Stable Isotope and Radiocarbon Analyses of a Black Deposit Associated with Pictographs at Little Lost River Cave, Idaho

Stable Isotope and Radiocarbon Analyses of a Black Deposit Associated with Pictographs at Little Lost River Cave, Idaho

A glossy, black deposit covers much of the ceiling and walls of Little Lost River Cave No. 1, Idaho. This site is of
particular interest because of the red, orange, and yellow pictographs underlying the coating. Carbon and nitrogen
stable isotope analysis has allowed us to better understand the nature and origin of the deposit. With a 13C value of 20·1‰, 15N value of +14‰, and a C/N ratio of 5·6 after the removal of inorganic carbon from the sample matrix, the deposit appears to have been derived from animal tissue, not plant. Plasma chemical extraction of carbon from the
organic material in another sample of the deposit, followed by accelerator mass spectrometric analysis, yielded a
minimum uncalibrated radiocarbon age of 299050 for the paintings. This preliminary evidence suggests that the deposit may be cooking residue.

A glossy, black coating covers many red, orange, and yellow rock paintings (pictographs) at Little Lost River Cave No. 1 in southeastern Idaho. The deposit is a shiny, malleable coating that is located only in the back chamber of the cave. The coating overlying the pictographs makes them visible only when viewed from certain angles with artificial
light. Artificial light is necessary because the shelter is deep enough that sunlight inadequately illuminates the area under question. Black deposits found in caves and on rock shelter walls are common worldwide and include a variety of types and origins. Black deposits on limestone and marble buildings and monuments (Amoroso & Fassina, 1983; Urzı`, Krumbein & Warschei, 1992) and specifically in caves (Smith, Bouchard & Lorblanchet, 1999) have been shown to contain several different materials, including manganese minerals (Moore, 1981; Peck, 1987), coloration from black fungi or bacteria (Diakumaku et al., 1995; Gorbushina et al., 1993; Groth et al., 1999), oxalate crusts (Edwards et al., 1992; Russ et al., 1996), decomposing plant materials or humates, tar, and bat or pack-rat guano (Hill, 1982).

From our preliminary analyses, the black deposit at Little Lost River Cave No. 1 does not appear to be any of these types of previously studied materials. Instead, it appears to be similar to a decomposed olive residue identified as a black deposit at Cueva del Encajero, Spain (Saiz-Jimenez & Hermosin, 1999). Though similar,
we believe that the black deposit at Little Lost River Cave No. 1 is of animal origin, perhaps a cooking residue.
A sample of the black deposit was effervescent when placed in acid (suggesting the presence of carbonates) and a fine black residue remained insoluble in acid and in water. An electron probe microanalysis (EPMA) with both energy- and wavelength-dispersive X-ray capabilities strongly suggests that the matrix of the deposit contains organic material (Steelman et al., in the press, 2001). The black coating consists of carbonates and iron oxide/hydroxide dust clasts uniformly suspended in a carbon-rich supporting matrix. Also from EPMA, substantial amounts of chlorine and boron were observed both throughout the black deposit matrix and a 1–2 m accretion on the surface of the ‘‘clean’’ rock substrate. Stable isotope analysis here shows that some of the carbon in the sample is organic, confirming a conclusion reached much earlier (Fichter et al., 1955).

Given that the alternative source of carbon in the black deposit in Little Lost River Cave No. 1 is organic, and assuming that it was formed from contemporaneous carbon, this material can be used to establish a relevant minimum age for those paintings covered by it. While radiocarbon dating is a possibility, the incorporation of carbon from old wood or old charcoal sources into the deposit is an issue. To obtain a meaningful radiocarbon date, the deposit must either be the result of anthropogenic activity (either deliberate or inadvertent) or a geological event that occurred in the Holocene. If the deposit were an application of geologically derived ‘‘dead’’ hydrocarbons (e.g. tar or pitch) or modern carbon, a radiocarbon age would not relate to the time of painting activity at the site.

In this current study, we conducted carbon and nitrogen elemental and isotopic analyses and measured the radiocarbon activity of the black deposit in Little Lost River Cave No. 1. We measured the amount of organic carbon in a sample of the deposit. Stable isotope data suggest an animal origin for this organic material, while the radiocarbon date establishes the deposit as an Archaic archaeological event.

Little Lost River Cave No. 1 (Site 10BT1) is located in Butte County, Idaho, within the Little Lost River drainage about 6 miles from the southeastern tip of the Lemhi Range, one of a series of three fault block ranges in east-central Idaho. The cave, estimated at about 5650 ft above sea level, is a solution cavity in dolomitic limestone. The inorganic carbonates that make up the limestone were probably formed during the Carboniferous Age (300 million years old) and contain only ‘‘dead’’ carbon (i.e., no 14C remaining). The southeast opening is at the base of a limestone
outcrop and at the top of a gentle colluvial slope (Figure 1). With an opening about 15 m across and between 2 and 3 m tall at the highest point, the cave is roughly 17 m in length towards the northwest (Butler, 1981a). The cave has two chambers separated by a low ceiling, but the black deposit occurs only in the back chamber, which opens up to a height of about 2 m.
Little Lost River Cave No. 1 was first studied archaeologically in 1954 with an exploratory excavation conducted by Idaho State College (Fichter et al., 1955; summarized by Butler, 1981a). Before its excavation, local artifact hunters vandalized the site and then later brought the cave to the attention of the college. Further excavations were carried out in 1990 Figure 1. Photograph showing the entrance of Little Lost River Cave No. 1 located near Howe, Idaho.
1190 K. L. Steelman et al. by archaeologists from the University of Alberta, Canada (Gruhn & Bryan, 1990). The 1990 report concluded from point types (Humboldt Concavebased, Northern Side-notched, Elko Corner-notched, and stemmed indented base points) that the main occupation phase at the site was during the Middle Prehistoric Period (4000 – 400).

Both excavations reports mention pictographs in the cave and a black glossy coating that covers much of the walls and ceiling of the cave. A partial chemical analysis in the 1954 report identified the substance to be organic in nature, containing carbon, hydrogen, and oxygen. The pictographs in the cave were ‘‘rediscovered’’ in 1999 during a BLM cost-share project with Archaeographics to record all rock art sites in the Black Canyon Wilderness Assessment Area. Since then, the cave has been mapped to show the location of the pictographs. However, the reflective glare of the black coating made photography and drawing of the individual panels in the back chamber a challenging experience (Figure 3). It was only after careful scrutiny of each image under a variety of light sources that the superimposition of the pictographs in relation to the deposit could be determined. Photo electronic imaging techniques applied to scanned negatives and slides of the pictographs helped further clarify many of the motifs.

A cross-section shows the location of a pictograph pigment layer underneath the black deposit (Figure 4). Thus, the age of the deposit corresponds to a minimum date for the pictographs. Located in the Black Canyon Wilderness Area, the site rests in the territorial domain of the Lemhi Shoshone and the Shoshone-Bannock tribes. Although the Shoshone believe they have always lived in this area, multiple hypotheses purport to explain human
and cultural migration and influence in the Snake River Plain from Archaic hunter-gathers to a Fremont presence to Shoshone (Swanson, 1972; Butler, 1981a, b, 1986; Adovasio, Andrews & Fowler, 1982; Holmer, 1994). Considering these varied hypotheses, a significant contribution to the cultural history of the Snake River Plain can be made if the material in the black deposit yields an accurate minimum radiocarbon age for the pictographs and if the stylistic and ethnographic evidence can identify a cultural influence for the images in Little Lost River Cave No. 1.

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