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QUANTITATIVE RECONSTRUCTIONS OF NEAR-SHORE ENVIRONMENTS OVER THE LAST 2000 YEARS IN VESTFIRDIR, NW ICELAND: EVIDENCE FROM HIGH-RESOLUTION STABLE ISOTOPE ANALYSES OF MOLLUSCS

Dietrich, Kristin A  University of Saskatchewan.
PATTERSON, WILLIAM P  University of Saskatchewan.
Andrews, John T  University of Colorado.
Jennings, Anne  University of Colorado.
Ogilvie, Astrid  University of Colorado.

Several near-shore sediment cores collected off the coast of Vestfirdir, the northwest peninsula of Iceland, contain abundant fossil molluscs dating back at least 2000 years. Twenty-four Arctic bivalves were collected from notable warm and cold intervals (based on other proxy data) in the cores, in order to establish a high-resolution record of seasonal variation during these periods. Bivalves have been micromilled using a three-dimensional, computer-controlled micromilling device that generates carbonate powder concordant with growth bands of the shells at ~weekly resolution (~1,200 samples to date). Age control of the specimens is provided by interpolation between AMS radiocarbon dates of the cores. Temperatures were calculated using standard temperature-fractionation relationships. Example data from several bivalves are presented below in Figures 1-3.

    Data reveal a general trend in both summer (maximum) and winter (minimum) temperatures (Fig. 4). Mean temperatures calculated for both seasons are highest in the oldest shell (dated at 80BC), decreasing by 460AD, and subsequently increasing in the late 800s (approximately the onset of the Medieval Warm Period). Temperatures decrease again towards the youngest shell dated at 1562AD (during the Little Ice Age). The greatest variability is found in summer maximum temperature, which ranges from 6.9 and 7.0C in years 460AD and 1562AD respectively, to a maximum of 12.4C in 870BC.

Variation in carbon isotope values can be attributed to species-specific differences in metabolic rate, dietary influences and temperature effects on metabolism.



Figure 1. Thyasira flexuosa specimen B99-341_66.25 from 870AD displaying maximum summer temperatures in excess of 12 degrees.


Figure 2. Thyasira flexuosa specimen MD99-2266_112 from 876AD demonstrating the resolution obtained by analyzing all samples recovered from a mollusc. Temperatures in this period range from less than 1C to nearly 9C at the onset of the Medieval Warm Period.


Figure 3. Macoma calcarea specimen MD99-2266_9.5 from 1562AD. This mollusc from the Little Ice Age displays the lowest temperatures of all molluscs sampled to date.


Figure 4. Summary plot of initial data demonstrating variation in summer maximum and winter minimum temperatures over the last ~2,100 years for the North Atlantic.


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