Mihaela C. Melinte¹, Marcos A. Lamolda² and Kunio Kaiho³

¹ National Institute of Geology and Marine Geoecology, Dimitrie Onciul 23-25, 70318 Bucharest, Romania;
e-mail: melinte@geoecomar.ro
² Facultad de Ciencias-UPV, Campus de Lejona, 48940 Lejona, Spain; e-mail: gpplapam@lg.ehu.es
³Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan
e-mail: kaiho@dges.tohoku.ac.jp

Since the appearance of the paper of Alvarez et al. (1980), a remarkable number of works, focused on the Cretaceous/Tertiary boundary interval, have been published. Detailed investigations of many groups of organisms have pointed out the presence of a mass extinction around the K/T Boundary, bioevent yielded especially by the planktonic faunas and floras. This mass extinction is, in many areas, synchronous with high geochemical anomalies of Ir, Os, Ni, etc.

The huge amount of collected data represents a base for the beginning of one of the most exciting debates in the geological community. A significant question related to the K/T mass extinction is if this is a catastrophic or a gradual one and if the time of this bio-event could be accurately appreciated. Was the mass extinction produced by a single major event (the Chicxulub meteoritic impact) or by many coincident causes (such as strong tectonical movements, high volcanism, modification of the atmospheric composition or changes in the distribution pattern of palaeoclimatic zones)?

One of the most sensitive groups of organisms, which might offer some answers related to the character of the K/T mass extinction, are the calcareous nannofossils. Romein (1977) published the first calcareous nannoplankton study on the Caravaca section. Later Romein & Smit (1981) related nannoplankton and stable isotopes with the K/T boundary extinction.

The aim of this paper is to present a detailed qualitative and quantitative distribution of the calcareous nannofloras around the K/T boundary interval at the Barranco del Gredero section, Caravaca (Betic Cordillera of SE Spain), as well as to provide a palaeoenvironmental interpretation of the gathered data.

The studied sequence across the K/T boundary at Caravaca (between –25 cm and +40 cm), covers the last 8.1 k.y. of the Cretaceous and the first 29.9 k.y. of the Tertiary (Arinobu et al., 1999). The nannofloral diversity and abundance is high in the lower part of the sequence studied, then starts to decline. Following Arinobu et al.’s (1999) sedimentation rate, the first decline in nannoflora, identified in the studied samples, occurred around 4 k.y. before the K/T event. An important nannofloral shift was observed in the green transitional layer (sample –0.3/0 cm, covering around 0.15 k.y. prior to the K/T Event), but the main nannofloral decline is observed in the rusty lamina interval, included in the planktonic foraminifera Guembelitria cretacea Zone (P0 after Smit, 1977; sample 0/+0.5 cm, Kaiho & Lamolda, 1999). A recovery of the Cretaceous nannofloras, just above the fallout lamina, followed by a progressive diminishing of these taxa, was observed in the studied Tertiary sequence, a pattern which suggests that most of the Cretaceous nannofossils are reworked in the base of the Danian. Thoracosphaera spp. and Braarudosphaera bigelowii have their first bloom within the interval between +15 and +22 cm above the K/T. The "incoming" species Biantholithus sparsus, Cyclagelosphaera alta, and Neobiscutum parvulum have their first appearance within the K/T boundary interval of Caravaca before the first successive blooms of Thoracosphaera spp. and Braarudosphaera bigelowii. If the above mentioned blooms point out special environmental conditions (decreasing salinity, lack of nutrients, unstable surface water temperatures, low productivity) it means that a new nannofloral speciation took place even under these stressful conditions.

Concerning the character of the observed nannofloras, it is a Tethyan dominant one as shown by the common occurrence of the nannofossils Ceratolithoides kamptneri, C. aculeus, Cylindralithus spp., Lithraphidites quadratus, Micula murus, and M. prinsii. It is noticeable that several species more related to high latitudes are present such as Prediscosphaera stoveri, Reinhardtites levis, Kamptnerius magnificus, Eiffellithus gorkae, and Nephrolithus frequens, which may imply an interfingering of a cold oceanic water mass with the warm Tethyan ones in the studied area, around the K/T boundary event.

A comparison with the nannofloral distribution identified across the K/T boundary in the same section of Caravaca by Romein (1977), as well in the nearby Agost section (Posphical, 1995) was also realized.

References

Arinobu, T., Ishiwatari, R., Kaiho, K., and Lamolda, M.A. 1999. Spike of pyrosynthetic polycyclic aromatic hydrocarbons associated with an abrupt decrease in d ¹³C of a terrestrial biomarker at the Cretaceous-Tertiary boundary at Caravaca, Spain. Geology, 27, 723-726.

Kaiho, K. and Lamolda, M. 1999. Catastrophic extinction of planktonic foraminifera at the Cretaceous/Tertiary boundary evidenced by stable isotopes and foraminiferal abundance at Caravaca, Spain. Geology, 27, 355-358.

Pospichal, J. J. 1995. Cretaceous/Tertiary boundary calcareous nannofossils from Agost, Spain. In Flores, J., A. and Sierro, F., J. (Eds): Proceedings of the 5th INA Conference in Salamanca, 185-217.

Romein, A.J.T. 1977. Calcareous nannofossils from the Cretaceous/Tertiary boundary interval in the Barranco del Gredero (Caravaca, Prov. Murcia, SE Spain). Proceedings of the Koninklijke Nederlandse Akademie ven Wetenschappen Amsterdam, serie B, 80, 256-279.

Romein, A.J.T. and Smit, J. 1981. The Cretaceous-Tertiary boundary: calcareous nannofossils and stable isotopes. Proc. Koninklijke Nederlandse Akademie van Wetenschappen, 84, 295-314.

Smit, J., 1977. Discovery of a planktonic foraminiferal association between the Abathomphalus mayaroensis zone and the "Globigerina" eugubina zone at the Cretaceous/Tertiary boundary in the Barranco del Gredero (Caravaca, SE Spain): a preliminary report. Proc. Koninklijke Nederlandse Akademie van Wetenschappen, 80, 280-301.