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How Determine the Overall Uplift Rate of the Alps

Tuesday, 10. November 2009, 21:38:30

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A couple of days ago I posted about the uplifting of the alps. A paper in the newest issue (Volume 21, Number 6, December 2009) of Basin Research describes how you can get a reasonable estimate of the uplift rate - or in other words determine the average Alpine exhumation rate. Exhumation is a term for the process that returns deeply buried rocks to the surface.

In the study fission-track thermocronology was used to reconstruct the long-term exhumation history. I described how fission-track dating works in a post back in March this year (2009), and invite you to read that post, if you have problems with the following discussion of the so-called lag time concept.

The following figure modified after Bernet (2009) hopefully illustrates the basic idea.

In uranium bearing minerals (e.g. zircon) spontaneous fission of uranium-238 releases energy, with the two fission products speeding away in opposite directions and electrons fleeing away from their path and leaving a single trail - a fission-track - in the crystal. The older the crystal, the more tracks. Fission-tracks are however sensitive to heat. If the temperature is sufficiently high for a sufficiently long time, any existing fission track will disappear. This “closure temperature” lies around 240°C for zircon. At temperatures lower than this the fission-track clock starts running. So on its way upwards when a rock with zircons cools below the closure temperature of the zircon fission-tracks system the clock is started and continues to tick to the surface. At the surface the rock is weathered and eroded, and the resulting grains are transported by glaciers and rivers down to their final deposition as sediments. The “lag time” is the time between the time of closure and the time of deposition and mainly represents the time needed to exhume the rock to the surface.

The depositional age is derived from the sediment in which the zircons are sampled, and in this case based on biostratigraphy (biozones) - with an estimated error margin of one million years.

So time of closure (“cooling age”) minus time of deposition (“depositional age”) = time of exhumation (the time of erosion and transport is regarded as geologically instantaneous).

This concept was described in details by Garver et al. (1999). There are 3 generl requirements for the use of this method.
1) No subsequent heating (higher than the closure temperature) that could have reset the “clock”
2) The detrital grains can be dated with sufficient precision
3) No active volcanism during the time of deposition (with possible input of much younger volcanic zircons contaminating the sediment samples).

In the study in question 24 medium to coarse-grained sandstone samples were collected from different stratigraphic horizons from the latest 36 million years. Obviously the result is an average, and maybe I should mention that different parts of the Alps have been uplifted at different rates (at different times). The overall average exhumation rate was calculated to be between 200 and 300 meters per million year on a regional scale.

References:
Bernet et al.
Exhuming the Alps through time: clues from detrital zircon fission-track thermochronology
Basin Research (2009) 21, 781–798
doi: 10.1111/j.1365-2117.2009.00400.x

Garver et al.
Exhumation history of orogenic highlands determined by detrital fission track thermocronology.
In: Exhumation Processes: Normal Faulting, Ductile Flow, and Erosion. Special Publication from the Geological Society of London 1999





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Alps Growing or Shrinking?

Friday, 6. November 2009, 16:36:48

A couple of months ago I wrote about a new hypothesis by Egholm et al. according to which the maximal mountain height correlate closely with climate-controlled gradients in snowline altitude rather than with tectonic activity. In Denmark this hypothesis seems to have lead to a heavy discussion about the uplift/erosion history of the Norwegian mountains between geologists at the University of Århus versus geologists at the University of Copenhagen, and I am looking forward to hear more about this dispute at the Nordic Geological Winter Meeting in Oslo in January.



But how about the Alps? A paper in the latest volume of the science magazine "Tectonophysics" (No. 474, S.236-249) seems now to prove that today's uplifting of the Alps is driven by strong climatic variations. The formation of the Alps through the collision of the two continents Africa and Europe began about 55 million years ago. By now the Alps are shrinking just as quickly in height, as they are growing. Due to the erosional work of glaciers and rivers about exactly the same amount of material is eroded from the Alps as added by uplifting.

Swiss geodesists, who have been measuring the Alps with highest accuracy for decades, have observed, that the Alp summits, as compared to low land, rise up to one millimetre per year. Over millions of years a considerable height would have to result. Researchers from the GFZ German Research Centre for Geosciences have calculated that the mountains eroded concurrently at almost exactly the same speed.

Though the Alps are constantly rising, it is no longer the plate forces but the strong climatic variations since the beginning of the so-called quaternary glacial before approximately 2.5 million years, to which mountain slopes in particular have been reacting so sensitively. To-days rise is attributed to the melting of Alpine glaciers.

Well, today it is probably just a question of isostasy, i.e. gravitational equilibrium between the earth's lithosphere and asthenosphere, meaning that the Alps "float" at an elevation which depends on their thickness and density. The burning question is, why the Alps stopped “growing”, where they did, during their collision phase. Was this only a question of isostasy or was the “snowline effect” at work?





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Largest Alpine Karst Area In Scandinavia

Thursday, 5. November 2009, 10:37:26

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The Norwegian Glomfjellet hosts the largest alpine karst area in Scandinavia. The Glomfjell karst consists of some 200 km² metacarbonates containing bands of relatively pure calcite marble (’grey marble’) interbedded with mica schist and less purer, micaeous marble (’yellow marble’).

The karst topography includes caves, dolines, bogazes, and karren.

It is planned to be a “Verneområde” (Area of Conservation) and is in this context described in an article at the Norwegian Geoportalen. Now, although the article is written in Norwegian it contains some really nice pictures of karst and other geologic features in the area for everybody to enjoy - just scroll downwards. So DO click here.

Latitude and longitude 66° 47' 8.95" N 14° 12' 8.04" E

Note:
A bogaz is a type of formation in which the rock (usually limestone) was eaten away along a joint to form a long, narrow cleft or ravine. They often have steep, almost vertical sides. This is exactly the same thing as a grike, except bigger. Boğaz is Turkish for throat.
Karren are furrows or channels formed on the surface of soluble bedrock by dissolution of a portion of the rock. (Also known as lapies.)

In Norwegian:




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Franz Josef Land (in the Arctic)

Wednesday, 4. November 2009, 20:07:28

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Few people know where Franz Josef Land is, or have ever heard of it. Except for a few Russian army border guards, Franz Josef Land is totally uninhabited. It has no native inhabitants. This is not because the area is small - Its 191 islands have a total area of 16,134 km². It is however situated far up north in the Arctic - and was furthermore completely closed to visitors for roughly 60 years until 1991. It is the most northerly group of islands associated with Eurasia and the Eurasian continental plate. Gakkel Ridge further north is normally seen as on the border zone between The Eurasian and the North American plate.

The situation may change now that the sea ice in the Arctic is melting and icebreakers in a few years perhaps no longer may be needed to reach the islands (Most of the Russian icebreakers in the area are - as far as I know - nuclear icebreakers). Since 2005 cruises are organised from Murmansk.

The archipelago is dominated by Jurassic to Tertiary basalts, but there are also late Triassic and Jurassic sediments lying in near horizontal strata. Fossil tree trunks show that the climate here was warmer in the Mesozoic. The basalt layer above the Jurassic sediments is up to 500 m thick and part of a Large Igneous Province (LIP) formed in the Cretaceous, and called the High Arctic Large Igneous Province (HALIP). This is a major Late Cretaceous large igneous province located in the Arctic. It includes the Ellesmere Island Volcanics, Strand Fiord Formation, Alpha Ridge, Franz Josef Land and Svalbard. These areas were closer to each other in the Cretaceous - since then the spreading ridge (Gakkel Ridge) has brought them further apart. A multitude of tectonic fault lines has broken the archipelago into many relatively small islands.

The age of the HALIP volcanic rocks is rather uncertain, but a long period of magmatism between ca. 136 and 80 million years ago has been suggested for the whole LIP.

From 15 June 2009 the archipelago became part of the newly established Russkaya Arktika National Park.







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Dinosaurs died out, but what about Insects?

Thursday, 29. October 2009, 10:11:16

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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





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Bulletin of the Geological Society of Denmark now Open Access

Monday, 26. October 2009, 14:45:45

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Good news from the Geological Society of Denmark (DGF). All papers published on the DGF website will now be Open Access for free download. The pages will be updated in the coming weeks.

The Bulletin of the Geological Society of Denmark is a peer-reviewed journal written in English. The Bulletin publishes contributions of international interest in all fields of geological interest, with a natural emphasis on results of new work on material from Denmark, The Faroes and Greenland.



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Unnatural Global Warming

Monday, 26. October 2009, 08:45:09

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Is the current climate change simply a natural variation like others that have occurred throughout geologic time. Some people still believe it, but more and more evidence points to human beings being (at the very least partly) responsible. The current global warming is in other words not natural. There is an abbreviation for the phenomenon - AGW (anthropogenic global warming).

In a paper, titled "Recent Changes in a Remote Arctic Lake are Unique Within the Past 200,000 Years", sediments retrieved by University at Buffalo geologists from a remote Arctic lake show that recent variations are unlike those that have been seen during previous warming episodes. The environmental transformations going on in the Arctic are dramatic.

The 20th century is the only period during the past 200,000 years in which aquatic indicators reflect increased warming, despite the declining effect of slow changes in the tilt of the Earth's axis which, under natural conditions, would lead to climatic cooling.

Reference:
Recent Changes in a Remote Arctic Lake are Unique Within the Past 200,000 Years
by Axford et al.
Proceedings of the National Academy of Sciences
Published online before print October 19, 2009
doi: 10.1073/pnas.0907094106
(requires a subscription or payment of US$10.00)





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Greenland and Oil

Saturday, 24. October 2009, 14:54:00

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So-called self-government began in Greenland on 21 June 2009. Greenland wants, fair enough, more independence to explore its own natural resources. On the other hand it needs to exploit the same natural resources to “buy” more independence. Is this a vicious circle?

Roughly 76 percent of the voters in a referendum in 2008 wanted greater self-rule; on June 21 2009, they got it. But as part of that self-rule deal, Denmark will end up reducing its annual subsidy to Greenland—about €7.300 per person, representing about 60 percent of the island’s budget. Hence the high hopes for oil revenue. Some estimates, including those of the U.S. Geological Survey, suggest Greenland’s coastal waters could hold anywhere from 16 billion to 47 billion barrels of oil, or 800,000 barrels for every man, woman, and child.

The race has begun. 13 oil firms have been pre-qualified for the first license round for 14 enormous blocks in the Baffin Bay area off the west coast of Greenland. A second round is planned for 2012.

The water depths are typically 300-400 metres, not particularly deep by today’s standards, but the wells are still expected to cost about 67 million € a time, and drilling is likely to be confined to the summer months. After all it is still a bit dark, cold and rough in the Arctic winters.

Where will the money go? and what will the impact be on the pristine environment?



In Danish:




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Algae Key To Mass Extinctions?

Friday, 23. October 2009, 13:10:25

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There are lots of hypotheses about what causes mass extinctions, one more fanciful than the other. Two ideas taken more seriously are volcanoes and cosmic impacts. Now a new hypothesis suggests algae may be the killer behind the world's great fauna annihilations.

The idea was introduced in "Hypothesis for the role of toxin-producing algae in Phanerozoic mass extinctions based on evidence from the geologic record and modern environments" , a paper published in the March 2009 issue of the peer-reviewed journal Environmental Geosciences, and also presented in a talk on 19 October 2009, at the annual meeting of the Geological Society of American in Portland, Oregon, USA. (The full paper by Castle and Rodgers from March can be downloaded from this Clemson University site).

Castle and Rodgers have spent two years analyzing data from ancient stromatolite structures finding evidence that blue-green algae, which produce poisons and deplete oxygen, were present in sufficient quantities to kill off untold numbers of plants and animals living on land or in the sea. The geologic record demonstrates a pronounced increase in abundance and environmental range of algae, including stromatolitic cyanobacterial mats, coincident with the first four of the five major Phanerozoic (the last 545 million years) mass extinctions. During these past events of algal expansion, population decline of animals could have been caused by effects of algal blooms, including algal-produced toxins, at a scale sufficient to generate a fossil record of mass extinction.


Mass extinctions have often been attributed to climate changes, sea level, volcanic activity, and asteroids. Castle and Rodgers claim that these causes are contributors, but algae were the mass killer.

Environmental changes such as climatic warming, sea level fluctuation, and increased nutrient supply may have promoted algal blooms over vast expanses of marine to freshwater environments. and indeed, in my humble opinion toxic algae may just be another contributor together with other factors leading to (too) harsh environmental conditions.


Schematic profiles illustrating the influence of climate-induced sea level change on algal growth. (A) Sea level is low, shelves are narrow, and water temperatures are less favorable for algal growth during periods of cool global climate. (B) During periods of warm global climate, sea level is high resulting in extensive areas of shallow marine and coastal environments favourable for algal growth. Warm water temperatures promote the growth of algal blooms, domal and columnal stromatolites, and stromatolitic mats, which increases the potential for toxin production and release.





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Large Igneous Provinces used for Plate Reconstructions

Thursday, 22. October 2009, 08:53:01

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Palaegeography is mainly a question of Where was What When. As to the “When”, dating of rocks is of course extremely important. Because of recent advances in the recovery and U-Pb dating (uranium-lead dating) of baddeleyite (a rare zirconium oxide mineral) it is now possible to obtain precise ages for the vast majority of the short duration, huge volume, igneous events, known as Large Igneous Provinces (LIPs) that are widespread on all the ~50 main possible 'puzzle pieces' of the Earth's past plates.

Deccan Traps and Siberian Traps are famous examples of such LIPs, because of their possible role in mass extinctions.

Paleocontinental reconstructions are critical to providing a tectonic context for major ore deposits. They are also important to understanding the full context of sedimentary basins. More broadly, robust paleocontinental reconstructions and the recognition of supercontinents through time are essential for inter-block tracing of all geological and structural features of the lithosphere, including their deep mantle roots, and for providing a paleogeographic framework for assessing Earth’s geodynamic evolution and climatic variations through time.

Precisely dated units can be assembled into a LIP 'barcode' (like a supermarket barcode) that characterizes each 'puzzle piece'. By matching these 'barcodes' and using LIP units (particularly their dyke swarms) as piercing points, it is possible to compare and reconstruct the dispersed component pieces into past supercontinents.


Global barcode of LIPs through time. Abbreviations: NA, North America; SA, South America; EU, Europe; AF, Africa; AS, Asia; Au, Australia; PA, Pacific Ocean. Modified after Ernst et al. (2005).





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