ASSESSING AEOLIAN INPUTS TO A LATE PALEOZOIC SHALLOW MARINE SECTION

Fine-grained dust and loess influence global climate via radiative forcing and atmospheric moisture and the biosphere by providing nutrients. We are researching variations in atmospheric dustiness during the late Paleozoic and assessing effects of dustiness on the biosphere. Our goals are to constrain spatial and temporal variations in atmospheric “dustiness,” dust provenance, and influences between dust and ecosystem composition to clarify the relationships among atmospheric dustiness, climate, and the biosphere.

Here we present preliminary data from two time intervals (Moscovian or middle Pennsylvanian) from the Copacabana Formation, Madre de Dios Basin (Bolivia). Two sections of ~15 meters each representing carbonate inner platform environments were processed to isolate the fine-grained silicate mineral fraction (SMF). Given the proximity to arc volcanism there is potential for ash to disrupt the aeolian dust signal; initial work is aimed at assessing the volcanic versus continental inputs to the basin as well as addressing our primary goals. Visual inspection and initial physical - chemical measurements of the dust-loess size fractions indicate differences between the upper and lower sections, as well as between ash-rich and ash-poor intervals. The lower section has intervals with up to 72 weight-% suspected ash and SMF (non-ash) of 1-7%. The suspected ash layers in the lower section have larger grain sizes (20-40 μm) and distinct ¹⁴³Nd/¹⁴⁴Nd isotopic compositions (ε_Nd = -3.5, -3.8) compared to the ash-poor intervals (~8 μm, ε_Nd = -5.0 to -8.8). The upper section, free of visible ash layers, has high SMF (mean 4-7% up to 41%) with similar size distributions (6-11 μm) and ε_Nd (-7.4 to -7.7) to the lower section, but more radiogenic ⁸⁷Sr/⁸⁶Sr compositions (0.797 versus 0.71 - 0.75). Qualitatively, the ash-influenced intervals exhibit abundant evidence for biological activity (bioturbation). The combination of distinct physical and isotopic characteristics of the ash and SMF in the sections and sub-intervals, and the correlation with biological productivity, suggests that (1) deconvolving ash and aeolian inputs to the marine biosphere is possible and (2) dust provenance may influence nutrient availability.