Surface water oxygenation and bioproductivity – A link provided by combined chromium and cadmium isotopes in Early Cambrian metalliferous black shales (Nanhua Basin, South China)
Introduction
The Ediacaran-Cambrian transition was a crucial period for the evolution of early animals in Earth's history characterized by substantial changes in the biosphere (Xiao and Laflamme, 2009). In particular, the largely immobile and softbodied fauna typical of the Ediacaran was replaced by a Cambrian small shelly fauna and marks the onset of the ‘Cambrian Explosion’ (e.g. (Butterfield, 2011; Knoll and Carroll, 1999; Li and Xiao, 2004; Morris, 2000; Shu, 2008). A manifestation of marine changes that most probably promoted these evolutionary changes of life forms is reflected by large global fluctuations of carbon isotopes during the Ediacaran-Cambrian transition (e.g. Chen and Feng, 2019; Ishikawa et al., 2008; Ishikawa et al., 2013; Li et al., 2009; Zhu et al., 2006). In addition, records of marine redox changes, e.g., revealed by non-traditional stable isotopes (Mo, Cr, Hg, U) and other redox sensitive element (RSE) studies of marine archives (carbonates, shales, iron formations) deposited during this transitional period (Canfield et al., 2007; Frank et al., 2019; Frei et al., 2017; Hohl et al., 2019; Lehmann et al., 2016; Lehmann et al., 2007; Parnell et al., 2014; Wei et al., 2018a; Wei et al., 2020; Wen et al., 2009; Xu et al., 2019; Yin et al., 2017), possibly point to a period in Earth's history characterized by ecosystem instabilities as consequence of rapid continental reorganization (Kirschvink et al., 1997) with water mass instability, stratified basin waters, and pronounced fluctuations in the supply of nutrients to the oceans, and so provide a link to ecological and environmental changes and potential climate changes that ultimately led to the Cambrian explosion of life forms.
Molybdenum isotope evidence from black shales speaks for an increase of marine oxygenation during the transitional Late Ediacaran - Early Cambrian period, reaching oxygen levels in the Early Cambrian that are comparable to those in modern oceans (e.g., Chen et al., 2015; Cheng et al., 2016; Cheng et al., 2017; Kendall et al., 2015). In contrast to oxygenated surface waters, the deep oceans may have remained largely anoxic and even euxinic (e.g., Canfield et al., 2008; Fan et al., 2018; Jin et al., 2016; Sahoo et al., 2016). Recent uranium isotope data in carbonates imply large redox fluctuations during the Ediacaran-Cambrian transition (Dahl et al., 2017; Dahl et al., 2019; Tostevin et al., 2019; Wei et al., 2018a; Zhang et al., 2018a) and also point to the prevalence of extensive ocean anoxia during the terminal Ediacaran, seemingly associated with the decline of Ediacaran biota (Zhang et al., 2018a).
Following studies aimed at investigating the cycling of Cd in the modern ocean system, the Cd isotope system has been applied in studies addressing biogeochemical metal cycling in ancient marine sediments, with the aim to investigate the effects and extent of primary productivity in the paleooceans. FeMn crusts (Horner et al., 2011; Schmitt et al., 2009), non-skeletal carbonates (Hohl et al., 2017; Hohl et al., 2020; John et al., 2017), stromatolites (Viehmann et al., 2019), black shales (Georgiev et al., 2015; Hohl et al., 2019; Zhang et al., 2018c) and organic-rich mudrocks (Sweere et al., 2020) have been studied for this purpose.
It was early recognized (e.g., Boyle et al., 1976; de Baar et al., 1994) that Cd distribution in the modern ocean reveals a vertical gradient, similar to the macronutrient P, with pronounced depletion in the surface waters due to uptake of Cd by marine primary producers in the euphotic zone of surface ocean waters and its removal to deeper water environments by organic material (OM).
Subsequent work aimed at investigating and characterizing the isotope composition of dissolved Cd in the modern ocean has shown that there is a vertical gradient in ε114Cd (ε114Cd = [114/110Cdsample/114/110CdNIST] - 1) × 10′000; NIST = reference solution 3108; (Abouchami et al., 2013) values from heavy to light isotope signatures in the surface oceans, and that deep ocean waters are characterized by a rather homogenous light Cd isotope composition of around ε114Cd = 4 +/1 (Abouchami et al., 2011; Conway and John, 2015a; John et al., 2018; Ripperger et al., 2007; Xue et al., 2013). The surface ocean Cd isotope gradient is seen as a consequence of the preferential uptake of the lighter Cd isotopes by primary producers, leaving the surface water isotopically heavy, typically with ε114Cd values between ~6–10 (Abouchami et al., 2014; Conway and John, 2015a, Conway and John, 2015b; John et al., 2018; Ripperger et al., 2007), but with exceptional values of up to 40 (Ripperger et al., 2007). The combination of the long residence time of Cd in the oceans (50 kyr; (Morford and Emerson, 1999), its incorporation into phototrophic organisms thriving in surface waters and its release from sinking OM by effective remineralization in deeper waters is capable to explain, at least in parts, the isotopically light Cd signatures in deep waters (Conway and John, 2015a; Ripperger et al., 2007). More recent work has shown that Cd concentrations and isotope compositions of modern deep ocean waters are largely controlled by circulation (advection of pre-formed signatures) rather than by remineralization processes though (e.g. (Abouchami et al., 2014; Janssen et al., 2017; Middag et al., 2018; Sieber et al., 2019a; Sieber et al., 2019b; Xie et al., 2017).
Due to reductive removal of dissolved hexavalent Cr into suitable sediment archives, potentially under anoxic marine conditions, the enrichment of authigenic Cr in ancient sediments has been used to reconstruct paleo-oceanic redox states (e.g. (Reinhard et al., 2014). A comprehensive and up-to-date summary of studies dedicated to constraining modern and past redox fluctuation in the hydrosphere-atmosphere system by using the Cr isotope system has recently been published by Wei et al. (2020), and we here refer to the literature cited in this respective contribution. In the study of Bruggmann et al. (2019) addressing the Cr isotope system in the water column and sediments across an oxygen minimum zone of the Peruvian margin, it became evident that δ53Cr (δ53Cr = 53/52Crsample/53/52CrNIST − 1) × 1000; NIST = reference solution 979) values of sediments deposited in permanently anoxic waters are different from the δ53Cr values of sediments deposited in oxic bottom waters (δ53Cr = 0.77 ± 0.19‰ vs. 0.46 ± 0.19‰, 2σ, respectively). These authors suggested that sediment Cr concentrations and δ53Cr values are influenced by water column redox (e.g. by reductive dissolution and transport of Fe oxides) and/or by early diagenetic processes (e.g. redistribution of Cr during phosphatogenesis). Similarly, the study of an oxygen minimum zone in the eastern sub-tropical Atlantic Ocean by Goring-Harford et al. (2018) led these authors to propose that the Cr isotope compositions of authigenic marine precipitates deposited in deep water in the open ocean mimic the Cr isotope signature of seawater, whereas signatures measured in shelf sediments deposited under more oxic conditions are potentially susceptible to diagenetic remineralization or re-oxidation of organic material with accompanying isotopic effects by Cr(III) – Cr(VI) transformations. Last but not least, the study of anoxic sediments in the Cariaco Basin by Reinhard et al. (2014) revealed a good agreement between the δ53Cr signatures of authigenic Cr in euxinic sediments and the Cr isotope signatures of modern open Atlantic Ocean seawater. These findings also led these authors to propose that sediments deposited in anoxic marine settings have the potential to record and preserve first-order trends in seawater δ53Cr compositions.
The Yangtze Block in South China has been used as a natural laboratory for studies aimed at the reconstruction of marine redox fluctuations during the Ediacaran-Cambrian transition (e.g., Cheng et al., 2016; Cheng et al., 2017; Frank et al., 2019; Nishizawa et al., 2019; Wei et al., 2018b; Wei et al., 2020; Wei et al., 2018c). Hohl and others (Hohl et al., 2019) present combined trace metal systematics and Cd isotope compositions of sequential leachates from OM-rich shales of the Early Cambrian Niutitang Fm. (Zhongnancun Section, Guizhou Prov., SW-China). Their study builds on the characterization of Cd isotope signatures of different mineral phases in these shales and contributed substantially to the evaluation of the Cd isotope tracer system as a potential proxy for paleoproductivity in general, but in particular during the Early Cambrian.
Chromium isotopes, in combination with Ce anomalies, were recently applied to carbonates from the Late Ediacaran-Early Cambrian Dengying and Zhujiaqing Formations in the northeastern Yunnan Province (Wei et al., 2020). In addition, (Frank et al., 2019) conducted a study on marine black shales and cherts from the Early Cambrian Jiumenchong Formation and Ediacaran Liuchapo Formation in the Guizhou Province, in which these authors apply chromium isotopes in combination with REE + Y patterns to constrain redox fluctuations across the Precambrian – Cambrian transition. In both studies, strongly positively fractionated Cr isotope compositions of these sediments, in combination with significantly negative Ce anomalies, are consistent with an increased oxygenation of the atmosphere and the inner shelf surface waters of the Yangtze Platform during the Ediacaran-Cambrian transition, and (Wei et al., 2020) propose that the first appearance and subsequent diversification of the Cambrian animals may have benefited from this oxygenation event. A decrease from δ53Cr values of ~1‰ in the Ediacaran Liuchapo Formation to typical igneous values at the top of the Early Cambrian Jiumenchong Formation was interpreted by Frank et al. (2019) to potentially record a shift from a dominantly authigenic Cr input into the sediments at the end of the Ediacaran to a dominantly detrital Cr input during the Early Cambrian, which these authors proposed to reflect a shift from dominantly oxidative weathering to more aeolian weathering. A similar study by Hohl et al. (2020) was aimed at constraining Cd isotope signatures in sequential leachates of OM-rich carbonates of the Cryogenian Datangpo Formation in northeastern Guizhou Province, in an attempt to calculate the Cd isotope signature of the ambient Cryogenian surface seawater of the restricted Nanhua Basin.
Molybdenum isotopes were used in the studies by Lehmann et al. (2007) and Xu et al. (2012), and chromium isotopes in the study of Lehmann et al. (2016) were applied to Early Cambrian metalliferous black shales in the Hunan and Guizhou Provinces. These studies revealed that metal enrichment of some MoNi sulfide-rich black shales happened as a consequence of combined redox cycling and bottom-water/sediment-interface scavenging under euxinic conditions of a number of redox sensitive trace and noble metals. Accumulation of a number of redox-sensitive and particle reactive elements, according to these authors, is seen as a consequence of oxidation (remineralization) of organic matter settling from the photic zones of the respective water columns, by their fixation with sulfide under denitrifying and sulfate-reducing conditions.
Similarly, Yin et al. (2017) report elevated Hg concentrations in sulfide-rich black shale and phosphorite in the lowermost part of Early Cambrian black shale sequences on the Yangtze Platform. These authors hypothesized that upwelling of nutrient-rich waters from the open ocean resulted in high bioproductivity in the photic zone with concomitant scavenging of Hg from seawater.
The above mentioned studies form the point of departure for our study which, to our best knowledge, is the first in its kind to combine chromium and cadmium isotope data on ancient marine archives with the aim to reconstruct and link the redox state of surface seawater with effects implemented by bioproductivity fluctuations in the water column. Our novel working hypothesis thus builds on the expectation that chromium and cadmium isotopes in suitable chemical and OM-rich marine sediment archives, in combination, have the power to mirror environmental conditions in respective euphotic zones of oceans and more restricted basins. Since chromium and cadmium have a resembling removal pathway via phytoplankton, accompanied by fractionation of their isotopes, we envisage that the systems, ultimately tied to oxygen levels, render the double isotope system potentially interesting for the reconstruction of bioproductivity fluctuations in ancient oceans and basins, and linking these to climate change over respective depositional time periods.
Section snippets
Geological setting
The Yangtze Platform in the Late Neoproterozoic (Ediacaran) and Early Cambrian is a passive margin with some Archean core fragments. Marine carbonate rocks define a shallow shelf facies (carbonate platform) in the northwest with a transition to a deeper flysch facies with black shales in the southeast (Zhao and Cawood, 2012) (Fig. 1; Note that this present-day configuration is rotated clockwise by 180° compared to paleogeographic reconstructions). The Late Neoproterozoic dolomite of the
Materials and methods
We analyzed a total of 14 samples from the regionally distributed Early Cambrian polymetallic black-shale unit (Niutitang Formation) on the Yangtze platform for their cadmium isotope composition. Twelve of these samples were analyzed previously for bulk chromium isotope compositions and major and trace element concentrations (Lehmann et al., 2016). Two additional samples (ZG 1 and ZG 2), for which major and trace element data were at our disposal, and for which we also measured cadmium isotope
Results
Major and trace element data, together with chromium and cadmium isotope data of HF-HNO3 bulk rock-, 3 N HNO3-, and aqua regia (in case of cadmium analyses) leachates are in Table 1.
Besides reporting values for the 3 N HNO3 leachates which we, in analogy to the studies by Frank et al. (2020) and Reinhard et al. (2014), consider to selectively release a predominantly authigenic signal, we also estimate authigenic Cr concentrations ([Cr]auth) in bulk analyses (using the [Cr/Al] value of an
Authigenic cadmium and chromium components
Our study of MoNi, V-rich and barren black shales from the lower Niutitang Fm. reveals that 3 N HNO3 leachates are capable of releasing most chromium and cadmium (with exception perhaps of sample ZH-1) from the bulk samples. Our results, therefore, are compatible with the leach spectra for chromium in anoxic marine sediments (muds and clays) from the Cariaco Basin (Reinhard et al., 2014), and can be extended here to also cover the cadmium inventory of such sediments. Although in the sequential
Conceptual depositional model
Mo-Ni-rich sulfidic shales and V-rich shales are here considered lateral stratigraphic equivalents (Lehmann et al., 2016), deposited in sub-basins with different redox stratifications. In some locations though, V-rich shales also occur in stratigraphically older positions of the same basin, as is the case with our samples from the Huangjiawan mine, indicating a change in the depositional environment within a specific sub-basin, as also advocated by Pi et al. (2013). The individual sub-basin
Conclusions
Stratigraphically equivalent V- and MoNi -rich shales of the Niutitang Fm. were deposited in a continental margin setting on shallow open shelf and a deeper anoxic sub-basin, respectively, of the Nanhua Basin (Yangtze Platform) during the Early Cambrian. The extraordinary metal enrichment is seen as a response to increased nutrient supply to the surface waters triggered by enhanced late Ediacaran oxidative weathering of continental hinterlands in the aftermath of the latest Neoproterozoic
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by the Independent Research Fund Denmark [grants 4181-00002B and 8021-00002B] and by the Carlsberg Foundation [grants 2012-01-071 and CF17-102] to RF and by the National Science Foundation of China [grant 41972072] to LX. We acknowledge the help we received from Toni Larsen and Cristina Nora Jensen in the lab and the TIMS support from Toby Leeper. We thank the four anonymous reviewers, and the editor Oleg Pokrovsky, who provided detailed and constructive comments and
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