1 Introduction
During the past decade the international project GEOTRACES has
undertaken an unprecedented effort to improve our knowledge of the
distribution and biogeochemical cycles of trace elements and their
isotopes (TEI) in the ocean (Anderson 2020). Trace metals are required
in many metabolic functions (Sunda 2012) and as such, biogenic particles
that are generated in the upper ocean are one of the main players
regulating the internal cycling of so called “bioactive” trace
elements (TE). TE are incorporated into particles through biological
uptake and/or passive adsorption; they can then be remineralized,
desorbed and/or ultimately exported. Particle sinking is one of the
mechanisms of downward transport of chemical elements (Boyd et al.
2019). Large biogenic particles such as phytoplankton aggregates or
fecal pellets are major vectors in the downward transport of particulate
organic carbon (POC), but lithogenic particles can also play a role by
ballasting aggregates and reducing remineralisation (Lemaitre et al.
2020). Due to their role in the control of atmospheric
CO2 (Antia et al. 2001), carbon vertical export fluxes
have been extensively studied, yet TE export fluxes have been
considerably less investigated.
Vertical fluxes of particulate TE can be determined in different ways
(McDonnell et al. 2015). Among them, moored sediment traps have been
used since the 1970s to measure vertical fluxes of sinking material in
the ocean. Important characteristics of vertical fluxes were revealed by
this approach, but possible biases and limitations were also identified
leading to the delivery of best practises (Buesseler et al. 2007).
Taking these recommendations into account for deployment of the moorings
and their design, moored sediment traps are powerful tools and are quite
unique to capture long term variability (months to years) of sinking
fluxes including TEs (Kremling and Streu 1993; Huang and Conte 2009;
Kuss et al. 2010; Conte et al. 2019). In the northern Sargasso Sea, the
oceanic flux program (OFP) provides the longest time series for
elemental composition of export fluxes at three depths. Data collected
between 2000 and 2015 were used to build mean seasonal cycles for 19
elements with a monthly temporal resolution (Conte et al. 2019). Based
on the assumption that the elemental composition of the upper
continental crust approximated lithogenic material composition in the
traps, elemental fluxes were partitioned between different phases
(organic matter, carbonates, lithogenic, authigenic). Two components
have been identified as main drivers of the seasonal dynamics of the
elemental fluxes. One of them was coupled to the seasonal cycle of
primary production and surface export. The other one was related to
internal processes associated with chemical scavenging and particle
aggregation (Conte et al. 2019).
Based on a 13 year time series of elemental flux composition at 2000 m
in the North East Atlantic, the mean annual cycles (monthly resolution)
of 13 elements were reported (Pullwer and Waniek 2020). Overall,
depending on the element considered, weak or no seasonality was
detected. This was likely due to a small biological signal at the
seasonal level, which was further damped by interannual variability of
environmental conditions in surface waters. The depth of the traps also
likely contributed to mask a clear seasonal signal at this site.
In the Southern Ocean, moored sediment traps were also widely used to
investigate carbon export dynamics (Honjo et al. 2000). However, studies
of the seasonal dynamics of TE export are rare. The seasonality of
particulate export fluxes of 8 TE was studied in the polynya of Pridz
Bay (Sun et al. 2016). Seasonal variations of Cu, Zn and Cd were mainly
driven by ice coverage and biological production whereas fluxes of Al,
Fe and Mn mainly derived from continental debris were controlled by ice
melting and freezing processes.
Further investigations of the seasonal export dynamics of TE, with high
temporal resolution are therefore required. We have addressed this
challenge in a productive region of the Southern Ocean, the Kerguelen
plateau where iron fertilisation leads to a marked seasonal pattern of
carbon export (Rembauville et al. 2015a; Blain et al. 2020). In this
context, we aimed to study and understand the various processes
impacting the stoichiometry and the magnitude of these export fluxes,
including the seasonal dynamics that can facilitate the partitioning of
TE export between different carrier phases.