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Volume 4, issue 2
Biogeosciences, 4, 195–203, 2007
https://doi.org/10.5194/bg-4-195-2007
© Author(s) 2007. This work is licensed under
the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

Special issue: Coupling biogeochemistry and ecology to fluid dynamics in...

Biogeosciences, 4, 195–203, 2007
https://doi.org/10.5194/bg-4-195-2007
© Author(s) 2007. This work is licensed under
the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

  15 Mar 2007

15 Mar 2007

Manganese content records seasonal upwelling in Lake Tanganyika mussels

D. Langlet, L. Y. Alleman, P.-D. Plisnier, H. Hughes, and L. André D. Langlet et al.
  • Section de Pétrographie-Minéralogie-Géochimie, Musée Royal de l'Afrique Centrale, Leuvensesteenweg 13, 3080 Tervuren, Belgium

Abstract. Biogenic productivity of Lake Tanganyika is highly dependent on seasonal upwellings of cold, oxygen-depleted, nutrient-rich deep waters. We investigated the shell of freshwater bivalve Pleiodon spekii as a geochemical archive of these periodic hydrological changes tuned by the monsoon regime. The results of a three-year-long limnological and geochemical survey of the coastal waters performed on the dissolved and particulate fractions were compared to LA-ICP-MS profiles of Mn in five aragonitic shells from the same lake location. Three shells present very similar Mn/Ca profiles dominated by a peak that matched the concomitant increase of Mn and chlorophyll a in surface waters during the 2002 upwelling, while a shell collected during 2003 dry season detect both 2002 and 2003 upwelling events. Larger shells showing an extremely reduced growth display more than 8 Mn/Ca peaks suggesting at least an 8-year-record of seasonal changes in water composition. We postulate that Mn/Ca in shells record the conjunction of an increase of biological activity with supplied of dissolved Mn and nutriments in coastal waters, resulting in an enhanced assimilation of biogenic Mn-rich particles. By combining the most recent generation of laser ablation system and the powerful High Resolution ICP-MS, the spatial resolution could be improved down to 5 to 10 µm crater size and end up in a better constrain of the relative variations of the annual Mn peaks. Such an approach on P. spekii from Lake Tanganyika has definitively a great potential to provide recent and past records on primary productivity associated with the monsoon climate system.

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