Marcus P. S. Badger

High resolution alkenone palaeobarometry indicates relatively stable pCO2 during the Pliocene (3.3 to 2.8 Ma)

Temperature reconstructions indicate that the Pliocene was approximately 3°C warmer globally than today, and several recent reconstructions of Pliocene atmospheric CO2 indicate that it was above pre-industrial levels and similar to those likely to be seen this century. However, many of these reconstructions have been of relatively low temporal resolution, meaning that these records may have failed to capture variations associated with the 41 kyr glacial–interglacial cycles thought to have operated in the Pliocene. Here we present a new, high temporal resolution alkenone carbon isotope-based record of pCO2 spanning 3.3–2.8 Ma from Ocean Drilling Program Site 999. Our record is of high enough resolution (approx. 19 kyr) to resolve glacial–interglacial changes beyond the intrinsic uncertainty of the proxy method. The record suggests that Pliocene CO2 levels were relatively stable, exhibiting variation less than 55 ppm. We perform sensitivity studies to investigate the possible effect of changing sea surface temperature (SST), which highlights the importance of accurate and precise SST reconstructions for alkenone palaeobarometry, but demonstrate that these uncertainties do not affect our conclusions of relatively stable pCO2 levels during this interval.

Extreme warming of tropical waters during the Paleocene–Eocene Thermal Maximum

The Paleocene–Eocene Thermal Maximum (PETM), ca. 56 Ma, was a major global environmental perturbation attributed to a rapid rise in the concentration of greenhouse gases in the atmosphere. Geochemical records of tropical sea-surface temperatures (SSTs) from the PETM are rare and are typically affected by post-depositional diagenesis. To circumvent this issue, we have analyzed oxygen isotope ratios (δ18O) of single specimens of exceptionally well-preserved planktonic foraminifera from the PETM in Tanzania (∼19°S paleolatitude), which yield extremely low δ18O, down to 3 °C during the PETM and may have exceeded 40 °C. Calcareous plankton are absent from a large part of the Tanzania PETM record; extreme environmental change may have temporarily caused foraminiferal exclusion.

Causes of ice age intensification across the Mid-Pleistocene Transition

Significance Conflicting sets of hypotheses highlight either the role of ice sheets or atmospheric carbon dioxide (CO2) in causing the increase in duration and severity of ice age cycles ∼1 Mya during the Mid-Pleistocene Transition (MPT). We document early MPT CO2 cycles that were smaller than during recent ice age cycles. Using model simulations, we attribute this to post-MPT increase in glacial-stage dustiness and its effect on Southern Ocean productivity. Detailed analysis reveals the importance of CO2 climate forcing as a powerful positive feedback that magnified MPT climate change originally triggered by a change in ice sheet dynamics. These findings offer insights into the close coupling of climate, oceans, and ice sheets within the Earth System. During the Mid-Pleistocene Transition (MPT; 1,200–800 kya), Earth’s orbitally paced ice age cycles intensified, lengthened from ∼40,000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ∼43 to ∼75 μatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.

No substantial long-term bias in the Cenozoic benthic foraminifera oxygen-isotope record

Author(s): Evans, David; Badger, Marcus PS; Foster, Gavin L; Henehan, Michael J; Lear, Caroline H; Zachos, James C

Erratum: Addendum: Plio-Pleistocene climate sensitivity evaluated using high-resolution CO2 records

This corrects the article DOI: 10.1038/nature14145

Three dimensional coupled model approaches to terrestrial methane cycling during Paleogene greenhouse climates

The biogeochemical cycling of methane presents a major challenge for paleoclimate scientists, as although this critical greenhouse gas (GHG) has the forcing potential to drive significant changes in the Earth System, there are no proxy methods for reconstructing it’s ancient atmospheric concentration. This is especially important as biogenic methane emissions are controlled by environmental conditions such as temperature and precipitation, and so methane has significant power as a positive or negative feedback on global climate change. Understanding how methane emissions and cycling acted in the high pCO 2 greenhouse worlds of the Paleogene potentially bridges the gap between our understanding of other, better (although arguably still poorly-) constrained GHGs and global temperature. The recent application of advanced three dimensional global modelling strategies to the problem of Eocene trace GHG concentrations has begun to show how important these may be in high-CO 2 worlds (Valdes et al, 2005; Singarayer et al, 2011; Beerling et al, 2011), suggesting that as much as 2.7 °C of global warming may be contributed. This approach couples the unified Hadley Centre climate model (HadCM3L) with Sheffield Dynamic Global Vegetation Model (SDGVM; Beerling et al, 1997) to simulate trace gas emissions, and the atmospheric chemistry model STOCHEM to simulate the concentration of methane and ozone in the troposphere Here we present early results of a project to extend and develop the results of these earlier works, with revised and improved biogeochemistry and vegetation models, and broader consideration of boundary and initial conditions. These new results will be compared to indirect proxy methods to determine Paleogene methane emissions using lipid biomarker carbon isotopes in Eocene wetland settings (preserved as lignites) to probe past methanogenic and methanotrophic populations and constrain ancient methane emissions in high-CO 2 worlds.