Salvatore Valastro

Late Quaternary climates and environments of the Edwards Plateau, Texas

Abstract Fossil vertebrate, pollen, and plant macrofossil data from the Edwards Plateau, Texas and throughout the southcentral United States permit reconstruction of regional changes in temperature and effective moisture. Full-glacial temperatures were significantly cooler than those of today, at least 6°C during the summer months, but by ca. 13,000 yrs B.P. summer temperatures were within 2–3°C of present values. There was more effective moisture during the full-glacial period than at any time since then. During the late-glacial, ca. 14,000–10,500 effective moisture first decreased then increased, while the early to middle Holocene was dominated by a protracted decrease in effective moisture. This long-term trend culminated in conditions that were drier than modern during the early part of the late Holocene from ca. 5000 to 2500 yr B.P. Conditions were more mesic than present from ca. 2500 and 1000 yr B.P., while the modern drought-prone climate has characterized the last 1000 years. Fossil vertebrates and the characteristics of cave fill sediments show that late Pleistocene and Holocene changes in temperature and moisture regimes were coupled with vegetation changes and a gradual degradation of upland soils. During the full- and late-glacial much of the upland landscape was covered by thick, deeply weathered reddish clay-rich soils and an open savanna vegetation with a mixed tall and short grass understory. Changes to Holocene climatic conditions promoted a diminished vegetation cover and initiated the gradual degradation of soil mantles, whereas minimum effective moisture during the earlier part of the late Holocene resulted in upland landscapes that were covered by a mixture of short grasses and scrub vegetation, and the near complete removal of the remaining soil mantle. Vegetation changes during the last 2500 years are poorly known, but the upland landscape has consisted of exposed bedrock with little soil cover. Comparison of empirical data and the results of paleoclimate models shows good correspondence for the full-glacial through middle Holocene when climate system boundary conditions that drive model simulations were substantially different from today. Climate models also provide plausible physical explanations for climatic and environmental changes identified by empirical data. Our synthetic reconstruction, based on both empirical data and climate model simulations, provides a framework for evaluation of the impacts of climatic and environmental changes on earth surface processes and landforms, and a basis for discussion of prehistoric human adaptations to different environmental conditions.

Isotopic ecology of deer bones

Abstract The carbon and oxygen isotopic composition of CO 2 evolved from the apatite fraction of bones from freshly killed deer, and the carbon isotopic composition of bone collagen is controlled by environmental factors. Deer from high latitudes have δ 13 C CO 2 values of about −15‰ relative to PDB and δ 18 O CO 2 values of about −3‰. A fairly constant difference (Δ) of about 8‰ exists between δ 13 C collagen and δ 13 C CO 2 , and the bones all contain about 3.4 wt.% CO 2 . Deer from more arid areas are systematically enriched in both 13 C and 18 O, but Δ values and the amount of CO 2 in the bones are unaffected. Most Pleistocene deer bones we have analyzed have isotopic compositions inconsistent with recent kills despite the fact that the weight % CO 2 in the bone (after leaching with acetic acid to remove calcite) and X-ray diffraction data are unchanged. In most cases the CO 2 derived from apatite is unsuitable for paleoecologic or 14 C age determinations, and in several cases the δ 13 C of collagen appears to have been altered as well.

Fluvial response to Late Quaternary climatic and environmental change, Edwards Plateau, Texas

Abstract Radiocarbon-controlled late Pleistocene to modern stratigraphic frameworks for fluvial systems that drain the Edwards Plateau, westcentral Texas, coupled with reconstruction of climatic and environmental changes, permit development of a model for the evolution of fluvial landscapes during the past 20,000 years. During this time, large valleys of the Edwards Plateau were characterized by channel aggradation and sediment storage from 20 to 14,000 yrs B.P., deep excavation of bedrock valleys from ca. 14 to 11,000 yr B.P. and deposition of extensive and complex valley fills during the last 11,000 yr. Valley fills contain three distinct unconformity-bounded allostratigraphic units representing episodes of channel aggradation and floodplain construction during the early to middle (ca. 11,000–5000 yr B.P.) and late Holocene (ca. 5000–1000 yr B.P.), and development of the incised channels and associated narrow floodplains of the last 1000 years. Early to middle Holocene alluvial deposits were derived from proximal sources within the respective drainage basins, whereas late Holocene deposits include sediments derived from distal sources. Sediment supply to major valleys axes has been extremely limited during the last 1000 years. Alluvial sequences of the Edwards Plateau record fluvial responses to climatically-driven changes in discharge regimes and the concentration of sediments along valley axes. Allostratigraphic units define time periods when the concentration of sediment exceeded stream power, resulting in sediment storage, whereas unconformities record widespread morphological and sedimentary adjustments. Unconformities between late Pleistocene alluvium and Holocene valley fills record bedrock valley incision in response to decreased sediment loads associated with slope stability in the uplands. By contrast, unconformities within the Holocene valley fill record floodplain abandonment accompanied by continued channel migration and sediment storage, but little additional bedrock valley cutting. Episodes of floodplain abandonment occurred as a result of decreased flood magnitudes following shifts to drier climatic conditions at ca. 5000 and 1000 yr B.P. Fluvial responses to climatic change were conditioned by a progressive degradation of upland soils that caused increases through time in the flashiness of flood events, which in turn led to changes in processes of floodplain construction. Flood events on late Pleistocene and early to middle Holocene rivers were, for the most part, contained within channel perimeters, and floodplains were constructed by lateral migration. By contrast, late Holocene rivers were characterized by deep overbank flooding and floodplain construction by vertical accretion. High magnitude floods were most significant from ca. 2500 to 1000 yr B.P. when large chute channels were cut and filled on floodplain surfaces, and soils developed on previously stable terrace surfaces were buried by up to 2 m of fine sands and muds.

Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas

Investigations in the lower Colorado River Valley, Gulf Coastal Plain of Texas, have resulted in the development of a spatially and temporally controlled history of changes in channel and flood-plain erosional and depositional processes. When combined with paleoclimatic and stratigraphic data from the upper Colorado River drainage and the record of glacio-eustasy in the Gulf of Mexico, this study permits evaluation of the relative influence of different external controls on channel and flood-plain behavior, the development of alluvial landforms, and the development of alluvial stratigraphic sequences. Late Pleistocene and Holocene alluvial deposits of the lower Colorado River have been subdivided into allostratigraphic units, with chronological control afforded by radiocarbon ages. In the bedrock-confined valley, up to 10 m of late Pleistocene (∼20,000-14,000 yr B.P.) sediments referred to as the Eagle Lake Alloformation (ELA) underlie a terrace at 17-20 m above the present-day channel. Deposition of the ELA was followed by bedrock valley incision, then deposition of a complex Holocene valley fill referred to as the Columbus Bend Alloformation (CBA). Columbus Bend Allomembers 1 and 2 (CBA-1 and CBA-2) underlie a terrace at 12-14 m above the present-day channel. CBA-1 was deposited ∼12,000-5,000 yr B.P., whereas CBA-2 was deposited ∼5,000-1,000 yr B.P. Columbus Bend Allomember 3 (CBA-3) consists of channel and flood-plain deposits that represent the past 600 yr of activity. Allostratigraphic units within the lower Colorado valley correlate with allostratigraphic units in major valley axes of the upper Colorado drainage and with records of climatic and environmental change, suggesting that alluvial deposits record basinwide responses to climatically controlled changes in discharge regimes and sediment supply. Basal unconformities for Holocene valley fills, however, appear to be 1,000-2,000 yr younger in the upper Colorado drainage than they are in the lower Colorado valley. This time-transgressioe episode of bedrock valley cutting was initiated by climatically controlled reductions in sediment supply, but conditioned by limits on rates of up-stream propagation of incision through a large drainage basin. By contrast, unconformities within Holocene valley fills document time-parallel episodes of flood-plain abandonment and soil formation, but little additional bedrock valley cutting, and indicate decreased flood magnitudes following shifts to drier climatic conditions. Flood-plain morphology and sedimentary facies changed through time in response to changes in climate coupled with a protracted degradation of upland soil mantles, which altered the rate at which precipitation was transferred to stream channels as runoff. During the late Pleistocene through middle Holocene, runoff was filtered through deep upland soils, floods were for the most part less flashy and contained within channel perimeters, and flood plains were constructed by lateral migration without significant vertical accretion; hence, the ELA and CBA-1 contain few vertical accretion facies. Exposure of bedrock surfaces during the late Holocene resulted in increased flood stages, deep overbank flooding, and construction of flood plains by vertical accretion; hence, CBA-2 and CBA-3 contain thick vertical accretion facies. Allostratigraphic units and bounding unconformities persist through the bedrock-confined valley to the Quaternary alluvial plain, but stratigraphic architecture changes substantially in the downstream direction as a result of the last glacio-eustatic cycle. On the alluvial plain, late Holocene CBA-2 and modern CBA-3, deposited contemporaneously with the present interglacial highstand, onlap and bury the ELA and CBA-1, which were emplaced during the last full glacial lowstand and the transgression that followed.

Reexamination of postglacial vegetation history in northern Idaho: Hager Pond, Bonner Co.

Abstract Hager Pond, a mire in northern Idaho, reveals at least five pollen zones since sediments formed after the last recession of continental ice (>9500 yr BP). Zone I (>9500-8300 yr BP) consists mainly of diploxylon pine, plus low percentages of Abies, Artemisia , and Picea . SEM examination of conifer pollen at selected levels in the zone reveals that Pinus albicaulis, P. monticola , and P. contorta are present in unknown proportions. The zone resembles modern pollen spectra from the Abies lasiocarpa-P. albicaulis association found locally today only at high elevation. Presence of whitebark pine indicates a cooler, moister climate than at present, but one which was rapidly replaced in Zone II (8300-7600 yr BP) by warmer, drier conditions as inferred by prominence of grass with diploxylon pine. Zone III (7600-3000 yr BP) was probably dominated by Pseudotsuga menziesii , plus diploxylon pine and prominent Artemisia and denotes a change in vegetation but continuation of the warmer drier conditions. Beginning at approximately 3000 yr BP Picea engelmannii, Abies lasiocarpa , and/or A. grandis and diploxylon pine were dominants and the inferred climate became cooler and moister concomitant with Neoglaciation. The modern climatic climax (Zone V), with Tsuga heterophylla as dominant, has emerged in approximately the last 1500 yr.

Pleistocene glaciation of volcano Ajusco, central Mexico, and comparison with the standard Mexican glacial sequence

Abstract Three Pleistocene glaciations and two Holocene Neoglacial advances occurred on volcano Ajusco in central Mexico. Lateral moraines of the oldest glaciation, the Marques, above 3250 m are made of light-gray indurated till and are extensively modified by erosion. Below 3200 m the till is dark red, decomposed, and buried beneath volcanic colluvium and tephra. Very strongly to strongly developed soil profiles (Inceptisols) have formed in the Marques till and in overlying colluvia and tephra. Large sharp-crested moraines of the second glaciation, the Santo Tomas, above 3300 m are composed of pale-brown firm till and are somewhat eroded by gullies. Below 3250 m the till is light reddish brown, cemented, and weathered. Less-strongly developed soil profiles (Inceptisols) have formed in the Santo Tomas till and in overlying colluvia and tephra. Narrow-crested moraines of yellowish-brown loose till of the third glaciation, the Albergue, are uneroded. Weakly developed soil profiles (Inceptisols) in the Albergue till have black ash in the upper horizon. Two small Neoglacial moraines of yellowish-brown bouldery till on the cirque floor of the largest valley support weakly developed soil profiles with only A and Cox horizons and no ash in the upper soil horizons. Radiocarbon dating of organic matter of the B horizons developed in tills, volcanic ash, and colluvial volcanic sand includes ages for both the soil-organic residue and the humic-acid fraction, with differences from 140 to 660 yr. The dating provides minimum ages of about 27,000 yr for the Marques glaciation and about 25,000 yr for the Santo Tomas glaciation. Dates for the overlying tephra indicate a complex volcanic history for at least another 15,000 yr. Comparison of the Ajusco glacial sequence with that on Iztaccihuatl to the east suggests that the Marques and Santo Tomas glaciations may be equivalent to the Diamantes glaciation First and Second advances, the Albergue to the Alcalican glaciations, and the Neoglacial to the Ayolotepito advances.

Holocene vegetation history of the Okanogan Valley, Washington

Abstract Haploxylon pine(s) and Artemisia dominated the initial vegetation in front of the receding Okanogan Lobe until ca. 10,000 yr B.P., as revealed by two pollen records in north-central Washington. After 10,000 yr B.P. the macroclimate became warmer throughout the Okanogan drainage as diploxylon pines and Artemisia increased. The Mount Mazama eruption at ca. 6700 yr B.P. is recorded as two stratigraphically separate and petrographically distinct tephra units at Bonaparte Meadows. While there are apparent short-term changes in the vegetation coincident with the ashfall(s), Artemisia continues to dominate the Okanogan Valley until ca. 5000 yr B.P. By 4700 yr B.P. the modern vegetation, dominated by Pseudotsuga menziesii , had become established around Bonaparte Meadows.

Calcified microbial mats date prehistoric canals—Radiocarbon assay of organic extracts from travertine

A methodology for radiocarbon dating of calcified microbial mats has been developed and successfully applied in establishing a preliminary chronology for the prehistoric carbonate-encrusted canals of the Tehuacan Valley of Puebla, Mexico. In contrast to traditional carbonate dating techniques, this method focuses on the organic carbon component of the calcareous sediments. This method is based on the assumptions that (1) the carbon being dated is derived from the organisms associated with travertine buildup on canals, (2) these organisms are incorporated into the travertine contemporaneous with calcite precipitation, and (3) this carbon is then sequestered from most subsequent contamination and secondary fractionation. Evidence supporting these assumptions includes δ13C values, cultural chronologies, and stratigraphic relationships consistent with expectations.

A late paleo‐Indian/Early archaic water well in Mexico—Possible oldest water‐Management feature in the new world

A filled prehistoric water well discovered at the village of San Marcos Necoxtla, Puebla, Mexico, may be the oldest directly dated water-management feature in the Americas. The ∼10 m stratigraphic section exposed at this remarkable site records 18,000+ yr of deposition, erosion, water-table and hydrochemical fluctuations, and ≥10,000 yr of continual occupation. Temporal control is afforded by a multicomponent cultural chronology and radiocarbon assays by conventional and experimental techniques. The ∼10 m wide, ∼5 m deep well was excavated, utilized, maintained, and filled with cultural material between <9863 and ≫5950 yr B.P., a span of ≪3913 yr. Ages of other reported Late Pleistocene(?) to Middle Holocene wells in the New World are based on indirect or questionable dating, or are ≤6600 yr. Because of its age and continuity of occupation, the San Marcos Necoxtla well site may help define the nature of the peopling of the Americas and the advent of New World agriculture.