Indigenous Sea Gardens have been Adapting to Sea Level Change for at least Four Millennia

Abstract Indigenous peoples along the northwest coast of North America have maintained sea gardens (clam gardens), an ancestral form of mariculture, for at least four thousand years. While their occurrence is being increasingly reported, the degree to which Holocene sea level change has obscured existing ancestral structures along parts of the coast was unknown. We used intertidal and subtidal archaeological excavations, ground-penetrating radar, and radiocarbon dating to assess sea garden wall structure, thickness, and site formation processes at two coastal sites in southern British Columbia, Canada. By comparing sea garden wall depth to the local relative sea level history, we found that sea gardens have been continuously modified and maintained for at least four millennia, and that the morphology of sea gardens is different in areas of transgressing versus regressing shorelines. Moreover, we found that different local sea level histories will influence the visibility of these monumental features today. These observations, woven with Indigenous knowledge, demonstrate that not only is sea gardening an enduring, adaptive cultural practice that has been resilient to changes in sea level position for millennia, but also offers a strategy for climate change adaptation today.