Variations in the Properties of Extractable “Humic Matter” and Associated Kerogen in Sediments through Geologic Time: Their Significance for Precambrian Biological Evolution and Paleoecology

ABSTRACT

Chemical properties and δ¹³C values of benzene/methanol-extractable “humic matter” and associated kerogen in a large, diverse collection of Precambrian and Phanerozoic sediments from different parts of the world showed complex systematic variations through geologic time, reflecting major developments in the history of Precambrian life, and different kinds of sediment yielded similar patterns of variation. Moreover, certain data differentiate clearly between glacial and nonglacial detrital sediments, or between lacustrine and marine sediments, and some data suggest the occurrence of Precambrian land life. The abundance of aromatic groups and the proportion of aliphatic to aromatic groups in the extracts showed little variation from the early Archean (ca. 3.3 Ga) to the mid-Proterozoic (ca. 1.6-1.3 or 1.3-1.1 Ga), whereupon they increased sharply, peaked ca. 1.1-0.900 Ga, and then plunged to a minimum in the late Proterozoic (ca. 0.800 Ga) or early Phanerozoic. This is interpreted as indicating that cyanobacteria were the dominant photoautotrophs until the mid-Proterozoic, when algae evolved, proliferating until the late Proterozoic, whereupon their populations were depleted by herbivorous metazoans. Nitrogenous aromatic material increased to a maximum ca. 3.4–3.3 Ga and then decreased steadily to ca. 1.3 Ga, suggesting that early Precambrian cyanobacteria were enriched in photoprotective as well as photosynthetic tetrapyrrole pigments owing to the lack of ultraviolet radiation-shielding atmospheric O₂ and O₃. The concentration increased again starting ca. 1.3–0.800 Ga, reflecting the rise of algae, peaked ca. 0.680 Ga, and dropped catastrophically to a much lower value in the Cambrian (ca. 0.510 Ga), suggesting mass mortality at the Precambrian-Cambrian boundary.

KEYWORDS:

• extractable humic matter
• kerogen
• Precambrian biological evolution
• Precambrian paleoecology
• sedimentary organic matter

Acknowledgments

The samples, information about depositional environments, and sample ages (except for updated ages extracted from the literature) were provided by the late P. Cloud (Biogeology Clean Laboratory, Department of Geological Sciences, University of California, Santa Barbara), with whom I was affiliated as a Research Associate from 1970 to 1972 and in whose laboratory I prepared the samples for analysis. The research was supported by grants from National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and National Research Council (NRC) of the USA (awarded to P. Cloud) and by funds provided by Environment Canada (currently known as Environment and Climate Change Canada), a department of the government of Canada. The chemical analyses of kerogen were done by C.B. Moore (Jackson and Moore, 1976), and the carbon isotope analyses were performed by P. Fritz and R. Drimmie (Jackson et al. 1978). Parts of Figures 1, 4, and 5 were published by Jackson (1975) and are republished here together with related but previously unpublished material, as it was necessary to include them for purposes of comparison. More detailed acknowledgements have been published elsewhere (Jackson 1973, 1975; Jackson and Moore 1976; Jackson et al. 1978).

Additional information

Funding

Environment and Climate Change Canada.