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Terraces through time: Reconstructing fossil beaches in southern California

San Nicolas Island is a strange, far-away place very familiar to a surprising number of Californians. Thanks to Scott O'Dell's Island of the Blue Dolphins, this island — the most remote of California's eight Channel Islands — and it's native Nicoleño people have been engrained into the imaginations of many elementary school children. My own mind was captivated by this story in the fourth grade when I had the opportunity to conduct fieldwork on San Nicolas Island with Daniel Muhs (U.S. Geological Survey) and my adviser Seth Finnegan in July 2015 I was thrilled! Descending from hundreds of feet above the island’s landing strip I was already able to spot the very reason for my fieldwork- Pleistocene fossil beaches.

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San Nicolas Island from above showing its terraced coastline. Marine terraces - records of ancient beaches - are formed by the powerful erosional energy of waves. They are relatively flattened geomorphological features that can serve as convenient pre-leveled platforms for human infrastructure. Hence, San Nicolas Island’s naval base airstrip (the island has been a naval base since the 1940’s) lies atop the island’s seventh terrace, which is the island’s most apparent marine terrace. Photo by D. Günther

Emily standing on a concretion jutting out just below San Nicolas Island’s youngest marine terrace (~80,000 years old).

Emily standing on a concretion jutting out just below San Nicolas Island’s youngest marine terrace (~80,000 years old). Photo by Seth Finnegan

Carved by the powerful energy of ancient waves, over 11 Pleistocene fossil beaches are terraced (hence their geological name "marine terrace") over the landscape of San Nicolas Island's modest 23 square miles. The youngest fossil beach (~80,000 years old) sits just above present-day sea level and the oldest (~1,200,000 years old) lies atop the island's highest elevation. Fossil mollusc shells — very similar to the kinds you find along California beaches today — abound within these marine terraces. Differences in the species compositions and abundance of these mollusc shells record dynamic ecological changes that occurred in response to glacial-interglacial climatic change during the Pleistocene.

Close-up of marine terrace sediments from one of the island’s oldest marine terraces (~1,200,000 years old). Fossil preservation on this terrace is exceptionally good- with original shell color preserved on many specimens.

Close-up of marine terrace sediments from one of the island’s oldest marine terraces (~1,200,000 years old). Fossil preservation on this terrace is exceptionally good- with original shell color preserved on many specimens.

My goal on San Nicolas Island is to collect fossil shells from the lowest three marine terraces — which record the last full interglacial cycle (~120,000 – 80,000 years ago). In particular, I am collecting well-preserved fossil Callianax biplicata (common name, purple olive shell) specimens. Using these fossil shells, I am reconstructing paleoenvironmental conditions during the last interglacial period through the use of stable isotopes. The reason this is possible is because shells grow by semi-continuously depositing layers of calcium carbonate. In the same way scientists use tree rings to chronicle the life a tree, I am using shell growth layers to reconstruct the environmental conditions experienced during the lives of molluscs that lived during the last interglacial period.

After collecting fossil C. biplicata from the terraces of San Nicolas Island, Sydney Minges (UCB Integrative Biology and Earth Planetary Sciences undergraduate student) and I sampled tiny holes along shell growth lines and analyzed these samples for carbon and oxygen stable isotope ratios at UC Berkeley's Center for Stable Isotope Biogeochemistry. Taken together, these isotope ratios can be used to reconstruct changes in seasonal, annual, and inter-annual seawater conditions and temperature during the last interglacial period. When combined with paleoecological species abundance and composition data, these paleoenvironmental data will allow me to test whether species lived in environmental regimes during the last interglacial period that are quite different from conditions they experience today, or whether species have tracked their environmental niches from the last interglacial period to the present day.

emily-collage

Left: Emily sampling fossil C. biplicata (purple olive shell) from a terrace on San Nicolas Island; the majority of white shells in photo are C. biplicata specimens. Photo by Seth Finnegan. Top Right: C. biplicata modern shell (specimen in ~1 cm in length). C. biplicata are the most abundant shells on both modern and Pleistocene beaches in southern California. Bottom Right: Sectioned fossil C. biplicata shell; small holes along right side of shell are spots that are being sampled for stable isotope analysis. Photo by Sydney Minges.

San Nicolas Island is only one of my dissertation study areas. Ultimately, I hope to reconstruct the paleoenvironmental and paleoecological conditions of the last interglacial period along much of the coast of southern California. The uniqueness of this Channel Island's geology and biota will leave a lasting impression. Aside from its extensive marine terraces and rich archaeological record, San Nicolas Island also boasts ghostly caliche forests, adorable dwarfed gray foxes called "island foxes", and some of the most pristine rocky intertidal habitats in southern California. Through my work reconstructing the paleoenvironmental and paleoecological characteristics on San Nicolas Island and elsewhere in southern California, I hope to establish a pre-human baseline for how shallow marine environments respond to climate change.

Emily and Dan Muhs on a marine terrace with abundant Giant Coreopsis plants. Photo by Seth Finnegan.

Emily and Dan Muhs on a marine terrace with abundant Giant Coreopsis plants. Photo by Seth Finnegan.

Island fox on San Nicolas Island. Island foxes are dwarfed relatives of mainland California’s gray fox. Adult island foxes weigh about 4 pounds. Photo curtesy of the Island Conservancy.

Island fox on San Nicolas Island. Island foxes are dwarfed relatives of mainland California’s gray fox. Adult island foxes weigh about 4 pounds. Photo curtesy of the Island Conservancy.

Caliche forest that dates to the Last Glacial Maximum; fossil root casts- many of which are Giant Coreopsis- are visible. Caliche is a sedimentary rock made of calcium carbonate cement. Photo by Emily Orzechowski.

Caliche forest that dates to the Last Glacial Maximum; fossil root casts- many of which are Giant Coreopsis- are visible. Caliche is a sedimentary rock made of calcium carbonate cement. Photo by Emily Orzechowski.

Intertidal sea urchins living in holes bored into Eocene sandstone on San Nicolas Island.

Intertidal sea urchins living in holes bored into Eocene sandstone on San Nicolas Island.

This work is generously supported by grants from The UC Museum of Paleontology, National Sigma Xi, Berkeley Chapter of Sigma Xi, The UC Berkeley Department of Integrative Biology, The Evolving Earth Foundation, The American Philosophical Association, the American Association of Petroleum Geologists, the American Museum of Natural History, and the Geological Society of America.