CHASING GLACIERS: CHEMICAL WEATHERING TRENDS IN A POSSIBLE NEOPROTEROZOIC GLACIAL SEQUENCE FROM TIGRAI, N. ETHIOPIA

The Late Neoproterozoic Era spanned a series of unprecedented glacial-interglacial (freeze/fry) fluctuations of Earth's climate, which uniquely define the Cryogenian Period. Competing hypotheses, under the popularized “Snowball/Slushball Earth” (SEH) moniker, invoke extreme variations in greenhouse gas concentrations that profoundly influenced Earth's hydrologic cycle, and therefore the intensity of chemical weathering. Weathering indices (WI) expressing the ratio of immobile-to-mobile cations preserved in detrital aluminosilicates should record these variations, but this potentially informative paleoclimatic tool has yet to be widely applied in Cryogenian studies.

Here, we evaluate the weathering history of the Tambien Group, a prospective Sturtian age (ca. 775-720 Ma) Cryogenian sequence from N. Ethiopia (Tigrai Provence). The Tambien Group is a metasedimentary succession of intercalated slates and limestones, capped locally (Negash) by a diamictite of probable glacial origin. It overlies an older island arc metavolcanic sequence (Tsaliet Group), the presumed source terrain for Tambien slates. Compositional (XRF, XRD) and petrographic analyses of 60 samples from the Tsaliet-Tambien lithostratigraphic sequence at three localities (Negash, Mai Kenetal, Abergele) comprise our composite dataset. Preliminary WI values [Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW)] for the Negash Upper Tambien transition to diamictite deposition significantly decrease (~97 to ~72). The basal Mai Kenetal transect also shows a transition to lower weathering rates (PIA and CIW values drop from ~97 to ~86), and a potential cap carbonate immediately overlying this sequence may indicate an earlier Tambien glacial sequence.

These up-section decreases are consistent with waning weathering intensity, as speculated for SEH glacial transitions. Although these results suggest that WIs may be useful for future SEH studies, important caveats remain. Uncompensated carbonate can greatly reduce calculated WI values through non-silicate Ca addition. Source rock provenance also determines applicable weathering indices and their interpreted degrees of weathering. Ongoing XRD and petrographic evaluation will further assess these variables.