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

The diagenetic and preservational history can affect the [delta]^sup 13^C of preserved plant tissues. Carbohydrates are the chemical components least resistant to diagenesis and have [delta]^sup 13^C values that arc more positive than [delta]^sup 13^C^sub plant^ whereas lignin and lipids are the most resistant to diagenesis and have ^sup 13^C-depleted [delta]^sup 13^C values compared with bulk plant (see Grocke 1998). Spiker & Hatcher (1987) indicated that fossil plants should have ^sup 12^C-enriched carbon-isotopic compositions (1-2[per thousand]) compared with the original (or modern) values because of the preferential loss of carbohydrates during diagenesis. Hence, most fossil C^sub 3^ plants should have carbon-isotope compositions of -28 to -29[per thousand]. However, most studies indicate that fossil coals and plants have [delta]^sup 13^C values between -24 and -25[per thousand], indicating enrichment in ^sup 13^C relative to a modern average of - 27[per thousand] (Grocke 1998). The reason for this is not entirely clear but it may, in part, be due to ^sup 13^C enrichment during coalification and charcoalification (Colombo et al. 1968; Jones & Chaloner 1991).

Scholle & Arthur (1980, p. 83) were the first to suggest that carbon-isotope excursions discovered in marine carbonate 'would be communicated through changes in the [delta]^sup 13^C values of atmospheric CO2 to land plants'. During the last decade a number of studies have shown that despite the great variability in the carbon-isotopic composition of modern plants and potential post-depositional effects, it is possible to observe the same patterns through time in fossil wood as those patterns seen in marine carbonate and organic matter (e.g. Hasegawa 1997; Grocke et al. 1999; Hesselbo et al. 2000, 2002, 2003; Ando et al. 2002; Becrling & Royer 2002; Heimhofer et al. 2003), thereby confirming that the ocean-atmosphere carbon cycle was coupled during much of the Mesozoic.

Early Cretaceous carbon-isotope stratigraphy

The Barly Cretaceous was a time of significant perturbations to the isotopic composition of both the shallow-oceanic and atmospheric carbon reservoirs. Strata of Aptian and younger age have been extensively studied, resulting in a now well-defined carbon-isotope stratigraphy for this time period (e.g. Scholle & Arthur 1980; Wcissert & Breheret 1991; Bralower et al. 1999; Jenkyns 1995; Menegatti et al. 1998; Weissert et al. 1998; Erba et al. 1999; Grocke et al. 1999; Jenkyns & Wilson 1999; Jahren et al. 2001; Luciani et al. 2001; Ando et al. 2002; Heimhofer et al. 2003).

However, the carbon-isotope stratigraphy of older Cretaceous sediments (Berriasian-Barremian) is less well studied. A significant positive carbon-isotope excursion occurs at about the Valanginian-Hauterivian boundary in both bulk carbonate and marine organic matter at several Tethyan and Atlantic localities (Lini et al. 1992; Fourni et al. 1994; Michalik et al. 1995; Hennig et al. 1999; van de Schootbrugge et al. 2000; Wortmann & Weissert 2000). Hauterivian carbon-isotope records generally show a broad, long-term negative trough, which is followed by a broad positive excursion that spans the Barremian (e.g. Lini et al. 1992; Fourni et al. 1994; Weissert et al. 1998; Erba et al. 1999; Hennig et al. 1999; van de Schootbrugge et al. 2000).