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| Titel |
Calcium isotope evidence for pulses of increased continental weathering during the early Toarcian (Early Jurassic) |
| VerfasserIn |
Guillaume Suan, Jean-Michel Brazier, Vincent Balter, Laurent Simon, Emanuela Mattioli |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095950
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| Publikation (Nr.) |
 EGU/EGU2014-11428.pdf |
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| Zusammenfassung |
| The Toarcian interval is punctuated by a number of episodes of environmental changes and
mass extinctions that are considered as some of the most severe of the Mesozoic
era. Significantly, the corresponding strata record marked negative carbon isotope
excursions that point to pulses of massive injection of isotopically light carbon to the
superficial reservoirs. Potential causes of these perturbations include gas hydrate
dissociation, wildfires, and massive inputs of thermogenic and volcanogenic carbon
related to the onset of volcanic activity of the Karoo-Ferrar province. All these
scenarii imply large increases in chemical weathering rate as key drivers of the
accompanying biotic and environmental perturbations (e.g., productivity-driven anoxia and
coastal eutrophication). Nevertheless, detailed examination of most likely cause(s) of
these events has been hampered by the uncertainty surrounding the timing and
intensity of coeval changes in continental weathering. In this study, we reconstruct
changes in continental weathering during the Toarcian using new calcium isotope
ratios δ44-42Ca of brachiopods and bulk rock sediments from the Peniche section in
Portugal. The data reveal two marked (>0.4permil) negative Ca-isotope excursions
near the Pliensbachian-Toarcian transition (Pl-To) and at the base of the levisoni
ammonite Zone recording the Early Toarcian Oceanic Anoxic Event (T-OAE).
The comparison of the brachiopod and bulk rock Ca isotope data indicates that
these excursions reflect changes in the global Ca-isotope composition of seawater
rather than changes in the dominant mineralogy of calcifying organisms. Mass
balance calculations suggest that the Ca-isotope excursions recorded across the
Pl-To transition and T-OAE interval can be explained by the Ca inputs from rivers
corresponding respectively to 90% and 34% of the initial mass of oceanic Ca. Based
on these values, the injection of tens of thousands of gigatons of carbon with a
C-isotope composition comprised between –5oand –20oappears as the most
likely cause of the C-isotope excursions characterizing these two events. These
results point to the massive injection of volcanogenic carbon by the Karoo-Ferrar
volcanism as the main cause of the Toarcian environmental and biotic changes. |
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