Research ArticleIsotopic evidence for changes in the mercury and zinc cycles during Oceanic Anoxic Event 2 in the northwestern Tethys, Austria
Introduction
Understanding Oceanic Anoxic Events (OAEs, Schlanger and Jenkyns, 1976) and their initiation mechanisms can help us to predict the consequences of current anthropogenic climate change, including marine deoxygenation (Keeling et al., 2010). Mesozoic OAEs were characterized by high atmospheric CO2 levels (pCO2), perturbations to the global carbon cycle, rapid global warming, and widespread anoxic–euxinic conditions in the oceans (Arthur and Sageman, 1994; Barclay et al., 2010; Jenkyns, 2010; O'Brien et al., 2017; Takashima et al., 2006). OAE 2, which occurred at the Cenomanian–Turonian boundary (CTB; ca. 94 Ma), is the most widespread and well-defined OAE, represented by worldwide deposition of organic-rich marine mudstones, referred to as “black shales” (Arthur and Sageman, 1994; Jarvis et al., 2011; Schlanger and Jenkyns, 1976). Super-greenhouse temperatures (“hothouse”) at the onset of the OAE 2, termed the “Cretaceous thermal maximum”, have been widely linked with large igneous province (LIP) volcanic activity (Du Vivier et al., 2014; Jenkyns, 2010; Jenkyns et al., 2017; O'Brien et al., 2017; Turgeon and Creaser, 2008).
Numerous studies suggest that mercury (Hg) anomalies can be a useful indicator of LIP volcanism in the geological record (Grasby et al., 2019; Percival et al., 2021; Percival et al., 2018; Sanei et al., 2012; Shen et al., 2020; Sial et al., 2013). However, Hg signatures in OAE 2 reference sections in different locations are inconsistent, indicating that the records were affected by several factors, such as the depositional environment and the paleogeographic conditions (Percival et al., 2018; Scaife et al., 2017). It also remains unclear whether volcanic eruptions were the sole source of anomalous Hg, or if other Hg sources and pathways were significant. The heterogeneous nature of Hg anomalies from different sites hinders our understanding of the Hg-cycle perturbation and the OAE initiation mechanism. Recent studies suggest that Hg stable isotopes serve as a powerful proxy for tracing the source and pathway of Hg anomalies (e.g. Grasby et al., 2017; Grasby et al., 2019; Shen et al., 2019a; Shen et al., 2019d; Yao et al., 2021). However, to date, a Hg isotope study on OAE 2 sedimentary records is lacking.
The occurrence of organic-rich black shales and/or pronounced carbon isotope excursions (CIEs) during OAE 2 could indicate elevated marine primary productivity, potentially facilitated by an increased flux of bio-essential elements from hydrothermal activity associated with LIPs (Erba, 2004; Leckie et al., 2002; Snow et al., 2005). However, there is no direct evidence for the causal link between volcanism and primary productivity. A range of transition metals act as micronutrients, required by all microbial life (Wyatt et al., 2014), and their availability in seawater may affect marine primary production (Morel et al., 1994; Sinoir et al., 2012). Zn is one such bio-essential element, and Zn isotopes (δ66Zn) in the sediment record can potentially reveal processes controlling water-column Zn distributions and reveal constraints on global oceanic mass balance (e.g. Little et al., 2016; Little et al., 2014; Liu et al., 2017). Recently, high-resolution δ66Zn curves across OAE 2 from marine carbonate records in the proto-North Atlantic and adjacent areas, such as Eastbourne, Raia Del Pedale (Sweere et al., 2018) and Tarfaya (Sweere et al., 2020), and the south of the eastern Tethys region, such as southern Tibet, Gongzha (Chen et al., 2020), have revealed perturbations as well as spatial variation in δ66Zn.
In this study, we present Hg and Zn abundance and isotopic records from a pelagic Cenomanian-Turonian section at Rehkogelgraben, Austria, which was located in the Penninic Ocean along the distal northern continental margin of the western Tethys (Gebhardt et al., 2010; Pavlishina and Wagreich, 2012; Wagreich et al., 2008). The aims of this investigation are to: 1) test if the Hg anomaly coeval with OAE 2 is recorded in this region; 2) reconstruct the paleo-redox conditions and paleogeographic setting of the Penninic Ocean; and 3) report Zn isotopic variation across the OAE 2 interval in the Rehkogelgraben section. These new data will be compared with global records to assess the timing, magnitude, spatial extent and synchroneity of geochemical signals during OAE 2, to help us understand the potential mechanisms behind this event.
Section snippets
Geological setting
The present study deals with pelagic sediments exposed at Rehkogelgraben in northern Austria that form part of the Ultrahelvetic Unit of the Austrian Eastern Alps (Wagreich et al., 2008). This deep-water pelagic unit, together with the more shallow-water deposits of the Helvetic shelf to the north, was originally part of the northern European margin of the so-called Penninic Ocean (Fig. 1; Alpine Tethys of, e.g., Schmid et al., 1996), an oceanic branch that connected the northwestern Tethys to
Materials and methods
The biostratigraphy and carbon-isotope stratigraphy of the Rehkogelgraben section have been described in detail by Wagreich et al. (2008) and Gebhardt et al. (2010). We use the same set of samples in this study. Samples were carefully checked to avoid visible veins and fractures and were ground to a ~ 200 mesh with an agate mortar that was carefully cleaned with Milli-Q water (18.2 MΩ·cm), before chemical analyses.
Hg chemostratigraphy
The carbonate and organic carbon isotope data presented here, as well as the definition of OAE 2 CIE in the Rehkogelgraben section, are taken from Wagreich et al. (2008) and Gebhardt et al. (2010). In the Rehkogelgraben section, our new Hg concentration data show two orders of magnitude variation from 1.9 to 325 ppb. There is a clear increase in Hg concentration from an average of 15.4 ppb prior to the OAE 2 interval (0–2.26 m) to an average of 60 ppb within it (2.26–3.7 m), with a first
Hydrographic setting of the Rehkogelgraben section
The degree and nature of water exchange between the Penninic Ocean, where the studied section was located, and the open ocean (proto-North Atlantic and adjacent areas) is of critical importance in understanding the stratigraphic Δ199Hg and δ66Zn patterns (Scaife et al., 2017; Sweere et al., 2020). The depositional conditions and processes in the tectonically complex northwestern Tethyan paleoceanographic system, including the Penninic Ocean, can be evaluated using a Mo-UEFs scatter plot and
Conclusions
This study provides isotopic evidence for changes in the Hg cycle across OAE 2 in the pelagic Rehkogelgraben section from the northern margin of the western Tethys. The Hg content enrichments, the absence of correlation between Hg content and OM, Mn-Fe-oxyhydroxides and/or clay minerals, and overall positive Δ199Hg excursion during the early part of OAE 2 indicate a volcanic origin. The Hg peak coincides with an Os-isotope excursion, which indicates that the Hg anomaly records a global
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.
Acknowledgments
Hanwei Yao acknowledges Kaibo Han, Huifang Guo, Di Chen for assistances in chemical analyses. This work has been funded by the National Natural Science Foundation of China (Grant Nos. 41888101, 42072118). Michael Wagreich acknowledges funding by UNESCO IGCP projects 609 and 710, and by the International Programs of the Austrian Academy of Sciences. This work contributes to IGCP 739. Rosalie Tostevin acknowledges support from the NRF–COE in Palaeosciences and BIOGRIP. We would also like to thank
References (84)
- et al.
A re-assessment of elemental proxies for paleoredox analysis
Chem. Geol.
(2020) - et al.
Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems
Chem. Geol.
(2004) - et al.
Environmental analysis of paleoceanographic systems based on molybdenum–uranium covariation
Chem. Geol.
(2009) Immobilization of mercury by pyrite (FeS2)
Environ. Pollut.
(2008)Cadmium, zinc, copper, and barium in foraminifera tests
Earth Planet. Sci. Lett.
(1981)The trace metal content of recent organic carbon-rich sediments: implications for Cretaceous black shale formation
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2006)- et al.
The expression of early Aptian to latest Cenomanian oceanic anoxic events in the sedimentary record of the Brianconnais domain
Glob. Planet. Chang.
(2018) - et al.
Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth
Earth Planet. Sci. Lett.
(2013) Marine 187Os/188Os isotope stratigraphy reveals the interaction of volcanism and ocean circulation during Oceanic Anoxic Event 2
Earth Planet. Sci. Lett.
(2014)Ca isotope stratigraphy across the Cenomanian–Turonian OAE 2: links between volcanism, seawater geochemistry, and the carbonate fractionation factor
Earth Planet. Sci. Lett.
(2015)
Calcareous nannofossils and Mesozoic oceanic anoxic events
Mar. Micropaleontol.
Paleoceanographic changes at the northern Tethyan margin during the Cenomanian–Turonian Oceanic Anoxic Event (OAE-2)
Mar. Micropaleontol.
Mercury as a proxy for volcanic emissions in the geologic record
Earth Sci. Rev.
Contemporaneous massive subaerial volcanism and late cretaceous Oceanic Anoxic Event 2
Earth Planet. Sci. Lett.
The oceanic mass balance of copper and zinc isotopes, investigated by analysis of their inputs, and outputs to ferromanganese oxide sediments
Geochim. Cosmochim. Acta
Sr, Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate
Geochim. Cosmochim. Acta
Turonian Oceanic Red Beds in the Eastern Alps: concepts for palaeoceanographic changes in the Mediterranean Tethys
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Cretaceous sea-surface temperature evolution: Constraints from TEX86 and planktonic foraminiferal oxygen isotopes
Earth Sci. Rev.
Quantifying the missing sink for global organic carbon burial during a Cretaceous oceanic anoxic event
Earth Planet. Sci. Lett.
Biostratigraphy and paleoenvironments in a northwestern Tethyan Cenomanian-Turonian boundary section (Austria) based on palynology and calcareous nannofossils
Cretac. Res.
The importance of volcanic emissions for the global atmospheric mercury cycle
Atmos. Environ.
Mercury distribution in relation to iron and manganese in the waters of the St. Lawrence river
Sci. Total Environ.
Interactions between mercury and dissolved organic matter––a review
Chemosphere
Mercury in marine Ordovician/Silurian boundary sections of South China is sulfide-hosted and non-volcanic in origin
Earth Planet. Sci. Lett.
Mercury enrichments provide evidence of Early Triassic volcanism following the end-Permian mass extinction
Earth Sci. Rev.
Sedimentary host phases of mercury (Hg) and implications for use of Hg as a volcanic proxy
Earth Planet. Sci. Lett.
Mercury isotopic composition of hydrothermal systems in the Yellowstone Plateau volcanic field and Guaymas Basin sea-floor rift
Earth Planet. Sci. Lett.
Mercury as a proxy for volcanic activity during extreme environmental turnover: the Cretaceous-Paleogene transition
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Isotope geochemistry of mercury in source rocks, mineral deposits and spring deposits of the California Coast Ranges, USA
Earth Planet. Sci. Lett.
Zinc- and cadmium-isotope evidence for redox-driven perturbations to global micronutrient cycles during Oceanic Anoxic Event 2 (Late Cretaceous)
Earth Planet. Sci. Lett.
Trace metals as paleoredox and paleoproductivity proxies: an update
Chem. Geol.
Analysis of marine environmental conditions based onmolybdenum–uranium covariation—applications to Mesozoic paleoceanography
Chem. Geol.
Anoxic vs dysoxic events reflected in sediment geochemistry during the Cenomanian–Turonian Boundary Event (Cretaceous) in the Umbria–Marche Basin of central Italy
Chem. Geol.
Calcareous nannoplankton, planktonic foraminiferal, and carbonate carbon isotope stratigraphy of the Cenomanian–Turonian boundary section in the Ultrahelvetic Zone (Eastern Alps, Upper Austria)
Cretac. Res.
The influence of temperature, pH, and growth rate on the δ18O composition of inorganically precipitated calcite
Earth Planet. Sci. Lett.
Environmental influences on the chemical composition of shales and clays
Phys. Chem. Earth
Legacy impacts of all-time anthropogenic emissions on the global mercury cycle
Glob. Biogeochem. Cycles
Marine black shales: depositional mechanisms and environments of ancient deposits
Annu. Rev. Earth Planet. Sci.
Carbon sequestration activated by a volcanic CO2 pulse during Ocean Anoxic Event 2
Nat. Geosci.
Mass-dependent and -independent fractionation of Hg isotopes by photoreduction in aquatic systems
Science
Mercury isotopes in earth and environmental sciences
Annu. Rev. Earth Planet. Sci.
Effect of intense weathering and postdepositional degradation of organic matter on Hg/TOC proxy in organic-rich sediments and its implicationsfor deep-time investigations
Geochem. Geophys. Geosyst.
Cited by (3)
Paleoenvironmental changes across the Mesozoic–Paleogene hyperthermal events
2023, Global and Planetary ChangeRecent advances on zinc isotopes in Earth science
2023, Dizhi Xuebao/Acta Geologica Sinica