Chapter 1
47
quantities of size fractionated particles (see GEOTRACES cookbook
1
and McDonnell et al.,
2015). They can operate at any depth and, at a same station many ISP can be deployed along
a cable off the side of the ship. The pumping time is typically 2-4 hours and 1000-2000 L of
seawater can be filtered depending on particle concentration. The two commercial systems
used in the present work are Challenger and McLane and the latter can be configured with two
pump heads equipped with different filters that can be subsampled for multiple measurements.
It is important to note that suspended particles sampled with ISP are not necessarily the ones
that sink and can lead to substantial biases in estimates of export flux. Moreover, the size is
not always related to density and ISP sampling can include living and non-sinking
phytoplankton or micro-zooplankton (Lalande et al., 2008; Puigcorbé et al., 2015). Other
methodological issues are the high pressure differential created within ISP that may cause a
rupture of aggregates (Gardner et al., 2003) and the adsorption of dissolved matter onto the
membranes filters (Zhou et al., 2016) which both can bias the size structure of particles and
their elemental concentrations.
After ISP collection, particle concentrations can be measured and combined with the
210
Po or
234
Th export flux in order to deduce a particulate elemental export flux. This latter method is
used and described in this thesis (see section 2.2.1).
- Sediment traps
Sediment trap collect directly sinking particles during their transit to depth, into successive cups
placed into a carousel hermetically sealed from ambient water. The cup position switches in
order to collect a time series or to separate the sampling between the elements of interest
(POC versus trace elements for example). Often, poison (formalin or mercury) is added to the
cups in order to stop the bacterial activity or to kill zooplankton swimmers that enter in the cups
(Lee and Fisher, 1992; Lee et al., 1992). However, this poison can change qualitatively the
trapped organic matter (Liu et al., 2006) or contaminate for trace element determinations
1
http://www.geotraces.org/science/intercalibration/222-sampling-and-sample-handling-protocols-for-geotraces-
cruises
Chapter 1
48
(McDonnell et al., 2015). Different types of traps can be deployed: moored traps, usually
deployed in the deep sea and used for time series (Antia et al., 2001; Honjo et al., 2008;
Rembauville et al., 2015a, 2015b); drifting traps usually deployed in the upper water column
(Laurenceau-Cornec et al., 2015a); free-drifting traps that are able to sample at a density
horizon (Buesseler et al., 2007; Lampitt et al., 2008); polyacrylamide gel-filled sediment traps
to examine sinking flux characteristics (particle type, number, size; Ebersbach and Trull, 2008;
Laurenceau-Cornec et al., 2015a). Sediment traps allow the direct quantification of export
fluxes but they tend to undercollect slower sinking particles due to physical advection, in
particular the moored sediment traps. Also, the sampling of swimmers, even when killed by
poison, can alter the particle composition and the related flux (McDonnel et al., 2015;
Buesseler et al., 2007).
To sum up, the collection of the sinking material by sediment traps or ISP is not without
difficulties knowing the contamination sources of each element of interest (specifically in this
study: POC, PN, BSi, and particulate trace metals), the physical processes as the lateral
advection perturbing the particle trajectories or the inclination of the particle collector, the
solubilization or the loss of material during collection or recovery, and the biological interactions
such as zooplankton grazing, migration and excretion which can remove or add to the
particulate flux instantly and locally.
1.3.2. Determination of remineralization fluxes
The attenuation of the particulate organic matter concentration with depth can be observed
using sediment traps, in-situ pumps or the
234
Th deficit method between the surface and the
mesopelagic layers (Marsay et al., 2015; Usbeck, 2002) but other tools can estimate the
oxygen consumption and thus quantify the remineralization fluxes:
- Bacterial respiration
The bacterial respiration can be determined from dissolved oxygen consumption in dark
seawater incubations. The dissolved oxygen consumption is determined by Winkler titration
Chapter 1
49
(Christaki et al., 2014; Lefèvre et al., 2008), then the oxygen consumption rate can be
estimated by integrating the bacterial respiration in a set depth layer. However, the
determination of the bacterial respiration is mostly limited to the upper 200 m of depth because
of sensitivity issues (detection limit varying from 0.06 to 0.5 µmol O
2
.L
-1
.d
-1
; Arístegui et al.,
2005; Robert, 2012).
- Oxygen Utilization Rate (OUR)
Surface oxygen concentrations are close to saturation with atmosphere and oxygen can be
supplied to the water column by the physical transport of oxygenated surface waters to depth.
The OUR can be estimated by dividing the apparent oxygen utilization (AOU) by the age of the
water mass, i.e. the elapsed time since the water mass was in contact with atmosphere
(Jenkins, 1982). The AOU provides a measure of the oxygen undersaturation, due to the
remineralization, and is calculated by subtracting the observed oxygen concentration to the
preformed (or saturated) oxygen concentration (Broecker and Peng, 1982). The integrated
OUR can then be converted into a carbon remineralization flux using the Redfield molar ratio
C/O
2
(127/175; Feely et al., 2004; Sonnerup et al., 2014).
- Other tracers
Remineralization processes are a source of changes in the elemental concentrations and
isotopic compositions. For example, ammonium is produced during remineralization and is, in
general, rapidly transformed into nitrate (nitrification) but an ammonium accumulation indicates
an imbalance between the two processes and thus an important remineralization (Sigman et
al., 2009). Stable isotopes provide information on the relationship between the dissolved and
particulate phases such as uptake, scavenging and remineralization. In surface waters, the
increase of the isotopic signatures of dissolved compounds (NO
3
-
, trace metals) can be related
to the biological uptake of lighter isotopes while below the surface, the remineralization
contributes to the lower isotopic signatures by releasing the lighter isotopes in solution (Hoefs,
2010).
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