(Appendix 1) Crustal helium-4 abundance and flux data for ODP Hole 130-806B

The mass accumulation rates (MARs) of aeolian dust in the ocean basins provide an important record of climate in the continental source regions of atmospheric dust and of the prevailing wind patterns responsible for dust transport in the geologic past. The incorporation of other terrigenous components such as volcanic ashes in seafloor sediments, however, often obscures the aeolian dust record. We describe a new approach which uses the delivery rate of crustal 4He to seafloor sediments as a proxy for the mass accumulation rate of old continental dust which is unaffected by the addition of other terrigenous components. We have determined the flux of crustal 4He delivered to the seafloor of the Ontong Java Plateau (OJP) in the western equatorial Pacific over the last 1.9 Myrs. Crustal 4He fluxes vary between 7.7 and 30 ncc/cm**2/kyr and show excellent correlation with global climate as recorded by oxygen isotopes, with high crustal 4He fluxes associated with glacial periods over the entire interval studied. Furthermore, the onset of strong 100 kyr glacial-interglacial climate cycling is clearly seen in the 4He flux record about 700 kyrs ago. These data record variations in the supply of Asian dust in response to climate driven changes in the aridity of the Asian dust sources, consistent with earlier work on Asian dust flux to the northern Pacific Ocean. However, in contrast to previous studies of sites in the central and eastern equatorial Pacific Ocean, there is no evidence that the Inter Tropical Convergence Zone (an effective rainfall barrier to the southward transport of northern hemisphere dust across the equator in the central and eastern Pacific) has influenced the delivery of Asian dust to the OJP. The most likely carrier phase for crustal helium in these sediments is zircon, which can reasonably account for all the 4He observed in the samples. As a first order estimate, these results suggest that the mass accumulation rate of Asian dust on the OJP over the last 1.9 Myrs varied from about 4 to 15 mg/ cm**2/kyr. In contrast, previous studies show that over the same interval the total MAR of terrigenous dust (i.e. Asian dust plus local volcanics) on OJP varied between about 34 and 90 mg/ cm**2/kyr.

(Table 2) Analyses of prominent ash layers recovered at DSDP Site 18-178, Gulf of Alaska

A sequence of ash layers recovered from site 178 of the Deep Sea Drilling Project in the Gulf of Alaska was studied to determine the nature of highly explosive volcanic eruptions associated with the Aleutian Arc and Alaskan Peninsula during the last 8 m.y. The major-element chemistry of 25 distinct ash layers was determined. When the analyses are plotted on conventional major-element variation diagrams, the unusual, highly evolved, calc-alkalic characteristics of the ashes are apparent. Perhaps more significantly, there is a good correlation of certain indices of the degree of chemical evolution of each ash (SiO2 content and Larsen index) with sample age. Both parameters vary cyclically, with maximum values of both indices occurring at present, 2.5, and about 5.0 m.y. ago. The cause of the cyclic activity, as well as discontinuous volcanic activity reported for other areas by other investigators, is not precisely known. However, we suggest that variable rates of subduction provide a viable hypothesis for discontinuous volcanic activity associated with convergent plate boundaries.

(Table 1) Soil density and concentration of 10Beryllium and 26Aluminium in samples from two ash deposits in the McMurdo Dry Valleys

The McMurdo Dry Valleys, Antarctica (MDV) are among the oldest landscapes on Earth, and some landforms there present an intriguing apparent contradiction such that millions of years old surface deposits maintain their meter-scale morphology despite the fact that measured erosion rates are 0.1-4 m/Ma. We analyzed the concentration of cosmic ray-produced 10Be and 26Al in quartz sands from regolith directly above and below two well-documented ash deposits in the MDV, the Arena Valley ash (40Ar/39Ar age of 4.33 Ma) and the Hart ash (K-Ar age of 3.9 Ma). Measured concentrations of 10Be and 26Al are significantly less than expected given the age of the in situ air fall ashes and are best interpreted as reflecting the degradation rate of the overlying sediments. The erosion rate of the material above the Arena Valley ash that best explains the observed isotope profiles is 3.5 ± 0.41 x 10**-5 g/cm**2/yr (~0.19 m/Ma) for the past ~4 Ma. For the Hart ash, the erosion rate is 4.8 ± 0.21 x 10**-4 g/cm**2/yr (~2.6 m/Ma) for the past ~1 Ma. The concentration profiles do not show signs of mixing, creep, or deflation caused by sublimation of ground ice. These results indicate that the slow, steady lowering of the surface without vertical mixing may allow landforms to maintain their meter-scale morphology even though they are actively eroding.