Cadmium isotopes in Late Ediacaran–Early Cambrian Yangtze Platform carbonates – Reconstruction of bioproductivity in ambient surface seawater

Abstract

Cadmium (Cd) isotope signatures (expressed as δ114Cd values) in seawater and in suitable modern and ancient sediments have been proposed as a useful tracer for paleobioproductivity and for constraining Cd as a micronutrient. This study contributes to the calibration of Cd isotope compositions in ancient shallow-water carbonates and proposes a combination with chromium (Cr) isotopes to link bioproductivity and ocean redox condition. We analysed 64 carbonate samples from the Jiulongwan, Gaojiaxi-Yanjiahe and Xiaotan sections of the Yangtze Platform, South China, covering the Shuram negative carbon isotope excursion (SCE) and Ediacaran-Cambrian transition (ECT), for Cd concentrations [Cd] and Cd isotopes. The results of this study show δ114Cd values ranging from −0.9‰ to +0.6‰, averaging −0.20 ± 0.65‰ (2σ, n = 56) and [Cd] with substantial variations from 0.003 μg/g to 2.78 μg/g. Using the experimental fractionation factor for Cd into calcite (−0.45‰), we calculate an average ambient surface seawater δ114Cd during the entire studied period of +0.24 ± 0.65‰, largely covering today's surface and deep ocean water compositions. The δ114Cd of ambient seawater during the SCE is reconstructed to an average value of +0.48 ± 0.40‰. This elevated signature possibly reflects the result of increased nutrient upwelling and concomitant enhanced bioproductivity levels in the upper photic surface waters during this period, leading to an increased oxygenation of the surface water. The carbonates deposited during the post-Shuram-Wonoka interval reveal a strong correlation between [Cd] and [Zn] and a correlation between degree of negative Ce anomalies and positive δ114Cd excursions, which point to strongly oxidized surface waters and concomitant biotic removal of isotopically light Cd isotopes into phytoplankton. It remains unclear why surface waters during the Late Ediacaran—Early Cambrian did not exhibit more positively fractionated Cd isotope signatures as expected from elevated bioproductivity during this transitional period.