I. Molybdenum isotopes: Potential paleoredox probes. II. Iron isotope variations in natural materials measured using high mass resolution multiple collector ICPMS. III. Extraterrestrial iridium, sediment accumulation and the habitability of the early Earth's surface.

摘要

Part I. The molybdenum isotope system is emerging as an indicator of global redox conditions. The basis of this application is the relatively small (0.5‰; delta97/95Mo) fractionation between Mo in seawater and sediments accumulating under euxinic conditions, and the comparatively large (∼2‰) fractionation between Mo in seawater and ferromanganese crusts and nodules (Barling et al., 2001; Siebert et al., 2003). The latter effect probably dominates Mo isotope fractionation in the oceans. The Mo isotope composition of seawater should therefore vary over geologic time with the extent of global ocean anoxia: shifts toward lighter values would reflect decreased removal to Mn oxides associated with expanded anoxia. Such variations may be recorded in delta97/95Mo of black shales. A shift of 1.1‰ toward values lighter than today is seen in mid-Proterozoic black shales, consistent with expanded anoxia in the oceans at this time.; Part II. We present the first measurements of Fe isotope variations in chemically purified natural samples using high mass resolution multiple-collector inductively coupled plasma source mass spectrometry (MC-ICPMS). High mass resolution allows polyatomic interferences at Fe masses to be resolved (especially, 40Ar14N+, 40Ar16O+, and 40Ar 16OH+). These findings demonstrate the utility of high resolution MC-ICPMS for high precision Fe isotope analysis in geologic and other natural materials. They also highlight the importance of internal monitoring of mass bias, particularly when using routine methods for Fe extraction and purification.; Part III. We have measured the concentrations of Ir and Pt in ancient meta-sediments from Akilia Island, southern West Greenland, to obtain the first direct constraints on the extraterrestrial mass flux at the Earth's surface before 3.8 Gyr. The lunar impact record indicates this was a time of heavy bombardment, hence heightened Ir and Pt abundances are possible in sediments from this time. The observed concentrations of Ir and Pt in the Akilia meta-sediments are extremely low. The observations are explained if rapidly accumulating sediments sampled stochastic bombardment by an impactor population, such that exogenous Ir and Pt are concentrated in rare horizons. This explanation has two consequences. (1) Evidence of massive impact events may be rare in sediments older than 3.8 Gyr despite heavy bombardment. (2) Conditions at the Earth's surface may have been conducive for extensive accumulation of sedimentary sequences and for microbial life during most of the period 4.15--3.8 Gyr.