A continental shelf perspective of ocean acidification and temperature evolution during the Paleocene-Eocene Thermal Maximum

Abstract

A rapid and large injection of isotopically light carbon into the ocean-atmosphere reservoirs is signaled by a negative carbon isotope excursion (CIE) at the Paleocene-Eocene boundary ~56 m.y. ago. To better understand the extent of ocean warming and acidification associated with the carbon injection we generated elemental and isotopic records of surface and thermocline planktonic foraminifera across the Paleocene-Eocene boundary from an expanded section along the Mid-Atlantic coastal plain, New Jersey (USA). Ocean temperatures (derived from magnesium/calcium paleothermometer) document a lag in thermocline warming relative to surface waters, implying a progressive deepening of the mixed layer in addition to global warming. A similar magnitude of acidification (as recorded by boron/calcium, B/Ca) on the shelf compared with open ocean sites confirms widespread acidification of the surface ocean. An increase in seawater alkalinity after the CIE, as recorded by B/Ca in planktonic foraminifera, likely played an important role in neutralizing the added carbon, possibly minimizing benthic extinction along the shelf.

Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions

“…This suggests that Paleocene foraminifers living in seawater with lower [B] T than in the modern ocean would record overall lower B/Ca ratios. Paleocene M. velascoensis recorded B/Ca ratios clustering around 60–70 μmol/mol before the PETM carbon system perturbation, reaching values as low as 30 μmol/mol during peak acidification [ Babila et al , ; Penman et al , ]. Babila et al [] note that this is much lower than analogous modern surface dwelling species such as T. sacculifer and G. ruber , which typically record B/Ca ~90–140 μmol/mol.…”

“…Paleocene M. velascoensis recorded B/Ca ratios clustering around 60–70 μmol/mol before the PETM carbon system perturbation, reaching values as low as 30 μmol/mol during peak acidification [ Babila et al , ; Penman et al , ]. Babila et al [] note that this is much lower than analogous modern surface dwelling species such as T. sacculifer and G. ruber , which typically record B/Ca ~90–140 μmol/mol. The lower [B] T of the surface ocean during the Paleocene [ Lemarchand et al , ], as well as low surface ocean pH and thus B(OH) 4 − [ Penman et al , ; Anagnostou et al , ], are thus in qualitative agreement with the lower overall B/Ca ratios of Paleocene foraminifera.…”

“…If foraminifers did fully “bleach” at the PETM, it is conceivable that this could have a small effect on B/Ca records. However, analyses of δ 11 B and δ 13 C of different size fractions of symbiont‐bearing species across the PETM do not provide any evidence for a reduction of the symbiont association [ Penman et al , ; Babila et al , ], arguing against a significant “bleaching” effect. Careful analysis of isotopic trends with size is advisable however to constrain this effect in future studies.…”

“…Recently, however, Edgar et al [] showed that diagenetic alteration caused B loss from Eocene planktic foraminifers displaying frosty preservation, in comparison to contemporaneous glassy specimens. Babila et al [] measured B/Ca from a shallow marine section spanning the PETM off the New Jersey Margin, in which glassy foraminifers were used for analysis. The authors note that the magnitude of the B/Ca excursion is very similar to the excursion observed at Site 1209, suggesting instead that diagenetic overprinting at the open ocean site is not a major contributor to the B/Ca signal, lending confidence to the size of the excursion recorded at Site 1209.…”

“…This is because large, consistent B/Ca excursions are observed across this event at multiple geographically disparate sites, suggesting that B/Ca responded to global ocean acidification due to rapid carbon input [ Penman et al , ; Babila et al , ]. Occurring globally at ~55.93 Ma [ Westerhold et al , ], the PETM is often touted as the closest analog to anthropogenic carbon release and warming [ Zeebe et al , ], as it is characterized by widespread dissolution of seafloor carbonates, a large negative carbon isotope excursion, pronounced warming, and planktic foraminiferal B/Ca and δ 11 B excursions [ Zachos et al , ; Penman et al , ; Babila et al , ]. These geochemical records suggest extreme ocean acidification, with an intensity that is larger than estimated from models [ Penman et al , ; Babila et al , ].…”