Germanium incorporation into sponge spicules: Development of a proxy for reconstructing inorganic germanium and silicon concentrations in seawater

Measurements of germanium (Ge) in deep-sea sponge skeletons are presented for sponges collected by dredge and for spicules isolated from a range of deep-sea sediment cores. Germanium to silicon (Si) ratios (Ge / Sisp) for sponge silica ranged between 0.075 and 0.380 μmol/mol, which are much lower than the present-day seawater Ge / Si ratio of 0.7 μmol/mol. A plot of Ge / Sisp versus estimated seawater Ge (and Si) concentration produced a linear relationship with the Ge content of spicules increasing with seawater Ge concentration. Plots of Ge / Sisp versus depth for both dredged sponges and sediment-bound spicules produced oceanic profiles similar to those of dissolved Ge (and Si) concentration. To explain the fractionation seen in the sponge Ge / Sisp data two models are presented. The first model used to interpret the data assumes that sponges only respond to the Ge content of the ambient seawater, implying that, Ge incorporation into sponge silica is independent of the seawater Ge / Si ratio up to a Si concentration of about 100 μmol/L. This model is consistent with the 68Ge uptake results of Davie et al. [E.I. Davie, T.L. Simpson, R. Garonne, Experimental germanium incorporation into siliceous sponge spicules, Biol. Cell 48 (1983) 191–202] for cultured spicules. Their results showed that the incorporation of Ge in sponge silica is only dependent on the Ge concentration of the water in which a sponge is growing. The second model used to explain the data assumes that Ge / Sisp fractionation results from subtle differences in the uptake kinetics of Ge and Si. While the assumptions used by each model to describe the data are different, it is possible to use sponge Ge / Sisp data to reconstruct palaeo-Ge concentrations using model I, and to reconstruct palaeo-Si concentrations using both models. Palaeo-Si concentrations estimated using both models are in good agreement.