Paper No. 307-6
Presentation Time: 9:15 AM
USING IN SITU GEOCHEMISTRY TO INVESTIGATE THE DEPTH HABITAT OF AMMONITES
LINZMEIER, Benjamin J.1, LANDMAN, Neil H.2, SESSA, Jocelyn A.3, PETERS, Shanan E.4, ORLAND, Ian J.5, KITAJIMA, Kouki5, KOZDON, Reinhard6 and VALLEY, John W.5, (1)Department of Earth and Planetary Sciences, Northwestern University, Technological Institute, 2145 Sheridan Road, Evanston, IL 60208; Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, (2)Division of Paleontology (Invertebrates), American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, (3)Department of Invertebrate Paleontology, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA 19103, (4)Department of Geoscience, University of Wisconsin–Madison, 1215 W. Dayton St, Madison, WI 53706, (5)WiscSIMS, Department of Geoscience, University of Wisconsin–Madison, 1215 W Dayton Street, Madison, WI 53706, (6)Lamont-Doherty Earth Observatory, Columbia University, 209 Geoscience, 61 Route 9W - PO Box 1000, Palisades, NY 10964, blinzmeier@wisc.edu
Ammonites are the most iconic of extinct mollusks. Their wide spatial distribution and biostratigraphic utility make them ideally suited to investigate past climate. To date, stable isotope analyses of ammonites have focused on adult shells that can be sampled by bulk methods and have suggested adults had a nektobenthic mode of life. Eggs and hatchlings are thought to occupy a shallower position in the water column. Stratigraphic occurrence and analogy to modern cephalopods supports this hypothesis, but stable isotope analysis has not previously been applied to this question, primarily because the volume of material necessary for conventional techniques precludes sampling the diminutive embryonic shell (~700 µm diameter). Here we test for the depth that ammonite eggs and hatchlings inhabited by using secondary ion mass spectrometry (SIMS) for
in situ sampling of the shell wall compared to environmental context.
Materials from the Maastrichtian Fox Hills and Owl Creek Formations of North America were sampled. Samples were assessed for diagenesis using backscattered electron and cathodoluminescence imaging. Original nacreous aragonite microstructure is visible. Electron backscatter diffraction confirmed aragonite mineralogy. Fossils from the Fox Hills Formation were from a single concretion. Samples from the Owl Creek Formation spanned several meters of section. Published bulk measurements from the location show no stratigraphic trend in δ18O, but ~1‰ range. Surface and bottom water conditions inferred from δ18O analysis of coocurring organisms.
Hatchling ammonite δ18O is variable within and between individuals at both sites. Shell wall precipitated before hatching in the Fox Hills Hoploscaphites suggest a benthic development (~-2‰ change at hatching) and carbon isotope data suggest influence of terrestrial runoff or the egg yolk carbon source at hatching (~-5‰ change at hatching). These pre- to post-hatching differences suggest the relationship between egg size and development depth amongst modern coleoids may not apply to ammonites. Owl Creek material shows no change in δ18O between varix and post-hatching shell wall, but a wide range of δ18O especially in later whorls (2‰ in whorls 4 or more). Our data demonstrate the utility of in situ geochemical methods to better understand life history of mollusks.
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