Fig 1.
Locations of clam garden study sites superimposed on a hillshaded and contoured LiDAR digital elevation model (average point density of 18.2 pts/m2).
The LiDAR data were obtained and processed by Rob Vogt of the University of Northern British Columbia Lidar Research Group and Derek Heathfield of the Hakai Institute Coastal Sand Ecosystems program.
Fig 2.
A) Location of trenches 1 and 2 at Single Wall Site, KB14-05. Vertical core KB14-05-B was extracted from non-walled beach sediments seaward of trench 2 (arrow). Vertical core KB14-05-A was extracted from terrace sediments ~2 m inland from the edge of the photo (arrow). B) and C) show the locations of optical samples KB14-05-1 and KB14-05-2. Sample KB14-05-1 was collected from sediments immediately underlying the lowest rocks of the clam garden wall in trench 1. Sample KB14-05-2 was collected from sediments between rocks within the clam garden wall, 50 cm below the terrace surface in trench 2.
Fig 3.
A) Location of trenches 1 and 2 at Double Wall Site, EbSh-5. The best maximum age estimate for wall construction was obtained from a radiocarbon age of 980–1362 cal years BP from a Saxidomus gigantea shell collected from below the wall in trench 1. Optical sample EbSh-5-1 was taken from sediments immediately below the upper wall in trench 2. The Minimum Age Model (MAM) age of this sample is consistent with a radiocarbon age of 3619–3969 cal years BP obtained from a S. gigantea shell immediately next to the optical dating sample. The correspondence between the MAM-based age and the radiocarbon age determinations gives us confidence in our optical dating methods.
Fig 4.
A) Triple Wall Site, EbSh-58. Optical sample EbSh-58-1 was taken from sediments immediately below the middle wall. The ages of the youngest aliquots measured from this sample (i.e., the Minimum Age Model age of 1.43–2.40 ka) is consistent with a radiocarbon age of 1279–1673 cal years BP from a Nucella sp. shell collected from the surface of the pre-garden beach underlying the highest terrace, suggesting that the pre-garden beach surface underlying both the middle and upper walls is ~1200–2400 years old. These ages provide the best maximum age estimates of wall construction on the middle and highest terraces. The youngest Finite Mixture Model (FMM) component (2.83–4.91 ka) is consistent with a pre-wall construction radiocarbon age of 3336–3799 cal years BP from a S. gigantea shell in growth position found trapped below the middle wall. The older FMM component (16.5 ± 1.2 ka) dates back to early postglacial time, and likely records the deposition of outwash sand by glaciers proximal to site EbSh-58.
Fig 5.
A conceptual diagram illustrating the sampling strategy for optical dating clam gardens.
All methods (i–iv) were applied at site KB14-05 (Single Wall Site) while one method (i) was applied at sites EbSh-5 (Double Wall Site) and EbSh-58 (Triple Wall Site). Not drawn to scale.
Table 1.
Radiocarbon ages from excavations of three clam gardens on Quadra Island, BC (2013–2015).
Table 2.
Optical ages from three clam gardens on Quadra Island, BC (2014–2015).
See Table 3 for more information.
Table 3.
Sample De values, OD values, fading rates and optical ages1.
Reported fading rates are weighted mean values from 12 multi-grain aliquots measured from each sample.
Fig 6.
Multi-grain aliquot De distribution of samples EbSh-58-1 and KB14-05-A1.
These samples are thought to contain mixtures of grains from sedimentary layers of different ages. The radial plot and histogram of EbSh-58-1 are plotted in (A) and (B), respectively. The radial plot and histogram of KB14-05-A1 are plotted in (C) and (D), respectively. Finite Mixture Model components 1 and 2 are shown as red dashed lines. The Minimum Age Model De value of sample EbSh-58-1 is shown as a solid black line, and was calculated after excluding two anomalously low De values (outliers).
Fig 7.
IRSL measurements and optical ages from core samples extracted near trench 2, site KB14-05.
Due to compaction, the cores were only partially filled with sediment (brown shading) before extraction. Sensitivity-corrected IRSL measurements were made from subsamples collected every 2 cm along the length of the core. Age estimates were obtained above and below the horizon where the IRSL signal intensity increases dramatically from almost negligible levels in core KB14-05-A. This dramatic increase is interpreted to represent the pre-garden beach surface. Age estimates were also obtained from near the surface and near the base of core KB14-05-B. The ages of optical sample KB14-05-1 and a S. gigantea shell collected from below the wall in trench 1 are shown for comparison.