Diagenetic Attenuation of Carbon Isotope Excursion Recorded by Planktic Foraminifers During the Paleocene‐Eocene Thermal Maximum

Abstract

Earth surface temperatures warmed by ~5 °C during an ancient (~56 Ma) global warming event referred to as the Paleocene‐Eocene thermal maximum (PETM). A hallmark of the PETM is a carbon isotope excursion (CIE) signaling the release of massive amounts of 13C‐depleted carbon into the ocean‐atmosphere system, but substrate‐specific differences in the CIE magnitude are a source of uncertainty for estimating the mass of carbon emitted. Here we report that secondary ion mass spectrometry (SIMS)‐based in situ measurements of δ13C in minute (7 μm) domains of planktic foraminifer shells (Ocean Drilling Program Site 865, central Pacific Ocean) yield a CIE that is ~2‰ larger than that delineated by conventional “whole‐shell” δ13C values for this same PETM record. SIMS‐based measurements on diagenetic crystallites yield δ13C values (~2.8‰) that fall between those of pre‐CIE and CIE planktic foraminifer shells, indicating that the crystallites are an amalgamated blend of pre‐CIE and CIE carbonates. This suggests that diagenesis shifts the whole‐shell δ13C compositions of pre‐CIE and CIE foraminifers found in samples straddling the base of the PETM interval toward the intermediate δ13C composition of the crystallites, thereby dampening the amplitude of the isotopic excursion. The diagenetic process envisioned would be most consequential for carbonate‐rich PETM records that have suffered chemical erosion of pre‐CIE carbonate. Given that the domains targeted for SIMS analysis may not be pristinely preserved, we consider the 4.6‰ excursion in our SIMS‐based δ13C record to be a conservative estimate of the full CIE for surface ocean dissolved inorganic carbon.

Archaeal lipid biomarker constraints on the Paleocene-Eocene carbon isotope excursion

“…These factors similarly affect the carbon isotopic composition of marine phytoplankton biomass 24 and thus bias reconstructions from bulk sediment organic matter (often towards larger CIEs), in addition to offsets resulting from varying admixtures of terrigenous organic matter into marine sediments 19 . By contrast, δ 13 C records of bulk carbonate and foraminifera may underestimate the magnitude of the CIE due to biases imposed by carbonate dissolution 25 , diagenesis 26 , and vital effects 27 . Collectively, these uncertainties confound not only the quantification of carbon emissions during the PETM 16 , but also our understanding of temporal release patterns 11,17,28 , climate sensitivity 15 , and ecosystem recovery 10 .…”

The benthic foraminiferal δ34S records flux and timing of paleo methane emissions

“…The mount was subsequently impregnated with epoxy to fill any exposed void and polished with 1 μm Al 2 O 3 powder (Mark V Laboratories). We note that our protocol is similar to other published methodologies to prepare foraminiferal cross-sections for ion microprobe analysis (Glock et al 58 ; Kozdon et al 59 ), even if our sample preparation was optimized for mounting and polishing relatively small C. neoteretis specimens. Prior to ion microprobe work, the mount was gold coated and each foraminiferal shell was imaged using a Zeiss Auriga scanning electron microscope (SEM) at the University of Rochester (Rochester, NY) using a voltage of 20 kV.…”

Constraints on the onset duration of the Paleocene–Eocene Thermal Maximum

“…[ 75 ] and a −4.6‰ atmospheric δ 13 C excursion after Kozdon et al . [ 76 ] and Diefendorf et al . [ 77 ].…”