Research

My lab group aims to answer questions about Earth’s climate system by reconstructing how climate changes have unfolded in the past. We perform all stages of paleolimnological research, from site selection and field work to interpreting sediment stratigraphy and chronology to developing and interpreting paleoclimate proxies.

Current Research Foci

 

Testing and improving paleolimnological methods for reconstructing Arctic climate

Current projects in the lab are investigating the use of oxygen isotopes in organic materials to reconstruct past lakewater isotopic composition; determining the sources of lipids preserved in lake sediments and controls on their hydrogen isotopic composition; and convening an international collaboration to improve understanding of chironomid species distributions (and thus their use as paleoenvironmental proxies) around the Arctic. See for example Ph.D. student Jamie McFarlin’s NSF DDRI grant description or Ph.D. student Everett Lasher’s website.

 

Holocene temperature history of Greenland — and impacts of climate change on Greenland’s glaciers
The Greenland Ice Sheet margin near Kangerlussuaq.

The Greenland Ice Sheet margin near Kangerlussuaq.

We are currently conducting research in several sectors of Greenland to reconstruct Holocene temperature histories. My students and I are pursuing the use of biological assemblages, stable isotopes of faunal remains, sedimentary plant biomarkers, and glaciofluvial/glaciolacustrine sediments as proxies for past climate. Despite Greenland’s global importance (e.g., as a source of sea level rise), and a long history of ice core research, Greenland’s Holocene climate history is surprisingly poorly known. Lab members are currently busy analyzing Holocene sediment cores collected in 2016 from remote organic lakes in southernmost Greenland in 2016 (part of a NSF-funded CAREER project) and cores collected from mountain-glacier-influenced and organic lakes in 2015 near Nuuk with funding from the National Geographic Society.

Field work and some of the goals of our SW Greenland project near Nuuk are described in a National Geographic Explorers’ blog post by embedded Medill journalist Bryce Gray.

Collaborations with glacial geologists near Thule, Scoresby Sund and Jakobshavn/Ilulissat are designed to compare our estimates of the magnitudes and rates of past temperature fluctuations with corresponding reconstructed changes in the Greenland Ice Sheet and nearby ice caps. Ultimately, one goal of this work is to help answer the urgent question, “How (and how quickly) do Greenland’s glaciers respond to a warming climate?” For a taste of some early results integrating glacial geology and paleolimnology, see these papers on early Holocene ice sheet retreat in South Greenland; Holocene ice cap history near Scoresbysund; temperature reconstructions near Ilulissat/Jakobshavn in West Greenland; and the response of Jakobshavn Isbrae to Holocene temperature changes.

 

Climate perspectives from multiple interglacials
A diatom from Lake CF8 was the cover model for this issue of PNAS.

A diatom from Lake CF8 was the cover model for this issue of PNAS.

The Arctic is changing dramatically before our eyes — but how unusual are these changes from a geologist’s perspective? And to what extent were the impacts of early Holocene warmth in the Arctic typical of warm periods in general? Are there sediments preserved in our study areas that record the behavior of the arctic system during even warmer times (perhaps the Last Interglacial, i.e., Eemian)? I am interested in chasing down valuable glacial-geologic settings where lake sediments from multiple interglacials are preserved in situ in extant lakes around the glaciated arctic, despite proximity to ice sheets, thanks to past polythermal and cold-based ice sheet advances. This approach previously proved fruitful on Baffin Island in Arctic Canada, where cold-based and thus minimaly erosive advances of the Pleistocene Laurentide Ice Sheet have preserved interglacial sediments in modern lakes that extend back at least 200,000 years.¬†Papers in PNAS, Geology, and Geological Society of America Bulletin summarized our resulting highly unusual multi-proxy reconstructions of arctic environmental conditions over multiple interglacials.

Currently,we are working to recover and analyze the first such records from lakes in northwest Greenland, where paleoclimate reconstructions can be compared with those from the nearby North Eemian (NEEM) ice core.

Other Ongoing and past projects

 

Syntheses of Arctic climate reconstructions
Sampling surface sediments, Baffin Island.

Sampling surface sediments, Baffin Island.

The whole is greater than the sum of its parts — so I have participated in numerous collaborative efforts to collect and collate published arctic paleoclimate reconstructions from researchers around the world, to make these diverse raw data more readily available to fellow scientists and the general public, and to draw robust conclusions about the Arctic’s overall climate history based upon the resulting large data sets. Recent such efforts include 2016 review papers summarizing Holocene paleoclimate data from Greenland and the Canadian Arctic, and from Alaska and the Northwest Territories. Older examples include this Science paper presenting a pan-Arctic 2000-year-long temperature reconstruction, the publically available Arctic Holocene Climate Proxy Database, and this paper describing the unique database.

 

Tracking the Aleutian Low through millennia in Alaska
Examining tephra (volcanic ash) layers in a sediment core, on Adak Island in the central North Pacific Ocean.

Examining tephra (volcanic ash) layers in a sediment core, on Adak Island.

This NSF-funded collaboration with Northern Arizona University investigates how past climatic changes affected atmospheric circulation in the North Pacific region. Inferring changes in circulation is a good deal more complex than reconstructing a single parameter (e.g., temperature) at a single site: To track changes in circulation over time, we have collected a range of proxy data from several strategically located field sites forming a transect across southwest to south-central Alaska. The Polar Field Services newsletter summed up our 2010 field work in the central Aleutian Islands and I’ve posted some¬†field shots here. A 2012 paper summarized results from a collaborative study sites in the Ahklun Mountains of southwest Alaska, and results are forthcoming from additional collaborative sites in the Aleutians and south-central Alaska.

 

Paleoecology in the Peruvian Andes

Can insect (chironomid) remains be used to reconstruct past temperatures at high-elevation sites in the tropical Andes? A midge pilot study, part of an NSF-funded collaboration with Dartmouth College and the University of Cincinnati (and now Open University in the UK), is aiming to answer this question. This work is part of a larger project, led by Dr. Meredith Kelly at Dartmouth and designed to clarify the Holocene climate history of the region around Quelccaya Ice Cap, the largest tropical glacier complex in the world. Charcoal and pollen analyses from a Quelccaya study site are also underway at NU, and should yield insights about how climate, vegetation, and fire history have been linked through the Holocene.

You might be wondering: Why midges?

Subfossil chironomid head capsules collected from lake sediments on Baffin Island.

Subfossil chironomid head capsules collected from lake sediments on Baffin Island.

Photomicrograph of chironomid genus Heterotrissocladisus.

Photomicrograph of chironomid genus Heterotrissocladisus.

Perhaps the most unusual specialty of our lab is the analysis of chironomid remains. At many sites, we document and interpret the shifting abundances of various species of chironomids, and at others we take a close look at the chemical composition of their remains. Chironomids (the insect family more commonly known as non-biting midges, Insecta: Diptera: Chironomidae) are a diverse and nearly ubiquitous family of two-winged flies. Chironomids spend much of their life cycle as aquatic larvae. The larval head capsules of chironomids are chitinous and usually well preserved in lake sediments. (In fact, they are often the most abundant type of organic remains in the sediments we study, especially in sediments from very high latitudes.) Distinctive mouth parts and other features make many head capsules recognizable to at least the generic level. Because many chironomid taxa have temperature-dependent habitat distributions, fossil chironomid assemblages are useful as paleotemperature indicators. All of these qualities — their remarkable abundance and diversity, widespread excellent preservation in lake sediments, success at living in arctic and high-elevation lakes, and temperature sensitivity — make chironomids exceptionally useful for paleoclimate research.