Abstract
The Cenomanian–Turonian anoxic event or ‘Oceanic Anoxic Event 2’ (OAE 2), occurring c. 94 million years ago, marks one of the most severe climatic disturbances in Earth’s history and is associated with widespread ocean de-oxygenation, or anoxia. Constraining, in detail, the timing, magnitude and progression of ocean anoxia during OAE 2 improves understanding of how the complex ocean–atmosphere system is likely to respond to today’s high atmospheric CO2 levels, and includes global warming, ocean acidification, sea-level rise, and ocean anoxia, the latter of which is gaining increasing traction with the public. Such understanding is crucial in order to forecast and ultimately find ways to mitigate climate change effects.
Reconstructions of environmental perturbations during OAE 2, and other climatically important events throughout Earth’s history, are commonly derived on the basis of the geochemical records for organic-rich black shale and carbonate sediments. These include the traditionally used carbon (C)-isotope stratigraphic record, which constrains major reorganisations of the carbon cycle, from which ocean anoxia can be indirectly inferred. More recently, the concentration gradients and stable isotope systems of redox-sensitive metals can provide more direct information on ocean de-oxygenation changes during past OAE. However, the systematics of these metal stable isotope systems are not always well constrained, leading to complexities in interpreting the palaeo-record. Furthermore, there is incomplete knowledge of the timing, duration and character of OAE 2 due to biases towards study sites in the northern hemisphere sites and locations at low latitudes where black shales and carbonates tend to form.
In this study, the utility of the uranium (U)-isotope system, a novel and recently emerged palaeo-redox tracer, was expanded through an investigation of the U-isotope systematics in a classical black shale OAE 2 sequence outcropping in Furlo, Italy that was originally deposited in the northern hemisphere in the former Tethys Ocean. The degree to which organic-rich black shales reflect the marine U-isotope signature is not well understood. Therefore, U-isotope ratios and elemental concentrations in black shales from the Furlo OAE 2 sedimentary sequence were investigated to unravel the mechanisms controlling the U-isotope fractionation factor between black shales and ambient seawater. This study indicates that the U-isotope records of black shales can provide reliable information on ocean redox changes for palaeo-environmental reconstruction, provided local influences on the U-isotope fractionation factor between seawater and sediments can be excluded or characterised. Likely parameters that influence U-isotope fractionation behaviour during black shale deposition include local redox conditions, the precise location of U reduction, watermass restriction and the reduction pathway from U(VI) to U(IV). The Furlo U-isotopic results also suggest a global two-fold expansion in the size of the anoxic and euxinic (anoxic and sulphidic) marine sinks during OAE 2 compared to today, and is one of only three such constraints that exist for OAE 2 based on U-isotope records.
While OAE 2 has been extensively studied in the low- to mid-latitude regions of the northern hemisphere, little is known about the evolution and extent of ocean anoxia in the southern hemisphere and especially at higher latitudes. To help address this knowledge deficit, high-resolution C-isotope datasets were generated for two southern hemisphere sites in New Zealand, which reveal a ~2 ‰ positive excursion that represents the global change in the carbon cycle associated with OAE 2. In particular, this excursion records the geochemical expression of the ‘Plenus Cold Event’, a transient cooling and partial climate recovery event midway through OAE 2, which previously has not been well documented in the southern hemisphere. On the basis of the new C-isotope data, a cyclostratigraphic age model was devised, which constrains the duration of OAE 2 to 930 ± 25 thousand years (ky) and implies a minimum duration of 200 ± 25 ky for the Plenus Cold Event, beginning 30 ± 13 ky after the initial onset of the CIE. The lithologies and ultra-low organic carbon content in the New Zealand section imply locally oxic conditions in the southern palaeo-Pacific Ocean, although there is some indication that dramatic changes in weathering regime and/or intensity may have given rise to a unique environmental response to OAE 2 in this region.
The New Zealand sites comprise grey and red shales, which in contrast to organic-rich black shales and carbonate lithologies that have limited geographical coverage, form across a wide range of latitudes, including mid to high latitudes. However, as these archives pose additional challenges, such as an overwhelming detrital, land-derived contribution, they are an underexplored archive for palaeo-redox studies. On the basis of the new high-resolution C-isotope stratigraphies obtained for the two New Zealand sequences, a series of partial leaching experiments were conducted on selected sediments in order to selectively extract the authigenic, seawater-derived fraction from these sediments, thought to be the iron- and manganese-(oxyhydr)oxide phase that records the marine redox-chemistry at the time of deposition. This was achieved with a cold 6 M hydrochloric acid extraction procedure, and was subsequently applied in moderately high resolution to the grey and red shales of both New Zealand sections. The authigenic fraction was extracted and chemostratigraphic records of metal concentrations and U-isotope gradients were obtained. The geochemical datasets from one New Zealand section appear to largely reflect changes in global marine redox conditions during the first half of OAE 2 and capture the climatically important onset of OAE 2 and the ‘Plenus Cold Event’. These records indicate that oceanic redox fluctuations along the continental margin of New Zealand mimicked those of the global ocean during OAE 2 and provide the first records of their type for the southwest sector of the palaeo-Pacific Ocean. In contrast, the authigenic fraction could not be reliably extracted from the other New Zealand section due to the dominance of detrital material in the sediments, which confounds the interpretation of the geochemical results. Together, these findings demonstrate that grey and red shales are promising archives for palaeo-redox studies, provided the proportion of the authigenic phase is of sufficient magnitude to overcome the detrital contribution. The results of this study highlight the influence of OAE 2 on the global environmental conditions, including the remote reaches of the southern palaeo-Pacific Ocean.