Examining pelagic carbonate-rich sediments as an archive for authigenic uranium and molybdenum isotopes using reductive cleaning and leaching experiments

Abstract

2020. Examining pelagic carbonate-rich sediments as an archive for authigenic uranium and molybdenum isotopes using reductive cleaning and leaching experiments. Chemical Geology 539 , 119412.

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes

“…Pre-PETM U/Ca are similar to modern bulk pelagic carbonates but higher than modern foraminifera 39,40 . These U/Ca match better with the higher partition coefficient of abiotic calcite compared to biogenic calcite 39 , suggesting that the majority of carbonate-associated U resides in syn-sedimentary authigenic cements 40 . Sites 865 and 401 demonstrate remarkable invariance and reproducibility in pre-PETM δ 238 U CAU , with an average of −0.36 ± 0.06‰ (2SD) that is similar to modern seawater (−0.39 ± 0.01‰; ref.…”

“…Site 690 appears to also show invariant baseline values before the PETM, but with a lower δ 238 U CAU of −0.64‰. This difference might reflect a higher proportion of adsorbed (isotopically light) U in the authigenic carbonate at Site 690, such as has been inferred for a modern pelagic bulk carbonate sample 40 , or U associated with planktic organic matter 44 that could be incorporated into authigenic calcite. The same explanation could also apply to the single anomalously low value at Site 865.…”

“…Samples were processed and measured at ETH Zurich (ETHZ; Sites 865, 401, and 690) and the Open University (OU; Site 865). At ETHZ, ~600 mg of dry sample powder was leached using 2 × 40 mL of 1 M ammonium acetate (pH 5) for 24 h at room temperature in order to selectively dissolve the bulk carbonate fraction 40 . The excess volume of acid ensures near complete carbonate dissolution with this method.…”

Uranium isotopes in marine carbonates as a global ocean paleoredox proxy: A critical review

“…Differential equations Ocean-atmosphere C dA/dt = Fd -Fw + Fox -Fmorg -Ftorg + FLIP mol C yr-1 Ocean P balance dP/dt = FPw -FOrgP -FFeP -FCaP mol P yr-1 Ocean U balance dU/dt = Friv -Fanoxic -Fother mol U yr-1 C isotope balance dδA/dt = (Fin × (δin -δA) + FLIP × (δLIP -δA) -Forg × (-Δ))/A ‰ yr-1 U isotope balance dδU/dt = (Friv × (δriv -δU) -Fanoxic × Δanoxic -Fother × Δother)/U Clarkson et al, (2018Clarkson et al, ( , 2019 Stirling et al, (2007,2015) Dahl et al, (2014, 2017) Tissot et al, (2015) Lau et al, (2016, 2017 coral, speleothem, chimney 0.6 M or 1 M sodium acetate, pH > 5 10 % CH 3 COOH rock surface contaminats were leached by 10 % acetic acid surface contaminants were removed by ultrasonic agitation with distilled ethanol diluted HNO 3 Chen et al, (2018a, b) 1M HNO 3 Zhang et al, (2018aZhang et al, ( , b, c, 2019aZhang et al, ( , b, 2020a rock surface washed by MQ-water White et al, (2018) Table A four-columns uranium purification method for carbonate U isotope analysis (Dahl et al, 2014(Dahl et al, , 2017(Dahl et al, , 2019. This purification protocol is intended for extremely low U samples such as brachiopod shells.…”

The uranium isotopic record of shales and carbonates through geologic time

“…The carbonates samples were then crushed into fine powder using an agate mortar and pestle, and the carbonate content was estimated from major element abundances determined on compacted powder pellets using a JEOL JSM-5800LV scanning electron microscope (SEM). The carbonate content was used to calculate the amount of acid to add to each sample to fully digest the carbonate fraction without leaching detrital phases (Tissot et al 2018;Clarkson et al 2020;Zhang et al 2020b).…”