Accretion of Asian dust to Hawaiian soils: Isotopic, elemental, and mineral mass balances

摘要

Hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs, including sea salt aerosols and Asian dust. We use soil mineralogy and radiogenic isotope geochemistry (Sr and Nd) to evaluate the impact of Asian dust on a chronosequence of Hawaiian soils. Dust becomes an important constituent of soils 20 ky and older. Near-surface (< 50-cm depth) horizons contain as much as 30% quartz, a mineral absent from local parent material. Basaltic Sr and Nd isotope signatures in these horizons have been completely overprinted by Asian dust signatures, with Sr-87/Sr-86 ratios as high as 0.723 and epsilon (Nd) values as low as -7. REE patterns in these soils are indistinguishable from that of average upper continental crust. Quartz abundance and Nd isotopes provide two independent tracers of long-term dust additions to the chronosequence soils. The two tracers indicate a minimum long-term average dust accretion rate of 125 mg cm(-2) ky(-1) at the 150 ka chronosequence site, roughly a factor of three higher than Holocene dust accumulation rates estimated from North Pacific sediment cores. We find that the mass of dust preserved in these soil profiles does not increase with age in soils 150 ka and older, requiring a loss mechanism for accreted dust. On the basis of the geomorphic stability of these sites, observed preferential loss of dust-derived mica relative to quartz, and estimates of soil Si leaching rates we argue that chemical weathering is the dominant loss mechanism for dust from these soils. Dust has a profound effect on the budgets of elements that are susceptible to leaching losses. Dust becomes the dominant source of soil nutrients Si and P in the oldest, most intensely weathered soils. We calculate a dust-derived P input flux of 0.8 mg cm(-2) ky(-1), and a dust-derived Si input flux of 35 mg cm(-2) ky(-1). Si leaching fluxes are high (1400 mg cm(-2) ky(-1)) in the youngest (2 ka) soils and drop systematically with soil age to a value that closely balances the dust-derived Si input flux by 4100 ka. Extremely refractory elements such as Nb, which are concentrated in soils by residual enrichment processes, are much less readily impacted by dust addition. Although dust has had a pronounced impact on Sr and Nd isotopic budgets and on soil mineral composition at the 150 ka site, dust cannot have contributed >4% of the total Nb contained in this soil profile. Copyright (C) 2001 Elsevier Science Ltd. [References: 42]