INTEGRATED ANALYSIS OF SHELL GROWTH, MICRO-STRUCTURE, AND OXYGEN ISOTOPE COMPOSITION IN CAMBRIAN LINGULID BRACHIOPOD SHELLS FROM THE UPPER MISSISSIPPI VALLEY, WISCONSIN

For much of the Mesozoic and Cenozoic record, well-preserved fossils provide critical constraints on both local paleotemperatures and on aspects of atmospheric/oceanic circulation. Studies extending further into geological time require fossils of exceptional preservational quality, with candidates required to pass stringent tests scanning for possible diagenetic alteration. Here we report the analysis of some extraordinarily well preserved, color-banded fossilized shells from two lingulid brachiopod species, Lingulella ampla (Owen, 1852) and Dicellomus politus (Hall, 1861), from the Eau Claire Formation of the Upper Mississippi Valley (WI). Interest in these fossils as paleothermometers is sparked by recent reports of icy shoreline conditions in local rocks some 5 my younger in broadly similar facies.

Analysis of shell microstructure in L. ampla reveals banding occurring at two scales; broad scale banding varying from ~30 to ~120 mm apparently represents abrupt disruptions of the deposition of shell material, indicating that the growth of these species was not continuous and finer scale banding at ~7 mm apparently records more cyclic growth, and may represent a vestige of the original organo-phosphatic lamellar structure. Investigations using XRD, CL, and SEM-EDX reveal that original shell material (apatite) and original shell structures (i.e. laminae and cell polygons) are preserved and that the broader scale laminae correlate with external growth bands visible on the valve surface.

Interestingly, preliminary isotopic analyses of the δ¹⁸O_p on two L. ampla specimens indicate very little variation in isotopic composition along the axis of growth (~17.20‰). As diagenetic process appear unlikely to have been responsible for this homogenized signal the lack of isotopic variability appears to suggest that these brachiopods deposited their shells with a remarkably consistent range of δ¹⁸O values. This might either reflect the influence of vital effects, in which the brachiopods actively controlled their δ¹⁸O value, or that during intervals of shell manufacture local seawater conditions were remarkably stable. Thus, if the δ¹⁸O values accurately represent local conditions they suggest that temperatures were remarkably constant during the growing periods represented by the shell material.