Scott A. Minor Kevin M. Schmidt Jonathan Hoffman James B. Paces 2023 <p>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 (<i>Pycnodonte</i> sp. [oyster] and <i>Lyropecten</i> sp. [scallop]) and crustaceans (<i>Balanus</i> sp. [barnacle]). Results show a wide range of ⁸⁷Sr/⁸⁶Sr values, indicating that shell materials experienced varying degrees of diagenetic alteration. Strontium concentrations and ⁸⁷Sr/⁸⁶Sr values in subsamples of <i>Pycnodonte</i> shell show correlations between original shell material and a secondary component having lower strontium concentrations and less radiogenic (lower) ⁸⁷Sr/⁸⁶Sr. In contrast, all <i>Lyropecten</i> shell analyses yielded a uniform ⁸⁷Sr/⁸⁶Sr 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 <i>Balanus</i> 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 ⁸⁷Sr/⁸⁶Sr values consistent with the least radiogenic values in <i>Pycnodonte</i> subsamples. A simple mixing model between two calcite end-members can explain observed <i>Pycnodonte</i> data, although actual diagenetic processes likely involved secondary dissolution/reprecipitation or strontium ion exchange between shell material and pore fluid. Data indicate that only <i>Lyropecten</i> subsamples have retained their original ⁸⁷Sr/⁸⁶Sr 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.</p> application/pdf 10.3133/sir20235026 en U.S. Geological Survey Strontium isotope chronostratigraphic age of a sirenian fossil site on Santa Rosa Island, Channel Islands National Park, California reports