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The oceanic anoxic event of the Cenomanien-Turonien boundary

The oceanic anoxic event of the Cenomanien-Turonien boundary. (Furlo, northern Apennines). Olivier Jacquat Under the direction of Prof. Föllmi and Dr. Adatte. Institute of geology, Neuchâtel. What we will see today :. Overview of the C/T boundary Travel to Furlo (Italy)

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The oceanic anoxic event of the Cenomanien-Turonien boundary

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  1. The oceanic anoxic event of the Cenomanien-Turonienboundary (Furlo, northern Apennines) Olivier Jacquat Under the direction of Prof. Föllmi and Dr. Adatte Institute of geology, Neuchâtel

  2. What we will see today : • Overview of the C/T boundary • Travel to Furlo (Italy) • Biozonation with foraminifera • X-ray analysis on rock and clay • Isotope vs TOC in a modelling • Phosphorus • Conclusions

  3. CENOMANIAN-TURONIAN ANOXIC EVENT (OAEII) The most extensive of the Mid Cretaceous Bio-events (93,5 Mioa). Coincides with highest sea-level of the Phanerozoic (Transgression). Coincides with highest global warming of the last 115 Ma. Rapid increase in accumulation OM rates where euxinic conditions reaching the photic zone resulted in excellent preservation of OM This burial led to 40-80% reduction in PCO2 levels (below 500ppmv) Large positive global carbon-isotope excursion in both carbonate and organic matter, caused by a major perturbation of the carbon budget due to extensive burial of OM in black shales. One of the biggest source rocks of the world (e.g. Venezuela, Brazil). Modest extinction event (7% of families, 26 % of genera. (From Kaufman & Hart, 1995)

  4. Preservation model (Demaison, Shimkus et al.) e.g. Black sea Productivity model(Caron, Kühnt et al.) e.g. upwelling zone

  5. “ enhanced productivity or/and better preservation?” Kuhnt et al., 1990 • enhanced OM accumulation • in equatorial zones, high TOC • relatively high sed. rate • Slope environments (Cassis), low TOC • high sed. rate • Continental margin basin and deep sea, • very high TOC values, but extremely • low sed. rate. (Italy, Spain) Kuhnt et al., 1990 TOC accumulation rates in Grams per square meter and year

  6. Main features - Second order mass extinction • Increased CO2 • Widespread anoxia -Reduced Sr isotopes values -∂13C positive excursion -Global warming, but possible short cooling -Huge sea-level transgression • Increase in oceanic crustal production • coincident with those perturbations due to • large mantle plume related volcanic provinces • over a period of 2-3Ma. Enriched trace-elements Hallam and Wignall, 2000

  7. CTB OAE II Possible scenario • Disruption of the biosphere at CTB due to • oceanic crust production (trace metal release), • sea level transgression and disruption of • circulation patterns. • Warming, increase in CO2, runaway • greenhouse climate, increase in • productivity contributing to anoxia and • mass extinction. - Greenhouse was put in reverse by the increased productivity, removal of C from the atmospheric reservoir and buried as organic carbon in black shales Estimation of the productivity at CTB ??? Hallam and Wignall, 2000

  8. Late Cenomanien paleo-environements : Modified from Dercourt et al., 1993 Furlo: Paleo-depth: 1000m, pelagic conditions, sluggish circulation, reduced sedimentation rate during CTB event (0.20 [cm/Ky])

  9. Geographic situation of Furlo :

  10. Furlo section :

  11. Biofacies of the CTB at Furlo : Dominance of mudstones with planktic foraminifera of “K” type; specialized, complex and large forms (e.g. Dicarinella),with scarce radiolarian inputs Dominance of radiolarian sands and laminated OM, diatoms, planktic foraminifera of “r” type; small opportunistic and low O2 tolerant forms (e.g.Heterohelix) Dominance of mudstones with planktic foraminifera of “K” type; specialized, complex and large forms (e.g. R. cushmani),with scarce radiolarian inputs

  12. Biostratigraphy : H.helvetica TURONIAN No carbonates 200 μm ? W.archeocretacea 200 μm Premature disappearance of R.cushmani due to dissolution ? Where is the boundary ? CENOMANIAN R.cushmani Isotope analysis help us ! 200 μm

  13. Chemo-stratigraphy of the CTB : Disappearance of R. cushmani (well before the first ∂13C peak) is an artifact probably due to intensive dissolution process. Satisfactory correlation with the well dated Pueblo section Timing of the CTB event can be constrained using ∂13Corg peaks

  14. Mineralogy : bulk rock : a) Bonarelli depleted in carbonates b) Quartz and Opal increase.

  15. Mineralogy : clay (fraction 2μ) :

  16. Nature and amount of organic matter at Furlo : Marine Terrestria/ altered well preserved OM, of marine origin, in the entire B.E Higher OM coincides with ∂13C maxima in the upper part of the Bonarelli level

  17. Productivity vs Preservation : Mass Accumulation rate : MAR [g/m2/ky] TOC [%] MAR TOC [g/m2/ky] TOC : 2-20% MAR TOC : 0.1-0.25 [g/m2/y] Very low MAR TOC due to low sed. rate Significant decrease in TOC mass accumulation rate in the top of the Bonarelli Event (artifact due to an overestimate time of the interval CTB-FAD of H. helvetica??)

  18. Productivity vs Preservation Modelling

  19. Productivity versus Preservation : Stella modelling results C) B) A) A)3-4 x increase of productivity B)High TOC for the same productivity; better preservation due to increased anoxia, higher HI C)Decreased MAR TOC for the same productivity, but higher HI (Decreased PCO2 ?,miscalculation of the timing: Tragic use of helvetica FAD ????)

  20. Phosphorus : P [mg/g] MAR [g/m2/ky] Bound to OM >Preservation Bound to skeletal matter >Productivity Phosphorus speciation !

  21. Remarks : • Model based on present data (sarmiento et al.) • A more precise knowledge of CO2 interactions is required • Model = simplification of the environment • Quantitative approach, test of qualitative.

  22. CONCLUSIONS Italian deep-sea sections can be chemostratigraphically correlated when usual biostratigraphic markers are lacking ! These sections are condensed but rather complete. Isotopic and MAR TOC fluctuations can be resolved by the modelling if ages are well constrained (do not use FAD of H. helvetica!). Increased productivity cannot be the only factor for high MAR TOC, preservation is also important. Depending on local conditions, additional factors may have to be carefully examined. Therefore, productivity modelling must be applied in every kind of local setting. Other productivity proxies such as Phosphorus and Ba must be used for a better understanding of the CTB events.

  23. Thank you ! Bibliography : -Caron M., Robaszynski F., Amédro F., Baudin F., Decochnik J.-F., Hochuli P., Von Salis-Perch Nielsen K. & Tribovillard N., (1999) Estimation de la durée de l'événement anoxique global au passage Cénomanien/Turonien. Approche cyclostratigraphique dans la Formation Bahloul en Tunisie centrale. Bull. Soc. géol. France, 170, 2, 145-160 -Demaison G. J. and GT. Moore (1980) Anoxic environments and oil source bed genesis,Organic Geochemistry, 2, Issue 1, 9-31. -Kuhnt W. and Wiedmann J. (1995) Cenomanian-Turonian source rocks: Palaeobiogeographic and paleoenvironmental aspects. In: Palaeogeography, palaeoclimatology and source rocks (Edited by A.Y. Huc), AAPG Studies in Geology 40, 213-231. -Shimkus K., Trimonis E. (1974) Modern sedimentation in Black Sea. Degens ET, Ross DA (eds) The Black Sea-geology, chemistry and biology. AAPG member, 20,249-278.

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