Re-evaluation of using rhodolith growth patterns for paleoenvironmental reconstruction: An example from the Gulf of Panama

Abstract

Rhodoliths are a proxy data source for understanding environment and climate variation in tropical to arctic oceans. A clear understanding of growth pattern and rate is required to interpret time-series proxy data. Variability in growth on multiple-axes of a single rhodolith has not been extensively studied. This investigation compares algal band counting and dating methods found in the literature, and has implications for detecting biases in branch selection for growth and geochemical analyses. Panamanian rhodoliths (Lithothamnion sp.) were collected at the Archipelago de Las Perlas, Gulf of Panama (GOP). Alizarin Red S (ARS) stain was used to study growth rates and banding patterns for 10 branched samples randomly selected for microcosm experiments over a 172-day period. DNA analysis was used to help determine the dominant species present in the sample group and reduce the possibility of inter-sample variation. Samples were overturned every 2weeks for a multi-axial growth test and were grown under ambient seawater conditions except for the addition of LED lighting. Growth post-ARS staining was measured along the long (A), intermediate (B), and short (C) axes. Bisected branches show banding correlated with growth rate while banding was highly variable within an individual and between individuals. Average growth band widths in both the pre- and post-ARS stained growth were approximately the same, suggesting that non-environmental factors, such as overturning or biological influences, can cause the formation of growth increments. Differences in growth pattern and rate suggest that careful inspection of growth formation is needed before pursuing rhodolith-based time series proxy research.