The Cambrian Explosion Was Enabled by Plate Tectonics
Abstract
Life was present on Earth by 3.6 or 3.5 Ga, but until the latest Proterozoic was limited to single-celled organisms. Soft-bodied multicellular organisms first appeared about 600 Ma during Ediacarian time, known only from soft-sediment impressions; it is unclear even whether they were plants or animals. Then came the Cambrian explosion of evolution, starting about 540 Ma. Within about 50 m.y. most phyla of modern animals arose, including the chordates to which we belong. Some animals were a meter long. The causes of evolution across those two great thresholds, about 600 and 540 Ma, have been subjects of much debate. The answer may lie in the onset of plate tectonics.
A multitude of geological and geophysical data suggest that plate tectonics began only in the latest Proterozoic. The Archean was an era of internally mobile crust, with vertical motions resulting from variations in density, reflected in the deposition and collapse of basins. Tonalite-trondhjemite-granodiorite (TTG) crust formed by the hydrous partial melting of a mafic protocrust. Archean crust is granite and greenstone with no modern analogue. The Phaenerozoic has been the era of plate tectonics. Near the end of the Proterozoic, downward recycling of fusible components enabled a weak asthenosphere to develop over which lithospheric plates could slide; shallow materials cooled and densified enough to subduct in a plate-like fashion. Paleomagnetic measurements, proposed to constrain pre-Phanerozoic plate tectonics, have been questioned in recent studies; Earth's internal strong dipolar magnetic field may only have developed at ~600 Ma. Numerical modeling of convection in the core and data relating to pre-Phanerozoic magnetic field strength suggest that Earth's strong dipolar field may have developed only when the solid inner core began to form, an event that probably occurred in the latest Proterozoic. If this is correct, the evidence for plate tectonics in pre-Phanerozoic eons is weak. Organic evolution may have been enabled by the late-Proterozoic changes in lithospheric dynamics that began plate tectonics and radically changed atmospheric composition. Development of Earth's strong dipolar magnetic field at the same time largely blocked cosmic rays and high-energy ultraviolet light to allow profusely varied life to exist on Earth's surface.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP54A..04H
- Keywords:
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- 0448 Geomicrobiology;
- BIOGEOSCIENCES;
- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCES;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE