Between a rock and a soft place: Using optical ages to date ancient clam gardens on the Pacific Northwest

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.

3500 years of shellfish mariculture on the Northwest Coast of North America

“…Prior to ~2500 years ago, the correspondence of sea level with wall height and below wall clams suggests that clam gardens in our study area are at least ~3500 years old. These ages are in line with the ages of other archaeological marine management features on the Northwest Coast [28, 29] and extends the ages for clam gardens reported previously [8, 12].…”

“…Here we build on our previous research on the dating of clam garden construction and use [8, 12] and thus the role of clam gardens in ancient social-ecological systems. Our expanded survey and excavations demonstrate that past clam gardeners built three different types of clam gardens, depending on the pre-existing beach geology and ecology.…”

Historical Ecology of Cultural Keystone Places of the Northwest Coast

“…; Neudorf et al. ). Identifying and dating fish traps and clam gardens is essential to understanding the larger history of CKPs—especially given the trickiness of studying more ephemeral management features and the potential importance of these management features to fisheries today.…”

Exploring community-based marine aquaculture as a coastal resource management opportunity in Nova Scotia, Canada

“…The Mi'kmaq First Nation, one of the predominant Indigenous peoples in Atlantic Canada, are no exception, as it has been estimated that, before European settlement, 90% of their dietary needs were obtained from aquatic resources (Miller 2004;McMillan and Prosper 2016). Ancient examples of what could be considered CBMA, such as the clam gardens of Indigenous peoples in the Pacific Northwest (estimated to be well over 1000 years old), reveal that the concept is not new, and the revival of such a coastal resource management strategy could benefit modern society (Augustine and Dearden 2014;Deur et al 2015;Neudorf et al 2017). Although this model is far from industrial aquaculture, it fits within the broad definition of co-management.…”