Dig Deeper! Why Do We Care about Lake Sediments?

by Alicja Bonk

Summer at the lake 

Many people, when asked what they think about when they hear the word "lake", immediately answer "swimming", "summer" or "fish" (especially if the person asked is a fishing enthusiast). In this post, I will try to show you that lakes have even more to offer. To be more specific, I will explain why lake sediments in particular are of high interest to many people. 


Don't be distracted by all these fine-looking fish. The hero of this story is stinky mud.


Why does all of this mud matter? 

The answer to this question is quite complex. Lakes are considered as one of the best natural archives to track environmental and climatic changes because they store all information available not only from their direct catchment (Fig. 1) but also from a bigger area. It might come as a surprise to some, but climatic fluctuations, fires, human activities (eg. industry, agriculture) and many other factors can be tracked with the use of lake sediments.

The biggest advantage of lake sediments is that the life cycle of a lake is relatively long and often spans thousands of years. Other archives like, for example, tree rings have a similar sensitivity to lakes, but their life cycle is much shorter. The oldest known tree, Old Tjikko (spruce, Norway), is 9,500 years old, while the oldest lake, Lake Baikal (Russia), was formed over 25 million years ago! What you have to know, though, is that they are only exceptions. Usually, neither trees nor lakes exist that long. All this is not to say, of course, that non-lake archives are useless. They are also valuable and tell us a lot of stories about the past, but from different time perspectives.

The second advantage of lake sediments is their on-land location. Marine sediment records cover much longer periods, but their accessibility is worse due to water depth and high costs of sediment collection.

The lake sediment is, therefore, a good start for further reconstructions of temperature, solar activity, precipitation and many other proxies that help scientists to predict future changes and to model scenarios of the Earth's future. Valuable predictions, however, are possible only when we know exactly what has happened in the past - and, how fast. Before a scientist says "Eureka!", there is a lot of work to be done.



Fig. 1: The sources of particles at the lake bottom (after [1])



Reach the bottom 

Most of us probably don't like to hear the words "you've reached the bottom", but to a person working on a coring platform, this is music to their ears (Fig. 2). It usually takes a lot of days (sometimes weeks!) to retrieve the set of sediments that will cover the whole history of the lake development. It means, in most cases, that scientists are looking for lakes in which depositions started after the last glaciation in a given area. Once it is done, the real fun begins.



Fig. 2: Sediment coring scheme, modified after [2]


What does sediment look like? 

Each lake is different and so are sediments. Usually, sediments consist of a brownish to  greenish, stinky mixture of in- and organic components. Sometimes, however, annually laminated (varved) sediments are discovered. This is a real cherry on the cake for the scientists. 


... varves? 

The first definition of varves was used to describe rhythmically deposited clays in a proglacial environment by a Swedish geologist, De Geer (1912). A century later, the meaning was extended to include all annually laminated sediments deposited on continents and in the ocean [3].
Generally speaking, one varve consists of at least two layers which represent two different seasons and are treated as one calendar year.

To gain a better impression of what varves look like, let's imagine the slice of a tree trunk that consists of tree rings. Each couple of dark and pale layers represents one year. A similar composition can be observed in the case of varves (Fig. 3).

Unfortunately, varved sediments are rare and unique because of several conditions that have to occur to allow varves to form and preserve in sediment. Ideally, the lake should be in a sheltered location, its area should be relatively small in comparison to lake depth and the lake bottom should not have steep slopes. Otherwise, the sediment can be mixed by the wind and dwelling organisms or disturbed by slumps.


Fig. 3: One year in a tree trunk (A); one year in varved sediment (B)



To count or not to count? 

Lakes with annually laminated sediments are of the highest importance because they allow us to develop chronologies (and track changes) with an annual or even seasonal resolution [4]. In other words, we can study the appearance of different phenomena in calendar years. This is possible because varve formation follows the hydrological year: In a temperate zone, where four different seasons are observed, the “new” varve starts to form in spring, immediately after ice cover disappearance, and lasts until the next spring, with different accumulation rates throughout the year [4]. We observe different layers as a result.

At this point, one might think  "What's the big deal? All you are doing is counting lines!". Unfortunately, it is not that easy. Sometimes the record is disturbed, sometimes there is a lack of sediment, other times varve structures are really complex [4].

It is worth mentioning that a good chronology is crucial for further reconstructions, thus it should be as reliable and accurate as possible. For that reason, apart from counting varves, there is a set of different dating methods that should be used to verify chronologies. Some of the problems connected to varve counting and other dating methods, such as C-14 and Pb-210, have been described in scientific papers [5], [6]. 


A crystal ball 

Scientists aren’t fortune-tellers and don’t read the Earth’s past and future from a crystal ball. To say something about future changes, they must work with solid and reliable data. Sometimes our knowledge is insufficient or data is not good enough, but one thing is certain: Unraveling past changes is the key to understanding the future. 


Take home messages:
1. Due to their long life cycles, lakes give us an opportunity to study past climatic and environmental changes.
2. Varved lake sediments are considered as one of the best natural archives that allow tracking past changes in high resolution. 
3. The chronology based on varve counting is provided in calendar years, but it has to be verified by external methods.
4. Reliable and accurate chronologies are vital for further analysis and reconstructions.



Alicja Bonk, PhD
Polish Academy of Sciences, Poland  

If you have questions or comments concerning Alicja’s post, please leave a comment below or send her an email. You can also connect with her on ResearchGate.



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References 
1. Sturm M, Lotter F (1995) Lake sediments as environmental archives. EAWAG News E
2. Zolitschka B, Francus P, Ojala AEK, Schimmelmann A (2015) Varves in lake sediments – a review. Quaternary Science Reviews. Vol 117. doi.org/10.1016/j.quascirev.2015.03.019
3. Zolitschka B (2007) Varved lake sediments. In: Elias SA (ed) Encyclopedia of Quaternary Science. Elsevier, pp 3105-3114. doi.org/10.1016/B0-44-452747-8/00065-X
4. Bonk A. Tylmann W, Amann B, Enters D, Grosjean M (2015) Modern Limnology and Varve-Formation Processes in Lake Żabińskie, Northeastern Poland: Comprehensive Process Studies as a key to Understand the Sediment Record. Journal of Limnology. 74(2): 358–370, DOI: 10.4081/jlimnol.2014.1117
5. Bonk A, Tylmann W, Goslar T, Wacnik A, Grosjean M (2015) Comparing varve counting and 14C-AMS chronologies in the sediments of Lake Żabińskie, Poland: implications for accurate 14C dating of lake sediments. Geochronometria 42:159–171.DOI: 10.1515/geochr-2015-0019
6. Tylmann W, Bonk A, Goslar T, Wulf S, Grosjean M (2016) Calibrating 210Pb dating results with varve chronology and independent chronostratigraphic markers: problems and implications. Quat Geochronol 32:1–10. DOI: 10.1016/j.quageo.2015.11.004

Comments

  1. This is really informative and accessible. Thank you.

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