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 Calif ornia. The chronology represents cellulose from five-year ring groups pooled from multiple radii of multiple trees. The most dramatic isotop ic event in the chronology appears from A.D. 1080-1129, when deltaC-13 values are depressed to levels approximately 2sigma below the mean fo r the period A.D. 925-1654. This isotopic excursion appears to represe nt a real event and is not an artifact of sampling circumstances; in f act, a similar excursion occurs in a previously-reported, independent delta-C-13 chronology from bristlecone pine. By carbon isotope fractio nation models, the shift to low deltaC-13 values is consistent with ab undant soil moisture, permitting leaf stomata to remain open, and allo wing ready access of CO2 from which carbon fixation may discriminate m ore effectively against C-13 in favor of C-12. According to this model , the C-13-depleted 50-yr isotopic excursion represents the wettest pe riod in the White Mountains in the past 1000 yr, during which isotope- reconstructed July Palmer Drought Severity Indices averaged approximat ely +2.2.