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Techniques for assessing spatial heterogeneity of carbonate δ 13 C values: Implications for craton‐wide isotope gradients
Author(s) -
Garrecht Metzger J.,
Fike David A.
Publication year - 2013
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/sed.12033
Subject(s) - geology , carbonate , diagenesis , excursion , isotopes of carbon , sedimentary rock , overprinting , carbon cycle , paleontology , stable isotope ratio , mineralogy , total organic carbon , chemistry , tectonics , environmental chemistry , ecology , physics , organic chemistry , quantum mechanics , ecosystem , biology , political science , law
The sedimentary record of carbonate carbon isotopes (δ 13 C carb ) provides one of the best methods for correlating marine strata and understanding the long‐term evolution of the global carbon cycle. This work focuses on the Late Ordovician Guttenberg isotopic carbon excursion, a ca 2·5‰ positive δ 13 C carb excursion that is found in strata globally. Substantial variability in the apparent magnitude and stratigraphic morphology of the Guttenberg excursion at different localities has hampered high‐resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle. This work investigates the magnitude, spatial scale and sources of isotopic variability of the Guttenberg excursion in two sections from Missouri, USA . Centimetre‐scale isotope transects revealed variations in δ 13 C carb and δ 18 O carb greater than 2‰ across individual beds. Linear δ 13 C carb to δ 18 O carb mixing lines, together with petrographic and elemental abundance data, demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components. These patterns facilitated objective sample screening to determine the ‘least‐altered’ data. A δ 18 O carb filter based on empirical δ 18 O carb values of well‐preserved carbonate mudstones allowed further sample discrimination. The resulting ‘least‐altered’ δ 13 C carb profile improves the understanding of regional as well as continental‐scale stratigraphic relations in this interval. Correlations with other Laurentian sections strongly suggest that: (i) small‐scale variability in Guttenberg excursion δ 13 C carb values may result in part from local diagenetic overprinting; (ii) peak‐Guttenberg excursion δ 13 C carb values of the Midcontinent are not distinct from their Taconic equivalents; and (iii) no primary continental‐scale spatial gradient in δ 13 C carb (for example, arising from chemically distinct ‘aquafacies’) is required during Guttenberg excursion‐time. This study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for δ 13 C carb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry.