First International Symposium on Carbon Cycle and bio-diversity during the Cretaceous

Waseda, January 25-29, 2000

Invited lectures

The Cenomanian-Turonian Boundary Event:
a case history about fossil and stable isotope record in western Europe

Marcos A. LAMOLDA
Facultad de Ciencias-UPV, Campus de Lejona, 48940 Lejona, Spain

It is widely accepted now that there was a relatively short-lived, but major palaeoceanographic event in the latest Cenomanian, sometimes referred to as the Cenomanian-Turonian Boundary Event (CTBE), among other things the CTBE involved (1) widespread deposition of sediments enriched in organic carbon, (2) a major positive d¹³C excursion with values rising to as much as 2.0-2.5 ‰ above background levels, and (3) a significant faunal turnover.
The Cenomanian-Turonian boundary sections at Dover, Eastbourne, southern England, and Menoyo, northern Spain are compared; Eastbourne and Menoyo sections are among thickest outcrops of the CTBE in western Europe. All of them are rhythmic throughout, and rich in macro-, micro- and nanno-fossils. Geochemical and biostratigraphic data are all related to detailed (rhythmic) lithostratigraphy, allowing correlation with the global boundary stratotype section to within 20000 years and accurate estimates of rates of changes. The CTBE was initiated by a sharp fall in sea level which produces a subplenus erosion at both English sections, followed by a gradual recovery over 100000 years to higher level in the latest Cenomanian, as shown by declining rate of accumulation of terrigenous clastics through the Plenus Marls; a similar trend is found at Menoyo.
Very detailed stable isotope curves for oxygen and carbon are presented. A very precise stable isotope stratigraphy, at Eastboune, has established the most detailed d¹³C curve available, which can be related to both lithostratigraphy and biostratigraphy. This curve consists of: a pre-excursion background, a build-up in two phases of rapid increase in d¹³C values separated by a trough in bed 4 of the Plenus Marls, a plateau phase from bed 8 of the Plenus Marls to Meads Marl 6, a recovery phase from Meads Marl 6 to Holywell Marl 3, and a new post-excursion background thereafter. This curve is well recognized at Dover and Menoyo, too.
Key macrofossil bioevents can be recognized and related to the detailed lithostratigraphy, including all NW European late Cenomanian and early Turonian ammonite zones, the incoming of inoceramid bivalves of the genus Mytiloides, and several pulse faunal events. Patterns of occurrence of coarse fraction benthic foraminifera (>250 µm) show two peaks of extinction corresponding to the two build-up phases of the carbon excursion. Benthic and planktic foraminifera and calcareous nannofossils all show declines in diversity and abundance across the CTBE. Key biomarker events for the CTB include: first occurrences of the ammonites Fagesia catinus and Mammites nodosoides, the bivalve Mytiloides, the planktic foraminifera Helvetoglobotruncana praehelvetica, H. helvetica and Marginotruncana renzi, and the calcareous nannofossils Quadrum intermedium, Q. gartneri and Eprolithus octopetalus, and last occurrences of the ammonite Sciponoceras, the planktic foraminifera Rotalipora greenhornensis and R. cushmani, the calcareous nannofossils Corolithion kennedyi, Axopodorhabdus albianus, Lithraphidites acutus, Rhagodiscus asper and Microstaurus chiastius. Planktic and benthic foraminifera, ostracods and calcareous nannofossils all show significant decrease in diversity and abundance across the CTBE, what is related to a decreasing productivity. Planktic and benthic foraminifera show significant size decrease as well at Eastbourne. The d¹³C excursion is the result from increased rates of burial of organic carbon which sequestered nutrients and caused the mentionned decline in productivity. Cyclostratigraphy allows very precise correlation of isotopic and bio-events with other sections in Europe and North America.

The Coniacian-Santonian boundary at the Olazagutía Quarry, northern Spain:
bioevents, biostratigraphy and stable isotope record

Marcos A. LAMOLDA¹, Mihaela C. MELINTE² and Christopher R.C. PAUL³
(1) Fac. Ciencias-Universidad del País Vasco, (2) Institutul Geologic al României, (3) University of Liverpool

The quarry at Olazagutia, northern Spain, exposes a thick, continuous section across the Coniacian-Santonian boundary in richly fossiliferous, almost uniform, marly-chalk sediments. The section measured and sampled extends from approximately 19 m below, to 19 m above the boundary as indicated by the first appearance of Cladoceramus undulatoplicatus (Roemer) Stable isotope samples were taken at approximately 1 m intervals; micro- and nanno-fossil samples approximately every 2 m.
For stable isotope analyses samples were dried overnight, powdered and 3 mg subsamples were used were roasted in a low pressure plasma oven for four hours to remove organic matter. Gaseous CO₂ was released by reacting the powder with 2 ml anhydrous 100 % orthophosphoric acid in a constant temperature bath at 50^oC for at least three hours. An acid fractionation factor of 1.00928 was used. Samples were calibrated using NBS 19 and are expressed as per mil (‰) variation relative to the VPDB standard. Sample reproducibilities are better than 0.1 ‰ for both oxygen and carbon.
Both nannofossils and microfossils (foraminifera) were prepared following standard semiquantitative procedures, e.g., nannofossils smear-slides were mounted with Canada balsam and analyzed with a polarizing light-microscope at 1600 X magnification. Semiquantitative analysis was performed by counting 250-300 specimens on each slide, in a longitudinal transverse. Nannofloral preservation is moderate to good. Only a few specimens showed evidence of overgrowth and/or dissolution, so specific identification was not significantly hindered. Assemblages are diverse with between 45-55 taxa.
A preliminary study of the calcareous nannofossil and planktonic foraminiferal assemblages (Lamolda et al., 1999) allows us to locate the boundary, defined by the first occurrence (FO) of Cladoceramus undulatoplicatus (Roemer), very close to bases of both the Dicarinella asymetrica and Sigalia carpatica planktonic foraminiferal biozones The successive sequence of bioevents: FO of Lithastrinus grillii Stradner, FO of Calculites ovalis-obscurus, common FO of Micula concava (Stradner) Bukry, FO of D. asymetrica (Sigal), FO of S. carpatica Salaj & Samuel, and FO of Arkhangelskiella etmophora Bukry, characterize the Coniacian-Santonian transition. In particular, at Olazagutia the FO of C. undulatoplicatus occurs between the underlying FOs of C. ovalis-obstusus, L. grillii and Lucianorhabdus ex gr. cayeuxii, and the overlying FO of the planktonic foraminifera D. asymetrica.
Carbon isotope values show a gradual and continuous decline through the sampled interval from a maximum of +3.05 ‰ (sample 2) to +2.68 ‰ (sample 45), with one or two slight fluctuations. In particular, there is a relatively large drop from 2.98 ‰ to 2.71 ‰ over four samples just below the first appearance of Cladoceramus undulatoplicatus. Above this level values increase again to 2.91 ‰ over the next four samples.
Oxygen values range from -2.26 to -2.84 ‰ over the same interval and also show a weak trend to more negative values up section. There is a significantly more negative value just below the first appearance of Cladoceramus undulatoplicatus and the most negative value of all six samples higher up, but both are of little significance.
Jenkyns et al. (1994) published long term stable isotope curves for the Upper Cretaceous of southern England and Italy. These are probably the best published curves with which to compare our results. In England carbon isotope values decline from a peak in the Mid Coniacian to a minimum near the mid Santonian, with one or two brief reversals. Similarly, in Italy carbon values decline through the same interval, but show a more marked drop adjacent to the Coniacian-Santonian boundary as inferred on macrofossils. (The boundary inferred from microfossils is significantly lower down). Jenkyns et al. (1994, fig. 11) used smooth carbon isotope curves to suggest the best correlation between Britain and Italy. This correlation shows a decline in carbon values across the C-S boundary.
Oxygen isotope curves for both England and Italy are much more ‘noisy’ and show an overall trend to slightly less negative values across the Coniacian-Santonian boundary. The fluctuations and differences in trends are probably due to oxygen isotope values being much more easily affected by diagenesis than carbon isotope values (Marshall, 1992).
Overall, carbon and oxygen stable isotopes do not show a significant excursion which can be used unequivocally to identify the Coniacian-Santonian boundary. Nevertheless, the broad trend in carbon values is consistent with results from southern England and Italy, and smoothed carbon isotopes curves can be used to aid correlation, as Jenkyns et al. (1994) have demonstrated. Used in conjunction with biostratigraphic data (Lamolda, et al. 1999), carbon isotope curves help refine global correlation.

References
Jenkyns, H.C., Gale, A.S. and Corfield, R.M. 1994. Carbon- and oxygen-isotope stratigraphy of the English Chalk and Italian Scaglia and its palaeoeclimatic significance. Geological Magazine, 131, 1-34.
Lamolda, M.A., Melinte, M.C. and Peryt, D.1999. Datos micropaleontológicos preliminares sobre el límite Coniaciense-Santoniense en Olazagutía (Navarra, España). Revista Española de Micropaleontología, 31, 337-345.
Marshall, J.D. 1992. Climatic and oceanographic isotope signals from the carbonate rock record and their preservation. Geological Magazine, 129, 143-160.

Grupo de Trabajo Español - 434