Soil weathering and human activities dominate trace elements in a stalagmite from Shandong Peninsula in coastal North China during the last millennium

Abstract

Trace elements in speleothems are important proxies for paleoclimate reconstructions. Water-rock interaction (WRI) and prior calcite precipitation (PCP) are two mechanisms mostly used to interpret trace elements in speleothems. It is usually assumed that higher ratios of trace element to calcium (X/Ca) indicate enhanced WRI and/or PCP effects and in turn drier climates. In this study, the Mg/Ca, Sr/Ca, Ba/Ca, and ⁸⁷Sr/⁸⁶Sr ratios of stalagmite KY1 (collected from Kaiyuan Cave in Shandong Peninsula, coastal North China) were determined and the mechanisms dominating the X/Ca and ⁸⁷Sr/⁸⁶Sr ratios were investigated. The results indicate that 1) the soil layer overlying Kaiyuan Cave is an important source of trace elements in stalagmite KY1, contributing more than a half of Sr in stalagmite KY1; and 2) chemical weathering of the soil layer, instead of the two widely used mechanisms WRI and PCP, dominates the variations of the Mg/Ca, Sr/Ca, and Ba/Ca ratios in stalagmite KY1 during the last millennium. Before ∼1670 CE, chemical weathering of and trace element release from the soil layer were controlled naturally by climates. Warm-humid climates strengthened chemical weathering of the soil layer, releasing relatively more trace elements and eventually resulted in higher X/Ca ratios in stalagmite KY1. Cold-dry climates were responsible for lower X/Ca ratios. After ∼1670 CE, human activities played a key role on the X/Ca ratio variations in stalagmite KY1. Enhanced reclamation led possibly to deforestation, reduced residence time of groundwaters in the soil layer, weakened chemical weathering of the soil layer, resulting in lower X/Ca ratios in stalagmite KY1. This study underlines the importance of the soil layer to accurately interpret trace elements in speleothems.