James Pierson

East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins

The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lakes water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after ≈70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations.

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.

Late Pleistocene luminescence chronology of loess deposition in the Missouri and Mississippi river valleys, United States

The loess stratigraphy of the mid-continental U.S. is an important proxy record for the advance of the North American ice sheet into the catchment of the Mississippi, Missouri and Ohio rivers. One of the most outstanding problems is deciphering the age of the loess deposits in this area during the Late Pleistocene. We used multiple-aliquot additive dose procedures under infrared stimulated luminescence (IRSL) and single aliquot regeneration (SAR) protocols to resolve ages of loess at the Loveland Silt type locality, Iowa, the Pleasant Grove School section, Illinois, and the Bonfils Quarry section, Missouri. Radiocarbon dated levels in the Peoria Loess and Roxana Silt were used to test the accuracy of the IRSL and SAR methods. SAR on polymineral and quartz extracts yielded underestimates in age by 20–55%, whereas IRSL gave concordant ages to the 14C control and was used to date the loess sequences. The oldest loess, the Bonfils Silt, gave IRSL ages between 159 and 264 ka and are considered non-finite estimates because the luminescence response was near saturation with additive β dose. The Loveland Silt at the type locality exhibited considerable luminescence in growth with additive β dose and yielded the mean age of 159±14 ka (n=4). The Teneriffe Silt gave a mean IRSL age of 93±5 ka (n=4) which, overlaps at two σ with a previous TL age and indicates deposition sometime between 100 and 80 ka. The Roxana Silt yielded stratigraphically consistent IRSL ages between ca. 60 and 30 ka. Luminescence and radiocarbon dating indicate that there were four distinct periods of loess deposition in the Mississippi and Missouri river basins at ca. 180–140 ka, 100–80 ka, 60–30 ka, and 25–12 ka which are concordant with periods of meltwater input to the Gulf of Mexico, another proxy of glaciation in mid-continental North America.

Aeolian sand depositional records from western Nebraska: landscape response to droughts in the past 1500 years

The Great Plains is dominated by presently stabilized dune fields that are indicators of extreme drought in the late Holocene. This study focused on deciphering the timing of reactivation of dunes in western Nebraska. Stratigraphy adjacent to dune-dammed lakes reveals aeolian sand separated by palaeosols, indicating mobilization of aeolian sand followed by landscape stability. The chronology of aeolian-sand depositional events is constrained using the luminescence-based, single aliquot regeneration method, providing resolution to relate dune movement to tree-ring and palaeolimnologic records of drought. There are at least a six aeolian depositional events in the past 1500 years, with apparent mean ages of 1390+130, 670 +70, 470 +40, 240 +40, 140 +20 and 70+10 yr. All study sites show evidence for aeolian accumulation in the twentieth century, potentially reflecting the 1930s drought. Significant aeolian activity is coincident with the tree-ring-identified sixteenth-century megadrought, indicating widespread landscape impacts.

Stratigraphy and chronology of Mississippi Valley loess in western Tennessee

The stratigraphy and chronology of loess deposits in the Mississippi Valley provide an important proxy record of the number and timing of glacial advances into the upper Mississippi drainage basin. The stratigraphy of loess and paleosols from two exposures in northwestern Tennessee reveals three major loess units that are correlated with the Loveland Loess, Roxana Silt, and Peoria Loess—from oldest to youngest. Detailed measurements of clay and secondary (pedogenic) iron contents suggest that the Loveland Loess may consist of multiple loess units which have been pedogenically welded to one another to form a composite Sangamon Soil. The mineralogy of the coarse silt fraction from one exposure reveals both primary mineralogical differences between the Peoria Loess and the Roxana Silt and pedogenic overprinting by the Sangamon Soil. Grain size and iron data suggest that the degree of pedogenic alteration increases from west to east, away from the loess source area, which probably reflects the role of sedimentation rate on the degree of pedogenic alteration of loess. However, only the Peoria Loess shows the expected exponential eastward decrease in thickness; the thickness of older loess units appears to be controlled more by the occurrence of localized erosion than by distance from source area. Most samples are resistant to solar resetting of the thermoluminescence signal, which, if unrecognized, could yield thermoluminescence ages that significantly underestimate the true age of the deposits. The best match between thermoluminescence age using the total bleach method and radiocarbon age of the Peoria Loess and Roxana Silt is achieved when either long durations of optical bleaching by sunlight (≥32 hr) or artificial ultraviolet (UV) light are applied to define the residual thermoluminescence level. Partial bleach analysis indicates that long-duration optical bleaching by sunlight or UV lamp bleaching does not systematically overbleach samples of Peoria Loess and Roxana Silt. Three thermoluminescence age estimates between 38 ± 4 (±1σ) and 53 ± 5 ka from the basal half of the Roxana Silt, and six age estimates between 18 ± 2 and 25 ± 3 ka from the basal half of the Peoria Loess are consistent with numerous finite radiocarbon dates from around the central Mississippi Valley. Thermoluminescence age estimates suggest that the oldest of the intra-Loveland Loess units is 127 ± 12 ka (one age) and may have been deposited during marine oxygen isotope stage 6. The younger two of these loess units were deposited between 113 ± 10 and 70 ± 6 ka (three ages). This latter age range for loess that underlies the top of the presumed Sangamon Soil is problematic because it requires that pedogenesis may have begun as late as approximately 70 ka. Moreover, loess deposited between about 120 and 70 ka suggests that the southern margin of the Laurentide ice sheet was within some part of the upper Mississippi Valley during this interval. This conflicts with some glacial geologic evidence, but it is consistent with global sea level and δ18O records of ice volume, which indicate several intervals of moderate glaciation during this time.

Formation of linear and parabolic dunes on the eastern Snake River Plain, Idaho in the nineteenth century

Abstract Geomorphic and stratigraphic evidence show periods of distinct hydrologic excess and deficit sometime in the past 500 years on the eastern Snake River Plain. Natural exposures at the current shore of Mud Lake contain a lacustrine silty sand beneath eolian sands; the latter deposit is expressed at the surface as hairpin parabolic and linear dunes. These dunes can be traced into and across lakes, a clear sign of recent aridity. Two shallow-water gastropods and two wood fragments from the lacustrine silty sand yield the respective AMS 14 C ages of ca. 705 and 988 year BP, and 195 and 305 year BP. The offset in ages indicates a lake hard-water effect of aquatic gastropods, with the wood ages providing the closest age constraint. Optically stimulated luminescence single aliquot regeneration ages of 150±30 and 120±20 year BP on quartz grains from the overlying eolian sands further constrains a drop in lake level and subsequent aridity to the 19th century. This lacustrine–dune sequence is probably the geomorphic expression of a distinct wet period followed by a severe drought between AD 1845 and 1856, which has been documented in the dendroclimatic records for the Rocky Mountains and the western Great Plains. Lake fluctuations and dune formation is associated with greater subdecadal variability in spring precipitation, than recorded in the past 100 years.

Assessing the accuracy of thermoluminescence for dating baked sediments beneath late Quaternary lava flows, Snake River Plain, Idaho

Baked sediments beneath lava flows on the Snake River Plain, Idaho, with independent age control by either 14C or K/Ar dating were analyzed to evaluate the accuracy of the thermoluminescence (TL) technique. The age of flows ranges from ∼2 to 100 ka and multiple TL analyses by the total bleach method yielded ages that overlap at one sigma with independent chronologic control. The TL signal of one sample of baked sediment beneath a lava flow with an inferred age of at least 641±54 ka was near saturation, perhaps reflecting a relatively high environmental dose rate, and is not datable by TL. This study underscores several major limitations of luminescence geochronology, the natural spatial and temporal variability in environmental radioactivity and the susceptibility of silicate minerals to the growth and retention of a luminescence signal. Despite these limitations, the results demonstrate the utility of luminescence geochronology to date volcanic eruptive events during the Quaternary.

Holocene lake level history of the Tangra Yumco lake system, southern-central Tibetan Plateau

Massive carbonate banks representing ancient lacustrine deposits are exposed in the catchment of the lake Tangra Yumco (southern-central Tibetan Plateau) and nearby lake Xuru Co. Nine sediment samples were taken below and above these lacustrine deposits to determine periods of changing lake level using optically stimulated luminescence (OSL) applying a multiple-aliquot regeneration (MAR) protocol. According to facies and stratigraphy, samples below the carbonate banks indicate a rising and samples from above a falling lake level. The results indicate that the rising lake level of Tangra Yumco passed the sampling location at 10.5 and 2.1 ka while a falling lake level passed the sampling location at 0.9 ka. The rising lake level of Xuru Co passed the sampling location at 7.9 and 1.7 ka and a falling lake level at 0.5 ka showing a similar trend as at Tangra Yumco. Combining these results with recalculated cosmogenic nuclide ages and previously published feldspar luminescence data allows the establishment of a Holocene lake level reconstruction for Tangra Yumco, which is unique for the southern-central Tibetan Plateau. The lake level of Tangra Yumco crested a lake level highstand of 181–183 m above the recent lake level prior to 8.5 ka and has generally fallen after 8.5 ka, with a minor lake level rise at 2.1 ka. Lake level variations at Tangra Yumco occur simultaneously with other lakes on the Tibetan Plateau indicating that variations were controlled by monsoonal dynamics with a moist early Holocene and a successive reduction of available moisture thereafter. The average rate of the lake level rise between 10.5 and 8.5 ka is at least 0.05 m a−1 (compared with a modern value of 0.38 m a−1), providing valuable insights in the monsoonal impact on lake level change on the southern-central Tibetan Plateau.