A COMPARISON OF TECHNIQUES FOR ANALYSIS OF MG/CA RATIOS IN THE HIGH-LATITUDE CORALLINE ALGAL PROXY CLIMATE ARCHIVE CLATHROMORPHUM

The coralline alga Clathromorphum compactum is an important annual to sub-annual resolution archive of Arctic and Subarctic environmental conditions. Annual Mg/Ca cycles measured in C. compactum’s high Mg calcite growth increments have been used as a proxy for past temperatures and sea ice coverage going back more than 600 years. Several methods of analysis of coralline algal Mg/Ca climate time series along the direction of growth have been employed in past studies including laser ablation inductively coupled mass spectrometry (LA-ICP-MS), electron microprobe, and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). Here we present a comparison of the application of these techniques for coralline algal environmental reconstructions. Comparing several adjacent LA-ICP-MS transects measured along the direction of growth in the same sample indicate high data reproducibility (R²=0.58, n=1526, p<0.001). A comparison to spot analysis by electron microprobe along transects with three different step sizes showed that a loss of amplitude of the annual Mg/Ca cycles occurred when measurement spacing was more than 10% of the annual growth rate. This means that in slow growing Arctic specimens (annual growth rates ~100 μm) a minimum of 10 data points/year of electron microprobe spot measurements are needed in order to capture annual maxima and minima. While SEM-EDS produces Mg/Ca data creating a time series comparable to the other methods, and we demonstrate it’s ability to analyse small (0.5 μm) spot sizes allowing for higher spatial resolution measurements. Data from LA-ICP-MS, electron microprobe, and SEM-EDS all produced similar Mg/Ca cycles, with average values from all three methods within two standard deviations of each other. This suggests that all of these methods can be used to produce reliable results when analyzing coralline algae for past climate reconstruction.