TY - JOUR
T1 - Large isotopic variability at the micron-scale in ‘Shuram’ excursion carbonates from South Australia
AU - Husson, Jon M.
AU - Linzmeier, Benjamin J.
AU - Kitajima, Kouki
AU - Ishida, Akizumi
AU - Maloof, Adam C.
AU - Schoene, Blair
AU - Peters, Shanan E.
AU - Valley, John W.
N1 - Funding Information:
Fieldwork and sample collection were supported by NSF grant EAR-1121034 to ACM and BS. Lab work was partially supported by NSF grant ICER-1440312 to SEP and the Department of Geoscience at the University of Wisconsin-Madison . WiscSIMS is supported by the NSF grants EAR-1355590 and EAR-1658823 and the University of Wisconsin-Madison . KK and AI were supported by the National Aeronautics and Space Administration under Grant NNA13AA94A issued through the Science Mission Directorate. JWV was supported by U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under award number DE-FG02-93ER14389 . We thank Brian Hess for sample preparation and John Fournelle and Bil Schneider for SEM imaging and EPMA. Mike Spicuzza calibrated calcite and dolomite standards by acid digestion and GSMS. We thank Clara Blättler and Anne-Sofie Ahm, who provided comments on a pre-draft of this manuscript. We thank 4 anonymous reviewers and Editor Lou Derry for their help and comments in making this work a better manuscript. We are grateful to the owners of Beltana Station for access to their land during fieldwork in South Australia.
Publisher Copyright:
© 2020 Elsevier B.V.
PY - 2020/5/15
Y1 - 2020/5/15
N2 - Ediacaran-aged (635–541 million years ago) marine sediments contain a large negative carbon isotope (δ13C) excursion, in which carbonate δ13C values reach −12‰ (VPDB). Known as the ‘Shuram’ excursion, many workers have interpreted this δ13C record as an unprecedented perturbation to the global carbon cycle, leading to speculation about a causal connection to the broadly contemporaneous rise of animal life. Others have interpreted the δ13C signal as a product of diagenesis, thereby minimizing its relevance for understanding the evolution of metazoans. Here, we present SEM imaging and in-situ δ13C and δ18O values measured by secondary ion mass spectrometry (SIMS) to assess these competing hypotheses in the Wonoka Formation of South Australia. Our results from the minimum of the excursion show that rounded sedimentary grains of calcite have δ13C values between −12.8 to −10.6‰ and δ18O values between −17.8 to −15.5‰ (VPDB). Euhedral dolomite that appears to have grown unimpeded in open sedimentary pore spaces also is present. These early-stage dolomites are interpreted as early authigenic in origin and have δ13C values that reach +5‰, requiring a formation fluid with a substantially different δ13C composition from basin waters or bulk sediment. Together, these results provide little evidence for the hypothesis that a late diagenetic overprint has generated the ‘Shuram’ excursion in the Wonoka. Instead, they suggest the presence of a large carbon isotopic gradient in the surface environment, with shallow waters capable of precipitating carbonates with very low δ13C (down to −12‰) and deeper shelf and/or marine pore waters generating carbonates with positive carbon isotope values (up to +5‰). Because negative isotope excursions of similar magnitude are found in widely dispersed Ediacaran basins, it is likely that this gradient was characteristic of shelf environments of this period and that a still-unknown global process led to the ‘Shuram’ excursion in shallow water carbonates.
AB - Ediacaran-aged (635–541 million years ago) marine sediments contain a large negative carbon isotope (δ13C) excursion, in which carbonate δ13C values reach −12‰ (VPDB). Known as the ‘Shuram’ excursion, many workers have interpreted this δ13C record as an unprecedented perturbation to the global carbon cycle, leading to speculation about a causal connection to the broadly contemporaneous rise of animal life. Others have interpreted the δ13C signal as a product of diagenesis, thereby minimizing its relevance for understanding the evolution of metazoans. Here, we present SEM imaging and in-situ δ13C and δ18O values measured by secondary ion mass spectrometry (SIMS) to assess these competing hypotheses in the Wonoka Formation of South Australia. Our results from the minimum of the excursion show that rounded sedimentary grains of calcite have δ13C values between −12.8 to −10.6‰ and δ18O values between −17.8 to −15.5‰ (VPDB). Euhedral dolomite that appears to have grown unimpeded in open sedimentary pore spaces also is present. These early-stage dolomites are interpreted as early authigenic in origin and have δ13C values that reach +5‰, requiring a formation fluid with a substantially different δ13C composition from basin waters or bulk sediment. Together, these results provide little evidence for the hypothesis that a late diagenetic overprint has generated the ‘Shuram’ excursion in the Wonoka. Instead, they suggest the presence of a large carbon isotopic gradient in the surface environment, with shallow waters capable of precipitating carbonates with very low δ13C (down to −12‰) and deeper shelf and/or marine pore waters generating carbonates with positive carbon isotope values (up to +5‰). Because negative isotope excursions of similar magnitude are found in widely dispersed Ediacaran basins, it is likely that this gradient was characteristic of shelf environments of this period and that a still-unknown global process led to the ‘Shuram’ excursion in shallow water carbonates.
KW - Ediacaran Earth history
KW - SIMS
KW - Shuram excursion
KW - carbon isotopes
UR - http://www.scopus.com/inward/record.url?scp=85081645314&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85081645314&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2020.116211
DO - 10.1016/j.epsl.2020.116211
M3 - Article
AN - SCOPUS:85081645314
SN - 0012-821X
VL - 538
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
M1 - 116211
ER -