The Toarcian Oceanic Anoxic Event and the Role of Climate
Oceanic anoxic events (OAEs) refer to periods in the Earth’s history when portions of oceans became depleted in dissolved oxygen at depth and for a few thousand years (Ait-Itto, 2018). OAEs have not happened for thousands of years. However, paleo-records show that they happened several times in the past. OAEs are associated with high paleo-temperatures (Bailey et al., 2003), several mass extinctions (Danise et al., 2013) and high rate of organic carbon burial (Hesselbo et al., 2007 ; Suan et al., 2015). These mass biotic extinctions include some species that paleo-climatologists use as time markers in biostratigraphic dating.
We presently focus on the Toarcian oceanic anoxic event (T-OAE), occurring around 183Ma before present. The T-OAE has been extensively studied in the past three decades, and yet the forcing mechanisms behind OAEs are still unclear. OAEs are linked to elevated levels of greenhouse gases (Hermoso et al., 2012), climate warming (Bailey et al., 2003) and release of methane hydrate (Percival, 2015). Some argue that enhanced volcanic activity could be an external trigger responsible for OAEs (Percival, 2015 ; Hermoso et al., 2012). Therefore, it is especially relevant to study the causes and implications of these events in time of global change and rapid warming.
In this paper, we first describe how one can identify OAEs with stable isotopes and biomarkers, and what are the limits of such paleo-climate proxies. We then explore how climate perturbations may have triggered OAEs. We conclude by wondering whether OAEs could happen in a close future.
2. Proxies: detection of OAEs and limits
In this section, we describe paleo-climatic proxy uses and limits in the context of Oceanic Anoxic Events (OAEs). OAEs are characterized by the presence of black shale deposition on a global scale (Korte et al., 2011). These dark strata come from high burial rates of organic carbon (Hesselbo et al., 2007) and are associated with high productivity and/or poorly oxygenated deep oceanic waters, and temperature shift (Bailey et al., 2003). Such features can be identified by proxies, which are indirect extraction of climate signals. To understand proxy data, paleo-climatologists must first understand the relationship between proxy and climate signals (Ruddiman, 2008).
2.1 Marine fossils
Low Mg-calcite fossils – such as brachiopods, belemnites, foraminifera, bivalves – can be used for climate reconstructions (Korte et al., 2015). However, because the seafloor is younger than 170Ma, it is impossible to find such fossils in the ocean corresponding to the Toarcian OAE (~183Ma). Therefore, remains from the continents are used instead.
When studying marine fossils, it is important to consider the possible alteration and decay of the samples. Diagenesis can affect fossils by altering their composition during the transformation of sediments into rocks. Low Mg-calcite fossils are more resistant to diagenesis and thus more suitable for climate reconstructions. Chemical weathering can similarly affect the quality of the samples. Additionally, the vital effect must be considered when choosing appropriate samples: each specie can deviate from the equilibrium in a specific way. The restricted use of only one marine specie allow for more consistent time series without needing any correction (Sharp et al., 2007).
Marine belemnites seem to mark the top of the T-OAE by a severe reduction in its population, which is interpreted as a result of extensive anoxic deep waters in the Cardigan Bay Basin, UK (Xu et al., 2017 ; Danise et al., 2013). Marine fossils can also be used to reconstruct past seawater temperature by studying their carbonate oxygen and carbon isotope ratio.
2.2 Oxygen isotopes and temperature shift
Oxygen is abundantly present in Earth climate system – in the atmosphere, oceans, lakes and ice sheets – mainly as 16O and 18O. The ratio between 16O and 18O (?18O) gives information about the climate such as air temperature and elevation of ice sheets. Seawater stable isotopes of oxygen are available from marine carbonates (CaCO3) and can be used to re-create past climate and carbon dioxide variations (Sharp, 2007).
The T-OAE is characterized by a temperature shift (Bailey, 2003). A 2015 oxygen isotope dataset from the Laurasian seaway area highlights a temperature rise in early Toarcian of up to 7?C in mid-latitudes (Korte et al., 2015). However, another oxygen isotope dataset from the Paris Basin does not coincide with a temperature change but with a change in mineralogy. Yet, even though the absolute values of ?18O is affected by diagenesis, there is still a notable relative negative excursion (Hermoso et al., 2012). Further support to the temperature increase is provided by a higher soil-formed-kaolinite to illite ratios in Cardigan Bay Basin (Xu et al., 2017).
2.3 Carbon isotopes
12C and 13C are both stable carbon isotopes, they are present naturally in the atmosphere, hydrosphere and biosphere. The ratios between those two isotopes (?13C) is a valuable tool for climate reconstruction (Sharp, 2007).
The T-OAE organic-rich black shale strata coincides with a succession of carbon isotope excursions: Hesselbo (2000) indicates a short initial negative excursion preceding a main positive excursion in ?13Ccarb(VPDB). Data from the Paris Basin further supports Hesselbo’s findings, and reveals a -6‰ negative carbon isotope excursion in ?13Ccarb(VPDB) due to a higher light carbon content in the atmosphere-ocean system (Hermoso et al., 2012). The Cardigan Bay Basin data shows a -7‰ negative carbon isotope excursion in ?13CTOC during the Toarcian (Xu et al., 2017).
Timing between carbon and oxygen isotope excursions are not always obvious and sometimes no correlation can be made between the two records. In such case, no correlation can be drawn between temperature change and ocean carbon burial (Korte et al., 2015). Moreover, there are regional differences in term of magnitude and timing of the carbon isotope excursions (Korte et al., 2015).
The carbon isotope excursions have been detected in carbonate, marine organic matter and wood. Wood is an additional marker that can be used to complement other proxies. Wood particles are found together with marine organisms: once trees die, their remains are carried away down to the oceans. It is a marker for terrestrial climate change and demonstrates that climate warming during the T-OAE has been felt in both marine and terrestrial environments (Korte et al., 2015).
3. Role of climate
We described shortly a few methods to identify the T-OAE with stable isotopes and biomarkers. We have seen that the T-OAE is a global scale event affecting both marine and terrestrial environments. What has triggered this severe global change?
3.1. Carbon-cycle perturbations
The negative carbon isotope excursion suggests a release of isotopically light carbon in the atmosphere, likely from a rapid release of methane from gas hydrates (Hesselbo et al., 2000). While the positive carbon isotope excursion suggests an increased burial of light isotope carbon rich organic matter (Hesselbo et al., 2007). Both of these excursions display a disturbance in the carbon-cycle which could be related to a change in greenhouse gases emissions or weathering rates (Ruddiman, 2008).
3.2. The volcanic hypothesis
Hermoso (2012) argues for a volcanic trigger: high volcanic activity prior to the T-OAE could have resulted in higher CO2 level in the atmosphere. Hermoso (2012) then points at the timing between the isotopically light carbon negative excursion and high volcanism to propose a causal link between the two phenomena. This could have lead to a destabilization in methane hydrates and eventually the T-OAE (Hermoso et al., 2012). In support to the volcanism hypothesis, Hg content and Hg/TOC ratio have been used as a proxy for volcanic activity across 6 basins (5 in Europe and one in Argentina) and show positive excursions in both absolute Hg content and Hg/TOC ratio. Both of these excursions coincide with the carbon isotope excursions characteristic of the T-OEA (Percival, 2015).
3.3. The ocean circulation hypothesis
Korte (2015) proposes an alternative to the atmospheric greenhouse gas and volcanic hypotheses to explain Jurassic climate changes. A modification in ocean circulation caused by lithosphere doming could have had a substantial role and trigger deep changes in climate, such as the one observed in the Toarcian (Korte et al., 2015).
These hypotheses about the T-OAE probable forcings are not mutually exclusive, and it seems reasonable to advance that they all acted in concert to some extent.
The T-OAE’s black shale layer coincides with a temperature increase, a negative and then positive carbon isotope excursion, and massive extinctions across the globe. Possible climatic forcings have been proposed: carbon-cycle perturbations, increased volcanism, methane hydrate release, and change in ocean circulation patterns. The origins of OAEs are still unclear and it is likely that these forcings are all linked.
One can make a connection between sudden warming in the Toarcian and contemporary global warming. According to the IPCC Technical Summary, the last hundred years have been marked by unprecedented surface and ocean warming. Atmospheric greenhouse gases have reached record concentrations compared to the past 800 kyr. Additionally, oceanic oxygen has decreased since the 1960s due to warming-induced stratification. These oxygen-depleted ocean areas have been named “Dead zones” and it is likely that they will become larger over time (Danise et al., 2013 ; Stocker et al., 2013). Although some species might benefit or adapt to the lack of oxygen, it is likely that the overall effect on oceanic ecosystem communities and services would be negative (Danise et al., 2013). Therefore, it seems relevant to encourage research on possible causes and mechanisms responsible for ocean anoxic events to gain a better understanding of possible global warming consequences on modern oceans.