A model for the formation of layered soda-straw stalactites

Abstract

Climate records based upon instrumental data such as rainfall measurements are usually only available for approximately the last 150 years at most. To fully investigate decadal-scale climate variation, however, these records must be extended by the use of climate proxies. Soda-straw stalactites (straws) are a previously under-utilised potential source of such data. In this contribution we investigate the structure and formation of straws and look at some issues that may affect the reliability of straw-based palaeoclimate records. We use laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analysis to document surface contamination features that have the potential to obscure annual trace element variations, and develop a method to reveal the underlying layering. We also use LA- ICP-MS to map the two-dimensional trace element distribution in straws. These maps reveal straw-layer geometry, in which layers are widest at the outside edge of the straw, narrowing and becoming almost parallel on the interior of the straw.Based upon these observations, we present a model for the formation of straws of this type, where rapid degassing of CO2 from the drip extending below the straw forms the wider outer layers. Summers are defined by increased layer widths and higher trace element contents relative to winter layers. In palaeoclimate studies, where such annual variations can be used to construct time-lines, we suggest that, ideally, the outside surface of the straw be analysed where the trace element content difference is greatest and layering is widest.The terminal phase of one straw (FC-02) shows decreasing layer widths and increased trace element contents. These features may also be representative of soda-straw responses to drought-induced decreases in percolation water.

Keywords

• soda-straw stalactites
• palaeoclimate
• contamination