Silica banding in the deep-sea lithistid sponge Corallistes undulatus: Investigating the potential influence of diet and environment on growth

We present detailed records of trace metals and carbon isotopes to understand siliceous spicule formation in the deep-sea lithistid sponge Corallistes undulatus Lévi and Lévi, 1983 (Demospongiae: Corallistidae). X-ray analysis of two longitudinal sections removed from the lamellae of the cup-shaped sponge revealed 144 and 137 light and dark density band-pairs, respectively, within the siliceous skeleton. Four portions of silica were removed along one of the sections for silicon-32 (³²Si) dating in order to constrain the overall extension rate of the sponge. Although there was some variability in the ³²Si data, the overall age established using these data indicated that the sponge was between 135 and 160 yr old. This agreed well with the counts of density band-pairs, indicating that these band-pairs represent an annual deposition of layers of silica in the desma skeleton. Links between silica deposition and growth (food supply) were established using the Δ¹⁴carbon (C) and δ¹³C signatures of organic material trapped with the spicule matrix and the zinc content of the silica. A carbon budget based on these results indicated that the amount of fresh, labile surface export organic carbon reaching C. undulatus was not sufficient to support its growth. The Δ¹⁴C results for organic carbon trapped in the silica desmas, deposited after the 1960s, supports this assertion; only a small atmospheric nuclear weapons 'bomb' spike was observed in the Δ¹⁴C data. Taken together, the Δ¹⁴C, δ¹³C, and trace-metal results all indicate that the organic carbon source to C. undulatus is likely to be a mixture of fresh, labile, surface-derived material and older, perhaps sediment-derived material, with the latter being dominant.