Grain size of Miocene volcanic ash layers from Sites 998, 999, and 1000: Implications for source areas and dispersal

Document Type

Conference Proceeding

Date of Original Version



Crystal size measurements have been carried out on tephra fall layers of Miocene to Holocene age from Sites 998, 999, and 1000 in the western Caribbean Sea. Maximum crystal size is used as a proxy for the grain-size characteristics of the layers and an index of atmospheric dispersal from source eruptions. Crystal sizes range from 50 to 650 μm with the majority falling between 200 and 300 μm. All three sites exhibit a coarsening in the grain size of tephra layers with increasing age to the early Miocene that broadly correlates with an increase in the frequency of layers. Analysis of the present lower and upper level atmospheric circulation in the western Caribbean suggests that the layers were derived from source eruptions to the west of the sites somewhere in the Central American region. Minimum distances to these sources are on the order of 700 km. Crystal sizes in tephra layers at these distances are consistent with their derivation from energetic pyroclastic flow-forming eruptions that ejected tephra to stratospheric levels by large-scale ignimbrite- and Plinian-style plumes. Coarsening of the layers during the Miocene peak of explosive volcanism cannot be attributed to any major change in paleowind intensity and is taken to represent the occurrence of more energetic eruptions that were able to disperse tephra over larger areas.

Publication Title, e.g., Journal

Proceedings of the Ocean Drilling Program: Scientific Results