Closed‐system marine burial diagenesis: isotopic data from the Austin Chalk and its components

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Abstract
The carbon and oxygen isotopic composition of the Austin Chalk was examined in cores representing a range of depths from surface to 3000 m in order to document the effects of burial diagenesis on carbon and oxygen isotopic composition. Low magnesium calcite oysters were separated (from 500 um wide areas) and analysed to estimate the starting composition of Cretaceous marine sediment. These gave an average value of ‐2·5%δ18O; + 2·0%δ13C (PDB). The compositions of micrite, intergranular cement, and fracture cement were analysed, and their deviation from this original marine composition was evaluated to document the progression of chalk diagenesis. Interestingly, micrite exhibits only minor variation in composition from marine values despite present burial depth ranges in excess of 3000 m. The average deviation from δ18O marine is less than 1·5. Furthermore, intergranular cement and particularly fracture cements, which occur only in the deepest cores and which clearly post‐date micrite lithification, are generally indistinguishable from micrite in composition. Isotopic compositions exhibit no correlation with depth of burial despite abundant petrographic evidence of deep burial diagenesis. This uniformity in composition is interpreted as reflecting a closed, rock‐dominated diagenetic system in which the compositions of precipitated carbonate cements were controlled by the composition of dissolving carbonates during lithification. As such, the composition of burial cement is not representative of the rock‐water temperatures during precipitation.Thus, in the context of isotopic analyses from other carbonate systems, unless the degree of openness of the diagenetic system is known, oxygen isotopic signatures of cements cannot directly be converted to the rock‐water temperatures at which they were precipitated unless the composition of the ambient porefluid is also known.