Red Sea Rhodoliths as Environmental Archives: A Novel Method to Overcome Historical Challenges in Climate Reconstruction

Global environmental change threatens the persistence of coral reef systems. Long term in-situ environmental data is required to contextualize reef-scale thermal variability and future organismal resilience. These data can be derived from skeletal growth patterns of various marine calcifiers, most commonly corals, bivalves, and foraminifera. Free-living coralline algae, or rhodoliths, are largely underutilized biogenic archives which record environmental data in their growth increments. Many traditional attempts to utilize rhodoliths as (paleo-) environmental proxies have failed to produce viable reconstruction data due to difficulties in physical sectioning from nonlinear branching patterns and gaps in the resulting chronologies from unpredictable growth interruptions. Here we present a novel method of non-destructively deriving reef-scale annual mean sea temperatures using composite increment width profiles from microtomography scans of rhodoliths from the central Red Sea. Compiled profiles of rhodolith growth increment width were strongly positively coupled with mean annual temperature (R = 0.63), with a positive relationship between resemblance to corresponding temperature profiles and number of branches compiled. By circumventing inaccuracies in chronologies from unpredictable growth interruptions in rhodoliths, this novel method allows for the derivation of more accurate reconstructions of mean annual reef-scale sea temperatures using a previously inaccessible archive.