A Simple Proxy System Model of High-Latitude Encrusting Algal Oxygen Isotope Composition (δ18O)

The calcitic skeletons of marine crustose coralline algae yield important reconstructions of oceanic change spanning the past several centuries. The alga Clathromorphum spp. is abundant in coastal regions throughout the mid-to-high latitude northern hemisphere, forms an annually layered structure, and may grow for 850 years or more. In this alga, the stable oxygen isotopic composition (δ18O) of the skeleton reflects seawater temperature and/or δ18O composition of the seawater during the time of calcification. Here, we build and assess a simple, empirical proxy system model (PSM) of algal δ18O to better evaluate incorporation of the environmental parameters into the skeleton of the algae for future paleoclimate reconstructions. To develop the coralline algal PSM, we first determine the response season (in which the alga incorporates the ambient environmental signals) for four specimens spanning the algal growth range in the northern hemisphere. We find that the alga significantly records ambient seawater temperatures during summer months, but shows no relationship with salinity. This yields a simple, univariate proxy system model of pseodoalgal-δ18O = αSST + error, where α is the slope of the algal-δ18O to SST relationship determined by regressing temperature during the response season with algal-δ18O across the habitat range. We find the coefficients vary within a narrow range from -0.135 to -0.17, depending on the algal location and gridded dataset used, supporting a coherent relationship between seawater temperature and algal-δ18O. Using the average coefficient and associated uncertainty range, we create pseudoalgae records for four locations within the Clathromorphum spp. northern hemisphere range to compare with the observed algal records and assess variability and secular trends over the past century.