Evidence for increased cool season moisture during the middle Holocene

Isotopic analyses of three stalagmites from Cold Water Cave, northeast Iowa, United States, reveal nearly identical delta (super 13) C trends from ca. 7 to 2 ka. However, delta (super 18) O patterns differ by as much as 3 per mil from 5.7 to 3.2 ka. These disparate delta (super 18) O values reflect evaporative (super 18) O enrichment in meteoric water prior to infiltration, suggesting that previously calculated temperatures based on a single Cold Water Cave stalagmite overestimated middle Holocene warming. The coincidence of elevated middle Holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle Holocene was marked by a predominance of cool-weather precipitation during a period of increased overall aridity.