TY - JOUR
T1 - Geological evolution of the marine selenium cycle
T2 - Insights from the bulk shale δ82/76Se record and isotope mass balance modeling
AU - Mitchell, Kirsten
AU - Mansoor, Sannan Z.
AU - Mason, Paul R D
AU - Johnson, Thomas M.
AU - Van Cappellen, Philippe
PY - 2016/5/1
Y1 - 2016/5/1
N2 - Bulk δ82/76Se values of representative marine shales from the Paleoarchean to the present day vary between approximately -3 and +3‰ with only local deviations beyond this range. This muted Se isotope variability in the shale record contrasts with the relatively large fractionations associated with abiotic and microbial Se oxyanion reduction seen in experimental studies. Long-term temporal trends in the bulk shale data do not directly correlate with changes in redox conditions of the global ocean, although a minor but significant shift towards more negative formation-averaged δ82/76Se values appears to track oxygenation of the deep ocean at the end of the Proterozoic. We hypothesize that extensive δ82/76Se variability in the shale data was suppressed due to the early emergence of biological assimilatory uptake and the resulting persistence of low seawater Se concentrations, coupled with small authigenic Se outputs throughout most of geological time. In the modern ocean, Se is an essential micronutrient with a relatively short residence time of about 11,500 yrs. The marine Se cycle is dominated by assimilation into biomass and subsequent recycling in the water column and surface sediments, i.e. processes that result in only minimal isotopic fractionation. We suggest that similar processes dominated back through the geological record to Archean times. Our model shows that paleoceanographic information could in principle be extracted from proxy data on the Se isotopic composition of seawater, once isotopic differences can be readily discerned between individual sedimentary Se pools.
AB - Bulk δ82/76Se values of representative marine shales from the Paleoarchean to the present day vary between approximately -3 and +3‰ with only local deviations beyond this range. This muted Se isotope variability in the shale record contrasts with the relatively large fractionations associated with abiotic and microbial Se oxyanion reduction seen in experimental studies. Long-term temporal trends in the bulk shale data do not directly correlate with changes in redox conditions of the global ocean, although a minor but significant shift towards more negative formation-averaged δ82/76Se values appears to track oxygenation of the deep ocean at the end of the Proterozoic. We hypothesize that extensive δ82/76Se variability in the shale data was suppressed due to the early emergence of biological assimilatory uptake and the resulting persistence of low seawater Se concentrations, coupled with small authigenic Se outputs throughout most of geological time. In the modern ocean, Se is an essential micronutrient with a relatively short residence time of about 11,500 yrs. The marine Se cycle is dominated by assimilation into biomass and subsequent recycling in the water column and surface sediments, i.e. processes that result in only minimal isotopic fractionation. We suggest that similar processes dominated back through the geological record to Archean times. Our model shows that paleoceanographic information could in principle be extracted from proxy data on the Se isotopic composition of seawater, once isotopic differences can be readily discerned between individual sedimentary Se pools.
KW - Geological evolution
KW - Isotopic mass balance modeling
KW - Marine biogeochemical cycling
KW - Selenium
KW - Stable isotopes
UR - http://www.scopus.com/inward/record.url?scp=84959358552&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2016.02.030
DO - 10.1016/j.epsl.2016.02.030
M3 - Article
AN - SCOPUS:84959358552
SN - 0012-821X
VL - 441
SP - 178
EP - 187
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -