Why We Study Sediment Cores:
by Paul Garrison, Wisconsin Department of Natural Resources
Lake folks often get into lively discussions over what the lake used to be like...more plants, fewer plants, clear water, murky water... Is there any way to really know for sure? Well, the answer is yes! In fact we can have a good idea of what lakes used to be like hundreds of years ago with a science called Paleolimnology
Paleolimnology, sometimes referred to as “History in the Mucking,” is the interpretation of past conditions and processes in lake basins. Each year since their formation, a steady rain of sediments, plant pieces, creature parts and other materials settle to the bottom of our lakes leaving a record of past conditions. Over the eons this fascinating record has remained safe and sound in the sediments waiting for someone who could unlock its secrets. In addition to providing background information on a lake, the sediments hold a record of natural and human disturbances that have occurred in the lake. The response of the lake to these disturbances provides insight into how the lake functions, and provides a better understanding of the significance of trends observed with modern monitoring programs.
Obtaining a Sediment Core
To extract the lake’s past history a sediment core is usually collected using some type of a hollow tube. The tube is pushed into the lake bottom and a cap is remotely placed on top of the tube to hold the sediment in place until it can be returned to the surface. Once collected, the core is sectioned into intervals usually of 1-2 cm. Each of these sections represent sequential chapters in time, with the top of the core being today and the bottom of the core indicating some time in the past. The time frame of interest in paleolimnological studies often is the last 200 years, which typically covers the impacts of European settlement.
Questions Answered with Paleolimnology
The science of paleolimnology can reveal many secrets from a lake’s past life. Most natural and human disturbances affecting lakes can be evaluated using paleolimnological approaches. Nutrient increases and acid rain are issues that have been extensively documented and studied with paleolimnology. Paleolimnology can give us details on the severity of human impacts and tell us within a few decades of when the impacts began. The genesis of events such as erosion, organic pollution from sewage treatment plants and animal feed lots can all be documented. Paleolimnologists can tease many messages from the sediments, such as how many and what types of aquatic plants grew in the lake, changes in species composition, past fish populations, or the frequency of algal blooms. Most recently, trends in climate change are being studied with paleolimnological techniques.
Dating Sediment Cores
An accurate sediment chronology is an essential part of a paleolimnological study. Cores are usually dated to establish the timing of past environmental changes and to determine the rate of input of materials into a lake. Paleolimnologists can accurately date sediment layers because they can be cross-checked against known historical events.
Cores are typically dated by analyzing a series of samples from the surface to a core depth that corresponds with 200 years ago. The most common dating technique for sediments deposited within the last 200 years is the lead-210 technique. Lead-210 is a naturally occurring atom that exhibits radioactivity. It enters lakes primarily through precipitation and dry deposition (i.e. dust), following the decay of an atmospheric gas called radon-222 (radon gas).
Several methods can be used to corroborate the lead-210 dating technique. Testing of atomic weapons has left stratigraphic markers (layers) in the sediments of all lakes around the world. These markers include cesium-137 (a byproduct of atmospheric nuclear testing). Atmospheric testing by the U.S.S.R. peaked in
1963. It then dramatically declined after the implementation of the Nuclear Test Ban Treaty that same year.
In southern Wisconsin another marker that is often used to confirm sediment dates is arsenic (yes, arsenic). Sodium arsenite was used during the 1950s-60s in a number of Wisconsin lakes to control aquatic plants. Records are available that document how much was applied (Lake Tides Vol. 31, No. 2, 2006). The peak in arsenic concentration in the core corresponds with the time of maximum application.
Another dating marker that can be traced in lake sediments is stable lead. It was used in leaded gasoline until its removal in the mid-1970s and provides another time marker to check on the lead-210 dating technique.
A Record in Glass
Fossils are one of the guides to the past life of a lake. The fossils used most are diatoms. These are a special type of algae that possess cell walls made of silica. Silica is the same as glass so diatoms can be preserved for thousands of years in the sediments. Diatoms are particularly useful because most of them live under well-known environmental conditions. This makes them ideal to characterize what past environmental conditions were like when they were living, such as phosphorus concentrations. Diatoms have been used to estimate trends in phosphorus, acidification, color, salinity and plant communities. Studies have allowed us to determine that some lakes are naturally acidic while others have become acidic as a result of human activities. Other algal groups such as certain blue-green and green algae are also preserved in the sediments.
Other fossils that are deposited and are useful for re-creating past environments are aquatic insect larvae and zooplankton. Insect larvae can be used to track changes in a lake’s oxygen content over time. Zooplankton are often eaten by fish and invertebrates, so changes in their numbers allow us to infer past fish populations. For example, a decline in large zooplankton is an indication of an increase in plankton-eating fish such as perch or bluegills.
Paleolimnology is a powerful tool to discover where a lake has been and maybe predict where it is going. This buried treasure of information is stored in lake sediments and we can read the sedimentary records like the pages of an ancient book. Look for more information on paleolimnology in future Lake Tides.