2000 Annual Science Report
Harvard University Reporting | JUL 1999 – JUN 2000
Redox-Sensitive Metals and Environmental Evolution
There are three primary components to the ongoing research at the University of Rochester in collaboration with the Harvard Team:
a) Conceptual study of the impact of the changing Precambrian redox environment on the availability of key nutrient metals (especially Fe and Mo).
b) Measurement of concentrations of redox-sensitive metals in Proterozoic sediments to place constraints on atmosphere/ocean redox evolution.
c) Exploration of the use of metal isotope fractionation (esp. Fe and Mo) to study paleoredox conditions and changes in the biological use of metals through time.
Component (a) has developed into a draft manuscript coauthored by Anbar and Knoll. This manuscript discusses the likely impact on Fe and Mo abundances of a sulfidic Mesoproterozoic ocean, as postulated by D. Canfield. Because of the extreme redox sensitivity of Fe and Mo, these metals would have been scarce in seawater under such conditions with profound consequences for the biological nitrogen cycle and eukaryote evolution. These concepts were presented at the Fall ’99 AGU Conference. This component is ahead of schedule, as it was not among the proposed goals. Published results are anticipated in Year 3.
Component (b) has involved efforts by Anbar, Arnold, Ramon and Barling to develop and apply methods to measure Mo, Re, U, V and other trace metals in Proterozoic carbonaceous shales. These methods involve sample digestion and analysis using an inductively coupled plasma mass spectrometer (ICP-MS) at the University of Rochester. This work is being coordinated with the similar efforts of H. D. Holland (also a Harvard Team member), who is focussing on Archean shales. Both projects are aimed at looking for evidence of changes in the geochemical cycling of redox-sensitive metals as a means of constraining the redox evolution of the environment. At Rochester, methods have been finalized in the past 12 months, and work has begun on Mesoproterozoic sediments obtained by A. Knoll from the McArthur Basin, Australia. This component is slightly behind schedule because of the delayed start in funding, but should ramp up in Year 3, with published results anticipated in Year 4.
Component© is a completely novel application involving extensive development of new analytical methods, and determination of the mechanisms by which metal isotopes are fractionated in nature. This work involved Anbar, Barling, Roe, Knab, Polizzotto, Carti and Nealson. In Year 2, we developed methods for Fe and Mo isotopic analysis, and observed variations in natural materials. Mo variations suggest a possible application for paleoredox studies which will be a major area of emphasis in Year 3. In work sponsored primarily by the JPL/Caltech Team, we demonstrated that nonbiological chemical processes can fractionate Fe isotopes. This has important implications for biomarker applications. These projects led to presentations at both the 1999 AGU and GSA conferences, the 1999 V. M. Goldschmidt Conference, and a publication in Science in April, 2000.
In addition, during this period we submitted for publication (to JGR- Planets) a manuscript on the influence of impacts on on the Hadean (> 3.85 Ga) surface environment. This manuscript is an outgrowth of Ir and Pt measurements in Akilia Island metasediments conducted by Arnold in Year 1. Coauthors on this publication are S. Mojzsis (UCLA Team) and K. Zahnle (NASA Ames Team). Our interpretation of the data leads to the conclusion that the Earth’s surface was habitable during most of the “Late Heavy Bombardment”.
PROJECT MEMBERS:Ariel Anbar
E. (Beth) Holman
RELATED OBJECTIVES:Objective 5.0
Describe the sequences of causes and effects associated with the development of Earth's early biosphere and the global environment.
Define climatological and geological effects upon the limits of habitable zones around the Sun and other stars to help define the frequency of habitable planets in the universe.
Determine the resilience of local and global ecosystems through their response to natural and human-induced disturbances.