Suigetsu > Topics > Varves

Varve Counting

Of paramount importance to the significance of the Lake Suigetsu sediments is the presence throughout much of the sedimentary profile of annually deposited laminae, or "varves". These varves provide a high resolution, independent age scale against which palaeoenvironmental data (including the radiocarbon dataset) can be directly compared. Counting of the Lake Suigetsu varves is being carried out by combining two complementary methods: those of thin section microscopy (at the GeoForschungsZentrum, Potsdam) and high-resolution X-ray fluorescence and X-radiography (at the Institute of Geography and Earth Sciences, Aberystwyth University).

The varves are composed of alternating layers of diatom-rich sediment and Mn-enriched siderite [(Fe,Mn)CO3] deposited predominantly after lake overturn in autumn. The siderite layers are distinct both optically, when viewed in thin section, and geochemically, as high density peaks in Fe and Mn. A multi-parameter approach is being employed for geochemical counting using high resolution XRF and X-radiographic measurements with the PeakCounter software, which was specifically developed for this project.

In sections of the core where varves are not preserved sufficiently, a newly developed interpolation algorithm has been utilised. The interpolation approach is based on an automated analysis of frequency distributions of annual sub-layers from these problematic sections, allowing an estimate of the sedimentation rate that is unbiased by those of neighbouring sections.

The two methods also provide complementary data on varve composition and micro-facies changes, therefore providing crucial information about past processes in the lake and its catchment. This includes the identification of detrital event layers with both methods (e.g. turbidites and tephras), allowing for their precise removal prior to age-depth model reconstruction.

Publications

In 2012, two papers were published discussing the dual varve counting methods applied to the SG06 sediment core, as well as the automated interpolation approach applied:

Additionally, Staff et al. (2013) have published a more general description of the way in which the varve counting of SG06 has been, and is going to be, integrated with data from the alternative chronological techniques applied to the core.

PAGES logo A useful reference point for other archives of varved sediment is provided by the PAGES (Past Global Changes) varve working group's varve image portal.