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

Early Cretaceous carbon cycling

The ability to make geologically sound correlations between the data from the Isle of Wight and the Tcthyan reference curve supports the hypothesis that the ocean-atmosphere carbon cycle was coupled during the Early Cretaceous, as previously demonstrated by several workers (i.e. Grocke et al. 1999; Ando et al. 2002; Heimhofer et al. 2003). The trend towards more negative fossil-wood carbon-isotope values from the mid-Barremian (at the base of the Vectis Formation) onwards may be the result of increased outgassing of isotopically depleted volcanic CO2 driven by increased crustal production (Larson 1991; Larson & Erba 1999) and coinciding with the beginning of local transgression, which may well be an expression of long-term eustatic sealevel rise. There is a hint of more positive fossil-wood carbon-isotope values in the lower part of the Wealden at Worbarrow Bay (between -400 and -300 m composite depth) that may be part of the late Valanginian-early Hauterivian positive carbon-isotope excursion. If this were strengthened by further observations, then the Valanginian-Hauterivian event would indeed represent a global carbon-cycle perturbation.

An implication of the present work is that even during periods when the global carbon-cycle was relatively stable, the carbon-isotopic composition of atmosphere CO2 faithfully tracked the long-term trends of the oceans. This result, in conjunction with independent estimates of atmospheric pCO^sub 2^ (e.g. Robinson et al. 2002), is important for understanding the operation of the relatively steady-state carbon cycle during the Hauterivian-Barremian, before the massive perturbations of the mid-Cretaceous (Aptian-Turonian). This is significant for carbon-cycle modelling, as it suggests that the Tethyan carbonate-carbon isotope records are indeed representative of the entire exchangeable carbon reservoir. Some studies have suggested that it may be possible to use [delta]^sup 13^C^sub plant^ to calculate the carbon-isotopic composition of palaeo-atmospheric CO2 ([delta]^sub 13^C^sub atm^; Arens et al. 2000; Grocke 2002), an important variable for calculating pCO^sub 2^ from the [delta]^sup 13^C of soil carbonate (see Robinson et al. 2002 and references therein) and of importance to carbon-cycle models. However, there may be several problems inherent in the method of calculating [delta]^sup 13^C^sub atm^, in particular the effect of changing pCO^sub 2^ on [delta]^sup 13^C^sub plant^ (see Beerling & Royer 2002; Robinson 2002; Hesselbo et al. 2003). Until these issues are resolved, it will not be possible to calculate [delta]^sub 13^C^sub atm^ from [delta]^sup 13^C^sub plant^.

Conclusions

(1) Fossil wood samples from the non-marine Wessex Formation (Wealden Group, Lower Cretaceous, southern England) have carbon-isotope values in the range that is expected for Mesozoic C^sub 3^ plants.

(2) Carbon-isotope data from the upper Wessex Formation sections on the Isle of Wight show progressively more positive values up-section. When these data are combined with previously published data from the overlying Vectis Formation it becomes apparent that there is a broad positive excursion in [delta]^sup 13^C values, which peaks at about the Wessex Formation-Vectis Formation boundary. This broad excursion can be correlated with a similar excursion in the carbon-isotopic composition of Barremian Tethyan carbonates.