PROBING THE NEOPROTEROZOIC HYDROLOGIC CYCLE FROM SILICATE WEATHERING INDICES: APPLICATIONS FOR SNOWBALL EARTH?

The Snowball Earth hypothesis (Hoffman et al., 1998) predicts drastic changes in the hydrologic cycle from near “shut downs” associated with widespread/global glaciations to intense hothouse conditions involving extensive chemical weathering. Silicate weathering indices, designed to quantify stratigraphic trends in weathering from bulk compositions, represent prospective tools for investigating these predictions. Typically, these compare abundances of mobile (Ca, Na, Mg, K) versus immobile (Al, Ti) cations from silicate source rock.

We evaluated the utility of five common silicate weathering indices [Chemical Index of Alteration (CIA), Nesbitt and Young, 1982; Plagioclase Index of Alteration (PIA), Fedo et al., 1995; Chemical Index of Weathering (CIW), Harnois, 1988; Weathering Index of Parker (WIP), Parker, 1970; and Vogt's Residual Index (V), Vogt, 1927] to interpret weathering trends in reconnaissance samples from the Tambien group of N. Ethiopia. The Tambien Group is a Sturtian age (ca. 740-720 Ma) metasedimentary succession of intercalated slates and limestones, capped locally by a diamictite of probable glacial origin (Miller et al., 2003). It overlies an older island arc metavolcanic sequence (Tsaliet Group), the presumed source terrane for Tambien slates.

Major and minor element compositions and mineralogy of Tambien slates from four localities, were determined by XRF, ICP-OES, and XRD. Four of the five silicate weathering indices show up-section decreases in values: CIA 78-68, PIA 92-74, CIW 94-78, V 4-1.4, implying decreased weathering with increasing proximity to the diamictite. Interestingly, Sr isotopic compositions of well-preserved Tambien limestones also decrease up-section (0.7067 to 0.7059) with proximity to the diamictite interval. This trend indicates a relative increase in hydrothermal Sr input, which could be interpreted as a decrease in continental weathering due to widespread glaciation.

The weathering indices combined with Sr isotope trends possibly indicate a slowing of the hydrologic cycle approaching the diamictite, consistent with the Snowball Earth Hypothesis. The apparent stratigraphic weathering trend preserved in Tambien metasediments shows potential for further evaluations in other prospective Snowball Earth sequences, but future work should include more systematic stratigraphic sampling.