Paleoenvironmental reconstruction in La Paz Bay, Gulf of California: Evidence from δ18O in Chione californiensis

Oceanic characteristics of the Holocene are used to understand climatic patterns and phenomena that affect marine and human communities. Likewise, past marine conditions can be reconstructed from surface sea temperature (SST), using stable oxygen isotopes in bivalve shells. The objective of this study was to calculate Holocene summer SSTs for La Paz Bay, by analyzing delta O-18 of C-14 dated bivalve shells (Chione californiensis) from a Holocene camp site located in Canada de La Enfermeria, Baja California Sur, Mexico. Aragonite was extracted from the shells' umbo, representing the summer growth season during the first year of life. delta O-18 value of C. californiensis is -1.9 +/- 0.1 parts per thousand at present, and varied between -1.3 parts per thousand and -2.6 parts per thousand during the last 9 ky. In 9469 BP, 8396 BP, and 7708 BP, delta O-18 values were similar to those of the present. In 7857 BP, 7805 BP, and 7804 BP, delta O-18 was O-18 depleted (0.6-0.9%4, indicating warmer summer SSTs versus the present. In 7070 BP, 6945 BP, and 2087 BP, delta O-18 was enriched in O-18 (0.3-0.4 parts per thousand), suggesting colder SSTs versus the present. This study coincides with other paleotemperature studies for the region and allows us to address the effect of changing SST on this marine resource, its use by human communities of the past, and its effects on human presence in the area with respect to climate variability.