LATE HOLOCENE CHANGES IN ATMOSPHERIC PATTERNS IN THE UPPER MIDWEST AS INFERRED FROM OXYGEN ISOTOPE RATIOS OF PEAT DEPOSITS

Sedimentological studies of beach ridge sequences that are commonly present along the coastline of Lake Superior indicate large and relatively rapid lake-level fluctuations over the past 4,700 years, which resulted from the interplay of climate, differential isostatic rebound, and outlet dynamics. The presence of peat deposits in swales between the beach ridges offers an opportunity to assess the role of climate because the isotopic composition of cellulose extracted from peat can be used to assess changes in water balance, moisture sources, and/or local hydrological conditions. Despite its great potential as a paleoclimatic proxy, the factors that control the isotopic composition of cellulose in modern plants are still uncertain.

Because the enrichment of oxygen-18 in moss cellulose may reflect site-specific conditions, we collected modern Sphagnum mosses in the Lake Superior area and samples of their potential water sources, namely rainwater, swale water, and groundwater. To assess the regional and temporal variability of isotope values, samples were collected in three locations along the shoreline of Lake Superior throughout two entire growing seasons.

Isotopic results show a marked seasonality in the oxygen isotopic composition of swale water with values ranging from -14.2 to -6.8 per mil. Despite this seasonal variability, oxygen isotopic composition of moss cellulose shows a constant value of 20±2 per mil throughout the entire sampling period, indicating the synthesis of cellulose early in the growing season.

Peat deposits formed from about 3140 to 3270 B.P., a period of higher lake levels in the region, exhibit values enriched by 6 per mil relative to modern isotope values of Sphagnum. Based on the invariance of the modern isotope composition of Sphagnum, we interpret that the correspondence of higher isotope values and higher water levels from about 3140 to 3270 B.P. is related to an enhanced predominance of air masses over the region from the Gulf of Mexico.