CALIDAD DESCRIPCIÓN
3.8. REGISTRO DE DATOS POR EJE CAROTÍDEO.
Silica fluxes dominate the annual input of particulate material to the deep Southern Ocean as mirrored by the extensive accumulation of opaline sediments (De Master, 1981). Previous laboratory cultures (Takeda, 1998) and field observations (Hutchins and Bruland, 1998) have demonstrated that iron-replete cells take up less silica per unit carbon. Thus, the observation of low Si : C ratios in trap materials are occasionally interpreted as evidence of carbon export from a bloom where the previously iron-replete cells began to senesce and aggregate (Nodder and Waite, 2001). However, these ratios are likely skewed by the selective export of large species, empty frustules, and the presence of carbon-rich functional groups such as Phaeocystis, dinoflagellates etc.
Conversely, high Si:C ratios may not be accurately viewed as the sedimentation of iron- deplete diatoms either.
Rather than exclusively commenting on the magnitude of Si:C ratios, one can express their deviation away from the integrated (upper 60m) elemental molar ratios of the suspended particulate field, thus minimising any regional community structure biases. Si:C and Si:N ratios in sinking material at M8E, M8W, and M3.3 were all depleted relative to the overlying particle field (Fig. 3.13) compared to the remainder of the stations. As all the PELAGRA deployments occupied different depths, one has to use remineralisation co-efficients derived from empirical observations (Martin et al., 1987; Buesseler et al., 2007) to calculate organic carbon flux at 100m albeit with large uncertainties. Normalising Si measurements is even more difficult owing to the
complicated dissolution kinetics (Van Cappellen et al., 2002). The departure of Si:C
and Si:N ratios in sediment trap particles away from the ratios in suspended particulates displays an increase in both variables with depth (Fig. 3.14). The observed decoupling of silica and organic material suggests that downward POC flux from iron fertilised diatom blooms is preferentially remineralised relative to silica, as has been inferred from other studies (Nodder and Waite, 2001). These conclusions are consolidated upon inspection of the dissolved nutrient profiles of silicate and nitrate, at M8W for example these values are much lower (Si:N = 0.065) than they are at M2 and M6 (Si:N = 0.82). Our study suggests that the decoupling of the global silica and carbon cycles that is well known as a defining feature of the Southern Ocean (Nelson et al, 2002; Sarmiento et al, 2004) has its origins in the upper ocean.
It has been suggested recently (Matsumoto et al., 2002) that during glacial periods an
increase in aeolian iron supply induces the preferential uptake of nitrate relative to silicate in Southern Ocean diatoms, as demonstrated by incubation experiments (Takeda, 1998). The resulting excess silicic acid in Subantarctic Mode Waters is advected northward by Ekman transport and allows low-latitude diatom production at the expense of calcifying organisms. Models suggest that the export of diatoms over calcifying
Fig. 3.13 MRPar is the molar ratio of the suspended particulate phase integrated over
the upper 60m using depths of 5, 10, 20, 40, and 60m. MRTrap is the measured molar
ratios of exported particles captured in PELAGRA. The difference between these molar elemental ratios is plotted as the deviation away from zero which is marked with a dark grey horizontal line.
the order of 70ppm, which accounts for glacial / interglacial atmospheric CO2 cycles
(Archer et al., 2000). The silicic acid leakage hypothesis (Matsumoto et al., 2002;
Sarmiento et al., 2004) described above is dependant on the reduction of Si:N ratios in
Fe-replete diatoms. The central question to the silicic acid leakage hypothesis is what governs the Si:N values in Southern Ocean waters advected north? If it is the Si:N uptake ratio of 1 by marine diatoms as suggested by culture experiments (Hutchins and Bruland, 1998., Takeda, 1998) then this would allow excess silicic acid to reach low- latitudes. However, data from this study clearly show that diatom export from Fe- fertilised blooms experiences selective regeneration of nitrate over silica as a function of depth relative to the ambient particle field (Fig. 3.14), thus increasing the Si : N ratio of diatoms and consequently lowering the potential excess silicic acid in Subantarctic surface waters predicted from this hypothesis.
Fig. 3.14 MRPar is the molar ratio of the suspended particulate phase integrated over the
upper 60m using depths of 5, 10, 20, 40, and 60m. MRTrap is the measured molar ratios
of exported particles captured in PELAGRA. The difference between these molar elemental ratios is plotted as a function of depth. . Grey circles and black regression line for Si:C (y = 0.0117x – 1.0713; r2 = 0.80, n = 7, P<0.02) and black crosses and
dashed regression line for Si:N (y = 0.0715x – 6.4986, r2 = 0.90, n = 7, P<0.02)
In the deep-sea the correlation between opal fluxes and organic carbon fluxes is not as strong as the correlation observed between organic carbon and calcium carbonate fluxes
(Francois et al., 2002; Klaas and Archer, 2002), despite opal fluxes previously being
implicated as crucial vectors for POC flux (Ragueneau et al., 1994; Buesseler, 1998). It has been suggested from these data (Klaas and Archer, 2002) that calcium carbonate is the primary mineral responsible for the sedimentation of POC. It has also been shown that areas of high organic matter flux are characterised by high Si:C ratios with a high fraction of production exported. The further decoupling of Si:C ratios during downward
particle transport (this study; Nelson et al, 2002) shows how this can lead to low and
3.6 Conclusions
It is difficult to ascertain accurately the detailed processes of carbon export from single point measurements. The comparison of normalised carbon fluxes to contemporaneous measurements of production is often undertaken in many studies. This generally produces over- and under-estimates of export ratios which can only be sensibly interpreted within the context of temporal decoupling between production and export processes. In this study satellite-derived chlorophyll data and empirical observations between in-situ chlorophyll and integrated production have been used to generate retrospective estimates of production. Using assumed particle sinking velocities this approach provides a range of export ratios for comparison, and decoupling, as one would expect, was more prevalent in the productive regime north of the plateau.
It was important to normalise organic carbon fluxes to a nominal depth to enable full comparison of the data set. Instead of normalising fluxes by a single factor based on an empirical relationship that has no observational validity in the study area, this study had the benefit of employing normalising factors from similar environments derived from recently published work. Comparison of these two techniques reveal that the traditionally-used Martin curve may under- and over-estimated normalised fluxes at southern and northern stations respectively.
The measured and depth-normalised organic carbon flux at M8W north of the plateau was an order of magnitude larger than fluxes from all other stations. Based on observed production profiles this deployment seems to represent the best estimate of export from the fertilised region north of the plateau. Using a simple mass balance approach to account for seasonal depletion of dissolved silicic acid in surface waters and Si fluxes from the euphotic zone, potential export of organic carbon from the bloom area was
estimated to be in the order of 11-15 g C m-2. This is higher than previous estimates
obtained from mesoscale perturbation experiments and may reflect fundamental differences in methodical approach.
Diatom size correlates well with a range of calculated export ratios(100m) demonstrating
the significant role of large diatoms in export from a naturally fertilised bloom. The main diatoms involved in export from the surface are medium-large centric diatoms, particularly E. antarctica; although to the south of the plateau the heavily silicified pennate F. kerguelensis was most important, reflecting the regional differences in community structure and its effect on the magnitude of carbon export. Si:C and Si:N ratios of the particles normalised to the overlying particle field increase proportionally with depth, highlighting the preferential remineralisation of carbon and nitrogen over silica has its origins in the surface ocean.
It appears that events or regions of high surface export are characterised by a decoupling of production and export and these events are under-sampled and poorly represented in models. The following chapter will focus on the analysis of deep-sea sediment trap material recovered from the CROZEX study region.
Data
The data presented in Chapter 4 includes:
• Organic carbon measurements
• Total carbon measurements
• Biogenic silica measurements
• Sediment trap current speeds
• Diatom taxonomy of sediment trap samples
• Satellite-derived chlorophyll
The picking and splitting of all sediment trap samples was carried out by the author All biogenic silica measurements and organic carbon data was measured by the author.
Total carbon samples were prepared by the author but analysed on the elemental analyser at the University of Liverpool (UOL) due to equipment failure at the National Oceanography Centre, Southampton (NOCS). The agreement between total carbon measured at NOCS and UOL is presented in Appendix 1. Sediment trap current speeds were downloaded from the current meters by Jane Read (NOCS) and processed by the author. Diatom taxonomy samples were prepared by the author and counted by Alex Poulton, this was to maintain consistency within the CROZEX project. Satellite data was downloaded from SeaWifs by Hugh Venables (NOCS), validated according to Venables et al., (2007) and processed by the author.