Dissertation/Thesis Abstract

Investigating holocene climate change on the northern channel islands and cretaceous mosasaur ecology using stable isotopes
by Robbins, John A., Ph.D., Southern Methodist University, 2010, 255; 3438884
Abstract (Summary)

California mussels excavated from CA-SRI-147 yield δ18O values with an inverse relationship compared to the amount and size of these mussels. The long-term decline in both mussel amount and average size at the site is not found in inferred temperatures. Four cold intervals are observed. The first occurs when the mass of California mussels, brown turban snails, and black and red abalones present in the midden increases (∼6800 to 6500 cal BP). A drop in mussel mass and an increase in red abalone and turban snail occurred during the second (∼5500 cal BP). The third occurred when platform mussels became an important staple and California mussels decreased in both size and amount (∼4800 to 4400 cal BP). The fourth (∼4100 to 3700 cal BP) was followed by a hiatus in site occupation. No long-term trend in δ 13C is found, though a period of generally declining δ 13C found between ∼6200 to 3800 cal BP is observed in inferred productivity proxies from two sediment cores in the Santa Barbara Basin.

Sea surface temperature change during the Holocene inferred from all δ 18O values reported from the Northern Channel Islands reveals a variable pattern centering around ∼13.0°C. No overall trend over time is found. Several similarities between this nearshore temperature proxy and the offshore temperatures inferred from oxygen isotopes of foraminifera are found, including an increase in average marine temperatures over the last ∼1000 cal years. Two Early Holocene sites yield temperatures lower than most others. These low values fit well with those of foraminifera from the offshore sediment core AII-125 JPC-76. The modern spatial temperature gradient found in the Northern Channel Islands, where temperatures increase as one moves eastward, is not an obvious feature of the mussel dataset due to the variability in temperatures from San Miguel Island shells. Red abalone middens, argued as evidence of cool temperature periods, occur under a range of temperatures (∼3.5°C) during the interval from 6,500 to 5,500 cal BP, suggesting that temperature change alone is not responsible for these sites. Carbon isotopes are generally higher during this interval than the remainder of the Holocene, suggesting that red abalone middens may be the result of short-term increases in regional productivity.

Tooth enamel δ13C from extinct Late Cretaceous Mosasauridae generally decrease with increasing size of the individual (R2 = 0.52). This is likely the result of the decrease in primary producer δ 13C with increasing distance from shore, which propagates through the food web. Removal of the durophagous mosasaurs, which fed on lower trophic level taxa, increases this correlation (R2 = 0.63). A high correlation is found among macrophagous Mosasaurinae (R2 = 0.74), while a poor correlation is found among Plioplatecarpinae (R 2 = 0.23). Multiple species of large-bodied Tylosaurus generally fall within the body size trend (R2 = 0.35). The small-bodied Tethysaurus has low δ13C for its size (∼3 m). Depleted values from these genera and some members of Plioplatecarpinae may be the result of carbonate precipitation during prolonged or frequent diving. This is due to the build-up of CO2 in the bloodstream during long dive periods. The presence of avascular necrosis among taxa with depleted values suggests diving in nearshore waters rather than offshore surface foraging. An overall trend of decreasing δ13 C values for the Mosasauridae family during the Late Cretaceous is a feature not found in organic or inorganic δ13C records. This suggests that the decrease with time in mosasaur δ13C represents a change in behavior for the family.

Indexing (document details)
Advisor: Gregory, Robert T.
Commitee: Ferguson, Kurt M., Jacobs, Louis L., Rick, Torben C., Yapp, Crayton J.
School: Southern Methodist University
Department: Geological Sciences
School Location: United States -- Texas
Source: DAI-B 72/03, Dissertation Abstracts International
Subjects: Archaeology, Paleontology, Geochemistry
Keywords: Channel islands, Climate change, Foraging behavior, Isotope, Mosasaur, Sea surface temperature
Publication Number: 3438884
ISBN: 978-1-124-43286-1
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