Geology topics

Research about Santa Rosa Island

Source-linked reports with geographic coverage including Santa Rosa Island.

6 recordsLinked to original sources

USGS2025

New evidence for eolian activity and mammoths on Santa Rosa Island prior to the Last Glacial Maximum

Sea-level fluctuations due to the growth and decay of continental ice sheets of the Quaternary exert a strong influence on geologic processes along coastlines. The California Channel Islands are no exception to this, and many studies have been conducted that focus on the extremes of these glacial-interglacial cycles, such as the last glacial period (marine isotope stage [MIS] 2) and the last interglacial period (MIS 5). Far less attention has been paid to intermediate time periods between these extremes, such as MIS 4. Here, we present two very different geologic records from this time period that show how sea-level change affected the nature of sedimentation on the northern shore of Santa Rosa Island. On the northwestern coast of Santa Rosa Island, thick eolian sediments accumulated between ~80 ka and ~45 ka due to a lowered sea level that exposed carbonate-rich skeletal sands that had accumulated during the last interglacial period. On the central part of the northern coast of Santa Rosa Island, thick alluvial sediments were deposited between 80 ka and 47 ka, similar to the time of eolian sedimentation to the west. A mammoth tusk discovered within these deposits is only the third stratigraphically controlled mammoth fossil on the Channel Islands that dates prior to MIS 2. Its discovery adds to the evidence that mammoths migrated to the Channel Islands from mainland California prior to the Last Glacial Maximum.

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USGS2023

Strontium isotope chronostratigraphic age of a sirenian fossil site on Santa Rosa Island, Channel Islands National Park, California

Fossils in the order Sirenia (family Dugongidae) from Santa Rosa Island, part of Channel Islands National Park in southern California, provide rare temporal and spatial links between earlier and later evolutionary forms of dugongids, and add information about their dispersal into the northeastern Pacific region. Marine sedimentary rocks containing these fossils have characteristics of both the late Oligocene to middle Miocene Vaqueros Sandstone and the early to middle Miocene Rincon formation observed elsewhere. To determine a more precise age of the fossils, marine invertebrate shells were collected from the same exposures as the sirenian fossils for chronostratigraphic assessment using strontium isotope compositions and the well-calibrated seawater strontium evolution curve. Shells used for analysis were from bivalve mollusks ( Pycnodonte sp. [oyster] and Lyropecten sp. [scallop]) and crustaceans ( Balanus sp. [barnacle]). Results show a wide range of 87 Sr/ 86 Sr values, indicating that shell materials experienced varying degrees of diagenetic alteration. Strontium concentrations and 87 Sr/ 86 Sr values in subsamples of Pycnodonte shell show correlations between original shell material and a secondary component having lower strontium concentrations and less radiogenic (lower) 87 Sr/ 86 Sr. In contrast, all Lyropecten shell analyses yielded a uniform 87 Sr/ 86 Sr value (0.708440±0.000010 [2× standard deviation]) over a wide range of strontium concentrations (around 900 to 1,800 micrograms per gram [μg/g]). Results for Balanus shell subsamples show a range of strontium compositional behavior between the other two types of shell. Acetic acid leachates of sandy matrix confirm that diagenetic fluids had low 87 Sr/ 86 Sr values consistent with the least radiogenic values in Pycnodonte subsamples. A simple mixing model between two calcite end-members can explain observed Pycnodonte data, although actual diagenetic processes likely involved secondary dissolution/reprecipitation or strontium ion exchange between shell material and pore fluid. Data indicate that only Lyropecten subsamples have retained their original 87 Sr/ 86 Sr compositions, resulting in a best-estimate age of 20.08±0.11 million years ago (Ma) (±95-percent confidence interval [CI]). Although Dugongidae fossils have been found in Miocene and younger sediments along the west coast of North America, the Santa Rosa Island specimens represent some of the earliest and most accurately dated sirenian fossils in the region. Chronostratigraphic results also constrain the timing of the transgressional processes represented by shallow-water (Vaqueros Sandstone) to deep-water (Rincon formation) depositional environments.

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USGS2022

Diverse native island flora shows rapid initial passive recovery after exotic herbivore removal on Santa Rosa Island, California

Removing exotic vertebrates from islands is an increasingly common and potentially effective strategy for protecting biodiversity. Yet, surprisingly few studies evaluate large-scale effects of island removals on native plants. We surveyed 431 hectares of habitat in 7 canyons on Santa Rosa Island just after exotic herbivore control began (1994–1996), and again after two herbivore species had been eradicated and ~ 90% of herbivores removed (2010–2012). We searched for 68 endemic and/or rare native plant taxa, mapping and recording abundances for the 39 found. Initially most of these 39 species were absent from most canyons (79.9% species-canyon combinations). Nearly 35% of absences changed to presences by 2010–2012, while only 5.5% of presences changed to absences. Thirty-six of these 39 species increased in total area, and 38 increased in total abundance. Graminoids increased more and shrubs less than other life histories, for both apparent colonizations and abundances. Beta diversity and species turnover between canyons was high at both surveys. Although a diversity of the 39 located taxa showed substantial gains, nearly half remained uncommon in 2010–2012. These results reinforce the devastating effects of exotic vertebrate herbivores on island native plants, particularly long-lived, slow-growing species. They also demonstrate significant potential benefits of exotic herbivore removal even without other active restoration, not only for vegetative cover but for a number of rare taxa. Our surveys were more spatially extensive than most post-removal studies; high spatial turnover in these data suggests that larger-scale monitoring may be critical to capture full effects of exotic animal removal.

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USGS2018

Regeneration and expansion of Quercus tomentella (island oak) groves on Santa Rosa Island

Quercus tomentella (island oak) is an endemic species that plays a key functional role in Channel Island ecosystems. Growing in groves on highland ridges, Q. tomentella captures fog and increases water inputs, stabilizes soils, and provides habitat for flora and fauna. This cloud forest system has been impacted by a long history of ranching, and restoration efforts are underway that include erosion control, leaf litter capture, fog capture, and reforestation. To inform retoration efforts, we explored tree regeneration and the potential for Q. tomentella grove expansion on Santa Rosa Island. We delineated current and historic groves at Black Mountain by comparing stand maps from 1989 and aerial photographs from 1994 and 2015. We evaluated regeneration in the outlying areas by recording the location, diameter at breast height, number of trunks, height class, percent crown mortality, nurse plant species (if present), and reproductive status for all 4355 outlying seedlings, saplings, and mature trees. We defined outliers as individuals that were 15 m outside of the canopy of island oak groves. There are 14 groves at Black Mountain, and grove size expanded by an average of 36.9% (SD 18.5%) between 1994 and 2015. Nurse plants correlated positively with outlier tree height and reduced percent crown mortality. This effect is potentially due to increased fog drip from nurse plant species such as Quercus pacifica (island scrub oak), Heteromeles arbutifolia (toyon), and Baccharis pilularis (coyote brush). These results indicate that Q. tomentella is regenerating and that nurse plants can serve as catalysts for ecosystem restoration.

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USGS2009

Fire and vegetation history on Santa Rosa Island, Channel Islands, and long-term environmental change in southern California

The long-term history of vegetation and fire was investigated at two locations – Soledad Pond (275 m; from ca. 12 000 cal. a BP) and Abalone Rocks Marsh (0 m; from ca. 7000 cal. a BP) – on Santa Rosa Island, situated off the coast of southern California. A coastal conifer forest covered highlands of Santa Rosa during the last glacial, but by ca. 11 800 cal. a BP Pinus stands, coastal sage scrub and grassland replaced the forest as the climate warmed. The early Holocene became increasingly drier, particularly after ca. 9150 cal. a BP, as the pond dried frequently, and coastal sage scrub covered the nearby hillslopes. By ca. 6900 cal. a BP grasslands recovered at both sites. Pollen of wetland plants became prominent at Soledad Pond after ca. 4500 cal. a BP, and at Abalone Rocks Marsh after ca. 3465 cal. a BP. Diatoms suggest freshening of the Abalone Rocks Marsh somewhat later, probably by additional runoff from the highlands. Introduction of non-native species by ranchers occurred subsequent to AD 1850. Charcoal influx is high early in the record, but declines during the early Holocene when minimal biomass suggests extended drought. A general increase occurs after ca. 7000 cal. a BP, and especially after ca. 4500 cal. a BP. The Holocene pattern closely resembles population levels constructed from the archaeological record, and suggests a potential influence by humans on the fire regime of the islands, particularly during the late Holocene.

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USGS2009

Coastal change during Hurricane Dennis 2005

Hurricane Dennis made landfall as a Category 3 storm on Santa Rosa Island in the Florida Panhandle on July 10, 2005. Exposed to some of the strongest winds, Santa Rosa Island suffered erosion, as well as severe overwash. A storm surge of 2 m was recorded near Navarre Beach. The U.S. Geological Survey (USGS) and U.S. Army Corps of Engineers (USACE) are collaborating in a research project investigating coastal change that occurred as a result of Hurricane Dennis. The USGS acquired still oblique aerial photography both before and after hurricane landfall to better understand the impacts of extreme storms on coastal environments. On Tuesday, July 12, 2005, scientists conducted an aerial photographic survey of Santa Rosa Island, Florida, that was impacted by Hurricane Dennis. The photographs were compared to pre-Dennis photographs taken in July 2001 and after the landfall of Hurricane Ivan in September 2004 to illustrate extreme coastal change. On Santa Rosa Island, the storm eroded dunes and beaches, and overwashed roads. In Navarre Beach, parking lots and roads were covered with sand and dune walkovers damaged or destroyed.

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