Abstract
Rock-walled archaeological features are notoriously hard to date, largely because of the absence of suitable organic material for radiocarbon dating. This study demonstrates the efficacy of dating clam garden wall construction using optical dating, and uses optical ages to determine how sedimentation rates in the intertidal zone are affected by clam garden construction. Clam gardens are rock-walled, intertidal terraces that were constructed and maintained by coastal First Nation peoples to increase bivalve habitat and productivity. These features are evidence of ancient shellfish mariculture on the Pacific Northwest and, based on radiocarbon dating, date to at least the late Holocene. Optical dating exploits the luminescence signals of quartz or feldspar minerals to determine the last time the minerals were exposed to sunlight (i.e., their burial age), and thus does not require the presence of organic material. Optical ages were obtained from three clam garden sites on northern Quadra Island, British Columbia, and their reliability was assessed by comparing them to radiocarbon ages derived from shells underneath the clam garden walls, as well as below the terrace sediments. Our optical and radiocarbon ages suggest that construction of these clam garden walls commenced between ~1000 and ~1700 years ago, and our optical ages suggest that construction of the walls was likely incremental and increased sedimentation rates in the intertidal zone by up to fourfold. Results of this study show that when site characteristics are not amenable to radiocarbon dating, optical dating may be the only viable geochronometer. Furthermore, dating rock-walled marine management features and their geomorphic impact can lead to significant advances in our understanding of the intimate relationships that Indigenous peoples worldwide developed with their seascapes.
Highlights
Rock-walled archaeological features, such as fish traps and agricultural terraces, have the potential to provide rich insights into past relationships between people and their naturalPLOS ONE | DOI:10.1371/journal.pone.0171775 February 9, 2017Optical dating ancient clam gardens on the Pacific NorthwestThe Hakai Postdoctoral Fellowship and Hakai Masters Student Fellowship were granted by the Hakai Institution, which is run by Eric Peterson and Christina Munck
We show that infrared stimulated luminescence (IRSL) signals from terrace sediments can detect changes in sedimentation rates that reflect the time, and the rate of clam garden wall construction and build-up
Optical ages helped us constrain the time of initial construction of the clam garden wall at the Single and Triple Wall sites (KB14-05 and EbSh-58), as well as the terrace sedimentation rate and the time of terrace infilling at the Single Wall Site (KB14-05)
Summary
Rock-walled archaeological features, such as fish traps and agricultural terraces, have the potential to provide rich insights into past relationships between people and their natural. We know from the Indigenous terms for these features, as well as from local First Nations knowledge holders, that rolling rocks downslope was part of the regular traditional management of productive clam beaches [19, 20] Determining when these features were constructed, and understanding the timing and nature of the terrace infilling process will help us understand the temporal relationships between settlement patterns of ancient peoples and terrace construction, and the history of Indigenous subsistence strategies and land use along the coast. Three clam gardens on northern Quadra Island were sampled for radiocarbon and optical dating (sites KB14-05, EbSh-5 and EbSh-58) (Fig 1) Each of these gardens was formed by constructing rock walls on sloping sandy and gravelly beaches. We collected radiocarbon dates from shells below the walls or below the terrace infill to provide independent assessments of the ages of wall construction and maintenance
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