Rethinking the physical evidence for Arctic warm periods: When do trees, sea ice, and ice sheets make sense?
|Location||International Geological Congress,oslo 2008|
|Holding Date||11 October 2008|
The literature summarizing the Arctic Ocean borderlands contain clear stratigraphic evidence for forested conditions at intervals over the past 50 million years, recording the migration of treeline from High Arctic coastal locations within the Canadian Archipelago. Metasequoia forests of the peak Eocene gave way to a variety of biomass-rich circumarctic redwood forests by 46 Ma. Between 23 and 16 Ma, cool-temperate metasequoia forests dominated NE Alaska and the Yukon while mixed conifer-hardwood forests (similar to those of modern southern maritime Canada and New England) dominated the central Canadian Archipelago. By 16 Ma, these forests gave way to larch and spruce. From 5 to 3 Ma the braid plains of the Beaufort Fm were dominated by over 100 vascular plants including pine and birch, while other locations remained dominated by spruce and larch. Boreal conditions across northern Greenland and arctic Alaska are consistent with the presence of bivalve Arctica islandica in marine sediments capping the Beaufort Formation on Meighen Island at 80oN, correspond to the peak of Pliocene warming (~3.2 Ma). These sites span significant portions of the Late Cenozoic requiring that marine records from the Arctic Ocean be integrated with this information, within the limitations of the geochronology and the spatial patchiness. Apparently conflicting evidence from oceanographic and terrestrial sources requires an evaluation of non-analog or new analog GCM reconstructions. Nowhere in the Arctic do trees exist near shore where there is perennial sea ice; trees are found only in southerly reaches of regions with seasonal ice and is presently controlled by the July 7oC mean isotherm. The combination of spatial relationships between marine and terrestrial data should allow for a new comprehensive reconstruction of the past climate.
Deep drilling planned for early 2009 to acquire a continuous 3.6 My record at Lake El’gygytgyn in NE Siberia, 100 km north of the Arctic Circle will produce the first deep-time high resolution terrestrial record of its kind in the Northern Hemisphere for comparison with emerging results from IODP drilling on the Lomonosov Ridge and the McMurdo Ice Shelf Project (ANDRILL). Short pilot cores from Lake El’gygytgyn record millennial scale change attributable to orbital forcing, rapid global scale changes in ocean circulation and ice sheet dynamics. However, even for the last interglacial, 5e, we find a discrepancy between warmer than present land conditions, reduced coastal summer sea ice and winter ice extent at odds with evidence of continuous ice cover over much of the central Arctic, with one exception. Modeling efforts are now key to assessing means of reconciling temporally overlapping land and sea records. Planned deep drill cores from Lake El’gygytgyn will capture millennial-scale changes to which Antarctic records can be compared order to evaluate and model systemic teleconnections and leads/lags relative to insolation forcing.