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Aralar Mountain Range Marine Conditions 120 Million Years Ago

March 22, 2010

The Early Aptian (120 million years ago) was an age of intense volcanic activity on Earth, eruptions that emitted large amounts of CO2 into the atmosphere, thus causing a revolution in the carbon cycle. As a consequence, great changes happened in the whole of the terrestrial system. Researcher María Isabel Millán has studied how these changes happened in the marine environment of the Aralar mountain range (at that time it was under the sea) in the Basque Country, and found more than one surprise. She presented her conclusions in her PhD thesis at the University of the Basque Country (UPV/EHU).

Ms Millán’s thesis is entitled Record of Palaeoceanographic changes during the Early Aptian of the Aralar mountain range. Given its geological characteristics, Ms Millán suspected that the changes that took place in the Early Aptian period must have left traces in the sediments of the Aralar mountain range, which straddles the Basque provinces of Gipuzkoa and Navarre. She began to study the outcrops in detail. The researcher observed that the materials representing this period in Aralar are more significant than those studied to date, and this was the first surprise of this thesis. While in other parts of the world the sedimentary series of the Early Aptian that can be studied are some 20 metres thick, Ms Millán found up to 1,000 metres in Aralar.

An unparalleled event

One of the principal global changes that took place in the oceans of the Early Aptian was that known as OAE1a (Oceanic Anoxic Event 1a); that is, a sudden reduction in maritime oxygen at the ocean beds. In order to show that this phenomenon also occurred in Aralar, Ms Millán employed a number of methodologies. On the one hand, she used ammonite fossils, which give very precise dates and which have been found in abundance in these sediments. She also made use of analytical techniques for identifying rock enriched with organic material.

In fact, when something important happened in the atmosphere or in the ocean during that period, rocks known as black shales were deposited at the bottom of the sea. These rocks were enriched with organic material, leaving remains thereof in the sedimentary layers. Ms Millán also made use of stable isotopes of carbon, as their proportion depends on the origin and concentration of CO2 present at the time. It is precisely in the Early Aptian that a huge increase of volcanic-origin CO2 took place. She found evidence amongst the sedimentary rocks which were significant in terms of an OAE1 event, thus demonstrating that this phenomenon also had taken place in Aralar.

While looking for the OAE1, Ms Millán also came across another event which, to date, has only been found on Aralar. The point here is that she discovered an interval of rock enriched with organic material, which indicates an enormously significant geological event. However, the ammonites found in this interval show that this event does not correspond exactly to the period in which the OAE1 event occurred, but is a somewhat later (younger) event; from the Upper Early Aptian to be exact. Ms Millán believes it could be a sub-event within the more wide-ranging OAE1 one. It would be, therefore, a regional or local event, given that nothing similar has been found anywhere else to date, and it remains to be seen if anything equivalent does, in fact, exist.

Sudden collapse of the reef platform of Madotz

The biocalcification crisis is another of the representative phenomena of the changes in the Early Aptian. Specifically, in Madotz, in the south-east of the mountain range, there was a reef platform, equivalent to, for example, what is found in the coral reefs of Australia today. As the researcher explained, the biocalcification that commonly occurred on the reefs suddenly collapsed on this platform, coinciding with the OAE1a event and probably responding to an acidification of the oceans (as is happening today due to industrial-origin CO2). This, according to Ms Millán, is a clear reflection of the biocalcification crisis, given that the carbonated composition of the platform was drastically modified.

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