Current Projects

I am a co-leader of the PAGES/INQUA working group, PALeo constraints on SEA level rise (PALSEA3)


Arctic speleothem records of permafrost and climate
The Arctic is the fastest warming region on the planet, and melting permafrost there has the potential to dramatically amplify global warming because its frozen soils contain twice as much carbon as the atmosphere, which can be released as greenhouse gases upon thawing. One way to assess the vulnerability of permafrost to climate change is to reconstruct its response during past warm periods. Caves provide a novel archive to address this problem – speleothems require flowing water to form, and thus their presence in caves that are currently frozen imply thawed ground conditions in the past. Since speleothems can be dated with remarkable precision for the past 500 kyr and beyond, studying a number of caves across the Arctic should allow the extent of paleopermafrost thaw to be mapped out over the past several interglacial periods. In addition, oxygen isotopes records can be measured along the speleothems to reconstruct climate during the intervals when they grew – essentially extending the Greenland ice core record of Arctic climate, which extends only to the last interglacial, far back in time. David McGee (MIT), Corinne Wong (BC), and I are applying this approach to several caves in the North American Arctic, in collaboration with long-time Arctic cave scientists Derek Ford (McMaster) and Bernard Lauriol (Ottawa).


Cosmogenic nuclides in marine sediments
Paul Bierman (UVM) and I are measuring the first-ever in situ 10Be records from marine sediments to reconstruct the exposure and erosion history of glaciated continents. The basic idea is that 10Be is only produced in the top meter or two of bedrock exposed to cosmic radiation; once the land is covered, 10Be production ceases and erosion chews into deeper and therefore 10Be-poorer material. So, an ice-free landscape would have lots of 10Be in its rocks and sediments, a growing ice sheet would scrape this 10Be-rich material off, and any shorter interglacials thereafter would re-dose the surface with a new, smaller batch of 10Be only to be scraped off again by a later ice advance. These signals should have been shed off the landscape through time and recorded in nearby ocean sediments. We’re measuring 10Be in cores adjacent to Greenland and Antarctica to test this hypothesis and reconstruct the behavior of these ice sheets over the past 7 Myr. We’ve also measured 10Be in modern sediments emanating from the ice sheet and deglaciated basins today around Greenland to better understand the nature of this signal. Initial results look good, and this is an approach that could easily be extended to other ice sheets or the lower latitudes to reconstruct long-term erosion.

Paul and Alice (UVM MS '13) interviewed on Burlington, VT WCAX News, 7/10/12. Looking into climate change
Josh Brown's embedded reporting from the field in 2012. The Unbearable Lightness of Greenland
Vermont Quaterly writeup. Ice Sheets, Isotopes, and Musk-Ox Pizza


New England ice sheet dipsticks
Glacial geologists have spent decades dating the retreat history of the late Pleistocene ice sheets, tracing their individual outlines. Marine geologists, meanwhile, have produced detailed reconstructions of sea level, revealing the total volume of ice on the planet through time. Marrying these two records, area and volume, to obtain the thinning history of each ice sheet remains a grand challenge. Such information is essential for pinpointing the ice sheets and processes responsible for past sea-level jumps, understanding climate-ice sheet linkages, and providing much-needed tests for models attempting to simulate ice-sheet decay. Paul Bierman (UVM), Thom Davis (Bentley), and I are reconstructing the collapse of the Laurentide Ice Sheet in New England by measuring 10Be exposure ages of glacial boulders down numerous mountainsides. This is essentially the only region where the thinning history of this massive former ice sheet can be directly constrained, since it covered flat terrain nearly everywhere else.


The Holocene temperature conundrum
Colleagues and I recently developed the first reconstruction of global temperature spanning the Holocene from 73 proxy records (Marcott et al., 2013). This temperature curve exhibits a long-term cooling over the past several thousand years, presumably in response to Milankovitch cycles pushing us toward the next ice age. Interestingly, existing climate model simulations instead show warming over this time interval due to modestly rising greenhouse gases and shrinking ice sheets. Which is right? Perhaps the proxy records are biased toward certain seasons and do not provide a faithful record of mean annual temperature. Or maybe the models are missing key feedbacks that amplified the Milankovitch forcing, rendering them slaves to CO2 instead. Shaun Marcott (Wisconsin) and I plan to mine the vast body of Holocene proxy records that have been published from around the world to better understand the signals they contain and create a more robust global temperature reconstruction.



Holocene context for current tropical glacier extent

Retreating tropical glaciers are a poster child for global warming, but how anomalous is current ice extent relative to the past 10,000 years and what controlled their fluctuations during this time? Northern Hemisphere glaciers generally grew through the Holocene, whereas Southern Hemisphere glaciers appear to have retreated, each in response to local insolation forcing – but the situation in the tropics is less clear. Brent Goehring (Purdue) and I aim to apply a new paired cosmogenic isotope technique (Goehring et al., 2011) to recently exposed bedrock at the toe of several retreating tropical glaciers to determine the duration of the Holocene that each was smaller than today.