I study past changes in Earth's climate to further our understanding of forcings, responses, and feedbacks in the climate system. I use a combination of isotope geochemistry and numerical modelling to reconstruct and understand these changes. Some of my active research projects, and selected publications, are listed below.

Volcanic forcing of climate

Volcanic eruptions provide one of the major natural forcings of climate, and thus past eruptions are opportunities to study how the climate responds to external forcing. Our best records of past volcanic events come from sulfate peaks in ice cores, but many of the key attributes that determine the climate forcing associated with an eruption are hidden. I have been developing new techniques to measure sulfur isotopes in ice cores to reconstruct the climate forcing potential of unidentified past eruptions.

McConnell et al. 2020. Extreme climate after massive eruption of Alaska’s Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom. PNAS, 117(27), 15443.

Burke et al., 2019. Stratospheric eruptions from tropical and extra-tropical volcanoes constrained using high-resolution sulfur isotopes in ice cores. EPSL, 521: 113-119.

McConnell et al., 2017. Synchronous volcanic eruptions and abrupt climate change ∼17.7 ka plausibly linked by stratospheric ozone depletion. PNAS, 114: 10035-10040.

Ocean carbon cycling in the Last Glacial Maximum & deglaciation

The end of the last ice age between 20 and 10 thousand years ago marks the largest, most recent natural perturbation to the carbon cycle, with atmospheric CO2 concentrations rising by 90 ppm. The ocean played a key role in this transition, but the exact mechanisms remain elusive. I have been using geochemical tracers (e.g. radiocarbon and boron isotopes) in marine calcifying organisms such as foraminifera and deep-sea corals to reconstruct when and how carbon moved from the ocean to the atmosphere.

Chen et al., (2020). Persistently well-ventilated intermediate-depth ocean through the last deglaciation. Nature Geoscience, 13(11), pp.733-738.

Rae et al., 2020. Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific. Science Advances, 6(50), eabd1654.

Gray et al., 2018. Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean. Nature Geoscience, 11: 340-344.

Chen et al., 2015. Synchronous centennial abrupt events in the ocean and atmosphere during the last deglaciation. Science, 349:1537-1541.

Sea ice, circulation, and carbon dynamics in the Southern Ocean

The Southern Ocean plays a crucial role in setting the large scale global overturning circulation. Furthermore, the unique dynamics and biogeochemistry of this region contribute to its importance on glacial-interglacial carbon cycling. My research combines geochemical records from deep sea corals with dynamical models of this important region and highlights the importance that Southern Ocean sea ice has on circulation and carbon cycling on glacial-interglacial to centennial timescales.

Li et al., 2020. Rapid shifts in circulation and biogeochemistry of the Southern Ocean during deglacial carbon cycle events. Science Advances, 6(42), p.eabb3807.

Rae et al., 2018. CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales. Nature, 562: 569-573.

Burke et al., 2015. The glacial mid‐depth radiocarbon bulge and its implications for the overturning circulation. Paleoceanography, 30: 1021-1039.

Ferrari et al., 2014. Antarctic sea ice control on ocean circulation in present and glacial climates. PNAS, 111: 8753-8758.

Burke et al., 2012. The Southern Ocean's role in carbon exchange during the last deglaciation. Science, 335: 557-561.

The sequence of climate events resulting in the last deglaciation

The demise of the Laurentide and other major Northern Hemisphere ice sheets at the end of the last ice age and the concommitant rise in atmospheric CO2 was the result of a complex interplay of feedbacks and responses to forcings. Studying the sequence of events that led to this extreme change in climate allows us to investigate key processes and identify tipping points in the climate system. I have been working on improving marine chronologies over this time period, as well as synthesizing data for comparison to climate model simulations of the last deglaciation through the PMIP4 effort.

Ivanovic et al., 2018. Acceleration of northern ice sheet melt induces AMOC slowdown and northern cooling in simulations of the early last deglaciation. Paleoceanography and Paleoclimatology. 33: 807-824.

Ivanovic et al., 2017. Collapse of the North American ice saddle 14,500 years ago caused widespread cooling and reduced ocean overturning circulation. Geophysical Research Letters, 44: 383-392.

Weathering and its role in climate

The weathering of rocks on Earth's surface is intimately linked to the global carbon cycle, and as such it plays a key role regulating Earth's climate. However, whether weathering is a source or sink of CO2 to the ocean-atmosphere system depends on the rock that is being weathered, the source of acidity in the water, and the timescale being considered. Isotope measurements are ideal for distinguishing between different weathering sources, and can be used with an inverse model to determine weathering budgets in individual catchments or globally. My work on riverine sulfur isotopes has shown that the global weathering of pyrite has been previously underestimated, and thus that the CO2 weathering sink is likely overestimated.

Kemeny et al., 2021. Sulfate sulfur isotopes and major ion chemistry reveal that pyrite oxidation counteracts CO2 drawdown from silicate weathering in the Langtang-Trisuli-Narayani River system, Nepal Himalaya. Geochimica et Cosmochimica Acta, 294, pp.43-69.

Burke et al., 2018. Sulfur isotopes in rivers: Insights into global weathering budgets, pyrite oxidation, and the modern sulfur cycle. Earth and Planetary Science Letters, 496: 168-177.

Bhatia et al., 2011. Seasonal evolution of water contributions to discharge from a Greenland outlet glacier: insight from a new isotope-mixing model. Journal of Glaciology, 57: 929-941.