Interpreting environmental signals from the coralline sponge Astrosclera willeyana

Coralline sponges (sclerosponges) have been proposed as a new source for paleo subsurface temperature reconstructions by utilizing methods developed for reef-building corals. However unlike corals, they do not have density variations, making age determination difficult. In this study, we examined multiple elemental ratios (B / Ca, Mg / Ca, Sr / Ca, Ba / Ca, U / Ca) in the coralline sponge Astrosclera willeyana. We also measured skeletal density profiles along the outer “living” edge of the sponges and these data indicate significant thickening of skeletal material over intervals of 2–3 mm which corresponds to 2–3 years. This suggests that any skeletal recovered environmental record from A. willeyana is an integration of signals over a ∼2–3 year period. Sponge Sr / Ca seemed to hold the most promise as a recorder of water temperature and we compared Sr / Ca from 2 sponges in the Great Barrier Reef and one from Truk in Micronesia to their respective sea surface temperature record. The correlations were not strong (∼r = − 0.5) but were significant. It appears that the signal smoothing due to thickening or perhaps even some other control (biologic, biomineralization) on Sr skeletal partitioning limits the use of Sr / Ca as an indicator of water temperature in A. willeyana. Variations of B / Ca, Mg / Ca, Ba / Ca and U / Ca only have moderate to poor correlations to Sr / Ca and are not consistent between samples. They also do not show correlations with water temperature and may be more heavily influenced by the biomineralization occurring in the secondary thickening.