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Jurassic Forum » JURASSIC references & paper reviews » Jurassic around the world » Brazier et al., 2015 (Calcium isotope evidence for dramatic weathering increase)
Brazier et al., 2015
mhornДата: Wednesday, 18.02.2015, 17:20 | Сообщение # 1
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Brazier, J. M., Suan, G., Tacail, T., Simon, L., Martin, J. E., Mattioli, E., & Balter, V. (2015). Calcium isotope evidence for dramatic increase of continental weathering during the Toarcian oceanic anoxic event (Early Jurassic). Earth and Planetary Science Letters, Vol.411, P.164-176.

http://perso.ens-lyon.fr/vincent.balter/Articles/Brazier(EPSL)15.pdf

The early Toarcian was punctuated by pulses of massive carbon injection that are thought to have triggered, through increased greenhouse conditions, elevated continental discharge and nutrient input, marine anoxia, seawater acidification and species extinctions. Nevertheless, the mode and tempo of changes in continental weathering across this interval remains highly debated, leading to considerable uncertainty about the main causes of these perturbations. In this study we present calcium isotope measurements (δ44/40Ca) of well-preserved brachiopods and bulk rock samples from the hemipelagic strata of Pliensbachian–Toarcian age of Peniche in Portugal in order to constrain changes in the calcium cycle and hence changes in continental weathering during the early Toarcian. The data reveal a similar trend as carbon isotope data from the same section and show negative excursions of about 0.5 at the Pliensbachian–Toarcian transition (Pl–To) and at the base of the Toarcian Oceanic Anoxic Event (T–OAE) interval. The comparison of δ44/40Ca ratios recorded in brachiopods and bulk rock corrected for variable dolomite contribution indicates that these excursions reflect changes in the global isotopic composition of seawater rather than changes in the dominant mineralogy of calcifying organisms or in hydrological budget of the considered basin. Box modeling results suggest that the Pl–To and T–OAE δ44/40Ca excursions can be explained by a transient 90% decrease of carbonate accumulation due to seawater acidification followed by a 500% increase in continental weathering rates. The sharp increases in continental weathering inferred from the δ44/40Ca ratios seem overall consistent with lower Toarcian sedimentological and biotic records that document rapid crises in carbonate production followed by episodes of increased calcium carbonate burial. Nevertheless, the maximum of carbonate burial recorded by most NW European basinal successions occurs several hundreds of kyrs after that predicted by box modeling results. This mismatch either implies that the European records of carbonate accumulation do not reflect global trends or that the fundamental processes related to the removal of excess alkalinity caused by increased continental weathering are more complex than previously appreciated. Based on the amount of Ca input simulated by box modeling, the injection of tens of thousands of gigatons of carbon with an isotopic composition (δ13C) comprised between −6 and −14 appears as the most likely causes of the δ13C excursions characterizing these two events. These results indicate that environmental and biotic changes of the Pl–To and T–OAE were mainly caused a cascade of environmental changes triggered by the massive carbon emissions from the Karoo–Ferrar volcanism


Middle Jurassic - Lower Cretaceous ammonites & aptychi
 
Jurassic Forum » JURASSIC references & paper reviews » Jurassic around the world » Brazier et al., 2015 (Calcium isotope evidence for dramatic weathering increase)
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