Where on Google Earth #173

The last excursion of WoGE (pronounced WvoGoEe) showed us some important type localities - locations where for the first time a mineral was found and described, or where it is found in accessible and well recognizable specimens - so the last round for example was the mineral "Tyrolensis" - no wait, was it "Saussurite", or should we name it "Arduinoite" ?

No - even if a first chemical observations of a strange Mg-rich limestone was published in 1779 by the Italian geologist Giovanni Arduino, the honour to became named a mineral in 1792, and then a mountain range after him, pertains to a restless "voyageur".
With 26 years - after some troubles with justice- he decided to travel and visited different geological localities of the European continent (a early Woge-Player?), so in 1784 he came to the Italian province of Calabria, after the great earthquake. And being there, he visited another well known geological phenomenon of the region on the great island nearby - even when then you couldn't go skiing on the top of this geological feature.

Remaining at home in his country he could also have visited Mg-bearing black rocks (this time not related to his name) and so the geological feature that he discovered active in Italy - even when these features today are filled with a complete opposite element to the strange things in Italy. And there finally it is , the WoGE 173, with 500m in diameter:


But that's good so, because if something fells in this hole, it remains there, and when after 325ka climatologist cam to drill, they sometimes discover another warm "climate" - and name it after the locality they found it - so at end, it's just another type locality.

I think from the first clues you can quickly deduce what country ´s geology you have to study to find the geological "spot" - it would be sufficient to name the general context of this point, maybe the specific content of the "holes" is known better to quaternary geologist - but if you really good (and I expect nothing lesser) you can also explain why the keyword here is "climate".

"For any new players to Where on (Google) Earth, simply post a comment with latitude and longitude (or a description of the location) and write something about what the features in the picture are, or how they have developed. Also you need to explain how the keyword fits in there. If you win, you get to host the next one - with the new twist to the game: the location should be connected to the previous one by some common concept, or “keyword”."

Schott's Rule could be applied: former winners have to wait 1 hour for each WoGE they got right.

Good luck !

debris flow calendar

The past weekend strong rainfalls caused various debris flows in my near surrounding area, with significant damages and one roadmen missing after a debris flow hit the street he was clearing from detritus.

To understand where and when these events hit is vital for appropriate response tactics and risk evaluation for urban areas. Thereby, the frequency and magnitude of debris flow events are of especial interest, also in view of climate change and human impact.
Information for past debris flow events in historic time can be obtained by studying archives or contemporaneous eyewitness reports/images. Prehistoric events can be reconstructed by 14C-dating of buried soils, dendrochronolgy or lichenometry. The disadvantage of these approaches is their limited time span and coarse resolution.

In the bottom sediments of the lake of Braies, in the Dolomite Alps, another possible long term record was, and still is, studied (IRMLER 2003; IRMLER et al. 2006). The lake Braies is an alpine lake on 1.492m a.s.l. with a maximum area of nearly 36ha and a catchment area of 30 square kilometres. It is surrounded by mountains up to 2.800m, dominated by dolo- and limestone formations. Several debris flow cones extend from the slopes of the mountains to the southern and eastern shores of lake Braies.

View to south with the main debris flow cones entering the lake Braies.

Simplified geological map of the lake and surrounding area (after IRMLER 2003).

In thin sections recovered from cores taken from the bottom lake sediments between annual laminations several "event layers", representing debris flows, were recognised.
Entering the lake, the debris flow brought more fine sediments in the lake then the average sedimentation rate of some millimetres per year. Under the microscope graduated layers, with progressive fining upward sequence, from well-sorted fine to middle sand at the base to silt and clay on the top could be recognized. Load casts and flame like structures support reconstructed rapid deposition. These structures indicate that the sediment moved as underflow (hyperpycnal flow - density current) into the lake basin.
A second category of layers lacked the above mentionetd characteristics, nevertheless these layers show a graduation and are much thicker than the surrounding lamination - up to seven times. These sediments are interpreted as deposits of overflow currents (hypopycnal or homopycnal flow).

Example of the studied core with recognizable annual lamination (from IRMLER 2003).

Erosive contact between annual lamination and a debris flow layer. The base of a debris flow layer is usually very coarse and the single grains more or less the same size.
Photo C) and D) shows so-called "flame structures" and small grooves - caused by the erosion of a debris flow event (Picture size ca 3.9 mm), from IRMLER 2003.

With this approach a debris flow calendar for the last 2250 years could be reconstructed. (IRMLER et al. 2006). During this time the recurrence interval of debris flows varies between 1 and 127 years. At an average of every 16 years a debris flow was deposited. The comparison with climatic phases, from the "Medieval Warm Period" to the "Little Ice Age" showed no significant correlation of events in the catchment area of lake Braies with climatic phases.
The study shows that lake sediments represent a good archive for reconstructing debris flows. In doing so, the record provides the possibility of estimation from the past the threat posed by natural hazards and gives important data for future hazard prediction assessment.

References:

IRMLER, R.; DAUT, G. & MÄUSBACHER, R. (2006): A debris flow calendar derived from sediments of lake Lago di Braies (N. Italy). Geomorphology 77:69-78
IRMLER (2003): Seesedimente als natürliches Archiv zur Erstellung eines Murkalenders am Beispiel des Pragser Wildsees (Norditalien). Ph.D. Thesis, University of Jena, Germany.

Glacier Erosion

Forms of glacial erosion represent some of the most widely distributed and recognizable indicators for past glacier extent. The discovery that polished rock surfaces were formed trough glacier abrasion, and the subsequent mapping of this feature on valley floors very distant from recent glaciers, was vital for the support of the "glacial theory" in the mid-1800s by the geological community.

The glacial striations of Le Landeron on Lake Biel, visited by the participants of the excursion of the Société Géologique in 1838, in a representation of Agassiz's work Etudes sur les glaciers of 1840.

Abrasion is the process of (frictional) wear, produced by surface rubbing against each other, and is achieved in the subglacial environment by sliding of debris-charged ice across the rock-bed.
Roches moutonées are the classical example of subglacial bedrock erosion, with abrasion dominating on the upstream (stoss) side and plucking (material pull off) on the lee side. This results in a highly unsymmetrical form, with a plain, upward side and a step to vertical termination.


To be continued...

Geologists Who Say "Nye"

Roches moutonnées of the last glacial maximum seen by the "Geotope Fischbach" (Bavaria) with Nye channels (after a British physicist) - subglacial channels eroded by meltwater under high pressure in the limestone formation of the "Wettersteinkalk".

In warm-based glaciers, also called temperate glaciers - with the bottom near or slightly above the freezing temperature, water flows in or on the bottom of the glacier forming subglacial streams in channels, that finally join at the snout, forming a glacier outlet.

There are three types of subglacial channels, depending on such factors like glacier movement, bedrock topography and lithology:

N-channel or Nye-channel: incised in the underlying bedrock.

R-channel or Röthlisberger-channel: incised in the ice (and so not found in "fossil" form).

C-channel or Clarke-channel: partly incised in the bedrock and ice, a combination of the formerly mentioned types.

#19 AW & call for #20

The #19 Accretionary Wedge is up on Dino Jim´s Musing, and this time it will stimulate all your senses...

The next Accretionary Wedge will be hosted here - let's say because it's the #20 on 20.09.2009 (easy to remember), and after explaining how somebody becomes geologist, or how to teach geology, and why "bad" movies and cake are so important in geology - why not asking:

What remains to be discovered for future earth scientists what we (still) don't know about earth? What are the geological riddles that still lack answer - all questions are allowed - it could be a local anomaly, or a global phenomena, or something strange...(Naturally you can also include a possible answer to your problem).



And when you are not afraid to pass this edition of AW, we are still searching host's for the next edition in October.

Submit your question simply by posting a link as comment here or by mail.

Quaternary stratigraphy

The quaternary has a long-established tradition of sediment sequences divided on the basis of climatic changes influencing the deposited sediments, particularly sequences in central Europe and North America where divided with this approach.

The German pioneers PENCK & BRÜCKNER divided on the basis of glacial diamicton and nonglacial deposits the terrestrial stratigraphical sequence in the Alps into four main glacial (glaciation), and corresponding interglacial periods. This scheme was based on the identification of glaciofluvial sediments - attributed to glacials- that could be traced upstream until corresponding moraines. The fossil soils between these deposits were interpreted as interglacial weathering.
Although later workers added earlier events to this first sequence, the basic structure remained dominant for more than half a century - and seemed to be recognised all over the world.

In North- and Central Germany the relative division of the Quaternary is based - following the tradition- mainly on glaciofluvial gravels, fossil soils, Loess, moraines and limnic sediments.
The extensive mining for coal and gravel provided exceptionally vast outcrops to study these sediments.

Map with the discussed localities and the main recognized ice sheet stands.
See also: Gamsenberg / Westeregeln / Hundisburg / Bilzingsleben / Ehringsdorf /Weimar and Pennickental

The gravel pit of "Wallendorf" in Thuringia exposes typical, 12 to 5m thick gravel deposits - denominated "Wallendorfer Schotter". These gravels, with rocks outcropping in the Thuringian basin and Thuringian forest, were transported to this locality by the small rivers Ilm and Unstrut, that joined the greater Saale river.

Gravel pit with the "Wallendorfer gravel", attributed to the Holstein interglacial (ca. 350-300kyr).

The Wallendorfer gravels overlay with an unconformity a moraine (Elster) and varved sediments, and are overlay by remains of a younger moraine (Saale), so the unit was attributed to the "Hauptterassenkomplex", the sequence of fluvial terraces deposited during the classical Holstein - interglacial.
The decalcification of the lower part of the partly calcareous gravels, and also the studied mollusc assemblage, seems to confirm a temperate climate with ongoing weathering and soil development.
But the presence of lithic artefacts and bones of ice age steppe animals, like mammoth, auerochs and reindeer, also with ice wedge casts, demonstrates that the upper part of the gravels were already deposited during cold conditions and the beginning of a glacial.

Ice wedge cast in the upper sequence of the "Wallendorfer gravel". This features are typical for periglacial conditions.

Varved sediments and the moraine of the Elster glacial - the next "deeper" sediments in the stratigraphic column- can be observed in the gravel pit "Rehbach".

Gravel pit "Rehbach".

Here, in a proglacial lake, fine clayey sediments were deposited until the advancing ice sheet run down the previously deposited sediments. On the basis of the moraine reworked, probably transported in frozen state, clasts can be observed.

Varved lake clay -deposited in a proglacial lake.

The contact between the varved lake sediments (note the brown layers) and the massive, loamy moraine.

Sandy clast with some pebbles in the transition area between lake sediments and moraine, probably reworked and fluvial transported (in frozen state) material then incorporated at the basis of the moraine.

What seems like an easy recognisable alternation of glacial and interglacial sediments, can however present local conditions that can be very tricky. In the gravel pit Karsdorf a succession of polymict gravels is suddenly overlay by sand with gravel lenses. This seems inconsistent with any sudden climatic change, or stratigraphic pattern in the region.

Outcrop in the gravel pit "Karsdorf", sand is overlay by large scale lenses of pebbles and coarse grained rubble.

Shear imbrication typical for debris flows overlay sandy sediments of fluvial orogin.

Only the study of the large outcrop, the topography and the underlying geology solved this apparent mystery.
The upper sand-gravel-rubble unit represents an alluvial fan, that coming from the circumjacent limestone hills joined the former pathway of the Unstrut river, and probably forced the river to change flow direction.

The discussed division is fundamentally lithological. However sediments are not unambiguous indicators of climate (for example the "Wallendorf gravels extend from a "warm" climate to a "cold"), so that other evidence, such as fossil assemblages, characteristic sedimentary structures (including periglacial structures) or textures, and soil development must be also considered. Local and regional variability of climate complicates additionally the attribution of sediments to climatic phases. The correlation between stratigraphy of different localities is mostly only possible were large outcrops are present, or very distinct sediments were found.

The climatic subdivision of the Quaternary is not without problems, in contrast with the rest of the geological column, which is divided using time (chronostratigraphy). The relative correlation between terrestrial glacial and interglacial periods is difficult and restricted mostly geographically - especially the correlation with the well dated and accepted marine records, that at least shows 40 climatic oscillations during the Pleistocene, is still at the first steps.

References:

GIBBARD, P.L. (2006): Climatostratigraphy. In (ed): ELIAS, S.A.: Encyclopedia of quaternary science. Elsevier : 2819-2847
BERNHARDT, W.; THUM, J.; SCHNEEMILCH, M. & RUDOLPH, A. (1997): Flußschotter als Schaufenster in die Zeit der ältesten Besiedelung Mitteldeutschlands. Archäologie in sachsen-Anhalt Nr.6. Archäologische Gesellschaft in Sachsen-Anhalt, Halle
WEBER, T. (1996): Das Paläolithikum und das Mesolithikum in Mitteldeutschland. Archäologie in sachsen-Anhalt Nr.6. Archäologische Gesellschaft in Sachsen-Anhalt, Halle

Caves and the dream of long term records

Within the Alps, long term climate records, ranging to the last or even to the penultimate interglacial are exceptionally rare. We are simply lacking sediments of these periods - sediments of interglacial's were systematically eroded by the (re)advance of glaciers, lay hidden under younger sediments (mostly postglacial alluvional river deposits) or are simply not yet recognised.

In cave systems we are not (so) affected by the destructive power of glaciers, so there maybe can be found sediments ranging much deeper in time, with a record much completer then in the outside world.

Speleothem growth depends strongly from temperature and water chemistry, and water in liquid form depends strongly by temperature of the environment outside the cave. During a cold period, water will be trapped in form of ice on the surface, and water percolation in the underground will be very restricted or completely missing - the speleothem will stop to growth. During warming, and melting of ice, again water is available, and the speleothem goes on growing. So just the presence of speleothems can provide a first clue to reconstruct past climates. But even better - the isotopic composition of the precipitation changes with the amount of water trapped in ice shields - so measuring the relationship between the two oxygen isotopes (the "light" 16O and the "heavy" 18O) in the- from the water deposited - carbonates, can give a direct information's of extend of ice shields, and so climate, during the past. And the carbon isotopes 13C and 12C, also found in the carbonate, give hinds on vegetation and soil cover of the catchment area of the cave. Plants assimilate preferred the lighter isotope - a high values of delta13C indicate low or even negligible input of soil-derived organic carbon into the dripwater.

Spannagel cave is a large high altitude (entrance to the cave 2.531m, extending down in two main branches to ca. 2.200m a.s.l.) cave network with approximately 10km of length in the Zillertal Alps of Austria.

The "Hintertux glacier" with the morain of the last highstand (1850). The entrance of the Spannagel caves - and also the Spannagel hut- lies on the upper end of the left morain.

It is the largest out of a series of more than 30 caves that developed within the Jurassic marble that covers the "Zentralgneiss" - the tectonic uplifted gneiss core of the "Tauern window", and is itself overlie by the phengitic gneisses.

The area above the cave today is ice free, but it was covered partially by the Hintertux glacier until 1850, and covered entirely by up to 250 thick ice during the past glacial. U/Th dates in the cave showed that deposition of the speleothems occurred repeatedly during the past few hundred thousand years, and is still ongoing, thanks of the constant temperature (1-2°C) in the cave slightly over the freezing point of water.


A stalagmite in the cave, composed of dense, columnar calcite, apparently grew without significant interruption for ca. 50 ka, albeit at a very slow rate, during the penultimate interglacial. The oxygen isotope record shows three prominent maxima, representing three warm phases, separated by a long earlier and a shorter later cold period. The mid points of the transitions into the three warm phases occurred at 240 ± 3 (correlated subsequently with the MarineIsotopicStadium 7.5), 215 ± 2 (MIS 7.3) and 200 ± 3 ka (MIS 7.1) and the end of the interglacial (MIS 7/6 transition) was dated to 190 ± 3 ka.

During full glacial periods no speleothem growth could be found. During the transition of interglacial to glacial conditions the sudden drop of the isotopes values suggest a cooling, but speleothems growth continues. Comparing the curve of the oxygen isotopes with the carbon isotopes shows a remarkable pattern - both curves appear very similar, relatively high delta18O values indicating warm atmospheric conditions coincide with high delta13C values. This suggests very little if any vegetation at this cave during the warm periods, and so less favourable conditions than today - were at least a thin soil developed.
The later interglacial (MIS 5), and also the Holocene show data with higher values of delta18O, and a low delta13C value during the "high stand" of oxygen - high temperature and thick soil and vegetation cover.

Continuous stable isotope record of speleothem growth during the Marine Isotope Stage 7 at the high-alpine Spannagel Cave, Central Alps. SPÖTL et al. (2008): Spannagel Cave, Austria MIS 7 Speleothem Stable Isotope Data. Age is given in kyr BP, isotope ratio per mil VPDB).
Relatively high (reaching -8 to -9 0/00) delta18O values indicating warm atmospheric conditions, these values during the observed interglacial coincide with relative high delta13C values (>2 0/00). The 13C isotope derives mostly of anorganic sources (p.e. dissolving limestone), so high values are sign of lacking vegetation or organic rich soil cover. During "cold periods" complete lack of soil and vegetation produces the high peak between 230-220kyrs.

These facts let conclude that the penultimate interglacial posess three major climatic phases, with warmer periods separated by cooler periods. On average this interglacial was less warmer then the last interglacial or the Holocene in this altitude, with consequent lower equilibrium lines for glaciers. The catchment area of the Spannagel cave must be covered by ice, but the ongoing growths of speleothems demonstrate the presence of water - this implies warm based conditions beneath the ice. Only during full glacial conditions no speleothem deposit occurred.

References:

SPÖTL, C.; MANGINI, A.; BURNS, S.J., FRANK, N. & PAVUZA, R. (2002): Speleothems from the high-alpine Spannagel cave, Zillertal Alps (Austria). In (ed.) SASOWSKY & MYLROIE: Studies of Cave Sediments: Physical and Chemical Records of Paleoclimate
SPÖTL et al.(2006): The last and the Penultimate Interglacial as Recorded by Speleothems From a Climatically Sensitive High-Elevation Cave Site in the Alps. In SIROCKO, F. et al. (ed): The climate of past interglacial. Developments in Quaternary Science 7.
VOLLWEILER, N.; MANGINI, A.; SPÖTL, C.; SCHOLZ, D. & MÜHLINGHAUS, C. (2009: Stalagmites from Spannagel cave (Austria) and holocene climate. Geophysical Research Abstracts. Vol.11