Extracting growth rates from the nonlaminated coralline sponge Astrosclera willeyana using bomb radiocarbon

Abstract

The isotopic and chemical composition of the skeletons of hermatypic reef building corals have been used for many years to generate proxy time series records of surface oceanic conditions (water temperature and salinity). These types of records have recently been constructed using coralline sponges. Because they are found in a greater range of depths, coralline sponges have the potential to fill in gaps in our understanding of subsurface oceanographic variability. Using coralline sponges together with surface corals, we can obtain a threedimensional (3D) picture of oceanographic/climate variability. However, coralline sponges have one disadvantage compared to hermatypic reef building coral proxies in that most do not have annual density bands and need to be radiometrically dated for age determination. We have measured radiocarbon in 1 mm increments from Astrosclera willeyana sponges collected off the Central and Northern Great Barrier Reef (GBR) and from Truk in the Caroline Islands and compared these radiocarbon profiles to independently dated coral radiocarbon records to examine growth rate variability. Growth rates of the GBR sponges averaged 1.2 ± 0.3 and 1.0 ± 0.3 mm y−1 at north and central sites respectively, but the growth rate can vary by a factor of two over the life span of the sponge. The growth rate of the Truk sponge averaged 1.2 ± 0.1 mm y−1. These growth rates are faster than those measured for other Astrosclera willeyana sponges (0.2–0.7 mm y−1) (Moore et al. 2000; Wörheide 1998).