Tracing gypsiferous White Sands aerosols in the shallow critical zone in the northern Sacramento Mountains, New Mexico using Sr/Ca and 87Sr/86Sr ratios

Abstract White Sands (New Mexico, USA) emits gypsum dust with a unique chemical and mineralogical signature, providing an opportunity to investigate its loading and regional movement. This study tracked White Sands dust input to montane soils downwind in the northern Sacramento Mountains, and provided better understanding of the effects of gypsum dust on sources of bioavailable plant nutrients in an active critical zone. Four soil profiles were collected over bedrocks of different weatherabilities, from the most reactive limestone, mixed limestone, diorite, to the least reactive sandstone, as well as different atmospheric end-members (rain, regional dust, and White Sands gypsum). Annual dry deposition rate observed at the Sacramento Mountains was approximately at 11 g m−2 yr−1, up to an order of magnitude lower than those at the Chihuahuan Desert basins upwind and closer to dust sources at lower elevations. Elemental chemistry, Ca/Sr and 87Sr/86Sr of bulk soils were mostly controlled by those of bedrocks and modified by chemical weathering over carbonate substrates, whereas the atmospheric input was only detectable in shallow soils over the least reactive sandstone through addition profiles of Ca, Sr and Na. Although only making up a small portion of a bulk soil in mass, White Sands dust, along with wet deposition, dominantly controlled the Ca/Sr and 87Sr/86Sr ratios of plants and the water leachable fraction of the soils. Particles from White Sands, predominantly gypsum, were very soluble, and dissolved quickly in soils of the Sacramento Mountains but re-precipitated at the impermeable bedrock-soil interface. Although absolute concentrations of Ca and Sr in plants differed widely among four sites and among different local plant types, Sr/Ca ratios were largely controlled by plant type, and thus the rooting depth. This study highlighted the importance of regional dust and rainfall in loading nutrients even in a semi-humid environment.