Colloquium: Ian Orland

Ian Orland

NSF Postdoctoral Fellow, Department of Earth Sciences, University of Minnesota.

Abstract: Scientific understanding of past and future climate change is derived from a tapestry of instrumental, proxy and modeled climate records. Individually, each of these records tells a story about climate from a different spatial and temporal vantage point. Given trends in modern climate, there is a particular interest in studying the dynamics of abrupt climate change; high-temporal-resolution paleoclimate records from habitable (low-latitude, continental) settings are especially informative.

Speleothems are carbonate cave deposits that can preserve a geochemical record of local climate as they grow. Oxygen isotope ratios (d¹⁸O) and trace element concentrations in speleothem calcite can provide important paleoenvironmental proxies, including rainfall amount. Most speleothem paleoclimate studies, however, have been performed at a sampling resolution that corresponds to several decades or centuries of growth. As a result, sub-annual climate change is averaged and information is lost. Recent developments in micro-analytical techniques have opened exciting new lines of speleothem research; both ion microprobe analysis of d¹⁸O and laser-ablation ICP-MS analysis of trace elements are now possible at seasonal resolution.

These new analytical techniques were first combined in studies of speleothems that grew in Soreq Cave (Israel) between 34 ka and today. Seasonal-resolution geochemical analyses and imaging by confocal laser fluorescent microscope reveal distinct differences in both the pattern of fluorescent banding and the gradients of d¹⁸O and trace elements measured across single, annual growth-bands. Before 15 ka, results suggest a reduced gradient of seasonal precipitation in the Eastern Mediterranean region relative to the annual cycle of wet and dry seasons that is apparent throughout the Holocene. Detailed analysis of the Younger Dryas termination (an abrupt warming event at 11.6 ka) reveals a rapid onset to regional changes in atmospheric circulation. Fluorescent bands indicate that – as recorded by local rainfall – the termination spanned a minimum of 12 years.

Given the wide geographic distribution of caves and the rarity of seasonal-resolution paleoclimate records, the methods described above represent an important new approach for assessing past changes in seasonal climate around the globe.