Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills

Abstract

Carbonate concretion bodies, representing a number of morphological types, and associated calcite fracture fills, mainly from New Zealand, have been studied both organically and inorganically. Extracted organic material is dominated by a complex polymeric dark brown highly polar fraction with a subordinate less polar and lighter coloured lipid fraction. The relative proportion of the two fractions is the principal control on the colour of fracture fill calcites. Concretions are classified mainly by reference to their carbonate stable carbon and oxygen isotope and cation composition. Typical subspherical calcitic septarian concretions, such as those in the Paleocene Moeraki and the Eocene Rotowaro Siltstones, contain carbon derived mainly by bacterial sulfate reduction in marine strata during early diagenesis. Other concretions, including a septarian calcitic type from the Northland Allochthon, have a later diagenetic origin. Siderite concretions, abundant in the non‐marine Waikato Coal Measures, are typically dominated by methanogenic carbon, whereas paramoudra‐like structures from the Taranaki Miocene have the most extreme carbon isotope compositions, probably resulting from methane formation or oxidation in fluid escape conduits. Lipids from concretion bodies and most fracture fill calcites contain significant concentrations of fatty acids. Concretion bodies dominated by bimodally distributed n‐fatty acids with strong even‐over‐odd preference, in which long chain n‐acids are of terrestrial origin, have very low hydrocarbon biomarker maturities. Concretion bodies that lack long chain n‐acids often have higher apparent biomarker maturity and prominent α‐ω diacids. Such diacids are abundant in fracture fill calcites at Rotowaro, especially where calcite infills the septaria of a siderite concretion in the non‐marine Waikato Coal Measures, and support the view that fluid transport resulted in carbonate entrapment of the fracture‐hosted acids. Diacids also occur in Northland calcite concretion bodies, but not in their septarian fracture fill. Release from kerogen into migrating pore fluid during an early organic maturation stage is suggested as a plausible origin of the diacids. Their site of entrapment may have been serendipitous, depending on the timing of concretion body and fracture fill carbonate precipitation.