Particle Fluxes in the North-East Atlantic and Southern Ocean

Abstract

Concerns regarding the climatic implications of the increase in atmospheric CO2 concentrations
throughout the anthropocene have provided the impetus to obtain a mechanistic understanding
of oceanic processes and their role in regulating atmospheric pCO2. One important mechanism
is the functioning of the biological pump which partitions carbon between the atmosphere and
ocean reservoirs over relevant time scales. Current uncertainties revolve around the accuracy of
upper ocean particle flux measurements, and the effect of iron and ballast minerals on the
strength and efficiency of the biological carbon pump. This study documents the design and
deployment of a neutrally buoyant sediment trap (PELAGRA). In the north-east Atlantic
organic carbon fluxes were measured using this new technology and compared to indirect
estimates of export based on 234Th and nutrient budgets. The vertical fluxes of 234Th into the
traps were less than those estimated from the 234Th water column budget, which is interpreted to
be the result of previous export events removing 234Th from the water column and the lateral
advection of gradients of total 234Th/238U disequilibria confounding the Eulerian budgeting
approach adopted. Successful simultaneous deployments in July 2006 at different depths
provided a direct measurement of the attenuation of flux with depth, which at 1.8 is
substantially greater than the canonical value of 0.856. PELAGRA deployments in the
Southern Ocean were conducted as part of the CROZEX project, which examined the role of
iron supply on bloom dynamics and subsequent export. Using a mass balance approach to
account for the seasonal depletion of dissolved silica acid in surface waters and Si fluxes from
the euphotic zone, potential surface export(100m) of organic carbon from +Fe bloom area was
estimated to be in the order of 11-15 g C m-2, which is higher than previous estimates obtained
from artificial fertilisation experiments. The issue of temporal decoupling between production
and export processes was addressed by employing retrospective estimates of production.
Particle export efficiency in the +Fe region to the north of the plateau (25-70%) was higher than
similar estimates in the –Fe region (11-20%). Diatom size was well correlated with a range of
calculated export ratios(100m). The main diatoms involved in the export from the surface were E.
Antarctica in the +Fe region and F. kerguelensis in the –Fe region. E. Antarctica fluxes also
dominated deep-water (3000m) diatom fluxes in the +Fe region, and its importance is attributed
to the regions proximity to the Crozet Islands, where resting spores and dissolved iron are
advected into the bloom area during the winter. Deep-water carbon fluxes measured to the
south of the plateau. Deep-water carbon fluxes measured south of the plateau (0.09 g C m-2 yr-1)
are consistent with previous measurements in a similar environment. In the +Fe region to the
north, deep water fluxes were 0.4 g C m-2 yr-1 indicating that natural iron fertilisation can
increase the strength of the biological carbon pump by a factor of 4. Comparison of fluxes with
satellite-derived productivity also suggests that the efficiency of the biological pump in
transferring organic carbon to the deep-ocean is increased by a factor of 3 in the presence of
iron. The flux and composition of amino acids, in relation to the dominant mineral phases that
comprised the particulate flux in the NE Atlantic and the Southern Ocean was also examined.
The fraction of carbon that could be accounted for by the total hydrolysable amino acids varied
very little (20-30%) with sample composition. Protein amino acids were used to quantify the
degradation state of the settling particulate material. Specific amino acids seem to infer
diatomaceous rather than calcareous as the dominant organic matter source. Multiple linear
regression analysis reveals that mineral fluxes can only explain a very small amount of the
variability in amino acid composition, which does not support previous hypotheses that relate
mineral fluxes and organic carbon fluxes through the differential protective capacity of various
mineral phases.

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