Using the growth patterns and geochemistry of coralline red algae to develop proxies for paleoclimate reconstruction in tropical and subarctic environments
The purpose of this research is to further develop the use of encrusting and free-living forms (i.e. rhodoliths) of crustose coralline red algae (CCA) as geochemical proxy sources for paleoenvironmental and paleoclimate reconstructions. Rhodoliths (Lithothamnion sp.) from the Gulf of Panama (GOP), Panama were grown in a microcosm experiment to evaluate growth patterns and rates. Previous researchers concluded GOP rhodoliths grew annual bands and formed as a result of temperature and salinity fluctuations associated with upwelling and non-upwelling seasons. Our study shows that the rhodoliths can grow highly variable amounts of growth bands (n = 0-20) in <6 months and that the more dominant controls on growth are light and axial branch orientation to the light source. These results highlight the importance of calibration studies to develop paleo-archives. The trace elemental content of the GOP rhodoliths revealed that the Mg/Ca and P/Ca ratios were recorded congruently with the number of sub-annual growth bands produced during the experiment for 50% of the sample group. Both the Mg/Ca and P/Ca show moderate linear correlations and statistically significant relationships to annual growth rates for the experimental growth, all of which show no correlation to sea surface temperature (SST) or other seawater parameters. These observations imply that temperature is not the primary control on elemental uptake, and that the Mg/Ca ratios cannot be used to identify annual growth cycles in Lithothamnion sp. from this location. This is the first documentation of P/Ca ratios recorded in measurable concentrations in CCA. Unlike rhodoliths, encrusting CCA (Clathromorphum compactum) are a more reliable proxy source due to their ability to record seawater geochemistry in situ. Here we show that the oxygen isotopic composition (δ18O) of this species is capable of recording large freshwater pulses off western Greenland, allowing for a glacial meltwater reconstruction for 1924-2013. Extreme negative δ18O values (indicating increased freshwater input) align with the 2012 melt event where 98.6% of the Greenland Ice Sheet experienced melt during July 11-12, 2012. The 89-yr algal isotope record supports a trend of increased glacial melt, longer summer growing seasons, and overall subarctic SST warming in Godthåbsfjord, Greenland.