TY - JOUR
T1 - Investigating controls on boron isotope ratios in shallow marine carbonates
AU - Zhang, Shuang
AU - Henehan, Michael J.
AU - Hull, Pincelli M.
AU - Reid, R. Pamela
AU - Hardisty, Dalton S.
AU - Hood, Ashleigh v.S.
AU - Planavsky, Noah J.
N1 - Funding Information:
We thank Donald Penman, Dan Asael and Leanne Elder for assistance in the laboratory. We thank Bob Ginsburg for provision of Belize cement sample material. Mathis P. Hain is thanked for his help in estimating p K B ⁎ values in different solution chemistries. Peter Swart is thanked for XRD analyses. Simon D'Haenens is thanked for help with C–O isotope analysis. Jamie Foster is thanked for help with sample collection and many fruitful discussions. Thanks to Bahamas Marine EcoCentre for logistical support at Darby Island. Noah J. Planavsky acknowledges funding from the NASA Alternative Earths Astrobiology Institute, the Alfred P. Sloan Foundation and Yale University . Michael J. Henehan acknowledges financial support from the Yale Peabody Museum . Ashleigh v.S. Hood acknowledges support from a NASA Astrobiology Institute Postdoctoral Fellowship. We thank Simone Kasemann and two other anonymous reviewers for their input in improving the manuscript.
PY - 2017/1/15
Y1 - 2017/1/15
N2 - The boron isotope-pH proxy has been widely used to reconstruct past ocean pH values. In both planktic foraminifera and corals, species-specific calibrations are required in order to reconstruct absolute values of pH, due to the prevalence of so-called vital effects — physiological modification of the primary environmental signals by the calcifying organisms. Shallow marine abiotic carbonate (e.g. ooids and cements) could conceivably avoid any such calibration requirement, and therefore provide a potentially useful archive for reconstructions in deep (pre-Cenozoic) time. However, shallow marine abiotic carbonates could also be affected by local shifts in pH caused by microbial photosynthesis and respiration, something that has up to now not been fully tested. In this study, we present boron isotope measurements from shallow modern marine carbonates, from the Bahama Bank and Belize to investigate the potential of using shallow water carbonates as pH archives, and to explore the role of microbial processes in driving nominally ‘abiogenic’ carbonate deposition. For Bahama bank samples, our boron-based pH estimates derived from a range of carbonate types (i.e. ooids, peloids, hardground cements, carbonate mud, stromatolitic micrite and calcified filament micrite) are higher than the estimated modern mean-annual seawater pH values for this region. Furthermore, the majority (73%) of our marine carbonate-based pH estimates fall out of the range of the estimated pre-industrial seawater pH values for this region. In shallow sediment cores, we did not observe a correlation between measured pore water pH and boron-derived pH estimates, suggesting boron isotope variability is a depositional rather than early diagenetic signal. For Belize reef cements, conversely, the pH estimates are lower than likely in situ seawater pH at the time of cement formation. This study indicates the potential for complications when using shallow marine non-skeletal carbonates as marine pH archives. In addition, variability in δ11B based pH estimates provides additional support for the idea that photosynthetic CO2 uptake plays a significant role in driving carbonate precipitation in a wide range of shallow water carbonates.
AB - The boron isotope-pH proxy has been widely used to reconstruct past ocean pH values. In both planktic foraminifera and corals, species-specific calibrations are required in order to reconstruct absolute values of pH, due to the prevalence of so-called vital effects — physiological modification of the primary environmental signals by the calcifying organisms. Shallow marine abiotic carbonate (e.g. ooids and cements) could conceivably avoid any such calibration requirement, and therefore provide a potentially useful archive for reconstructions in deep (pre-Cenozoic) time. However, shallow marine abiotic carbonates could also be affected by local shifts in pH caused by microbial photosynthesis and respiration, something that has up to now not been fully tested. In this study, we present boron isotope measurements from shallow modern marine carbonates, from the Bahama Bank and Belize to investigate the potential of using shallow water carbonates as pH archives, and to explore the role of microbial processes in driving nominally ‘abiogenic’ carbonate deposition. For Bahama bank samples, our boron-based pH estimates derived from a range of carbonate types (i.e. ooids, peloids, hardground cements, carbonate mud, stromatolitic micrite and calcified filament micrite) are higher than the estimated modern mean-annual seawater pH values for this region. Furthermore, the majority (73%) of our marine carbonate-based pH estimates fall out of the range of the estimated pre-industrial seawater pH values for this region. In shallow sediment cores, we did not observe a correlation between measured pore water pH and boron-derived pH estimates, suggesting boron isotope variability is a depositional rather than early diagenetic signal. For Belize reef cements, conversely, the pH estimates are lower than likely in situ seawater pH at the time of cement formation. This study indicates the potential for complications when using shallow marine non-skeletal carbonates as marine pH archives. In addition, variability in δ11B based pH estimates provides additional support for the idea that photosynthetic CO2 uptake plays a significant role in driving carbonate precipitation in a wide range of shallow water carbonates.
KW - Bahama Bank
KW - boron isotopes
KW - micrite
KW - ooids
KW - pH proxy
KW - shallow marine carbonates
UR - http://www.scopus.com/inward/record.url?scp=85003875988&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85003875988&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2016.10.059
DO - 10.1016/j.epsl.2016.10.059
M3 - Article
AN - SCOPUS:85003875988
VL - 458
SP - 380
EP - 393
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
ER -