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.