A Late Miocene Reduction in the Spatial Breadth of Marine Tephra Occurrence: A Five-Basin Test of the Volcanic-Pulse Hypothesis

Bryan Keith Sell

Episodic "pulses" of explosive volcanism have been widely invoked to explain climatic and
biotic events across the Phanerozoic, but rarely tested against a high-resolution, multi-basin
occurrence record. I tested for non-random temporal clustering of explosive volcanism using
VOLCORE, a compilation of 34,696 visible tephra layers recovered by ocean drilling from
across the world’s ocean basins. Using a site-presence statistic designed to resist distortion from
unusually prolific sites and correcting for sampling effort via independently dated age-control
points, I find no evidence for discrete global pulses of anomalously high activity: every
candidate excess identified during exploratory analysis failed to replicate across basins or did not
survive correction for a bin-edge artifact. I instead find a robust, inter-basin reduction in the
spatial breadth of tephra occurrence—the fraction of independent drill sites recording any ash
layer—spanning approximately 6.5-13 Ma. This narrower geographic footprint of tephra
occurrence is present in the West Pacific, East Pacific, Indian Ocean, and Southern Ocean, and in
the Atlantic once one geologically atypical site is excluded and survives block-bootstrap
resampling at the level of independent drilling expeditions. A parallel analysis using raw
occurrence counts rather than site presence does not produce the same signal, a discrepancy
traced to a single expedition supplying over half of one region's window-period layers from a
single locality. I interpret this as evidence that the marine tephra record does not support discrete
volcanic pulses, but does support a genuine, multi-basin decline in the spatial extent of explosive
volcanism during the Late Miocene, distinct from any change in eruptive magnitude.