Proterozoic Organic Carbon — Its Preservation and Isotopic Record

摘要: The carbon isotopic composition of Proterozoic organic matter has been studied as a function elemental H/C ratio. Kerogen samples which have experienced various degrees postdepositional thermal alteration show correspondingly wide spread δ13C values. Setting minimum threshold 0.2 to eliminate the kerogens highest rank immediately results in marked contraction range δ 13C. Analysis time-dependence these data reveals smooth trend increasing from an Early average — 34 %o PDB about 30.5%o for Middle Proterozoic. This contrasts markedly with record carbonates, typically near 0%o PDB. Several explanations are offered this trend. Firstly, long-term decline occurred partial pressure atmospheric CO2 during An accompanying development biochemical pumps, raise internal concentrations photoautotrophs and allow maintenance high levels fixation, would cause partially closed system respect external reduction 13C fractionation. Alternatively, or perhaps additionally, evolutionary pressures toward optimizing kinetic parameters major carboxylating enzyme RUBISCO may accompanied by enzymic discrimination against Late display substantial variability. most likely reflects secular changes burial rates versus carbonate carbon.