Geology topics

Gregory W. L. Hodgins

Publications and source records attributed to Gregory W. L. Hodgins.

5 recordsLinked to original sources

USGS2018

Subseasonal variations in marine reservoir age from pre-bomb Donax obesulus and Protothaca asperrima shell carbonate

Two Donax obesulus and two Protothaca asperrima shells collected prior to the nuclear testing of the 1950's were micromilled at sub-seasonal resolution to yield new reservoir effect (ΔR) estimates for the coast of Peru. Shells from northern (4°40′S to 8°14′S) and central (13°52′S) Peru produced ΔR values of 123 ± 50 and 110 ± 49 years respectively. We found such values statistically indistinguishable from each other while reporting intra-annual ΔR variability along shells ontogeny in agreement with previously published regional ΔR values. This similarity suggests radiocarbon dating of short-lived (<1.5 years) mollusks shells is a useful tool for comparing marine radiocarbon signals. Additionally, our analysis of ΔR and δ 18 O changes along shells' ontogeny found a tenuous relationship between them. We are not the first one to report these phenomena in Peruvian upwelling , but we are the first to suggest that fast Lagrangian water mixing as a mechanism behind it.

Chemical Geology↗

USGS2017

Radiocarbon chronometry of Site QJ-280, Quebrada Jaguay, a terminal Pleistocene to early Holocene fishing site in southern Peru

Excavations in 1970, 1996, and 1999 at Site QJ-280, Quebrada Jaguay, in southern Peru, yielded enough dateable terrestrial plant material to establish an extensive radiocarbon chronology for the site. QJ-280 is one of oldest well-dated fishing sites in the Americas: it was occupied from the terminal Pleistocene to the mid-Holocene (about 13,000–8,300 calibrated years BP) based on 42 terrestrial radiocarbon dates, encompassing the Jaguay and Machas Phases of the local archaeological sequence. In addition to the terrestrial dates, radiocarbon measurements on valves of two marine surf clam ( Mesodesma donacium ) individuals from a single, well-dated mid-Holocene Manos Phase archaeological context have provided insight into marine upwelling conditions during the occupation of Quebrada Jaguay. The marine reservoir age varied between 130 and 730 14 C years during the brief lives of the two clams (up to 5 years each), and varied by up to 530 14 C years within an individual valve, suggesting strong and variable deep marine upwelling; conditions broadly similar to those that exist in coastal Peru today. These rapid variations in marine radiocarbon age suggest that marine radiocarbon dates from environments with variable upwelling could be skewed by up to hundreds of years.

Journal of Island and Coastal Archaeology↗

USGS2017

Chronology and ecology of late Pleistocene megafauna in the northern Willamette Valley, Oregon

Since the mid-19th century, western Oregon's Willamette Valley has been a source of remains from a wide variety of extinct megafauna. Few of these have been previously described or dated, but new chronologic and isotopic analyses in conjunction with updated evaluations of stratigraphic context provide substantial new information on the species present, timing of losses, and paleoenvironmental conditions. Using subfossil material from the northern valley, we use AMS radiocarbon dating, stable isotope (δ 13 C and δ 15 N) analyses, and taxonomic dietary specialization and habitat preferences to reconstruct environments and to develop a local chronology of events that we then compare with continental and regional archaeological and paleoenvironmental data. Analysis of twelve bone specimens demonstrates the presence of bison, mammoth, horse, sloth, and mastodon from ~ 15,000–13,000 cal yr BP. The latest ages coincide with changing regional climate corresponding to the onset of the Younger Dryas. It is suggested that cooling conditions led to increased forest cover, and, along with river aggradation, reduced the area of preferred habitat for the larger bodied herbivores, which contributed to the demise of local megafauna. Archaeological evidence for megafauna–human interactions in the Pacific Northwest is scarce, limiting our ability to address the human role in causing extinction.

Oregon↗

USGS2016

Centuries of marine radiocarbon reservoir age variation within archaeological Mesodesma Donacium shells from Southern Peru

Mollusk shells provide brief (<5 yr per shell) records of past marine conditions, including marine radiocarbon reservoir age ( R ) and upwelling. We report 21 14 C ages and R calculations on small (∼2 mg) samples from 2 Mesodesma donacium (surf clam) shells. These shells were excavated from a semi-subterranean house floor stratum 14 C dated to 7625±35 BP at site QJ-280, Quebrada Jaguay, southern Peru. The ranges in marine 14 C ages (and thus R ) from the 2 shells are 530 and 170 14 C yr; R from individual aragonite samples spans 130±60 to 730±170 1 4 C yr. This intrashell 14 C variability suggests that 14 C dating of small (time-slice much less than 1 yr) marine samples from a variable- R (i.e. variable-upwelling) environment may introduce centuries of chronometric uncertainty.

Radiocarbon↗

USGS2013

Marine radiocarbon reservoir age variation in Donax obesulus shells from northern Peru: Late Holocene evidence for extended El Niño

For at least 6 m.y., El Niño events have posed the greatest environmental risk on the Peruvian coast. A better understanding of El Niño is essential for predicting future risk and growth in this tropical desert. To achieve this we analyzed archaeological and modern pre-bomb shells from the surf clam Donax for the radiocarbon reservoir effect (ΔR) to characterize late Holocene coastal upwelling conditions in northern Peru (8°14′S). Mean ΔR values from these shells suggest that modern upwelling conditions in this region were likely established between A.D. 539 and A.D. 1578. Our radiocarbon data suggest that upwelling conditions ca. A.D. 539 were less intense than those in modern times. The observed coastal water enrichment in 14 C may be consequence of frequent strong El Niño events or extended El Niño–like conditions. These ΔR-inferred marine conditions are in agreement with proposed extended El Niño activity in proxy and archaeological records of ca. A.D. 475–530. Extended El Niño conditions have been linked to political destabilization, societal transformation, and collapse of the Moche civilization in northern Peru. A return to such conditions would have significant impacts on the dense population of this region today and in the near future.

Geology↗