Attaching dates to lake sediment cores: precise dating using varves

By: Rob Brown

There are many proxies paleoecologists use to determine past environments and communities (insects, pollen, diatoms, packrat middens, tree rings, etc.), many of which have been discussed on this blog previously. These proxies can be used to answer questions ranging from seasonal to millennial time scales. With the exception of tree rings, which were previously discussed on this post by Erin our reconstructions are often limited by errors in dating methods. However in some lakes, sediments are deposited in visible annual layers called varves. Varved sediments offer a unique situation where the temporal resolution necessary to determine annual to decadal changes relevant to a human lifetime can be achieved.

Figure 1. Varve sediment from Newbury, Vermont, USA. Note the alternating light and dark bands and different thicknesses. From Tufts University North American Glacial Varve Project

Figure 1. Varve sediment from Newbury, Vermont, USA. Note the alternating light and dark bands and different thicknesses. From Tufts University North American Glacial Varve Project

What are varves and where are they found?

Simply put, a varve is an annual layer of sediment that forms in distinct layers (Figure 1). A single year’s deposit includes a light (summer) layer and a dark (winter) layer.

Varves don’t form in all lakes, in fact they are found in very few. The main factor controlling varve formation is climate variability; there must be large seasonal differences in both temperature and precipitation. This sets up the succession of biotic life and the physical and chemical structure of the lake necessary to form the contrasting layers. Additionally, there needs to be no disturbance of the sediment once it is deposited. Processes such as underwater currents, sediment slumping (think underwater mudslides), degassing (air bubbles within the sediment), or bioturbation (organisms physically mixing the sediment) all mix the sediment layers and the annual deposits are lost (O’Sullivan, 1983). Continue reading

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Through the looking glass: how diatoms can reconstruct wind

By Kelsey

Few object are more beautiful than the minute siliceous cases of the diatomaceae: were these created that they might be examined and admired under the high powers of the microscope? ~ Charles Darwin

As Darwin remarks, diatoms are beautiful.  They have unique, intricate cell walls that help in their identification, because in addition to their beauty, they can tell a lot about past environmental conditions.  My research uses diatoms as a proxy (a preserved item that acts as a ‘natural archive,’ capable of telling us something about climate in the past) to explore past environmental conditions in lakes.  Diatoms are a type of single celled organisms called algae.  These organisms are found in many wet environments including soils, but I focus on diatoms in lakes.

Figure 1: Images of various species of diatoms. Image from Wikimedia Commons.

Figure 1: Images of various species of diatoms. Image from Wikimedia Commons.

Diatoms are unique from other types of algae because they have siliceous or glass-like cell walls, and therefore are well preserved in lake sediments.  This makes them good proxies of past climates.  Additionally, diatom species, like other algae have a variety of environmental preferences.  These preferences can range from mixed or stable water conditions, to high nutrient or light levels and provide the basis for climate inferences. Continue reading