Fossil-wood carbon-isotope stratigraphy of the non-marine Wealden Group (Lower Cretaceous, southern England)

Journal of the Geological Society, Jan 2004 by Robinson, Stuart A, Hesselbo, Stephen P

Abstract:Fossil-wood carbon-isotope data are presented for the Wessex Formation, a non-marine unit within the Lower Cretaceous (Valanginian-Barremian) Wealden Group of the Isle of Wight and Dorset, southern England. The carbon-isotope values have a range ([delta]^sup 13^C c. -26.6 to -19.8[per thousand]) that is consistent with that expected for Mesozoic C^sub 3^ plants. Consideration of the Isle of Wight fossil-wood carbon-isotope data together with data from Dorset allows construction of a composite fossil-wood carbon-isotope curve for almost the entire Wealden Group. The new carbon-isotope curve is in smooth continuity with data previously published from the overlying lagoonal Vectis Formation and marine Lower Greensand Group of Aptian age. A tentative correlation with a Tethyan reference carbon-isotope curve allows the provisional application of stage-level chronostratigraphy to the Wealden Group. Correlations suggest that the Wealden Group sediments are dominantly of Hauterivian and Barremian age and that the Valanginian is either condensed or partially missing. The carbon-isotope data presented indicate that even during times of relative carbon-cycle quiescence atmosphere CO2 faithfully tracks the carbon-isotopic composition of the oceanic reservoir.

Keywords: Early Cretaceous, chemostratigraphy, carbon isotopes, fossil wood, Wealden Group.

Realization that major carbon-isotopic change in the oceans is also expressed by the isotopic composition of higher plant fossils leads to the potential use of terrestrial carbon-isotope stratigraphy as a means to link the non-marine and marine records of environmental change during times of significant disturbance to the carbon cycle (e.g. Hasegawa 1997; Grocke et al. 1999; Hesselbo et al. 2000, 2002, 2003; Ando et al. 2002; Heimhofer et al. 2003). The Early Cretaceous (Berriasian-Barremian) carbon-isotope record is known from studies of Tethyan marine carbonates (e.g. Lini et al. 1992; Weissert et al. 1998) and it has been shown that there is a significant positive carbon-isotope event at about the Valanginian-Hauterivian boundary (Lini et al. 1992). The non-marine, fluvial Wealden Group of southern England contains fossil wood in local abundance and ranges in age from approximately the Berriasian-Valanginian to the Barremian, although the group lacks precise age constraints. In this paper we assess the utility of fossil-wood carbon-isotope stratigraphy in the intrabasinal correlation of the non-marine Wealden strata and the potential for extrabasinal correlation with the marine (standard) provided by Tethyan carbonates. The ability to correlate between a marine carbonate carbon-isotope curve and a fossil-wood carbon-isotope curve has important implications not only for stratigraphy but also for understanding the operation of the global carbon-cycle in the geological past and associated environmental changes on land.

Rationale and geological background

[delta]^sup 13^C of fossil wood and application as a chemostratigmphic tool

It is well known that in modern plants there is considerable variability in [delta]^sup 13^C values within a single individual, between individuals of the same species and between different species of plants. Modem terrestrial plants can be divided into three groups on the basis of their photosynthetic pathway: C^sub 3^ (Calvin-Benson cycle; includes temperate shrubs and trees), C^sub 4^ (Hatch-Slack cycle; predominantly grasses), and CAM (crassulacean acid metabolism; mainly succulents). C^sub 3^ and C^sub 4^ plants can be distinguished by their characteristic carbon-isotopic values (C^sub 3^ plants: -23 to -34[per thousand]; C^sub 4^ plants: -8 to -16[per thousand]; Bender 1971; Smith & Epstein 1971; Vogel 1993). CAM plants, however, use an intermediate pathway that can be modified in response to changing environmental conditions and hence have intermediate carbon-isotope values. Therefore, the use of fossil plants for chemostratigraphy is dependent upon the assumption that the palaeo-ecosystem did not contain a mixture of plants utilizing more than one photosynthetic pathway. Fortunately for studies of the Cretaceous, evidence from the fossil record suggests that C^sub 4^ plants did not evolve until the Miocene (Throughton et al. 1974; Ceding & Quade 1993; Ceding et al. 1993) although, according to some isotopic evidence, they may have appeared sporadically during earlier geological time (see Spicer 1989; Wright & Vanstone 1991; Kuypers et al. 1999). Mesozoic fossil plants generally have carbon-isotopic compositions consistent with a C^sub 3^ photosynthetic pathway (Bocherens et al. 1993; Grocke 1998).

Within modern C^sub 3^ plants there is variability in the carbonisotopic composition of the constituent parts of the plant. Leaves are consistently lighter than the branches to which they are attached by as much as 3[per thousand], and leaves themselves can vary by 4[per thousand] on a single tree (Leavitt & Long 1991; Schleser 1999). There is also considerable variation within twigs and branches (up to 2-3[per thousand]; Leavitt & Long 1986; Schleser 1999) and between early and late wood (typically 1-2[per thousand]; Leavitt & Long 1982; Loader et al. 1995). To add further complication, the [delta]^sup 13^C of plant material is affected by variations in temperature, salinity, water supply and local pCO2 (e.g. the canopy effect and altitude) (see Grocke (1998) for a review). From a chcmostratigraphic point of view analyses should clearly be restricted to consistent plant organs through a stratigraphic sequence, and only large reproducible patterns can be indicative of long-term shifts in atmospheric carbon-isotopic compositions.