Models of ecosystem development and response to environmental variation must incorporate change in vertical soil space as well as over time. Insufficient measurement of subsurface soil properties ...represents a major observational bias in ecosystem studies.
We address these changes in horizontal (time) and vertical (soil profile) space along a three‐million‐year, semi‐arid, piñon‐juniper woodland substrate age gradient with characteristic progressive and retrogressive ecosystem development phases and a shift from nitrogen (N) and water to phosphorus (P) limitation. We present a novel pedological approach using isotopic tracers and biogeochemical analyses to address fine root distribution, depth of plant uptake and relative nutrient availabilities.
We show that (a) the quantity of fine roots remains constant with ecosystem development but their distribution in the soil profile becomes increasingly deeper and less concentrated in the surface soil; (b) mean depth of tree uptake becomes deeper with substrate age and follows the relative availability of P as P‐limitation develops and (c) soil P transformations in the oldest soil profiles resemble the theoretical changes with age to produce a depth gradient of relative N and P availability.
Synthesis. The expanding role of deep roots in this model system is tightly linked to phases of ecosystem development and relative nutrient availability. The inclusion of whole soil profiles is vital to investigating the intersections of biota, soil and geologic substrate and developing a more complete understanding of ecosystem structure and function.
The distribution of fine roots and mean depth of plant uptake shifted increasingly deeper during ecosystem development. The inclusion of whole soil profiles is vital to investigating the intersections of biota, soil and geologic substrate and developing a more complete understanding of ecosystem structure and function.
The earliest known foragers to populate most of North America south of the glaciers ∼11,500 to ≥ ∼10,800 ¹⁴C yBP; ∼13,300 to ∼12,800 calibrated (Cal) years made distinctive “Clovis” artifacts. They ...are stereotypically characterized as hunters of Pleistocene megamammals (mostly mammoth) who entered the continent via Beringia and an ice-free corridor in Canada. The origins of Clovis technology are unclear, however, with no obvious evidence of a predecessor to the north. Here we present evidence for Clovis hunting and habitation ∼11,550 yBP (∼13,390 Cal years) at “El Fin del Mundo,” an archaeological site in Sonora, northwestern Mexico. The site also includes the first evidence to our knowledge for gomphothere (Cuvieronius sp.) as Clovis prey, otherwise unknown in the North American archaeological record and terminal Pleistocene paleontological record. These data (i) broaden the age and geographic range for Clovis, establishing El Fin del Mundo as one of the oldest and southernmost in situ Clovis sites, supporting the hypothesis that Clovis had its origins well south of the gateways into the continent, and (ii) expand the make-up of the North American megafauna community just before extinction.
Depositional models of ancient lakes in thin‐skinned retroarc foreland basins rarely benefit from appropriate Quaternary analogues. To address this, we present new stratigraphic, sedimentological and ...geochemical analyses of four radiocarbon‐dated sediment cores from the Pozuelos Basin (PB; northwest Argentina) that capture the evolution of this low‐accommodation Puna basin over the past ca. 43 cal kyr. Strata from the PB are interpreted as accumulations of a highly variable, underfilled lake system represented by lake‐plain/littoral, profundal, palustrine, saline lake and playa facies associations. The vertical stacking of facies is asymmetric, with transgressive and thin organic‐rich highstand deposits underlying thicker, organic‐poor regressive deposits. The major controls on depositional architecture and basin palaeogeography are tectonics and climate. Accommodation space was derived from piggyback basin‐forming flexural subsidence and Miocene‐Quaternary normal faulting associated with incorporation of the basin into the Andean hinterland. Sediment and water supply was modulated by variability in the South American summer monsoon, and perennial lake deposits correlate in time with several well‐known late Pleistocene wet periods on the Altiplano/Puna plateau. Our results shed new light on lake expansion–contraction dynamics in the PB in particular and provide a deeper understanding of Puna basin lakes in general.
Here we present the first use of calcareous microfossils to examine the late Quaternary paleoecology of the endorheic Pozuelos Basin (Argentina). Modern deposition in the basin centers on Laguna de ...los Pozuelos (LP), a shallow playa-lake that is fed by axial rivers and groundwater and dominantly accumulates siliciclastic sediments. Today, the distribution of limnocytherid and cypridoidean ostracodes across southern LP is strongly influenced by distance to the Río Cincel delta, whereas the northern end of the playa-lake is characterized by a paucity of ostracodes due to frequent sub-aerial exposure. Ten ostracode biofacies define a sediment core retrieved from LP, which reveal progressive changes in aquatic environments that varied in salinity, depth, and proximity to deltas over the late Pleistocene. Closed lakes occupied the basin from ∼ 37.6–30.7 ka, ∼ 28.0–25.0 ka, and ∼ 23.0–16.6 ka, whereas saline wetlands occurred when these lakes contracted. Extant LP has no analog in the late Pleistocene record; it formed after ∼ 7.2 ka, following a hiatus that removed the Pleistocene–Holocene transition. Paleoecological evidence indicates that the core site was influenced by deltaic inflows from the eastern basin margin until ∼ 24.3 ka, an area where today dry alluvial fans are found. Reorganization of the watershed by normal faulting, most likely at ∼ 18.0 ka, appears to have reduced the influence of these deltaic inflows. Extensional neotectonics, perhaps induced by incorporation of the Pozuelos Basin into the Andean hinterland, is a mechanism that along with tropical climate change is potentially important to water balance and ecology in high-altitude convergent orogenic basins.
Whether or not abrupt Younger Dryas climate change affected regional paleoenvironments and late Pleistocene hunter-gatherer populations is an important topic in the archaeology of the American ...Southwest. This paper reviews multiple, age-resolved proxy evidence to gauge the magnitude and direction of Younger Dryas Chronozone (YDC) environmental changes in different settings and systems. There is no record of YDC pluvial lake highstands in Arizona or New Mexico, but there are impressive records of vegetation, faunal, stable isotope, and geomorphological change coincident with the YDC. These correlate with important adaptive changes in human hunting and land use, as revealed in the analysis of the spatiotemporal distribution of late Pleistocene hunting technologies. Clovis and Folsom projectile point distributions do not support extant models of paleoenvironmental conditions in these interpretations. Significant cultural changes that coincide with the YDC include the Clovis-to-Folsom transition, the demise of mammoth hunting and the development of a highly successful emphasis on bison, increased regionalization, and the abandonment of the northwestern Chihuahuan and the Sonoran deserts by mobile, big-game hunters.
Without exogenous rock‐derived nutrient sources, terrestrial ecosystems may eventually regress or reach a terminal steady state, but the degree to which exogenous nutrient sources buffer or slow to a ...theoretical terminal steady state remains unclear. We used strontium isotope ratios (87Sr/86Sr) as a tracer and measured 87Sr/86Sr values in aeolian dust, soils, and vegetation across a well‐constrained 3 Myr semiarid substrate age gradient to determine (1) whether the contribution of atmospheric sources of rock‐derived nutrients to soil and vegetation pools varied with substrate age and (2) to determine if the depth of uptake varied with substrate age. We found that aeolian‐derived nutrients became increasingly important, contributing as much as 71% to plant‐available soil pools and tree (Pinus edulis) growth during the latter stages of ecosystem development in a semiarid climate. The depth of nutrient uptake increased on older substrates, demonstrating that trees in arid regions can acquire nutrients from greater depths as ecosystem development progresses presumably in response to nutrient depletion in the more weathered surface soils. Our results demonstrate that global and regional aeolian transport of nutrients to local ecosystems is a vital process for ecosystem development in arid regions. Furthermore, these aeolian nutrient inputs contribute to deep soil nutrient pools, which become increasingly important for maintaining plant productivity over long time scales.
Key Points
Shift from rock‐derived to atmospheric sources of Sr with substrate age
Plants acquire Sr from greater depths as ecosystem development progresses
Demonstrates that similar ecosystem processes occur across climate types
A dearth of reliably-dated paleolake records from the southern Basin and Range has limited knowledge of past water balance changes there, precluding a more complete understanding of late Pleistocene ...atmospheric circulation across western North America. Paleoshorelines in closed basins throughout the region can provide accurately dated records of local effective moisture variations, representing a largely untapped source of paleohydrologic information. This dissertation presents paleohydrologic reconstructions from depositional successions in two basins at 32°N, approximately 100 km apart: Willcox basin, in southeastern Arizona, and Playas Valley, in southwestern New Mexico. Also presented are the results of ¹⁴C dating of charcoal samples from the El Fin del Mundo Clovis archaeological site, in northwestern Sonora, Mexico. In depth analysis of these results allowed constraint of the "small sample effect" on the charcoal ages, found to be smaller than 1σ of analytical uncertainty. The magnitude of the problem in ages from miniscule shell samples in the Willcox and Playas chronologies was found to be similar. The successions record moist pluvial conditions from ~20-13 ka in Playas, and>37-11 ka in Willcox, with most dates younger than 19 ka--before which there is no solid evidence for lake transgressions. There is clear evidence for overlapping highstands between ~18.3 and 17.9 ka and a brief highstand of Cochise at ~12.9 ka, coinciding with Heinrich events H1b and H0, respectively. Temporal concordance between wet periods and perturbations in the North Atlantic ocean and/or southern Laurentide ice sheet supports the idea that abrupt paleoclimatic changes in the southwestern U.S. occurred in response to large-scale atmospheric linkages to the northern high latitudes. The H1b highstands fill a hiatus in ¹⁴C dates compiled from paleoshorelines throughout the western U.S., and correspond to the first part of a lowstand in paleo-Lake Estancia (35°N), in north-central New Mexico. Anti-phasing within New Mexico suggests that the newly documented highstands resulted from an increase in southerly-sourced precipitation. This is consistent with paleoenvironmental evidence from southern Arizona and New Mexico that points toward periodic intensification of the summer monsoon during the late Pleistocene.
A dearth of reliably-dated paleolake records from the southern Basin and Range has limited knowledge of past water balance changes there, precluding a more complete understanding of late Pleistocene ...atmospheric circulation across western North America. Paleoshorelines in closed basins throughout the region can provide accurately dated records of local effective moisture variations, representing a largely untapped source of paleohydrologic information. This dissertation presents paleohydrologic reconstructions from depositional successions in two basins at 32oN, approximately 100 km apart: Willcox basin, in southeastern Arizona, and Playas Valley, in southwestern New Mexico. Also presented are the results of 14C dating of charcoal samples from the El Fin del Mundo Clovis archaeological site, in northwestern Sonora, Mexico. In depth analysis of these results allowed constraint of the "small sample effect" on the charcoal ages, found to be smaller than 1σ of analytical uncertainty. The magnitude of the problem in ages from miniscule shell samples in the Willcox and Playas chronologies was found to be similar. The successions record moist pluvial conditions from ~20–13 ka in Playas, and >37–11 ka in Willcox, with most dates younger than 19 ka—before which there is no solid evidence for lake transgressions. There is clear evidence for overlapping highstands between ~18.3 and 17.9 ka and a brief highstand of Cochise at ~12.9 ka, coinciding with Heinrich events H1b and H0, respectively. Temporal concordance between wet periods and perturbations in the North Atlantic ocean and/or southern Laurentide ice sheet supports the idea that abrupt paleoclimatic changes in the southwestern U.S. occurred in response to large-scale atmospheric linkages to the northern high latitudes. The H1b highstands fill a hiatus in 14C dates compiled from paleoshorelines throughout the western U.S., and correspond to the first part of a lowstand in paleo-Lake Estancia (35°N), in north-central New Mexico. Anti-phasing within New Mexico suggests that the newly documented highstands resulted from an increase in southerly-sourced precipitation. This is consistent with paleoenvironmental evidence from southern Arizona and New Mexico that points toward periodic intensification of the summer monsoon during the late Pleistocene.
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