Dust, Volcanic Ash, and the Evolution of the South Pacific Gyre through the Cenozoic

Abstract:

Dust and volcanic ash play a critical role in past global climate by affecting cloud cover and ocean nutrients as well as responding to changes in tectonics, aridity, and wind. Because the eolian fluxes in the Southern Hemisphere are so low, subtle changes in the absolute flux of dust and volcanic ash may have a disproportionally large impact on climate. Our multi-site record of eolian dust and volcanic ash accumulation in pelagic clay of the South Pacific Gyre (SPG), gathered during IODP Expedition 329, shows that eolian fluxes varied by an order of magnitude over the Cenozoic and correlate with changes of tectonic processes and global climate.

We analyzed the concentrations of 37 elements in 138 bulk pelagic clay samples from 6 sites drilled throughout the SPG. Using multivariate statistical modeling of the geochemical dataset (e.g. Q-mode factor analysis and multiple linear regression) and a cobalt-based age model, we quantified the mass accumulation rate (MAR) of 6 end-members that comprise the SPG pelagic clay: dust, rhyolite, altered basalt, Fe-Mn oxyhydroxides, excess Si, and apatite.

Our record shows that Australian dust MAR begins at the Early Eocene Climatic Optimum ~50 Ma as global temperatures began to cool and Australia began to tectonically separate from Antarctica. The mid-Miocene has noticeably higher MAR of dust and ash at multiple sites. While a simultaneous increase in production is possible (i.e., more aridity and volcanic activity), the synchronicity may be more indicative of stronger winds in the Southern Hemisphere and increased material in the atmosphere during this time.

Heavier Mg isotopes occur at Site U1366 in samples that are composed primarily of hydrothermal deposition, excess Si, and volcanic ash. The Mg isotopic enrichment suggests that these components have undergone alterations to form authigenic aluminosilicates.