Oxygen Isotopes in Marine Shell

The ratio of the stable isotopes ¹⁸O and ¹⁶O in marine shell samples taken from archaeological sites in southern California and northern Baja California has been used to address several prehistoric research issues. These include the seasons during which marine resources were exploited, paleoclimatic changes, and the effects of freshwater outflow in marine environments. Similar studies are potentially applicable to sites in San Diego County.

John S. Killingley (1980, 1981) analyzed shell samples that had been collected by Carl L. Hubbs from Punta Minitas (LC-219), a site south of Ensenada in northwestern Baja California. Previous studies had shown that the proportion of ¹⁸O precipitated in shell carbonates decreases as temperature rises, and that the annual cycles in temperatures experienced by modern Mytilus californianus (California mussel) are closely mapped by the ¹⁸O/¹⁶O ratios in the growing shells. The Punta Minitas midden dated primarily between about 3000 and 2000 RCYBP (ca. 800 B.C.-A.D. 800), but samples came from levels associated with dates ranging between 5810 and less than 1500 RCYBP (ca. 4200 B.C.-post A.D. 1300). Killingley studied 14 specimens. Because the number of available samples of the preferred species, Mytilus californianus, was limited, several other species were also used, including Lottia gigantea (owl limpet), Thais emarginata, Septifer bifurcatus (bifurcate mussel), and Haliotis cracherodii (black abalone). Profiles of the temperature history of the shells during their final months of growth were developed by taking small subsamples of carbonate from near the margins of the shell and analyzing them with a mass spectrometer. The Punta Minitas shell were found to reflect harvesting primarily during the summer months. Statistically, it was projected with 95 percent confidence that between 56 and 98 percent of the shell population was collected during the warm period (April-September), and only 2 to 44 percent during the cool period (October-March). These results contradicted previous suggestions that prehistoric shellfish collection had been primarily a winter activity, because of the need to supplement meager terrestrial resources during that season and to avoid the toxic “red tide” (Gonyaulax catenella) phenomenon of spring and early summer.

Henry C. Koerper and Killingley (1998) analyzed 14 Mytilus californianus shells and two Protothaca staminea (Pacific littleneck) shells from Putuidem (ORA-855), a late prehistoric village site near San Juan Capistrano in Orange County. The investigators concluded that shellfish harvesting had occurred there primarily during the summer and fall, a little later in the year than at Punta Minitas. Analyses of several dozen specimens from seven other Orange County sites reportedly supported similar conclusions about seasonality (Dunbar et al. 1998; Ericson 1973).

In addition to providing profiles of relative seasonal temperature change, oxygen isotope studies have been used to track longer-term changes in absolute water temperature. Killingley compared the isotopic composition of the Punta Minitas shells with modern values and concluded that “there is evidence of a slightly cooler climate a few thousand years ago” (Killingley 1980:22). Based on oxygen isotope evidence from ORA-855, Koerper, Killingley, and R. E. Taylor (1985) concluded that water temperatures were about 3^oC cooler during the Little Ice Age (ca. A.D. 1400-1850) than at present.

Somewhat different research objectives were addressed by Miguel A. Téllez Duarte, Guillermo Avila Serrano, and Karl W. Flessa (2002) using oxygen isotope profiles for shells taken from middens near the head of the Gulf of California in northeastern Baja California. Téllez and his associates analyzed three Protothaca grata shells, two of them from San Felipe and the third from farther south at Campo Cristina, all dated to around 1300-1000 RCYBP (post A.D. 1500). Also used for comparison were isotopic profiles from a modern Chione (venus clam) shell and a semifossilized Mulinia coloradoensis (surfclam) shell from the Colorado River delta. In addition to temperature, the isotopic composition of marine shells may be influences by the input of fresh water, which has a lower ¹⁸O/¹⁶O ratio than sea water. More extreme summer lows in the ratio were attributed to the outflow of fresh water from the Colorado River, which was felt as far south as San Felipe but not at Campo Cristina.

PROSPECTS

Future investigations using ¹⁸O/¹⁶O analyses may be able to shed light on the season or seasons during which shellfish were exploited from coastal San Diego sites. Similar studies of shells at inland sites may clarify seasonal patterns in the logistical use of coastal resources. Oxygen isotope studies may clarify Holocene patterns of ocean temperature change, including the emergence of the El Niño-Southern Oscillation (ENSO) phenomenon. Shell from bay/lagoon species may help to reconstruct local variation in the seasonal input of fresh water and the lagoons’ evolution produced by the stabilization of sea level, progressive siltation, and the formation of barriers at their mouths.