Aptian Mystery Solved – Ontong Java Plateau Eruption Suggested to Have Promoted Climate Change and Ocean Anoxia Expansion

Australia: The Land Where Time Began

Aptian Mystery Solved – Ontong Java Plateau Eruption Suggested to Have Promoted Climate Change and Ocean Anoxia Expansion

Large quantities of CO₂ were released by the volcanic eruption that formed the Ontong Java Plateau. It is suggested by a reconstruction of CO₂ concentrations that climate change and the expansion of ocean anoxia were promoted by the eruption.

About 120 Ma a dramatic series of events occurred globally, temperatures surging by 4-8^o (Mutterlose et al., 2014; Ando, et al., 2008), and shifts in the concentration of oxygen and nutrient availability in the ocean that resulted in organic matter being accumulated rapidly in the ocean sediments (Jenkyns, 2010) during the Aptian Oceanic Anoxic Event 1a. Also at this time the main plankton that form shells in the open ocean, the nannoconids, abruptly reduced in average size and degree of calcification (Erba, 1994). The evidence suggesting a possible cause of these events is present on the deep ocean plains in the Pacific Ocean: this evidence is the remnants of the most voluminous magma extrusion that has been identified on Earth (Taylor, 2006) – the Ontong Java Plateau. It has been found there is a fingerprint of a CO₂ surge to the atmosphere from the Ontong Java Plateau volcanism, and shows its contribution to the Aptian oceanic anoxic event. Unexpectedly, the release of CO₂ was too late to have caused the calcification crisis in the nannoconids by acidification of the ocean.

A record of changes in the concentration of atmospheric CO₂ was constructed (Naafs et al., 2016) that spanned the Aptian Oceanic Anoxic Event 1a and the eruption of the Ontong Java Plateau from marine sediments that are now exposed in southern Spain. An unusually detailed view of the sequence of events was allowed by the rapidity of the sediment accumulation. The record depends on the ratio in fossil algal matter of ¹²C to ¹³C – a high abundance of CO₂ allowed photosynthesis to incorporate preferentially ¹²C, and therefore the shifts in carbon isotopes can be used to estimate concentrations of oceanic CO₂. The record produced by Naafs et al. reveals that the increase in CO₂ concentrations occurred in 2 steps. Carbon isotope indicators in a second sediment sequence from Egypt were used to confirm the main CO₂ transitions that were revealed in Spain, thereby verifying that the algae recorded faithfully a global signal in atmospheric CO₂ levels.

The osmium isotopic composition of the marine sediments provided the evidence pointing to Ontong Java Plateau as the source of the CO₂; with concentrations rising in sync with a shift in the osmium isotopic composition that has been attributed to the main phase of eruption of basalts of the Ontong Java Plateau (Bottini et al., 2012).

Concentrations of atmospheric CO₂ remained high for millions of years, though the Ontong Java basalts also provided the ingredients for its eventual elimination; a readily extractable source of key nutrients to the ocean was provided by the very large quantity of fresh basalt. The supply of nutrients to the ocean was increased by the weathering of this basalt, as well as the aggressive weathering of continental rocks under the warmer, wetter climate that had been triggered by the rise in concentration of atmospheric CO₂. A surge in the burial of organic carbon on the sea floor that was produced by the fertilisation of algae that lasted a million years, which removed part of the extra carbon from the atmosphere while promoting the consumption and eventual depletion of oxygen in the deep ocean. The weathering of basalt led to increased alkalinity in the ocean, which increased the removal of CO₂ from the atmosphere, in the form of carbonate sediments. It has been difficult to find direct evidence of this burial of extra carbonate, but evidence of a stronger weathering of silicate rocks contributing to the drawdown of CO₂ from the atmosphere at this time is supported by the lithium isotope records (Lechler et al., 2015). Ultimately, atmospheric concentrations of CO₂ returned to pre-event levels after about 2 million years.

Clear evidence has been found (Naafs et al., 2016) showing that CO₂ released by flood basalts was the main contributor to the climate events. This record shows that the Ontong Java Plateau did not decimate the nannoconids. It had been suggested previously that the volcanic CO₂ released had caused the acidification of the surface waters of the ocean, which then hindered the calcification of surface organisms and thereby caused, or contributed to, the nannoconids crisis (Erba et al., 2010). The CO₂ record of Naafs et al. shows that as well as the nannoconids being 60,000 years past their prime when the concentrations of CO₂ rose, the rate of rise of the CO₂ was too gradual and sustained to have significantly acidified the surface waters of the ocean. The nutrient fertilisation of the ocean occurred after the nannoconids crisis, which rules out the possibility that excessive amounts of nutrients could have altered the carbon isotopic record of the algae and marked an early rise of CO₂. Therefore the cause of the nannoconids crisis remains to be determined.

It has been considered that the Aptian was 1 of only a few potential analogues of the anthropogenic ocean acidification (Hoenisch, 2012). With pervasive acidification at this time being ruled out by Naafs et al. (Naafs, 2016), it appears that in the near future the climate will be entering unchartered waters.

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