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Cadmium isotope variations in Neoproterozoic carbonates - A tracer of biologic production?

S.V. Hohl1,2,

1Institut für Geologische Wissenschaften, Freie Universität Berlin, Germany
2State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing, China

S.J.G. Galer3,

3Max-Planck-Institut für Chemie, Abteilung Klimageochemie, Mainz, Germany

A. Gamper4,

4Museum für Naturkunde Berlin, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany

H. Becker1

1Institut für Geologische Wissenschaften, Freie Universität Berlin, Germany

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Hohl, S.V., Galer, S.J.G., Gamper, A., Becker, H. (2017) Cadmium isotope variations in Neoproterozoic carbonates - A tracer of biologic production? Geochem. Persp. Let. 3, 32-44.

• DFG Research Group FOR-736 “The Precambrian-Cambrian Biosphere Revolution” (Subproject Be 1820/4-2)

Geochemical Perspectives Letters v3, n1  |  doi: 10.7185/geochemlet.1704
Received 19 May 2016  |  Accepted 31 August 2016  |  Published 9 September 2016
Copyright © 2017 European Association of Geochemistry




Figure 1 Isotope and concentration data from Xiaofenghe section. Samples are drawn in equidistance according to their sample numbers in Table 1 for better visibility. (a-c) Carbon isotope data. (d) Carbonate ε112/110Cd (error bars = 2σ), grey bar = discrimination line for modern seawater values (Ripperger et al., 2007

Ripperger, S., Rehkämper, M., Porcelli, D. (2007) Cadmium isotope fractionation in seawater - a signature of biological activity. Earth and Planetary Science Letters 261, 670-684.

). (e) Salinity-corrected ε112/110Cd of seawater. (f) Bulk δ15N, yellow envelope = 0.2 ‰ uncertainty, grey bar = modern surface water. (g,h) Shale-normalised Y/Ho and Ce/Ce* in carbonates, grey bars represents discrimination of modern seawater values (Bau et al., 1995

Bau, M., Dulski, P., Möller, P. (1995) Yttrium and holmium in South Pacific seawater: vertical distribution and possible fractionation mechanisms. Oceanographic Literature Review 42, 955.

) and negative/positive Ce anomalies.
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Table 1 Stable isotope data; N and C concentrations; assorted shale normalised REE ratios and Mn enrichments relative to Cal-S from Hohl et al. (2015) Hohl, S.V., Becker, H., Gamper, A., Jiang, S.-Y., Wiechert, U., Yang, J.-H., Wei, H.-Z. (2015) Secular changes of water chemistry in shallow-water Ediacaran ocean: Evidence from carbonates at Xiaofenghe, Three Gorges area, Yangtze Platform, South China. Precambrian Research 270, 50–79. . D1-D4 = Doushantuo Fm., DG = Dengying Fm.
1 O and C isotopic data relative to VPDB; 2 Cd relative to NIST SRM 3108; 3 N relative to Air; 4 calculated following equations given by Lawrence and Kamber, 2006 Lawrence, M.G., Kamber, B.S. (2006) The behaviour of the rare earth elements during estuarine mixing—revisited. Marine Chemistry 100, 147–161. .
sampleHeight [m]Memberδ13Ccarb1δ18Ocarb1δ13CorgΔδ13ε112/110Cd22SE Cd [µg/g]δ15N3N [wt. %]TOC [wt.%]C/NΣREE [µg/g]Ce/Ce*4YN/HoNPrN/YbNEF Mn(Cal-S)



















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31.6D1-3.7-7.7

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270.831.41.130
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53.85D1-3.2-6.9

0.180.390.030.50.011






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812.1D2

-28.1



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Figure 2 (a) Mn vs. ε112/110Cd. Cap dolomites (open triangles) have high Mn concentrations and exhibit positive ε112/110Cd. (b) N vs. Cd isotopic compositions reveal negative trend. (c) Carbon vs. Cd isotopic compositions show slight positive correlation when extreme values (red squares) and cap dolomites are excluded. (d) Cd concentrations vs. ε112/110Cd values scatter along modern Southern ocean fractionation line (Abouchami et al., 2011

Abouchami, W., Galer, S.J.G., de Baar, H.J.W., Alderkamp, A.C., Middag, R., Laan, P., Feldmann, H., Andreae, M.O. (2011) Modulation of the Southern Ocean cadmium isotope signature by ocean circulation and primary productivity. Earth and Planetary Science Letters 305, 83–91.

), open circles represent coeval calculated ε112/110Cdsw; note that the seawater Cd concentrations cannot be estimated from those measured in the carbonates.
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Supplementary Figures and Tables

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Figure S-1 (a) Mn/Sr ratios are strongly elevated in the cap dolostones, presumably as a result of reducing pore-fluids, but show no correlation with Cd isotopic compositions. (b) K concentrations show a correlation with Cd isotope compositions in the Doushantuo IV member, however as in other members we do not report any dependency, as leaching of detrital minerals during the carbonate dissolution is considered negligible. (c) Y/Ho ratios in middle and upper Doushantuo correlate with Cd isotopic compositions. (d) N concentrations in the lower Doushantuo may reflect loss of N during diagenesis.
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Figure S-2 BSD picture of a Xiaofenghe section cap dolostone at 2.35 m stratigraphic height. The sample shows calcitic fillings within a formerly porous dolomicritic matrix. Subhedral (10-40 µm diameter) authigenic sulphide grains are abundant in samples from the basal Doushantuo cap dolostones. Scale bar is 100 µm.
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Figure S-3 (a) Assuming that fractionation of Cd into carbonate phases is salinity controlled, we used Y/Ho ratios in acetic acid leachates as a record for salinity variations. In modern estuarine mixing environments shale normalised Y/Ho ratios vary from 0.97 for river water (RW) to 2.96 for seawater (SW) (Lawrence and Kamber, 2006), confirmed by similar Y/Ho ranges obtained by other authors (e.g., Bau and Dulski, 1995). We used these two values as end members for the calculation of the corrected Ediacaran Cd isotope curve (Fig. 1e). A general 5 % uncertainty (2 RSD) was applied on the trace element concentration data. We calculated salinities for a case in which salinity and Y/Ho are linearly correlated (dotted line) and another case in which the correlation follows a conservative mixing line between the two end members (bold line). The average of the two cases (exemplified by the dashed orange line) was then used to calculate model salinity values. (b) From calculated salinities S we inferred ε112/110Cd shifts that result from Cd isotopic fractionation under variable salinity as shown experimentally by (Horner et al., 2011). Following their work, we assume a linear relationship between no shifts at S = 0 ‰ and a shift of -2.27 ε112/110Cd at S = 35 ‰ (αCaCO3–Cdaq = 0.9995773 ± 0.00006). Finally, with this information we were able to calculate a salinity independent ε112/110Cd curve of seawater that may have been in equilibrium with the carbonates at the time of their precipitation (Fig. 1e).
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