Starch granule analysis of bedrock metates in Warner Valley, Oregon

Starch-rich geophytes are a highly-valued food among many human societies. For example, Indigenous people in the northern Great Basin plan social activities around the seasonal foraging of bulbs, roots, and tubers. Despite such obvious dietary and cultural importance, the antiquity of geophyte use in the Great Basin remains difficult to establish. Herbaceous underground storage organs do not preserve well in the archaeological record. Therefore, most studies rely on indirect evidence to infer geophyte consumption by hunter-gatherers during the late Pleistocene/early Holocene.  It has been suggested that bedrock metates found among upland rock art sites in the northern Great Basin reflect seasonal geophyte exploitation over 14,000 years. Our study tests this hypothesis by analyzing starch residue extracted from bedrock metates at three archaeological sites in the uplands of Warner Valley, Oregon. Species of biscuit root (Lomatium) were collected in the field and sampled for starch. Systematic studies conducted on granules defined morphological characteristics were then applied to the identification of archaeological granules. Starch granules from geophytes, specifically Lomatium spp., were identified on metate surfaces at all sites, thereby providing direct evidence for the collection and processing of geophytes. These results support previous hypotheses regarding Paleoindian foraging strategies in the northern Great Basin.

Methods
Reference Materials. Our approach to identifying archaeological starch granules from bedrock metates began with a systematic study of Lomatium granules extracted from extant plant populations within the vicinity of the sites (Louderback et al. 2022b). Three Lomatium species (L. donnellii, L. macrocarpum, and L. triternatum) were collected and each taproot (n = 3 per species) was sampled for starch (Figure 3). Using randomly generated X-Y coordinates on the microscope stage, morphological characteristics (e.g., size, shape, surface features) of 100 starch granules from each taproot (n=900 granules total) were measured and described (see Louderback et al. 2022a for descriptions of morphological characteristics). The frequency of those characteristics occurring on granules was calculated and expressed as a number between 0.0 and 1.0. This approach allowed us to develop a set of statistically defined characteristics frequently observed in Lomatium granules, which can then be applied to the identification of archaeological granules.

Sample Collection in the Field. Starch samples were collected from the surfaces of 58 bedrock metates from all three sites (Barry Spring n=28, Corral Lake n=6, Long Lake n=24). To minimize contamination, nitrile-free disposable gloves were worn throughout the extraction process, a new pair for each sample (see Wilks et al, in prep). Loose sediment was brushed from metate surfaces prior to sampling. Approximately 50 ml of distilled water (DH20) was then added to the stone surface and cleaned with an ultrasonic brush. A sterile syringe was then used to transfer the DH20 and sediment to a sterile test tube labeled ‘metate surface’. This was repeated until the DH20 wash was clear.

Next, 50 ml of a 2.5 percent solution of sodium hexametaphosphate (Na-Hex) was added to the cleaned bedrock surfaces to deflocculate residue that was more deeply embedded in the stone’s interstitial matrix. After about one hour ~ 50ml of DH20 was added and a sterile electric toothbrush was used to vigorously clean the surface for five minutes. A sterile syringe was then used to transfer the resultant serum to a 50 ml test tube labeled ‘metate interstitial’.

Control Samples. Thirteen control samples (Barry Spring n=6, Corral Lake n=1, Long Lake n=6) were also collected from non-worked or non-cultural stone surfaces approximately 5–10 meters from the sampled archaeological bedrock metates. Figure 4 demonstrates the location of control samples in relation to sampled surfaces at Long Lake. These control samples were collected at each site and treated in the same way as the surface and interstitial starch samples. Control samples were labeled ‘control’ and numbered consecutively as they were collected in the field. The purpose is to compare the control sample residue to the residue extracted from the bedrock metate. Although non-cultural stone surfaces in the vicinity of bedrock metates may have been contaminated with starch from associated plant materials, this source would have insignificant levels of starch granules when compared to starch granules ground into the stone matrix of bedrock cultural features and artifacts.

 Laboratory Analysis. All samples were processed in the Archaeobotany lab at the Natural History Museum of Utah (NHMU). Each residue serum was sieved through a 125 μm mesh Endecott sieve into a beaker using deionized water (DiH20). Sample water >125 μm was discarded, while sampled portion <125 μm was retained and transferred to a 50 ml test tube and centrifuged for 3 minutes at 3000 RPM. The supernatant was then discarded, the sample pellet transferred to a new 15 ml test tube, re-suspended with ~10 ml of DiH20, mixed with a vortex, and centrifuged for 3 minutes at 3000 RPM. Approximately, 7 ml of Lithium heteropolytungstate (LST: specific gravity 2.2) was added, resuspended with a vortex mixer then centrifuged for 15 minutes at 1000 RPM. The starch samples were extracted from the surface of the heavy liquid using a pipette and transferred to a freshly labeled 15 ml test tube. To remove any residual heavy liquid each sample was rinsed 2–3 times with ~10 ml of DiH20, vortexed, and centrifuged for 3 minutes at 3000 RPM. The sample was then decanted and resuspended with ~7ml of acetone, mixed with a vortex, and centrifuged for 3 minutes at 3000 RPM. The acetone was decanted and samples were covered and left to dry overnight. Individual samples were reconstituted with 50% DiH20 and 50% glycerol and mounted on a glass slide for observation. Each slide was scanned in its entirety using a transmitted brightfield microscope fitted with polarizing filters and Nomarski optics (Zeiss Axioscope 2, Zeiss International, Göttingen, Germany). Starch granules were photographed under 400x magnification with a digital camera (Zeiss HRc) and measured with Zeiss Zen software.