How Dusty was the Early Paleogene Greenhouse World?

Models and limited proxy data suggest that overall hydrologic cycling was more intense during the early Paleogene, resulting in higher precipitation in temperate and high latitudes likely with greater vegetative cover than in drier periods. The lower aridity in temperate and tropical regions would have resulted in diminished dust transport to the ocean basins at these latitudes. However, a more intense hydrological cycle likely resulted in increased drying of the arid-to-semi-arid subtropical regions that may have enhanced the size of the available subtropical dust source regions. Because the accumulation of dust in remote, pelagic sediments is controlled by aridity in the source regions as well as the gustiness of the transporting winds, we can test the zonal pattern of aridity (dust availability) by reconstructing dust accumulation records from a meridional swath of deep-sea sedimentary sections. The available drill sites in the South Pacific provide a useful transect from ~60 to 30S over the study interval ~70-30 Ma. We estimated ²³²Th-based dust fluxes at DSDP Site 596 from published ²³²Th concentration data and sediment mass accumulation rates, and generated new ²³²Th-based dust fluxes from IODP Sites U1369, 1370 and 1371. Dust fluxes recorded at Site 596 vary between~5 and 12 mg/cm2/kyr with no apparent temporal trend from 70 to 30 Ma. Fluxes recorded at Sites U1369, 1370 and 1371 generally decrease over the interval ~65 to 50 Ma, but by ~50 Ma these sites recorded similar values as Site 596 to the north. While the range of values recorded at the southern sites overlaps, there is a general gradient in dust flux decreasing from west to east, with Site U1371 recording the highest overall fluxes and Site U1369 recording the lowest fluxes. The west to east decrease in dust flux suggests that the primary source of eolian material to the southern Pacific basin was to the west. This source region likely was Australia. However, the magnitude of the ²³²Th-based dust fluxes at all of the sites is low, indicating that the hypothesized Australian dust source region was not very arid from ~70 to 30 Ma.