Mass extinction 540 million years ago: Death came from the depths

Hydrogen sulphide poisoned habitats of the Ediacara fauna

Professor Thomas Nägler in front of the mass spectrometer, with which the molybdenum isotopes were determined. © University of Bern
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With the help of the metal molybdenum in sedimentary rocks, Swiss researchers have shown why about 540 million years ago on earth came to a large mass extinction of the first multicellular: The measured molybdenum isotopes indicate that the habitats of the so-called Ediacara fauna by the In the current issue of the science magazine "Nature", the scientists have poisoned the depth of oceans of rising hydrogen sulfide (H2S).

At the end of the Precambrian period, some 600 million years ago, a special group of multicellular molluscs dominated the oceans, the so-called Ediacara fauna. But a tremendous global event brought its supremacy to an abrupt end. For the mass extinction researchers discuss various theories, but only one explains the observations of the Bern researchers.

Oceans with hostile conditions

The latest findings of the geologist Professor Thomas Nägler from the University of Bern and his international team are based on the study of Cambrian black slates from China and Oman. Black slate and clay contain much carbon from dead organic matter and are produced from sludge. In such ocean regions life-threatening conditions prevail: It has hardly any oxygen and the water can contain large quantities of hydrogen sulfide (H2S).

Under the influence of hydrogen sulfide, the shale prefers to incorporate heavy molybdenum isotopes. Isotopes are atoms of an element with the same number of protons, but different numbers of neutrons and thus different weights. The measured isotope ratio of molybdenum in the black shales now shows, similar to a balance, the spread of H2S-rich areas compared to oxygen-rich areas in the ancient oceans.

Two ocean basins - a phenomenon

Nägler and his team measured the molybdenum isotopes in the individual slate layers from bottom to top, from the older layers to the younger ones. The Berner discovered a sudden increase in heavy molybdenum isotopes in both seas at the Precambrian-Cambrian border. display

In the younger strata, it would have been expected that, after the sudden increase, the content of heavy molybdenum isotopes would slowly decrease to normal levels. Contrary to expectations, however, the increase was followed by an extreme decrease, which ultimately yielded even lower values ​​than in the original composition: the scale of the isotope ratios had been unbalanced, fluctuated and oscillated only with time back at the initial values.

The R tsels solution is the water mixing

In order to find an answer to the extraordinary phenomenon, Martin Wille sought an explanation by means of computer modeling. His conclusion about the events 540 million years ago: A sudden event must firstly have greatly reduced the molybdenum concentration in ocean water. That is, the molybdenum dissolved in ocean water must be precipitated very quickly and stored in the black shale.

Secondly, there must have been a brief phase in which heavy molybdenum isotopes were preferentially precipitated, and third, a longer phase with the filling of lighter compositions.

Hydrogen sulfide (H2S) is considered to be the catalyst for this sudden change in the molybdenum isotope ratios at the Pr kambrium-Cambrian boundary. At the end of the Průkambrium, the oceans were similarly stratified as a covered lake. The water was hardly mixed. While there were oxic conditions on the water surface with oxygen and molybdenum oxide, enormous amounts of H2S were stored at depth. A mixing of the ocean led to a sudden rise of the highly poisonous water, which finally sloshed into smaller marginal sea basins and killed the Ediacara fauna living there.

Cause: Climate variability or plate tectonics

As a result, the hydrogen sulfide (H2S) led to the precipitation of the dissolved molybdenum. However, since H2S is oxidized very rapidly in contact with the oxygen in the atmosphere, the original state in the surface waters is rapidly restored. Thus, the isotope levels settled again and the marginal seas were again life-friendly. The extinction of the Ediacara fauna left countless habitats, which were immediately reoccupied with newly developed life the so-called "Cambrian explosion".

The question of why the ocean currents changed so much that it came to the mixing, geologists can not answer. However, changes in the ocean currents are not uncommon in the history of the earth. Causes can be, for example, climatic fluctuations or the closing or opening of sea lanes due to the migration of the continents.

(University of Bern, 29.05.2008 - DLO)