A MULTI-BASIN COMPARISON OF IRON SPECIATION RECORDS OF THE CAMBRIAN SPICE EVENT

The late Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) is recognized as a global oceanographic event recorded as positive shifts in the carbon (δ¹³C), and sulfur (δ³⁴S) isotopic records that are linked temporally with marine extinctions on several paleocontinents. While these trends are thought to result from an increase in the areal extent of marine anoxia and possibly euxinia (water-column H₂S), relatively little work has focused directly on characterizing the (local) redox conditions during the event. Here we employ iron speciation analyses to explore the redox history of the SPICE event from four different basins representing a range of depositional conditions (e.g., sedimentation rate, biological productivity, paleobathymetry, etc.) in an effort to tease apart local controls from its broader global expression. Specifically, we examined successions from Avalonia (central England), Baltica (southern Sweden), Laurentia (eastern U.S.), and North China (South Korea). Our iron speciation analyses broadly indicate a shift towards more reducing conditions during the SPICE at each location. However, with the exception of the location in Baltica, where persistent euxinia accompanied the event, the anoxic conditions we record elsewhere are dominantly ferruginous (water-column Fe²⁺) suggesting euxinia likely remained limited to areas where conducive environmental conditions existed. Importantly only the succession in Laurentia, deposited in a relatively shallow intrashelf basin, records a local shift from oxic to anoxic conditions during the SPICE. Intriguingly, Laurentia is also the lone paleocontinent of the four compared here to experience a significant pulse of marine extinction at this time, suggesting that the deoxygenation of shallower marine environments may have been a factor driving this event. These results provide a more nuanced picture of the heterogeneous redox conditions that accompanied the SPICE along with illustrating an important environmental mechanism that may help explain the observed pattern of biological turnover.