Atmospheric pCO ₂ and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremian, Lower Cretaceous, Wealden Beds, England)

Abstract

Nodular soil carbonates (calcretes) are present in overbank facies of Lower Cretaceous, non-marine Wealden Beds (Wessex Formation) of southern England. Field evidence suggests that these calcretes formed mostly under semi-arid Mediterranean-type climatic conditions. Typical calcrete fabrics, identified petrographically, include floating detrital grains, corroded grain margins and circumgranular cracks defining peds. Localized alteration of primary micrites is mainly associated with large cracks where early non-ferroan diagenetic cementation and neomorphism was focused. Diagenetic ferroan calcites occur as void fills and yield relatively light carbon-isotope and oxygen-isotope compositions ( 13 C= 15.0‰; 18 O= -6.3‰) compared to well-preserved micrite ( 13 C= -10.2‰; 18 O= -4.0‰). Precise definition of 13 C values for well-preserved micrites allow estimation of partial pressure of atmospheric CO 2 (pCO 2 ) for the early Barremian of 560 ppmV using a published diffusion-reaction model. The data suggest that atmospheric CO 2 was low during the mid-Early Cretaceous before rising to a previously defined mid-Cretaceous high. Data from calcretes in the Weald Clay highlight the need for selection of appropriate material and careful evaluation before pCO 2 calculations are attempted. The Weald Clay samples come from marshy palaeoenvironments where ingress of atmospheric CO 2 into the soil-zone was either reduced or prevented.

Cretaceous CO2 Decline and the Radiation and Diversification of Angiosperms

“…Estimates of atmospheric CO 2 change from Tajika's model, as shown in Fig. 7.3, indicate relatively lower CO 2 levels in the Hauterivian (132-127 Ma), late Cenomanian (ϳ95-93 Ma), and Campanian to Maastrichian (ϳ85-65 Ma) in agreement with independent proxy CO 2 (Kuypers, Pancost, and Sinninghe-Damste 1999;Robinson et al 2002;McElwain unpublished data) and climate (Frakes 1999) indicators for these times. Relatively higher CO 2 levels are estimated in the earliest Cretaceous (Tajika 1999).…”

“…It is noteworthy, however, that although the more recent and refined model, GEOCARB III, predicts lower concentrations in early and late Cretaceous CO 2 than does GEOCARB II, the general trends shown in both models are in good agreement and, in general, are well supported by independent proxy data (Chen et al 2001;Robinson et al 2002;Ekart et al 1999). Long-term carbon cycle models do not take into account shorter scale processes influencing the carbon cycle, such as increased organic carbon burial at oceanic anoxic events, which may decrease concentrations of atmospheric CO 2 (Kuypers, Pancost, and Sinninghe-Damste 1999), or rapid methyle hydrate release, which hypothetically would increase CO 2 (Hesselbo et al 2000;Jahren et al 2001).…”

Carbon isotopic evidence for organic matter oxidation in soils of the Old Red Sandstone (Silurian to Devonian, South Wales, UK)

“…In comparison, the plant assemblages from the latest Silurian to earliest Devonian Anglo-Welsh Basin are more abundant and diverse, with evidence of vascular plants (Edwards & Richardson 2004). It is notable that Lower Cretaceous calcretes of the Wealden Beds, UK, which were also deposited in partially waterlogged to marshy soils, have very comparable δ 13 C values (−9 to −12.5‰; Robinson et al 2002). There, Robinson et al (2012) suggested that the ingress of atmospheric CO 2 to the soils was low to negligible.…”

“…An exceptionally high value of 20000 ppmV (Royer et al 2001) might be appropriate if the soils were waterlogged when most of the carbonate precipitated. In their study of Lower Cretaceous calcretes formed in seasonally waterlogged soils, Robinson et al (2002) chose to apply an S(z) value of 10000 ppmV. However, this S(z) value was too low to allow palaeoatmospheric pCO 2 calculation from calcretes inferred to have formed in the wettest, marshy palaeoenvironments.…”

Stable Carbon and Oxygen Isotopic Composition of Mio-Pliocene Bivalve Shells and Calcareous Sediments in Nhila Anticline, Southeast Bengal Basin, Bangladesh

“…Carbon isotopic compositions of pedogenic carbonates have been shown to reflect changes in atmospheric pCO 2 levels over geologic timescale (Cerling, 1991;Robinson et al, 2002). Oxygen isotopic compositions of carbonates are much prone to alteration during diagenesis and/or post-depositional modifications (Hudson, 1977;Viezer, 1983).…”