Stable oxygen and carbon isotope compositional fields for skeletal and diagenetic components in New Zealand Cenozoic nontropical carbonate sediments and limestones: A synthesis and review

Abstract

The stable oxygen isotope composition (δ¹⁸O) of a precipitated carbonate depends mainly on the isotope composition, salinity, and temperature of the host fluid, whereas the stable carbon isotope composition (δ¹³C) reflects the source of CO₂ for precipitation, such as meteoric or sea water, shell dissolution, or various biochemical origins, including microbial oxidation of organic matter and methane. Despite the potentially complex array of controls, natural waters tend to show a characteristic range of isotope values which in turn are mimicked or tracked by the carbonate minerals precipitated from them. Consequently, plots of δ¹⁸O versus δ¹³C for carbonate materials can help identify their depositional and/or diagenetic environment(s). Here we compile isotope results for about 800 samples of carbonate skeletons, bulk sediments, fossils, limestones, cements, concretions, and veins spanning a range of ages (latest Cretaceous to modern) and locations in temperate‐region New Zealand. Despite some overlap, the isotope values for 21 defined categories of carbonate materials tend to group in discrete regions of the δ¹⁸O‐δ¹³C diagram, thereby providing important insights about their origin. New Zealand modern skeletal shelf carbonates plot in a distinctly different field from their tropical counterparts because of their heavier δ¹⁸O and narrower range of δ¹³C values, in accord with their nontropical bryomol skeletal facies. Cements in New Zealand temperate limestones are predominantly of burial or, more rarely, meteoric origin, but their nontropical heritage based solely on isotope composition becomes blurred and eventually lost as diagenesis proceeds. In common with many other global examples, siderite, calcite, and dolomite concretions have developed during shallow burial in a range of freshwater to marine depositional settings, the carbon originating mainly from early diagenetic, bacterially mediated reactions involving the decomposition of organic matter in bottom sediments. A summary δ¹⁸O‐δ¹³C diagram showing mean and standard‐deviation isotope values for the various New Zealand carbonate categories will form a basis for comparing and interpreting other carbonate materials and deposits.

Keywords:

• oxygen isotopes
• carbon isotopes
• New Zealand
• Cenozoic
• nontropical carbonates
• limestones
• skeletons
• fossils
• cements
• concretions
• calcite
• siderite
• dolomite