Freitag, 21. August 2009

Dating cave sediments

The decay chains between radioactive and a series of daughter isotopes is a useful tool to date sediments and rocks.
The radioactive decay process of uranium (238U and 235U) and thorium (in the form of either 230Th or 232Th) were investigated in 1938, but only in the 1950s and 1960s the method was applied to date lacustrine carbonates, marine sediments, corals and cave calcite deposits.

Thorium is a daughter product in the 238U decay chain. This element is much less soluble in water than uranium and is not found in groundwater, thus, speleothems (including flowstones, stalagmites and stalactites) formed in caves as a result of precipitation of calcium carbonate from aqueous solutions, will show an uranium-, but no thorium content. Thorium will be produced only as the uranium isotopes decay with time, providing a dating tool for such materials in the age range for some 100ka.

Speleothems are well suited for this dating method because their calcite
crystals are usually large and have little tendency to recrystallize after deposition, forming so a "closed" system, where no contamination can "enter", and no information can "escape".
When viewed in cross section many speleothems
display a prominent laminated structure (growth layers), caused by variation of the deposition conditions and so the deposited material (p.e. amount of fluid inclusions or organic matter).


The growth of speleothems depends not only from the presence of water, but also of it’s content of dissolved CO2. Before percolating through the bedrock, surface water is exposed first to atmospheric CO2 and then to soil gases enriched in biogenic CO2, from where most dissolved CO2 in groundwater comes.

The stratigraphy of the Conturines cave was recorded during the excavations conducted from 1988 to 1990, from 1996 to 1998 an
d in 2001. As excavation site the first encountered area with the scattered bones and skulls, where the small conduit opens in the much larger main hall of the cave – denominated conveniently “hall of skulls”- was chosen. The floor of the upper parts of the cave is covered by up to 2m thick flowstone that shows a fine lamination, maybe representing annual cycles.


It is overlain in turn by sand, fossiliferous dolomitic sand, again sand without fossil and finally large bl
ocks.
The presence of such thick flowstone implies, that there were abundant precipitation and conditions favourable to develop soil horizons and vegetation cover in the catchment area of the cave. This limits the period of flowstone genesis to an interglacial or a warm interstadial period.
The fossils of cave bear are found only in the sand overlying the flowstones, implying that this deposit is younger. Cave bears were herbiverous animals, but today vegetation can found only 1.000m lower. During the occupation of the cave by the cave bear, vegetation was aviable in immediate vicinity, this also let´s assume that this deposits represents another warm period, with a shrub- and treebelt extending much higher then today.


So at least we need two periods were the temperatures reached higher values then today on this site – this was the case during the interglacials of the Eemian (130 to 120kyr) and the Holstein (350-300kyr).
The comparation of the anatomical features of the Conturines bear with other high alpine extinct bear species seemed to confirm an minimal age between 65.000 to 30.000 years, implying a hypotetical warm and icefree period just before the glacial maximum 20.000 years ago.

So it is possible by the stratigraphy and the evolution “niveau” of the bear fossils to restrict the sedimentation phase in the cave to a period between 300kyr and 30.000 years ago.

It’s notable that the radiometric dating method confirmed in part this hypothesis. The dates of the flowstone samples resulted beyond the range of dating of 350kyr by the thorium - uranium method. The bone-bearing sands are much younger, the two oldest dates obtained by dating the bones are 87+-5kyr and 108+-8/-7kyr. Similar to the flowstone dates, the C14 method applied to the bones resulted in an age older then 39.000 years - beyond the limit of this method.
Even if this shifts the possible ages for the first sedimentation phase considerably to older ones, the flowstone still could be deposited during, or slightly before the Holstein, or – and this would be very exceptional, during the Cromer interglacial (800kyr).
Then a sedimentation phase follows, with the deposition of grey, yellow and red sand, containing no fossils, overlaid by the fossil bearing sand and rubble layer.
The radiometric results seems to confirm an age for the occupation of the cave by the bear during, or just after the Eemian.
Then erosion removes and reworks in part the fossiliferous sandy layers – channels forms, which later were refilled with an grey sandy material, again lacking fossils.


This example shows how the limitations of the different dating methods can be compensated in part by careful observations and the “strengths” of other methods, and many different results are needed to reconstruct the sedimentation history in a "restricted" environments like caves.


References:


SCHWARCZ, H. Speleothems. In (ed): ELIAS, S.A. (2006): Encyclopedia of quaternary science. Elsevier : 290-300
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.

Kommentare:

Lost Geologist hat gesagt…

Cool series you have here on caves and karst features. Meets my interests exactly. Go on! :-)

Are you doing research on these yourself?

David Bressan hat gesagt…

thank´s for the interest

I´m not a professional speleologist, but I visited some caves in the Alps where others have done research for paleoclimatology (Oxygen isotopes)or paleoecology - I´m collecting data and pictures that hopefully can be used for a documentation of the Quaternary geology of the central Alps - as addendum to glaciers and their deposits (my field of interests).

Thinking about karst I came across the fascinating "glacial carbonates" - maybe I will post somthing in future times.

Lost Geologist hat gesagt…

Glacial carbonates? I'll be curious. I read about ice in caves. If I am right in a cave in Romania that is 6000 years old. Never had a chance to check if its plausible though.