TRACKING HOLOCENE CHANGES IN THE SEASONALITY OF PRECIPITATION IN THE NORTHEASTERN UNITED STATES

Precipitation regimes differ around the world today not only in annual amount of precipitation, but also in the seasonal timing of precipitation. Reconstructing the seasonal timing of precipitation may, likewise, be useful for tracking the long-term evolution of atmospheric circulation. Here, I use lake-level estimates and compound-specific hydrogen isotope measurements to evaluate the mechanism underlying ecologically-significant changes in the precipitation regime of the northeastern United States and adjacent Canada during the Holocene. Lake level and pollen data from the Northeast pose an interesting problem because they indicate that moisture availability rose and fell multiple times when the known climatic controls (i.e., ice extent and insolation) underwent progressive (non-fluctuating) changes. Shifts in the seasonality of precipitation may be important for explaining the contrast because the progressive changes in ice extent and insolation could have produced predictable but complex seasonal responses. A leading hypothesis includes a summer-dry early-Holocene when the Laurentide ice sheet was present, followed by a slow insolation-driven transition from summer-wet to winter-wet conditions. As a first test of the hypothesis, I used a heuristic moisture-budget model to simulate level levels based on hypothesized changes in the seasonality of precipitation. The results match the Holocene observations. As a second test, I measured the hydrogen isotope ratios of terrestrial leaf waxes preserved in two lake sediment cores. The results are also consistent with the hypothesized changes. Therefore, the sub-orbital (millennial-scale) moisture-balance fluctuations probably represent non-linear, seasonal responses to orbitally-forced circulation changes.