A RECORD OF GLACIAL ARIDITY AND MILANKOVITCH-SCALE FLUCTUATIONS IN ATMOSPHERIC DUST FROM THE PENNSYLVANIAN TROPICS

Atmospheric dust is an important climatic agent as well as a valuable climatic archive. This study documents systematic variations in a single Upper Pennsylvanian, shallow-water carbonate buildup of the Midland Basin (West Texas) that was isolated from fluvio-deltaic input and consists almost entirely of carbonate bounded by sequence boundaries. Thin (0.02 to 1 m) intervals of dark, pyritic, siliciclastic mudrock with subtle pedogenic features occur commonly at the sequence boundaries, and are interpreted as loessite deposited during lowstands, primarily on the basis of paleogeographic isolation of the buildup. The silicate mineral fraction (SMF) extracted from the underlying carbonate, which includes quartz, clay, and trace feldspars and heavy minerals, varies in concentration (weight %) from 0.015% to 6.69% in the carbonate. This SMF contains both fine-grained detrital and authigenic silica interpreted to be eolian on the basis of geochemical, petrographic, and grain-size characteristics as well as paleogeography; hence, the SMF is considered a proxy for atmospheric dust. Whole-rock geochemistry suggests that the dust was at least partly derived from uplifts of the Ancestral Rocky Mountains with a possible minor contribution from active volcanic centers located to the (present day) south and southwest. The estimated mean accumulation rate (MAR) of dust in highstand carbonate facies (full interglacial) relative to carbonate facies that formed during relative fall in sea level suggest that dust flux increased 4 to 10 times accompanying the transition to glacial conditions. MARs in the siliciclastic mudrock, which accumulated at lowstand (full glacial) to incipient transgression, suggest glacial-phase increases in dust flux of nearly 400 to 4000 times relative to full interglacial conditions. Spectral analyses of dust abundance through the study section show variations consistent with Milankovitch forcing.