4
Chapter 5: Biogenic trace element export fluxes in the North Atlantic Ocean ..................... 181
Abstract .......................................................................................................................... 182
5.1.
Introduction ......................................................................................................... 183
5.2.
Methods .............................................................................................................. 185
5.3.
Results ............................................................................................................... 191
5.4.
Discussion .......................................................................................................... 203
5.5.
Conclusion .......................................................................................................... 221
Acknowledgements ........................................................................................................ 221
Chapter 6: Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency
of
particulate biogenic silica, nitrogen and iron export fluxes ............................................... 229
Abstract .......................................................................................................................... 230
6.1.
Introduction ......................................................................................................... 231
6.2.
Methods .............................................................................................................. 233
6.3.
Results ............................................................................................................... 242
6.4.
Discussion .......................................................................................................... 253
6.5.
Conclusion .......................................................................................................... 265
Acknowledgments .......................................................................................................... 266
Chapter 7: General conclusions and future directions ........................................................ 271
7.1.
Major findings ..................................................................................................... 272
7.2.
Comparison of the two study areas .................................................................... 275
7.3.
Perspectives ....................................................................................................... 279
7.4.
The fate of the BCP with global warming ........................................................... 282
References ........................................................................................................................... 287
5
Table of figures
Chapter 1:
Figure 1.1: The global carbon cycle. Arrows represent the fluxes in gigatons of carbon per year and the
boxes represent the reservoirs in gigatons of carbon. Black values are natural fluxes and red are
anthropogenic contributions (Diagram from http://www.ipcc.ch/, modified from (Sabine et al., 2004;
Sarmiento, 2002). ...................................................................................................................................24
Figure 1.2: The marine carbon cycle including the solubility and biological pumps with the pre-industrial
fluxes and average values for the 1980-1990s (in brown box). Fluxes are represented by arrows and
are in Pg C/year. Reservoir sizes are represented by the numbers in brackets
and are expressed in Pg
C. NPP is the net primary productivity (Modified from Sabine and Feely, 2007). ..................................27
Figure 1.3: Global
map of annual sea to air CO
2
exchanges (in µatm; Sarmiento and Gruber, 2006). 28
Figure 1.4: Simplified view of the biological carbon pump (S. Hervé, IUEM). .......................................29
Figure 1.5: Contribution of the different carbon pump components (the solubility pump ΔC
gas-ex
; the
carbonate counter pump ΔC
carb
; and the biological carbon pump ΔC
soft
) to DIC increase in the deep
ocean (Gruber and Sarmiento, 2006). ....................................................................................................31
Figure 1.6: Schematic view of the decrease of the downward POC flux with depth (Lampitt et al., 2008)..
................................................................................................................................................................34
Figure 1.7: Patterns of nutrient limitation. Backgrounds indicate annual average surface concentrations
of nitrate (a) and phosphate (b) in µmol.kg
-1
.To assist comparison, nitrate is scaled by the mean N:P
ratio of organic matter (that is divided by 16). Symbols indicate the primary (central circles) and
secondary (outer circles) limiting nutrients as inferred from chlorophyll and/or primary productivity
increases following artificial amendment of: N (green), P (black), Fe (red), Si (orange), Co (yellow), Zn
(cyan) and vitamin B12 (purple). Divided circles indicate potentially co-limiting elements. White outer
circles indicate that no secondary limiting nutrient was identified, which will be because of the lack of a
test (from Moore et al., 2013). ................................................................................................................39
Figure 1.8: Correlations between POC and mineral fluxes collected in 52 sediment traps below 1000 m
and around the world ocean (Klaas
and Archer, 2002). .........................................................................40
Figure 1.9: Attenuation of the carbon export flux from the mixed layer into the mesopelagic zone by the
microbial and zooplankton metabolisms (Steinberg et al., 2008)...........................................................43
Figure
1.10:
SeaWIFS
Chlorophyll-
a
concentrations
averaged
over
boreal
spring
(http://oceancolor.gsfc.nasa.gov/cgi/biosphere_globes). The red line indicates the GEOVIDE
transect. ..................................................................................................................................................51
Figure 1.11: SeaWIFS mean Chlorophyll-
a concentrations averaged over austral spring
(http://oceancolor.gsfc.nasa.gov/cgi/biosphere_globes). The red arrow indicates the Kerguelen Island
position. ..................................................................................................................................................52
Chapter 2:
Figure 2.1: Summary of the different sampling and analytical steps on samples collected during the
GEOVIDE cruise .....................................................................................................................................69
Figure 2.2: Perspex filtrations units used for the determination of Ba
xs
concentrations during the
GEOVIDE cruise .....................................................................................................................................70
Figure 2.3: CaCO
3
(a) pAl (b) and pBa (c) concentrations determined on samples
collected with Niskin
bottles and
in-situ pumps. The black lines represent the 1:1 slopes. ....................................................73
Figure 2.4: Depth profiles at Station 32 of particulate trace elements as determined using GO-FLO
bottles (blue diamond) and
in-situ pumps (red triangles). ......................................................................75
Figure 2.5: Radioactive
decay chain of
238
U (a=years; d=days; h= hours; m=minutes; s=seconds) ....77
Figure 2.6: Typical
profile of the
234
Th/
238
U ratio in seawater ................................................................77
Figure 2.7: Mesopelagic particulate biogenic Ba (Ba
xs
) versus calculated oxygen consumption rate
(JO
2
) for stations within the Southern Antarctic Circumpolar Current region (Dehairs et al., 1997). .....85
Figure 2.8: Typical Ba
xs
profile in
seawater, adapted from Jacquet, 2007. ...........................................86
Figure 2.9: Example of a barite crystal (indicated with the red arrow) obtained by scanning electron
microscopy, on the filter sampled at 600 m of Station 69 during the GEOVIDE cruise, and the
corresponding energy-dispersive spectra. .............................................................................................87