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Publication Detail
Testing palaeo-environmental proxies in Jurassic belemnites: Mg/Ca, Sr/Ca, Na/Ca, δ18O and δ13C
  • Publication Type:
    Journal article
  • Publication Sub Type:
    Article
  • Authors:
    McArthur JM, Doyle P, Leng MJ, Reeves K, Williams CT, Garcia-Sanchez R, Howarth RJ
  • Publisher:
    Elsevier
  • Publication date:
    2007
  • Pagination:
    464, 480
  • Journal:
    Palaeogeography, Palaeoclimatology, Palaeoecology
  • Volume:
    252
  • Print ISSN:
    0031-0182
  • Keywords:
    Palaeo-oceanography, belemnites, Toarcian
  • Addresses:
    Earth SCience, UCL
Abstract
We test the hypothesis that the noise in belemnite records through time results from changing climate.We do so by comparing palaeoproxies (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 fromthe stomach of amarine 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 lowerMg/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 palaeotemperature.
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