Does a strong pynocline impact organic-matter preservation and accumulation in an anoxic setting ? The case of the Orca Basin, Gulf of Mexico - Archive ouverte HAL Access content directly
Journal Articles Comptes Rendus Géoscience Year : 2009

Does a strong pynocline impact organic-matter preservation and accumulation in an anoxic setting ? The case of the Orca Basin, Gulf of Mexico

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Viviane Bout‑roumazeilles
Jean Carlos Montero Serrano
  • Function : Author
Olivier Lafon
  • Function : Author
  • PersonId : 929317
Aany Sofia
  • Function : Author
Lilly Thankamony
  • Function : Author
Melanie Rosay
  • Function : Author
Fabien Aussenac
Xingyu Lu
  • Function : Author
Julien Trébosc
Hervé Vezin
Jean-Paul Amoureux
  • Function : Author

Abstract

We show how the 29 Si NMR signals of dispersed inorganic nano-particles of laponite s can be enhanced by Dynamic Nuclear Polarization (DNP). The direct DNP enhances the signals of 29 Si nuclei near unpaired electrons, whereas the indirect DNP via 1 H enhances the signals of more remote sites. As a local and non-destructive technique, solid-state nuclear magnetic resonance (NMR) provides precious insight into the atomic-scale structure and dynamics of nanoparticles (NPs), i.e. materials with three external dimensions sized between 1 and 100 nm. 1,2 Nevertheless, the low sensitivity of NMR can preclude the observation of diluted species, such as the edge, corner, grafting or surface sites. The sensitivity limitation of NMR is even more acute for nuclei with long longitudinal relaxation times (T 1n), low natural abundance and/or low gyromagnetic ratio, such as 29 Si. 3,4 Herein, we show how the NMR signals of inorganic NPs dispersed in a frozen solution containing TOTAPOL 5 can be enhanced at high static magnetic field, B 0 , and under magic-angle spinning (MAS) using dynamic nuclear polarization (DNP). 3-12 Enhancements of 29 Si NMR signals in the order of 10 are reported in direct polarization (DP) and 1 H-29 Si cross-polarization (CP) experiments for laponite s (1), an industrial synthetic clay NP (see Fig. 1), with applications for hybrid materials and soft matter (cleanser, coating). 13-15 Hereafter, the DP and CP experiments with microwave irradiation are referred to as direct and indirect DNP, respectively. This protocol should become a standard for the DNP of NPs, since they are often dispersed in a liquid phase to prevent their aggregation. 16 The dispersion is supplementary to impregnation 4,17 and co-condensation, 18 which have been employed for high-field MAS DNP of porous solids or particle aggregates. 3,4,17-19 Hitherto, dispersion in frozen 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)-propan-2-ol (TOTAPOL) solutions has only been demonstrated for indirect 13 C and 15 N DNP of biological systems, such as bacteriophage or peptide mesocrystals. 10,11 We also show the complementarity of indirect and direct 29 Si DNP: in indirect DNP, 1 H spin diffusion distributes the polarization within the whole sample, whereas direct DNP enhances the signals of 29 Si sites nearer to TOTAPOL. The polarization buildups in direct and indirect DNP are compared and the polarization leakage is discussed. Fig. 2a shows how the indirect 29 Si DNP results in a 14-fold enhancement of 29 Si NMR signals of 1 dispersed in TOTAPOL solution. The concentration in unpaired electrons of 1 is lower than 100 nm (see the EPR spectrum in Fig. S1a, ESI †). Hence, in indirect DNP, the polarization is transferred from an exogenous TOTAPOL radical in the matrix to 29 Si nuclei of 1 via 1 H spin diffusion and 1 H-29 Si CP. The 29 Si NMR spectrum enhanced by indirect DNP displays two resolved 29 Si NMR signals. The intense peak at À94 ppm corresponds to the (SiO) 3 Si(OMg) (Q 3) sites, located inside the silicate framework, whereas the weak peak at À85 ppm is assigned to the (SiO) 2 Si(OMg)OH (Q 2) sites. 15 Fig. 1 (a) Schematic representation of 1 NPs, which are disk-shaped crystallites with a diameter of ca. 25-30 nm. 20 The specific surface area of 1 is 370 m 2 g À1. (b and c) Atomic-scale structures of the circular (b) and lateral (c) surfaces. The sheet of octahedrally coordinated magnesium atoms is sandwiched by two sheets of silicate.
The Orca Basin (an intraslope depression located in the Gulf of Mexico) collects sedimentary particles of terrestrial origin (clastic and organic particles mainly supplied by the Mississippi River) and of marine origin (biogenic productivity). The basin is partly filled with dense brines leached from salt diapirs cropping out on the sea floor, and is permanently stratified. A strong pycnocline induces anoxic bottom conditions, expectedly favorable to organic matter (OM) preservation. Here, we report on OM in the upper 750 cm below sea floor of Core MD02-2552 (Holocene). The organic content is dominated by marine-derived amorphous OM. The organic assemblage is unexpectedly degraded to some extent, which may be accounted for by a relatively long residence time of organic particles at the halocline-pycnocline at 2240 m. Thus the organic particles are temporarily trapped and kept in contact with the dissolved oxygen-rich overlying water mass. Lastly, the land-derived organic fraction shows co-variations with the land-derived clay mineral supply. To cite this article: N. Tribovillard et al., C. R. Geoscience 341 (2009). # 2008 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved. Résumé Une pycnocline concentrée at -elle un impact sur la préservation de la matière organique et sur son accumulation dans un dépôt anoxique ? Le bassin d'Orca, petite dépression située dans le Golfe du Mexique, reçoit des particules sédimentaires d'origine continentale (clastiques et organiques), véhiculées par le Mississippi principalement, et d'origine marine (productivité). Le bassin est partiellement rempli par des saumures issues du lessivage de diapirs salifères affleurant dans les fonds marins. La colonne d'eau y est très fortement stratifiée et le corps d'eau situé sous la halocline-pycnocline est totalement anoxique. Cette anoxie est a priori favorable à la préservation de la matière organique (MO). Nous étudions ici le contenu organique des premiers 750 cm de la carotte MD02-2552 déposés au cours de l'Holocène. Le contenu organique est dominé par la MO amorphe d'origine marine. L'état de préservation de la MO est étonnamment mauvais, compte tenu du contexte de dépôt. Cela est dû au fait que la MO est retenue durablement à l'interface de densité à 2240 m de profondeur, ce qui permet un long séjour dans des conditions encore relativement riches en oxygène dissous. Enfin, les particules organiques terrigènes montrent une distribution relativement bien corrélée à celle des minéraux argileux. Pour citer cet article : N. Tribovillard et al., C. R. Geoscience 341 (2009). # 2008. Publié par Elsevier Masson SAS. Tous droits réservés.

Dates and versions

hal-00371297 , version 1 (27-03-2009)

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Nicolas Tribovillard, V. Bout-Roumazeilles, Thomas Sionneau, J.C Montero Serrano, Armelle Riboulleau, et al.. Does a strong pynocline impact organic-matter preservation and accumulation in an anoxic setting ? The case of the Orca Basin, Gulf of Mexico. Comptes Rendus Géoscience, 2009, 341, pp.1-9. ⟨10.1016/j.crte.2008.10.002⟩. ⟨hal-00371297⟩
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