In addition to our work interpreting information about environmental changes, we work to develop our ability to extract geochemical and sclerochronological proxies archived in marine organisms.
In coralline algae, we’re using a proxy system model (PSM) approach to interprete changes in the oxygen isotopic composition of the skeleton (Ng et al., 2016). We’re collaborating with Diane Thompson to build a coherent and consistent approach to understanding how geochemical proxies for seawater temperature, water isotopic composition, and biological variability are incorporated into the coralline algae skeleton. We first identified that there is substantial variability in geochemical proxies that are not explained by environmental variability in our process of developing a robust Mg/Ca-temperature proxy (Williams et al., 2014). Here, we found that replication of Mg/Ca values within and among specimens was critical to amplify the environmental signature encoded in the algal chemistry. We applied this technique to reconstruct seawater temperature in the Aleutian archipelago through the past 342 years (Williams et al., 2017).
In gorgonian corals, we quantified reproducibility of carbon and nitrogen stable isotopic composition in a suite of gorgonian corals. We found that isotopic composition varies around the circumference of a single coral colony in contemporaneously formed skeleton, exceeding that of instrumental uncertainty (Williams et al., 2018). This suggests that either food source or nutrient processing differs around the coral. As a result, we recommend homogenizing a larger section of contemporaneous skeleton prior to sub-sampling for analysis to capture a more representative value for the coral at one point in time. We also found no systematic alteration of skeletal δ15N values in pre-treated skeleton, indicating that both δ13C and δ15N can be measured in pre-treated coral skeleton.