4 Discussion
Our parallel observations of the seasonal changes in the export fluxes
of different biological carrier phases, as defined hereafter, and of
trace metals, provide the opportunity to identify the the main factors
that control their export in this iron fertilized region of the Southern
Ocean.
The bulk composition of particles is usually partitioned between
different pools identified as particulate organic matter (POM), biogenic
silica (BSi), calcium carbonate (CaCO3), lithogenic and
authigenic material (Lam et al. 2015). The partitioning of TEs between
these different pools relies on two main hypotheses. First, one assumes
that it is possible to identify a chemical element or a chemical form of
the element that largely dominates one of the pools and has a minor
contribution to the others. For the lithogenic fraction, Al has been
extensively used, although Ti has recently gained interest in this
context when the potential source material and its chemical composition
are clearly identified. For the POM fraction, beside POC that is widely
used, phosphorus (P) is also selected as the reference element, because
it is a major contributor and has a mineral form (e.g. apathite) with
low abundance in seawater. In addition, P is measured simultaneously
with metals by analytical methods like Sector Field Induced Coupled
Plasma Mass Spectrometry (SF-ICP-MS) or X-ray Fluorescence (XRF)
synchrotron (Twining et al. 2003).
The second assumption is that for any given element, the ratio with the
reference element of a given fraction must be known or postulated. For
the lithogenic pool, the elemental composition of a representative
material can be used in order to determine enrichment factors. These
enrichment factors provide information on the extent to which TE are
associated with particles of lithogenic origin. In most studies, global
crustal composition or upper crustal compositions (Taylor and McLennan
1995) are used, but the composition of local mineral sources like desert
dust are also valuable (Kremling and Streu 1993). For the biogenic
fractions, CaCO3 and BSi determinations are
straightforward, but there are few experimental data to constrain the
ratio of a given TE to CaCO3 (TE/ CaCO3)
and BSi (TE/BSi) and therefore to derive directly the amount of metal
transported by these fractions. The issue is even more complicated for
POM due to the diverse composition of this fraction. When POM is
dominated by phytoplankton, an extension of the Redfield ratio to metals
can be considered, but there are large uncertainties in the
determination of phytoplankton TE/P ratios (Twining and Baines 2013).
Moreover, elemental ratios of dead microorganisms can largely differ
from those measured in living cells due to the dissolution and
remineralization rates that vary between elements. Consequently, TE/P
ratios in two important vectors of TE export, phytoplankton aggregates
(Twining et al. 2015) or fecal pellets (Fowler 1977) cannot easily be
inferred. For example, different types of particulate organic matter
(Lam et al. 2015) could influence surface adsorption of TEs (Balistrieri
et al. 1981) and ultimately the TE stoichiometry. Together, these
considerations result in a complex dynamic of TEs hosted in dissolved
and particulate pools. This is further complicated by the fact that the
magnitude of external sources and individual processes are subjected to
strong variations throughout the year (Sternberg et al. 2007; Hayes et
al. 2015).
In the following, we will discuss our findings from several points of
view. First, we will use an approach classically found in the literature
and summarised above that provides an estimate of the lithogenic
contribution to the TE flux. This approach allows to derive the flux not
supported by lithogenic carriers which can approximate the biological
contribution. Secondly, we will consider simultaneously several possible
carrier phases to extract the ones most probably associated with the
individual elements. This second approach will be used to investigate
further the role of different biological carriers. We will confront
these results with recent findings on the biological role of TE in both
autotrophic and heterotrophic microorganisms, as revealed by laboratory
or in situ omics-based studies.