Magnesium and strontium systematics in tropical speleothems from the Western Pacific

Abstract

We present bulk average Sr and Mg data for 13 speleothems from different locations in the western equatorial Pacific (Guam, Solomon Islands, and Vanuatu). These data plot on a single straight line in a graph of ln(Sr/Ca) vs ln(Mg/Ca) with a slope of ~0.9. A 22,000year record of Sr and Mg in one of these samples from Guam also plots with the same slope, suggesting that the process partitioning Mg and Sr within the Guam speleothem is the same as the one that partitions Mg and Sr between the different Pacific speleothems. We rule out temperature, growth rate, detrital phases, and sea-spray as likely mechanisms for this correlation. We construct mathematical models of limestone diagenesis and show that this cannot explain the slope of the correlation.Our favoured explanation for the correlation is calcite/water interaction in the form of prior calcite precipitation (PCP) and/or incongruent calcite dissolution (ICD). We present a formal mathematical model of PCP and prove that the slope of a graph of ln(Sr/Ca) and ln(Mg/Ca) is given by KdSr−1KdMg−1. A similar equation is derived for ICD in a companion paper (Sinclair, 2011). Using published values for KdSr and KdMg, this slope is calculated to be ~0.88, in excellent agreement with our observations. Because the slope is independent of solution and host-limestone composition it can be used to diagnose calcite–water interaction in individual cave water studies where host rock composition is unknown, or in speleothems for which no field drip water data or host-rock geochemistry exists. Approximately half of published drip and speleothem Mg+Sr studies plot within error of this slope.We hypothesize that the overall trend in our Pacific speleothem data results from the individual dripwater systems evolving from a roughly similar initial rock composition set by late Quaternary reef limestone. In the Guam speleothem, the broad peak in Mg and Sr centred on the early–mid Holocene reflects a change in hydrology (rainfall), with the most likely scenario being that dry conditions prevailed on Guam at this time.