Particle size distributions of aeolian sediments from five ecosystem types at the Jornada Basin, New Mexico, USA

Vegetation protects surface sediments from wind erosion. Vegetation cover varies across landscapes, especially in arid lands where grasses are patchy and shrublands with open intershrub spaces tend to dominate: intershrub spaces tend to experience greater aeolian erosion than nearby grasslands. There are five major ecosystem types at the Jornada Basin Long-Term Ecological Research (LTER) site in south-central New Mexico, USA: mesquite dunes, black grama grasslands, creosote shrublands, tarbush alluvial flats, and grass-dominated playas. We describe dry particle size distributions (PSDs) of material collected in BSNE aeolian sediment traps in late spring - early summer 2006 in these five ecosystem types. Four towers per site, three sites per ecosystem, collected sediment at 5, 10, 20, 50, and 100 cm heights. Sediment PSDs were determined in dry state with a Malvern Mastersizer 2000 laser diffractometer. The PSDs at 5 and 10 cm were generally very similar across the sites, most sites having > 80% sand-sized particles (>50 µm). The exceptions were the one playa site and one tarbush site, where 35% and 40% of the sediment at 5 cm was silt and clay (<50 µm), respectively. Samples at 20 cm height from the creosote and grassland sites, one mesquite site, and the other two tarbush sites were all similar to those from 5 and 10 cm, while sediments at 50 and 100 cm shifted progressively finer. Sediments at 50 and 100 cm heights above the playa were similar, with fewer <10 µm particles than at 5 and 10 cm, but with more 10-50 µm particles than at lower heights. A similar pattern was seen at the tarbush site, with the 20 cm sample having more 10-50 µm particles but fewer >500 µm particles than at 5 and 10 cm. Two mesquite sites had nearly identical PSDs at 5, 10, and 20 cm heights, with the sediment at 50 cm slightly finer than lower heights, and the 100 cm height strongly shifted to finer particles. The same pattern was seen at an artificially de-vegetated site. Shifts in PSDs with height are consistent with a transition between saltation (coarser particle)-dominated transport and suspension (finer particle)-dominated transport. For most of the sites at Jornada, the transition occurred either at ~20 cm or between 20 and 50 cm. The tarbush site and playa site had unusually large percentages of fines at 5 cm, where larger particles are expected to dominate. These sites have aggregated fines which aerodynamically act as larger particles, and thus are too coarse to suspend. Our earlier work found that these two sites were important potential dust sources due to the high percentages of fines. Two mesquite sites had the transition at or above 50 cm, as did the de-vegetated site. Despite similar PSDs, horizontal mass flux at the de-vegetated site was about ~2x that of the mesquite site with the largest intershrub distances. This suggests that although the transported particles have similar size distributions, the mesquite dunes at the Jornada LTER provide some protection to aeolian erosion, albeit less than other vegetation types provide.