The Palaeocene/Eocene isotopic excursion of organic carbon (delta C-13(org)) in the continental palaeoenvironment at Varangeville (Haute-Normandie, Paris basin)

The late Palaeocene carbon isotope excursion (C.I.E.) is often regarded as the best means of correlating marine and continental deposits. The few isot opic studies carried out in continental environments were based on pedogeni c carbonate [Koch et al., 1992], or on organic matter. Sinha [1997] took up this subject starting from the outcrops on the coast of the English Channe l at Varangeville, where marine sequences biostratigraphically constrain th e isotopic excursion. His work documents a negative delta C-13(org) excursi on value similar to - 27 parts per thousand PDB. The present work points out that it is necessary to study more complete sec tions than those studied by Sinha. A section named Phare d'Ailly has been s ampled in detail (figs. 1,2,3), In this section, for which detailed analysi s of sedimentology, palaeontology and organic matter facies establish the c ontinental nature of the palaeoenvironment, isotopic analysis of organic ma tter reveals a very negative delta C-13(org) excursion value similar to - 3 0 parts per thousand PDB. The P/E interval synthesized in figure 2 shows three main groups, the strat igraphy of which is strongly constrained between the calcareous nannofossil Zones NP8 and NP11, The marine Thanetian facies belong to Zones NP8 and NP 9. Above. the "Sparnacian" (Mont Bernon Group) is divided into 5 units refe rred to as SP. Unit SP2 is attributed to the Peckichara disermas Charozone, equivalent to Zone NP9. For unit SP4, an indirect correlation with Zone NP 10 may be deduced. The upper part of the Varangeville Formation is known fo r its nannofossil association attributed to Zone NP 11. We may conclude fro m these observations that the sparnacian sediments are synchronous with NP9 -10 Zones and that they are effectively located in the time interval of the delta C-13 excursion. The SP1 and SP2 sediments were analysed for carbonate content, grain size d istribution, clay mineralogy and total organic content (T.O.C.), using stan dard laboratory methods: (fig. 3). An optical specification of the organic matter has been obtained from semiquantitative analysis of the organic matt er facies. The survey of macro- and microfossils has supplied complementary supports for isotopic analysis (seed, wood and charcoal). The isotopic measurements were carried out on the bulk sediment and on comp lementary supports with a mass spectrometer in continuous flow delta+ (Finn igan Mat). The measurement precision is better than 0.1 parts per thousand and the reproducibility is about 0.15 parts per thousand. Washing residues provide Gasteropoda. Bivalvia, Ostracoda. Characea. as wel l as fruits and seeds, microcodiums and otolith. All the palaeontological d ata illustrate a continental biota of a pure lacustrine environment. quiet and shallow. in a hot and humid climate. The organic matter facies analysis confirms the absence of any marine influence and documents a palaeoenviron ment of lakes and ponds having an anoxic floor. Thus the carbon of the orga nic matter is considered to be continental and their isotopic variations li nked to those of the atmospheric carbon. In the Phare d'Ailly section. delta C-13(org) values ranges between - 24 pa rts per thousand and - 30 parts per thousand (fig. 5. With the exception of its upper part L-1, the SP2 unit exhibits delta C-13(org) values between - 24.5 parts per thousand and - 26 parts per thousand. The lacustrine limest ones at the base of unit SP2 show delta C-13(org) values fluctuating around - 24.6 parts per thousand. Similarly, the palaeosol presents very homogene ous delta C-13(org) values but slightly more negative (- 25.5 parts per tho usand). These values are compatible with C-3 photosynthetic flora. The lignite L-1 is characterized by a prominent negative isotopic excursion with apparently double structure (fig. 6). A low amplitude (about 2 parts per thousand) first excursion (A(1) : - 27.3 parts per thousand) is evident in the lower part (L-1a) between the top of the palaeosol (A(0) : - 24.9 p arts per thousand) and the middle part (L-1b) Of this complex (A(2) : - 25. 3 parts per thousand). However the palaeontological data shows an environme ntal change in L-1b that can explain the less negative values A(2). In cons equence, the A(1) excursion is probably an artefact due to a short environm ental modification. The second excursion (of about 4 parts per thousand) oc cupies the upper part (L-1c) and shows of a progressive decrease of the iso topic values from A(2) to a minimum (A(3) : - 29.7 parts per thousand) and then an increase to less negative values (A(4) : - 25.4 parts per thousand) in the uppermost lignite layer. The negative delta C-13(org) Shift observed in the Phare d'Ailly section is in part the same as that seen by Sinha in 1997 (fig. 6). However. the isot opic negative CA(3) value (- 26.89 parts per thousand), recorded in the org anic matter of an Unio shell, is only the beginning of the very negative va lues. In the Phare d'Ailly section the negative A(3) excursion and the foll owing increase to A(4) are completely recorded. We demonstrate that the delta C-13(org) excursion value is about - 30 parts per thousand PDB and the shift amplitude is: similar to - 4 parts per thou sand. Considering its well constrained chrono-stratigraphic position in bio chron NP9, we may assume that the negative A(3) excursion of the Phare d'Ai lly section is an equivalent of the negative delta C-13 excursion widely-kn own in the marine realm and terrestrial carbonates: in the late Palaeocene- early Eocene transition. The negative excursion A prevails over the A(1) dr op CA(1) artefact in the composite section and confronts us with the proble m of previously established correlations. The Phare d'Ailly section is marked by a continental evolution from a fluvi al environment to a lacustrine one, which evolves finally in a vast peat bo g. So it is right to consider that the carbon of the organic matter is terr estrial and that the isotopic fluctuations are related to those of the atmo spheric carbon except for the A(2) excursion. Evidenced in a well-dated. well-exposed and permanently outcropping sequenc e, the isotopic excursion of the Phare d'Ailly section may be considered as the regional reference of one of the major events which typified the Palae ocene/Eocene transition.