Early to Middle Holocene Estuarine Shellfish Collecting on the Islands and Mainland Coast of the Santa Barbara Channel, California, USA

Estuaries have long been important habitats for coastal peoples around the world, with the diversity of estuarine habitats and organisms supplying people with important food and other resources. For example, estuaries provided shellfish for Neanderthals in the Mediterranean some 42,000 years ago (Stringer et al. 2008) and estuarine and coastal resources may have helped fuel the colonization of Sahul (Pleistocene Australia, New Guinea, and Aru Islands) some 50,000 years ago (Bird et al. 2019). In California and elsewhere on the North American Pacific Coast, estuarine resources were also important in early coastal economies and were part of a broadening of human use of a range of coastal habitats from the terminal Pleistocene through the Middle Holocene (Erlandson 1994; Erlandson et al. 2019; Graham et al. 2003; Jones & Hadick 2016; Jones et al. 2019; Masters & Aiello 2007). Estuaries continued to be of importance to human subsistence economies through historic times in parts of California, but they appear to have been particularly important during the Early and Middle Holocene (especially ~9000–5000 cal BP) when sea level rise resulted in the formation of a series of estuaries, including in the Santa Barbara Channel Region (SBC) (Erlandson 1985, 1994, 1997; Glassow 1997, 2020a; Reeder-Myers 2014a). As Hildebrandt et al. (2020) note, however, modeling of the SBC coastline demonstrates that estuaries were not ubiquitous, forming in some areas at different times. Estuaries were long thought to largely be a feature of mainland coastal California with the offshore Channel Islands lacking such habitats and people focusing subsistence efforts on rocky intertidal and kelp forest resources (Erlandson 1994; Rick et al. 2005). We now know that at least one estuary (Abalone Rocks) existed on California’s Santa Rosa Island, which is supported by archaeological data, shoreline modeling, and paleoecological research (Anderson et al. 2010; Braje et al. 2019; Cole & Liu 1994; Rick et al. 2005). Despite a few studies exploring the importance of estuarine habitats on Santa Rosa Island during the terminal Pleistocene to Middle Holocene, there has been limited research comparing human use of Channel Island estuaries to the estuarine systems on Rick, TC. 2020. Early to Middle Holocene Estuarine Shellfish Collecting on the Islands and Mainland Coast of the Santa Barbara Channel, California, USA. Open Quaternary, 6: 9, pp. 1–14. DOI: https://doi.org/10.5334/oq.86


Introduction
Estuaries have long been important habitats for coastal peoples around the world, with the diversity of estuarine habitats and organisms supplying people with important food and other resources. For example, estuaries provided shellfish for Neanderthals in the Mediterranean some 42,000 years ago (Stringer et al. 2008) and estuarine and coastal resources may have helped fuel the colonization of Sahul (Pleistocene Australia, New Guinea, and Aru Islands) some 50,000 years ago (Bird et al. 2019). In California and elsewhere on the North American Pacific Coast, estuarine resources were also important in early coastal economies and were part of a broadening of human use of a range of coastal habitats from the terminal Pleistocene through the Middle Holocene (Erlandson 1994;Erlandson et al. 2019;Graham et al. 2003;Jones & Hadick 2016;Jones et al. 2019;Masters & Aiello 2007). Estuaries continued to be of importance to human subsistence economies through historic times in parts of California, but they appear to have been particularly important during the Early and Middle Holocene (especially ~9 000-5000 cal BP) when sea level rise resulted in the formation of a series of estuaries, including in the Santa Barbara Channel Region (SBC) (Erlandson 1985(Erlandson , 1994(Erlandson , 1997Glassow 1997Glassow , 2020aReeder-Myers 2014a). As Hildebrandt et al. (2020) note, however, modeling of the SBC coastline demonstrates that estuaries were not ubiquitous, forming in some areas at different times.
Estuaries were long thought to largely be a feature of mainland coastal California with the offshore Channel Islands lacking such habitats and people focusing subsistence efforts on rocky intertidal and kelp forest resources (Erlandson 1994;Rick et al. 2005). We now know that at least one estuary (Abalone Rocks) existed on California's Santa Rosa Island, which is supported by archaeological data, shoreline modeling, and paleoecological research (Anderson et al. 2010;Braje et al. 2019;Cole & Liu 1994;Rick et al. 2005). Despite a few studies exploring the importance of estuarine habitats on Santa Rosa Island during the terminal Pleistocene to Middle Holocene, there has been limited research comparing human use of Channel Island estuaries to the estuarine systems on the adjacent mainland Jazwa et al. 2013;Rick et al. 2005Rick et al. , 2006Wolff et al. 2007).
In this paper, I present new quantified shellfish data from excavations at two sites dated between ~7 450 and 5960 cal BP (CA-SRI-77 and CA-SRI-192) on eastern Santa Rosa Island, California that contain a variety of estuarine and rocky coast shellfish species (Figure 1). These data are placed in the context of contemporary Early and Middle Holocene sites on Santa Rosa Island and the adjacent mainland coast to document the importance of estuarine versus outer coast shellfish habitats on island and mainland southern California between about 9000 and 5000 cal BP. These data demonstrate the value of zooarchaeological research for informing past environmental change and human subsistence strategies in complex coastal systems.

Context and Background
The Santa Barbara Channel region is situated on the southern California Coast from Point Conception in the west to Point Mugu in the east. The region is part of the Transverse Range and trends east-west, rather than north-south like most of the Pacific Coast. The four Northern Channel Islands (NCI) are about 20 to 44 km offshore and mark the southern boundary of the SBC. The region is known for its diverse and productive marine ecosystems and organisms, including kelp forests, rocky shores, sandy beaches, and estuaries. During the Pleistocene and through about 9000 years ago, the NCI were connected in one large superisland known as Santarosae which, although closer to the mainland coast, was always separated by a watergap of at least 7 km Reeder-Myers et al. 2015). Rising sea level from the glacial to interglacial created numerous estuaries in the Santa Barbara Channel region, which were exploited by people since at least the terminal Pleistocene, including on Santa Rosa Island Jazwa et al. 2013;Rick et al. 2005).
Santa Rosa Island currently sits ~4 4 km offshore and is about 217 km 2 in area. The island contained at least one estuary in the past, the Abalone Rocks paleoestuary and marsh complex located in the vicinity of Old Ranch and Old Ranch House canyons on the eastern end of the island (Cole & Liu 1994;Rick et al. 2005). The Abalone Rocks Paleoestuary/Marsh produced paleoecological records spanning roughly 7000 calendar years, suggesting marine/estuarine conditions through the Middle Holocene (Anderson et al. 2010;Cole & Liu 1994). The Abalone Rocks Paleoestuary is unique because it is the only well documented estuary on the Channel Islands with associated shell middens and human harvest. Other estuarine habitats likely existed in the terminal Pleistocene and Early Holocene near the now submerged Crescent Bay area (which encompasses Abalone Rocks) of Santa Cruz and Santa Rosa islands, with subbottom profiles raising the possibility of estuarine deposits in the Santa Cruz Channel (see Braje et al. 2019). In addition to the presence of an estuary, Old Ranch Canyon is a well-watered drainage with abundant freshwater, ranking as the second most productive Santa Rosa Island drainage because of flow, size, resilience and other variables (Jazwa et al. 2016).
The NCI have produced evidence of human occupation dated to 13,000 cal BP, with a site near Point Conception on the mainland coast dated to ~1 0,700 cal BP, demonstrating some of the earliest coastal settlement in the Americas Gusick & Erlandson 2019;Lebow et al. 2015). The SBC and NCI also contain evidence of human occupation more or less continuously across the Holocene through the early 19 th century (Erlandson & Rick 2002;Gamble 2008;Rick et al. 2005). The Chumash of the SBC and NCI are well known for their sophisticated maritime hunting and fishing strategies and use of a variety of marine and terrestrial habitats. On the Channel Islands, marine fishes and shellfish from kelp forest and rocky intertidal habitats were the focus of human subsistence strategies since initial colonization through the 19 th century, although people also harvested a wide range of island terrestrial plants (Braje 2010;Gill & Hoppa 2016). On the mainland, people appear to have had more balanced economies with a greater inclusion of terrestrial animal foods (e.g., deer) that were absent on the Channel Islands (Erlandson & Rick 2002).

Materials and Methods Site descriptions and chronology
CA-SRI-77 is a multi-component shell midden located on the east side of Becher's Bay roughly 3-4 km from Abalone Rocks. Dense Late Holocene midden deposits are exposed at CA-SRI-77 in a road cut and along the sea cliff.
A few surface depressions visible in the northern site area appear to be the remains of Late Holocene house depressions. The site's Middle Holocene component is exposed on a bench in the sea cliff and stratigraphically below the dense Late Holocene deposits about 1 m below the site surface (Figure 2). This lower deposit is visible for roughly 15 m in the sea cliff and extends for an unknown distance to the east. The lower midden deposits are roughly 10-20 cm thick and contain scattered pockets of shellfish remains, including estuarine and rocky intertidal taxa and lithic artifacts (Rick et al. 2005). Four radiocarbon dates from the site document an occupation at roughly 7450 years ago and Late Holocene occupation between about 1150 and 1000 cal BP (Kennett 2005(Kennett , 2005Rick 2009;Rick et al. 2005). Radiocarbon dating of a Chione undatella shell from the lower component provides a 2 sigma calibrated age of 7620-7280 cal BP, centering on 7450 cal BP (Figure 3, Table 1). CA-SRI-192 is located on a gently sloping hill overlooking the Abalone Rocks Marsh. Shell midden deposits sit on top of the hill and down slope, with a small rockshelter also located on site (Figure 4). Our work at the site demonstrates that, like CA-SRI-77, the site has at least two components, one containing mostly Mytilus californianus shell, as well as a single piece fishhook on top of the plateau and another with estuarine taxa located downslope (Rick 2019;Rick et al. 2005). Two radiocarbon dates obtained from shells collected in an auger hole excavated on top of the hill demonstrate a Late Holocene occupation around 2500 cal BP (Rick 2019). Three additional radiocarbon dates were obtained from the estuarine midden and provide a 2 sigma range of 6800 to 5660 cal BP (Figure 3, Table 1). However, the shellfish samples reported here come from Unit 1, with two dates that suggest a more constrained occupation between 6260 and 5660 cal BP, centering on 5960 cal BP. Collectively, the deposits from CA-SRI-77 and CA-SRI-192 represent a roughly 1000 to 1500 year period of time centered between 7450 and 5960 cal BP.

Field methods
In 2003, two 15 L bulk samples (Bulk Samples 1 and 2) were excavated in intact midden deposits exposed in the sea cliff at CA-SRI-77. These samples were spaced about 3 m apart on the sea cliff and excavated by hand and trowel using measured buckets to obtain volume. All samples were screened over 1/16-inch mesh in the field, with 1/8-inch residuals analyzed for this study. Data from Bulk Sample 1 were presented in Rick et al. (2005), but the data from Bulk Sample 2 have not been reported.
Excavations at CA-SRI-192 were conducted in 2005. Unit 1/Unit 1a was excavated as two 50 L bulk samples (100 L total volume) from a 50 × 50 cm area in archaeological deposits located in a small bench on the hill. Because of the eroding nature of the deposits and lack of  stratigraphic variation, these bulk samples were collected from the same area down to a depth of 60 cm, with Unit 1 representing the upper 30 cm and Unit 1a representing the lower 30 cm. These midden deposits were extremely dense and all residuals were poured over 1/8-inch mesh in the field. The Unit 1/1a samples were processed separately in case of any variation in constituents noted in the midden. Two augers were also excavated in the Late Holocene deposits at the site, demonstrating a midden dominated by Mytilus californianus with no estuarine shell to a depth of about 80 cm (Rick 2009(Rick , 2019.

Laboratory procedures
All excavated samples were returned to the lab for processing and analysis. Residuals were water screened, with 1/8-inch samples forming the focus of research. Students sorted the 1/8-inch residuals into general categories of shellfish, fish, mammal, bird, other vertebrate remains, artifacts, botanical remains, etc. Shellfish dominated the assemblages at both sites, with only trace amounts of vertebrate faunal remains, which is generally consistent with other Early to Middle Holocene sites in the Abalone Rocks area (Jazwa et al. 2013;Rick et al. 2005). For the two 15 L samples from CA-SRI-77, all 1/8-inch and larger materials were completely analyzed. Given the large volume of materials from CA-SRI-192, all 1/4-inch materials were completely processed and a random 25% sample by weight of 1/8-inch residuals was analyzed. All faunal remains were identified using comparative collections at Southern Methodist University and the Smithsonian Institution, with taxonomy following the Integrated Taxonomic Information System (www.itis.gov). Habitat information for shellfish follows Morris et al. (1980) with those taxa from estuarine habitats starred in Tables 2 and 3. All faunal materials were weighed in grams and MNI were calculated using sided hinges for bivalves and spires for gastropods. Following previous shellfish syntheses in the region, we focus on raw weights to explore the abundance and significance of various taxonomic categories (Braje et al. , 2012. While several studies have shown the strengths and weaknesses of various approaches to quantifying archaeological shellfish remains (see Giovas 2009), the use of weight allows for broad regional comparison across a number of other studies on Santa Rosa Island and the adjacent mainland. Because of a lack of stratigraphic variation and limited differences in constituents between the two bulk samples from CA-SRI-77 and the two contiguous units at CA-SRI-192, these samples have been combined for each site, with data for the separate units available in Tables S1 and S2.
Changes in taxonomic richness (number of taxonomic categories), diversity, and evenness at CA-SRI-77, CA-SRI-192, and other estuary sites near Abalone Rocks Jazwa et al. 2013;Rick et al. 2005Rick et al. , 2006Wolff et al. 2007) were estimated using the Shannon-Weaver diversity (H') and evenness (V') function: H' = -∑ (p i ) (log e p i ), where p i is the relative abundance (weight) of an individual shellfish taxonomic category divided by the total weight of all the taxonomic categories and V' = H'/log e S, where S is the total number of taxonomic categories (Reitz & Wing 2008: 111-112). The Shannon-Weaver function provides a quantitative means to evaluate how diverse and evenly distributed a particular shellfish assemblage is, which can help further evaluate environmental change and human shellfish collecting strategies across space and through time. To further understand the importance of estuarine compared to outer coast shellfish on the adjacent mainland and Santa  Table S2 for individual units). Rosa Island, we use % of total shell weight from a range of island and mainland sites (Figure 1), with mainland data relying on recent synthesis and recalculations by Hildebrandt et al. (2020).
Taxonomic richness and Shannon-Weaver diversity and evenness values from CA-SRI-77, CA-SRI-192, and four other estuarine site components from the Channel Islands document some temporal variation ( Figure 5; Table S3). The richness values are somewhat consistent through time between 20-24 species, with a low of 10 at CA-SRI-84/187 (Rick et al. 2005) and a peak of 44 at CA-SRI-666 (Erlandson et al. 2020a). The low value may be a result of small sample size, while the higher value at CA-SRI-666 is influenced somewhat by analytical differences, with many species of limpet, gastropod, and chiton identified to species at CA-SRI-666, but only identified to more general taxonomic categories in the other studies. Diversity values range from 0.7 to 1.9 and evenness from 0.19 to 0.64, with CA-SRI-77 having the highest values for each category and the other sites generally clustering closer together. This is reflected in the relatively even distribution of shellfish from open coast and rocky shore habitats at CA-SRI-77 whereas most other sites tend to be skewed by a much higher abundance of open coast compared to estuarine species.
When placed in the context of 5 additional site components with estuarine harvest on Santa Rosa Island, and  Table S1 for individual units). 12 site components from the adjacent mainland, the data from CA-SRI-77 and CA-SRI-192 inform broader patterns of the importance of estuarine shellfish compared to the outer coast between about 9000 and 5000 years ago on the mainland and 8000 and 5900 years on the islands (Figure 6, Tables S3 and S4). These data illustrate that on Santa Rosa Island between 4.1% and 49.7% of shellfish come from estuarine habitats. Although the 8000 year old component at CA-SRI-666 is only 6.5% estuarine shellfish, the other two early sites, SRI-84/187 and SRI-77 have 33.9%, and 49.7%, respectively, with percentages dropping quickly through time after this. These data stand in sharp contrast to the 12 mainland assemblages, where estuarine shellfish make up between 16.5% and 96.5% estuarine shellfish (Figure 6). Five of the 12 sites are above 90% estuarine shellfish and four others are 59% or higher.

Discussion
Archaeologists working in California have long been interested in the importance of estuaries for human subsistence and settlement, beginning with early research on San Francisco Bay shell middens and more recently by investigating the importance of estuaries for driving early human (Paleocoastal) settlement and subsistence ( Table S3 for raw data and references). the Santa Barbara Channel, California, USA Art. 9, page 8 of 14 intensity of human focus on estuarine resources varied considerably by location and in relation to major climatic events, such as sea level rise during the Pleistocene/Holocene transition that was crucial in the formation of many estuaries in the SBC, including Abalone Rocks (Erlandson 1994(Erlandson , 1997Glassow 2020a;Jazwa et al. 2013;Jones et al. 2019;Reeder-Myers 2014a;Rick et al. 2005). Radiocarbon dating of archaeological sites on eastern Santa Rosa Island demonstrates that Abalone Rocks was a focus of human settlement and subsistence between about 8000-5000 years ago (Rick 2009), which roughly corresponds with the timing of estuarine conditions documented in paleoecological cores (Anderson et al. 2010;Cole & Liu 1994). Work at two sites on Santa Rosa Island (CA-SRI-708 and CA-SRI-670) indicates that Abalone Rocks or another yet to be documented estuary was utilized by people on the island both well prior to the 8000 years ago and perhaps considerably later than 5000 years ago Minas 2017). These two sites are located near Water Canyon on the north coast of Santa Rosa Island about 3 km west of CA-SRI-77 and ~7 km from the current Abalone Rocks Marsh. CA-SRI-708 is a shell midden that appears to be dominated by Mytilus californianus, but with smaller amounts of estuarine shell. Three dates on Chione undatella shells document its age around 11,300-10,850 cal BP . No quantified data are available from this site, but these data suggest that estuaries were important for some of the earliest peoples to live on the Channel Islands, though it is unclear precisely where these shellfish may have been obtained and raises the possibility of other estuaries beyond Abalone Rocks and including western Santa Cruz Island, especially in the now submerged Crescent Bay .
At the other end of the spectrum, Minas (2017) documented trace amounts of Ostrea lurida and Saxidomus nuttalli in the Late Holocene shell midden CA-SRI-670. Midden samples demonstrate that Mytilus californianus and other rocky intertidal shellfish dominate the assemblage, with estuarine shell comprising less than 1% of the shellfish assemblage by weight. A direct date on an oyster shell from this deposit ranges from ~9 35-730 cal BP, raising the possibility that there was an estuary somewhere on the island at this late date (Minas 2017). S. nuttalli can be found in sheltered beaches on the outer coast, but oysters are generally only found in bays and estuaries (Morris et al. 1980). Given that these species occur in trace amounts they could have also been traded in from the mainland, but testing at other sites in the area is needed to determine if estuarine shellfish were present and exploited in the Late Holocene.
Calculation of taxonomic richness, diversity, and evenness from Abalone Rocks sites and estuarine versus open coast shellfish abundance from Santa Rosa Island and mainland sites supplies a means to evaluate the importance of estuaries to Early and Middle Holocene peoples throughout the SBC. Data on taxonomic richness from Abalone Rocks sites are generally consistent, suggesting a similar suite of shellfish that were harvested with two exceptions (CA-SRI-84/187 and CA-SRI-666) that may be from sample size and analytic procedures, rather than human diet (Figure 5). The biological diversity of the five assemblages calculated using the Shannon-Weaver function is fairly low in the earliest two assemblages, with a peak in diversity Figure 6: Abundance of estuarine shellfish as a percentage of total shell weight (excluding terrestrial and undifferentiated shell) at island (top) and mainland (bottom) sites from ~8 000-5960 cal BP on the islands and ~8 880-4900 cal BP on the mainland (see Tables S3 and S4 for raw data and references). Rick: Early to Middle Holocene Estuarine Shellfish Collecting on the Islands and Mainland Coast of the Santa Barbara Channel, California, USA Art. 9, page 9 of 14 and evenness at the 7450 cal BP occupation of CA-SRI-77 followed by a decline through the two latest estuary components at about 5960 cal BP. This decline later in time may relate to the closure of the estuary with people focusing on the rich Mytilus californianus beds available throughout these site occupations. Whatever the case, regardless of time period or site location, M. californianus is the most abundant taxon at most Abalone Rocks sites, demonstrating that the estuary was likely never more than an important supplemental focus of island human diets. In this sense, the presence of an estuary on eastern Santa Rosa Island added to the attractiveness of the area for human settlement, providing a range of habitats for human subsistence (kelp forest, rocky intertidal, open sandy coast, etc.), but was not the sole reason for settlement (Jazwa et al. 2013;Rick et al. 2005).
SBC archaeologists have long recognized the importance of estuaries in mainland coastal California throughout the Holocene, including some estuaries that may have only persisted during the Early to Middle Holocene (Erlandson 1991(Erlandson , 1994(Erlandson , 1997Glassow 1997Glassow , 2020bReeder-Myers 2014a). Mainland estuaries like the Goleta Slough provided shellfish and other resources throughout the Holocene, with peak abundance often occurring in some Late Holocene or even Historic period middens (Glassow 2020a;Hildebrandt et al. 2020). Beyond the Goleta Slough, most mainland SBC estuaries, particularly those in the western Santa Barbara Coast were most productive during the Early to Middle Holocene as evidenced by several sites with estuarine shellfish through about 5000 years ago that then precipitously decline or disappear (Erlandson 1997). Decades of research has detailed this pattern on the mainland, which has often stood in contrast to the Channel Islands, where only one estuary has currently been documented and its significance was still relatively poorly documented. Quantified data from seven island site components and 12 on the mainland coast demonstrate that, for the most part, the more extensive estuarine systems on the mainland were far more important to human subsistence than they were on Santa Rosa Island (Figure 6). Even on the western Santa Barbara Coast, where estuaries were likely never as productive as the Goleta Slough, estuarine shellfish abundance in all four Early Holocene site components there exceed 90%, whereas only one site on Santa Rosa Island even approaches 50% with the rest at 33% or substantially lower. Adding to this, is the fact that some mainland estuarine assemblages also contain a wider range of species that inhabit estuaries and in some cases other habitats (Argopecten aequisulcatus [scallop], Cerithedeopsis californica [horn shell], and Macoma nasuta [bent-nosed clam]) that, to my knowledge, have yet to be identified in Abalone Rocks island assemblages in any significant abundance (see Erlandson et al. 2020b;Glassow 2020b). These data illustrate some important differences between Early to Middle Holocene island and mainland people, with estuaries being a significant focus of subsistence on the mainland, and islanders generally focusing on the rich rocky intertidal and kelp forest habitats for more than 10,000 years (Braje 2010;Kennett 2005).
Another important difference between Early Holocene mainland and island communities is the apparent lack of the Milling Stone Horizon on the Channel Islands compared to the mainland coast where the Milling Stone Horizon was widespread (Erlandson 1994). As Erlandson (1991) noted, Early Holocene Milling Stone peoples on the SBC mainland often took advantage of estuarine shellfish in tandem with seeds and other plant foods that could be processed with manos and metates identified at early sites. Manos and metates are rare and the Millingstone Horizon is absent from the NCI, with early people instead focusing largely on geophytes (corms) that required different processing methods, including at Abalone Rocks sites (see Erlandson et al. 2020a;Gusick 2012Gusick , 2013Gusick and Erlandson 2019). These differences are important and show just how distinct early islanders and mainlanders were in terms of basic subsistence strategies and technologies.
What about changes in individual shellfish species and their relation to potential evolution and the development of the Santa Rosa Island estuary? Colten (1989) argued that increases in Chione spp. relative to Saxidomus nuttalli may indicate an increase in mudflats in the Goleta Slough. This suggestion was revisited by Hildebrandt et al. (2020), Erlandson et al. (2020b), andGlassow (2020b), with S. nuttalli and Protothaca spp. representing an immature estuary, whereas Chione spp. and, to an extent, Ostrea lurida may represent a more mature estuary. Comparison of the abundance of the four major estuarine shellfish genera at seven Abalone Rocks components provides a means to test potential changes in habitat in the Santa Rosa Island estuary (Figure 7, Table S5). These data document fluctuations in the importance of various taxa through time. S. nuttalli reaches peak abundance in the oldest two assemblages dated to roughly 8000 cal BP at CA-SRI-666 and CA-SRI-84/187, with a decline at 7450 cal BP and generally lower abundance through about 5900 years ago, although one site demonstrates about 50% S. nuttalli around 6200 cal BP. O. lurida and Chione spp. are generally more abundant in middens after about 7500 years ago. Protothaca spp. which can be found in a range of habitats is generally low in all sites, but peaks in one of the latest assemblage at 5980 cal BP. It is conceivable that these patterns reflect a shift to a more mature estuarine environment ~7 450 cal BP. Alternatively, they could also represent more localized focus on obtaining shellfish from specific habitats.
Complicating perspectives on changing Abalone Rocks Estuary conditions is that CA-SRI-708 demonstrates estuarine shellfish exploitation by ~1 1,000 years ago, including Chione spp. and O. lurida . While quantified data are not available from this site, these data provide preliminary support for a terminal Pleistocene mature estuary on Santa Rosa Island, perhaps at Crescent Bay . The 8000-6000 year old sites described here all post-date the breakup of Pleistocene Santarosae Island by over 1000 years, and likely represent the exploitation of Abalone Rocks. Currently, there is a gap between CA-SRI-708 (~11,000 cal BP) and CA-SRI-666 and CA-SRI-84/187 (~8000 cal BP) of roughly 3000 years without any known estuary sites. Future research should explore the possibility of sites that could fill this gap and help understand the changing coastal paleogeography of the dynamic eastern Santa Rosa and western Santa Cruz Island landscapes.

Conclusions
Estuaries were an important component of diverse Early and Middle Holocene subsistence strategies on the SBC mainland Coast and Channel Islands. Rocky intertidal shellfish dominate at most sites on the Channel Islands, but the Abalone Rocks Estuary/Marsh provided an important source of food and allowed people access to a diverse array of resources in the eastern Santa Rosa Island area. People opportunistically exploited Abalone Rocks Marsh and then, after its closing sometime around 5000 years ago, people shifted to other resources, especially rocky shore and kelp forest foods (Jazwa et al. 2013;Rick et al. 2005). People also shifted to Tivela stultorum at nearby CA-SRI-209 obtained from open coast sandy beaches ~4 730-4360 cal BP, but trace amounts of Tivela stultorum have been identified at an Early Holocene site in the nearby Carrington Point area and a few other early and later sites on San Miguel and Santa Cruz islands (Reeder-Myers 2014b; Rick & Eliot 2013;Thakar 2011). Similar changes in habitat exploitation are evident in some mainland sites too, documenting the wider pattern of people adjusting subsistence pursuits and decision-making related to changing environments throughout the SBC (Reeder-Myers 2014a; Vellanoweth and Erlandson 2004). In this case, the most apparent changes are shifts between rocky shore, surfswept sandy beach, and estuarine habitats that manifested at different times in different places, often correlating with broader environmental changes (e.g., El Niño frequency, sea level rise, etc.)(see Masters & Aielo 2007).
The data from Santa Rosa Island and the SBC mainland highlight the global significance of estuaries in the human past. The high biodiversity, mix of different organisms and ecosystems, and sheltered waters were attractive areas around the world that, along with other coastal ecosystems, may have helped fuel the Late Pleistocene spread of people along the Pacific Coast of the Americas and the colonization of Sahul (Bird et al. 2019;Erlandson et al. 2019). Estuaries are dynamic and strongly influenced by climate change, particularly changes in sea level, freshwater input, and infilling with alluvium. The significance of estuaries globally cannot be overstated, with sites across the Holocene documenting human use of estuarine habitats from Britain to Northumberland, Canada, the Chesapeake Bay, United States of America, Southwest Mexico, coastal Peru, Queensland, Australia, Tanzania, Madagascar, and Thailand and broader Southeast Asia (Bell & Neumann 1997;Beresford-Jones et al. 2018;Douglass 2017;Higham 2017;Lelièvre 2017;Pollard 2008;Reeder-Myers et al. 2016;Smith & McNiven 2019;Voorhies 2004). The data from Abalone Rocks and the SBC add to these global patterns, demonstrating that estuaries were important for human settlement and subsistence in the Holocene on all continents except Antarctica, and illustrating the importance of archaeological research to evaluate long-term environmental change in dynamic coastal environments.

Data Accessibility Statement
All data used for this study are presented in the main text or the online supplement. The CA-SRI-77 and CA-  collections are housed at the Department of Anthropology, Santa Barbara Museum of Natural History under Accession CHIS-442.

Additional Files
The additional files for this article can be found as follows:  Table S5 for raw data. •