SPATIAL AND TEMPORAL PATTERNS OF MID-HOLOCENE DRY PERIODS IN EASTERN NORTH AMERICA: ATMOSPHERIC LINKAGE AND AIR MASS INTERACTION?

Understanding spatial and temporal patterns of past climate changes is important in inferring climate forcing mechanisms and testing climate models.Lake-level fluctuations of closed-basin lakes are direct indicators of changes in atmospheric moisture balance, while other multiple proxy sedimentary data, especially stable isotopes, may shed lights on possible cause of these changes.Holocene peak dry periods show a transgressive trend in timing from 8-4 ka (radiocarbon time scale) in Minnesota to 5-2 ka in southern Ontario.Synthesis of regional water isotopes from the Midwest to Ontario shows a decreasing trend of dD and d¹⁸O in the mid-Holocene, suggesting progressively expanded influence from Pacific airflow. However, recent paleohydrological studies in New York, New England and southern Quebec demonstrate a different and complex timing of Holocene droughts.As indicated by synoptic patterns of instrumental climate records, influences of airflow from the Gulf of Mexico and Atlantic Coast have a dominant geographic limit. This limit might have shifted during the Holocene, as a result of varying boundary conditions (e.g., insolation, land covers, sea-surface conditions).Here we expand our work further east into northeastern Pennsylvania and northwestern New Jersey to constrain timing of Holocene dry period using multiple proxy records.Preliminary results from our ongoing effort at Lake Lacawac in the Pocono Mountains of northeastern PA (lat. 41º22’57’’ N, long. 75º17’35’’ W; altitude of 439 m asl; maximum water depth of 13 m, lake area of 0.21 km²) show low lake levels at ~6-3 ¹⁴C ka. This interpretation was supported by bracketed ¹⁴C dates of shallow-water deposits (sandy layers in southeast side of the lake, and detritus layers in north side) within deep-water fine organic-rich sediments from cores along a transect, and by ground penetrating radar profiles showing truncated signals at these horizons. These results appear to support the idea of further eastward extension of dry Pacific air during the mid-Holocene, which is in contrast with present-day climate patterns.Sedimentary isotope analysis would help in constraining moisture source regions and possible cause of this emerging climate pattern.