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).

There are some characteristics of lakes that help set up the necessary conditions. Relatively deep lakes with a small surface area and flat bottom have a greater chance of not experiences underwater currents and slumping. Also, lakes that are thermally stratified, separated into a warm upper layer and a cool bottom layer, for long periods of time can lead to the cool lower layer becoming anoxic (lacking oxygen). This can prevent the organisms that can mix the sediment from becoming established (O’Sullivan, 1983).

How are varves formed?

There are different types of varves, clastic and organic, each one forming in by a slightly different process.

Clastic varves are found in arctic regions, where the sediment input is dominated by annual freeze-thaw cycles. Larger sediment particles (sand) are deposited in the summer, while smaller (silt or clay) remain suspended until ice covers the lake. This leads to the alternating bands of light summer sediments and dark winter sediments (Figure 2; Zolitschka, 2007)

Instead of layers of different sediment types, organic varves alternate layers of lightly colored algal remains and dark highly organic sediment. These varves are found more in vegetated temperate regions. In the spring nutrients promote blooms of diatoms, a type of algae encased in a silica cell wall that fossilizes in sediments. In some cases, if the right chemical composition is present in the lake, calcite will also deposit in the sediments.  Chrysophytes, another group of algae, form siliceous cysts at the end of their life cycle that is often found at the end of the fall. These algal remains, combined with any calcite that may precipitate out of the water column, form the lightly colored summer layer, while increased organic runoff in the fall and winter form the darker colored winter layer (Figure 2; Zolitschka, 2007)

How to determine dates?

If the varves extend to the surface unbroken, the chronology can be determined from the surface as the present accumulation and absolute dates are possible. However, there are often inconsistencies or periods of missing layers. In these cases a second independent approach is used, 14C dating or known disturbance events (e.g. volcanic ash) can be used if visible. Precise ages can then be determined by combining these independent dating metrics with varve counts.

Figure 2. Differences between clastic and organic varve formation (In Zolitschka, 2007, from Sturm & Lotter, 1995)

Figure 2. Differences between clastic and organic varve formation (In Zolitschka, 2007, from Sturm & Lotter, 1995)

What are varves used for?

Varves have been recognized as a tool for determining chronology in the sediments since the early 1900’s (De Geer, 1912). However with the advent of 14C dating, their use became less popular. It wasn’t until more recently that varved sediments have seen a resurgence as a source of paleoecological data.

The advantage varved sediments have over other sediments is the ability to determine precise temporal changes, sometimes down to the season. This is useful to researchers trying to ask questions related to climate change, vegetation and fire history, erosion, pollution, and archaeology (e.g. Moore et al. 2001, Larsen et al. 1998, Hicks et al. 1994).

Since the layers are annual segmented, it is known that the material wasn’t further mixed after deposition. This is a critical assumption of paleoecology termed superposition, sediment along with any material (e.g. pollen, diatoms, charcoal, etc.) deposited in chronological order. In other words younger sediment is lying on top of older sediment. With varved sediments this can be observed, since the annual layers are visible, instead of assumed.

Imprecise time measurements are often the limiting factor in paleoecological studies. Coupled with an interdisciplinary multi-proxy approach, varved sediment records could provide a unique opportunity to further the understanding of abrupt change on ecosystems.

References

De Geer, G. (1912). A geochronology of the last 12,000 years. Proceedings of the International Geological Congress Stockholm 1, 241–257.

Hicks, S., Miller, U., and Saarnisto, M. (1994). Laminated sediments. Journal of the European Study Group on Physical, Chemical, Biological and Mathematical Techniques Applied to Archaeology 41, 148.

Larsen, C. P. S., & MacDonald, G. M. (1998). An 840-year record of fire and vegetation in a boreal white spruce forest. Ecology79(1), 106-118.

Moore, J. J., Hughen, K. A., Miller, G. H., & Overpeck, J. T. (2001). Little Ice Age recorded in summer temperature reconstruction from varved sediments of Donard Lake, Baffin Island, Canada. Journal of Paleolimnology25(4), 503-517.

O’Sullivan, P. E. (1983). Annually laminated lake sediments and the study of Quaternary environmental changes. Quaternary Science Reviews 1, 245–313.

Sturm, M., and Lotter, A. (1995). Lake sediments as environmental archives. EAWAG News 38E, 6–9.

Tufts University North American Glacial Varve Project (2010). Retrieved March 25, 2013 from, http://eos.tufts.edu/varves/default.asp

Zolitschka, B. (2007). Varved lake sediments. Encyclopedia of quaternary science. Elsevier, Amsterdam, 3105-3114.

 

 

 

 

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