Diagenetic evolution and mineral composition of Fe/Mn layers in the sediments of Lake Baikal
Lake Baikal in Siberia is one of the world’s great ancient lakes and home to a
unique variety of more than 2500 endemic species ranging from diatoms, sponges
to salmonid fish and the Baikal freshwater seal. Most of the catchment of
540’000 km2 with its ~350 rivers is only scarcely populated and thus contributes
little to anthropogenic contamination of the lake. The age of Lake Baikal is
estimated to 30 to 40 million years, the surface area is 31’500 km2 (3/4 the
size of Switzerland), and with 1637 m it is the deepest lake on earth. It lies
in the tectonic rift zone between Europe and Asia and has never been permanently
ice covered for the last ice ages. Hence, its sedimentary deposits of more than
7 km bear a treasure of information on continental paleoclimates.
More information about Lake Baikal
In many areas of Lake Baikal distinct layers of Mn and Fe oxide accumulations are found as layers, concretions or crusts in the upper sediment. The magnitude of these accumulations ranges from hardly visible enrichments to slate-like consolidated crusts of more than 1 cm thickness (Figure 1), and nodule-like pieces (Figure 2). Seemingly, these accumulations are located right at the redox interface of the sediment which is indicated by the black to red-brownish colors above and the blue-gray color below, and confirmed by measurements of redox potential. Within certain regions of the lake floor, remains of such layers are buried in the sediment and bear the possibility to be used as proxy indicators of past global changes in climatic conditions, as relics of them have been found in sediment layers as old as 65’000 (and up to 85’000) years.
|
|
Figure 1, left: Core 24 (2001) from the CONTINENT RIDGE, North Basin of Lake Baikal. Newly forming Fe/Mn band in 6 cm depth. Older crust at the redox boundary in 15-16 cm depth. Figure 2, right: Core 25 (2001) from the CONTINENT RIDGE, North Basin of Lake Baikal. Thick, nodule-like concretion within the oxidized zone. |
|
||
At present it is not clear under what circumstances the crusts are created and buried. Among the possible forcing factors are: change of the MAR of organic carbon, change of the sedimentation rate and/or the fraction of allochthonous input changing the MAR of Fe and Mn, a decline in the provision of the bottom water with oxygen, deposition of turbidites, increase of crust density and subsequent decrease of sediment porosity and, thus, diffusivity due to ageing, and others.
The aim of this project is the investigation of the geochemical lifecycle and the mineral structures of the authigenic ferromanganese layers. The riddle to be solved is: are the Fe/Mn layers formed under steady-state diagenetic conditions of reductive dissolution and oxidative precipitation, or are they an indicator of changing sedimentary inputs? In the former case such concretions represent ideal model systems to study the kinetics and mechanisms of geochemical processes at redox boundaries. In the second case the prevalence of recent and older relict layers could be used as proxy indicators for environmental change in this unique continental archive. We will gather information on the dissolution, precipitation, and the timescale of diffusive processes within the sediment by sampling of sediment and porewater with high spatial resolution and subsequent chemical analyses. Major and trace metal contents of undisturbed core sections will also be analyzed with spatial resolution in the sub-millimeter range by scanning methods. With the rate estimates of diagenetic formation and dissolution of Fe/Mn layers, and the vertically changing diffusion coefficients of Fe2+ and Mn2+ across the Fe/Mn layers, the temporal dynamics of the formation and dissolution of Fe/Mn layers, and their dependence on key parameters will be evaluated using a dynamic sediment model.
Funding
Collaborators
- Andrea Ulrich (Empa)
- Andreas Vögelin (Eawag)
- Martin Schmid (Eawag)
- Mike Sturm (Eawag)
- Limnological Institute, Irkutsk
- Institute for Earth-Crust Science, Irkutsk