Oxygen isotopic exchange potentially provides a tempera- ture indicator back through 80% of Earth’s history. Cherts
0
Largest values at a given time define the upper envelope, which may provide (for certain conditions) the surface temperature
500 40 Phanerozoic
Proterozoic
Archean 38
36 34 32 30 28 26 24 22 δ18O
20 18 16 14 12 10 8 6 4 2 0
−2
1,000 1,500 2,000
Time before present (millions of years)
2,500 3,000 3,500 4,000
Figure 6.3A possible temperature history of the Earth is reflected in the oxygen isotopic ratio in samples of chert. Plotted against time in Earth’s history is the so-calledδ18O, which is the difference of the ratio18O/16O measured in the sample to that in a reference standard, normalized by the reference standard. As is common to avoid having to use decimals and zeros, the values are all multiplied by 1,000. As discussed in the text, an estimate for the surface temperature of the Earth is given by the upper envelope of the curves. Geologic era, which we introduce in Chapter 8, are labeled and their boundaries indicated by dashed lines. From Knauth (2005).
are hard rocks, composed largely of very-fine-grained silica.
Silica is formed from silicon and oxygen: SiO2. It can occur as bands in limestones, as nodules, or in other physical forms.
Cherts form in a wide range of environments, precipitating directly out of rivers or ocean waters, or forming from rocks that are subjected to mild increases in temperature and pres- sure.Biogenicchert, that is, chert made by organisms such as sponges or radiolaria that secrete silica, is probably the most abundant.
Of interest to us here is that the oxygen isotopic content of the chert bears a definite relationship to that of the envi- ronment in which it is made. If precipitation occurs in an ocean environment, then the18O content of the silica decreases with increasing temperature in a manner that can be quanti- fied in the laboratory. Essentially, the chert, which preserves very well as a sediment through time, acts to record the ambi- ent water temperature through the oxygen isotopic enhance- ment during its formation. High temperatures in the water from which the chert is precipitated lead to lower 18O/16O ratios in the chert, while low temperatures lead to high18O/16O ratios.
Unfortunately, using cherts as indicators of the surface tem- perature of Earth is extremely complicated because cherts form in so many different environments and the18O/16O values may be altered in ways that have nothing to do with the surface temperature. In the 1970s geochemists Paul Knauth at Arizona State University and Donald Lowe at Louisiana State University attempted to use cherts to determine ancient ocean temperatures in spite of these difficulties. They argued that, for most (but not all) types of chert, processes during or after formation would tend to lower the18O/16O enhancement in cherts relative to the
value obtained during precipitation from ocean waters. There- fore, for a collection of cherts of a given age, the cherts with the highest18O/16O values should most nearly reflect equilibra- tion with ocean waters during formation. Hence, the cherts with the highest18O/16O value at a given time provide a measure of Earth’s ocean temperature.
A relative temperature history of the Earth from cherts is shown in Figure 6.3 from Knauth (2005). If one calibrates the oxygen isotopic data with the recent temperatures of 10 to 15◦C, the drop in18O/16O going back in time implies a temperature of 55 to 85◦C in the first third of the Earth’s history. Particularly striking is the sharp change in global temperature at about 2.5 billion years ago. This sharp drop in temperature occurs roughly in the time range where other types of geologic evidence sug- gest that the Earth experienced at least two episodes of dramatic global cooling in which ice covered much of the Earth (“snow- ball Earth”; see Chapter 19). Also, as Knauth (2005) points out, the earliest life forms, based on study of the relationships in the genome of organisms existing today (Chapter 12), preferred environments as warm as those implied by the earliest chert data.
Some qualifications must be applied to this analysis. First, the chert samples were formed over a range of latitudes, leading to the concern that one is mixing latitudinal and time variations in temperature. As we discuss in Chapter 19, however, warmer ice-free climates, which have dominated over most of Earth’s history, experienced much less variation of temperature with lat- itude than we experience today. The second issue is more seri- ous, and has to do with whether the oceanic value of18O/16O has really been constant over time. One source of variation are the episodes of massive glaciation interspersed throughout Earth’s
history that are mentioned above. Formation of glaciers alters the baseline18O/16O value in the oceans. Further, the baseline
18O/16O abundance may have been lower than today’s during the first quarter of Earth’s history, based on the chemistry of the most ancient cherts, which would explain the discontinuity in
18O/16O at 2.5 billion years ago without invoking a sharp drop in temperature. Finally, one must remember that the surface tem- perature is interpreted to be provided by the chert samples with the highest18O/16O out of a large range. There is no guarantee that the uppermost value actually corresponds to the ambient sur- face temperature, even though this is a reasonable assumption for the later data. If areas of high geothermal activity were more prevalent in the ancient past than today, it is more likely that the older samples, even those with the largest18O/16O, are affected by, or indeed predominantly reflect the environment of, these hot spots. That many of the samples reflect conditions in and near hot spots, such as hydrothermal vents on the ocean floor, is supported by measurement of silicon isotopic ratios, which vary in the chert samples over time in a way that suggests alteration in hot vents.
The chert story illustrates how important it is to be cautious and skeptical when extracting conclusions from a single type of data. Unfortunately, other evidence for Earth’s temperatures in the first quarter of its history is extremely sketchy. The intrigu- ing conclusion from the chert data, that at least portions of the Earth’s oceans were as warm if not warmer than today, is
consistent, at least, with the extensive geologic evidence that liquid water was stable on Earth at least 3.8 billion years ago.
Although this may not seem surprising, it is of some significance because stellar models argue that the Sun was much less lumi- nous at that time than it is today. The geologic record tells us that this early warm period was punctuated (or even terminated) by glacial epochs, in which ice covered much or perhaps all of the Earth, beginning about 2.9 billion years ago. Evidently the Earth’s climate underwent an adjustment that we do not under- stand, or periods dominated by ice existed earlier (but were not recorded in the chert data); however, the geologic evidence is too scant to record this.
We discuss thefaint early Sunproblem in Chapter 14. For now, it suffices to note that the interpretation of the oxygen isotopes in chert as indicating high surface temperatures on the early Earth create a paradox because they require early Earth to have been significantly warmer than at present when, in fact, the Sun was significantly dimmer. Spacecraft images of Mars and direct measurement of rocks at the Martian surface also indicate that our neighboring planet was hotter earlier in its history than it is today.
This dual-planet dilemma regarding dramatic climate change early in the history of the planets, in the face of the faintness of the Sun at the time, represents a major puzzle that we must tackle later in the book.
Summary
Stable isotopes of major elements, that is, isotopes that do not decay measurably over Earth’s history, can be used to track the climate history of our planet. In order to use stable isotopes, the given element must be commonly present in sediments or life forms, it must have more than one stable isotope whose separation depends on temperature, the altered isotopic ratio must be preserved in a time-ordered or datable way for a long time, and the isotopic ratios must be measurable. For recent cli- mate, isotopes of carbon, oxygen, and hydrogen are available.
Carbon has two stable isotopes, of mass 12 and 13, respec- tively, and the lighter isotope is preferentially incorporated from atmospheric carbon dioxide into carbohydrates produced in plants by photosynthesis. Thus, during warm periods, when less land is covered by ice and more rainfall occurs, allowing more plant activity, the ratio of the heavier to the lighter iso- tope,13C to12C, is enriched in the atmospheric carbon dioxide.
This record is preserved by shell-forming organisms that take up the atmospheric carbon for their shells, and upon dying become part of the sediments on the ocean floor. Oxygen and hydrogen isotopes record climate based on the difference in
propensity for evaporation between the lighter and heavier isotopes. Because the temperature drops more steeply from equator to pole during cold periods relative to warm ones, preferentially more of the lighter isotope is extracted from the ocean water in cold times and sequestered at the poles as ice.
Thus, in colder times the enrichment of the heavier isotope in the ocean water is larger than in warmer times. For oxygen this is recorded in shells; for hydrogen the record is in the cores of ice deposited during colder climates and preserved in Antarctica and elsewhere. An oxygen isotopic record of ancient climate exists in silicon–oxygen rocks called cherts. The cherts form by precipitation from ocean water, and the isotopic ratio of oxygen is altered as a function of temperature during the precipitation into the mineral phase. The cherts suggest that temperatures in the ocean were higher in the early history of the Earth than is the case today; however, the chert record may be contaminated by a number of factors other than the mean ocean temperature, including the effect of high-temperature
“hydrothermal” vents.
Questions
1. Why is it important to use more than one isotopic system to determine the history of Earth’s surface temperature?
2. What is it about the possible differences between12C and
13C that would lead to a preferential uptake by planets of the former compared to the latter? Is this a phenomenon of chemistry? If so, could abiotic chemical processes in, for example, the atmospheres of Mars (Chapter 15) or Saturn’s moon Titan (Chapter 16) exhibit the same sort of fraction- ation of the isotopes? Or is this a possible way to detect biological versus purely abiotic chemical processes?
3. The carbon and oxygen isotopic ratios in shell-forming organisms is not entirely independent of oceanic conditions;
rather, these ratios might be altered by the amount of hydro- gen carbonate ions (ions with the formula HCO−3 historically
called bicarbonates) in the oceans (see Chapter 14 for a dis- cussion of the chemistry). As the carbonate ions (which have the formula CO−3) become more abundant in the ocean, the values of13C/12C and18O/16O decrease in the shells. Since hydrogen carbonate ions are primarily produced from ero- sion of rock by rainfall, and then ends up in the oceans by river runoff, what might you predict would be the direction of this effect given that rainfall is decreased during colder epochs? How might you correct for this effect in determining past ocean temperatures from the shells of organisms?
4. In using cherts to determine global temperatures in the past, how would you test the claim that the oceanic18O value has been constant over Earth’s history?
General reading
Considine, D. M. (ed.) 1983. Cherts. InVan Nostrand’s Scientific Encyclopedia. Van Nostrand Reinhold, New York, p. 624.
Kasting, J. F. and Kirschvink, J. 2012. Evolution of a habitable planet. InFrontiers of Astrobiologyed. C. Impey, J. Lunine and J. Funes. Cambridge University Press, in press.
References
Jouzel, J. and Merlivat, L. 1984. Deuterium and oxygen 18 in precip- itation: modeling of the isotopic effects during snow formation.
Journal of Geophysical Research89, 11,749–57.
Knauth L. P. 2005. Temperature and salinity history of the Precam- brian ocean: implications for the course of microbial evolution.
Palaeogeography, Palaeoclimatology, Palaeoecology219, 53–
69.
Knauth, L. P. and Lowe, D. R. 1978. Oxygen isotope geochem- istry of cherts from the Onverwachte group (3.4 billion years), Transvaal, South Africa, with implications for secular varia- tions in the isotopic composition of cherts.Earth and Planetary Science Letters41, 209–22.
P¨alike, H. and Hilgen, F. 2008. Rock clock synchronization.Nature Geoscience1, 282.
Prahl, F. G. and Wakeham, S. G. 1987. Calibration of long- chain alkenones as indicators of paleoceaongraphic conditions.
Nature330, 367–9.
Shackleton, N. J. 1986. Paleogene stable isotope events.Paleogeog- raphy, Paleoclimatology, Paleoecology57, 91–102.
Spero, H. J., Bijma, J., Lea, D. W., and Bemis, B. E. 1997. Effect of seawater carbonate concentration on foraminiferal carbon and oxygen isotopes.Nature390, 497–500.
Van den Boorn, S. H. J. M., van Bergen, M. J., Nijman, W., and Vroon, P. Z. 2007. Dual role of seawater and hydrothermal fluids in Early Archean chert formation: evidence from silicon isotopes.Geology35, 939–42.
Vostok Project Members. 1995. International effort helps decipher mysteries of paleoclimate from Antarctic ice cores.EOS76, 169.
7
Relative age dating of cosmic and terrestrial events: the cratering record
Introduction
The absolute dating techniques of Chapter 5 rely on very pre- cise laboratory analyses of rock samples. For Earth, an abun- dance of accessible samples exists. However, with respect to the rest of the solar system, only meteorites, small bits of aster- oidal and cometary debris – interplanetary dust particles (IDP), and samples from the Moon have been delivered to terres- trial laboratories for age analyses. One class of meteorites, the Shergottites–Nakhlites–Chassigny (SNC), may have been ejected from Mars by collision with one or several asteroids.
Aside from these cases, we have no known samples of mate- rial from large bodies in the solar system and thus cannot date major geologic events on the surfaces of the bodies in an abso- lute fashion.
Instead, scientists userelativedating techniques to infer time histories of the moons and planets in the solar system, and they rely primarily on the record of bombardment, or cra- tering, of the surfaces of these bodies. We describe this technique and the physics of cratering in the present chap- ter. In addition to providing a foundation for inferring key aspects of the solar system’s history, this discussion provides a good foundation for the presentation in Chapter 8 of rela- tive age dating on Earth, which relies on geologic processes other than cratering but for which the principles are much the same.