Major Wet Interval in White Mountains Medieval Warm Period Evidenced in δ13C of Bristlecone Pine Tree Rings

A long δ13C 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 δ13C values are depressed to levels ∼ 2σ 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 δ13C chronology from bristlecone pine. By carbon isotope fractionation models, the shift to low δ13C 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 13C in favor of 12C. According to this model, the 13C-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 ∼ +2.2.