Determining Timescales and Paleoenvironments of Quaternary Human Evolution Using Stable and Radiogenic Isotopes
Assessing the covariance of human biological and cultural evolution with major climate fluctuations requires independent, precise, and accurate chronologies documenting environmental change in association with rare fossils and archaeology. This thesis aims to improve and diversify geochronological tools and associated paleoenvironmental characterization of paleoanthropological sequences using two of the most precise and accurate chronometers applicable in the Quaternary Period, 40Ar/39Ar and U-Series (i.e., 230Th) geochronology, combined with light stable isotopes, to reconstruct timescales and terrestrial paleoenvironments at paleoanthropological sites.
The first chapter refines the age of the Alder Creek sanidine (ACs) fluence monitor which is frequently utilized to determine 40Ar/39Ar ages of volcanic rocks in association with hominid fossil-bearing and archaeological deposits in eastern Africa. The age of ACs is significantly different if anchored by astronomical tuning (tACs = 1.1848 ± 0.0006 Ma, σ) than if anchored solely by constraints from radioactive decay (tACs = 1.1891 ± 0.0008 Ma), possibly due to 1) leads and lags in the deposition of astronomically tuned sequences, 2) inaccuracy of decay constants, and/or 3) assumptions inherent to each dating method. The second chapter outlines the use of C, N, and O stable isotopes of ostrich eggshells to reconstruct local paleoenvironments at African archaeological sites. Combined with novel 230Th burial dating of ostrich eggshells, these may provide precisely dated paleoenvironmental records for sites up to ~ 500 ka. The light stable isotope composition of ostrich eggshells from two eastern African archaeological sites ~50 – 4 ka, which include diachronous occurrences of the Middle to Later Stone Age transition, suggest a mosaic of local environments through time and space that is not apparent from global or regional paleoenvironmental records. In the third chapter, I apply 230Th dating to coral abrader artifacts from a stratified archaeological sequence at Tangatatau Rockshelter, Mangaia (Cook Islands). I compare the results with a recent Bayesian 14C chronology for the site and develop screening criteria to identify reliable 230Th dates from buried contexts at other sites in an effort to develop more precise ways of dating Polynesian expansion. Precise 230Th dates from coral abraders support early Polynesian arrival to Mangaia (1011.6 ± 5.8 CE) and the arrival of the sweet potato (Ipomoea batatas) to no later than 1361–1466 CE, indicating that trans-Pacific voyaging had introduced this South American native plant to the Cook Islands by that time. The techniques used in this thesis demonstrate the power of integrative research in isotope geochemistry to address questions about geological processes and human evolution.