MAJOR WET INTERVAL IN WHITE MOUNTAINS MEDIEVAL WARM PERIOD EVIDENCED IN DELTA-C-13 OF BRISTLECONE-PINE TREE-RINGS

A long deltaC-13 chronology was developed from bristlecone pine (Pinus longaeva) at the Methuselah Walk site in the White Mountains of California. The chronology represents cellulose from five-year ring groups pooled from multiple radii of multiple trees. The most dramatic isotopic event in the chronology appears from A.D. 1080-1129, when deltaC-13 values are depressed to levels approximately 2sigma below the mean for the period A.D. 925-1654. This isotopic excursion appears to represent a real event and is not an artifact of sampling circumstances; in fact, a similar excursion occurs in a previously-reported, independent delta-C-13 chronology from bristlecone pine. By carbon isotope fractionation models, the shift to low deltaC-13 values is consistent with abundant soil moisture, permitting leaf stomata to remain open, and allowing ready access of CO2 from which carbon fixation may discriminate more effectively against C-13 in favor of C-12. According to this model, the C-13-depleted 50-yr isotopic excursion represents the wettest period in the White Mountains in the past 1000 yr, during which isotope-reconstructed July Palmer Drought Severity Indices averaged approximately +2.2.