Testing palaeo-environmental proxies in Jurassic belemnites: Mg/Ca, Sr/Ca, Na/Ca, delta 18O and delta 13C

McArthur, J.M.; Doyle, P.; Leng, Melanie ORCID:; Reeves, K.; Williams, C.T.; Garcia-Sanchez, R.; Howarth, R.J.. 2007 Testing palaeo-environmental proxies in Jurassic belemnites: Mg/Ca, Sr/Ca, Na/Ca, delta 18O and delta 13C. Palaeogeography, Palaeoclimatology, Palaeoecology, 252 (3-4). 464-480.

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We test the hypothesis that the noise in belemnite records through time results from changing climate. We do so by comparing palaeo-proxies (Mg/Ca, Sr/Ca, Na/Ca, δ18O, δ13C) in two species of Toarcian belemnites, Acrocoelites (Odontobelus) vulgaris (14 specimens) and Acrocoelites (Acrocoelites) subtenuis (10 specimens), deposited in a period of time that was short relative to the timescale of climate change. The specimens are interpreted to have been regurgitated from the stomach of a marine predator and so were deposited together in a geological instant of Early Jurassic time. In A. (O.) vulgaris, isotopic and elemental palaeo-proxies group tightly, suggesting that this belemnite occupied a restricted environmental niche. In A. (A.) subtenuis, elemental and isotopic compositions spread more widely, suggesting that this species occupied a greater range of habitats. Possible sexual dimorphism in A. (O.) vulgaris is reflected in bimodal Mg/Ca and differing intra-rostral distribution of Mg in putative dimorphs; concentrations decline by 30% from apical line to rim in specimens with high Mg/Ca, but are largely invariant radially in specimens with lower Mg/Ca. Sexual dimorphism in A. (A.) subtenuis yields a bimodality in size, Mg/Ca, and intra-rostral distribution of Mg; larger specimens have a higher Mg/Ca, and Mg concentrations that increase along radial traverses from apical line to rim, whilst the smaller specimens show a mid-rostral minimum in Mg concentration. Radial profiles of concentrations for Sr and Na vary little in all forms. Toarcian belemnites of these species thus show strong biofractionation effects on Mg/Ca, and little or no observable biofractionation of Sr/Ca or Na/Ca. It follows that much of the noise in belemnite records results from biofractionation, and from a cosmopolitan life-style of some belemnite species, rather than climate-change. For Toarcian belemnites, we suggest that Sr/Ca is the best indicator of palaeo-temperature.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: BGS Programmes > NERC Isotope Geoscience Laboratory
ISSN: 0031-0182
Additional Keywords: Jurassic, Belemnites, Paleoenvironments, Toarcian, Biofractionation
NORA Subject Terms: Earth Sciences
Date made live: 16 Dec 2009 16:15 +0 (UTC)

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