INTRA- AND INTER-OBSERVER RELIABILITY OF CORALLINE RED ALGA CLATHROMORPHUM COMPACTUM AS A SEA ICE COVER PROXY

The relationship between growth and elemental composition of the coralline red alga Clathromorphum compactum and regional sea ice cover is well established. When light access to the seafloor is blocked by sea ice, these photosynthetic algae precipitate Mg-rich calcite at a slower rate, thereby forming annual structural banding. Thus, the combined use of Mg/Ca ratios and algal growth increment widths has been used to reconstruct annual past sea ice cover going back multiple centuries. However, the accuracy of age models for algal sea ice reconstructions are dependent on observers’ interpretations of annual Mg/Ca cycles and associated growth increments. Furthermore, the current procedure includes calibrating and adjusting age models of individual time series against satellite-derived sea ice data available since 1979. However, the quality of reconstructions that extend further back than the satellite era may consequently contain a larger age error. Therefore, investigating the reliability and reproducibility of algal reconstructions is required to improve the accuracy of C. compactum as an annual sea ice cover proxy.

In this study, we examine the intra- and inter-observer variability of sea ice cover reconstructions provided by four C. compactum sclerochronologists. We present error margins at each stage of the interpretation process: 1) identification of Mg/Ca cycles post-standard-calibration, 2) identification of graphed Mg/Ca cycles superimposed on high-resolution images of growth increments, and 3) identification of Mg/Ca cycles after comparison with satellite records of sea ice concentration. We also examine the statistical significance of errors according to the length of records with ANOVA statistics and confidence intervals. We provide protocol refinement recommendations and additional steps to improve age models derived from the C. compactum proxy. Preliminary results suggest that inter-observer errors are produced using the current protocol and this significantly affects the accuracy of the age models by introducing artificial leads or lags. Therefore, age model uncertainties of existing algal records should be kept in mind when using this high-resolution past sea ice proxy data in other disciplines such as history, human geography, and archaeology.