Kate Aasen Rylander

Vegetation invasions into absolute desert: A 45;th000 yr rodent midden record from the Calama–Salar de Atacama basins, northern Chile (lat 22°–24°S)

Plant macrofossils, percentage abundance of grass taxa, fecal-pellet d13 C, and plant-cuticle contents from 49 fossil rodent middens dated by 14 C record changes in local vegetation and precipitation since 45 ka (calibrated or measured to thousands of calendar years before present) in the central Atacama Desert (lat 228‐248S) of northern Chile. The midden sites are along the hyperarid upper margin (2400‐3100 m) of the ‘‘absolute desert,’’ in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs. Conditions between 40 and 22 ka may have been at least intermittently dry, and possibly cooler, as implied by four middens with low species richness. We infer a large increase in summer rainfall between 16.2 and 10.5 ka on the basis of the lowering of steppe grasses by as much as 1000 m, prominence of C4 grasses and summer annuals, high species richness, and displacement of northern species at least 50 km south of their modern ranges. The precipitation increase was greatest for a cluster of middens between 11.8 and 10.5 ka. Abrupt drying, evident in a dramatic decrease in grass abundance, occurred after 10.5 ka at all four midden localities. Increased percentages of

A vegetation history from the arid prepuna of northern Chile (22^23‡S) over the last 13 500 years

The Quaternary paleoclimate of the central Andes is poorly understood due to numerous discrepancies among the diverse proxy records that span this geographically and climatically complex region. The exact timing,duration and magnitude of wet and dry phases are seldom duplicated from one proxy type to another,and there have been few opportunities to compare climatic records from the same proxy along environmental gradients. Vegetation histories from fossil rodent middens provide one such opportunity on the Pacific slope of the Andes. We previously reported a vegetation history from the upper margin (2400^3000 m) of the absolute desert in the central Atacama Desert of northern Chile. That record identified a distinct wet phase that peaked between 11.8 and 10.5 ka,when steppe grasses and other upland elements expanded as much as 1000 m downslope,and a secondary wet period during the middle to late Holocene (7.1^3.5 ka). The latter wet phase remains controversial and is not as readily apparent in our lowelevation midden record. We thus sought to replicate both phases in a midden record from the mid-elevations (3100^ 3300 m) of the arid prepuna,where slight precipitation increases would be amplified. Midden records from these elevations identify conditions wetter than today at 13.5^9.6,7.6^6.3,4.4^3.2 and possibly 1.8^1.2 ka. Dry phases occurred at 9.4^8.4 ka and possibly at ca. 5.1 ka. Present floras and modern hyperarid conditions were established after 3.2 ka. The records from the two elevational bands generally match with some important differences. These differences could reflect both the discontinuous aspect of the midden record and the episodic nature of precipitation and plant establishment in this hyperarid desert. : 2003 Elsevier Science B.V. All rights reserved.

Late-Holocene rodent middens from Rio Limay, Neuquen Province, Argentina

Pollen analysis of late-Holocene amberat deposits from two caves near the forest-steppe ecotone in northern Patagonia documents a major shift from Austrocedrus-Nothofagus forest to steppe shrub assemblages some time after 1800 and before 1300 BP. The probable explanation of the reduction of tree taxa calls for either drier summers or intensified land use or a combination of both.