Carbon isotopes record over time

Bjerrum and Canfield (2004) have used the relationship in Fig. 5.13 to calculate the ratio of organic carbon burial to the total amount of carbon buried in the sedimentary record over time. They show (Fig. 5.13b) that organic carbon burial was less in the Archaean than at the present day. These authors also calculated the effects of a third carbon isotope reservoir - that of carbonate in the oceanic crust - and showed that in this case the degree of organic carbon burial was much less in the Archaean than at present. This result would imply that oxygen levels were low in the Archaean, not because... 

One further difference between the tissues should be noted briefly—that of turnover—which holds implications for the nature of the isotopic signal recorded and its interpretation. Bone is constantly resorbed and reformed during life, i.e., it turns over , whereas enamel and dentine do not, although secondary dentine can be later accreted. Enamel and dentine form during a discrete period in the individual s life. This means that carbon isotope dietary signals in bone, for both collagen and apatite, reflect diet integrated over years, whereas those in enamel and dentine increments reflect diet at time of formation. 

No remarkable alterations were shown in carbon and sulfur cycles or in atmospheric oxygen concentrations since the Latest Proterozoic. The coupling of carbon and sulfur biogeochemical cycles is supported by systematic covariations in the isotopic records of sedimentary rocks over Phanerozoic time. 

Specifically, the isotopic composition of the carbon compounds in the oldest sediments supports the conclusion that the geochemical cycle of carbon evolved into a biogeochemical cycle by the time of the deposition of these rocks. Thus, a fully developed biogeochemical carbon cycle may date from 3.8 billion years ago. Also, the morphological record of microbial (prokaryotic) life has been shown to proceed over at least 3.5-3.8 billion years of geological history. 

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