Geological Society of America

T11 Oxygen and Ecosystems from the Proterozoic to the Paleozoic

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Room 607

A PALEOZOIC RECORD OF URANIUM ISOTOPES CONSTRAINING FRACTIONATION FACTORS IN FERRUGINOUS SETTINGS

Paper number: T11-14
Program: Technical Sessions
Time: 11:30 AM – 11:45 AM

Authors

Devon B. Cole Presenting Author
Yale University, New Haven, CT
Daniel J Wilkes
Lawrence University, Appleton, WI
Martha Longley
Yale University, New Haven, CT
Xiangli Wang
Yale University, New Haven, CT
Erik A. Sperling
Stanford University, Palo Alto, CA
Andrew Knudsen
Lawrence University, Appleton, WI
Noah Planavsky
Yale University, New Haven, CT

Abstract

Uranium isotopes in shales are now widely used as a marine redox proxy through much of Earth’s history. However, the current understanding of fractionation factors during the reduction of U and removal to siliciclastic sediments is lacking. Foremost, there are minimal constraints on the fractionation of U isotopes under ferruginous conditions, which are thought to be relatively widespread in the Paleozoic based on evidence Fe speciation studies. Here we present more than 300 new U isotope data points from primarily the Paleozoic, along with additional data from the Precambrian and modern sediments. We find that; 1) there are a large range of values in both temporally equivalent strata and within individual formations, 2) based on maximum fractionations, there is evidence that seawater was generally within 0.1 ‰ of modern, 3) average fractionations in ferruginous settings have likely been overestimated in global isotope mass balance models. In sum, we suggest that these U isotope values are consistent with a large solution space dependent on the dominant reduction pathway, mass accumulation rates, and organic matter loading and are consistent with both largely oxygenated and largely anoxic depositional.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304742

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