Earth geologic record

Fig. 9.8 Stratigraphic presentation of 813C values for organic material extracted from the Arundal clay formation, Maryland. Error bars reflect the standard deviation for three replicate analyses. The dashed line represents the boundary between the early and middle Aptian eras ( 125 to 112 megayears BP) established from the geological record. The gray arrow highlights the isotope shift of interest (Reprinted from Jahren, A. H. et al., Earth Planet. Sci. Lett. 236, 691, (2005), Copyright 2005, with permission from Elsevier)...

Radioactive isotopes have thus enabled scientists to reconstruct the history of the Earth and its environs over billions of years. But stable isotopes are also an indispensable part of the geoscientist s inventory. In particular, their measurement in the geological record has revolutionized our perception of how the planet s climate system works and how it has changed over time. 

Much of what is currently known about the Earth s climate comes from the application of stable isotopes collected from ocean drill cores in marine sediments (e.g., Zachos et al. 2001). These isotopic data sets provide detailed records of how the Earth s oceans have responded to changing climate and are extremely valuable in assessing global climate histories down to millennial scales. Similar detailed isotopic records for terrestrial systems are, however, uncommon and frequently continuous terrestrial climate records that span millions to tens of millions of years are not preserved in the terrestrial geologic record. With the advent of paleoaltimetry studies targeted directly at the coupled isotopic effects of changes in climate... 

Figure 9.4 shows one interpretation of the atmospheric CO2 history of Earth over a time period of 100 million years. The geologic record of atmospheric CO2 will be addressed in detail in Chapter 10 of interest here is the possibility that humankind activities of fossil fuel burning and land use practices could lead to future atmospheric CO2 levels rivaling those of the past. Furthermore the future time scale of atmospheric CO2 change may be shorter than any period of CO2 change the Earth has experienced in 100 million years. 

Van Houten F.B. and Bhattacharyya (1982) Phanerozoic oolitic ironstones-geologic record and facies. Ann. Rev. Earth Plan. Sci. 10,441-458. 

Very little is certain on organic matter formation in the atmosphere or in the ocean of the primitive Earth as a result of lightning, volcanism, or other energy sources. Since no preserved geological records exist from the concerned era, there is no certainty about the reactor conditions (physical parameters kind, concentration and/or fluxes of reagents). The endogenous synthesis of... 

The fascination with the abundances of the atomic nuclei is that they inform of ancient events. The events that are recorded in their populations depend upon the material sample in question. In the crust of the Earth, they record its geologic evolution. Silicon in that crust is much more abundant than iron, for example, because the Earth s crust is sandy, whereas its iron sank to the Earth s core during its early molten state. In the Earth s oceans the elemental abundances reflect their solubilities in water. In the Earth s atmosphere, their numbers reflect their volatilities. And so it goes. Such abundance-sets reflect and record the geophysical history of the Earth and the chemical properties of the chemical elements. Atmospheric carbon dioxide (C02) and methane (CH4) record an extra wrinkle, the impact of human beings on the Earth s atmosphere. 

Mars is the only planet, besides Earth, that provides geomorphic evidence for the past operation of a hydrologic cycle. Today, the surface of Mars is cold and dry, but landforms generated by movement and ponding of water or ice indicate that brief but extensive episodes of aqueous activity punctuated the martian geologic record. These features include sinuous valleys. 

It is clear that the carbon cycle and global climate are linked in many ways throughout the history of the Earth. But it is equally apparent that the complex interactions evident in the geologic record defy simple attribution of cause and effect. Collection of more data and further analysis and modeling will continue to improve our understanding of these interactions, which are now so important to the near-term relationship between human activities and the global environment. 

Some of the eroded channels on Mars resemble terrestrial riverbeds, but some show evidence of a violent past. They seem to have been formed by enormous flash floods, perhaps caused when a Martian lake broke through a collapsing natural feature such as a rock wall and cascaded across the land. Several such inciderrts are documerrted in the geologic record c i Earth. 

Alonso-Zarza, A.M. (2003) Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record. Earth-Science Reviews 60, 261-298. 

This improbability is the crux of the matter. The scientific method can be paralyzed by problems that require understanding the very improbable occurrences that result from very, very large numbers of throws of the dice. Sometimes we can understand the statistics of the problem sometimes we cannot. How likely is it that a comet will hit the earth We now have good geological records. How likely is it that a star will explode into a nova There are many, many observable stars, and we now understand the statistics of nova formation quite well. 

How can we know about the early history of the Earth What evidence do we have for this period of Earth history, and how reliable is this evidence In this section we review the nature of the early geological record, enquire about the types of data that might be extracted from it and assess the quality of the information available. Of course in addition to the geological record, there is also much information to be gained about the early Earth from the planetary record. This is discussed in Chapter 2. 

A very specific challenge in the Archaean geological record are those rocks which are without modern counterparts. One example is the ultramafic rock komatiite, first discovered as a lava in the Barberton area of South Africa. At the time of their discovery, in the early 1960s, ultramafic lavas were thought to be an impossibility. Over the past 30 years we have learned a great deal about komatiites and they are discussed in detail in Chapter 3 (Section 3.2.1.2). Nevertheless, why they are more common in the early history of the Earth than at the present is an important scientific question. 

First, the geological record is incomplete. This is particularly true for the first 750 Ma of Earth history, about which we know very little. As was shown in the preceding section, there is very little rock record preserved from before 3.8 Ga. The dynamic nature of our modern planet has destroyed any evidence of... 

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