield et al, ; Skinner & Shackleton, ).…”

Section: Introductionmentioning

confidence: 99%

Southwest Pacific Vertical Structure Influences on Oceanic Carbon Storage Since the Last Glacial Maximum

Clementi

Sikes

2019

Lower atmospheric CO2 concentrations during the Last Glacial Maximum (LGM; 23.0–18.0 ka) have been attributed to the sequestration of respired carbon in the ocean interior, yet the mechanism responsible for the release of this CO2 during the deglaciation remains uncertain. Here we present calculations of vertical differences in oxygen and carbon isotopes (∆δ18O and ∆δ13C, respectively) from a depth transect of southwest Pacific Ocean sediment cores to reconstruct changes in water mass structure and CO2 storage. During the Last Glacial Maximum, ∆δ18O indicates a more homogenous deep Pacific below 1,100 m, whereas regional ∆δ13C elucidates greater sequestration of CO2 in two distinct layers: enhanced CO2 storage at intermediate depths between ~940 and 1,400 m, and significantly more CO2 at 1,600 m and below. This highlights an isolated glacial intermediate water mass and places the main geochemical divide at least 500 m shallower than the Holocene. During the initial stages of the deglaciation in Heinrich Stadial 1 (17.5–14.5 ka), restructuring of the upper ~2,000 m of the southwest Pacific water column coincided with sea‐ice retreat and rapid CO2 release from intermediate depths, while CO2 release from the deep ocean was earlier and more gradual than waters above it. These changes suggest that sea‐ice retreat and shifts in Southern Ocean frontal locations contributed to rapid CO2 ventilation from the Southern Ocean's intermediate depths and gradual ventilation from the deep ocean during the early deglaciation.

“…These cores all record lower δ 13 C cib values than PC493 during past glacial periods (Figure ; Ninnemann & Charles, ; Ullermann et al, ). These cores are today bathed in CDW and were located above the depth of AABW during at least the last glacial period (Basak et al, ). To explore the long‐term trends in SO paleocirculation, we have smoothed these East Pacific Rise records to 2‐kyr resolution and averaged them to form a ‘stack’ of central Pacific SO CDW of δ 13 C (Figure ).…”

Section: Discussionmentioning

confidence: 99%

Paleocirculation and Ventilation History of Southern Ocean Sourced Deep Water Masses During the Last 800,000 Years

Williams

Hillenbrand

Piotrowski

et al. 2019

Most conceptual models of ocean circulation during past glacial periods invoke a shallowed North Atlantic‐sourced water mass overlying an expanded, poorly ventilated Southern Ocean (SO)‐sourced deep water mass (Southern Component Water or SCW), rich in remineralized carbon, within the Atlantic basin. However, the ventilation state, carbon inventory, and circulation pathway of SCW sourced in the Pacific sector of the SO (Pacific SO) during glacial periods are less well understood. Here we present multiproxy data—including δ18O and δ13C measured on the benthic and planktic foraminifera Cibicidoides wuellerstorfi, and Neogloboquadrina pachyderma, and productivity proxies including percent CaCO3, total organic carbon, and Ba/Ti—from a sediment core located in the high‐latitude (71°S) Pacific SO spanning the last 800 kyr. Typical glacial δ13C values of SCW at this core site are ~0‰. We find no evidence for SCW with extremely low δ13C values during glacials in the high‐latitude Pacific SO. This leads to a spatial gradient in the stable carbon isotope composition of SCW from the high‐latitude SO, suggesting that there are different processes of deep‐ and bottom‐water formation around Antarctica. A reduced imprint of air‐sea gas exchange is evident in the SCW formed in the Atlantic SO compared with the Pacific SO. A spatial δ13C gradient in SCW is apparent throughout much of the last 800,000 years, including interglacials. A SO‐wide depletion in benthic δ13C is observed in early MIS 16, coinciding with the lowest atmospheric pCO2 recorded in Antarctic ice cores.