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.