Capturing the global signature of surface ocean acidification during the Palaeocene–Eocene Thermal Maximum

Abstract

Geologically abrupt carbon perturbations such as the Palaeocene-Eocene Thermal Maximum (PETM, approx. 56 Ma) are the closest geological points of comparison to current anthropogenic carbon emissions. Associated with the rapid carbon release during this event are profound environmental changes in the oceans including warming, deoxygenation and acidification. To evaluate the global extent of surface ocean acidification during the PETM, we present a compilation of new and published surface ocean carbonate chemistry and pH reconstructions from various palaeoceanographic settings. We use boron to calcium ratios (B/Ca) and boron isotopes (δB) in surface- and thermocline-dwelling planktonic foraminifera to reconstruct ocean carbonate chemistry and pH. Our records exhibit a B/Ca reduction of 30-40% and a δB decline of 1.0-1.2‰ coeval with the carbon isotope excursion. The tight coupling between boron proxies and carbon isotope records is consistent with the interpretation that oceanic absorption of the carbon released at the onset of the PETM resulted in widespread surface ocean acidification. The remarkable similarity among records from different ocean regions suggests that the degree of ocean carbonate change was globally near uniform. We attribute the global extent of surface ocean acidification to elevated atmospheric carbon dioxide levels during the main phase of the PETM.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

New constraints on massive carbon release and recovery processes during the Paleocene-Eocene Thermal Maximum

“…This decrease is consistent with a rapid (similar to the CIE onset rate), global, and sustained (similar to the CIE body duration) acidification of the surface ocean. At ODP Sites 1209, 865, 401, and 1263, a record of the boron isotopic composition (δ 11 B, a direct proxy for seawater pH) of surface-dwelling planktic foraminifers corroborates the acidification suggested by B/Ca records (Penman et al 2014, Gutjahr et al 2017, Babila et al 2018, and allows for its quantification. Due to the differing sensitivity of δ 11 B at different pH, the estimate for ΔpH across the PETM is a function of assumed initial pH, but for a reasonable assumption of pre-event (Paleocene) pH, the ∼1‰ decrease in planktic δ 11 B across the P-E boundary represents acidification of approximately 0.3 pH units (Penman et al 2014, Babila et al 2018.…”

“…At ODP Sites 1209, 865, 401, and 1263, a record of the boron isotopic composition (δ 11 B, a direct proxy for seawater pH) of surface-dwelling planktic foraminifers corroborates the acidification suggested by B/Ca records (Penman et al 2014, Gutjahr et al 2017, Babila et al 2018, and allows for its quantification. Due to the differing sensitivity of δ 11 B at different pH, the estimate for ΔpH across the PETM is a function of assumed initial pH, but for a reasonable assumption of pre-event (Paleocene) pH, the ∼1‰ decrease in planktic δ 11 B across the P-E boundary represents acidification of approximately 0.3 pH units (Penman et al 2014, Babila et al 2018. This falls within the higher end of the range of simluated ΔpH assuming various carbon inputs (Panchuk et al 2008, Zeebe et al 2009, Ridgwell and Schmidt 2010.…”

“…However, the event lasted at least an additional 100 000 years (Rohl et al 2007, Murphy et al 2010 and observations of carbonate chemistry during the remainder of the CIE and the recovery interval can be used to constrain an emissions trajectory and recovery for a full PETM scenario using LOSCAR. Three sets of observations argue for a sustained 'leak' of carbon after the initial large carbon release: the prolonged plateau of minimum δ 13 C (the 'body' of the CIE) in most records (Zeebe et al 2009), the sustained acidifications in records of planktic foraminifer δ 11 B and B/Ca globally (Babila et al 2018), both of which would otherwise begin to recover immediately following emissions cessation, and the delayed CCD overshoot at Site U1403 (Penman et al 2016), which would occur shortly (within a few tens of thousands of years) after the cessation of carbon emissions in the absence of a sustained leak.…”

“…The most direct evidence of ocean acidification comes from recent studies utilizing the boron concentration and isotopes in planktic foraminifera (Penman et al 2014, Babila et al 2016, Gutjahr et al 2017, Babila et al 2018. At ODP Sites 1209, 689, and 690 (Penman et al 2014, Babila et al 2018 (figure 1) and at sites along the New Jersey margin (Babila et al 2016), the B/Ca proxy, which herein is treated as a qualitative proxy for acidification (Allen et al 2011, Penman et al 2014, Uchikawa et al 2015, Haynes et al 2017, Uchikawa et al 2017, shows a rapid ∼30%-40% decrease at the event onset, followed by a plateau of low values and finally a recovery to near preevent levels in step with the carbon isotope excursion (CIE, figure 2). This decrease is consistent with a rapid (similar to the CIE onset rate), global, and sustained (similar to the CIE body duration) acidification of the surface ocean.…”

The Magnitude of Surface Ocean Acidification and Carbon Release During Eocene Thermal Maximum 2 (ETM‐2) and the Paleocene‐Eocene Thermal Maximum (PETM)

“…Most importantly, hyperthermals are linked to global changes in ocean carbonate chemistry as evidenced by decreases in deep ocean %CaCO 3 (Colosimo et al, 2006;Stap et al, 2009;Thomas & Shackleton, 1996;Zachos et al, 2005) and, for the PETM, independent records of surface ocean pH decrease (Babila et al, 2018;Gutjahr et al, 2017 ;Penman et al, 2014). Estimates of surface ocean acidification during the PETM are based on boron isotope values in planktic foraminiferal shells, which are directly controlled by ocean pH and closely related to atmospheric pCO 2 (Gutjahr et al, 2017;Penman et al, 2014).…”