TY - JOUR
T1 - The source of sulfate in brachiopod calcite
T2 - insights from μ-XRF imaging and XANES spectroscopy
AU - Richardson, Jocelyn A.
AU - Newville, Matthew
AU - Lanzirotti, Antonio
AU - Webb, Samuel M.
AU - Rose, Catherine V.
AU - Catalano, Jeffrey G.
AU - Fike, David A.
N1 - Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (#57548-ND2) for partial support of this research. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393).
PY - 2019/12/20
Y1 - 2019/12/20
N2 - Geochemical signatures preserved in sedimentary carbonate strata are
often used as archives for paleoenvironmental reconstructions. However,
diagenetic overprinting and/or muting of primary geochemical signatures
complicates the interpretation of these data. To avoid this issue,
geochemical techniques strive to target unaltered (or minimally altered)
carbonate components. The multi-layer low-Mg calcite shell composition
of articulate brachiopods are often the target in such studies because
they are commonly more robust to recrystallization than the shells of
other biomineralizers. Here, we have combined S K-edge μ-XRF imaging,
XANES spectroscopy and petrography to determine the source of sulfate in
the calcite lattice (carbonate-associated sulfate; CAS) of ancient and
extant brachiopods, in order to establish their suitability for use in
reconstructing seawater sulfate δ34S throughout the Phanerozoic. Both the extant brachiopod Terebratalia transversa
and fossil brachiopods display intra-specimen variability in sulfate
abundance parallel to the primary fabric, likely corresponding to
variations in growth rate. XANES spectroscopy identifies a majority of
the sulfate as inorganic. Additionally, XANES spectroscopy detected low
abundances of both reduced and oxidized organic sulfur species (thiol,
thioether, sulfoxide and sulfate esters) in all T. transversa samples and lesser abundances in a few of the fossil brachiopods. In T. transversa, inorganic sulfate and sulfate ester abundance increase towards the hinge of the valves. Bulk δ34SCAS
of the samples containing the fossil brachiopods are consistently more
positive than time-equivalent brachiopod-only values, likely reflecting a
mixture of CAS signals from homogenization of carbonate components
differentially affected by depositional environment and diagenesis. In
contrast, bulk δ34SCAS of modern T. transversa
is approximately 1‰ more positive than coeval seawater. Although
organic sulfate esters are found within the brachiopod shells, they are
only ever present as trace components. Our findings indicate that the
vast majority of sulfate in brachiopod shells is inorganic, sourced from
coeval seawater. This result supports the use of brachiopods as a
potential archive for a faithful CAS record of seawater sulfate
throughout the Phanerozoic. The characterization of in situ
organic sulfur compounds in both extant and fossil brachiopods indicates
the potential importance of various organic sulfur compounds in
mineralogical determination (and therefore fossil preservation) and
crystal orientation during brachiopod biomineralization throughout
geologic time.
AB - Geochemical signatures preserved in sedimentary carbonate strata are
often used as archives for paleoenvironmental reconstructions. However,
diagenetic overprinting and/or muting of primary geochemical signatures
complicates the interpretation of these data. To avoid this issue,
geochemical techniques strive to target unaltered (or minimally altered)
carbonate components. The multi-layer low-Mg calcite shell composition
of articulate brachiopods are often the target in such studies because
they are commonly more robust to recrystallization than the shells of
other biomineralizers. Here, we have combined S K-edge μ-XRF imaging,
XANES spectroscopy and petrography to determine the source of sulfate in
the calcite lattice (carbonate-associated sulfate; CAS) of ancient and
extant brachiopods, in order to establish their suitability for use in
reconstructing seawater sulfate δ34S throughout the Phanerozoic. Both the extant brachiopod Terebratalia transversa
and fossil brachiopods display intra-specimen variability in sulfate
abundance parallel to the primary fabric, likely corresponding to
variations in growth rate. XANES spectroscopy identifies a majority of
the sulfate as inorganic. Additionally, XANES spectroscopy detected low
abundances of both reduced and oxidized organic sulfur species (thiol,
thioether, sulfoxide and sulfate esters) in all T. transversa samples and lesser abundances in a few of the fossil brachiopods. In T. transversa, inorganic sulfate and sulfate ester abundance increase towards the hinge of the valves. Bulk δ34SCAS
of the samples containing the fossil brachiopods are consistently more
positive than time-equivalent brachiopod-only values, likely reflecting a
mixture of CAS signals from homogenization of carbonate components
differentially affected by depositional environment and diagenesis. In
contrast, bulk δ34SCAS of modern T. transversa
is approximately 1‰ more positive than coeval seawater. Although
organic sulfate esters are found within the brachiopod shells, they are
only ever present as trace components. Our findings indicate that the
vast majority of sulfate in brachiopod shells is inorganic, sourced from
coeval seawater. This result supports the use of brachiopods as a
potential archive for a faithful CAS record of seawater sulfate
throughout the Phanerozoic. The characterization of in situ
organic sulfur compounds in both extant and fossil brachiopods indicates
the potential importance of various organic sulfur compounds in
mineralogical determination (and therefore fossil preservation) and
crystal orientation during brachiopod biomineralization throughout
geologic time.
KW - Carbonate-associated sulfate
KW - CAS
KW - XANES spectroscopy
KW - μ-XRF imaging
KW - Sulfur speciation
KW - Brachiopod
U2 - 10.1016/j.chemgeo.2019.119328
DO - 10.1016/j.chemgeo.2019.119328
M3 - Article
SN - 0009-2541
VL - 529
JO - Chemical Geology
JF - Chemical Geology
M1 - 119328
ER -