4.1 Basalt is the main lithogenic carrier phase.
Using the results of the PCA for TE (Figure 6), we show that Y, Mn, Cr, Ti, Co, Th, Fe, Al and Zr have a similar seasonal export pattern. Some of these elements are well known as representatives of lithogenic matter (i.e. Ti, Cr, Zr, Y, Th and Al), while others like Mn, Co and Fe are also involved in biological processes. The quantitative estimate of the lithogenic fraction of the fluxes relies on both the choice of a reference element and a reference material of a known elemental composition. Al and Ti have been both used previously as reference elements. In the present study we will not use Al because it is known to be associated with diatom frustules (Ren et al. 2013) and a previous study above the Kerguelen plateau has shown that diatoms dominate during spring and summer (Blain et al. 2020). Therefore, Al is likely present in diatoms exported directly via aggregates or indirectly via fecal pellets. Using Al as the reference would therefore lead to an overestimation of the lithogenic fraction, while Ti can provide a more conservative estimate. We thus consider Ti as a reference element for the lithogenic fraction (Ohnemus and Lam 2014) and calculated the mass ratios FTE/FTi where FTEis the export flux of a given TE and FTi is the export flux of Ti collected in the same cup (Table 2).
The choice of the elemental ratio is also critical for the calculation of the lithogenic contribution to TE export fluxes. In the present study, for most of the elements associated mainly with a lithogenic carrier phase, Al, Fe, Cr, Co, Y, Zr, (Figure 6) the TE/Ti ratios (Table 2) are not significantly different (p=0.01) from the composition of basalt rocks collected around the Kerguelen plateau and islands (Kerguelen archipelago, Heard and Mac Donald Islands) (Weis et al. 1993; Yang et al. 1998). However, with the exception of Cr and Y, the TE/Ti ratios measured in the sediment trap differed largely from that typical of upper continental crust (UCC) (Taylor and McLennan 1995) (Table 2). Yet, the large differences for Fe/Ti and Al/Ti ratios resulted very likely from the high Ti content of island basalt (Prytulak and Elliott 2007). We also note that the Mn/Ti ratios are not significantly different from Kerguelen basalt, if a few basalt samples with low Ti (ratio<2) are excluded from this analysis. Therefore, derived basalt particles are likely the main contributors to the lithogenic export fluxes, although alteration of rocks and subsequent transformation during transport in terrestrial and marine environments could modify the chemical composition of lithogenic particles.
We calculated for individual elements the average TE/Ti based on: i) all cups and ii) only the first two cups and compared with the TE/Ti ratios in the UCC and in the Kerguelen basalt (Table 2). We then estimated the lithogenic contribution to TE export fluxes using equation 3:
FTElith= (TE/Ti) x FTi (Eq. 3),
where TE/Ti is the average ratio for the two first cups. In Figure 4, the projection of cups #1 and #2 presented the most negative score along PC1 suggesting that TE export fluxes collected in this trap were mainly driven by non-biological carriers. Moreover, the PCA of TE export fluxes (Figure 6) shows that the projections of cups#1 and #2 were located in the quarter of space that was related to a suite of TEs typically associated with basalt. This analysis of both PCAs clearly identify these cups as mainly associated with lithogenic material and suggests they are therefore the most appropriate to estimate a lithogenic elemental ratio for the sediment trap material. We note that including cups #3, #4 and #5 in the calculation of the individual elemental ratio would have resulted in a biased estimate due to the contribution of biological fluxes. We also calculated the residual export flux, which is not associated with lithogenic material for each element using equation 4:
Fxs = FTE-FTElith (Eq. 4)
and these residual fluxes are represented on Figure 7.
Our observations clearly underscore that the residual export fluxes of 6 elements (Zr, Co, Cr, Th, Fe, Al) estimated using this ratio are occasionally or consistently negative throughout the season (Figure 7). Regarding Fe, the order of magnitude of an expected biogenic flux based on the export flux of P (Pxs) can be estimated. Considering the highest values of Pxs of 50 µmol m-2d-1 in cup#5, and using a high estimate for the Fe quota (Fe:P=5 mmol mol-1 (Twining and Baines 2013)), one would expect 0.25 µmol m-2 d-1of biogenic Fe at the time of the peak flux. This represents around 0.5 % of the total flux of Fe measured (Figure 5), confirming that such low contributions cannot be detected using the calculation of the residual fluxes (Fexs Eq. 4). This result, together with the negative values of residual export fluxes, highlight that any contribution of a carrier phase (e.g. biological) other than basalt derived particles cannot be detected using this approach.