At sites occupied prior to 600 cal B.P., red abalone was harvested during two distinct portions of the year, whereas black abalone was harvested only in the late fall to early spring. Using 368 stable oxygen (δ 18 O) isotope samples from two Middle Holocene and three Late Holocene sites, we estimate season of harvest for 82 individual shells. This study reconstructs shellfish harvesting seasonality to elucidate details of subsistence intensification and potential human impacts on lower intertidal and subtidal red abalone (Haliotis rufescens) and intertidal black abalone (Haliotis cracherodii) on San Nicolas Island, California. With δ 18 O analysis becom- ing a routine part of archaeological research (Andrus 2012), our study demonstrates its potential for re-evaluating the intensity of hu- man occupation of small islands previously thought to have been short-term seasonal. Future isotopic and archaeological research will help better understand the extent of human occupation of Anacapa and other small con- tinental islands. Freshwater was a limiting factor on Anacapa, but could have been obtained from a spring on West Anacapa or possi- bly other sources (Rick 2011). However, these results suggest that if this was its primary function during the early portions of the Late Holocene that these stopovers oc- curred during all seasons of the year and by people conducting a wide range of activities. Our findings of a year-round occupation of CA-ANI-2 do not preclude the island from being used as a stopover by people from the mainland or Santa Cruz Island. Between about 35 cal BP, occupation of East Anacapa appears to have been relatively intensive, including three other nearby sites also dated to this time period (Rick 2006). These data suggest that we need to re-evaluate the nature of human occupation of Anacapa Island. The presence of a variety of shellfish, fish, bird, and marine mammal remains, and bone and stone artifacts also supports a more sustained occupation of CA-ANI-2. Isotopic evidence suggests CA-ANI-2 was occupied more extensively than previously believed. Seasonal distribution of CA-ANI-2 mussels includes summer ( n = 7), winter ( n = 7), fall ( n = 4), and spring ( n = 2), demonstrat- ing that CA-ANI-2 was occupied during each season (Table 1). The difference between PSST from the TGB and 12 mm of growth showed a minimum of 0.3 ◦ C and a maximum of ∼ 9 ◦ C. Mean PSST for most shells fall within 15– 18 ◦ C. Two mussel shells (CA-ANI-2# 15 and 20) yielded cooler estimated PSST between ∼ 12 and 16.5 ◦ C. Ranges and means for individual shells ( Figure 4) varied considerably with the warmest estimated PSST from shell CA-ANI- 2#13 and a range of ∼ 16–24 ◦ C. For all mussels, the minimum recorded PSST is 12.3 ◦ C and maximum 24.8 ◦ C with a mean of 16.6 ◦ C ( SD = ± 2.36). At CA-ANI-2, California mussels were harvested in a wide range of water tempera- tures (Online Appendix).

The directional changes between PSST values above or be- low the seasonal mean provide parameters to assign a season of harvest (Table 1). Seasonality for each shell was assigned where x equals the TGB and estimated PSST at time of collection and y equals the estimated PSST for the prior season at 12 mm of growth. Consequently, estimated seasonal ranges were modeled after modern SST distributions (Table 1). Estimated PSST for CA-ANI-2 shell profile range from ∼ 13 to 19.5 ◦ C or δ 18 O ∼ 0.8 and − 0.7, averaging 16.7 ◦ C (Figure 3) where modern SST average is 16.2 ◦ C ( ± 1 ◦ C). Modern SST for Anacapa ranges from ∼ 14 to 19 ◦ C ( Figure 3) with increasing water temperatures (16– 19 ◦ C) between late June and early September and decreasing (19–15 ◦ C) from late September to early December with further declines from late December to early March and an increase from late March to early June (Kennett 2005:56).

Modern δ 18 O values reported for California mussels from the Santa Barbara Channel region range between ∼ 0.8 and − 0.6 (see Kennett 1998:451–453).

Adjustments to the ice volume correction ( δ 18 O water (smow) ) are based on an ocean water sample ( δ 18 O = − 0.32 ) from Santa Rosa Island, providing a correction of − 0.25 (Robbins and Rick 2007). 3000 cal BP change in mean ocean water temperature ( 0m δ 18 O sw ) in the Pa- cific is ∼ 0.07 (see LaGrande and Schmidt 2009).

Psst meaning