Paper No. 4
Presentation Time: 4:30 PM-6:00 PM
TESTING PALAEO-ENVIRONMENTAL PROXIES (MG, SR, NA, d18O, d13C) IN THE TOARCIAN (LOWER JURASSIC) USING THE STOMACH CONTENTS OF A DINOSAUR MEAL OF BELEMNITES
MCARTHUR, John M.1, LENG, Melanie J.
2, DOYLE, P.
3, WILLIAMS, Terry
4, REEVES, Kevin
1 and GARCIA-SANCHEZ, Raquel
4, (1)Earth Science, UCL, Gower St, London, WC1E6BT, (2)NERC Isotope Geology Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom, (3)Earth Science, UCL, Gower Street, London, WC1E6BT, United Kingdom, (4)Mineralogy, Natural History Museum, Cromwell Rd, London, SW7 5BD, j.mcarthur@ucl.ac.uk
Accumulations of belemnites, mostly of
A. (A.) subtenuis and
A. (O.) vulgaris, in Toarcian (Early Jurassic) mudrocks of the Yorkshire coast, UK, represent the regurgitated stomach-contents of a marine reptile, and so two populations of belemnites that lived and died together in a geological instant. These populations can therefore be used to test the validity of palaeo-proxies in belemnites without the complicating effects of the passage of long periods of time.
Specimens of subtenuis contains more Mg, and Na, and less Sr, than do those of vulgaris, whilst being lighter in d13C and heavier in δ18O. Within each population, specimens have a restricted range of elemental composition but a large range in isotopic composition. The range in Mg/Ca, Na/Ca and Sr/Ca is around 10% (2 r.s.d.) or less, but values of δ13C range over 2.3 in subtenuis and 1.2 in vulgaris, whilst the range for δ18O is 1.9 in subtenuis and 1.3 for vulgaris. A bi-modality in the size of subtenuis represents sexual dimorphism and is reflected in different Mg/Ca values. Radial profiles of Mg/Ca in vulgaris are uniform from core to edge. In contrast, radial profiles of Mg/Ca in subtenuis show a strong dependence on distance along radial transects.
The inhomogeneities demonstrated here have obvious implications for palaeo-environmental interpretations derived from belemnite calcite.
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