The Geological Succession of Primary Producers in the Oceans

This chapter explores two records of past primary producers: morphological fossils and molecular biomarkers. Because these two windows on ancient biology are framed by such different patterns of preservational bias and diagenetic selectivity, they are likely to present a common picture of stratigraphic variation only if that view reflects evolutionary history. Diagenesis can obliterate fossils as well as preserve them: organic walls are subject to postdepositional oxidation and mineralized skeletons may dissolve in undersaturated pore waters. The molecular and morphological records of eukaryotic predominance in shelf primary production are mirrored by indications of relatively low cyanobacterial contributions. Fossils, molecular biomarkers, molecular clocks for individual clades, and the sedimentary silica record all disclose that the modern phytoplankton has Mesozoic roots. Paleozoic primary production is described in detail. Microfossil and biomarker molecules both suggest that green algae played a greater role in marine primary production than they have in the past 100 million years, and biomarkers also suggest a significant role for cyanobacterial production on continental shelves. The later part of the chapterdiscusses about proterozoic primary production and also explains the prokaryotic fossils, eukaryotic fossils, and proterozoic molecular biomarkers. The chapter concludes that improved understanding of Earth's redox history and the evolutionary record of animals and land plants will provide the framework needed to interpret the evolutionary history of marine photoautotrophs as it continues to emerge.