Instrumental records of climate - that is, records of measurements of temperature and other quantities by scientific instruments - date back at most a few hundred years. The first accurate thermometer was invented in 1654 by Ferdinando II de'Medici, and two hundred years passed before anything like a global network of reliable temperature measuring stations began to become available. Written historical records of such events as frost dates, encounters with sea ice and depictions of mountain glacier length provide some information about the climate of the past few millennia, but for the most part one must rely on climate proxies for information on what climate was doing a century or more ago. A climate proxy is any measurable thing preserved in the geological record of Earth or other planets, from which some aspect of climate can be inferred; to be useful, a climate proxy must come with a chronology, that is, some means of telling what period the proxy dates to. There is a vast and ever-improving array of climate proxies. We have encountered a few already. For example, the existence of river networks on the surface of Mars tells us that at some time in the past the surface of Mars must have been warm enough to support a liquid (probably water) flowing for long distances along the surface; in this case, the chronology at the time of writing comes from counting the number of craters superimposed on the features. Similarly, the existence of marine sedimentary deposits and stromatolites during the Archean provides compelling evidence that the Earth was warm enough to support open ocean water through much of its early history.
Some of the more intuitive kinds of proxies derive from plants and animals that live on land, since physiology places certain constraints on the conditions in which various organisms can grow or thrive. The presence of cold-blooded animals like crocodiles is a sure sign that winters cannot have been much below freezing for extended periods of time. Some kind of plants require tropical conditions to survive, while others require cooler conditions; even the shape of leaves provides information about temperature. A great deal of this evidence is preserved in the fossil record. Where there are trees, the width of tree rings provides a record of annual variations in the temperature of the growing season (though it is sometimes hard to distinguish temperature from rainfall effects). Land-based proxies are only available after the time at which a fairly diverse ecosystem established itself on land. While the first primitive plants colonized land as early as 470 million years ago (in the Ordovician period), and the first plants with stem structures able to conduct water appeared perhaps 430 million years ago (the Silurian period, a diverse land ecosystem did not really get underway until the Devonian period, which began about 416 million years ago. Once plants colonized land, animals in search of a free lunch followed not long afterwards.
One of the richest sources of information about past climates of the Earth comes from material preserved in sea floor sediments. Sediment is laid down in layers like the pages of a book, in which the history of Earth's climate can be read. The Earth is a dynamic planet, and sea floor is constantly being re-created at mid-ocean ridges, and likewise pulled down into the Earth's interior for recycling at subduction zones. For this reason, the deep-ocean marine sediment record is mostly limited to the past 100 million years, and is rather sketchy for the first half of that period. The oldest remaining deep-sea floor is about 180 million years old, and there is precious little of that. Near-shore deposits on continental shelves, on the other hand, can be uplifted and preserved for hundreds of millions, or even billions, of years. Many of the key marine deposits that tell us about the climate of the Neoproterozoic (about 600 million years ago) are now high and dry in Namibia, while others are found in Arctic Canada. These various deposits have a lot of individual personality and are known to geologists by names such as the Acasta Gneiss, the Warrawoona Formation, the Akademikerbreen or the Isua Greenstone Belt; we've met many of these already in the preceding sections.
Numerous aspects of the sedimentary record have been used to infer past conditions, and the ingenuity of paleoclimatologists is adding to the list all the time. Some sedimentary proxies involve the physical structure of the sediments, and are independent of chemistry or biology. Diamictites are a class of sediments known to be carried by land glaciers discharging into the ocean. They are a sure sign of very cold conditions, since it is only in such cases that a glacier can survive to sea level. Ice-rafted debris (IRD) is coarse material that can only be carried offshore by hitching a ride on icebergs or sea-ice. Dropstones - individual stones the size of pebbles and larger which fall with enough force to deform sediment layers - are a particularly striking form of ice-rafted debris. Inasmuch as stones do not float, they present very convincing evidence for ice. Continental dust in sediments provides an indication of the strength of wind, since larger particles require stronger winds to loft and transport them; the mineral composition of the dust can often mark where the dust came from, and hence the direction of the wind. Surveying the species of fossil algae found in sediments can provide information about the temperature of the layers in which the algae grew, since some organisms require colder temperatures while other require warmer temperatures.
A great deal of information can be gleaned from the chemical composition of the ocean, and of the microscopic creatures which dwell within it. Some chemical proxies do not rely on biology, and others make use of organisms mainly as nearly passive recorders of the composition of the ocean. Still other proxies are more intimately tied to biology, through the effect of temperature on the rate at which an organisms makes use of one element in preference to another. For example, the ratio of Magnesium (Mg) to Calcium (Ca) in corals and in the shells of certain micro-organisms is dependent on the temperature at which the organisms grew. The Strontium-Calcium substitution has been used similarly. Organic molecular proxies represent an important new source of data; it has been discovered that certain kinds of micro-organisms produce long-chain organic molecules with a somewhat variable chain-length which depends on the temperature at which the organism grows. When these molecules are robust enough to be preserved in the sedimentary record, the ratio of chain-lengths can be used to infer past temperatures. Alkenone and Tex-86 proxies fall into this class. They are useful only over time spans for which organisms producing the molecules exist, and can be presumed to have similar biochemistry to modern analogue organisms. Both alkenones and Tex-86 have been used to probe climates lying many tens of millions of years in the past, as well as more recent climates of the past few hundred thousand years.
If the chemical composition of a sedimentary sample has been altered by interaction with ocean water at some time after the sediment was first formed, then the information the sediment provides about past conditions is severely compromised. Post-depositional alteration is known as diagenesis, and it is a problem that plagues interpretation of geochemical sedimentary proxies. Paleoceanographers have had to become very clever sedimentary detectives in order to check for the nefarious effects of diagenesis, particularly when dealing with samples more than a few million years old. In some cases the existence of diagenesis has gone undetected for a decade or more, as will be discussed later in connection with the problem of hothouse climates.
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