39 structure and (ii) vehicle transportation for Bacteria and Archea. Aspect (i) would be studied in situ and in the laboratory to identify specific microbial strains associated with smectite clays. The second aspect (ii) would be then modelled. This could give further insights to understand the distribution of ‘motionless’ (e.g. at scale > μm-cm) Bacteria and Archea across ocean basins.
The bSiO2 frustule protects the intracellular content of diatoms (Smetacek, 2001), which guarantees a longer association between bSiO2 and OC of diatom origin. We also concluded that the association between BSi and POC formed via primary production was a prerequestite for the export of BSi-ballasted organic particles. The specific association between BSi and OC was further investigated in the Manuscript III and the role of TEP in bSiO2 dissolution assessed.
40 4. Affinities of 210Po,210Pb and 234Th for POC and TEP
The following answers have been given in the Manuscript IV:
(i) Which of the 234Th/238U and the 210Po/210Pb systems better traces POC and under which environmental settings?
We suggested that the trophic conditions of an ecosystem are a critical factor determining whether210Po or 234Th is most suitable to estimate POC fluxes. The affinity of 210Po and 234Th for POC is greatly affected by the presence of highly silicified organisms and lithogenic material, respectively. In our study the diagenetic state of the POM seems to especially influence 234Th. Therefore, in the Southern Ocean, in areas that are dominated by large diatoms, 234Th would be a better tracer for sinking POC; whereas in oligotrophic conditions, dominated by smaller, less silicified cells and detritus, 210Po and 234Th seem similarly appropriate. Finally, when non-biomineralized dominate the primary production 210Po would be the best choice.
(ii) What are the affinities of 210Po and 210Pb for TEP in natural samples?
All three radiotracers, 210Po, 210Pb and 234Th presented affinity for the bigger size fraction of TEP, independently of the depth or the sampled region. However, 234Th exhibited the highest affinity for TEP (> 1μm), which was 2.2 to 3.5 times greater than with 210Po. Therefore, when organisms that produce large amounts of TEP dominate the primary production, the
234Th/238U disequilibria method may have higher chance to over or under-estimate POC fluxes than the 210Po/210Pb method.
The influence of TEP on the over or under-estimations of POC fluxes will be further developed in a future manuscript (Robert et al., in preparation); where POC fluxes in the Arctic and the Southern Oceans will be calculated using the 210Po/210Pb system and the influence of TEP on the estimations compared.
The exploratory study presented in the Manuscript IV also raises further questions.
No affinity between the radionuclides and coomassie stainable particles (CSP), protein-rich particles, was reported in our sudy. Possibly, CSP are less surface active and do not scavenge trace elements to the same extent as TEP. Nevertheless, this result emphasizes the probable different cycling pathways of CSP and TEP. Future work includes the utilization of the the
41 size spectra of CSP (Manuscript IV) and TEP (data to be analyzed) to compare aggregation predictions from CSP and TEP in the Southern Ocean (Logan et al., 1995).
210Po and 210Pb were associated with the smaller fraction of TEP (0.2 - 1 μm) that is a bacteria-enriched fraction, whereas 234Th was not. This result was striking because it contradicts previous views about the cycling of 234Th (Honeyman and Santschi, 1989;
Santschi, 2003). The relation between 210Po and bacteria-enriched fraction also further supports the idea that 210Po could be a tracer for the bacteria production and the transfer of OC to higher trophic levels (Friedrich, 1997; Kim and Church, 2001; Stewart et al., 2008).
Last, but not least, not only the composition of organic particles but also the presence/absence of lithogenic and bio-minerals appeared to influence the efficiency of the
210Po/210Pb and 234Th/238U systems to estimate carbon export. Therefore, as already pointed out by Geibert and Usbeck (2004) it seems important to determine the affinity of these radionuclides for mineral particles in natural conditions. Geibert and Usbeck (2004) found high partition coefficients between thorium, protactinium and the smectite clay (SWy-2). We used the same clay in the Manuscript II, where we found that SWy-2 was efficiently trapping POC. Thus we chose to explore the affinity of 210Po, 210Pb and 234Th for SWy-2 in natural conditions. The partitions of 210Po, 210Pb and 234Th in the dissolved and the particulate pools in the absence and presence of SWy-2 (0.5 mg.L-1) were compared during several aggregation experiments using in situ seawater from the Atlantic sector of the Southern Ocean. In parallel, aggregates formation, POC, TEP, CSP, bacteria and virus abundances and activities were monitored. The results are currently under analyze and will be submitted in a future manuscript (Robert et al., in preparation).
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