Based on culture experiments (Sutton et al., 2013), it has recently been suggested that the diatom silicon isotope fractionation factor may be species dependent, and in particular that the genus Chaetoceros may have a significantly larger, and the species Fragilariopsis kerguelensis, a significantly smaller 30ε than the widely accepted value of ∼−1‰ (De la Rocha et al., 1997). Our previous Southern Ocean core top study included Antarctic Peninsula sediments containing Chaetoceros sp. and Pacific sector sediments containing F. kerguelensis (Egan et al., 2012). The size fractions analysed had up to 40% differences in the abundance of these diatoms (Fig. 5). However, there is no detectable δ30Si offset between size fractions in the core tops TAN127 or 385 (Fig. 5a and b), whilst in KC08A (Fig. 5c), there is a slight negative offset of the 12–20 μm fraction which is opposite to that expected if the abundance of Chaetoceros was the cause (the suggested larger fractionation would act to lower δ30Si in the smaller size fractions). Rather, the δ30Si values of size fractions between 2 and 20 μm display a good correlation (r2=0.92, Fig. 2a) with surface silicic acid concentration and, where the water source δ30Si and silicic acid concentration are known or can be reasonably estimated, converge on an apparent 30ε of ∼–1‰ (Egan et al., 2012; Fripiat et al., 2011c). This suggests that whilst inter-specific differences in fractionation factor exist in culture, in the natural environment the presence of such species does not yield a large offset in the δ30Si signature of the diatom population as a whole. Although a down core effect cannot be completely ruled out, the good agreement between the 2–10 μm and 10–20 μm size fractions in our record from Site 1090 (Fig. 6) and our previous core top study (Figs. 2 and 5) suggest that changes in the species specific fractionation factor are unlikely to be the dominant driver of Site 1090 diatom δ30Si variation.
diatom silicon isotope fractionation factor
30ε
∼−1‰
‰
diatoms
differences in the abundance
40%
%
fraction
12–20 μm
μm
size fractions
between 2 and 20 μm
μm
δ30Si values of size fractions between 2 and 20 μm
r2
0.92
δ30Si values of size fractions between 2 and 20 μm
apparent 30ε
∼–1‰
‰
size fractions
2–10 μm
μm
size fractions
10–20 μm
μm