New Zealand Journal of Geology and Geophysics abstracts

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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

CAMPBELL S. NELSON
ABIGAIL M. SMITH*

Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand

*Present address: Department of Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand.

Abstract  The stable oxygen isotope composition (d¹⁸O) of a precipitated carbonate depends mainly on the isotope composition, salinity, and temperature of the host fluid, whereas the stable carbon isotope composition (d¹³C) 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 d¹⁸O versus d¹³C 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 d¹⁸O-d¹³C 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 d¹⁸O and narrower range of d¹³C 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 d¹⁸O-d¹³C 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

New Zealand Journal of Geology and Geophysics, 1996, Vol. 39: 93-107

0028-8306/96/0093 $2.50/0 (c) The Royal Society of New Zealand 1996

PDF file of entire paper: medium quality (1337K); (scanned from paper original: notes about this process)

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