Sandra Kirtland Turner

Recovering the true size of an Eocene hyperthermal from the marine sedimentary record

[1] Hyperthermals—episodes of abrupt global warming associated with the massive injection of carbon into the oceans and atmosphere—represent possible analogs for future climate change. However, uncertainties in their magnitude, rate, and duration arising as a result of mixing processes and changes in carbonate preservation as the sediment record is formed complicate their use in constraining climate sensitivity and the role of carbon cycle feedbacks. Here, we use cGENIE, an Earth system model of intermediate complexity, to assess likely magnitude and rate of carbon input, taking a small hyperthermal event from the early-middle Eocene, C22nH3 (~49.2Ma) as a case study. We develop an iterative method combined with a sediment model simulating the formation and mixing of deep-sea sediments to converge on an estimate for the “true” magnitude of the carbon cycle perturbation in the atmosphere and ocean that drives the event. In inverting the � 0.95‰ benthic δ 13 Cexcursion recorded atOcean Drilling Program Site 1258, we obtain an estimate of at least � 1.45‰ for the atmospheric CO2 δ 13 C excursion that drove event C22nH3. We also assess controls on intersite variation of event shape in model sediments and find that sedimentation rate is the strongest determinant of modeled event size, with higher sedimentation rate sites recording the atmospheric signal more accurately. Our revised estimate for the size of C22nH3 implies a total carbon input almost two-thirds higher than would be deduced if the recorded δ 13 C excursion magnitude was taken at face value.

Pliocene switch in orbital-scale carbon cycle/climate dynamics

The high-frequency (periods of ~105 years) relationship between carbon and oxygen isotopes in benthic foraminifera—the two proxies most extensively used to reconstruct past changes in Earths carbon cycle and climate—shows two distinct patterns across the Cenozoic. The first, “glacial-style,” pattern associates negative excursions in δ13C with positive excursions in δ18O indicative of relatively cold temperatures and greater ice volume. The second, “hyperthermal-style,” pattern associates negative excursions in δ13C with negative excursions in δ18O indicative of warming. Here I assess the coherence and phasing of these high-frequency, orbital-scale cycles (in particular, the ~100 kyr eccentricity period) in δ13C and δ18O from multiple high-resolution benthic foraminiferal records spanning the last ~65 million years of Earth history in order to identify which of these patterns is most persistent across the Cenozoic and when the switch between these patterns occurred. I find that the glacial-style δ13C-δ18O pattern is a feature restricted to the Plio-Pleistocene, suggesting a fundamental change in the interplay between the carbon cycle and climate associated with the onset of Northern Hemisphere glaciation. This relative stability of the high-frequency relationship between δ13C and δ18O across most of the Cenozoic persists despite significant secular changes in climate and may suggest a dichotomous response of terrestrial carbon cycle dynamics to orbital forcing with a switch occurring in the last ~5 Myr.

A probabilistic assessment of the rapidity of PETM onset

Knowledge of the onset duration of the Paleocene-Eocene Thermal Maximum—the largest known greenhouse-gas-driven global warming event of the Cenozoic—is central to drawing inferences for future climate change. Single-foraminifera measurements of the associated carbon isotope excursion from Maud Rise (South Atlantic Ocean) are controversial, as they seem to indicate geologically instantaneous carbon release and anomalously long ocean mixing. Here, we fundamentally reinterpret this record and extract the likely PETM onset duration. First, we employ an Earth system model to illustrate how the response of ocean circulation to warming does not support the interpretation of instantaneous carbon release. Instead, we use a novel sediment-mixing model to show how changes in the relative population sizes of calcareous plankton, combined with sediment mixing, can explain the observations. Furthermore, for any plausible PETM onset duration and sampling methodology, we place a probability on not sampling an intermediate, syn-excursion isotopic value. Assuming mixed-layer carbonate production continued at Maud Rise, we deduce the PETM onset was likely <5 kyr.Single-foraminifera measurements of the PETM carbon isotope excursion from Maud Rise have been interpreted as indicating geologically instantaneous carbon release. Here, the authors explain these records using an Earth system model and a sediment-mixing model and extract the likely PETM onset duration.