Long-chain plant wax n-alkane hydrogen isotopic evidence for increased Little Ice Age aridity in the midcontinental United States

Abstract

We compare hydrogen isotopic measurements of long-chain leaf-wax n-alkanes (δ²H_w; C₂₇, C₂₉, and C₃₁) from Martin Lake, Indiana, USA, with a calcite-based reconstruction of the oxygen isotopic composition of precipitation (δ¹⁸O_p) from the same lake. We observe stable and high δ²H_w during the Common Era (last 2000 years), which we interpret as growing-season precipitation originating mainly from the Gulf of Mexico and Atlantic. During the Little Ice Age (LIA; 1200–1850 CE), δ²H_w values increased by 3–8 ‰, concomitant with a significant decrease in δ¹⁸O_p values by up to 12.5 ‰. Multiple proxy records for this time indicate persistent growing-season drought. We interpret these relatively high δ²H_w values, as compared to the δ¹⁸O_p values, as a signal of low relative humidity that resulted in an ²H enrichment in plant source water resulting in high δ²H values through enhanced plant water and/or soil evaporation. These results support the occurrence of low humidity conditions during the LIA in the midcontinental USA that also contributed to the marked decline of regional pre-Columbian Mississippian populations.