Transient cooling episodes during Cretaceous Oceanic Anoxic Events with special reference to OAE 1a (Early Aptian)

Abstract

The two major oceanic anoxic events of the Cretaceous, those of the Early Aptian (OAE 1a) and the Cenomanian-Turonian boundary (OAE 2), registered some of the highest temperatures reconstructed for the Cretaceous Period, and are thought to be related to the input of volcanically derived carbon dioxide from one or more Large Igneous Provinces. Widely distributed deposition of marine organic matter, the hallmark of OAEs, and intensified silicate weathering in response to a globally accelerated hydrological cycle and/or reaction of seawater with freshly extruded basalt, are both potential mechanisms whereby the content of atmospheric carbon dioxide could have been drawn down to promote cooling, on the assumption that this potential effect was not offset by increased addition of this volcanically derived greenhouse gas. Reduction in the supply of such carbon dioxide, with deposition of organic matter and silicate weathering continuing at the same rate, could also have produced cooling. A transient fall in temperature and increase in marine dissolved oxygen levels is well documented for OAE 2, in the form of the so-called Plenus Cold Event or Benthic Oxic Event, associated with southward invasion of certain boreal faunas and an increase in many redox-sensitive and/or chalcophilic elements in seawater caused by temporary loss of anoxic-euxinic sinks as basalt-seawater interaction continued apace. High-resolution studies of OAE 1a show at least three cooling episodes of probable global distribution, one of which (recorded in the stratigraphy of the so-called C4 Segment) is documented at high enough resolution to show correlation with an increase in carbon-isotope values that was probably due to a rise in the quantities of organic matter being buried globally, with consequent potential drawdown of atmospheric CO and/or reduction in volcanic input of this greenhouse gas. Both calcium- and lithium-isotope records suggest an increase in silicate weathering over the OAE 1a interval but current relatively low-resolution records cannot at present be tied to any one cooling episode, although the lowest Li-isotope values do derive from the C4 Segment. Evidence for reoxygenation of the ocean during the transient cooling episodes of OAE 1a is meagre, due to the lack of suitable sedimentary archives, although a negative sulfur-isotope excursion in a Pacific shallow-water carbonate section, which can be interpreted as due to oxidation of pyrite and/or sulfur-rich organic matter in the global ocean, suggests that this phenomenon may also have been a feature of the C4 Segment. Further work is required to elucidate the similarities and differences between OAE 2 and OAE 1a, but both phenomena are demonstrably global in reach and represent major disturbances to the carbon cycle with attendant effects on marine temperatures.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

Pulses of enhanced continental weathering associated with multiple Late Devonian climate perturbations: Evidence from osmium-isotope compositions

“…Together with this organic-carbon burial, the enhanced silicate weathering could also have resulted in a drawdown of CO 2 and consequential global cooling, which has also been reported for the two Kellwasser crises (e.g., Joachimski and Buggisch, 2002;Balter et al, 2008;Xu et al, 2012;Le Houedec et al, 2013;Huang et al, 2018). Thus, the pattern of enhanced continental weathering rates immediately prior to/during the onsets of the two Kellwasser crises is consistent with evidence of several other environmental perturbations in effect during those times, and follows a relationship between climate change, continental weathering, and/or marine anoxia that is similar to scenarios proposed for a number of other major events throughout the Phanerozoic Aeon (e.g., Kaiser et al, 2006;Bond and Grasby, 2017;Jenkyns, 2018). Importantly, these findings also support previous proposals that this weathering acted as an important trigger for degradation to the global environment during the Kellwasser events (Algeo and Scheckler, 1998;Averbuch et al, 2005).…”

Aptian carbonate platform development in the Southern Iberian Palaeomargin (Prebetic of Alicante, SE Spain)

“…Collectively, these data support a correlation between the termination of the Llopis platform and the onset of OAE1a (as now conventionally defined: e.g., Jenkyns, 2018). The Cau section, located 10 km to the south of the Seguilí section, and deposited in a more distal location within the Prebetic platform, records the complete Lower Aptian in the hemipelagic facies of the Almadich Formation (Castro, 1998;Aguado et al, 1999), and has provided a distinctive C-isotope stratigraphy, with recognition of the isotope segments defined by Menegatti et al (1998), (de Gea et al, 2003Moreno-Bedmar et al, 2012;Naafs et al, 2016;Ruiz-Ortiz et al, 2016).…”

Placing our current ‘hyperthermal’ in the context of rapid climate change in our geological past

“…The hyperthermal events of the Cretaceous are also associated with enhanced organic carbon burial and, characterized by such thick deposits of organic-rich black shale, they are frequently known by the alternative moniker 'ocean anoxic events' (OAEs; table 1). Jenkyns [21] shows that organic carbon burial and CO 2 drawdown by the deposition of black shales during both OAE-1a and OAE-2 (table 1) periodically exceeded the CO 2 addition to the system. This not only drove a reversal of the negative δ 13 C excursion associated with the start of the OEAs as the light isotope was preferentially incorporated in the buried organic matter, but also caused globally distributed rapid cooling to near pre-excursion temperatures and ocean re-oxygenation (e.g.…”