Germanium/Silicon and Silicon Isotope Records of Marine Diatom Opal from a Low Silicon Environment

Here we present a 7 to 70 ka record of coupled Ge/Si and δ30Si of diatomaceous opal from a low silicon environment. The diatom samples were collected from Eltanin sediment core E33-22 located north of the Antarctic Polar Front (APF) in the Southeast Pacific Ocean (120 W and 54.6 S). The surface water silicon concentration overlying this core site is <5 µmol L-1. The percentage of biogenic opal steadily increases with time from 8.5% (7 ka) to 29% (70 ka). This trend is consistent with other percentage opal records obtained from the subantarctic zone but contrasts records for cores located south of the APF, which show a decline across this period. The Ge/Si record for E33-22 ranges from 0.31 to 0.83 (x10-6) with the ratio steadily increasing with time. Again this trend contrasts Ge/Si records for cores located south of the APF which decrease with time i.e. 0.78-0.55 (x10-6)(Mortlock et al. 1991). In comparison, the δ30Si record for E33-22 is inversely correlated with the Ge/Si record from 7 ka to the last glacial period with values decreasing from 2.4 to 1.5‰. Across the Marine Isotope Stage 2/3 boundary (c. 27 ka) the δ30Si record increases to 2.6‰. Interpretation of these records suggests that Si utilisation, and perhaps the supply of Si into the subantarctic zone, was higher during the last glacial period. An increase in silicon utilisation is consistent with the silicon leakage hypothesis which requires the supply of silicon into the subantarctic zone to be higher during glacial times.