Organic matter in Cretaceous chalks from eastern England

Abstract

Upper Cretaceous chalks from eastern England and northern France and two representative chalks of post-Cretaceous age contain organic matter in the form of humic and fulvic acids, humin and bitumen. Levels of total humic matter range from ∼ 0.01% in the white chalks (> 95% CaCO 3) to 0.1% in the clayey and phosphatic chalk facies; they reach 1% in a sample of the Black Band (marking the Cenomanian-Turonian boundary) and in one of the post-Cretaceous samples (Eocene chalk from Israel). Less than 30% of the total humic matter in the samples was amenable to direct extraction, the bulk (and all the bitumen) being obtainable only after dissolution of carbonates or of all mineral phases (as humin). This points to a diffuse distribution of organic matter in the chalks, some being intracrystalline in the calcite, some being associated with clays. H/C atomic ratios of the humic materials range from 0.6 to 1.6 and are positively correlated with N/C (range 0.02–0.09). Humic acids most susceptible to direct extraction, especially those from the phosphatic chalks, exhibit the highest H/C and N/C ratios. This trend correlates with structural information obtained using Fourier-transform infra-red and pyrolysis-mass spectrometry (PY-MS). Both show that the humic acids are predominantly aromatic structures but those with the higher H/C and N/C ratios contain an abundance of aliphatic and peptide-type moieties. Other structural elements identified by PY-MS included methoxyphenols (derived from lignins) and acetamide (derived from n-acetylaminosugars, e.g. chitin). Humic acids with the highest N/C ratios contain up to 20 wt.% of hydrolysable amino acids, although the relative abundances of different amino acids in all the samples was the same and congruent to that of humic acids and phytoplankton debris from Recent sediments. Bitumens from the chalks consisted mostly (>80%) of high-molecular-weight compounds that were not subject to further analysis. Chromatography and gas chromatography-mass spectrometry showed that the residual low-molecular-weight fractions were dominated by the presence of normal and branched alkanes ( n-isomers in the range C 12–C 23) and by n-fatty acids (especially C 14, C 16 and C 18 types). The release of methoxyphenols during pyrolysis shows that all the humic acids (particularly those with low H/C ratios) contain some terrestrially derived organic matter. This probably forms the aromatic skeleton of the humic acids. Aliphatic and peptide-type moieties, mainly in the samples with higher H/C and N/C ratios, together with the amino acid patterns, provide evidence that this skeleton is augmented by marine-derived structural units. The presence of marine, and more particularly of plankton-derived organic matter in the humic acids, is substantiated by the n-alkane and n-fatty acid data. Humic acids from the chalks exhibit a negative relation between their H/C and O/C ratios, in part a reflection of the presence of terrestrial components. However, this trend also suggests microbial oxidation of lipid and peptide material in the humic acids in the original chalk sediments. Low levels of sulphur indicate that the most extensively altered humic materials (mainly from the white chalks) were subject to oxidation in an aerobic microbial regime. High levels of sulphur in lipid/peptide-rich humic acids from the phosphatic and Israeli chalks reflect preferential preservation due to less active anaerobic microbial conditions. The presence of humic acids (rather than kerogen) and the predominance of polymeric material in the bitumens shows that all of the chalks examined are catagenically immature. This is not unexpected in view of their burial history.