Date of Award

Summer 8-19-2022

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Interdisciplinary Program

Advisor

Daniel H. Sandweiss

Second Committee Member

Alice R. Kelley

Third Committee Member

Kirk A. Maasch

Additional Committee Members

Karl Kreutz

Alan Wanamaker

Abstract

El Niño-Southern Oscillation (ENSO) is a climate phenomenon with periodic events that impact humans in the study area along the North Coast of Peru. In this region, Eastern Pacific and Coastal El Niño events are associated with anomalously high sea surface temperature (SST), reduced upwelling, and heavy precipitation (Grados et al., 2018; Sandweiss et al., 2020). Major changes in ENSO occurred during the Mid- to Late-Holocene, ca. 5.8 and 2.9 ka, resulting in rapid increases in event frequency to modern conditions. To better identify human adaption to changing climate during the Holocene, we must improve our understanding of ENSO. Though limited climate records exist along coastal Peru, marine bivalves present unique opportunities for studying ENSO (Sandweiss et al., 2020). Common in coastal archaeological middens, they are among the most promising ENSO proxies in this region (Sandweiss and Kelley, 2012; Sandweiss et al., 2020). This research explores two understudied bivalves, Tivela hians and Anadara tuberculosa and determines their growth lines are discrete and can be sampled. Carbonate material from lunar periods in their outer shell is sampled and analyzed to investigate oxygen and carbon stable isotopes. Fractionation rates of oxygen isotopes in shell carbonate is temperature and salinity dependent and well understood (Branscombe, Schulting, Lee-Thorp, and Leng, 2021; Dettman, Reische, and Lohmann, 1999; Carré et al., 2013; Grossman and Ku, 1986). Resulting δ18Oaragonite records are evaluated using a standard SST equation and δ18Oaragonite-derived SSTs are compared to SST and salinity measurements from harvest locations. T.hians δ18O accurately represents measured SST in 75% of the study collection. Salinityvariations do not correlate with unexplained variance in one of the four specimens, which may be attributed to sampling error. δ18O in A. tuberculosa growth rings appears to be influenced by seasonal salinity gradients which form in the near-equatorial Gulf of Guayaquil, complicating the temperature-dependent relationship of oxygen isotope fractionation in carbonate shell material. More information about regional salinity is required to account for the impact of salinity on δ18O precipitation in A. tuberculosa, as well as to determine if visually identified growth disruptions represent slowed or stopped growth during temperature stress.

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