Geologic records of a warmer Arctic

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The geologic record includes examples of past climates that were much different from today, and the Eocene epoch (from 56 to 33.9 million years ago) included the warmest climates of the last 66 million years. During the Eocene, a series of abrupt warming events, known as “hyperthermals”, coincided with pulses of carbon injection into the atmosphere, which are recorded in the geologic record by changes in carbon isotope values. These events had significant impacts on the distribution of plant and animal communities, and analysis of the events provides insights into the feedbacks between changes in the global carbon cycle and climate extremes.

The largest hyperthermal event was the Paleocene-Eocene Thermal Maximum (PETM at ~55.5 million years ago), when a massive amount of carbon was injected into the atmosphere. Carbon injection has been hypothesized to affect the global hydrologic cycle and seasonal contrasts in temperature, and paleoclimate proxies from sediments deposited during hyperthermals provide evidence to evaluate those hypotheses.

USGS researchers and colleagues from Brandon University, Lamont-Doherty Earth Observatory, and Utrecht University recently analyzed the impact of hyperthermal events in the Arctic using sediment cores collected by the Integrated Ocean Drilling Program (IODP) Expedition. Analyses of pollen, biomarkers from soil bacteria, and other organic-walled microfossils from the sediment cores allowed the scientists to reconstruct vegetation, atmospheric temperature, and hydrology before, during, and after the PETM. 

During the Paleocene and early Eocene, the continents were arranged differently than today (Figure 1), and the Arctic Basin was more or less land-locked, with limited exchange between the Arctic Ocean and the Pacific and Proto-Atlantic Oceans. The coring site used in this study, IODP Site 302-4A, is located on Lomonosov Ridge, a continental fragment that broke away from the Eurasian continent ~57 million years ago. During late Paleocene-early Eocene time, the site is thought to have been in relatively shallow water, and pollen from nearby landmasses was preserved in the ocean sediments.

Pollen evidence (Figure 2) from the core indicates that late Paleocene Arctic climates were warmer than today, with mixed conifer-hardwood forests (with pine, spruce, and walnut) occupying landmasses near the coring site. Today, these areas are underlain by permafrost, and plant communities consist of dwarf shrubs, grasses, mosses, and lichens. 

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Bioclimatic analyses, based on climatic requirements of plant groups identified in the pollen record, indicate a generally warm late Paleocene climate (mean annual temperature averaging ~13˚C/55˚F). Independent analysis of lipid biomarkers also indicates a warm late Paleocene (14.6˚C/58˚F). In contrast, present day mean average temperatures in Greenland are currently ~4˚C/39˚F.

During the PETM hyperthermal event, broad-leaved swamp forests (with members of the cypress family, palms, and other warm temperate to subtropical plants) dominated the nearby landscape. Both bioclimatic analyses and biomarkers indicate that PETM mean annual temperatures increased by as much as 3.5˚C/5.4˚F, driven primarily by warmer winters. Analysis of other organic microfossils indicates that runoff of water and nutrients from the continents to the oceans also increased during the PETM, resulting in lower salinity, decreased oxygen content of water, and changes in algal communities in the Arctic Ocean.

After the peak of the PETM, forested wetland and lowland vegetation dominated the landscapes, and subtropical plants were absent. Mean annual temperatures decreased but remained warmer than the late Paleocene baseline, and normal marine conditions returned.

The study shows that the PETM injection of carbon to the atmosphere was accompanied by a significant warming of air temperatures, particularly during the winters. The resulting restructuring of plant and animal communities includes the northernmost known occurrence of palms and other taxa that are now native to tropical and subtropical latitudes.   Increased runoff of water and nutrients from the land to the ocean resulted in lower salinity and availability of oxygen in ocean waters, which affected the composition of algal communities that are the base of the marine food chain.

This research is part of a broader international effort to document the interactions between natural changes in greenhouse gas concentrations, climate, and plant and animal communities on land and in the ocean across the globe. Through studies of past abrupt events, earth scientists are developing large datasets that can be used to test results of global climate models that simulate past, present, and future climate. The results also provide a unique window on how the Earth system has responded to extreme events of the past - yielding insights on impacts of potential changes in the future.

The paper “Arctic vegetation, temperature, and hydrology during Early Eocene transient global warming events” was published in Global and Planetary Change.

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