Precambrian Dynamo Reversal Behavior from Polarity Ratios

Numerous speculations exist regarding the abundance of geomagnetic superchrons through Earth history, as a function of secular cooling and inner core growth; however, data have generally been evaluated only on a regional basis. Global assessments of geomagnetic polarity require robust continental reconstructions, and only recently have these been generated with paleomagnetic compatibility and kinematic viability. Geomagnetic polarity ratio is the average ratio of normal polarity to total polarity in a series of discrete paleopole observations. We compile paleomagnetic polarity ratios from 0-335 Ma and 900-2200 Ma, with the goal to compare geodynamo polarity ratio behavior from the Phanerozoic to the Precambrian and to assess whether Precambrian superchrons can be identified. First, we compute polarity ratio, represented as percent normal (%N), from a 0-335 Ma compilation of paleopoles and identify the best timescale of polarity ratio averaging (tau) for distinguishing the two known superchrons during this record. We find that for tau≈20 Myr both the Cretaceous Normal Superchron (CNS) and the Kiaman Reverse Superchron are discernible in the polarity ratio series. Second, we compile paleopole data from the global paleomagnetic database with strict quality criteria and age precision within +/-15 Myr (a nominal superchron duration). We find that the Precambrian %N series using tau=20 Myr does not contain obvious superchrons like the 0-335 Ma record, but there is evidence for low frequency modulation from periods with prolonged low (<30%) to high (>70%) polarity ratio, similar to the 0-335 Ma record. Third, we test whether it is possible for superchrons to be identified in the 0-335 Ma set with the much lower sampling frequency of the Precambrian data set. This is done by subsampling the 0-335 Ma record with a random set of Precambrian sample age spacings. The procedure is then reproduced in a bootstrap, producing an average superchron identification success rate. If the success rate is high then we expect it to be possible to identify superchron-like behavior in the Precambrian, and whether such behavior is quasi-periodic like the ~200 Myr period between the three known Phanerozoic superchrons.