OXYGEN ISOTOPIC ANALYSES OF SHELLFISH FROM CHANNEL ISLAND ARCHAEOLOGICAL MIDDENS: IMPLICATIONS FOR MIDDLE HOLOCENE CLIMATE VARIABILITY ON HUMAN SUBSISTENCE STRATEGIES

Stable oxygen isotopic analyses of three shellfish species (California mussel, Mytilus californianus; black abalone, Haliotis cracherodii; and red abalone, H. rufescens) are presented as a test of the Middle Holocene marine cooling hypothesis of Glassow (1993).  The marine cooling hypothesis suggests that cooler sea surface temperatures drove subtidal red abalones temporarily into the intertidal zone.  One hundred forty-one samples were analyzed from five Middle Holocene archaeological sites on San Miguel (SMI-161, -172, -396, -557) and Santa Rosa (SRI-191) islands to determine the variability of sea surface temperatures and the relationship between temperature and resource availability.

 

Site &	# of	d18O range      (avg. temp °C)
Date (cal BP)	Samples	California Mussel	Black Abalone	Red Abalone
SMI-557 (6565-5960)	15	-0.4 to +0.7      (14.3)	0.9 to 2.0      (9.0)	1.0 to 1.7      (9.8)
SMI-396 (5130-4440)	15	-0.1 to +0.9      (13.4)	0.9 to 1.7      (10.0)	1.4 to 1.8      (9.1)
SMI-161 (4420-4300)	30	0.3 to 1.0        (12.4)	n.a.	-0.4 to +2.3   (9.7)
SMI-172 (6440-6270)	25	-0.5 to +0.9      (13.7)	n.a.	0.8 to 1.7      (10.1)
SRI-191 (~5900)	41	-0.7 to +1.0      (13.0)	0.4 to 1.6      (10.4)	0.9 to 2.3      (8.8)
SRI-191 (~4300)	15	-0.5 to +1.1      (13.8)	n.a.	n.a.
Sites listed from coolest to warmest average sea surface temperatures (roughly west to east).

 

These data demonstrate a decrease in temperatures from surf zone mussels to intertidal black abalones and finally subtidal red abalones.  This trend seems unaffected by the changes in sea surface temperature recorded by surface foraminifera during the Middle Holocene (Kennett 2005), suggesting that another characteristic is primarily responsible for the depths of different resources.  Site SMI-557 has black abalone temperatures lower than those of red abalones, and is located in the coolest waters surrounding the Northern Channel Islands.  This suggests that spatial temperature variability caused by the mixing of the Oregonian and Californian currents is responsible for the depths of different shellfish species.  A new model is presented in which spatial temperature variations rather than changes in sea surface temperature over time are used to explain the nature and location of red abalone middens during the Middle Holocene.