olelog

The Guilin area in South China boasts some of the most spectacular tropical karst topography in the world, in my view only surpassed by Halong Bay in North Vietnam. When I visited Guilin some years ago and made a boat trip on the Li river, it was unfortunately raining (it often does I have been told). I have found a picture from Wikipedia that strongly reminds me of my visit to the area.



Guilin is surrounded by Devonian limestone representing a shallow marine carbonate platform extending nearly 40 km from east to west and nearly 80 km from south to north, and it is through this (karstified) carbonate platform that the river Li has has carved its river bed. Below is a generalised cross-section of a typical carbonate platform also taken from Wikipedia.



The sediment succession at Guilin also covers the Late Devonian mass extinction event at the Frasnian-Famennian boundary - this event (or possibly 2 closely spaced events) is also known as the Kellwasser event.

Reefs studied at the margin of the carbonate platform a few km west of Guilin show a transition from reefs built up by multi-cellular organisms during the Givetian to dominating microbial reefs and mounds around the Frasnian-Famennian boundary. During the Givetian massive corals were common as reef-builders, but at he Frasnian-Famennian boundary the reefs/mounds became dominated by microbes, other unicellular organisms and algae. Calcimicrobes were the dominating reef-builders throughout the Famennian, but declined near the Devonian-carboniferous boundary.

Calcimicrobes are calcareous colonial microfossils, which include many morphologically dissimilar organisms, whose effect in massive aggregations, in association with shelly multi-cellular organisms, was to lay down the earliest recognizable reef systems. The term was first applied by James & Gravestock in 1990.

The late Devonian shift from skeletal to microbial reef-builders is seen as a major event in Phanaerozoic reef evolution, generally associated with the Frasnian-Famennian extinction event. The Phanaerozoic covers roughly speaking the latest 542 million years.

I would also like to mention that the extinction only seems to have affected marine life. The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions. Please notice that the occurrence of microbial reef-building communities is not solely a function of the extinction of larger skeletal organisms.

Definitions?

I wish could give you a good definition of reef, but unfortunately the experts do not agree. Rest to say that reefs are not necessarily coral reefs, but e.g. often build up by other calcium carbonate precipitating organisms. The differences between reefs and mounds are also rather vague. Generally speaking however reefs are considered to be held up by a macroscopic skeletal framework, whereas mounds lack such a macroscopic skeletal framework - mounds are built by microorganisms or by organisms that don't grow a skeletal framework. Many reefs are a mixture of the two at various proportions. I suppose that what used to be called mud mounds in most cases actually are microbial mounds, and that microbial carbonate production has so far been underestimated. Anyway the microbial production of carbonate is now considered the most important contribution to the formation of Paleozoic mud mounds (Paleozoic Era: 542 - 251 million years ago).


Blue G marker = Guilin. Red H marker = Halong Bay

• http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.2010.01158.x/abstract
• http://www.agiweb.org/geotimes/july08/article.html?id=feature_karst.html

The Cretaceous–Paleogene extinction event, which occurred approximately 65 million years ago, was a large-scale mass extinction of animal and plant species in a geologically short period of time. The most famous victims were the dinosaurs. More than half of the species that lived in the sea died out at this time - including ammonites and rudists.

What happened to the insects?

Gunnar Ries at Amphibol commented in a post of 28 October 2009 some of the different causes used to explain the extinction event. His post is in German. This post drew my attention (thank you!) to a publication by a team of biologists form Bonn in the Proceedings of the Royal Society.

Previous studies of insect-damaged fossil leaves in the US Western Interior showed major plant and insect herbivore (plant feeding) extinction at the Cretaceous–Palaeogene boundary. The Bonn team studied leaf fossils from the middle Palaeocene Menat site, France, which has the oldest well-preserved leaf assemblage from the Palaeocene of Europe, to test the generality of the observed Palaeocene US pattern. Apparently the insects were harder hit in the US than in Europe, and where it took about 10 million years for the insect fauna in the US to recover, Europe did in half that time, namely only 5 million years.

The diversity and complexity of plant–insect interactions at Menat suggest that the net effects of the Cretaceous–Paleogene extinction were less at this greater distance from the Chicxulub, Mexico, impact site. Along with the available data from other regions, the study seems to show that the end-of-Cretaceous event did not cause a uniform, long-lasting depression of global terrestrial ecosystems. Rather, it gave rise to varying regional patterns of ecological collapse and recovery that appear to have been strongly influenced by distance from the Chicxulub structure.

This does not end the discussion, but seems to back up the Chicxulub hypothesis. Who makes the next goal?

Reference:
Wappler et al.
No post-Cretaceous ecosystem depression in European forests? Rich insect-feeding damage on diverse middle Palaeocene plants, Menat, France
Published online before print September 23, 2009
doi: 10.1098/rspb.2009.1255

Unfortunately NOT open access !

The Palaeocene Epoch is a geologic epoch that lasted from around 65 to around 56 million years ago.
The Palaeogene Period (that began around 65 and ended around 23 million years ago) on the other hand comprises the Palaeocene, Eocene and Oligocene

• http://rspb.royalsocietypublishing.org/content/early/2009/09/22/rspb.2009.1255.abstract

At the 2009 Portland GSA Annual Meeting (18-21 October 2009) Sankar Chatterjee will present a paper on the Shiva Crater (18 October 2009, 3:45-4:00 p.m.). The massive Shiva basin is a submerged depression west of India that is intensely mined for its oil and gas resources. Some complex craters are among the most productive hydrocarbon sites on the planet.

If Chatterjee is right this could be the largest, multi-ringed impact crater the world has ever seen, with a diameter of ~500 km. The diameter of the so far known largest impact crater, the Vredefort Crater in South Africa has a diameter of about 300 km. Furthermore the Shiva crater has an age that makes it a suspect for the killing of the dinosaurs ca. 65 million years ago.

According to Chatterjee it is the remnant of a giant meteorite impact that left high-resolution stratigraphic signals in the sedimentary and volcanic rocks such as shocked quartz, iridium anomaly, nickel-rich spinels, sanidine spherules, magnetic nanoparticles, high pressure fullerenes, megatsunami deposits, and melt lavas. If the author and his team are right, this is the largest crater known on our planet. The bolide may have been perhaps 40 kilometres in diameter - as compared to the bolide of between 8 and 10 kilometres that hit Yucatan Peninsula, and is commonly thought to have killed the dinosaurs.

The impact was so powerful that it led to several geodynamic anomalies: it fragmented, sheared, and deformed the lithosphere mantle across the western Indian margin and contributed to major plate reorganization in the Indian Ocean. It initiated rifting between India and Seychelles in the west and created the Laxmi Ridge; it shattered the Indian plate easterly along the Narmada-Son Rift extending 1500 km across, dividing the Indian shield into a southern peninsular block and a northern foreland block. Because of topographic barrier of the Western Ghat Mountain range, the impact-triggered tsunami was restricted along the Narmada-Son Rift at the KT boundary.

The team hopes to go India later this year to examine rocks drill from the center of the putative crater for clues that would prove the strange basin was formed by a gigantic impact.

The rest of us are waiting to hear more.

• http://www.geosociety.org/news/pr/09-54.htm
• http://www.physorg.com/news174827113.html
• http://thedragonstales.blogspot.com/2009/10/shiva-impact-rising-again.html
• http://www.eurekalert.org/pub_releases/2009-10/gsoa-gin101509.php
• http://gsa.confex.com/gsa/2009AM/finalprogram/abstract_160197.htm
• http://www.sawfnews.com/Health/60779.aspx

In Norwegian:

• http://www.forskning.no/artikler/2009/oktober/232020

PS of 21 October 2009:
The hypothesis met with sharp criticism. See
http://www.space.com/scienceastronomy/091018-dinosaur-crater.html

PS of 2 November 2009 - More about the Shiva Crater:
http://suvratk.blogspot.com/2009/11/end-cretaceous-how-many-impacts-how.html
and
http://books.google.co.in/books?id=3IORF1Ei3LIC&pg=PA35&lpg=PA35&dq=bombay+high+stratigraphy&source=bl&ots=ng1Gm3E1r-&sig=lYxnwzsvy96JmE821U2z40VDcKw&hl=en&ei=fLzmSvHfK5DOsQOf36TfBQ&sa=X&oi=book_result&ct=result&resnum=9&ved=0CB4Q6AEwCDgK#v=onepage&q=bombay%20high%20stratigraphy&f=false

The Triassic-Jurassic extinction approximately 200 million years ago is one of the five major extinctions in Earth's history. The cause has of course been widely discussed. An important factor in any discussion is the pace with which it took place. Was it catastrophically rapid - or did it occur gradually over a long period of time.

According to a study of Late Triassic biodiversity in East Greenland published in the journal Science of 19 June 2009 the decrease in plant species (in what is now East Greenland) was fairly abrupt and seemed to coincide with a period with increased atmospheric CO₂ levels and global warming. As there is no current way of detecting changes in sulfur dioxide in the past, it is difficult to evaluate whether sulphur dioxide, in addition to a rise in carbon dioxide, influenced the extinction pattern.

The authors find that the abrupt plant diversity loss is consistent with expected plant responses to a catastrophically rapid rather than gradual environmental change and argues against the currently favored extinction mechanisms invoking gradual CO₂-induced global warming due to slow release of CO₂ from the mantle associated with extrusion of basalt over an area of more than 10 million km², namely the Central Atlantic Magmatic Province in the eastern US, South America, and western Africa - during the breakup of Pangaea.

A rapid environmental change could however be related to sulphur dioxide aerosol released during volcanic eruptions in the Central Atlantic Magmatic Province. Several other mechanisms have been suggested over the years, including a meteorite impact, with the Manicougan impact as a possible candidate. Unfortunately for this hypothesis U-Pb zircon dating of the impact melt has proved that the crater has an age of 214 ± 1 million years. As this is 12 ± 2 million years before the end of the Triassic, the crater cannot be the cause of the Triassic-Jurassic extinction event.

As to increased atmospheric CO2 levels and global warming - If we compare with the situation today, it is expected that the level of carbon dioxide in the modern atmosphere may reach as high as two and a half times today's level by the year 2100. It is at exactly this level, namely 900 parts per million, that the ancient biodiversity crash was detected.

Reference:
Fossil Plant Relative Abundances Indicate Sudden Loss of Late Triassic Biodiversity in East Greenland
By McElwain et al.
Science 19 June 2009:
Vol. 324. no. 5934, pp. 1554 - 1556
DOI: 10.1126/science.1171706

• http://www.sciencemag.org/cgi/content/abstract/324/5934/1554
• http://www.eurekalert.org/pub_releases/2009-06/nsf-sci061809.php
• http://my.opera.com/nielsol/blog/2008/02/11/manicouagan-impact-structure
• http://www.geologytimes.com/research/Sudden_collapse_in_ancient_biodiversity_Was_global_warming_the_culprit.asp

Magnetostratigraphy is a field within stratigraphy that studies the magnetic characteristics of rock bodies. If the magnetic properties of rocks have measurable differences stratigraphically, that is, from one strata to the next, those differences can be used to identify their relationships and identify varying stratigraphic units. Stratigraphic units are known collectively as magnetostratigraphic units (magnetozones). The most useful magnetic property for magnetostratigraphy results from a change in the direction of the magnetization of the rocks. Crystals in rocks are magnetically aligned with the Earth’s magnetic field. The Earth’s magnetic field has changed over the eons and those magnetic alignments are ‘recorded’ in the crystals of rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. The change in the earth’s magnetic fields is caused by reversals in the polarity of the Earth's magnetic field, the Earth’s magnetic poles literally change locations. These reversals of Earth’s polarity have taken place many times during geologic history. (http://en.citizendium.org/wiki/Magnetostratigraphy)

The long procedure of sampling and analysis required to obtain a reversal stratigraphy for a succession of sedimentary rocks means that this technique is normally only used when other (biostratigraphic) methods cannot be used or a high-resolution stratigraphy is required. (Gary Nichols, Sedimentology & Stratigraphy, Blackwell, 1999)

Permian-Triassic gap in the fossil record?

Magnetostratigraphy has now been used to determine whether there was a Permian-Triassic gap in the fossil record of the Russian Ural Mountains.

The world’s single most severe mass extinction event which took place at the end of the Permian and start of the Triassic ages, some 250 million years ago. The extinction event, thought to be the result of runaway global warming, wiped out between 80-95 per cent of the planet’s species. Was this extinction event a real biological catastrophe or was it merely the result of gaps in the fossil record? So far it was thought that ten million years worth of rock from around that time was missing in Russia (we are talking about the continental uppermost Permian Russian stages, the Kazanian and Tatarian).

The scientists matched the magnetic record fossilised within the disputed Russian rocks with those from the rest of the World, demonstrating that the Russian rocks do indeed record the run-up to the event and the Permian/Triassic boundary and therefore the fossil losses in these rocks are part of the mass extinction.The sampled sections span the upper Guadalupian to Induan stages without any obvious break, so confirming the traditional view that the Tatarian is Late Permian in age.

Yet another piece matching in a larger jigsaw puzzle.

Reference:
Taylor et al.
Magnetostratigraphy of Permian/Triassic boundary sequences in the Cis-Urals, Russia: No evidence for a major temporal hiatus
Earth and Planetary Science Letters
Volume 281, Issues 1-2, 30 April 2009, Pages 36-47
doi:10.1016/j.epsl.2009.02.002
• http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4VS3P43-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=26333ea71795423cab79750f8e75508d
• http://www.bris.ac.uk/news/2009/6320.html