Graduation Year

2016

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Geology

Major Professor

Peter J. Harries, Ph.D.

Co-Major Professor

Jonathan G. Wynn, Ph.D.

Committee Member

Philip E. Van Beynen, Ph.D.

Keywords

bivalves, cretaceous, freshwater, monsoon, stable isotopes, unionids

Abstract

Global warming in response to increasing levels of atmospheric CO2 concentration (pCO2) has generated concern over the effects of increasing surface temperature on the hydrologic cycle. Investigating precipitation dynamics during past ‘greenhouse’ intervals provide important insights necessary to better constrain potential future climate scenarios. The Late Cretaceous greenhouse is characterized by elevated pCO2 and surface temperatures, with a prolonged cooling trend which initiated in the late Campanian and an associated 4th-order sea-level regression recorded in the Western Interior Seaway (WIS), providing an opportunity to examine the hydrologic cycle under conditions of changing temperature and sea-level. This study uses a sclerochronologic approach to examine δ18O and δ13C values in freshwater bivalves collected from two horizons separated by ~800 ka, to reconstruct the late Maastrichtian hydrology at a locality along the western shoreline of the WIS. To ensure the presence of primary calcium carbonate, valves were examined using Scanning Electron Microscopy (SEM). Bivalve δ18O and δ13C values reflecting coastal river compositions range from -10.5 to -2.8‰ and -8.2 to 5.6‰, respectively. A positive correlation between δ18O and δ13C values found in specimens lower in the section, reveals that the lowest δ18O values occurred during times of peak summer soil respiration, whereas the highest δ13C values of 5.6‰ record a marine influence, supporting rainout during a summer monsoon as the cause for the lowest δ18O values recorded in this group. The valves collected higher in the section have an alternating correlation between δ18O and δ13C and plot closer to high elevation precipitation values on a mixing diagram. The loss of the summer monsoon between the two unionid groups is likely in response to decreasing surface temperatures and the retreat of the seaway, providing insight into the potential for increased intensity of modern monsoons in response to increasing surface temperatures and sea-level rise.

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Geology Commons

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