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

The near-shore marine environment of Iceland during the last 2000 years is inferred from d¹⁸O and d¹³C values of 26 Arctic bivalves obtained from sediment cores from the northwest coast of Iceland. Proxy data from the cores reveal notable warm and cold intervals that we further characterize by selecting specimens from these intervals to obtain a high-resolution (~sub-weekly to weekly) record of seasonal variation. Fifteen bivalves have been micromilled using a three-dimensional, computer-controlled micromiller (~3500 samples to date). Micromilling paths are generated parallel to the natural growth bands of shells, to generate carbonate powder samples that represent discrete (~3-10 days) time periods. Temperature variation is the dominant control on seasonal changes in d¹⁸O and d¹³C values. Specimen age is established by dating the opposite valve of individual molluscs, or is based on sediment accumulation rates and interpolation between AMS radiocarbon dates.

Temperatures derived from d¹⁸O values using standard temperature-fractionation relationships reveal variation in both summer and winter temperature throughout the 2000-year period. Average summer maximum and winter minimum temperatures are relatively low from 210 to 120 BCE, increasing to the highest values of the last 2000 years at 80 BCE, and then decrease by about 460 CE. Temperatures increase and remain relatively warm from 870 to 1130 CE, then decrease to the lowest temperatures recorded at 1270 CE, and remain relatively low until about 1740 CE at which time they increase markedly. These changes in temperature are in agreement with historical documentary evidence. More recent shells will be milled to compare to the meteorological record, which extends back to 1830.

A seasonality index, defined by average peak temperature - average minimum temperature, was constructed to characterize temperature variability over the 2000-year period. Seasonality is generally lower (<4.4°C) for the first 1000 years, fluctuates from 900 to 1300 CE, then increases around 1460 CE to the highest seasonality overall (5.9°C). Variation in carbon isotope values reflects changes in metabolic rate related to temperature, as well as the isotope value of diet, which is influenced by feeding mode and may be taxon-specific.