Topics, Tools and Techniques in Paleoclimate Research

Dendroclimatology
Speaker: Andy Cohen, GEOS
April 7th 2004

Summary prepared by Peggy Barker

Andy started his talk by explaining that since lake records are nonlinear he likes to use the term indicator instead of the term proxy. From there he gave a brief summary of the types of archives used in lake studies, such as cores, outcrops, and geophysical records.

A brief description of the many ways in which core samples are taken was discussed before focusing on geophysical records. Seismic methods and Ground Penetrating Radar (GPR) are used to obtain the geophysical data. The maximum resolution from seismic data is around 1m. Unfortunately, there is a trade off between penetration and resolution. But, even though there is this trade off, it can still be determined how things were laid down. Because how things were laid down can be established, a pronounced cyclicity can be seen. This pronounced cyclicity is a hallmark feature of lakes.  This cyclicity is called Van Houten Cycles, and can be seen in rift lakes related to formation of the Appalachians and Atlantic Ocean basin. The Van Houten Cycles repeat about every 10m in rocks from the Hartford Basin, but this will depend on the sedimentation rate.

Similar cycles are seen in east African rift lakes. These cycles, known as Capart Cycles, recur every 50m to 70m, and have three parts.

  1. The first part is poorly stratified, and typically underlain by sediment cutting into the previous sediment layer.
  2. The second part is a relatively chaotic, coarse grained material that is shaped like a fan in the seismic data. It is interpreted as sublucustrian.
  3. The third part contains continuous layers that can be traced over the lake for tens of kilometers, and contains diatom rich muds. This part formed when the lake was at its highest level.

The question was asked: Why do we get cyclic sedimentation records?  Are the lake records driven by processes which are:

  1. Autocyclic? - The movement of delta with basin filling and subsidence
  2. Tectonic? - pulses of faulting
  3. Astronomical forcing? - Milankovitch precession

Therefore, we need to know something about current deposition. To do this the seismic layers must be dated. Two cores are taken from different locations within the lake to see if the same dates can be obtained. Age errors are within 10%-15%.  Case studies for two long-lived lakes, the Great Salt Lake, Utah and Lake Malawi, East Africa were mentioned. At the Great Salt Lake a 120m core was taken with 96% recovery. A model was made from this showing: Open fresh lake (lake level high) with an abrupt shift to a hypersaline lake with a more gradual shift to an open saline lake (lake level lower) to a Marsh (lake level is below core site).  At Lake Malawi four drill sites are planned, with the likelihood of dropping the one in the southern portion of the lake. Records for this lake show laminations with annual resolution. Again, a cyclicity can be seen for this lake.  We were left with this question for Lake Malawi:  Are the cyles due to precession or high latitude forcing?


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