Quaternary sea-level history and the origin of the northernmost coastal aeolianites in the Americas: Channel Islands National Park, California, USA

Abstract

Along most of the Pacific Coast of North America, sand dunes are dominantly silicate-rich. On the California Channel Islands, however, dunes are carbonate-rich, due to high productivity offshore and a lack of dilution by silicate minerals. Older sands on the Channel Islands contain enough carbonate to be cemented into aeolianite. Several generations of carbonate aeolianites are present on the California Channel Islands and represent the northernmost Quaternary coastal aeolianites on the Pacific Coast of North America. The oldest aeolianites on the islands may date to the early Pleistocene and thus far have only been found on Santa Cruz Island. Aeolianites with well-developed soils are found on both San Miguel Island and Santa Rosa Island and likely date to the middle Pleistocene. The youngest and best-dated aeolianites are located on San Miguel Island and Santa Rosa Island. These sediments were deposited during the late Pleistocene following the emergence of marine terraces that date to the last interglacial complex (~120,000 yr to~80,000 yr). Based on radiocarbon and luminescence dating, the ages of these units correspond in time with marine isotope stages [MIS] 4, 3, and 2. Sea level was significantly lower than present during all three time periods. Reconstruction of insular paleogeography indicates that large areas to the north and northwest of the islands would have been exposed at these times, providing a ready source of carbonate-rich skeletal sands. These findings differ from a previously held concept that carbonate aeolianites are dominantly an interglacial phenomenon forming during high stands of sea. In contrast, our results are consistent with the findings of other investigators of the past decade who have reported evidence of glacial-age and interstadial-age aeolianites on coastlines of Australia and South Africa. They are also consistent with observations made by Darwin regarding the origin of aeolianites on the island of St. Helena, in the South Atlantic Ocean, more than a century and a half ago.

Coastal dune hydrology and its influence on maritime occupation sites (12–0.2 ka) in a semiarid island, San Miguel, California, USA

“…One travertine‐lined gully water course, near the west end of San Miguel (Figure b) is located at site CA‐SMI‐606 (ABP; Figure ). The perched water course occurs above a marine terrace bench, associated with an uplifted MIS5 terrace, at 15 m MSL (Muhs et al, ; Peterson, Erlandson, et al, ). Adjacent shell middens date from ~10 to 1.8 ka (Table ), indicating a long‐term, though possibly episodic, occurrence of this seasonally ephemeral water source on the north side of San Miguel.…”

“…Compilations of occupation sites associated with eolian deposits in San Miguel Island (Figure ) are used in this study to establish relations between some site components, occupation ages, paleo‐elevations, and paleo‐shoreline distances. Eolian deposits include dune sand, loess, caliche subsoils, and dune deposit colluvium (Johnson, , ; Muhs et al, ; Peterson, Erlandson, et al, ). Site component associations, including marine shells and lithic artifacts, and ranges of site occupation ages (median 14 C ages converted to ka) are taken from Erlandson et al (, ), Braje et al (), and Table .…”

“…Modest island elevations and a prolonged sand supply from the northern shelf platform (Figure ) permitted a widespread cover of coastal dunes across San Miguel (Figure ; Johnson, ). Late‐Pleistocene dune deposits (>10 ka) represent eolian transport from the emerged shelf‐platform during marine low‐stand conditions (Muhs et al, ; Peterson, Erlandson, et al, ). Middle‐ to late‐Holocene eolian sand ramps (<10 m thickness) developed along the northern (windward) bluff slopes following slowing of the Holocene marine transgression after 9 ka (Peterson, Erlandson, et al, ).…”

“…Reactivations of the shallow dune deposits that cross San Miguel have likely followed episodic devegetation, including widespread vegetation stripping, dune destabilization, and soil erosion associated with historical overgrazing (Johnson, ; Rick, ). Today San Miguel generally lacks perennial lakes or streams (Engle, ), but it likely developed ephemeral freshwater catchments and drainages associated with island’s eolianites, calcium carbonate‐rich dune sand, and associated late‐Pleistocene caliche subsoils (Johnson, , ; Muhs et al, , ; Peterson, Erlandson, et al, ). Johnson () reported “large portions of the island (San Miguel) are covered with porous (dune) sand that serves as a good recharge aquifer, and it has recoverable groundwater.”…”

“…One travertine-lined gully water course, near the west end of San Miguel (Figure 7b) is located at site CA-SMI-606 (ABP; Figure 6). The perched water course occurs above a marine terrace bench, associated with an uplifted MIS5 terrace, at 15 m MSL (Muhs et al, 2018;Peterson, Erlandson, et al, 2017a). Adjacent shell middens date from 10 to 1.8 ka (Table 2) which is incised into the steep bluff slope, located above the reach of the Holocene dune sand at section S6 and below the eolianite aquifer deposits (~10 m thickness) at the bluff top near section S15.…”