Uranium Isotope Variations in Carbonates: Validating A New Paleoredox Proxy

Project: Research project

Project Details

Description

We propose to investigate a new paleoredox proxy in carbonate rocks: Variations in the relative abundances of long-lived isotopes of uranium (U) namely238U and 235U arising from redox-dependent isotope fractionation. In complementary studies, we developed and implemented analytical methodologies for high precision U isotope analysis, surveyed variations in nature (reported as #238U), and are characterizing the basic systematics of this new isotope system. Here, we propose to examine #238U variations in carbonate rocks deposited before, during and after one of the large ocean redox perturbations in the last 500 million years: The Permian-Triassic Boundary (PTB). Our motivations are: (a) to validate the use of U isotopes in carbonate rocks as a global ocean proxy by examining multiple sections during a period of known global ocean redox change; and (b) to see if this proxy provides new insights into the timing and tempo of ocean redox change during the PTB crisis, representing the largest mass extinction in Earth history. The proposed work builds on the recent discovery by the project team of a significant U isotope shift in carbonate rocks deposited in a single section at or shortly before the PTB extinction horizon (Brennecka et al., 2011). If we assume that these data accurately represent seawater #238U and that ocean #238U act as a global proxy, then the data from this pilot study are consistent with expectations of a global ocean redox perturbation. At the same time, they indicate that pervasive whole-ocean anoxia did not exist until at, or immediately preceding, the extinction horizon (within ~ 100 kyr). This inference is at odds with a scenario of protracted open-ocean anoxia commencing in the early or mid-Late Permian that is widely inferred based on changes in lithology in deep-marine sections. However, it is consistent with more recent interpretations that are informed by data emerging from other proxies in shallow-water sections. To test the assumption that #238U in ancient carbonates faithfully records the U-isotope composition of contemporaneous seawater, we will analyze #238U in different pools within carbonate rocks from multiple sections, separated by sequential extraction, and will compare variations in #238U with a variety of other geochemical parameters and proxies. This detailed investigation will allow us to deconvolve the effects of varying U sources and diagenesis on #238U signatures. Interpretation will be guided by a complementary NSF-supported investigation of #238U systematics in carbonate sediments in a modern analog environment (the Bahamas) being carried out by the PI and ASU Co-I. To test the assumption that #238U is a global ocean proxy, we will carry out these measurements in 10 carbonate sections from the Tethys Ocean and in 2 siliclastic sections from the Panthalassic Ocean. All samples are in our possession and some supporting analyses are already underway. If #238U is a valid global proxy then we expect to see the same variations in #238U in all cores, once corrected for the effects of changing sources and diagenesis. On the other hand, differences in #238U among the cores may provide new insights into the effects of PTB paleogeography on ocean mixing. This research is relevant to the 2008 NASA Astrobiology Roadmap. The development of novel paleoredox proxies helps us to: (1) understand the nature and distribution of habitable environments; (2) determine how life on Earth and its planetary environment have co-evolved through geological time; and (3) recognize signatures of life on other worlds and on the early Earth. FORM NRESS-
StatusFinished
Effective start/end date9/1/138/31/17

Funding

  • NASA: Goddard Space Flight Center: $619,292.00

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