Atmospheric dust archives and drives climate change. Dust preserved in marine and continental sediments and ice has shed light on recent climate change, and dust also impacts climate via direct and indirect effects on the amount of solar energy received at Earth's surface, and by fertilization that stimulates primary productivity and thus the carbon cycle. However, the character and magnitude of the aerosol effect remains a poorly constrained variable in climate models, thus limiting the predictive capability of these models. In this research, PIs propose to assess the 'dust effect' by investigating the geologic record of a particularly dusty interval on Earth. The late Paleozoic world, 300 million years ago, was remarkably dusty, with dust flux varying on both million-year and millennial scales. This time period is also attractive as the last time that Earth's climate was analogous to today's, with large polar ice sheets. Here, PIs propose to test the overarching hypothesis that the abundant dust played a significant role in driving changes in late Paleozoic climate and linked (e.g. biotic) systems, through direct, indirect, and feedback effects. They will investigate how dust flux, atmospheric circulation, and dust transport varied between glacials and interglacials, how dust forced changes in tropical climate, and how the biosphere responded to such high atmospheric dustiness. To address these questions, PIs are targeting two time slices in localities spanning the girth of the tropics. They will examine dust distribution, assess atmospheric dustiness and wind strength and direction, and use geochemistry to examine effects on marine life. They will correlate among localities using fossils and radioisotopic dating. PIs will use the data they collect as input for climate- and dust-modeling experiments, to assess the direct and indirect effects of dust on atmospheric behavior and undertake biogeochemical modeling aimed at assessing the impact of variable nutrient fluxes on cycling of carbon. Intellectual Merit-- Results of this research will provide a high-resolution reconstruction of climate for the tropics and reveal the effects of dust on climate and life in a world characterized by variable dust flux on various timescales, within a 'glacial' world like today's. Owing to the known importance but remaining uncertainty of the roles of dust and associated aerosols in the climate system, our data will have predictive utility in expanding our understanding of Earth-system behavior across geologic time, and will provide important constraints useful for improving climate modeling. Broader Impacts--This project will involve heavy student participation (graduate and undergraduate levels), cross-disciplinary training among geologists, geochemists, and climate modelers, both in the field and laboratory. Undergraduates (geology and education majors) and minority middle-schoolers will take part through mentoring programs. Data will be archived and shared using newly developed web-accessible tools. Finally, we will use results of this research to guide the development of a traveling exhibit on the 'Paleozoic Dust Bowl' in conjunction with the Oklahoma Museum of Natural History and incorporate results in an outreach course taught (by co-PIs) at the Museum.
The availability of iron, an essential micronutrient in both the microbiological cycling of nitrogen and in the fertilization of planktonic growth, promotes biological activity in many regions of the global ocean. Primary biological production—specifically, the resulting burial of organic matter—draws down the carbon dioxide content of the atmosphere and thus cools climate. Conventional wisdom argues that most iron is delivered to the ocean by eolian dust fluxes, which vary dramatically with climate, particularly on glacial-interglacial time scales. The major goal of this project was to explore the relationship between dust delivery to the ocean and associated iron cycling during the last great ice age—roughly 300 million years ago during the Carboniferous Period. This objective also required that we look at synchronous dust accumulation and its properties on land. Our results reveal the potential for massive delivery of dust-related, bioavailable iron to this ancient ocean, which likely stimulated further ice accumulation by lower carbon dioxide in the atmosphere. A natural complement to our study of the Carboniferous ice age was the need for a better understanding of dust-related iron in the modern and recent world, as recorded in terrestrial loess deposits and deep ocean sediments. These results, which focus on the amounts and mechanisms of iron bioavailability, provide the necessary context for interpreting the Carboniferous data—and for understanding the modern world. Of particular interest is a tantalizing, and still not understood, suggestion that Carboniferous dust from North America was exceptionally bioavailable relative to more recent dust, which speaks to potential differences in source regions and/or novel weathering processes and transport mechanisms. Our study also demanded consideration of alternative scenarios of iron delivery to the ocean, in particular those related more directly to large-scale glacial processes. For example, modern melting Arctic glaciers are yielding extreme sediment loads via pro-glacial streams, resulting in early sediment burial conditions in coastal regions dominated by iron cycling and potentially enhanced supply of bioavailable iron to the overlying waters. It is clear that ice ages provide multiple pathways for delivery of iron the ocean, and elevated supplies of this essential micronutrient impact climate in response.
