late holocene subsistence change and marine productivity on

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Late Holocene Subsistence Change and MarineProductivity on Western Santa Rosa Island, AltaCalifornia

Christopher Jazwa, Douglas Kennett & Danielle Hanson

To cite this article: Christopher Jazwa, Douglas Kennett & Danielle Hanson (2012) LateHolocene Subsistence Change and Marine Productivity on Western Santa Rosa Island, AltaCalifornia, California Archaeology, 4:1, 69-98, DOI: 10.1179/cal.2012.4.1.69

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California Archaeology, Volume 4, Number 1, June 2012, pp. 69–98. 69 Copyright © 2012 Society for California Archaeology. All rights reserved.

Late Holocene Subsistence Change and Marine Productivity on Western Santa Rosa Island, Alta California

Christopher S. Jazwa and Douglas J. Kennett Department of Anthropology, The Pennsylvania State University, University Park, PA 16802 ([email protected]) ([email protected])

Danielle HansonDepartment of Anthropology, Indiana University, Bloomington, IN 47405 ([email protected])

Abstract Institutionalized differences in social status developed on California’s northern Channel Islands from the Late Middle (A.D. 650-1150) to Late (A.D. 1300-1782) periods. This is associated with the proliferation of sedentary com-munities along the coasts of these islands and a number of important socioeco-nomic changes, including a greater emphasis on fishing and the production of non-food craft items. These changes were particularly rapid during the Middle to Late Period Transition (MLT; A.D. 1150-1300) and partly attributed to signifi-cant environmental change during this interval. Population-resource imbalances caused by decreased marine productivity, drought, or some combination of the two have been suggested as possible environmental triggers for the sociopolitical changes evident in the archaeological record at this time. Shell midden deposits at CA-SRI-15 provide a unique opportunity to test whether elevated sea surface temperature (SST) reduced marine productivity during the MLT and contributed to these population resource imbalances. This is because the site was occupied relatively continuously from the Late Middle Period through the Late Period. Faunal records and associated oxygen isotopic data from the site do not support the hypothesis that increased SST reduced marine productivity during the MLT.

Resumen Diferencias institucionalizadas en el estatus social desarrollaron en las Islas del Canal norteñas de California desde el periodo Tardío Medio (650-1150 D.C.) hasta el periodo Tardío (1300-1782 D.C.). Estas son asociadas con la proliferación de comunidades sedentarias a través de las costas de estas tres islas y con varios cambios socioeconómicos importantes, incluyendo un mayor énfasis en la pesca y la producción de artículos de artesanía. Estos cambios ocurrieron con rapidez especial durante la transición desde periodo Medio hasta el periodo

70 Christopher S. Jazwa, Douglas J. Kennett, and Danielle Hanson

Tardío (1150-1300 D.C.) y están parcialmente atribuidos al cambio importante medioambiental que ocurrió durante este intervalo. Se han sugerido desequi-librios entre recursos y poblaciones causados por una disminución de la produc-tividad marina, la sequia, o una combinación de estos dos como posibles gatillos ambientales para los cambios sociopolíticos que son evidentes en el registro ar-queológico durante esta época. El conchero de CA-SRI-15 ofrece una oportunidad única para probar si o no la temperatura elevada del mar había reducido la pro-ductividad marina durante la transición Medio/Tardío y había contribuido a los desequilibrios entre recursos y poblaciones. Esto es porque el sitio fue ocupado relativamente constantemente durante el periodo Tardío Medio hasta el periodo Tardío. El registro de restos de fauna y los datos de isotopos de oxígeno de este sitio no apoyan la hipótesis de que el crecimiento de la temperatura elevada del mar redujo la productividad marino durante la transición Medio/Tardío.

The influence of climatically driven environmental change and sociopoliti-cal evolution is of great interest to archaeologists. This is certainly the case on the northern Channel Islands of California, where a number of studies indicate chang-es in resource availability and quality through time that appear to coincide with climate change and a range of sociopolitical responses (e.g. Arnold 1992a, 2001a; Arnold and Graesch 2004; Braje et al. 2007; Erlandson and Jones 2002; Kennett and Kennett 2000; Kennett et al. 2007, 2008; Raab and Larson 1997; Figure 1). There are also changes in human subsistence, as well as altered resource abundance and changes in the size of target prey that did not result from climatic change, but is better explained by long-term impacts resulting from human predation (Braje et al. 2007; Erlandson et al. 2008; Kennett 2005; Rick et al. 2008; Winterhalder et al. 2010). However, the dynamics of human-environmental interactions periodically resulting in population-resource imbalances and sociopolitical change are compli-cated and in need of further study. The well-preserved and relatively undisturbed faunal records available on the northern Channel Islands provide a rich source of data for exploring past human interactions with the environment (Erlandson et al. 2011; Kennett and Kennett 2000; Rick et al. 2005).

One period of particular interest on the northern Channel Islands is the Mid-dle to Late Period Transition (MLT; A.D. 1150-1300). Arnold (1991, 1992a, 1997, 2001b; also see Arnold and Tissot 1993; Arnold et al. 1997) defined this period as an important interval of change toward greater sociopolitical complexity between the Middle (600 B.C.-A.D. 1150) and Late (A.D. 1300-1782) periods. She originally associated these changes with a period of elevated sea surface temperature (SST) inferred from changing radiolaria assemblages in sediments from the adjacent

Late Holocene Subsistence Change and Marine Productivity 71

Santa Barbara Basin (Pisias 1978). As a test of this hypothesis, Arnold and Tis-sot (1993) analyzed developmental variability in trema number allometry of black abalone shells from archaeological contexts at four sites spanning the late Middle (A.D. 900-1150) and Late periods on western Santa Cruz Island. They argued that the results of their analysis supported Pisias’s findings for warm SSTs between A.D. 1150 and 1300.

Arnold and Tissot (1993) further argued that increased SSTs reduced marine productivity and the overall extent of kelp forests. In turn, this led to the resource stress that stimulated sociopolitical change on the islands during the MLT. Kelp is particularly sensitive to elevated SSTs and reduction in the distribution of off-shore kelp forests and associated biota would have serious consequences for the islanders during this interval. Increased SSTs would also have changed the dis-tribution of various dietary species, including fish, shellfish, and sea mammals (Arnold 1992a, 2001a; Colten 2001; Pletka 2001). The dramatic increase in the standardization and quantity of Olivella biplicata bead manufacture and associ-ated stone tool industries that occurred at this time is often cited as evidence of craft specialization and increasing cultural complexity (Arnold 1987, 1992a, 1993, 2001a, 2001b; Arnold and Graesch 2001; Dietler 2003; Preziosi 2001).

Raab and Larson (1997) challenged Arnold’s model of sociopolitical change re-lated to elevated SST during the MLT. They argued that the archaeological record

Figure 1. California’s northern Channel Islands and the location of CA-SRI-15.


in the Santa Barbara Channel region during the MLT is not consistent with marine resource depression because of warm SST and reduced marine productivity. Instead, they associated this period with drought conditions that occurred during the Medi-eval Climatic Anomaly (Jones et al. 1999; Jones and Schwitalla 2008; Stine 1994) and gave rise to the nutritional and social stress indicators evident in the bioarchae-ological record at this time (Lambert 1993; Lambert and Walker 1991). These indi-cators include disease, malnutrition, and violence, which they argued was related to shrinking supplies of water and terrestrial foods on the islands and mainland. They also maintained that these trends developed over a longer period of time than just the MLT (Raab and Larson 1997:331-334). In addition, a well-dated oxygen isotope SST record from the Santa Barbara Basin indicates that the MLT occurred at the tail end of a longer climatic interval (A.D. 450-1300) characterized by cooler and more variable SSTs (Kennett 2005; Kennett and Kennett 2000).

The potential relationship between changing environmental conditions and the emergence of social hierarchies has been one of several key questions in Chan-nel Islands archaeological research, particularly with respect to our understand-ing of the Late Holocene. Although there are recent reports of sites dating to this time on both the northern and southern Channel Islands (Raab et al. 2009; Rick 2007; Rick and Erlandson 2001; Yatsko 2000), there are relatively few well-docu-mented and precisely dated sites on these islands known to date to the MLT (Ar-nold 1991:956, 1992a:76, 1992b:142; Munns and Arnold 2002:133). Therefore, any well-dated site that can be associated with this period has the potential to provide valuable information about the environmental and sociopolitical changes that occurred during this time. In this article, we present archaeological and envi-ronmental data from a site on the western end of Santa Rosa Island that spans this interval (CA-SRI-15; Figure 1). The site has deposits that firmly date to the MLT and it was also occupied during the preceding Middle Period and the following Late Period, providing the opportunity to assess the changes that occurred during this time at this location.

Background and Methods

CA-SRI-15 is located northeast of the western tip of Santa Rosa Island at Aba-lone Point (see Figure 1). House depressions visible on the surface of this large site initially suggested a late prehistoric occupation (Kennett 1998, 2005). The presence of Olivella callus beads and retouched triangular microdrills on the sur-face of the site are consistent with a substantial occupation during the Late Period (A.D. 1300-1782; see Arnold 1987:222-228). Based on the presence of the house depressions and Late Period artifacts (see Kennett 2005), Johnson (1999:64) ini-


tially believed that CA-SRI-15 was the historic village of Nimquelquel. However, historic artifacts have not been identified at this location and there is now some question as to whether it is this named village (John Johnson, personal commu-nication 2010). The Late Period occupation is most evident on the eastern side of the point (at Locus B), but more subtle surficial house berms on the western side of the point (at Locus A) and the presence of Olivella wall beads and trapezoidal microblades on the surface indicate a late Middle Period occupation (A.D. 650-1150; see Kennett 1998). Deep midden deposits exposed in the sea cliff suggest that deposits in Locus A could extend back earlier in time.

In 1996, two column samples (25 25 cm) were excavated from the exposed and eroding sea cliff faces at the site by Douglas Kennett and Don Morris. The first sample (Unit 1) was excavated from Locus A, the older of the two loci, dat-ing primarily to the Late Middle Period and MLT. The second sample (Unit 2) was excavated from Locus B, which dates to the Late Period. The sea cliff was cleaned and straightened at each locus and the stratigraphic units of interest were defined prior to excavation. The deposits were then excavated in culturally distinct levels using the visible stratigraphic profile as a guide. If culturally defined stratigraphic units were thick (>15 cm), they were subdivided and excavated in smaller arbitrary units. This was most common in the uppermost strata of Unit 1, and in Unit 2, where the cultural deposits were not easily differentiated stratigraphically. All ex-cavated materials were transported to the University of California, Santa Barbara (UCSB) to be water-screened, dried, and size-sorted (½-inch, ¼-inch, and ⅛-inch mesh) to assist with the identification process. Material from each successive screen size was sorted separately. More detailed sorting and checking were later conducted at the University of Oregon (UO).

Four types of data were obtained from the two column samples excavated in 1996 at CA-SRI-15. First, we submitted eight Mytilus californianus shells to Beta Analytic between April 1996 and July 1997 for standard radiometric dating (Ken-nett 1998:456). These dates were recalibrated in OxCal 4.1 (Bronk Ramsey 2009) using the most recent marine calibration curve, Marine09 (Reimer et al. 2009) and an updated ∆R value for the Santa Barbara Channel region (261±21 radiocarbon years; Brendan Culleton, personal communication 2012). The revised ∆R estimate incorporates five new AMS radiocarbon dates on pre-bomb (A.D. 1925) Olivella shells collected near Santa Barbara, with three existing dates on Mytilus reported by Ingram and Southon (1996; also see Culleton et al. 2006; Kennett et al. 1997), and is calculated with respect to the updated marine model age of the Marine09 curve. In OxCal, a Bayesian statistical model was used to further constrain error ranges on dates based on the relative stratigraphic position of the radiocarbon samples. This is discussed in greater detail below.


Second, 14 individual M. californianus shells were selected from 11 levels and were sampled to generate annual SST range profiles. Using a 0.5 mm dental drill at the Department of Geology at UCSB (Kennett 1998:197), d18O measurements were taken from these shells at 2 mm increments along the growth axis. Sample d18O was determined by mass spectrometry (Finnegan MAT-251) with instrument precision of 0.1‰ based on the reproducibility of a laboratory standard (NBS-19). The measurement of oxygen isotopic values is a well-established technique for determining SST (Glassow et al. 1994; Jones and Kennett 1999; Kennett 2005; Killingley and Berger 1979; Shackelton 1973). By taking samples along the growth axis of a shell, it is possible to determine the seasonal range of SST during the life of the individual.

Third, data were compiled about shell size for California mussels (M. california-nus), the most abundant mollusk species throughout the deposit. Measurements were taken for all shells with hinge fragments. Complete mussel valves were mea-sured and placed into 1-cm size classes. The size of fragmented valves was esti-mated using a template produced by White (1989) and following procedures out-lined in Jones and Richman (1995) for determining coarse size category data from mussel hinges. A recent study suggested that this approach has some limitations (Bell 2009), but it is currently the only method available for determining the size of fragmented materials. For each excavation level, the total number of shells from each size class was compiled separately for left and right hinges. The values for the side with the greater number of individual specimens were used for the analysis (minimum number of individuals).

Finally, shellfish and other faunal constituents were separated by species (when possible) and quantified along with other cultural materials by trained un-dergraduate and graduate students at UCSB and UO. For each level, all midden ma-terials collected in the ½-inch mesh was sorted in its entirety, as were a 100-gram subsample of ¼-inch material and a 15-gram subsample of ⅛-inch material. The residual bulk material was also sorted for artifacts, O. biplicata shells, M. california-nus hinge fragments, all bone, asphaltum, charcoal, and small gastropods. All oth-er materials were bagged separately. Shells from ½-inch, ¼-inch, and ⅛-inch mesh were sorted and weighed separately, but weights for each mesh size were com-bined together for each species by level for analysis. Meat weights were calculated for the most important dietary constituents using the multipliers summarized by Rick (2004:79). Because O. biplicata was important for its shell, which was used in the Santa Barbara Channel region for bead making rather than as a dietary com-ponent (e.g. Arnold and Munns 1994; Bennyhoff and Hughes 1987; King 1990), we analyzed this species by shell weight rather than meat weight. Paige (2000) analyzed all fish bones and sorted vertebrae to species.


To assess changes in taxonomic diversity (per Reitz and Wing 1999:235), we used the Shannon-Weaver function (Shannon and Weaver 1949):

H’ = –Spi*ln(pi)

where pi is the proportion of each taxon per stratum, level, or time period in terms of volumetrically corrected meat weight contribution.

All data were initially compiled by excavation level. Because the excavations were done stratigraphically, levels were not uniform in volume, so all shell and meat weight data were normalized to a volume of one cubic meter. For more meaningful analysis, all data from Unit 1 were then combined for each of the seven strata (A through H), and again normalized to one cubic meter. Mussels were sorted into size classes and each was assigned the midpoint of the size class. Average mussel size class was calculated for each stratum by weighting the average size classes for each of the levels that comprised it by the total number of mussel shells that were measured. Unlike Unit 1, all of the cultural materials in Unit 2 were found within a single stratum, preventing us from combining data by stra-tum without washing out variation throughout the vertical column. Additionally, because we were interested in changes in human activities that occurred between the Late Middle Period, MLT, and Late Period, we combined data in the same way as was done above for each of these time periods. We considered all levels of Unit 2 to be part of the Late Period, Strata A and B of Unit 1 to be part of the MLT, and Strata C through H of Unit 1 to be part of the Late Middle Period. These decisions were driven by our Bayesian analysis of radiocarbon dates discussed below.

Results

Stratigraphy and Chronology

There was distinct stratigraphy within the two different loci at CA-SRI-15. Eight clearly defined strata were identified in Locus A once the seacliff was cleaned and the entire vertical sequence was visible. Unit 1 was excavated stratigraphically from this locus. The larger strata (A, B, C, and F) were broken into arbitrary lev-els for ease of processing. Data from individual strata were later combined during the analysis phase. The shell midden deposit in Locus B was rich, but there was no clear internal stratigraphic differentiation, so Unit 2 was excavated in 10-cm arbitrary levels.

The radiocarbon dates from CA-SRI-15 provide strong evidence that the site was occupied during the Late Middle Period, the MLT, and the Late Period (Table 1). Additionally, the data suggest either that breaks in occupation were short or


that it was occupied continuously during this time. This is particularly evident when the data are stratigraphically modeled using OxCal 4.1, which uses a Bayes-ian statistical approach (Figure 2; see Kennett et al. 2011). Figure 2 shows that the strata can be associated with these three periods in a meaningful way. Strata C

Figure 2. Radiocarbon chronology for CA-SRI-15. Dates are calibrated using OxCal 4.1.3 (Bronk

Ramsey 2009) and the Marine09 calibration curve (Reimer et al. 2009) and presented as 2 ranges.

Dark gray distributions are stratigraphically modeled and light gray distributions are not.

Table 1. Radiocarbon Dates from CA-SRI-15.

Sample Unit StratumDepth (cm)

UncalibratedDate

(Years BP)Uncalibrated

Error

CalibratedRange (2)

Years BP

ModeledRange (2)

Years BP

Beta-96868 2 -- 25 1,140 60 625-382 608-384

Beta-96867 2 -- 40-50 1,270 60 680-496 669-505

Beta-95454 1 A 0-10 1,380 80 853-532 781-560

Beta-100511 1 B 19-30 1,440 60 881-632 881-655

Beta-107041 1 C 65-75 1,690 60 1,140-830 1,033-801

Beta-107042 1 E 80-95 1,690 70 1,160-811 1,093-880

Beta-92290 1 G 117-126 1,690 70 1,160-811 1,150-926

Beta-107043 1 H 126-135 1,760 90 1,260-879 1,265-985

Note: All dates calibrated using OxCal 4.1.3 (Bronk Ramsey 2009) and the Marine09 cali-bration curve (Reimer et al. 2009).


through H from Unit 1 most closely date to the Late Middle Period, Strata A and B of Unit 1 represent an occupation through the MLT, and Unit 2 dates to the Late Period. These data suggest that settlement at the site shifted to Locus B during the early part of the Late Period. Importantly, the data do not show a visible or extended abandonment of the site during the MLT. Because relatively few sites on the island are known from this period, CA-SRI-15 provides a rare opportunity to explore the changes that occurred during this transitional period.

Sea Surface Temperature Reconstruction

Oxygen isotope profiles for single shells provide information about the range in SST over the lifetime of the individual mollusk. This represents a good approxima-tion of the seasonal range in SST during the time a site was occupied (Kennett 1998:193-197; Shackleton 1973:138-139). When coupled with the chronological information presented above, these data can also be used to identify longer term fluctuations in SST on western Santa Rosa Island (Table 2; Figure 3).

Table 2. Oxygen Isotope Data from CA–SRI–15.

Temperature (ºC) Isotopic Ratio

Sample UnitLevel (cm) M

inim

um

Max

imum

Mid

poin

t

Ran

ge

Min

imum

Max

imum

Mid

poin

t

Ran

ge

CA-SRI-15-A1 1 0–10 10.94 15.30 13.120 2.180 0.208 1.286 0.7470 0.5390

CA-SRI-15-A20 1 19–30 10.77 15.25 13.010 2.240 0.219 1.332 0.7755 0.5565

CA-SRI-15-A214 1 19–30 9.17 14.18 11.675 2.505 0.475 1.753 1.1140 0.6390

CA-SRI-15-A215 1 30–40 11.57 15.89 13.730 2.160 0.068 1.124 0.5960 0.5280

CA-SRI-15-A216 1 30–40 11.19 17.77 14.480 3.29 –0.367 1.222 0.4275 0.7945

CA-SRI-15-A66 1 47–65 9.67 14.61 12.140 2.470 0.372 1.620 0.9960 0.6240

CA-SRI-15-A63 1 65–75 11.25 16.55 13.900 2.650 –0.086 1.208 0.5610 0.6470

CA-SRI-15-A65 1 80–95 10.02 16.49 13.255 3.235 –0.073 1.528 0.7275 0.8005

CA-SRI-15-A62 1 95–117 9.45 14.93 12.190 2.740 0.296 1.680 0.9880 0.6920

CA-SRI-15-SR48 1 117–126 13.25 17.53 15.390 2.140 –0.312 0.703 0.1955 0.5075

CA-SRI-15-A50 1 126–135 9.23 13.90 11.565 2.335 0.544 1.737 1.1405 0.5965

CA-SRI-15-A51 1 126–135 11.95 16.02 13.985 2.035 0.038 1.028 0.5330 0.4950

CA-SRI-15B-A1 2 10–20 10.12 15.95 13.035 2.915 0.054 1.502 0.7780 0.7240

CA-SRI-15B-A2 2 40–50 10.42 16.15 13.285 2.865 0.006 1.423 0.7145 0.7085


Major directional changes in SST are not visible in the record at CA-SRI-15. Greater instability is evident in this record from the earliest occupation of the site (Unit 1, Stratum H; A.D. 685-965, 2 range) through the MLT (Unit 1, Stratum B; A.D. 1169-1295, 2 range) than during the Late Period. During the Late Middle Period and MLT, the annual mean for the d18O isotope ranges between 0.43‰ and 1.14‰. Values during these periods fluctuate from -0.37‰ to 1.75‰. This corresponds with a mean annual SST range of 11.57ºC to 14.48ºC. SSTs overall are between 9.17ºC and 17.77ºC. In the later stages of the MLT (Unit 1, Stratum A; A.D. 1169-1390, 2 range) and during the Late Period (at Locus B), however,

Figure 3. Sea surface temperature data from oxygen isotopic profiles of Mytilus californianus shells.

Annual means and temperature ranges are plotted by stratum in Unit 1 and arbitrary level in Unit 2.


mean annual d18O isotope ranges are between 0.71‰ and 0.78‰. The overall range is between .01‰ and 1.502‰. Similarly, the annual mean water tempera-ture during this period is between 13.04ºC and 13.29ºC, with an overall range be-tween 10.12ºC and 16.15ºC. What is noteworthy here is that mean SST appears to stabilize at the end of the MLT and this trend continues through the Late Period. However, there are no clear trends through time toward warmer or colder SST or toward larger or smaller annual fluctuations in SST.

Trends in Mytilus californianus Size

Changes in M. californianus (California mussel) size through time can result from human predation, environmental change affecting shell growth, or both. M. cali-fornianus was the dominant mollusk species present throughout the assemblage in both shell weight and calculated meat weight. Using full shells and hinge frag-ments, it was possible to determine approximate shell size and calculate average shell sizes for each stratum (Jones and Richman 1995; White 1989). There are two long-term trends in M. californianus shell size (Tables 3 and 4; Figure 4). Average California mussel size decreases during the Late Middle Period from 4.92 cm in the earliest stratum (Unit 1, Stratum H; A.D. 685-965, 2 range) to 3.06 cm at

0204060801001401

2

3

4

5

6

7

8

020120 4060

Middle Period

0204060801001200

10

20

30

40

50

60

70

02040140 60

STR

ATU

M H

STR

ATU

M G

STR

ATU

M F

STR

ATU

M E

STR

ATU

M D

STR

ATU

M C

STR

ATU

M B

STR

ATU

M A

60 -

65 c

mbd

50 -

60 c

mbd

40 -

50 c

mbd

36 -

40 c

mbd

29 -

36 c

mbd

20 -

29 c

mbd

10 -

20 c

mbd

0 - 1

0 cm

bd

Middle to Late Period Transition

Late Period

Mea

t Wei

ght (

kg/m

)3M

ean

Size

(cm

)

Depth (cm)

UNIT 1 UNIT 2

Figure 4. Mytilus californianus size and meat weight plotted by stratum in Unit 1 and arbitrary

level in Unit 2. The solid size curve is the mean value and dotted curves are the mean±one standard

deviation. The meat weight multiplier used for Mytilus californianus is 0.298 (Erlandson 1994:59).


the beginning of the MLT (Unit 1, Stratum B; A.D. 1169-1295, 2 range). This was followed by an increase in average mussel size during the MLT and Late Period. Average shell size during the most recently deposited level (Unit 2, 0-10 cm) is 4.82 cm, with small fluctuations evident during the Late Period.

Changes in overall meat weight density of California mussel through time may provide clues to interpreting this pattern. With the exception of the two lowest stra-ta in Unit 1 (Strata G and H), there is a general decrease in California mussel meat

Table 3. Summary of CA-SRI-15 Shell Constituents by Size (for Mytilus californianus) and Shell Weight (in g).

Depth (cm) Si

ze (c

m)

Calif

orni

a M

usse

l

Bla

ck

Aba

lone

Red

A

balo

ne

Aba

lone

To

tal

Ow

l Li

mpe

t

Plat

form

M

usse

l

Sea

U

rchi

n

Turb

an

Snai

l

Tota

l

Unit 2

0–10 4.8 568.2 4.4 3.7 8.1 0.0 16.4 54.5 47.0 694.2

10–20 4.8 372.9 30.3 30.1 114.1 0.0 32.9 40.8 65.5 626.2

20–29 3.9 390.2 11.4 0.0 11.4 3.1 38.1 64.0 30.5 537.4

29–36 4.2 199.1 6.1 0.0 8.2 0.0 347.5 46.5 56.7 658.1

36–40 4.7 205.7 9.9 0.0 11.2 0.0 105.6 36.8 30.7 389.9

40–50 3.8 269.4 48.2 0.0 51.8 0.0 37.6 25.9 13.5 398.3

50–60 4.4 353.3 4.8 2.6 7.4 0.0 14.4 6.3 29.2 410.6

60–65 3.5 38.8 0.3 0.0 0.3 0.0 0.4 1.4 1.8 42.7

Unit 1

0–10 3.3 694.2 23.4 12.5 35.9 11.9 400.8 238.5 323.5 1,704.8

10–19 3.7 634.7 26.6 9.3 35.9 17.3 237.8 466.7 102.2 1,494.6

19–30 2.9 588.9 5.3 26.2 36.7 1.5 54.6 234.4 44.3 960.4

30–40 3.4 297.3 21.8 0.7 22.6 14.8 55.9 126.9 40.2 557.6

40–47 3.7 509.4 6.5 0.2 6.6 1.3 22.9 52.2 112.6 704.9

47–65 4.4 1,543.9 78.8 31.0 109.9 9.1 52.3 176.2 35.9 1,927.3

65–75 4.0 1,250.8 47.3 37.5 84.8 5.1 34.8 245.2 67.9 1,688.7

75–80 3.7 622.6 14.0 17.1 31.1 0.0 23.5 142.4 18.9 838.6

80–95 3.2 914.9 34.3 0.0 36.0 8.9 15.8 456.8 63.3 1,495.9

95–106 3.9 1,798.2 40.9 1.9 42.8 8.9 37.2 713.3 29.3 2,629.6

106–117 4.3 1,327.6 75.4 0.0 88.9 19.4 43.6 787.7 52.7 2,320.0

117–126 4.4 366.2 18.6 22.9 41.4 8.3 0.7 142.3 30.1 589.0

126–135 4.9 63.9 0.0 0.0 0.4 0.0 1.2 271.9 3.6 341.1

Note: Average size determined using a template from White (1989).


weight density during the Late Middle Period (67.75 kg/m3 to 20.12 kg/m3). Meat weight density for this species is low throughout the Late Period, with a general in-crease from 3.70 kg/m3 (Unit 2, 60-65 cm) to 27.09 kg/m3 (Unit 2, 0-10 cm). These two trends are suggestive of human predation pressure on average mussel size.

Dietary Trends

In general, fish remains dominate the CA-SRI-15 faunal assemblage as measured by meat weight (Tables 5 and 6; Figure 5a). The exceptions to this are the low-est levels of Unit 2, which are the poorest preserved and therefore may reflect an unusually large loss of small fish bone. During the Late Middle Period and MLT, fish comprises more than 70 percent of the meat weight density, and more than

Table 4. Summary of Shell Constituents Normalized to 1 m3 (g/m3) and Combined by Stratum for Unit 1.

Level Size

(cm

)

Calif

orni

a M

usse

l

Bla

ck

Aba

lone

Red

A

balo

ne

Aba

lone

To

tal

Ow

l Li

mpe

t

Plat

form

M

usse

l

Sea

U

rchi

n

Turb

an

Snai

l

Tota

l

Unit 2

0–10 cm 4.8 90,919 700 592 1,291 0 2,624 8,717 7,522 111,073

10–20 cm 4.8 59,656 4,845 4,816 18,256 0 5,262 6,534 10,476 100,186

20–29 cm 3.9 69,377 2,034 0 2,034 556 6,767 11,384 5,421 95,540

29–36 cm 4.2 45,514 1,398 0 1,883 0 79,431 10,632 12,967 150,428

36–40 cm 4.7 82,267 3,942 0 4,461 0 42,237 14,725 12,287 155,977

40–50 cm 3.8 43,108 7,714 0 8,288 0 6,024 4,152 2,161 63,733

50–60 cm 4.4 56,520 768 422 1,190 0 2,301 1,012 4,670 65,694

60–65 cm 3.5 12,418 96 0 96 0 122 460 561 13,656

Unit 1

Stratum A 3.4 111,900 4,213 1,838 6,052 2,456 53,779 59,387 35,844 269,417

Stratum B 3.1 67,518 2,067 2,051 4,516 1,241 8,419 27,525 6,439 115,658

Stratum C 4.2 151,045 6,063 3,139 9,202 708 5,025 21,652 9,891 197,522

Stratum D 3.7 199,232 4,486 5,459 9,946 0 7,521 45,577 6,063 268,339

Stratum E 3.2 97,592 3,662 0 3,844 950 1,690 48,725 6,756 159,558

Stratum F 4.1 227,336 8,455 137 9,574 2,060 5,876 109,166 5,958 359,969

Stratum G 4.4 65,102 3,273 4,065 7,339 1,472 128 22,106 5,353 101,500

Stratum H 4.9 11,369 0 0 79 0 210 48,341 637 60,636

Note: Average size determined using a template from White (1989).


50 percent during the Late Period. Nonetheless, there is an overall decrease in fish meat weight between the Late Middle (353 kg/m3; 73 percent) and the Late Period (220 kg/m3; 53 percent). Shellfish is the second most prominent faunal category throughout the Late Middle Period and the MLT, comprising 17 percent (84.52 kg/m3) and 22 percent (77.91 kg/m3) of the faunal assemblage, respectively.

Figure 5. Faunal data from CA-

SRI-15. Meat weight multipliers

are: 27.7 for fish; 15.0 for bird;

24.2 for sea mammal; 0.298 for

Mytilus californianus; 0.944 for

Haliotis cracherodii; 1.36 for Haliotis

rufescens; 1.36 for Lottia gigantea;

0.583 for Strongylocentrotus pur-

puratus; 0.365 for Tegula sp.; and

0.364 for Septifer bifurcatus (Rick

2004:79). Graphs: (a) Meat weight

by period of the major classes of

faunal species; (b) Meat weight by

period of the seven most abundant

shellfish species at the site;

(c) Meat weight by period of

Mytilus californianus and the sum

of Haliotis cracherodii, Haliotis

rufescens, Lottia gigantea, Septifer

bifurcates, and Tegula sp.


The Late Period faunal assemblage is dominated in total meat weight by sea mammal, which was a minor contributor to the assemblage during the Late Middle Period (7 percent; 34.55 kg/m3) and MLT (1 percent; 3.75 kg/m3). During the Late Period, sea mammal (39 percent; 164.15 kg/m3) appears to become more preva-lent as shellfish (8 percent; 35.06 kg/m3) decreases both in relative and absolute abundance. Although within the column samples it is possible that a single large sea mammal bone can artificially inflate the weight for a particular level, this is unlikely to be the case at this site given the fragmentary nature of pinniped bones

Table 5. Summary of CA-SRI-15 Midden Constituents by Raw Shell/Bone Weight.

Level (cm)

Shellfish Total

Raw wt (g)

Sea Mammal BoneBird BoneRaw wt (g)

Fish BoneRaw wt (g)

Olivella biplicata

Raw wt (g)Raw wt (g) Fragments

Unit 2

0–10 694.2 18.2 20 0.0 37.4 23.8

10–20 626.2 12.1 15 0.0 62.9 18.7

20–29 537.4 22.1 13 0.5 60.1 6.5

29–36 658.1 10.3 3 0.0 51.8 4.7

36–40 389.9 67.0 10 0.0 39.2 2.6

40–50 398.3 113.1 31 0.0 58.6 38.3

50–60 410.6 30.2 5 0.0 12.7 7.0

60–65 42.7 2.6 8 0.0 0.7 0.3

Unit 1

0–10 1704.8 0.4 1 0.3 78.9 11.6

10–19 1494.6 1.3 2 5.9 47.9 16.1

19–30 960.4 1.8 5 0.0 73.5 10.0

30–40 557.6 0.3 1 0.3 44.9 5.6

40–47 704.9 9.1 9 1.2 64.1 1.0

47–65 1927.3 4.6 7 1.0 98.1 1.4

65–75 1688.7 2.0 10 1.0 163.0 2.2

75–80 838.6 5.0 2 6.7 50.7 0.1

80–95 1495.9 23.0 53 3.3 95.3 0.2

95–106 2629.6 12.2 7 5.1 80.0 0.1

106–117 2320.0 5.8 6 2.0 119.6 0.9

117–126 589.0 22.4 28 12.7 90.9 0.4

126–135 341.1 0.7 2 8.9 19.6 0.3


from these excavations. Bird bone appears to be the least prominent contributor to the dietary assemblage during all three periods, although it shows an overall decrease through time from the Late Middle Period (2 percent; 10.58 kg/m3) to the Late Period (0.04 percent; 0.18 kg/m3). Bird bone was only present in one Late Period level (20-29 cm, Unit 2).

California mussel dominates shellfish meat weight within the faunal assem-blages of all three time periods (Table 4; Figure 5b). Therefore, trends in meat weight for shellfish loosely reflect those of this species. Because the meat weight density of mussel decreases with time, so does that of shellfish as a whole. Mytilus decreases from the Late Middle Period (42.15 kg/m3; 50 percent of shellfish) to the MLT (26.40 kg/m3; 34 percent of shellfish) and then again during the Late Period (17.59 kg/m3; 50 percent of shellfish). Sea urchin (Strongylocentrotus purpuratus) also decreases in importance through time with a more precipitous drop from the MLT (24.87 kg/m3; 32 percent of shellfish) to the Late Period (3.97 kg/m3; 11 per-cent of shellfish).

Table 6. Summary of Midden Constituents Normalized to 1 m3 (g/m3) and Combined by Stratum for Unit 1.

Level Shellfish TotalSea Mammal

Bone Bird Bone Fish BoneOlivella biplicata

Unit 2

0–10 cm 111,073 2,909 0 5,981 3,814

10–20 cm 100,186 1,928 0 10,067 2,997

20–29 cm 95,540 3,936 89 10,688 1,161

29–36 cm 150,428 2,350 0 11,849 1,072

36–40 cm 155,977 26,804 0 15,696 1,032

40–50 cm 63,733 18,101 0 9,381 6,126

50–60 cm 65,694 4,837 0 2,024 1,115

60–65 cm 13,656 816 0 208 90

Unit 1

Stratum A 269,417 147 520 10,674 2,329

Stratum B 115,658 162 26 9,017 1,188

Stratum C 197,522 718 144 14,864 212

Stratum D 268,339 1,603 2,138 16,221 32

Stratum E 159,558 2,454 355 10,164 26

Stratum F 359,969 1,309 517 14,521 78

Stratum G 101,500 3,977 2,261 16,165 78

Stratum H 60,636 121 1,579 3,486 53


Other shellfish species contributing to the diet include the large owl limpet (Lottia gigantea), red abalone (Haliotis rufescens), black abalone (Haliotis crachero-dii), platform mussels (Septifer bifurcatus), and turban snails (Tegula sp.). All of these species are present in small quantities, but show no clear trends through time. When these species are considered together (“minor species”), they contrib-ute substantially less than Mytilus during the Late Middle Period (13.22 kg/m3; 16 percent of shellfish), but during the MLT reach a maximum and overtake mus-sels in importance (26.63 kg/m3; 34 percent of shellfish; Figure 5c). This suggests dietary diversification during the MLT (discussed below). During the Late Period, the meat weight density of the “minor species” falls to 13.51 kg/m3 (39 percent of shellfish species), but it remains almost as prominent as California mussel.

The CA-SRI-15 fish remains were analyzed in detail by Paige (2000). Like shell-fish, the total meat weight of fish decreases through time from the Middle Period (353 kg/m3; 73 percent) through the MLT (272 kg/m3; 76 percent) and the Late Period (220 kg/m3; 53 percent). Nonetheless, fish is the dominant component for all periods, all strata of Unit 1, and all but the four lowest levels of Unit 2, where sea mammals overtake it in importance (Table 6). This may be partly because of greater disintegration of the smaller fish bones than sea mammal bones, particu-larly in the two lowest levels, but it is also related to an increase in the importance of sea mammals. The overall dominance of fish during the periods of occupation of CA-SRI-15 fits the overall patterns seen elsewhere on the islands at the same time. In the Santa Barbara Channel region, there is a general increase in the amount of fishing and decrease in the amount of shellfish collecting through time, a trend that is particularly pronounced in island contexts (Braje et al. 2007:741; Colten 2001; Glassow 1993; Kennett 2005; Kennett and Kennett 2000; Raab et al. 1995; Rick 2007; Rick et al. 2008:81). Although the decrease in fish meat weight over time at CA-SRI-15 seems to contradict this trend, shellfish meat weight decreases as well, as would be expected for the Late Holocene. However, changing rates of deposition at the site may also be responsible for these decreases.

Paige (2000) classified the fish vertebrae from CA-SRI-15 by species, noting that all fish identified at the site are common to both nearshore and kelp bed en-vironments, both of which are accessible from the site. In particular, the three fish that dominate the assemblage are rockfish (Sebastes sp.), surfperch (Family Em-biotocidae), and señorita (Oxyjulis californica), all of which are common in rocky nearshore and kelp bed habitats (Paige 2000:18). Using a method comparable to that of Rick and Glassow (1999), Paige (2000) determined the relative use of dif-ferent habitats, dividing the total vertebral count (Rick and Glassow [1999] used MNI) of each taxonomic group by the number of habitats in which each group commonly lives and summing this value across species for each potential habitat.


As a result, he noted a narrowing of habitat zones that were exploited during occu-pation of the site (Paige 2000:17-18). During the MLT, there was a shift from the more varied use of habitat zones of the Late Middle Period to one with an increase in importance of the nearshore habitat and a slight increase in kelp bed use; the use of midwater and surfzone habitats appears to have decreased at this time. This trend continued to a lesser extent into the Late Period.

Olivella Shell Bead Manufacturing Detritus

Olivella biplicata shells, which occur throughout the sequence, were used by the Island Chumash as raw material to produce beads (Arnold and Munns 1994; Ken-nett 2005; King 1990; Rick 2007). These shells were not included in our dietary analysis, but were an important part of the local economy and represent one means of expanding diet via exchange with the mainland. Many of the Olivella shells in the assemblage are broken, which likely occurred during bead production. The weight of this detritus increases through time (Table 6; Figure 6) from 114.19 g/m3 during the Late Middle Period, to 1,730 g/m3 during the MLT, to 2508.55 g/m3 during the Late Period. Broken Olivella shells are the least frequent at the beginning of the Late Middle Period, increasing dramatically during the MLT. This trend continues into the Late Period, suggesting that Olivella shell bead manufac-ture started to become important at CA-SRI-15 during the MLT and continued to expand at the site throughout the Late Period.

Figure 6. Olivella biplicata shell weight by period.


Discussion

The data presented in this article are consistent with previous observations for ma-jor cultural and socioeconomic shifts during the MLT. With the exception of the low-est two levels of Unit 2, materials in the CA-SRI-15 sequence are well preserved and undisturbed and it is one of the few excavated sites on Santa Rosa Island that was occupied throughout the MLT. Radiocarbon dates obtained from the site are consis-tent with a relatively continuous occupation from the Late Middle Period through the Late Period, with no clear breaks in the occupational sequence. Faunal remains reflect dietary decisions that were made by occupants of the site through time based on the changing availability of food items and their relative ranking. SST data ob-tained from oxygen isotopic analysis of marine shell allow us to assess whether dietary shifts are related to the changing availability of resources associated with environmental fluctuations, changes in human predation, or both.

We used OxCal to constrain the two-sigma ranges for the radiocarbon dates using known stratigraphic relationships between the samples. All of the radiocar-bon dates were modeled as a single sequence (see Figure 2) based on the stratig-raphy evident in Unit 1 (Locus A) and Unit 2 (Locus B). However, we made the assumption that the deepest radiocarbon sample from Unit 2 was more recently deposited than the highest sample from Unit 1. This assumption was based on the later dates for Unit 2 and the Late Period artifact assemblage evident at the base of this unit. Regardless, there is clear evidence of Late Middle Period and MLT oc-cupation in Unit 1 and Late Period occupation in Unit 2, with no evidence for a sig-nificant hiatus in use other than a lateral shift in the occupation from one side of Abalone Point to the other. The combination of radiocarbon dates on short-lived marine samples and careful stratigraphic excavation provides a strong chronologi-cal framework for assessing the relative impacts of environmental change and hu-man activity at this location.

Evidence from the faunal record at CA-SRI-15 indicates that humans did have an effect on the nearby intertidal community. This includes a pattern of decline and rebound in the size of California mussels. Braje et al. (2007) observed cycli-cal patterns of decline and rebound in mean mussel size at CA-SRI-147 over a sequence spanning the last 7,300 years. Along with the overall changes in mussel meat weight in the assemblage, they attributed these patterns to periodicities in human predation and resource switching (Braje et al. 2007). While the sequence at CA-SRI-15 does not have the time depth of CA-SRI-147, our data show a single de-crease and rebound in California mussel size that we argue is consistent with hu-man predation and a subsequent shift to other resources. During the MLT, mussel size reached a minimum, and there is evidence for a shift to other shellfish species,


an increase in fish diversity (see below), and a significant increase in Olivella shell bead production.

In the Santa Barbara Channel region, there is a general increase in the amount of fishing relative to shellfishing through time, a trend that is particularly pro-nounced on the islands (Braje et al. 2007:741; Colten 2001; Glassow 1993; Ken-nett 2005; Kennett and Kennett 2000; Raab et al. 1995; Rick 2007; Rick et al. 2008:81). The occupation of CA-SRI-15 occurred completely within the Late Ho-locene. Therefore, the dominance of fish throughout the assemblage is in line with what has been observed at other sites in the Santa Barbara Channel. In our limited time scale, there is not a clear relationship between fishing and shellfishing, as the meat weight density of both decreased over time. The number of house depres-sions visible on the surface of Locus B, combined with the extensive nature of midden deposits dating to this time, suggests that populations increased at this location. Therefore, the overall decrease in meat weight density of fish and shell-fish is likely related to the different character of deposition in Locus A and Locus B.

There is evidence suggesting that the fluctuations in the average size and rela-tive abundance of California mussel were associated with dietary shifts. This is significant both because of the dominance of this species in the assemblage at CA-SRI-15 and because of the relatively high rank of California mussels within ecolog-ical models for the islands (Braje et al. 2007). Resource depression is indicated by a decrease in both the size and abundance of mussels during the Late Middle Period and the MLT, with recovery during the Late Period. This was especially prominent in mussel size. The data also suggest that resource depression at this site, whether because of human activity or other factors, is associated with diversification. To assess this relationship, we used the Shannon-Weaver diversity function for shell-fish and fish individually (Figure 7). Because only fish vertebrae could be reliably differentiated to species and vertebrae only represent 5.4 percent of the total fish meat weight, we could not combine fish and shellfish for this analysis.

Paige (2000:19-21) calculated the Shannon-Weaver diversity index for the fish assemblage and showed that the lowest diversity occurred during the Late Mid-dle Period (1.45). Diversity increased during the MLT (1.73) and then decreased slightly during the Late Period (1.62), which does not appear to be simply a prod-uct of differential preservation of the material from different time periods. A simi-lar trend is evident in the shellfish assemblage. The diversity index for shellfish increases from the Late Middle Period (1.24) to the MLT (1.60) before decreasing again during the Late Period (1.54). In the case of shellfish, this reflects a decrease in meat weight and average size of the high-ranked California mussels that may have caused the occupants of the site to switch to other, lower ranked species dur-ing the MLT. As this species began to recover during the Late Period, it may have


resumed being a larger proportion of the diet, as reflected by the small decrease in diversity. However, there is no analogous metric to mussel size at the site to assess the effects of human predation on the most important fish species. Our observation that the highest diversity in the fish assemblage co-occurs with that of shellfish is consistent with the hypothesis that increases in human population placed pressure on available resources.

Changes in the faunal assemblage may also result from shifting environmental conditions. Changing SSTs in the Santa Barbara Channel region are known to in-fluence marine productivity and the availability of food resources. Because of the relatively uninterrupted occupation of CA-SRI-15 throughout the Late Holocene, oxygen isotope profiles taken from marine shells recovered from the site provide a record of SST near the western end of Santa Rosa Island. We can then compare these data with the midden assemblage from CA-SRI-15. Our data suggest that near western Santa Rosa Island, the MLT may have been associated with the stabi-lization of mean annual SST. Prior variability may have been associated with a de-crease in the availability and size of important marine species, such as California mussel. This in turn could have stimulated dietary expansion. After SST stabilized towards the end of the MLT, species like California mussels may have recovered, as indicated by increases in their average size during the Late Period. Regardless, these data do not support a clear pattern toward warmer or colder SST or toward larger or smaller annual fluctuations in SST.

Figure 7. Shannon-Weaver Diversity Index for shellfish and fish meat weight by period.


The SST record generated from marine shell from CA-SRI-15 is consistent with the curve generated by Kennett and Kennett (2000:383-384), suggesting highly unstable marine conditions between A.D. 450 and 1300. Both records also show greater stability after A.D. 1300. These data are not consistent with the findings of Pisias (1978) and Arnold and Tissot (1993) that MLT SSTs were warm. Paige (2000:22-23) argued that when looking at the overall fish assemblage, tempera-ture preferences of the dominant fish species at CA-SRI-15 are consistent with the SST curve of Kennett and Kennett (2000), rather than Arnold and Tissot (1993). At CA-SCRI-191, a site on the western end of Santa Cruz Island that also spans the Late Middle to Late periods, Pletka (2001:237) also observed that there was a relatively low proportion of warm water and southern fish species from the MLT.

The midden assemblage from CA-SRI-15 does provide strong evidence for an economic shift during the MLT consistent with many of Arnold’s original observa-tions. This is reflected most clearly in the dramatic increase in Olivella shell detri-tus from the Late Middle Period to the MLT. Olivella beads served as a medium of exchange for the Chumash inhabitants of the northern Channel Islands and ad-jacent mainland (Arnold 2001a, 2001b; Arnold and Munns 1994; Kennett 2005; King 1990; Rick 2007). Additionally, this bead industry, along with its associated craft specialization, was one of the factors associated with the development of cultural complexity in the Santa Barbara Channel region (Arnold 1987, 1992a, 2001a; Arnold and Munns 1994; Kennett and Conlee 2002; Munns and Arnold 2002). This is consistent with what has been found at other sites on the north-ern Channel Islands, which indicate a dramatic increase in shell bead manufactur-ing detritus at this time (Arnold 1987, 1990, 1992a, 1992b, 2001a; Arnold and Munns 1994; Munns and Arnold 2002:132-133).

In addition to a greater demand for beads during this period, the switch from the wall of the Olivella shell to the callus as the common source of bead blanks lim-ited the number of beads of the preferred type that could be obtained from a single shell to one, further increasing the quantity of shells needed for bead manufacture (Arnold and Munns 1994; King 1990:155). Additionally, Arnold (1987, 1992a:71, 2001a:18) has argued that shell bead manufacture beginning in the MLT was dominated by people on the northern Channel Islands, where as many as 95 to 99 percent of the beads for the Santa Barbara Channel region were produced.

We argue here that the increase in Olivella bead manufacture during the MLT represents a cultural adaptation to resource depression beyond simply exploiting different food resources. Because these beads were important to occupants on both the mainland and the islands, they expanded the possibility of trade across the Santa Barbara Channel. Most importantly for this study, they would have al-lowed islanders greater access to mainland food resources, particularly at a time


when island resources may have been depressed, whether from human impacts or environmental change. This includes both mainland faunal and plant resources. This in turn would represent a diversification beyond what was previously dis-cussed with the Shannon-Weaver calculations. Although we did not find evidence of mainland faunal or plant resources in the assemblage from CA-SRI-15, it is quite possible—especially in the case of plant resources—that they may not have survived because of processing or degradation, or may have been too small to be recovered for this analysis.

The faunal record at CA-SRI-15 is relatively consistent with general subsis-tence trends evident during the Late Holocene on the northern Channel Islands. Rick (2007) reported that nearshore fish dominated Late Holocene deposits on San Miguel Island (CA-SMI-468), but that another site (CA-SMI-481) closer to a sea mammal rookery on the western end of the island was dominated by pin-niped bone. He also observed that California mussels were always the dominant shellfish species in these assemblages (Rick 2007). Closer to CA-SRI-15 on Santa Rosa Island, Rick (2011) noted that shellfish and finfish assemblages both diver-sified during the Late Holocene. As was the case at CA-SRI-15, finfish were the most abundant species at CA-SRI-2 during this time. At CA-SMI-232, Braje (2010) observed an overall dominance of fish combined with an increased emphasis on California mussels. At CA-SCRI-191 on western Santa Cruz Island, Pletka (2001) found that nearshore fish were an important component from the Late Middle Period through the Late Period, but observed more diversity among fish habitats during the MLT than at CA-SRI-15.

Conclusion

CA-SRI-15 provides a rare opportunity to assess changes in subsistence, environ-ment, and sociopolitical organization that occurred on the northern Channel Is-lands during the MLT (A.D. 1150-1300). Radiocarbon dates provide evidence that the site was occupied continuously from the Late Middle Period (A.D. 650-1150) to the Late Period (A.D. 1300-1782). The faunal record from CA-SRI-15 provides evidence consistent with resource depression due to human predation associat-ed with expanding populations at the site and more generally on the island as a whole. Decline in California mussel size is coincident with the Late Middle Period, during which time this species was also most abundant in terms of meat weight. During the MLT, when California mussel reached a minimum in both size and meat weight, there is a diversification in shellfish species at the site. The fish as-semblage also diversifies at this time.

The record of SST obtained on marine shell from CA-SRI-15 is not consistent


with the model proposed by Arnold and Tissot (1993) that the MLT was a period of warm SST. Instead, there appears to have been a stabilization of SST during this period, which in turn may have stimulated recovery in the size and abundance of important species. The abundance and array of intertidal and nearshore fauna in the assemblage are also consistent with cool and highly productive marine condi-tions. This pattern of unstable SST during the Late Middle Period and MLT and more stable SST during the Late Period is consistent with offsite paleoclimatic evidence from the Santa Barbara Basin (Kennett and Kennett 2000).

The MLT was a period of important cultural change. This is best reflected at CA-SRI-15 by large increases in Olivella shell detritus that continued through the Late Period. We hypothesize that this allowed the occupants of CA-SRI-15 to di-versify their resource base beyond the available terrestrial and marine subsistence resources on the island and engage in more extensive trade with the mainland. More generally, this study provides an example of human adaptation to resource depression, whether from overexploitation or environmental change. This further allows us to better understand the subsistence and sociopolitical changes that oc-curred during an important but archaeologically underrepresented period in the history of human occupation of the Santa Barbara Channel region.

Acknowledgments

This research was supported by the National Science Foundation (SBR-9521974, Kennett), Channel Islands National Park (1443CA8120-96-003, Kennett), and the University of Oregon. We would like to thank Don Morris, Kelly Minas, Ann Hus-ton, and Channel Islands National Park for support of this research. Also, Brendan Culleton and Claire Ebert provided helpful feedback on aspects of this project. Nathan Craig and Margaret Brown Vega graciously translated our abstract into Spanish. Many thanks to Torben Rick, Terry Jones, and an anonymous reviewer for useful comments that helped us improve the manuscript.

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