Date of Award

August 2018

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor(s)

Linda C. Ivany

Subject Categories

Physical Sciences and Mathematics

Abstract

Use of the δ18O thermometer in deep time investigations is complicated by uncertainty in the oxygen isotopic composition of seawater and an increasing potential for diagenetic alteration with age. These concerns are particularly important when considering that δ18O values from Paleozoic marine carbonates are generally low and show a depletion trend with increasing age. Competing explanations for this trend include increasing alteration with age, evolution of seawater δ18O through time, and increasing ocean temperatures with age. Demonstrating the preservation of original mineralogy, thus eliminating diagenesis as a factor, is a primary hurdle in deconvolving potential causes. Here, we report data from serially sampled, Middle Pennsylvanian mollusks from the Appalachian Basin of Kentucky. XRD and SEM analyses indicate an aragonitic mineralogy with retention of primary microtextures, and δ18O data reveal regular cyclic variation over ontogeny, suggesting that original shell carbonate is preserved and records primary environmental (presumably seasonal) conditions over the life history of the animal. However, values are depleted, centering around -4.6‰, and intraannual variation is significant, spanning up to 2.2‰. Recent work on the Carboniferous of the U.S. suggested that negative values reflect a regional salinity trend toward more depleted values with increasing distance from Panthalassa. Our data extend the trend eastward, suggesting significant freshwater input closer to the Allegheny Front. However, the high degree of seasonality differs from published data in the basin, suggesting that mollusks record conditions in nearshore settings that receive seasonally variable contributions of isotopically depleted runoff. Although our data support the primary nature of low δ18O values from brachiopod shells, they also highlight the influence that δ18O values from epicontinental seas may have on the Paleozoic depletion trend.

Access

Open Access

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