Paper No. 9
Presentation Time: 10:15 AM
INVESTIGATION OF GROWTH PATTERNS AND CLIMATE SIGNALS USING ISOTOPE SCLEROCHRONOLOGY IN SHELLS OF TWO PLIOCENE MARINE BIVALVES:
HUDLEY, Joel W., Department of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599 and SURGE, Donna, Geological Sciences, University of North Carolina at Chapel Hill, 104 South Road, CB #3315, Chapel Hill, NC 27599, jhudley@unc.edu
The Pliocene was the last time in the Earth’s history when reduction in polar ice-sheets and higher sea levels were a consequence of higher atmospheric carbon dioxide concentrations comparable to levels projected for the late 21
st century. In order to investigate the climate variability during the Pliocene, variations in shell growth, together with oxygen and carbon isotope ratios of the bivalve mollusks
Glycymeris americana (Glycymeridae; aragonitic shell) and
Panopea reflexa (Hiatellidae; aragonitic shell) were measured. The purpose of the study is to identify whether these species, both large and abundant in United States Atlantic coastal plain fossilerous units, have prominent, annually formed growth lines comparable to rates estimated using isotope sclerochronology. Previous work on extant species of geoduck (
P. abrupta) and dog cockles (
G. glycymeris) indicate the genera have annual growth-lines, reach maximum lifespans of 40 to 160 years, and are suitable for regional SST reconstructions.
The fossil bivalves were collected from the Moore House and Members of the upper Yorktown Formation (3.5-2.5 Ma) in Virginia. At this time, sea level was higher than present, and the fully marine sediments of the Yorktown formation yielded a well preserved molluscan assemblage. Most specimens of G. americana and P. reflexa were collected articulated and unaltered by diagenesis (revealed by using acetate peels to evaluate original petrography and microstructure). Sclerochronological studies of the growth patterns and the oxygen isotope ratios clearly exhibited annual cycles confirming periodic formation of the shell material. Several specimens of both G. americana and P. reflexa reached ontogenetic ages of more than 100 years. The bivalve shells investigated here provide high resolution data on seasonal to decadal climate, and may therefore serve as an ancient analogue for predicted climate shifts along the United States Atlantic coastal plain.
