Detecting major ecological changes in the Late Holocene: Age and growth of archaeological Chione undatella from a Southern California Lagoon

Background/Question/Methods

Recent research has revealed significant changes in the Holocene littoral communities of the Southern California Bight resulting from climate change, sea level rise, and anthropogenic impacts. Marine resources comprised a major portion of coastal Native American’s diet and faunal remains in archaeological shell middens represent resources from local habitats. Around 3,300 ybp there is a sharp decrease in shell middens in San Diego County, one of the common hypothesis to explain this reduction is the closure of highly productive costal lagoons. Once isolated from the ocean, lagoons would become anoxic and primary productivity would be reduced, decreasing the marine resources available to Native Americans. The lagoon dwelling venus clam Chione undatella was important to the diet of Native Americans that would have been negatively impacted by lagoon closure. To test if the decline in archaeological middens was the result of environmental forcing or social change, the size of C. undatella was used as a proxy for resource exploitation and growth rate of C. undatella was used as proxy for lagoon productivity. This study used archaeological material excavated from Los Penasquitos Lagoon, San Diego, and based on thirteen radiocarbon dates spanning from 2,000 to 7,000 ybp.

Results/Conclusions

Results from 495 shells show no detectable change in the average size of C. undatella between 2,000 and 7,000 ybp. However, the variance increased from 2,000 to 4,000 ybp, with both larger and smaller clams harvested, relative to 4,000-7,000. While the average size did not change the growth rate of these clams was lowest between 3,000 and 3,500 ybp. The reduction in growth rate supports the hypothesis of reduced marine productivity leading to a reduction in coastal Native American archaeological sites. Interestingly, while reduced growth is seen there is not a commensurate reduction in average size but there is increased variance of shells sizes. One explanation of this pattern is that during times of lower productivity foraging effort increased and selectivity decreased, resulting in wider range of sizes harvested. However, hypotheses invoking changes in human behavior are not directly testable. None the less, these data represent a quantitative method to test archaeological hypotheses and increase the knowledge of physical forcing on ecosystem productivity.