Lake sediment (mud) accumulates continuously at the bottom of many lakes, meaning that the deeper you go into the mud, the further you go back in time. This mud contains different types of fossils that can be used to reconstruct changes in the lake, surrounding terrestrial environment, and climate. Working with lake sediment requires careful collection and preservation of a “core” of mud, dating of the core, and physical description, extraction, and analysis of fossils in the cores. The fossil “proxies” we are working with include charcoal, silicaceous remains of diatoms, phosphorus, carbon and nitrogen content as well as abundance of organic matter, and magnetizable minerals.
Lake Sediments, Coring and Preservation
Sediment in a lake has two origins. It may be generated within the lake (autochthonous, e.g. dead algae, fish, fish poop) or from the outside (allochthonous). Allochthonous inputs may include organic matter but also include silt, sand, clay, and other inorganic material that either wash in or are blown into the lake. Sediment usually doesn’t accumulate (or only does so slowly) near the shore, but rather builds up in deeper waters where wave action, ice scouring, or disturbance by animals (bioturbation) is reduced. In general, sediment accumulates more rapidly in deeper waters, where the sediment is said to be “focused.” In many northern MN lakes only 10-30 cm of sediment (or less) may have accumulated in the last 150 years, contrasted with 75 cm up to 3 m in southern MN. A change from sticky, stinky mud to odor-free, non-sticky mud (termed “gytja”) can be used as a marker for the onset of logging or farming in area around a lake.
To remove a lake sediment core, we use what is termed a “drive-operated surface corer.” This instrument involves a clear polycarbonate tube that is attached via the “head” to a series of 1.5m long rigid rods that are screwed together and allow us to push the tube into the mud.
Before lowering the corer into the water, a piston is inserted into bottom of tube. The piston is connected to a cable that extends to the surface. The cable is tied off on the boat, which prevents it from going deeper into the water. When the corer is pushed into the mud (we start the “drive” above the sediment to capture the “sediment-water interface”), it is pushed past the piston, creating a vacuum that (a) makes it easier for the sediment to move in the corer and (b) prevents the sediment from falling out (as long as the cable is held tightly) when the corer is brought to the surface.
Once the core is returned to the surface, a second plug/piston is placed in the bottom and the core is returned to the shore. The sediment is extruded by pushing the corer past the second piston (which is supported by a series of wooden rods). The sediment is extruded in 2-4 cm sections, placed in plastic sample cups, and stored in a coldroom at 4oC.
Sediments can be dated using a number of tools. These may include basic changes in the physical characteristics of the sediment (sediment deposited in last 150 years tends to be sticky and stinky), the presence of ragweed or Russian thistle pollen, or isotopic dating. For older sediments, we can evaluate changes in the abundance of 14C for dates and for younger sediments we can use an isotope of lead (lead-210). Lead-210 may originate from the atmosphere or directly from the soil. The older the sediment, the less atmospheric lead-210 it contains. Lead-210 can provide highly accurate dates for sediments that range in age upwards of 100-150 years. Dr. Dan Engstrom at the St. Croix Research Station, Science Museum of Minnesota performs our lead-210 dating.
Charcoal is a product of incomplete combustion, and charcoal from fires may be dispersed to adjacent lakes via wind or water. Changes in charcoal abundance indicate changes in the intensity or frequency of fires, which in turn may be a product of variation in the local vegetation (prairie vs. forest) or climate (wet vs. dry).
Charcoal is angular, shiny, brittle, and often retains anatomical structure. Our analysis of charcoal involves soaking 1 mL samples of sediment in 10% potassium hydroxide (to break up mud) and then gently sieving the sediment through a 180 micron sieve. We then look at the charcoal that was trapped on the sieve using a dissecting microscope (at x20 power) and quantify the size and shape of the charcoal particles with a digital camera connected to the scope and imaging software.
In simple terms, “magnetic remanence” is the magnetization that is left behind in a material after exposure to an external magnetic field. Changes in the magnetic remanence characteristics of sediment can be used to describe changes in the concentration and size of magnetic materials in the sediment. In turn, these changes can be used to infer changes in erosion into the lake and/or the activity of bacteria, which in part reflect variation in water depth and/or lake temperature. We take our measurements at the Institute for Rock Magnetism, and our two measurements are Isothermal Remanent Magnetization (sediment is exposed to a strong DC field) and Anhysteric Remanent Magnetization (sediment is exposed to combined DC and AC fields). The ratio of ARM/IRM increases as the size of magnetic particles decreases.
Biogenic Silica and Phosphorus
Diatoms are single-celled algae that build beautiful glass (silica) shells. They are common in lakes and often a major component of lake productivity (especially in the autumn and spring). The amorphous silica diatoms produce is termed “biogenic” and, when preserved, can provide a record of past changes in lake productivity. To analyze biogenic silica, we dry some of the sediment and then digest a small known quantity in a 1% solution of sodium carbonate. The dissolved silica is then analyzed “colorometrically,” which means that an ammonium molybdate solution is added to the dissolved silica. The more silica present, the darker the solution becomes.
Phosphorus is a major contributor to lake productivity, and large increases in phosphorus have been a major contributor to eutrophication and water quality decline in Minnesota and elsewhere. To analyze phosphorus, we digest a small known amount of dried sediment in 30% hydrogen peroxide and strong acid. The phosphorus that is released during the digestion is then analyzed colormetrically, as described above for silica.
“Organic matter” is another term for “dead stuff.” We quantify the amount of organic matter within sediment samples by burning a known quantity of sediment at 550oC for one hour and then recording the decrease in weight. More organic matter means more mass is lost during combustion. We also measure the C and N content of the sediment by combusting samples at high temperatures (+1200oC) in an element analyzer and then measuring the amount of carbon dioxide and nitrogen gas that is produced. Carbon/nitrogen ratios of <10 are common for sediments dominated by algae, while higher ratios (>15 or more) are typical for sediments that contain leaves and other material that washed in from the surrounding area.
1 m = 100 cm = 39.37 in= 3.28 ft
1 km = 1000 m = 0.62 miles
1 L = 1000 mL = 1.06 qt = 0.264 gal = 33.81 fl oz
1 kg = 1000 g = 2.20 lb
1 lb = 16 oz = 453.6 g
1 ppm = 1 mg/L = 1 mg/mL
1 ppb = 1 mg/L = 1 ng/mL
T[celsius] = (5/9)*(T[Fahrenheit ]-32)