Читать книгу Earth Materials - John O'Brien - Страница 76

Three isotopes of carbon occur naturally in Earth materials: carbon‐12 (¹²C), carbon‐13 (¹³C) and the radioactive carbon‐14 (¹⁴C), used in radiocarbon dating. Each carbon isotope contains six protons in its nucleus; the remaining mass results from the number of neutrons (six, seven or eight) in the nucleus.¹²C constitutes >98.9% of the stable carbon on Earth, and¹³C constitutes most of the other 1.1%.

When organisms synthesize organic molecules, they selectively utilize¹²C in preference to¹³C so that organic molecules have lower than average¹³C/¹²C ratios. Enrichment of the organic material in¹²C causes the¹³C/¹²C in the water column to increase. Ordinarily, there is a rough balance between the selective removal of¹²C from water during organic synthesis and its release back to the water column by bacterial decomposition, respiration, and other processes. Mixing processes produce a relatively constant¹³C/¹²C ratio in the water column. However, during periods of stagnant circulation in the oceans or other water bodies, disoxic–anoxic conditions develop in the lower part of the water column and/or in bottom sediments. These conditions inhibit bacterial decomposition (Chapters 11 and 14) and lead to the accumulation of¹²C‐rich organic sediments. These sediments have unusually low¹³C/¹²C ratios. As they accumulate, the remaining water column, depleted in¹²C, develops a higher ratio of¹³C/¹²C. However, any process, such as the return of vigorous circulation and oxidizing conditions, that rapidly release the¹²C‐rich carbon from organic sediments, is associated with a rapid decrease in¹³C/¹²C. By carefully plotting changes in¹³C/¹²C ratios over time, paleo‐oceanographers have been able to document both local and global changes in oceanic circulation. In addition, because different organisms selectively incorporate different ratios of¹²C to¹³C, the evolution of new groups of organisms and/or the extinction of old groups of organisms can sometimes be tracked by rapid changes in the¹³C/¹²C ratios of carbonate shells in marine sediments or organic materials in terrestrial soils.

¹³C/¹²C ratios are generally expressed with respect to a standard in terms of δ¹³C. The standard once again is the¹³C/¹²C ratio of the Pee Dee Belemnite, or PDB. δ¹³C is usually expressed in parts per thousand (mils) and calculated from:

Box 3.1 illustrates an excellent example of how oxygen and carbon isotope ratios can be used to document Earth history, in this case a period of sudden global warming that occurred 55 million years ago.