Sites forlorn: dating intervals of abandonment at three shell middens on Santa Cruz Island, California using Bayesian chronological models
Introduction
Mobility is a crucial component of human adaptations to environments. Climate, population size, and technology—among other cultural and economic factors—structure movement to and between resource locations. Archaeologists seek to unravel this dense web of interactions by examining accretions of cultural material that accumulates sequentially over time (Stein et al., 2003:297). Consideration of variable mobility patterns, and the factors that influence them, requires chronological precision capable of establishing the true order of events within a given context, and discerning whether events in different contexts are actually contemporaneous (Culleton et al., 2012:1573). Thus, rigorous chronological control is central to inferences about mobility strategies. Repetitive cycles of occupation, abandonment, and reoccupation of favored locales, complicate this pursuit. Stratigraphically complex sites, such as shell middens, are notoriously difficult to decipher (Stein et al., 2003:298). However, several major methodological, analytical, and statistical improvements for building archaeological site chronologies have coalesced during the last decade (see Bayliss, 2009, Bronk Ramsey, 2008a, Bronk Ramsey, 2008b, Buck, 2004). In this paper, I draw upon these advances to establish and interrogate high-precision chronologies for three multi-component shell middens from Santa Cruz Island, California. This refined chronological framework illuminates intervals of site abandonment and facilitates comparison with detailed paleoenvironmental and subsistence reconstructions to shed light on Late Holocene mobility patterns on the Northern California Channel Islands.
The rationale for a Bayesian approach to constructing site chronologies is not only to arrive at more realistic age estimates for individual measurements (sensu Bayliss and Whittle, 2007, Bicho et al., 2012, Culleton et al., 2012, Jazwa et al., 2013, Kennett et al., 2011, Nakamura et al., 2013, Outram et al., 2010, and many others) but also to provide a statistical framework for further analysis (Bronk Ramsey, 1998:463,472). Although the individual dates are of considerable significance, archaeologists are most interested in the temporal parameters of cultural processes and human behavior inferred from chronometric data (Nicholls and Jones, 2001:503). As soon as conventional radiocarbon dates are received from the lab, archaeologists begin the process of interpreting these data. Inevitably, this involves a series of assumptions, some as simple as to whether a series of dates represents multiple dates of a single synchronous event or whether those same dates represent multiple sequential events. An experienced archaeologist will judge the former or the latter to be more plausible based on prior information, such as stratigraphic relationships between contexts containing dated samples (see Bronk Ramsey, 1995:426; 2008a:264). Use of these priors informs interpretation, regardless of whether they are made explicit.
A Bayesian approach to chronology building forces archaeologists to make explicit assumptions about the priors, and provides a quantitative framework to formalize and test assumptions. Agreement indices for the individual calibrations and for the model as a whole quantify the correspondence between the prior and posterior distributions, while accounting for the correlation between parameters (Bronk Ramsey, 2000:201; 2009:357). This statistical measure allows archaeologists to evaluate the validity of assumptions, such as the assumed stratigraphic integrity of a site, on which the model is constructed. A second statistic, convergence, measures the degree to which there is a truly representative and stable solution to the model (for more information, see Markov Chain Monte Carlo samplers, Bronk Ramsey, 1995:429; 2001:359; 2009:353). These two statistics allow archaeologists to evaluate both the reliability and stability of constructed chronological models. Moreover, a well-constructed model can produce reliable date estimates (boundaries) for events not directly dated by single radiocarbon determinations, such as the beginning and end of deposition (Bayliss et al., 2007:6), and can be queried to derive useful information about the temporal duration of an activity (span) or hiatuses between events (intervals). “Interrogating the model”, as Bronk Ramsey (1998:463) refers to such queries, affords archaeologists the opportunity to more effectively extract and evaluate information regarding the significance of temporal patterns. Herein lies the true value of constructing Bayesian site chronologies—the opportunity to move beyond visual inspection of groups of calibrated dates (Bayliss et al., 2007:8–9; Whittle and Bayliss, 2007:21).
In this paper, I begin with a Bayesian approach to integrate non-quantitative contextual information (the priors), with probability distributions from radiocarbon dates (the standardized likelihood) to trim confidence intervals and refine age estimates (the posteriors) for three depositional sequences. However, I focus analysis and discussion on the temporal information derived from querying the final chronological models.
Section snippets
Background
The Late Holocene represents a period of demographic, cultural, and economic development for the Chumash of the Santa Barbara Channel Region of California. Significant population increase (Glassow, 1999:56; Kennett et al., 2009:310) parallels the development of new maritime technologies (Arnold, 2007, Gamble, 2002, Rick et al., 2002, Rick et al., 2004), craft specialization (Arnold, 1992, Arnold, 2001, Arnold, 2004), and the emergence of social hierarchies (Winterhalder et al., 2010). Most of
Materials and methods
The explicit purpose of my excavation at CA-SCRI-236, CA-SCRI-823, and CA-SCRI-568 was to collect samples appropriate for detailed chronological, paleoethnobotanical, zooarchaeological, and isotopic analyses. These samples provide the foundation for evaluating diachronic variation in mobility and associated patterns of resource use. My principal concern was to ensure comparability among the assemblages recovered from each site. At least one volumetrically controlled unit was excavated by trowel
Results
In the resulting chronological models, the posterior modeled date ranges (displayed in black) are trimmed slightly in comparison to the standard calibrations (displayed as an outline). The site sequence-based models (Fig. 2, Fig. 3, Fig. 4, Fig. 5) provide estimates for the beginning and end of each period of site occupation; whereas the period phase-based model Fig. 6 provides general estimates for the beginning and end of occupation associated with each temporal period. The agreement index
Discussion
The protracted hiatus in site occupation that occurred between the terminal Early Period and Middle Period (ca. 2629–1698 cal. BP) at CA-SCRI-236, CA-SCRI-823, and CA-SCRI-568 coincides with recent recognition of a 1500 year gap in the occupation of interior site locations across Santa Cruz Island ca. 2750 BP-1250 cal. BP (Perry and Glassow, 2014:16)3
Conclusion
Bayesian chronological models highlight and provide formal age estimates for two distinct intervals of localized abandonment at three substantial habitation sites located in the western sector of Santa Cruz Island. The earlier, more protracted hiatus following terminal Early Period occupation at each site location represents a temporal datum for a series of significant cultural, demographic, and technological developments throughout the region. If the chronological sequences identified at these
Acknowledgments
This article stems from a paper presented at the SAA's in 2013 in a Symposium organized by Robert L. Kelly and Nicholas Naudinot. The final manuscript has benefitted greatly from comments and criticism provided by Michael Glassow, Barbara Voorhies, Danielle Kurin, Anne Pisor, and two anonymous reviewers. Douglas Kennett and Brendan Culleton provided not only the variable local reservoir correction factors but have also provided a sounding board throughout the development of these chronological
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