Creation Science Explained
How to put the 'Paleo' in Paleoclimatology:
Isotopic Records from Speleothems
Jonathan Baker, M.S. Geology
Caves are perhaps the most fascinating recorders of Earth's recent
climate. Though not the most popular proxy—being stuck in a world of
paleoclimatology where tree rings and ice, lake, and marine cores
make all of the headlines—caves have the potential to record
rainfall and soil data at high resolution for thousands of years.
The results are not only locked away in dark rooms, safe from the
elements, but are contained within some of the most beautiful rock
formations known to us:
And that is why we take hammers to them, saw them in half, and mount
them on a micro-drilling stage in the isotope geochemistry lab.
Paleoclimate records from stalagmites
By way of preface, I am slightly biased in my attitude, because I've
spent the past year analyzing isotopic records from stalagmites
around North America. But if you were to consider my position for a
moment, I don't think you would disagree. Consider, for example, how
a cave forms. Precipitation (or spring meltwater) trickles down
through a carbonate aquifer, picking up metal cations (like calcium)
and bicarbonate anions along the way. Steady drips of groundwater
quickly lose their carbonate concentration to the cave atmosphere by
-degassing as they hang from the cave roof (or from
stalactites). When the drip hits the floor, further degassing
initiates the precipitation of aragonite or calcite (CaCO3
Give the process tens to hundreds to thousands of years, and you
have a stalagmite with concentric laminae that reach toward the
As it turns out, the carbon and oxygen isotopic chemistry of the
laminae depends primarily on rainfall source and amount, as well as
soil activity. We can test these hypotheses by comparing isotopic
records from very recently formed stalagmites with
human/instrumental climate records, or by comparing the isotopic
chemistry of rainwater to dripwater to aragonite in stalagmites over
several years. In general, oxygen isotopes are depleted in 18
(heavy oxygen) during wet periods and enriched in 18
during dry periods, but the source of precipitation also plays a
role (high vs. low latitude; Atlantic vs. Pacific). Therefore,
speleothem records from North America record not only rainfall
amount, but migration of the Gulf Stream, El Niño cycles, and other
Depending on the residence time of the aquifer (i.e. how long, on
average, the water takes to get from rainfall to 'cave'-fall), the
groundwater will mix thoroughly with that from the past month to the
past several years. This means that isotopic inputs from rainfall
represent a weighted average for that time interval—good news for
the paleoclimatologist. Also, most carbonate ions in groundwater are
dissolved within the upper soil horizons during the wet season, so
one may track soil processes as well.
Both the hydrological and geochemical processes behind speleothem
formation are now very well understood. With few exceptions,
stalagmites have been proven faithful proxies of climate. If the
sampling process were not so destructive, I believe they would also
gain some popularity.
High-resolution age dating of speleothems: answering the 'when'
of cave formation
Understanding the climatic significance of isotopic ratios in
stalagmites is great, but unless we know when
formed, the records are quite useless. So how does one discern the
'paleo' in paleoclimate? If you've ever had the opportunity to visit
a cave set up for guided tours (Cave of the Winds, Colorado and
Timpanogos Cave, Utah are on my list), the tour guide likely pointed
out a speleothem that had been measured over time: "You see, 50
years ago, this guy was 5 cm shorter! So stalagmites grow about 1 mm
per year, and since now it's 105 cm tall, it must have been growing
This approach is simple and intuitive, and in some cases may provide
a decent approximation of stalagmite growth. But the fact is, the
rate of growth for individual stalagmites can vary over time, due to
fluctuations in climate. For example, high amounts of rainfall and
soil activity can promote speleothem growth. Low ambient CO2
and high ambient temperature in the cave can also promote growth by
increasing the rate of precipitation in each drop. Since we know all
of these factors will change over the life of a speleothem, we need
a more precise method of dating.
Unfortunately, the popular notion that stalagmite growth-rates are
simply extrapolated, like above, has caused young-Earth critics to
focus on examples of
rapid stalactite growth
—to make that case that
are compatible with a young-Earth, Flood model.
But the arguments typically go like this: we know that speleothems
can form rapidly under favorable conditions; therefore, all
speleothems formed rapidly under favorable conditions. The informal
logical fallacy is rarely challenged, because few people are
familiar with actual method used to date speleothems.
Uranium-thorium (U-series) dating of speleothems
Most speleothems are originally precipitated as aragonite (calcium
carbonate). But like any mineral, the aragonite is bound to contain
some impurities. Magnesium, strontium, sodium, barium, and lithium
are incorporated in trace amounts. As an aside, the ratio of calcium
to these trace elements serves as an independent proxy of climate,
occasionally used by ambitious geochemists. One of the most
important trace elements, however, is uranium.
Why uranium? Because uranium is radioactive, and decays into thorium
at a constant, known rate. By analyzing the current ratio of uranium
and thorium isotopes, one can estimate the absolute age of laminae
in speleothems. More specifically, the ratio of 234
(parent) to 230
Th (daughter) is measured. But the ratio
does not change like an hourglass model with time (as in the
radiocarbon, K-Ar, and U-Pb systems), since the daughter product is
also radioactive, and decays even faster than the parent. Let's take
a closer look.
Money matters: a financial analogy
Imagine that you set up a bank account with $1,000 in savings and $0
in checking. Every month, 1% of the savings amount is transferred to
checking, but 5% of the checking amount is...donated to charity. In
this scenario, the money in savings represents 234-Uranium, and the
money in checking represents 230-Thorium. Both accounts are
constantly decaying at a constant rate, unique to each account, that
depends on the residual balance. The money spent to charity
represents the daughter product of thorium decay, which is neither
measured in the rock nor this analogy.
At the end of the first month, zero dollars are donated to charity,
because the checking account has zero dollars available. But 1%, or
$10, will be transferred from savings to checking. The new balance:
$990 in savings; $10 in checking. So at the end of the second month,
5% of $10, or 50 cents, will be donated to charity, and $9.90
transferred from savings to checking. The new balances: $980.10 in
savings; $19.40 in checking. Easy enough?
In geology, we actually measure the ratio between the isotopes (i.e.
$ in savings divided by $ in checking). If we know the rate of decay
(what % is lost each month), and the original balance in at least
one of the accounts, we can back calculate the time that has passed
since the experiment started. Below, I have plotted the experiment
over 100 months:
The yellow line represents the ratio between the two accounts. As
you can see, the ratio changes very quickly at first, but eventually
flattens out to equilibrium (hence the name "Uranium-Thorium
Disequilibrium Dating"). This means that if one were to estimate the
time passed based on the current ratio between the accounts, that
estimate would be more precise at time = 0–30 months than at time =
30–100 months. Correspondingly, U-Th disequilibrium ages are most
precise up to ~500,000 years, after which the change in 234
is too small to be detected.
Another limit occurs in very young samples, since the mass
spectrometer is unable to detect thorium at exceedingly low
concentrations. Thus ideal samples are uranium-rich to begin with,
and are at least several years to several thousand years old.
Personally, I have seen very precise (±1%) age estimates from U-rich
samples, however, even between 0 and 100 years old.
Depending on the scientific importance of the sample, and given that
each age datum costs ~$500 to analyze, between 2 and 20 U-Th dates
are taken along the growth axis. This allows the paleoclimatologist
to construct an age model for each speleothem, and attach real ages
to isotopic records.
But aren't there a few assumptions involved?
Yes, some assumptions are made. That is how science progresses. But
fortunately for us, most of those assumptions can be
1) How do we know the initial ratio of U/Th isotopes? In oxic
environments, uranium is fairly soluble and thorium is very
insoluble. Since stalagmites form out of dissolved constituents of
groundwater, we should expect very little, if any, thorium to be
originally present (i.e. $0 in checking).
2) Does this assumption always hold? On the contrary, we expect this
assumption never to hold, in the absolute sense. There will always
be at least some
thorium present. So to account for this, we
measure the ratio of 238
U to 232
Th (two common
isotopes). Both isotopes are radioactive, but their half-lives (4.5
and 14.05 billion years, respectively) are much longer than that of
Th (75,380 years), and may be considered stable on
shorter geologic timescales. Using the 238
ratio, the 232
Th ratio, and the total
concentration of uranium, we can estimate the initial concentration
of 230-thorium. Typically, this value is insignificant, and will
only change the age estimates by a maximum of 1% if left
. To put this in perspective, imagine that I started
the experiment above with $1.50 in checking. In this case, the age
estimate would be off by less than a few days.
3) How do we know whether any uranium or thorium was lost since
crystallization? In speleothems, this is rarely a concern, since
most ages fit very well into a growth model (i.e. they get
progressively older along the axis, and result in globally
correlated paleoclimate records). But if this assumption were
challenged, one could use trace element data, petrography, and
cathodoluminescence to test whether recrystallization of the
speleothem caused a loss of soluble trace-elements. Also, any loss
of uranium is likely to be localized, through microfractures in the
speleothem. In this case, model ages taken from those points will
show up as anomalous, and result in an unrealistic growth-rate
curve. It is simply unreasonable to expect that uranium loss
occurred systematically, shifting all the ages by a proportional
4) How do we know the decay rates for both isotopes has remained the
same? This is a matter of quantum physics, and a sound one at that.
There is no reason to expect decay rates to change. If this were to
happen, however, during the life of the speleothem, then the growth
model would shift dramatically at a point, making it appear as
though the speleothem started to grow many times faster or slower.
Are caves and speleothems consistent with the Flood model?
In short, no. The Flood model must consider modern caves and
speleothems as post-Flood features. Even if one were to allow for
the unrealistic scenario of accelerated nuclear decay during, the
caveat would not apply to speleothems. Since thousands of
speleothems have been dated beyond 5,000 years, there remains a
significant challenge to young-Earth Flood geologists.
We can also consider speleothem records in the larger climatic
context. For example, speleothem records match up very well with ice
core records (dated by counting annual layers), marine/lake core
records (dated by counting annual layers and radiocarbon methods),
and tree ring records (same as above). Thus we have multiple
independent methods yielding essentially the same result. Such
concordance highly corroborates the use of each method to track the
Earth's climate history, and thoroughly falsifies the Flood model.
This article was originally posted by Jonathan Baker on his blog,
Questioning Answers in Genesis