Trace metal and carbon isotopic variations in cave dripwater and stalagmite geochemistry from northern Borneo

Abstract

We investigate stalagmite trace metal ratios and carbon isotopic composition (δ13C) as potential paleoclimate proxies by comparing cave dripwaters, stalagmites, and bedrock composition from Gunung Mulu and Gunung Buda National Parks in northern Borneo, a tropical rainforest karst site. Three year long, biweekly time series of dripwater Mg/Ca, Sr/Ca, and δ13C from several drips at our site are not correlated with rainfall variability, indicative of a relatively weak relationship between hydroclimate and dripwater geochemistry at our site. However, combining all of the dripwater geochemical data gathered over four field trips to our site (N > 300 samples), we find that drips with highly variable Mg[Sr]/Ca have relatively invariable δ18O values close to the mean. We hypothesize that increased residence times translate into reduced variance in dripwater δ18O through mixing in the epikarst as well as increased Mg[Sr]/Ca values through increased calcite precipitation in the epikarst. Mg/Ca, Sr/Ca, and δ13C time series from three overlapping stalagmites that grew over the last 27 kyrs are characterized by strong centennial‐scale variations, and bear little resemblance to previously published, well‐reproduced δ18O time series from the same stalagmites. The only shared signal among the three stalagmites' geochemical time series is a relative decrease of 1‰ in δ13C from the Last Glacial Maximum to the Holocene, consistent with a transition from savannah (C4) to rainforest (C3) conditions documented in nearby records. Taken together, our study indicates that stalagmite Mg[Sr]/Ca ratios are poor indicators of hydroclimate conditions at our site, while stalagmite δ13C exhibits some reproducible signals on glacial‐interglacial timescales.