Recent Projects

²³⁰Th/U burial dating of ostrich eggshells (OES)

Established chronometers do not fully meet the needs of researchers investigating Middle Stone Age (MSA, ~300 – 30 ka) sites that co-occur with the first fossils of H. sapiens, provide evidence of accelerated human behavioral innovation, and coincide in time with geographic range expansion of H. sapiens outside Africa. MSA sequences commonly contain ostrich eggshells (OES) which consist of ~2-mm thick, low-Mg calcite with 1-3% organics and are amenable to ¹⁴C dating. OES are also geochemically suitable for ²³⁰Th/U dating, though previous attempts neglected to account for secondary U uptake from soil pore fluids upon burial. Our novel approach to ²³⁰Th/U dating of OES explicitly recognizes U as secondary and has produced U-Th dates concordant with ¹⁴C dates on splits of ~10 – 50 ka OES from Lukenya Hill, an East African rock shelter site. From experiments and blind dating tests with other chronometers at ~10 archaeological sites in Sub-Saharan Africa, we have produced ages which agree with other chronological constraints, preserve stratigraphic principles, and refine the chronologies of localities containing evidence of human biological and cultural evolution up to ~150 ka. Our first paper was recently published on the technique, and we have since produced reliable, reproducible ages for a number of paleoanthropological sites (Niespolo et al., 2021a, Niespolo et al., 2021b, Mackay et al., in press at Nature Ecology & Evolution). We are excited to collaborate with paleoanthropologists in these efforts and to explore sites hosting ostrich eggshells.

Stable isotopes in ostrich eggshells as paleoenvironmental proxies

Paleoenvironmental change is commonly invoked as a factor in the development of modern human behaviors and the successful expansion of H. sapiens out of Africa. Paleoenvironmental information from archaeological sequences is central to addressing such questions. Ostrich eggshell (OES) are common in many African archaeological sequences and may be dated by ¹⁴C and U-series methods. In eggshells of modern ratites (large flightless birds including the ostrich and emu), the δ¹³C in eggshell calcite and the δ¹³C and δ¹⁵N in the total organic fraction (TOF) of eggshell have been shown to vary systematically across climate gradients in South Africa and Australia: δ¹⁵N varies inversely with mean annual precipitation (MAP) and δ¹³C co-varies with the C isotopes of vegetation. Thus, if primary C and N isotopic signatures are preserved, assemblages of OES can provide dated records of paleovegetation and paleoprecipitation at archaeological sites.

Below: Recent analyses of OES from Ysterfontein 1 rockshelter (South Africa) utilized these proxies to interpert local paleoenvironmental changes during shelter occupation ~120-113 ka (Niespolo et al., 2021a, PNAS).

Refining chronologies of Polynesian colonization and settlement with U-Th dating of coral artifacts

Polynesian archaeology has relied primarily on ¹⁴C dating to establish the timing of human colonization of the eastern Pacific and the tempo of ensuing cultural development and human-environmental interactions. Despite advancements in ¹⁴C dating including accelerator mass spectrometry, selection of short-lived plant remains to avoid in-built age, and the use of Bayesian models to refine ¹⁴C-based chronologies, large uncertainties introduced during calibration to calendar ages can severely limit the resolution of ¹⁴C dates in this context. ²³⁰Th dating of coral abraders, which are common to many Polynesian archaeological sequences, can potentially provide much more precise dates. The first suite of results are presented in this paper from Tangatatau rock shelter, on Mangaia Island (below, red star) - stay tuned for additional records coming from sites shown in the blue stars below.

Intercalibration of the Alder Creek sanidine standard for ⁴⁰Ar/³⁹Ar geochronology

The Alder Creek Sanidine (ACs) has yielded published ages spanning a range of ~2%, creating a challenge to determining precise and accurate ages of Quaternary unknown samples. In our recent paper in Quaternary Geochronology, we outline an intercalibration study with astronomically tuned tephra to calibrate both directly and as a stepwise intercalibration to Fish Canyon sanidine (FCs), which has an astronomically tuned age of 28.201 ± 0.023 Ma (σ). Since samples were co-irradiated, we utilize the intercalibration factor R to deal directly with ⁴⁰Ar*/³⁹Ar_K ratios (called F values in the paper) to yield an R matrix (seen below). This concept is useful not only to relate two samples measured under the same conditions, but it also aides conversion of ages from other published work whose data was calibrated to other standard (fluence monitor) ages. Combining these results with similarly treated data from other laboratories yields interlaboratory R values and an interlaboratory Astronomical age of ACs. Using the interlaboratory R value relating ACs to FCs, we also provide an optimization age for ACs. Work is ongoing to understand the difference in these two calibrations.

Above, Left: Intercalibration factors (R values) for two commonly used standards in ⁴⁰Ar*/³⁹Ar_K geochronology, ACs and FCs, and for two astronomically-dated tephra, Messâdit 4 (Mellila Basin, Morocco; Kuiper et al., 2008), and A1 (Faneromeni, Crete; Rivera et al., 2011). Above, Right: Interlaboratory ages of ACs. Figure content from Niespolo et al., 2017.

Geochronology of hominid evolution and archaeology in the Middle Awash, Ethiopia

The Middle Awash project has provided crucial insights in the field of human evolution and has expanded our understanding of the emergence of hominids in eastern Africa. The ages of type specimens have been determined with ⁴⁰Ar/³⁹Ar ages of intercalated tephra. I am currently working on dating tephra to constrain ages of previously undescribed hominid and archaeological deposits in the Middle Awash.

Above: Left, map of the the northern sector of the Middle Awash study area, Ethiopia; Right: Stratigraphic sections hosting dated horizons, fossils, and archaeology ranging in age from >158 ka to ~21 ka in the localities labeled on the map. Localities and river catchments labeled in the maps are discussed in Niespolo et al 2021b, PNAS.

The Fucino Basin Preliminary Drilling Program, Italy

This Fucino Basin drilling project is an international collaboration aiming to develop a continuous paleoclimate record from the ancient Fucino Lake that may date as far back as 2Ma. The preliminary results were recently published in Scientific Drilling (Giaccio et al., 2015). Figures below are from that publication, found here.

Above, Left: Map of location of Fucino lake relative to other lacustrine records in Europe of similar or overlapping ages; in center are the calculated and modeled 40Ar/39Ar ages for tephra intercalated in cores (After Manella et al., 2019, QSR)

Geochemistry of Guatemalan Jadeitites and Provenance of Maya Jade Artifacts

Thesis title: Mineralogy, Geochemistry, and Stable Isotopes of Guatemalan Jadeitites: a New Method to Determine the Provenance of Mesoamerican Jade Artifacts (Link to Abstract Here)

In my Master of Science thesis (Niespolo, 2014), I examined geochemical heterogeneities of jadeitites from the Motagua Valley, Guatemala, to infer metasomatic alteration in a subduction zone setting. Using a suite of geochemical analyses, including mineral phase composition and trace-element analysis of jadeite, coupled with stable isotope values of major minerals in jadeitites, I concluded that there may be an alternative protolith of jadeitite than that previously proposed in the literature. In addition, goechemical results showed that pairing oxygen stable isotope composition of jadeite with trace-element abundances of a specific group of elements may distinguish between geological source areas within the Motagua Valley. This discrimination is useful for identifying the provenance of jade artifacts recovered far from their geologic sources in Maya and other Mesoamerican archaeological sites.

Above, Left: Oxygen and Hydrogen isotope calculations for the fliud interacting with the protolith to form jadeitites: these values were calculated using stable isotope compositions of jadeite, albite, and white micas in jadeitite rocks from the Motagua Valley, Guatemala. Data from this project (Niespolo, 2014) and a previous study (Johnson & Harlow, 1999) conclude a similar fluid origin. Right: A cartoon model of jadeite formation in the subduction zone (For more information, see the AGU 2014 Poster)