Precise ages of the Réunion event and Huckleberry Ridge excursion: Episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core

- 40Ar/39Ar ages of lavas define a singular Réunion excursion at 2.200 Ma.
- The Feni subchron is bounded by polarity reversals at 2.153 and 2.115 Ma.
- 40Ar/39Ar and U-Pb dating indicate age of Huckleberry Ridge excursion is 2.077 Ma.
- A temporal clustering of dynamo instabilities resembling those of the Brunhes chron.
- Terms “Réunion I” and “Réunion II” should be abandoned.

The Réunion event is one of the earliest recognized periods of normal polarity within the reversed Matuyama chron. Named for the site at which it was first discovered on Réunion Island, it has since purportedly been found globally in both volcanic rocks and sediments, and thus has become a key chronostratigraphic marker. However, geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration, but also the number of Réunion events. New 40Ar/39Ar ages from eight Réunion Island lavas in three distinct sections are indistinguishable from one another, thereby placing the event at 2.200±0.007/0.010 Ma (±2σ analytical/total uncertainty, note this format is used throughout the paper). The paleomagnetic behavior recorded at two of the island sites shows that the opposite (normal) polarity was reached and sustained for a period during which several lava flows were erupted. Whether this can be classified as a very short subchron bounded by a rapid set of back-to-back reversals, or as a special case of a geomagnetic excursion, is unclear. Hence, we choose to continue labeling the dynamo activity recorded by these Réunion Island lavas as an “event”. This event preceded a ∼38 kyr period of normal polarity that we name the Feni subchron after its locality of discovery at ODP site 981. The Feni subchron was succeeded by the Huckleberry Ridge excursion for which 40Ar/39Ar sanidine and U-Pb zircon ages of 2.077±0.001/0.003 Ma and 2.084±0.012/0.013 Ma, respectively, from member B of the Huckleberry Ridge tuff in Idaho, are in agreement. These findings suggest that the full normal polarity recorded on Réunion Island is a singular brief period of unstable field behavior at the onset of a ∼125 kyr bundling of dynamo instabilities from 2.20 to 2.07 Ma. Disturbances to the axial dipole component of earth's magnetic field during this period, and by analogy similar periods of temporally-clustered excursions during the early and late portions of the Brunhes chron, may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner-core or core-mantle boundaries; a proposition that might be tested by future numerical dynamo simulations.