TY - JOUR

T1 - Surface water oxygenation and bioproductivity – A link provided by combined chromium and cadmium isotopes in Early Cambrian metalliferous black shales (Nanhua Basin, South China)

AU - Frei, Robert

AU - Lehmann, Bernd

AU - Xu, Lingang

AU - Frederiksen, Jesper Allan

PY - 2020/10/5

Y1 - 2020/10/5

N2 - The intrinsic link between atmospheric oxygenation, surface seawater redox and bioproductivity in the upper water column of oceans offers to combine cadmium with chromium isotopes in marine archives to deduce information on past controls of biological metal and nutrient cycling. We here use a novel approach in combining chromium and cadmium isotope signatures in metalliferous and organic matter (OM) rich shales from the Early Cambrian Niutitang formation (south China) to reconstruct the surface water redox and cycling of bioessential metals, in a period directly following the Cambrian animal evolution. The inventory of authigenic Cd in the black shales is controlled by their sulfide abundance, whereas authigenic Cr is dominantly controlled by iron(oxy-)hydroxides and OM. Cadmium isotopes measured on bulk sediments and 3 N HNO3 leachates exhibit ε114Cd values from −1.5 to +4.5, with Mo–Ni rich sulfidic shales revealing elevated values > + 2.5. Chromium isotopes define δ53Cr values from +0.4 to +1.7‰, with V-rich non-sulfidic shales yielding more positively fractionated signatures than Mo–Ni rich sulfidic shales. The negative δ53Cr-ε114Cd correlation trend recorded in the black shales suggests a high-ε114Cd sulfide endmember of authigenic Cd preserved in anoxic bottom conditions. The other endmember is constituted by OM with the isotopically heavy Cr and light Cd signals reflecting efficient phytoplankton uptake in the photic zone. The endmembers constrain the Cd and Cr isotope signatures of surface waters in the Early Cambrian Nanhua Basin to ε114Cd of ~ +7 to +17, and to δ53Cr of ~ + 0.7 to +1.9‰. These values are compatible with those of modern ocean surface waters and attest to a Cambrian surface ocean water sustaining elevated primary bioproductivity in the aftermath of late Ediacaran snowball Earth glaciations. Our study emphasizes the potential of the Cr–Cd double tracer in studies aimed at the reconstruction of paleoproductivity and (bio)geochemical metal cycling in marine paleo-basins.

AB - The intrinsic link between atmospheric oxygenation, surface seawater redox and bioproductivity in the upper water column of oceans offers to combine cadmium with chromium isotopes in marine archives to deduce information on past controls of biological metal and nutrient cycling. We here use a novel approach in combining chromium and cadmium isotope signatures in metalliferous and organic matter (OM) rich shales from the Early Cambrian Niutitang formation (south China) to reconstruct the surface water redox and cycling of bioessential metals, in a period directly following the Cambrian animal evolution. The inventory of authigenic Cd in the black shales is controlled by their sulfide abundance, whereas authigenic Cr is dominantly controlled by iron(oxy-)hydroxides and OM. Cadmium isotopes measured on bulk sediments and 3 N HNO3 leachates exhibit ε114Cd values from −1.5 to +4.5, with Mo–Ni rich sulfidic shales revealing elevated values > + 2.5. Chromium isotopes define δ53Cr values from +0.4 to +1.7‰, with V-rich non-sulfidic shales yielding more positively fractionated signatures than Mo–Ni rich sulfidic shales. The negative δ53Cr-ε114Cd correlation trend recorded in the black shales suggests a high-ε114Cd sulfide endmember of authigenic Cd preserved in anoxic bottom conditions. The other endmember is constituted by OM with the isotopically heavy Cr and light Cd signals reflecting efficient phytoplankton uptake in the photic zone. The endmembers constrain the Cd and Cr isotope signatures of surface waters in the Early Cambrian Nanhua Basin to ε114Cd of ~ +7 to +17, and to δ53Cr of ~ + 0.7 to +1.9‰. These values are compatible with those of modern ocean surface waters and attest to a Cambrian surface ocean water sustaining elevated primary bioproductivity in the aftermath of late Ediacaran snowball Earth glaciations. Our study emphasizes the potential of the Cr–Cd double tracer in studies aimed at the reconstruction of paleoproductivity and (bio)geochemical metal cycling in marine paleo-basins.

KW - (bio)geochemical metal cycling

KW - Cadmium and chromium isotopes

KW - Early Cambrian

KW - Metalliferous black shales

KW - Yangtze Platform

U2 - 10.1016/j.chemgeo.2020.119785

DO - 10.1016/j.chemgeo.2020.119785

M3 - Journal article

AN - SCOPUS:85088231028

VL - 552

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

M1 - 119785

ER -