The Silurian Mulde Event and a scenario for secundo–secundo events

Abstract

Graphic correlation using graptolites and conodonts provides a high-resolution timescale for correlating from coastal to deep oceanic sections and, thereby, also a detailed record of the sequence of changes during the Mulde Secundo-Secundo Event. That interval includes sedimentary facies otherwise unknown in older Wenlock to early Ludlow strata on Gotland. The identified sequence of changes includes a detailed record of, in order: two extinctions (Datum points 1 and 1·5); widespread deposition of carbon-rich sediments extensive enough to cause a δ¹³C increase of c. 4.8‰, the onset, maximum and end of a sea-level fall and rise of at least 16 m during 30 kyr; a third extinction (Datum 2); a disaster fauna; and a slow faunal recovery. Thus, a secondary result of the event was a weakened greenhouse effect triggering a glaciation: the Gannarve Glaciation (new term). The order of changes proves that regression did not cause the extinctions. Faunal and sea-level changes, as well as the sedimentary succession, fit well with predictions based on an oceanic model. Extinctions were primarily caused by a severe drop in primary planktonic productivity, causing starvation among planktonic larvae in non-coastal settings. The Grötlingbo Bentonite (new term), the thickest in the Wenlock of Gotland, was deposited across the basin shortly after Datum 2. Temporal resolution is high enough to permit some comparison with Quaternary glaciations.