Earth and Environmental Science Transactions of the Royal Society of Edinburgh

Research Article

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

Lennart Jeppssona1 and Mikael Calnera2

a1 Department of Historical Geology, Division of Geology, Sölvegatan 13, SE-223 62 Lund, Sweden. e-mail: Lennart.Jeppsson@geol.lu.se

a2 Department of Historical Geology, Division of Geology, Sölvegatan 13, SE-223 62 Lund, Sweden. e-mail: Mikael.Calner@geol.lu.se

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.

(Received July 23 2001)

(Accepted September 06 2002)

KEY WORDS

  • Conodonta;
  • extinctions;
  • glaciation;
  • graptolites;
  • high-resolution stratigraphy