Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion -New Dates

Geodynamo Recharged

About 560 Ma there was a transition from a weak, erratic geomagnetic field to one that is more stable inferred from palaeomagnetic measurements which have suggested that the inner core may have solidified about 560 Ma, which is much later than has been believed.

Driscoll, P. (2019). "Geodynamo recharged." Nature Geoscience 12(2): 83-84.

During the Ediacaran the geomagnetic field of the Earth was of ultra-low intensity which inferred that the inner core was young

The age of the solid inner core of the earth has been an enduring mystery. Thermal conductivity values that are plausible, though contrasting, lead to growth of the inner core ages of initiation that span 2 Gyr (billion years), from about 0.5 to more than 2.5 Ga. A direct probe of past core conditions is provided by palaeomagnetic data, though heretofore field strength data were lacking for the younger inner core ages of onset that had been predicted. In this paper Chen et al. present palaeointensity data from the Ediacaran, about 565 Ma, Sept- Îles intrusive suite measured on single plagioclase and clinopyroxene crystals that hosted single-domain magnetic inclusions. A time-averaged dipole moment of ~0.7 x 1022 A m2, the lowest value that has been reported to date for the geodynamo from extant rocks and more than 10 times smaller than the strength of the field of the present. 2 polarities that had unusually high angular dispersal (S=~26^o) at a low latitude were defined by palaeomagnetic directional studies of these crystals. As well as 14 other directional datasets that suggest a hyper-reversal frequency, these extraordinary low field strengths suggest an anomalous field behaviour, which is consistent with geodynamo simulations, high thermal conductivities and an onset age of the Ediacaran inner core growth.

Bono, R. K., et al. (2019). "Young inner core inferred from Ediacaran ultra-low geomagnetic field intensity." Nature Geoscience 12(2): 143-147.

New Ages have been found for the Ediacaran-Cambrian Boundary strata in south china have been found by U-Pb Dating

The Ediacaran – Cambrian transition is a critical interval which marks drastic biological, oceanic and geochemical co-evolutions in geological history, though it has been constrained in South China. In this paper Chen et al. present 2 new sets of sim U-Pb zircon ages from the Ediacaran boundary strata (Dengying, Liuchapo and Niutitang formations) that had been deposited on the slope-basin environments of carbonate platforms. In the basal and mid-upper Liuchapo Formation, respectively, the first direct age set for the Ediacaran-Cambrian boundary in South China is provided in slope and basal settings. The base of the overlying Niutitang Formation provided 2 more U-Pb ages, 524.2 ± 5.1 Ma and 522.3 ± 3.7 Ma which indicated that this widespread unit in South China was deposited was deposited about 20 Ma, after the onset of the Cambrian.

Chen, D., et al. (2015). New U–Pb zircon ages of the Ediacaran–Cambrian boundary strata in South China., Terra Nova, 27, 62–68, 2015

Cambrian Explosion – Marine redox Fluctuation as a Potential Trigger

During the latest Neoproterozoic and Early Cambrian, the diversification of metazoans has been attributed to, among other factors, a progressive rise in the surface levels of oxygen. Recent results have, however, questioned the idea of a prominent rise in atmospheric oxygen or a major or unidirectional shift in the marine redox landscape across this interval. In this paper Wei et al. present carbonate-associated uranium isotope data from the Upper Ediacaran to the Lower Cambrian marine carbonate successions. Evidence is provided by these data for short-lived episodes of widespread marine anoxia near the transition between the Ediacaran and the Cambrian and during Cambrian Age 2, about 525 Ma. Wei et al. suggest that biotic turnover and the ecological reconstruction that resulted, that was triggered by marine redox fluctuations, and not progressive oxygenation, were dominant drivers of the Cambrian Explosion. Wei et al. suggesting that episodes of harsh environmental conditions against a backdrop of Proterozoic-Phanerozoic oxygenation of the ocean on the eve of the Cambrian Explosion could have, by way of promoting the reconstruction of ecosystems, spurred the diversification of the Cambrian Evolutionary Fauna.

Wei, G., et al. (2018). Marine redox fluctuation as a potential trigger for the Cambrian explosion.

[We would be interested if someone could tell us if it is being considered that the closeness in time between the onset of the much stronger geomagnetic field about 560 Ma could have led to the Cambrian Explosion that involved the development of such an expansion of more complex life forms in the changed conditions. We assume that the change from the ultra-low geomagnetic field intensity of the Ediacaran to the much higher intensity geomagnetic field after about 560 Ma would be much more conductive to complex life, as the radiation reaching the surface of the Earth would have been much reduced.] MHM & EHK.

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