Isotopic evidence for changes in the mercury and zinc cycles during Oceanic Anoxic Event 2 in the northwestern Tethys, Austria

Highlights

• The first Hg isotope record of OAE 2 from the Penninic Ocean, north-western Tethys

• Hg anomaly during the early part of OAE 2 is derived from LIP volcanism.

• The main controls on the Zn-cycle show spatial variation.

• Spatial and temporal variations in Hg and Zn isotopes during OAE 2

Abstract

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. Widespread deposition of organic-rich shales during OAE 2 has been attributed to a rapid rise in atmospheric CO₂, global heating, and marine anoxia triggered by intense large igneous province (LIP) volcanism. Here, we present new Hg and Zn elemental and isotopic analyses from samples spanning OAE 2 in a hemipelagic section from Rehkogelgraben, Austria, which was part of the north-western Tethys. We compare our data to existing records from a range of sites to constrain the relative timing, magnitude and geographic extent of the perturbation. We find a prominent Hg concentration peak and an overall positive Δ¹⁹⁹Hg excursion, with no correlation between Hg content and organic matter (OM), Mn-Fe-oxyhydroxides, and/or clay minerals. We interpret this to indicate a terrestrial volcanic origin of Hg. The Hg excursion is coincident with an osmium (Os) isotope excursion, and together, this supports a global period of intense LIP volcanism. The δ⁶⁶Zn record from the Rehkogelgraben section decreases abruptly by ~0.5‰ prior to the onset of OAE 2, a change recorded consistently among all reference sections. Combined with the Hg data, we interpret this to result from isotopically light Zn sourced from LIP activity. However, the second negative excursion in δ⁶⁶Zn during the Plenus Cold Event (PCE), which is recorded in the proto-North Atlantic and adjacent areas and has been attributed to Zn released from OM during re‑oxygenation, is not recorded in this section. We suggest that the cool, oxygenated deep water mass did not invade the Penninic Ocean in the northwestern Tethys. Alternatively, this excursion could be missing in our section due to the presence of carbonate-free sediments during the PCE. After the PCE, the positive excursion in δ⁶⁶Zn recorded in all sections reveals a recovery of the atmosphere-ocean system. Our findings highlight the significance of spatial and temporal variations in Hg and Zn isotopes during OAE 2.

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.