Little Islands Recording Global Events: Late Quaternary Sea Level History and Paleozoogeography of Santa Barbara and Anacapa Islands, Channel Islands National Park, California

Abstract

Marine terraces are common on the Pacific Coast of North America and record interglacial high-sea stands superimposed on either stable or tectonically rising crustal blocks. Despite many years of study of these landforms in southern California, little work on terraces has been conducted on the two smallest of the California Channel Islands, Santa Barbara Island (SBI) and Anacapa Island (ANA). Presented here are new field and laboratory data on the ages, paleontology, and sea level history of marine terraces of these two islands. On both islands, the lowest marine terraces have shoreline angle elevations of ~11 m above sea level. Amino acid geochronology shows that terrace deposits on both islands host fossils of two ages, one group dating to the ~120-ka high-sea stand and the other group likely dating to the ~100-ka high-sea stand. A mix of fossil ages is consistent with the paleontology as well, with SBI in particular showing a faunal assemblage that includes both extralimital southern and southward-ranging species (inferred to be from the ~120-ka high-sea stand) and extralimital northern and northward-ranging species (inferred to be from thẽ 100-ka high-sea stand). Fossil mixing from these two high-sea stands supports the hypothesis that glacial isostatic adjustment (GIA) processes have left a strong imprint on the geologic record of sea level history in southern California. Nevertheless, the elevations of these terraces and that of a low terrace on Santa Cruz Island indicate that modeled GIA estimates of paleo-sea level for the peak of the last interglacial period at ~120 ka could be too high. Future development of models of GIA effects on the Pacific Coast of North America will need to consider geologic records, such as those from SBI and ANA, in refining reconstructions of sea level history.

Controls on range shifts of coastal Californian bivalves during the peak of the last interglacial and baseline predictions for today

“…This scenario posits that warm continental temperatures intensified Ekman transport and coastal upwelling during MIS 5e, thus creating relatively cool conditions within exposed outer-coast environments (Muhs and Kyser 1987;Muhs et al 1992;Ortlieb et al 1996), but warmer-than-present conditions within embayments sheltered from cool upwelling currents (Bakun 1990;Muhs et al 1992;Bakun et al 2015). The pattern of MIS 5e southern extralimital occurrences lends some support to this hypothesis (Table 1): 16 of 21 southern extralimital species in the confidently dated MIS 5e database were found within paleo-embayment environments (in some San Pedro localities [Muhs and Groves 2018], Newport Beach [Kanakoff and Emerson 1959;Powell et al 2004], and Carmel Valley [Kern 1971] localities), implying that outer-coast SSTs were more similar to today even while embayments were warmer.…”

“…Pleistocene invertebrate fossils from southern California's uplifted coastal marine terraces have been studied for more than 150 years and provide an excellent opportunity to analyze the responses of coastal ecosystems to major episodes of climate change (e.g., Conrad 1855; Valentine 1961; Lindberg and Lipps 1996; Muhs and Groves 2018). Up to 17% of species occurring in this exceptionally complete fossil record (Valentine 1989) are no longer present in the California region (Pacific coast from 34.4°N to 27.8°N; Valentine and Jablonski 1993).…”

“…Extralimital occurrences appear to track climatic cycles, with extralimitals emigrating from the north during colder climate states and from the south during warmer climate states (Valentine 1961; Valentine and Jablonski 1993; Roy et al 1996; Powell et al 2000; Muhs et al 2002, 2012, 2014; Muhs and Groves 2018). Extralimital species from warmer-than-present climate states thus have potential to provide information about which species may be expected to recolonize once regional climates sufficiently change to allow them to track their environmental tolerances back into their paleo-ranges.…”

“…Although this record has exceptional promise for unraveling the drivers of species range shifts, due to the historical difficulty of independently dating marine terrace deposits (see discussions in Muhs et al 2002, 2012, 2014; Muhs and Groves 2018) most previous investigations of range dynamics have been conducted at the composite mid–late Pleistocene level (e.g., Lindberg and Lipps 1996; Roy et al 1995, 1996, 2001) and have not been able to resolve range dynamics during discrete climate states. In addition, there are still open questions about which factors are most important in determining species range shifts.…”

“…Here we take advantage of advances in late Pleistocene geochronology (Muhs et al 2002, 2006, 2012, 2014; Muhs and Groves 2018) and satellite measurements of recent sea-surface temperatures and productivity to examine the modern-day characteristics of extralimital species that expanded their ranges into the California region (Fig. 1) during the warmest climate state of the last 300,000 years: the peak of the last interglacial, Marine Isotope Substage (MIS) 5e (~120,000 years ago; Shackleton and Opdike 1973).…”