Ancient ocean analogues

 

 

In September 2009, Adrian Gilli and Stefanie Wirth visited Lake Cadagno, Switzerland, equipped with a drill rig to collect two long sediment cores from the deepest part of the lake. These cores hold important information about the history of the lake since the last ice age 12,500 yr ago.

 

Wirth and Gilli gathered a team of specialists with diverse backgrounds including microbiology, ecology, quaternary geology and geochemistry. This led to a rigorous study of the water column conditions and microbial structure in the lake and constitue a cross-calibration of the newest techniques applied in the fields.

The sediments preserve the environmental history of the lake, its biology, and ecological conditions. With core scanning x-ray fluorescence, we measured molybdenum concentrations at 1 mm resolution (corresponding to <1 yr on average) in sediment cores and found a marked transition to anoxic and sulfidic water column conditions occurring 9800 yr ago, after a period of intense Mn cycling in the lake.   To our astonishment, Cadagno has conserved anoxic conditions 100-fold longer than previously known.

Molecular analyses (DNA and pigments) record the presence of anoxic phototrophic communities and confirm the Mo-based evidence for anoxic and sulfidic water column conditions, while providing temporal details on periods dominated by green sulfur bacteria (GSB) and purple sulfur bacteria (PSB). These communities have different adaptation to light and nutrients, but always require anoxic bottom waters.

With the new reconstruction, one can learn how microbial and biogeochemical processes in the sediments (diagenesis) alter the chemical signals over 10 kyr time scales. This improves our ability to use Lake Cadagno as a calibration site for new techniques that allow us to understand anoxic conditions on early Earth.

References:

Dahl et al. GCA (2010): Molybdenum cycling in Lake Cadagno

Wirth et al. GCA (2013): 9800 years of euxinia