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 δ¹³C trends from ca. 7 to 2 ka. However, δ¹⁸O patterns differ by as much as 3‰ from 5.7 to 3.2 ka. These disparate δ¹⁸O values reflect evaporative ¹⁸O 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.