NEAR-FREEZING LOW-LATITUDE SURFACE WATERS DURING PENNSYLVANIAN GLACIATION: GLENDONITES FROM EQUATORIAL WESTERN PANGAEA

We report the first absolute constraints on Pennsylvanian sea-surface temperatures from low-latitude Western Pangaea and initial climate model results to simulate these temperatures. Glendonites occur in two shallow subtidal carbonate intervals (meter-scale) within the lower Moscovian (upper Atokan) succession of the Bird Spring Formation in Arrow Canyon, Nevada. These carbonates were deposited when southeastern Nevada lay within a few degrees of the equator. Glendonites are calcified and silicified pseudomorphs after the hydrated CaCO₃ phase ikaite, which forms naturally only in waters < 7 ºC and typically < 4 ºC.

Glendonites in the Arrow Canyon carbonates occur in mid- to upper-parts of two shallowing upward meter-scale cycles. Lower facies of cycles are heterozoan wackestones that contain scattered phosphatized grains and abundant calcite-filled molds after sponge spicules. Upper facies of cycles are photozoan packstones and grainstones that contain coated grains, red and green algae, dm-scale Auloporid tabulate corals, and desiccated tidal laminites. Glendonites occur in spiculitic wackestones and in the muddiest zones of photozoan packstones. Sedimentologic evidence indicates that these glendonites grew below wavebase but locally within the photic zone. The co-occurrence of phosphatized grains, abundant spicule molds, and cool-water faunas (red algae, thin-walled brachiopods) with glendonites suggests that cold, nutrient-rich waters welled up onto the Bird Spring Shelf.

Near-freezing surface waters in this marginal seaway place important constraints on climate models for this time. Our simulations of this time period using an atmosphere-only global climate model (GENESIS) fail to predict such cold temperatures. GENESIS simulates near-equatorial, minimum sea-surface temperatures of ~20 °C in an experiment with low pCO₂ (140 ppmv) and a large continental ice sheet on southern Gondwana. We have begun a series of new ocean-atmosphere simulations for this time period that include ocean circulation in order to further investigate the origin of exceptionally cold, low-latitude surface waters implied by glendonites in Arrow Canyon.