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Mass Extinction events caused by a fluctuating sea level

Worldwide stratigraphic sections and postulated sea level curves show that generally, there were gradual sea level fluctuations from early Triassic times to the late Jurassic. 

The effect of rising sea-level from Restless Earth, disasters of nature, National geographic society (1997)

There was a major regression toward the end of the Triassic, which was followed by a transgression at the beginning of the Jurassic.  This was large enough to have possibly caused the Triassic/Jurassic mass extinction, but the sea level change is so gradual that it does not explain why some of the recorded extinctions were so rapid. 

There were several short lived,  rapid sea level fluctuations during this time and this is what is more likely to have caused such extinctions.  Sea level rise or fall would lead to severe changes in primary productivity and a loss of habitat for shallow marine species, such as the conodonts and ammonoids.          

sealev.jpg (40779 bytes) Image of Vail sea level curves through time.

CLICK on the image to enlarge.

Picture from Sedimentary Geology by D.Prothero and F Schwab (1997)

The fossil record shows that ammonoid populations correlate extremely well with these sea level fluctuations.  This suggests that toward the end of the Triassic the fluctuating sea level changes exerted increased strain upon species living in the environment, in their battle for survival.  This supports the theory that there may possibly have been several extinction events, and during times of change in sea level, different species became extinct or had a huge decrease in population numbers.

The sea level fluctuations continued in the same fashion until the end of the Jurassic.  Why then did the extinctions occur toward the end of the Triassic?  Possibly some other factor contributed alongside the strain exerted upon species from changing sea level during this time, to cause the extinctions.

It is likely though, that this fluctuating change in sea level did have an effect upon many species at the Triassic-Jurassic boundary.

  How do we know?

Changes in lithologies along stratigraphic sections suggest environmental change.  The types of lithological change we observe suggests the type of environmental change which occurred.

A good indication that a transgression has occurred is the presence of laminated organic-rich shale deposits.  This rock type is very fine grained and would have been deposited where oxygen is not present.  Bacteria cannot live in an anoxic environment thus, in their absence organic material is not broken down, and therefore gets preserved.  A deep-sea, environment of deposition is located in the basal waters of oceans,  this is the type of location you may expect to find the rock types mentioned above. These deposits have been found at various locations globally at Triassic-Jurassic boundaries, so there is no doubt that there was a eustatic transgression during this time.

Stratigraphic sections of the late Triassic have been taken worldwide and support abrupt changes in sea level during this time.  Sections in Europe, North America, and the Arctic show there was generally global regression, followed by a transgression at the beginning of the Jurassic Period. 

It still remains unsolved, why these sea level fluctuations occurred.

Reference:

M.R. House. (Nov. 6, 1989), Ammonoid Extinction Events  Philosophical Transactions of the Royal Society of London.  Series B, Biological Sciences, v 325, Issue 1228, Evolution and Extinction, 3007-325.

 

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