5 Drivers of Continental Magmatism (Posters)

Tuesday, 23 May 2023
8:00 AM - 6:00 PM
C/D Ballroom

THE HYPOTHESIS OF LARGE-SCALE TEPHRA FALL PRESENTS A COMPLEX EARTH SCIENCE CHALLENGE THAT DEMANDS AN INTEGRATED SCIENTIFIC EVALUATION

Paper number: 5-3
Program: Theme Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

The Large-Scale Tephra Fall (L-STF) hypothesis proposes that exploded volcanic tephra filled the atmosphere in huge volumes more often than is appreciated by geoscience. Large Igneous Province (LIP) emplacements contributed eolian ash to augment arc-generated volcaniclastics and form black shales near times of Oceanic Anoxic Events (OAEs). Before the Phanerozoic, black shales are linked to LIPs in the “boring billion” at 1,383 Ga. Rapid fluctuations in benthic oxygen levels, not persistent anoxia, characterized the OAE depositional environments of the Eagle Ford, the Middle Eastern Qusaiba, and the Appalachian Basin late Devonian shales. The term “OAE” has persisted beyond its usefulness; a more accurate descriptor is “Global Magmatic and Volcanic Event” (GMVE).

Similar to compounds (ringwoodite) that supplied oxygen to form earth’s oceans, oxidized iron has been identified deep in the earth’s mantle. Already oxidized iron erupted onto earth’s surface became Banded Iron Formations (BIFs). BIFs, those rocks considered proof of “ferruginous oceans” and the Great Oxygenation Event (GOE), are not evidence of those things, but evidence of primitive earth’s large-scale mafic volcanic tephra fall. BIF chert bands are from altered volcanic glass. Haugaard et al (2016) writes, “...the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways, thereby making the Joffre BIF poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses.”

In organic-rich shales, U, V, and Mo are supplied by diagenetic alteration of volcanic tephra. The authigenic minerals they form did not precipitate from saturation concentrations in the water column, therefore are not “hydrogenous” (Tribovillard, 2006) and cannot accurately be used to assess paleo-oxygenation levels of the ocean water column.

When paleoclimatologists observe increased siliciclastic deposition into a basin and conclude the cause is climate-induced “enhanced continental weathering”, they are most often ignoring massive volcaniclastic explosions into that upstream drainage basin.

Understanding that L-STF occurred will simplify and improve our understanding of many aspects of geological science once an integrated scientific evaluation has been accomplished.

Geological Society of America Abstracts with Programs. Vol. 55, No. 5

doi: 10.1130/abs/2023RM-387709

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