Signatures of the Toba super-volcano eruption in Borneo stalagmite geochemistry: a multi-proxy approach

Abstract:

Instrumental climate data, paleoclimate data, and climate models show that large volcanic eruptions cause significant global cooling by injecting reflective aerosols into the upper atmosphere, increasing planetary albedo (see Robock et al., 2000 and references therein). As such, these eruptions provide an empirical constraint on the relationship between radiative forcing and climatic response – a key uncertainty in numerical simulations of future climate change. However, the overall magnitude of a large eruption’s climatic effects, and their regional expression, remain highly uncertain, and are the subject of heated debate in the peer-reviewed literature (Robock et al., 2005; Timmreck et al., 2009; Mann et al., 2012a, 2012b, 2013; Anchukaitis et al., 2012). As the largest eruption of the last 2 million years, the Toba eruption ~74 thousand years before present (kybp) on Sumatra presents an opportunity to probe the climatic responses associated with a massive perturbation to the Earth’s radiative balance. Here we present trace metal as well as isotopic data from U/Th-dated stalagmites from Gunung Mulu National Park, in northern Borneo, across the Toba depth horizon. Previously published timeseries of stalagmite oxygen isotopes (Carolin et al., 2013) document a significant positive anomaly contemporaneous with the Toba super-eruption (interpreted as dry conditions at the site), but the degree to which Toba contributed to this anomaly has remained uncertain. We present new synchrotron-based micro x-ray fluorescence data showing the presence of distinct horizons in the Toba time interval with elevated concentrations of Fe, Mn, and Co, possibly indicating the presence of Toba ash in the samples. Working with an array of trace elements, we compare the stalagmite geochemistry in these horizons to the geochemistry of 1) distal ash samples from the Younger Toba Tuff (Smith et al., 2011), 2) clays isolated from the Gunung Mulu caves, and 3) depth horizons marked by brown layers and/or known hiatuses. We illustrate the promise of synchrotron-based fluorescence for identifying trace volcanic ash in stalagmites, with potentially broad applications in paleoclimate.