Temperatures from Fossil Shells
Climate science required the invention and mastery of many difficult techniques.
These had pitfalls, which could lead to controversy. An example of the ingenious technical work
and hard-fought debates underlying the main story is the use of fossil shells to find the
temperature of oceans in the distant past. For other examples, see the essays on Uses of Radiocarbon Dating and Arakawa's Computation Device.
| The oceans swarm with tiny plankton,
including countless foraminifera (nicknamed "forams"), single-celled
animals that scavenge with pseudopods wiggling through holes in their
shells. When forams die, their tiny shells drift down into the ooze
of the seabed and there endure for ages, so numerous in some places
that they form thick deposits of chalk or limestone. Different species
can be identified under the microscope by the striking architecture
of their shells, as elaborate as candelabra. Wolfgang Schott, inspecting
findings of the German Meteor oceanographic expedition of
1925-27, realized that the species whose shells were found in the
muck of the seabed depended sensitively on the temperature of the
water where the creatures had lived. The mix of foram species could
serve as a thermometer of past climates.(1)
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A typical foram
| In the 1950s, the nuclear chemist
Harold Urey devised another way to use the shells to measure ancient
temperatures. He found he could take the temperature of an ancient
ocean by measuring the oxygen that forams built into their shells.
The rare isotope oxygen-18 is a bit heavier than normal oxygen-16,
and biologists had shown that the amount of each isotope that a foram
takes up varies with the temperature of the water. The isotopes were
fossilized with the shells, and the ratio of isotopes (O18 to O16)
could be determined with the new and exacting techniques of mass spectrometry.(2)Urey
and his team at the University of Chicago refined these tools, developed
for nuclear studies, and applied them to calcite in fossils. They
found plausible temperatures clear back to the Cretaceous era,
more than 100 million years ago.(3)
| Many problems had to be solved along the way. First of all, cores
had to be extracted from the sticky sediments of the ocean floor without
disturbing the layers. Börge Kullenberg solved the problem for
a Swedish Deep Sea Expedition in 1947. He put a piston inside a tube
and pulled the piston up to suck in sediment while the tube was being
shoved into the seabed. Kullenberg could recover cores more than 20
meters long. Back in the lab, somebody would put a sample of the muck
under a microscope and tease out a few hundred of the shells, each
no bigger than the period at the end of this sentence. Next, pure
carbon dioxide gas had to be extracted from the organic compounds
without altering the ratio of isotopes, typically by grinding down
the shells and roasting the powder in a stream of helium gas. Then
the oxygen isotopes in the gas could be analyzed in a mass spectrometer.
Workers needed to be very careful to avoid contamination by any other
source of gas, such as their own breath. All these processes had to
be reduced to a routine that a lab technician could execute reliably
hundreds of times over. Urey, already a Nobel Prize winner, called
it "the toughest chemical problem I ever faced."(4*)
| In 1955 Urey’s
student Cesare Emiliani conquered the problem. His data lay within
slimy cylinders of mud and clay, totaling hundreds of meters in length,
extracted from the seabed in recent years and carefully stored away
in oceanographic institutions. Emiliani relied on the loan of cores
pulled up by various expeditions (in this case from the Oceanographic
Institute at Göteborg, Sweden, the Lamont Geological Observatory
in New York and the Scripps Institution of Oceanography in California).
He was also helped by advice about locations on the sea-floor where
the sediments had been laid down most regularly and continuously.
And he required technical assistance on chemistry and so forth from
fellow members of Urey's research group. From each of many hundreds
of layers of sediment in the cores, he took a sample and found its
oxygen-isotope temperature. The product was a remarkable record of
temperature changes stretching back nearly 300,000 years.(5)
This sort of work needed money. Emiliani, for example, received funds from the U.S. Office of
Naval Research, the Atomic Energy Commission, and the National Science Foundation. The
work also needed a stock of cores, carefully preserved in their thousands (eventually a great
many thousands) as long rods of damp clay. One pioneer of climate history studies, Nicholas
Shackleton in England, used cores drilled by the pathbreaking Challenger
expedition, a century back, before turning to Lamont cores. The autocratic founder and director
of Lamont, Maurice Ewing, had insisted that the institution's two ships pull up cores regularly
wherever they happened to wander on the seven seas. He was criticized for taking more cores
than anyone could analyze, but he stored them up on the principle that somebody, someday,
would learn something from them.(6)
| While Emiliani and others pursued the oxygen
isotopes, others continued to estimate temperatures from the assemblage
of foram species found in a given layer in a core. Carbon-14 dating
could lay out an accurate chronology for the changes — provided
one could assemble a collaboration including oceanographers willing
to share their deep-sea cores, a radiochemistry laboratory capable
of determining the dates, and an expert in the arcane skill of foram
shell identification. A leading such expert, David Ericson, collaborated
with others at Lamont to dispute Emiliani's temperature curves. They
cast doubt, first, on his timing of the changes, and second, more
serious still, on his claim that past ice ages had brought a huge
drop in the Caribbean Sea's temperature. Emiliani in return cast doubt
on Lamont's reports, pointing out among other things how the smearing
of sediment layers by burrowing worms ("bioturbation") could have
biased their results. Debate broke out at scientific meetings and
in journals, where Emiliani strenuously defended his position, loth
to admit error. Corrections that an outside observer may find unproblematic
can strike some scientists as an unjust attack on their proudest accomplishments.
| Such debates were nothing new. The ice ages, as one expert ruefully
remarked, had been "a battleground for scientists" for over a century.
A main reason for the fierce controversies, he suggested, was the
"close connection with the evolution of man."(7) Many people, including in particular Emiliani,
thought that the timing of ice ages must say something fascinating
and important about how our species had emerged.(8)
A decade passed before scientists resolved the discrepancy between oxygen-isotope and foram
temperatures. There were many possible sources of confusion. In particular, as Emiliani himself
had noted, the temperature you got from a given species of foram reflected where it lived in the
sea, whether near the surface or in the colder depths. You had to take it on faith that in the distant
past a given species had lived at the same depth as its present-day descendants, despite any
changes in currents, salinity, etc. But the problem turned out to be even subtler. The long-term
fluctuations of the oxygen isotopes in foram shells were not caused mainly by changes of the
water's temperature at all. Alongside the biophysics of the forams, another kind of physics had
been at work.
| Back in 1954, Willi Dansgaard had pointed
out that the ratio of O18 to O16 in snow would depend on a variety
of factors. He had realized that processes in clouds might act like
the distilleries that people used to concentrate particular isotopes.
The processes would be influenced by the temperature of the seas where
the water evaporated, for the heavy isotope would have a harder time
evaporating than the lighter one. You also had to consider what happened
as the water traveled on the winds, and the temperature of the clouds
where the moisture crystallized into snow.(9) When snowfall built up continental ice sheets, the process
had withdrawn from the oceans more of the lighter isotope than the
heavier one. Thus no matter what the temperature of the water where
the forams lived, during a glacial period their shells wound up with
less of the lighter isotope. The changes that Emiliani had detected
reflected the changing volume of the planet's ice sheets. (Over later decades more refined analysis of how temperature and ice volume changes affect the isotope balance found the weight of each depended on complex circumstances; on average each accounts for about half the isotope change.)
| Emiliani discussed the problem at some length
in his 1955 paper, but he had not thought a large correction was needed.(10) For decades he vigorously defended his original conclusions.
Other scientists gradually decided he was mistaken. The tipoff was
the fact that the isotope variation was found not just in regions
where cooling would be expected, but everywhere in the world's oceans.
In fact, the water temperature in the Caribbean had probably dropped
only a reasonable 2°C or so. Nevertheless, as one of Emiliani's
critics acknowledged, his work remained "of inestimable value." Indeed
"its value is in a sense enhanced by the certainty that it is a time-sequence
for terrestrial glacial events, rather than oceanographic events."(11*)
There was nothing extraordinary in such a combination of discovery and error. Every great
scientific paper is written at the outside edge of what can be known, and deserves to be
remembered if there is a nugget of value amid the inevitable confusion.
| Meanwhile the techniques advanced. For example,
Shackleton spent a decade working out a way to measure oxygen isotopes
in minuscule samples, combining tireless attention to detail with
ingenious detective work. (Among other things, he discovered that
his instrument "remembered" previous measurements, for its copper
tubing absorbed a trace of oxygen. He had to replace the copper with
stainless steel.) This opened the way to detailed measurements on
the less abundant forams that lived in the deep
sea.(12) As an alternative and a check, Ericson
and others continued to develop the "thermometer" given by an assemblage
of foram species. This method had great potential, but also countless
details and pitfalls. The workers learned to notice and exploit subtle
effects, such as a change in the shell's manner of coiling (left-handed
to right-handed) for a particular species at a particular temperature.(13)
Analyzing foram assemblages in the best cores, researchers confirmed
the oxygen-isotope results for the precise timing of glacial cycles.(14)
| The advances in technique
inspired a bold plan to create maps of the world's ocean surface temperatures
at given epochs in the past. The complexities were so great that it
took another two decades of vehement argument to get even partial
agreement about just what the forams had to tell about ancient temperatures.
As usual in geophysics, no technique stood on its own. It was the
agreement among different types of evidence, once this was finally
thrashed out, that convinced scientists they were getting down to
the true facts.(15)
Past Cycles: Ice Age Speculations
1. Schott (1935).
2. Urey (1947).The biological fractionation of isotopes ("Dole effect") was first noted by Dole (1936).
3. Urey et al. (1950).
4. "Toughest:" Emiliani (1958),
p. 54; Emiliani (1955), p. 539, lists crucial
breakthroughs: in mass spectrometry (Nier), in extracting carbon dioxide
from the carbonate (McCrea), and in calcium compound extraction (Epstein).
See p. 548. BACK
5. Emiliani (1955).
6. McNutt (2000), pp. 54, 60;
Wertenbaker (1974), pp. 103-05.
7. Ericson and Wollin (1968), p.
8. Emiliani (1956); Emiliani (1958).
9. Dansgaard (1954); Dansgaard (1964).
10. Emiliani (1955), pp. 543-44.
11. The dominance of ice volume was pinned down by Shackleton (1967), quote p. 17; see also Dansgaard and Tauber (1969); Imbrie
and Kipp (1971); Emiliani (1992), complaining that
Shackleton's conclusion was "almost universally (and uncritically) accepted" and arguing that at
least part of the effect is indeed temperature; for discussion Bradley
(1985), pp. 179-80.
12. Shackleton (1965); that
this opened the way is attested by Broecker (1995), p. 285.
13. Changes of direction were reported, but only tentatively
connected with climate shifts, in Ericson et al. (1955).
14. E.g., Ericson and Wollin
(1968); the key work was Imbrie and Kipp (1971); for a
general review of "transfer functions" to deduce temperatures from assemblages of species, see
Sachs et al. (1977).
15. Emiliani and Ericson were involved again. A summary with
references is Bradley (1999), pp. 223-26.
© 2003-2011 Spencer Weart & American Institute of Physics