Carbon isotope comparisons are a useful tool for dating fossils and age of sediments. The increase in carbon fourteen in the atmosphere due to the activities in the nuclear age has necessitated correction factors for more recently produced materials. But there is another factor affecting ratios of carbon isotopes that needs to be considered in some cases.
Marine sediments
I became aware of one factor while considering the isotope aging of carbon in pogonophoran tubes as a method of aging pogonophorans. The likelihood that the tubes were made from fossil food in the sediments would make them look older than the ratio would indicate made me abandon that approach.
Fresh-water sediments
Date of continental glacier recession may be more recent than some studies indicate for a somewhat similar reason. Carbon used for photosynthesis, by plants in fresh-water lakes, may have the ratio distorted, to values indicating older dates of formation by intrusion of carbon, in compounds depleted of radioactive carbon, carried in groundwater passing through ancient limestone to springs in lakes.
The broad leaved species of pond weeds (Potamogeton spp.) are often encrusted with lime in moderately alkaline lakes. They could be analyzed to provide a partial correction factor of the ratio of fossil carbon to atmospheric carbon in such locations if they contributed to formation of surrounding bogs.
The rocks of lake shores in such locations may also provide a clue. They can be recently formed by algae such as the ones below from Fish Lake in Allegan County, Michigan.
Marl concretions from Fish Lake
Cross-section of one of the marl concretions from Fish Lake
Two rocks are shown in the upper picture. The left one is shown split open in the lower picture and the green of the algae is evident in the outer layers, but less evident in the inner layers. Most of the marl concretions making up the shallow sediment surface along the shore were much smaller in size. These photos were taken in the summer of 1954 on a limnology class field trip from the Michigan State University Kellogg Biological Station on Gull Lake.
LAWRENCE LAKE
Lawrence Lake is a lake in the same glaciated region. Robert G. Wetzel (1975, Limnology, Saunders, Philadelphia) notes on page 40 that "groundwater largely from springs contributed nearly 40 per cent of the annual water income to calcareous Lawrence Lake Michigan" and about ten percent was from precipitation and the remainder was from streams. If the source of water in streams is from springs the streams could also contribute to distortion of ratios of carbon as compared to the atmospheric source, where the radioactive isotope is naturally produced. The half-life for the decay of radioactive carbon is about 5730 years. So plants from Lawrence Lake might start with a ratio indicating they were formed almost 4000 years before they really were formed.
From his own research, Wetzel, page 367, says "carbonate deposits encrusting the submersed parts of macrophytes in calcareous hard water often exceed the weight of the plant material". On the same page he noted some of the carbon dioxide, bicarbonate, and calcium process producing the encrustation. It is presumably similar to the same process of photosynthesis involvement in the algae producing the "rocks" illustrated above.
Dr. Wetzel took over as resident limnologist a few years after I took limnology. He produced much research, especially based on examples at Lawrence Lake. His work at that time contained one of the few available studies of relative contributions of different components of lake ecosystems. It was noted for that reason in an environmental conference I attended in Miami, Florida, over forty years ago. But large lakes would have a higher proportion of their productivity coming from their plankton communities if they have a greater proportion of deeper areas.
ICE AGE ANIMALS
I am not familiar with studies dating the demise of the mastodons and mammoths of North America. If the dates were determined by radiocarbon dating of their tissues, it could be erroneously older if their food was primarily or largely aquatic vegetation. The dating of the retreat of the glaciers opening up land for them might also be dated erroneously older if the dating used aquatic vegetation or bog material.
Joseph G. Engemann Emeritus Professor of Biological Science, Western Michigan University February 5, 2015
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