8. Speleothems, U-series dating and growth frequency analysis

8. Speleothems, U-series dating and growth frequency analysis
Zusammengefasst von: Piet Nordhoff
8.1 Introduction
Cave formations or speleothems are one of several kinds of cave interior deposits,
the others being in situ breakdown materials and deposited clastic sediments
transported mechanically into the cave. A speleothem (“spelaion”=cave,
“thema”=deposit) is a “secondary mineral deposit formed in caves” (MOORE, 1952)
derived from cool water (or hot water, in geothermal settings) circulating the karst.
Since these cave dripstones are formed as part of the meteoric water cycle, their
variations in growth rate and composition reflect environmental changes on the land
surface. Cave deposits often escaped radical surface alterations (e.g. glaciations),
and possess a remarkably accurate dating potential. Using uranium-series dating
techniques, speleothems are important palaeoclimatic archives for the terrestrial
environment, complementing the marine and ice-core records. The climatic proxies,
that can be deciphered from speleothems are growth rate, stable isotope composition
(δ18O, δ13C), organic (humic) matter and trace element composition, as well as
luminescent laminae.
As the excursion is going to visit some caves like the famous “Jeita Grotto” ~10 km
NE of Beirut, some basic information about the origin of speleothems and especially
its age control via U/Th series dating will be provided. Although more than 200 cave
minerals are known (HILL and FORTI, 1996), only calcite, aragonite and gypsum are
considered as common, whereas calcite is by far the most important cave mineral
and narrowed down by WHITE (1976) as constituting mostly of a low magnesium
calcite.
8.2 Carbonate Deposition
The main processes by which carbonate formations are deposited are via CO2
degassing and by evaporation. The seeping water characterized by a high
pedogenically derived pCO2 (high dissolved CO2-load) enters the cave atmosphere
with a relative low pCO2, almost reflecting surface atmosphere conditions (rarely
more than the factor 10; EK et al. 1969). Consequently CO2 will slowly degas from
the seeping water until equilibrium with the caves pCO2 is reached. The degassing of
CO2 leads to an oversaturation with respect to calcite/aragonite (or other minerals in
solution) and results in its precipitation until new chemical equilibrium is reached. If
evaporation occurs, the CO2 removal will be comparably faster, often resulting in
speleothems with microcrystalline, porous and soft crystalls. As most caves display in
its deep interior a relative humidity of above 95% and an annual constant
temperature, degassing of CO2 without evaporative influences is considered to be the
primary course of cave carbonate deposition (WHITE, 1976).
8.3 Stalactites and Stalagmites
The principal forms and widely used classification of speleothems are given in
Abb.30.
1.
Dripstone
and 2.
Erratic 3.
Sub-aqueous
flowstone forms:
forms:
forms
a) stalactites
a) shields
a) rimstone pools
b) stalagmites
b) helictites b) concretions
c) draperies
c) botryoidal c) pool deposits
forms
d) flowstone sheets
d)
d) crystal linings
anthodites
e) moonmilk
Abb. 30: The compromise listing of speleothem forms (WHITE, 1976). Underlined forms are
summarized below.
Stalactites
The fundamental form for speleothems are soda straw stalactites (and draperies) and
are formed from drips before they fall from ceilings and walls. The degassing of
single drips of seepage water produce a ring of calcite about ~5 mm in diameter
(CURL, 1973). CURL’S analysis shows that the diameter is related to the
gravitational force on the pendant drip and the surface tension of the liquid through
the dimensionless Bond number ( B0 ):
B0 =
ρ gd 2
σ
Bond number ( B0 ); ρ = density of the solution; g = gravity; d = diameter of the straw;
σ = surface tension of the solution
The calcite ring grows downwards as a hollow tube (one ring below the other) made
of a single crystal with its c-axis oriented down the straw (Abb. 31). Seepage of water
occurs through grain boundaries and along cleavage cracks in the wall and leads to
deposition on the outside and consequently to a gradual thickening of the soda straw.
The central channel may become obstructed by crystal growth in the orifice or via
introduction of foreign matter. This gradually forces the feeding water to percolate
down the outer flanks of the soda straw. This results in a gradual transformation of
the soda straw type to a more or less conical or carrot shaped style. A section, cut
perpendicular to the growth axis, displays mostly a layering of alternating rings
somewhat similar to growth rings trees. C-axes orientation can be subparallel to the
central channel, radiate from it, or be beeing randomly distributed, depending on driprate and constancy of deposition (Abb. 31; White, 1976).
The maximum dimensions of stalactites are limited by the strength of attachment to
the cave roof (and therefore on the strength of the rock) and the dimension of the
cave.
Abb. 31: The growth of soda straw stalactites
depend on the rate and the constancy
of deposition.
A slow and constant calcite precipitation
leads to an internally monocrystalline
straw without necessarily changing its
outer morphology with the c-axis of calcite
grains parallel to the axis orientation of the
proto-straw,
whereas
faster
and/or
fluctuating growth conditions result in
randomly orientated grains. (WHITE,
1976)
.
Stalagmites
Drips falling on the floor continue to deposit most of its calcite load. That leads to the
build up of stalagmites (or flowstones) whose growth laminae thin away from the drop
impact point. Much effort has been carried out to correlate the outer stalagmite
morphology to environmental conditions during calcite deposition: Following WHITE
(1976) constant drip rates, water hardness and cave atmosphere promote uniform
diameter stalagmites, while decreasing deposition rates result in conical forms.
FRANKE (1961, 1963, 1965, 1975) categorized the form of stalagmites into three
groups, where uniform diameter stalagmites (1) with preferred new growth at the top
(not down the outer flanks) are promoted by constant drip rates, solute
concentrations of the feedwater and cave atmosphere. The relation of the available
CaCO3 ( C0 ), flow rate ( Q ) and rate of vertical growth ( ż ) may also be expressed as:
C0 Q
Πż The increase in diameter is proportional to Q and the drip fall height
(CURL, 1973, GAMS, 1981)
D=2
Periodic fluctuations in growth height and greater fall heights lead to a greater splash
at the drip impact point. This is likely to produce stalagmites with terraced or
corbelled thickening, forming e.g. palm tree trunks (2). The most common type is the
conical or tapered form (3), attributed either to a decreasing growth rate (FRANKE,
1961, 1965) or increasing fall height (GAMS, 1981) resulting in an increasing calcite
precipitation from a waterfilm down the flanks with a smooth transition to the cave
floor/flowstone sheets.
8.4 Dating of speleothems and growth frequency analysis
Speleothems are most commonly dated via uranium-series disequilibrium methods
(generally 230Th/234U). Isotopes of U, leached from the carbonate bedrock, are coprecipitated as uranyl carbonate with the calcite of the speleothems. Two natural
parent isotopes, 238U and 235U, exist. These have exceptionally long half-lives, and
decay by emission of α and β particles to stable lead isotopes 206Pb and 207Pb (Abb.
32). Intermediate daughter products such as 234U and 226Ra are also suitable for
dating. During weathering, more daughter 234U is released than the parent 238U and
235
U. All three species become oxidized and are transported in solution as complex
ions which may be precipitated in calcite. Daughter products like 231Pa and 230Th are
insoluble and may be bond to the charged surface of clay particles or organic free
particles. Thus, assuming the speleothem contains no clay or insoluble detritus,
which are potential carriers of detrital Th, the activity ratio of 234U to its decay product
230
Th is the result of 234U decay since its crystallization (HARMON et al.,1975).
The method can be applied to atime range of 350 ka to 10 ka B.P. Precautions have
to be taken to ensure a reliable age estimate, for instance, that any sample
containing more than 1 % of acid-insoluble detritus rejected, or that any signs of
recrystallization would suggest, that the sample may not have remained in a closed
system. Today the minimum amount of U in calcite samples for providing reliable
results are 30-50 ppb (personal commun. WIEGAND, 2003).
Abb. 32: The decay of 238U, 235U, and 232Th (WHITE, 1997).
Dating begin and termination of speleothem growth by using samples from large
areas enables the creation of regional chronologies which may indicate large-scale,
climatic related controls of growth periods (e.g. HENNIG et al., 1983).
Case study:
A large number of speleothem U-series dates, collected from caves in northern
England revealed quite similar periods of speleothem growth. Major growth phases
from 130 to 90 ka B.P and 15 ka to the present (Abb. 33) can clearly be indentified.
Deposition seems to have ceased entirely from 165 to 140 ka B.P. and from 30 to 15
ka B.P., when the area was too cold for water circulation in the caves (GASCOYNE,
1992, GORDON et al. 1989).
Abb. 33: Histogram of ~180 U-series dates on
speleothems from the North of
England.
The Speleothem formation was associated
with warm interglacial or interstadial conditions
when groundwater circulation was not impeded
by temperatures below freezing. No samples
were dated between ~140-165 ka B.P. or from
15-30 ka B.P., thus indicating cold, glacial
conditions(GASCOYNE, 1992)
8.5 References
R.L. Curl (1973): Minimum diameter stalagmites.- NSS Bull., 35, 1-9.
C. Ek, S. Gilewska, L. Kaszowski (1969): Some analyses of the CO2 – content of
the air in five Polish caves.- Z. Geomorphol., 13/3, 226 – 286
H.W. Franke (1961): Formgesetze des Höhlensinters.- Rass. Speleol. Italiana,
Mem. 5, 185-202.
H.W. Franke (1963): Formprinzipien des Tropfsteins.- Proc. III Internat. Congr.
Speleol., 2, 63-71.
H.W. Franke (1965): The theory behind stalagmite shapes.- Studies in Speleo.,
1, 89-95.
H.W. Franke (1975): Sub-minimum diameter stalagmites.- NSS Bull., 37, 17-18.
I. Gams (1981): Contribution to morphometrics of stalagmites.- Proc. 7th. Int.
Congr. Speleo., Sheffield, 205-208.
M. Gascoyne (1992): Paleoclimatic determination from cave calcite deposits.Quaternary Science Reviews, 11, 609-632.
D. Gillieson (1996): Caves: Processes, Development and Management.Blackwell, 324 pp.
D. Gordon, P.L. Smart, D.C. Ford, J.N. Andrews, T.C. Atkinson, P.J. Rowe,
N.S.J. Christopher (1989): Dating of Late Pleistocene interglacial and
interstadial periods in the United Kingdom from speleothem growth
frequency.- Quaternary Research, 31, 14-26.
R.S. Harmon, P. Thompson, H.P. Schwarcz, D.C. Ford (1975): Uranium-series
dating of speleothems.- National Speleological Society Bulletin, 37, 21-33.
G.J. Hennig, R. Grün, K. Brunnacker (1983): Speleothems, travertines and
paleoclimates.- Quaternary Research, 20, 1-29.
C.A. Hill, P. Forti (1986): Cave Minerals of the World.- Huntsville, ALA: Nat.
Speleo. Soc.
G.W. Moore (1952): Speleothem-a new cave term.- Natl. Speleol. Soc. News,
11, 4, 1-3.
W.B. White (1976): Cave minerals and speleothems.- IN: T.D. Ford and C. H.D.
Cullingford (eds.): The science of Speleology. London: Academic Press,
pp. 593.
W.M. White (1997): Geochemistry.- online textbook Cornell University.