Lee C. Nordt

The Buttermilk Creek Complex and the Origins of Clovis at the Debra L. Friedkin Site, Texas

A large artifact assemblage dating to 15,000 years ago lies beneath a Clovis assemblage in central Texas. Compelling archaeological evidence of an occupation older than Clovis (~12.8 to 13.1 thousand years ago) in North America is present at only a few sites, and the stone tool assemblages from these sites are small and varied. The Debra L. Friedkin site, Texas, contains an assemblage of 15,528 artifacts that define the Buttermilk Creek Complex, which stratigraphically underlies a Clovis assemblage and dates between ~13.2 and 15.5 thousand years ago. The Buttermilk Creek Complex confirms the emerging view that people occupied the Americas before Clovis and provides a large artifact assemblage to explore Clovis origins.

High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs

The Triassic successions of the Colorado Plateau preserve an important record of vertebrate evolution and climate change, but correlations to a global Triassic framework are hampered by a lack of geochronological control. Tuffaceous sandstones and siltstones were collected from the Upper Triassic Chinle Formation exposed in the Petrified Forest National Park, Arizona, USA, within a refined stratigraphic context of 31 detailed measured sections. U-Pb analyses by the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the Chinle fluvial system. The base of the Blue Mesa Member of the Chinle Formation is placed at ca. 225 Ma, and the top of the Petrified Forest Member is placed at 208 Ma or younger, bracketing an ∼280-m-thick section that spans nearly the entire Norian Stage of the Late Triassic. Estimated sediment accumulation rates throughout the section reflect extensive hiatuses and/or sediment removal by channel erosion. The new geochronology for the Chinle Formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria. A mid-Norian age (ca. 219–213 Ma) for the distinctive Sonsela conglomeratic sandstone bed constrains the Adamanian-Revueltian land vertebrate faunachron boundary. Our new data permit a significant time overlap between the lower Chinle sequence and the dinosauromorph-rich Ischigualasto Formation of northwestern Argentina. Near-contemporaneity of the trans-American deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the Americas.

Mass-balance reconstruction of a modern Vertisol: implications for interpreting the geochemistry and burial alteration of paleo-Vertisols.

Abstract Utilizing identical sampling and analytical techniques, the morphological and chemical characteristics of a modern Vertisol (Houston Black series, central Texas) can be directly compared with an Upper Mississippian paleo-Vertisol from the Appalachian basin (Pennington Formation, east-central Tennessee). Mass-balance reconstructions suggest retention of primary pedochemical patterns in the paleo-Vertisol, including patterns of soil volume change (strain) and transport functions (translocations) of many major and trace elements. Retention of primary pedochemical patterns suggests that Vertisols constitute nearly closed systems during burial diagenesis. Chemical and mineralogical changes associated with burial diagenesis of the paleo-Vertisol include oxidation of organic carbon (OC), illitization of smectites, dehydration and recrystallization of Fe–Mn oxyhydroxides, and dolomitization of pedogenic calcite. Significant differences in the chemical behavior of gilgai microhigh and microlow pedons in modern Vertisols have implications for interpretation of geochemical data obtained from paleo-Vertisols. Overall wetter soil conditions and variable redox potential under gilgai microlows promote greater depths of leaching and mobility of redox-sensitive trace elements, including Co, Cr, Cu, Mn, Ni, and V. Gilgai microhighs behave as evaporative “wicks” that draw moisture and soluble phases towards the soil surface, resulting in precipitation of metal hydrosylate complexes and sulfates (gypsum) at the capillary fringe and shallower depths of leaching and fixation of trace elements. Paleoprecipitation estimates from paleosols, based on the depth to the top of the pedogenic carbonate horizon, should therefore utilize field, petrographic and geochemical data for characterizing maximum depths of leaching, loss or gain of exchangeable bases, and calcification, rather than relying solely upon field data.

New weathering index improves paleorainfall estimates from Vertisols

Vertisols are clayey, shrink-swell soils that are widely recognized in the rock record, thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations. We assessed the weathering performance of the four major base-forming oxides (CaO, MgO, Na 2 O, K 2 O) along a modern Vertisol climosequence spanning a strong precipitation gradient, and discovered that the concentration of bulk soil CaO and MgO yields the strongest correlation to mean annual precipitation (MAP). Based on this fi nding, we introduce the CALMAG weathering index, defi ned as Al 2 O 3 / (Al 2 O 3 + CaO + MgO) × 100, which improves rainfall estimates for Vertisols relative to the wellestablished CIA-K (chemical index of alteration minus potassium) weathering index. Rather than documenting the hydrolysis of weatherable minerals common in many other soil orders, in Vertisols CALMAG principally tracks the fl ux of calcium and magnesium sourced from calcium carbonate, detrital clay, and exchangeable Ca 2+ and Mg 2+ . Application to two Mesozoic paleosols reveals that in drier climates CIA-K yields higher MAP estimates than CALMAG, but that the reverse is true in wetter climates. This work improves paleorainfall estimates from Vertisols and suggests that a family of weathering indexes is needed for different paleosol types.

Eustatic Control on Alluvial Sequence Stratigraphy: A Possible Example from the Cretaceous-Tertiary Transition of the Tornillo Basin, Big Bend National Park, West Texas, U.S.A.

Paleosol-bearing alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A., and exhibit a three-tier hierarchy of depositional cyclicity. Meter-scale, fluvial aggradational cycles (FACs) occur as fining-upward successions that are gradation- ally overlain by paleosols or are sharply overlain by the coarser- grained base of the succeeding FAC without an intervening paleosol. FACs stack into decameter-scale, fluvial aggradational cycle sets (FAC sets) that also fine upward, and from base to top contain either a grad- ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity. Longer-period trends of FAC thickness, lithologic proportions, paleosol maturity, and paleosol drainage indicate that two complete, and two partial, hectometer-scale fluvial sequences occur within the study in- terval. From base to top, each sequence is characterized by an asym- metric increase and decrease in FAC thickness, a decrease in the pro- portion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic epi- sodes of channel avulsion and stability, and longer-term avulsive chan- nel drift within the alluvial valley, respectively, fluvial sequences may coincide with third-order sea-level changes within the North American Western Interior Seaway. As such, the Cretaceous-Tertiary (K-T) transition within the Tornillo Basin may provide an example of me- gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession. Thickening and thinning successions of FACs record a third-order period of accelerating (transgressive- equivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage- to lowstand-equivalent). Se- quence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorly- drained paleosols are associated with the transgressive-equivalent por- tion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand- and falling stage-equivalent.

Vertisol Carbonate Properties in Relation to Mean Annual Precipitation: Implications for Paleoprecipitation Estimates

Previous publications combining the properties of multiple soil orders show that depth to carbonate (DTC) increases systematically between 350 and 1000 mm of mean annual precipitation (MAP). We hypothesize that carbonate in Vertisols (clay‐rich, shrink‐swell soils) respond differently to water flux than other soil orders because of lower permeability. To test this hypothesis, we compiled soil description and characterization data from multiple published sources across a late Pleistocene climosequence of the coast prairie of Texas to assess the relationship between MAP (700–1400 mm) and DTC. The DTC of carbonate nodules represents an index of accumulation and the DTC of calcium carbonate equivalent (total carbonate <2.0 mm diam.) an index of leaching. The DTC for 1%, 2%, and 5% abundances were assessed using regression analysis. The R2 values were highest for the DTC of 2% nodules and of 1% calcium carbonate equivalent in Vertisol microlows. Surprisingly, relatively high R2 values were calculated for regression between MAP and DTC in Vertisol microhighs, whereby the relationship is expressed as a parabolic curve and DTC is shallowest in the central part of the climosequence where gilgai expression is greatest. When compared with previous MAP‐DTC relationships, it is clear that Vertisols retain carbonate into rainfall isohyets exceeding 1400 mm, >400 mm higher than the preservation of carbonate in other soil orders. When replotted, the use of DTC to estimate paleoprecipitation with previous equations underestimates MAP in a Mississippian paleo‐Vertisol microlow by approximately 32% at a DTC of 100 cm for 5% nodules. Other paleosol proxies also project greater rainfall than previous DTC equations in this paleo‐Vertisol.

Quantifying pedogenic carbonate accumulations using stable carbon isotopes

Abstract Four pedons from a late Quaternary chronosequence developed in calcareous alluvium in central Texas were investigated to assess the ability of the stable C isotope method to partition and quantify pedogenic carbonate accumulations. To quantify pedogenic carbonate accumulations with this method, δ 13 C values of bulk, pedogenic, and parent carbonate must be known. For each pedon, δ 13 C values of bulk carbonate were measured on a horizon by horizon basis. The parent carbonate end-member for all pedons was approximated by averaging the bulk δ 13 C values for all horizons from the weakly developed floodplain soil. The diffusion model of Cerling (1984) and Quade et al. (1989) was used to estimate the pedogenic carbonate end-member. Quantification of pedogenic carbonate accumulations by the isotopic method was compared to quantitative estimates conducted by field morphology, binocular light microscope point counts, and thin-section point counts. Results suggest that the isotopic method is superior to other methods of quantifying pedogenic carbonate accumulations. Whole-soil pedogenic carbonate accumulations as calculated by the isotopic method for the chronosequence were: (1) 1 to 4 vol.% by 2000 years; (2) 4 to 15 vol.% by 5000 years; and (3) 1 to 12 vol.% after 15,000 years. These results reveal net soil carbonate loss through time for central Texas climates. This corroborates similar conclusions drawn for the same chronosequence using mass balance analysis to calculate the flux of carbonate. Net carbonate loss through time also indicates that these soils do not serve as long-term carbonate C sequesters.

Paleosol barometer indicates extreme fluctuations in atmospheric CO2 across the Cretaceous-Tertiary boundary

We present an atmospheric p CO 2 ( p is partial pressure) curve showing extreme fluctuations for the interval between ca. 77 and 63 Ma in southern Alberta, Canada, using a paleosol barometer. Paleosol carbonate nodules (micrite) were collected from 40 Bk horizons among 6 stratigraphic sections for stable carbon isotope analysis. Based on results from the study area, declining atmospheric p CO 2 from 1200 ppmV (V is volume) in the Campanian to 780 ppmV in the Maastrichtian correlates with Late Cretaceous climate cooling and falling sea level as documented in global records. The remarkable rise in atmospheric p CO 2 near 65.5 Ma (1440 ppmV) correlates with volcanic activity associated with the Deccan Traps, rising sea level, and warmer global climates. The decline in atmospheric p CO 2 (760 ppmV) at the Cretaceous-Tertiary boundary and subsequent sharp rise into the Danian (1000 ppmV) occurred during static terrestrial temperatures and sea level. This work provides compelling evidence that atmospheric p CO 2 curves modeled for the Phanerozoic do not offer the resolution needed to understand environmental conditions during catastrophic events in Earth9s history.

Pedogenic carbonate isotopes as evidence for extreme climatic events preceding the Triassic-Jurassic boundary: Implications for the biotic crisis?

The Triassic-Jurassic boundary is associated with widespread marine and terrestrial extinctions, but there is disagreement regarding the existence and extent of climatic changes that may have driven the biotic crisis. Here, we apply quantitative isotopic climate proxies in order to construct two age-equivalent, relatively continuous temperature and p CO 2 records that span the eight million years preceding the Triassic-Jurassic boundary and that supersede previous terrestrial records in temporal resolution. The δ 18 O data suggest that mean annual temperatures (MAT) increased by 7–9 °C from the late Norian to the Rhaetian in association with the peak increases in p CO 2 levels. The δ 13 C data suggest relatively low late Norian p CO 2 levels ( 1500 ppmV), and at least two periods of extreme p CO 2 levels (~3000 ppmV) preceding the Triassic-Jurassic boundary. These estimates are consistent with a recent Late Triassic climate model that suggests the effects of increased p CO 2 levels on Pangea would cause severe climatic consequences, including a global MAT increase of 6 °C (>10 °C in some regions). While it is possible that periods of increased aridity could have resulted in erroneously high estimates of both temperature and p CO 2 levels, it is likely that climate was still fluctuating during the end of the Triassic. Although our data precede the Triassic-Jurassic boundary, many studies conclude that the mass extinction took place over a more prolonged period beginning in the Late Triassic. Thus, climate may have been a significant driving mechanism of the Late Triassic extinctions.

Late Quaternary temperature record from buried soils of the North American Great Plains

We present the first comprehensive late Quaternary record of North American Great Plains temperature by assessing the behavior of the stable isotopic composition (δ 13 C) of buried soils. After examining the relationship between the δ 13 C of topsoil organic matter and July temperature from 61 native prairies within a latitudinal range of 46°–38°N, we applied the resulting regression equation to 64 published δ 13 C values from buried soils of the same region to construct a temperature curve for the past 12 k.y. Estimated temperatures from 12 to 10 ka (1 k.y. = 1000 14 C yr B.P.) fluctuated with a periodicity of ∼1 k.y. with two cool excursions between −4.5 and −3.5 °C and two warmer excursions between −1 and 0 °C, relative to modern. Early Holocene temperatures from ca. 10–7.5 ka were −1.0 to −2.0 °C before rising to +1.0 °C in the middle Holocene between 6.0 and 4.5 ka. After a cool interlude from 4.2 to 2.6 ka, when temperatures dropped to slightly below modern, another warm interval ensued from 2.6 to 1 ka as temperatures increased to ∼+0.5 °C. A final decline in temperature to below modern occurred beginning ca. 0.5 ka. Cooler than present temperatures in the Great Plains indicate telecommunications with cool-water episodes in the Gulf of Mexico and North Atlantic potentially governed by a combination of glacial meltwater pulses and low solar irradiance.