Delays, Discrepancies, and Distortions: Size‐Dependent Sediment Mixing and the Deep‐Sea Record of the Paleocene‐Eocene Thermal Maximum From ODP Site 690 (Weddell Sea)

Abstract

The deep-sea sedimentary record of Ocean Drilling Program Site 690 (Weddell Sea) has figured prominently in the study of an ancient (~56 Ma) global warming event referred to as the Paleocene-Eocene Thermal Maximum (PETM). Yet discrepancies in the timing, amplitude, and structure of the isotopic excursions marking the PETM exist between the bulk-carbonate and planktic foraminifer stable isotope (δ 13 C and δ 18 O) records of this reference section. A recent study invoked size-dependent sediment mixing (SDSM) to reconcile the discrepancies between these parallel δ 13 C records. Here we report supplementary stable isotope data compiled from size-segregated planktic foraminifer shells that further elucidate the effects of SDSM on the Site 690 PETM section. Our records reveal a stratigraphic sequence where the carbon (CIE) and oxygen (OIE) isotope excursions marking the onset of PETM conditions are first registered by larger shells, confirming that smaller-sized pre-CIE shells have been more intensely mixed and preferentially displaced up-section into the overlying CIE interval. The size-dependent manner in which the CIE and OIE are recorded by asymbiotic subbotinids indicates that the transitory loss of size-dependent δ 13 C signatures in photosymbiotic taxa (acarininids) at Site 690 is an artifact of SDSM. Synthetic bulk-carbonate δ 13 C records generated with two end-member (pre-CIE versus CIE materials) sediment-mixing curves readily reproduce the series of "steps" seen in published bulk-carbonate δ 13 C records of the CIE, suggesting that these fine-scale features may also be artifacts of SDSM and varying rates of carbonate sedimentation.

Photosymbiosis in planktonic foraminifera across the Paleocene–Eocene thermal maximum

“…4). Similarly weak isotopic enrichments are recorded in pre-PETM A. subsphaerica at Site 690 (Hupp et al 2019;Hupp and Kelly 2020) and in pre-PETM acarininids from ODP Site 689 (Si and Aubry 2018). Across all Southern Ocean studies, Acarinina shows no significant size-δ 13 C relationship in the middle of the core-PETM, during which Acarinina δ 13 C values collapse onto those of co-occurring Subbotina.…”

“…It is common for the largest individuals of photosymbiont-bearing species to show weaker size-δ 13 C fractionation effects than smaller individuals, which is attributed to ontogenetic effects such as gametogenic calcification, which could account for differences among size fractions. Hupp et al (2019) and Hupp and Kelly (2020) proposed a similar pattern during the onset of the PETM at Site 690 to be a preservational artifact of differential mixing depths and abundance changes across size fractions, an explanation that works equally well to explain the disconnect in isotopic changes across size fractions at Site 401, but not the differential response of acarininids and morozovellids to the PETM.…”

“…Across all Southern Ocean studies, Acarinina shows no significant size-δ 13 C relationship in the middle of the core-PETM, during which Acarinina δ 13 C values collapse onto those of co-occurring Subbotina. Both trends are captured by the record of Hupp et al (2019) and Hupp and Kelly (2020), which have greater temporal resolution around the onset of the PETM. The shape of the acarininid collapse (occurring in the largest size fraction first and resulting in a strongly negative size-δ 13 C relationship during the collapse) emphasizes the importance of sediment mixing and population abundance in structuring the perception of biological change (Kirtland Turner et al 2017;Hupp and Kelly 2020).…”

Size‐Fraction‐Specific Stable Isotope Variations as a Framework for Interpreting Early Eocene Bulk Sediment Carbon Isotope Records

“…Hupp and coworkers recently reported that cm‐scale discrepancies between bulk carbonate‐ and planktic foraminiferal δ 13 C values can arise from size‐dependent sediment mixing (SDSM), which may affect the stratigraphic position of a CIE onset for different size fractions (Hupp & Kelly, 2020; Hupp et al., 2019). The record presented here extends from 58 Ma to 50 Ma, spanning ∼25 meters with a mean sampling resolution of ∼0.5 m; thus, the trends reported here are much more protracted that those expected to arise from SDSM.…”

“…Despite the excellent chemostratigraphic correlation shown amongst bulk sediment δ 13 C across widely separated sediment sections, the timing of onset of major climatic events sometimes appear to be offset between bulk sediment‐ and planktic foraminiferal δ 13 C records (Kirtland‐Turner et al., 2017). Such discrepancies could result from post‐depositional size‐dependent sediment mixing―broadly a taphonomic effect―which results in delays in the onset of isotope excursions recorded by finer foraminifers compared to the ones with larger tests (Hupp & Kelly, 2020; Hupp et al., 2019). These post‐depositional effects may affect interpretation of high‐resolution isotope records.…”