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After that time, large settlement sites become more apparent in the later Holocene Lepofsky unpublished data. With over 15 km of rock walls [ 15 ] and more than 15 ha of clam habitat [ 9 ], Kanish and Waiatt bays house among the highest density of clam gardens currently known on the Northwest Coast.
These mariculture features are found both on beaches with soft sediments containing pre-existing clam habitat, as well as along bedrock shorelines where boulder wall constructions have encouraged new beaches and clam habitat to form. Gardens tend to be located along the sides of semi-protected inlets with strong tidal currents and away from the heads of bays or deltas.
Local knowledge indicates that First Nations maintained clam garden walls into the 20 th century. Understanding the general characteristics and formation history of clam gardens is a requisite step for dating wall construction and maintenance events and thus for understanding the local development of mariculture. In general, a clam garden consists of a boulder wall constructed at or near the lowest tide line. Wall height was augmented and maintained through on-going rolling of rocks down to the lowest intertidal whenever people dug clams [ 6 ]. The wall increases butter clam Saxidomus gigantea and littleneck Protothaca staminea habitat by creating a shell-hash and sediment filled terrace at particular tidal zones [ 8 , 18 ].
Today, these species of clams can be 2—4 times more productive in clam garden beaches than in non-walled beaches in the same area [ 7 , 19 ]. Within the general form of clam gardens, we identify three variations in construction that occur in isolation and in various combinations, depending on the geomorphology of particular sites Figs 2 and 3.
The three forms are: Form 1- those built on soft sediment beaches with already existing clam habitat Fig 2 , Form 1 a,b ; Fig 3A ; Form 2- those built on flat bedrock outcrops Fig 2 , Form 2 a,b ; Fig 3B ; and Form 3- those created along a section of steep, eroding, bedrock shoreline where people have levelled the eroding boulders to create a flattened platform Fig 2 , Form 3 a,b ; Fig 3C.
The majority of Form 1 gardens consist of a single wall at one tidal height, but some sites have multiple walls and terraces at different tidal heights Fig 3 , d and e. In Form 2 gardens, there may have been small pockets of sediment on the bedrock shelf that could have provided clam habitat prior to wall construction. In contrast, in Form 3, no prior clam habitat could have existed. As we discuss later, the creation of beach habitat where there was none prior is significant both in terms of cultural and natural history, and in the selection of suitable dating material.
Collectively, these forms reflect that people engineered many possible geomorphic settings and ecosystems to maintain and increase the production of their staple marine foods. Falling sea level during the Holocene means that the base of many clam garden walls, particularly those built on soft sediment, are often exposed during the lowest tidal windows today.
Form 1. Clam gardens built on soft sediment beaches; 1a Original unmodified beach surface with existing clam habitat prior to wall construction; 1b Clam garden wall and terrace on same beach. Note clams and barnacle scars from original beach covered by clam garden wall and terrace sediment. Note also some boulders with barnacle scars from original beach moved into clam garden wall during construction.
Stippled light coloured sediment behind wall indicates shell hash and coarse sediment that accumulated after wall was built, thereby expanding clam habitat; Form 2. Clam gardens built on bedrock shelf; 2a Original bedrock shoreline; 2b Wall built on same bedrock to create clam garden terrace.
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Note base of wall is not always accessible at low tide; Form 3. Clam gardens built on steep eroding bedrock shoreline; 3a Original steep, eroding bedrock shoreline. Rocks from mid to upper intertidal and supratidal are moved down slope to create levelled terrace at the elevation for optimal clam habitat.
Note base of wall is usually not accessible at low tide. We conducted subsurface testing, sample collection, and radiocarbon dating at nine clam garden sites, six in Kanish Bay and three in Waiatt Bay Fig 1. Archaeological work was conducted under permits — and — from the BC Archaeology Branch. All samples if not consumed entirely by radiocarbon analysis are presently housed at Simon Fraser University. No human subjects or vertebrates were involved in this study. We focused part of our sampling effort on sites that we believed would yield dates associated with older clam gardens, when sea level was higher.
In addition, we tried to sample beaches in different geomorphological contexts. Our initial sampling efforts focussed on Form 1 gardens but towards the end of our project we expanded our efforts to include Forms 2 and 3. While we tested fourteen gardens, we retrieved suitable dating samples from the nine gardens included in this paper. Our strategy was to target dating contexts that yielded samples associated with the initial construction of the wall, and early use of the clam garden beach.
To examine stratigraphy and extract datable samples at Form 1 clam gardens, we dug shovel tests in the terrace and excavated 1—1. We sought datable material such as barnacle scars, clams, whelks, limpets, and wood at the base of the wall that could be temporally linked to initial wall construction, or for similar samples within the wall and terrace that could be associated with ongoing garden construction, maintenance, and use i.
When the water table allowed, we excavated through the wall or terrace and into underlying original beach deposits. On the original beach surface, we looked for organic materials that would have been trapped by wall construction or terrace infilling and therefore would provide a maximum age for when the clam garden wall was initiated. Note the rounded and sub rounded boulders typically found in the intertidal zone a , compared to the angular boulders in the wall foundation that are typical of eroding bedrock near the tree line b.
In the Form 2 and 3 clam gardens where we could not access the base of the wall during low tide, we dug shovel tests in the terrace down to bedrock or boulder substrate. Within the terraces of all the Forms, our strategy was to find burrowing shellfish specimens that established themselves in the sediments that began accumulating after the initial wall was built. In Form 2 and 3 gardens, we also sought surface dwelling invertebrates that inhabited the bedrock or boulder base immediately prior to sediment infill.
In Form 2 gardens built on bedrock shelves, we were cognizant that there could be naturally accumulated sediment and clams that could pre-date wall construction. In Form 3 walls, after some trial and error, we looked explicitly for portions of the terrace that were part of the newly constructed anthropogenic landform and could not have existed prior to the building of the wall—that is, those portions of the gardens that were built out beyond the original beach edge.
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Depth below surface was recorded for each collected sample. Our understanding of clam garden formation history, and our ability to examine the temporal relationship of dated samples to clam garden construction and past sea levels, required collecting precise elevations of the radiocarbon samples, wall bases, and wall heights. Elevation data were acquired using a survey grade GPS Topcon GR5 and real time kinematic methods, with elevation values referenced to local chart datum tidal heights. Elevations of all excavation units were extracted from the DSM.
Positional accuracy was calculated based on statistical analysis of our control and SfM software results. SfM survey accuracy typically degrades over larger areas [ 21 ]. Therefore, tidal stage reference points were added to boundary areas located more than m away from GCP targets to improve vertical accuracy. Due to unclear stratigraphy and high water tables we were not able to collect suitable dating samples from the Form 2 gardens. In total, we analyzed sixty-one radiocarbon samples from the nine sites S1 Table , S2 Table.
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All samples have allowed us to establish sampling guidelines for dating clam gardens S1 File in areas of falling sea levels. We discuss each group of unsuitable and suitable samples below followed by a discussion of how the relative sea level history and tidal heights of the samples and walls can also be used to ascertain clam garden ages. All radiocarbon dates discussed in the text are the median calibrated cal BP dates.
Twenty-six samples from complicated depositional contexts could not be tied definitively to clam garden construction or use and thus were removed from our age assessments of the gardens S1 Table. These samples do, however, help us understand the best contexts for dating clam gardens S1 File. The samples fall into four different contexts. The first set is composed of very young samples from within and below the walls that were extracted too close to the wall exterior to offer insight into wall construction or use sample , , , That is, we realized that modern juvenile shellfish and barnacle spat can easily settle within the air spaces between the outer more exposed boulders.
Our inference that this zone is biologically active is reinforced by the fact we found live clams up to 1. As we came to understand clam garden construction, we directed our sample collection to the base of those walls or parts of walls that were several rocks tall preferably in excess of 30 cm—the maximum burrowing depth of clams , densely packed, and encased in sediment to avoid recent clams that may have burrowed into or under the wall. A second set of removed samples are those that are in secondary contexts and thus pre-date the wall construction event.
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For example, we learned that clam samples most useful for dating are those that died in growth position i. In two instances, we sampled clams from within the wall that were not in growth position , Their ages are older than other clams from secure dating contexts below the same section of wall, leading us to hypothesize that people used the older clams as wall-fill along with the pebbles and cobbles that are common wall building materials Fig 4.
The third set of removed samples comes from shellfish specimens that we later identified as, or found associated with, invasive species and were buried within the terrace as a result of recent sedimentation perhaps associated with 20 th century logging , The final group of removed samples are barnacle scars , , and clams , , , , , , , , , , , , , , , from distinct paleobeach deposits located well below and dating much earlier than the clam garden walls and terraces.
These clam samples represent early Holocene specimens that were lying on or within the old beach surface for thousands of years before being buried by the clam garden wall or sediments. We surmise that these death assemblages form during periods of relative sea level stasis, when beach sediments and organisms accumulate in the paleo-environmental record. The paleo-shells are a fundamental component of our study reconstructing the shellfish ecology of each beach prior to clam garden construction [ 9 , 22 , 23 ] and have been used to help refine late Pleistocene and early Holocene sea level history [ 13 ], but play only a limited role in this discussion on dating clam gardens.
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We suspect that some of our later Holocene samples 4,—3, year range discussed below may have also accumulated during a period of relative sea level stasis, just prior to or concomitant with construction of the first clam gardens. Radiocarbon calibrations and curve produced in OxCal 4. Note there are no below wall samples from sites WB08 and WB02 as these are Form 3 clam gardens and wall bases were inaccessible. Radiocarbon calibrations and probability plots produced with OxCal 4. IntCal 13 atmospheric and Marine 13 marine [ 27 ]. Samples recovered from the terraces or within the walls were deposited sometime during the use of the garden.
Three additional samples that age the use of clam gardens include a barnacle scar and a whelk , found immediately below the clam garden terrace sediments and in shovel tests well inshore and upslope of the wall sample type 4 in S2 Table and S1 File. While these samples are below the terrace sediments, and in the cases of samples and , on the surface of the former pre-garden beach, we consider these samples to be use dates as it would have taken some unknown amount of time after wall construction for these samples to be covered by sediment.
Determining when these samples were deposited relative to initial wall construction requires understanding how terraces were in-filled with sediment post-wall construction. Our current hypothesis is that sedimentation first occurred behind the wall and then progressed upslope; however we do not know the rate or pattern of this progression. The below wall samples are composed of clams, whelks, and a limpet that were trapped during wall construction when the first boulders were placed at the base of the walls.
As such, these samples provide lower constraining ages for clam garden wall construction sample types 5 and 6 in S2 Table and S1 File ; Fig 7. In our study, these samples fall within two time periods. Three of the six specimens from below the wall are non-burrowing organisms and would have lived and died on the beach surface. Because these three are epifaunal we can be confident that they date the old beach surface at or near the time of wall construction.