Elizabeth M. Walsh1, Anitra E. Ingalls1 & Allen J. Milligan2

1Oceanography, University of Washington, 2Botany and Plant Pathology, Oregon State University

How to Get a Date with a Diatom: Compound-Specific Isotope Analyses of Frustule-Bound Organic Matter as Proxies for Ocean Conditions

The Southern Ocean is hypothesized to play an important role in mediating changes in glacial-interglacial atmospheric carbon dioxide levels.  Biological uptake of carbon dioxide in the Southern Ocean, a currently iron-limited region, is controlled by diatom primary production.  The siliceous shells, or frustules, of diatoms act as ballast, allowing diatom organic matter to sink below the euphotic zone and be buried in sediments.  Much of the Southern Ocean sediment is nearly 100% diatomaceous material.  Despite the importance of Southern Ocean diatom primary production in regulating global CO2, little is known about diatom paleoproductivity on glacial timescales, as paleoceanographic analysis of diatom age and productivity has been methodologically difficult and unreliable.  It may be possible to circumvent issues of contamination and diagenesis by analyzing the content and isotopic composition of organic matter bound inside diatom frustules, and thus protected from outside influences. In this study, the organic matter from diatom cultures grown in nutrient rich and iron-limited conditions was characterized, and biomarkers identified.  This technique, combined with compound-specific radiocarbon analyses of the biomarkers, was then applied to cores from the Southern Ocean.

Joe R. Melton1, Jed O. Kaplan2, & Michael Whiticar1

1Earth and Ocean Sciences, University of Victoria, 2WSL/EPFL Switzerland

Simulated Wetland Methane Carbon Isotope Dynamics Over an Abrupt Climate Change

Recent measurement of ice core delta 13C-CH4 and deconvolution of the global CH4 budget during abrupt climate change events has led to a widespread speculation regarding the mechanisms behind observed rapid changes in atmospheric CH4 concentrations. Using a paleoclimate scenario from an intermediate complexity climate model and a dynamic global vegetation model, we simulate changes in the magnitude and carbon isotope composition of CH4 emitted from wetlands. In a control scenario, global wetland CH4 emissions are dominated by the tropics. During full glacial climate conditions of low sea level, tropical continental shelves supported wetland areas larger than those at present giving a heavier (more positive) wetland delta 13C-CH4 value. Preliminary results indicate that the boreal region possess a comparatively lighter methane isotopic ratio that could become important through climate changes. The differing response of the tropical vs. boreal wetlands can have important implications to the global wetland delta 13C-CH4. These results may partially explain why observed delta 13C-CH4 appear stable even at times of rapidly changing atmospheric CH4 concentrations.

Shelley A. Kunasek, B. Alexander, E.J. Steig, M.G. Hastings, & J.C. Jarvis

Earth and Space Sciences, University of Washington

Measurements and Modeling of ∆17O of Nitrate in a Snowpit from Summit, Greenland

Measurements of ∆17O of nitrate and sulfate in polar ice cores may provide information about changes in paleoatmospheric oxidant concentrations associated with glacial-interglacial climate changes.  In this study, quantitative interpretation of nitrate ∆17O (∆17O(HNO3)) in polar snow and ice is explored.  Measurements of ∆17O(HNO3) from a snowpit at Summit, Greenland are compared to predictions by an atmospheric chemical box model forced by the GEOS-Chem global 3-D geochemical transport model and available surface observations.  Measured values of nitrate ∆17O spanning 2.5 years (July 2003 – March 2006) range from 24.7‰ in summertime to 33.0‰ in wintertime, while model results show a larger range (18.9-31.5‰).  Agreement between observed and modeled results is excellent for winter, when O3 oxidation pathways dominate.  The large discrepancy between summertime box model results and measurements of nitrate ∆17O is best explained by horizontal advection of nitrate from oceanic air masses.  This suggests that ∆17O(HNO3) is representative of atmospheric chemistry on at least a regional scale and is not dominated by local oxidation chemistry and post-depositional processing.  A sensitivity study of the box model reinforces that ∆17O(HNO3) is most sensitive to the balance of oxidant concentrations in summertime and enables a critical assessment of the potential for quantifying oxidant concentrations from ice core ∆17O(HNO3).

Mariana Huerta & Cathy Whitlock

Earth Sciences, Montana State University

Postglacial Fire and Climate History of Northern Yellowstone National Park Inferred from High-Resolution Charcoal and Pollen Records

The Pacific Northwest and West Coast of the United States, under the influence of the subtropical high pressure system during the summer, have a low summer to winter precipitation ratio (summer-dry conditions).  The southwestern states and Great Plains are affected by summer monsoonal flow from the Gulfs of California and Mexico and have a high summer to winter precipitation ratio (summer-wet conditions).  The boundary between these regions runs through Yellowstone National Park, with summer-dry conditions in the southwestern and summer-wet conditions in the northeastern parts of the park.  Previous studies suggest intensification of the summer-dry and summer-wet patterns in Yellowstone occurred during the early Holocene when increased summer insolation caused atmospheric circulation patterns to strengthen.  In order to test the hypothesis that these precipitation regimes were strengthened across all of Yellowstone, fire and vegetation records were constructed using a 7-m-long sediment core from Blacktail Pond, a site in the northern summer-wet part of the park.

Four radiocarbon dates and the Mazama Ash were used to develop a 15,000 year chronology from Blacktail Pond.  Charcoal particles > 125μm diameter were extracted in contiguous 1- and 0.5-cm intervals that spanned 30 years on average.  The charcoal record was decomposed into background and peak components to reconstruct fire occurrence around the site.

Following deglaciation, fire activity at Blacktail Pond was low but charcoal accumulation rates increased substantially and fire frequency rose to 8 episodes/1000 years after 12,500 yr BP.  Frequency was high (15 episodes/1000 years) from 10,000 to 7000 yr BP, dropped to 7 episodes/1000 years between 7000 and 1000 yr BP and increased over the last 1000 years to 13 episodes/1000 years.

The fire history from Blacktail Pond suggests that this site was under the influence of summer-dry conditions during the early Holocene and shifted to summer-wet conditions only during the late Holocene.  Thus, the boundary between summer-wet and summer-dry conditions in Yellowstone has not remained stationary throughout the Holocene.  This boundary moved in a southwesterly direction during the middle Holocene, likely the result of changes in atmospheric circulation patterns linked to decreased summer insolation.

Aaron Donohoe

Atmospheric Sciences, University of Washington

A Proposed Mechanism for Observed Abrupt Climate Change Events in Greenland 

The temperature record deduced from Greenland ice cores provide the most dramatic observational record of abrupt climate change with repeated instance of 12-15 C warmings over the course of less than a decade. These events occur in the absence of abrupt external forcing and therefore suggests a regime transition in the circulation of the climate system. It is argued here that the Greenland ice core dust records give the clearest picture of these regime transitions across abrupt climate change events. Each regime of circulation is associated with a dramatically altered mid-latitude atmospheric circulation. Evidence from the Greenland ice core stable isotopes and accumulation rates indicate that the temperature change across abrupt climate change is dominated by changes in winter temperature.

These lines of evidence suggest that abrupt climate change events predominantly reflect a change in the stationary wave pattern and storm tracks in the Northern Atlantic atmosphere. Two different atmospheric circulation regimes in the North Atlantic are found and analyzed in a general circulation model forced with slightly different ice sheets over North America. The different circulation regimes in the general circulation model are consistent with the changes in temperature, dust, and accumulation seen across abrupt climate change events in the Greenland ice core. We therefore propose that abrupt climate change events reflect transitions between the two circulation regimes isolated in the general circulation model.      

Brian Rose

Atmospheres, Oceans and Climate, Massachusetts Institute of Technology

Sea Ice, Wind, and Ocean Currents: Feedbacks and Instabilities in Ice Age Climates

The polar sea ice caps are both shaped by, and help to shape, the global circulation of winds and ocean currents. Changes in ice cover affect the energy balance of the planet through the albedo effect, but can also drive changes in the large-scale wind patterns (location of storm tracks and jets) by altering the low-level heating of the atmosphere. Shifts in the surface wind have implications for the wind-driven ocean currents and heat transport, which in turn affect the sea ice cover. I will discuss interactions between the various components of this feedback loop, and their implications for the large-scale climate. In the colder climate of the ice ages, the sea ice extended farther equatorward, and thus would have interacted more strongly with the atmosphere’s main mid-latitude baroclinic zone that shapes the global wind system. I will use some idealized models to illustrate the feedback between ice, wind and ocean currents, and discuss how it could have destabilized the glacial climate, leading to rapid, large scale advance and retreats of the ice cover. This mechanism may help explain the observations of enhanced climate variability during glacial times.