Estimated Mean Global Temperature
FIGURE I. I Generalized temperature history of the Earth plotted as relative departures from the present global mean from Frakes et al., 1992 . Studying the proxy record of paleodimate is rather like looking through a telescope held the wrong way around for recent periods there is evidence of short-term climatic variations, but these cannot be resolved in earlier periods. It is abundantly clear from the paleodimate record that abrupt changes have occurred in the global climate system at certain...
Dating Methods Involving Chemical Changes
Two general categories of dating methods are based on chemical changes within the samples being studied since they were emplaced. The first involves amino-acid analysis of organic samples, generally used to assess the age of associated inorganic deposits. The method may also be used to estimate paleotemperatures from organic samples of known age. The second category encompasses a number of methods that assess the amount of weathering that an inorganic sample has experienced. They are primarily...
Quantitative Paleoclimatic Reconstructions Based On Pollen Analysis
Paleoclimatic reconstruction from fossil pollen spectra is based on the notion that, as vegetation distribution is largely determined by climate, it should be possible to use that distribution as represented in the fossil pollen spectra to reconstruct past climate. Large databases of surface pollen samples generally from the surface sediments of lakes are now available for many parts of the world see Appendix B and these have enabled pollen assemblages to be calibrated directly in terms of...
Feedback Mechanisms
Interactions within the climate system often involve complex, nonlinear relationships. All components of the climate system are intimately linked or coupled with all other components, such that changes in one subsystem may involve compen satory changes throughout the entire climate system. These changes may amplify the initial disturbance anomaly or dampen it. Interactions that tend to amplify the disturbance are termed positive feedback mechanisms or processes they operate in such a way that...
C Cpdb
A change in 814C of only l o i.e., a change in 813C of 0.5 o corresponds to an age difference of -8 yr. Consequently, if two contemporaneous samples differed in 813C by 25 o, they would appear to have an age difference of -400 years Olsson and Osadebe, 1974 . To avoid such confusion, it has been recommended that the 13C value of all samples be normalized to -25 o, the average value for wood. By adopting this reference value, comparability of dates is possible. This is particularly important in...
Age ka
FIGURE 6.51 An estimate of the rate of glacial meltwater entering the world ocean, derived from the first derivative of the sea-level record shown in Fig. 6.50. Two chronologies are shown one is in ,4C yr, derived from the l4C-dated coral series Fairbanks, 1990 and the other is in calendar years, based on a calibration of the l4C record by 230Th 234U-dates. Two major pulses of meltwater are seen mwp-IA occurred first, mwp-IB occurred later Bard etai, 1990 . event was related to meltwater from...
Former Vegetation Distribution From Punt Macrofossils
It is not uncommon for plant macrofossils to be found far beyond the range of the particular species today. Where climatic controls on present-day plant distributions are known, their former distribution may be interpreted paleoclimatically, from dated macrofossils. Fluctuations of three major biogeographical boundaries have been studied in considerable detail using macrofossils the arctic treeline the alpine treeline and the lower or dryness treeline of semiarid and arid regions. In each case,...
Antarctic Bottom Water
Globorotalia tumida Globorotalia menardii ' Globigerinoides ruber Sphaeroidinella dehiscens' Pulleniatina obliguiloculata Globorotalia tumida Globorotalia menardii ' Globigerinoides ruber Sphaeroidinella dehiscens' Pulleniatina obliguiloculata 5000 FIGURE 6.34 Changes In percentage abundance of several diagnostic planktonic foraminifera in equatorial Atlantic core tops with increasing water depths due to differential dissolution.The more corrosive Antarctica Bottom Water dissolves poorly...
CLIMATE AND CLIMATIC VARIATION Gcy
FIGURE 2.16 Variations of eccentricity, obliquity, precession, and the combination of all three factors ETP over the last 800,000 years with their principal periodic characteristics indicated by the power spectrum to the right of each time series upper diagram . Below is the time series of July solar radiation at 10, 65, and 80 N expressed as departures from A.D. 1950 values . Note that the radiation signal at high latitudes is dominated by the 41,000 year obliquity cycle whereas at lower...
Paleotemperature Records From Alkenones
Certain marine phytoplankton of the class Prymnesiophyceae, most notably the coccolithophorid Emilyania huxleyi, respond to changes in water temperature by altering the molecular composition of their cell membranes. Specifically, as water temperature decreases, they increase the production of unsaturated alkenones ketones . Cells contain a mixture of long-chain alkenones with 37, 38, or 39 carbon atoms n-C37 to n-C39 , which are either di- or tri-unsaturated designated for example, as C37.2 and...
Yearly Pollen Influx
8 years represented by 1 cm thickness 50 000 grains deposited on a cm2 surface each year 25 years represented by 1 cm thickness 800 grains deposited on a cm2 surface each year FIGURE 9.8 Examples of pollen influx pollen flux density calculations from measurements of pollen concentration and the rate of accumulation of the sediment matrix Davis, 1963 . 1993b R. S. Webb et al., 1993 . Consequently, pollen flux density studies have taken a back seat to pollen percentages in most modern...
Ocean Circulation Changes And Climate Over The Last Glacialinterglacial Cycle
Analysis of 813C and Cd Ca in benthic foraminifera and 8lsO in planktic forams reflecting temperature and or salinity changes in surface waters have enabled circulation changes to be reconstructed over the last glacial-interglacial cycle in some detail longer-term changes in 813C are examined by Raymo et al. 1990 . These studies indicate that significant changes in the thermohaline circulation of the oceans have occurred. Although production and circulation of NADW was similar to today in the...
Dissolution Of Deepsea Carbonates
Throughout the deep ocean basins of the world, a major factor affecting preservation of carbonate tests is the rate of dissolution at depth. The oceans are predominantly undersaturated with respect to calcium carbonate at all depths below the upper mixed layer the zone above the thermocline Olausson, 1965, 1967 . After death of the organisms, deposition of the test on the ocean floor leads to dissolution in the undersaturated water Adelsack and Berger, 1977 . Pteropod tests composed of calcium...
Marine Sediments and Corals
6.2 Paleoclimatic Information from Biological Material in Ocean Cores 193 6.3 Oxygen Isotope Studies of Calcareous Marine Fauna 198 6.3.1 Oxygen Isotopic Composition of the Oceans 199 6.3.2 Oxygen Isotope Stratigraphy 206 6.3.4 Sea-level Changes and hlsO 215 6.4 Relative Abundance Studies 216 6.5 Paleotemperature Records from Alkenones 233 6.6 Dissolution of Deep-sea Carbonates 236 6.7 Paleoclimatic Information from Inorganic Material in Ocean Cores 242 6.8 Coral Records of Past Climate 247...
Snowlines And Glaciation Thresholds
In regions of permanent snow accumulation, it is possible to identify an altitudinal zone separating the lower region of seasonal snow accumulation from the upper region of permanent snow. The term zone is used because from year to year the actual boundary or snowline will vary in elevation, depending on the particular FIGURE 7.9 Paleotemperature reconstruction for Europe based on periglacial features right column compared to other long proxy data records Maarleveld, 1976 . FIGURE 7.9...
Stable Isotope Analysis
The study of stable isotopes primarily deuterium and 180 is a major focus of paleodimatic research. Most work has been on stable isotope variations in ice and firn, and in the tests of marine fauna recovered from ocean cores. However, increasing attention is being placed on other natural isotope recorders, such as speleothems FIGURE 5.1 Location of principal ice-coring sites in the Canadian Arctic and Greenland, and in Antarctica. FIGURE 5.1 Location of principal ice-coring sites in the...
Relative Abundance Studies
The possibility of reconstructing paleoclimates by using the relative abundance of a particular species, or species assemblage, in ocean sediment cores was first proposed by Schott 1935 . Schott recognized that variations in the number of Globorotalia menardii a foraminifera characteristic of subtropical and equatorial waters are indicative of alternating cold and warm intervals in the past. However, 30 yr were to elapse before the availability of relatively long undisturbed cores and improved...
Second Edition
University of Massachusetts Amherst, Massachusetts Amsterdam Boston Heidelberg London New York Oxford Paris San Diego San Francisco Singapore Sydney Tokyo Rock Art at Zalat el Hammad, Southeastern Sahara, Northwest Sudan. Rock art from Zalat el Hammad 17 50' N, 26 45' E , a circular group of fractured sandstone hills, in the Wadi Howar, southeastern Sahara. The Wadi 1100 km long and 10 km wide is a now defunct watercourse which 10,000-2000 yr ago was the Nile's largest tributary from the...
Timescales Of Climatic Variation
Climate varies on all timescales and space scales, from interannual climatic variability to very long-period variations related to the evolution of the atmosphere and changes in the lithosphere. Examples of known climatic fluctuations are shown in Fig. 2.14. In this diagram, each row represents an expansion, by a factor of ten, of each interval on the row above it. Thus, one can envisage short-term high-frequency variations nested within long-term lower-frequency variations Webb, 1991 ....
Loess
Loess is a deposit of wind-blown silt that blankets large areas of the continents. It is characteristically creamy-brown in color and calcareous, consisting predominantly of quartz feldspars and micas Pye, 1984, 1987 . Geographically, loess is extensive in the North American Great Plains, south-central Europe, Ukraine, central Asia, China, and Argentina Fig. 7.1 . In North America, loess deposits are related to formerly extensive outwash deposits from the Laurentide ice sheet and to the...
Lake Sediments
Lakes accumulate sediments from their surrounding environment and so sediment cores recovered from lakes can provide a record of environmental change. Accumulation rates in lakes are often high, so lake sediments offer the potential for highresolution records of past climate, providing they can be adequately dated. Lake sediments are made up of two basic components allochthonous material, originating from outside the lake basin and autochthonous material, produced within the lake itself....
Mipt Index Paleoclimatology
When I wrote the first edition of Quaternary Paleoclimatology in the early 1980s the field of paleoclimatology was still in its infancy. Since then there has been an explosion of interest and research on the subject. It is amazing to realize that in the early 1980s AMS radiocarbon dating was hardly being used, we knew nothing of Dansgaard-Oeschger cycles and their relationship to the North Atlantic thermohaline circulation, the significance of Heinrich events had not been fully recognized, the...
Lake Windermere Interstadial 1
FIGURE 8.7 Reconstructed July paleotemperatures based on insect remains in areas of the southern and central British Isles since the last Ipswichian interglacial. Annual temperature ranges are also shown Coope, l977b .The period before 50,000 yr B.R dashed line is very uncertain and there may have been a more gradual, monotonic decline in temperature from 120,000 yr B.R to 60,000 yr B.P. any climatic amelioration Coope, 1975b . Evidently the Coleoptera were sufficiently mobile that they could...
Thermohaline Circulation Of The Oceans
Circulation of water at the ocean surface is largely a response to the overlying atmospheric circulation which exerts drag at the surface. However, circulation of deeper waters in the oceans of the world is a consequence of density variations, which result from differences in temperature and salinity brought about by sensible and latent heat fluxes, precipitation, and runoff at the ocean surface this is termed the thermohaline circulation. In areas where surface waters become relatively dense,...
Speleothems
Speleothems are mineral formations occurring in limestone caves, most commonly as stalagmites and stalactites, or slab-like deposits known as flowstones. They are composed primarily of calcium carbonate, precipitated from ground water that has percolated through the adjacent carbonate host rock. Certain trace elements may also be present often giving the deposit a characteristic color one of these, uranium, can be used to determine the age of a speleothem, as discussed in what follows....
Maps of Modern Pollen Data
Extensive studies of modern pollen rain and modern vegetation have been made for North America by Davis and Webb 1975 , Webb and McAndrews 1976 , Webb et al., 1978 , and Delcourt et al., 1984 , and for Europe by Huntley and Birks 1983 . In these studies, the amount of modern pollen of a particular genus was expressed as a percentage of the total pollen accumulation at each site isopolls lines of equal percentage pollen representation were then mapped for each major genus and superimposed on...
Radiocarbon Years Bp 1
FIGURE 8.1 A reconstruction of Holocene treeline fluctuations in southwestern Keewatin, Northwest Territories, Canada.Treeline position is based on radiocarbon-dated tree macrofossils in situ north of the present treeline, and on dates on buried forest and tundra soils north and south of the modern treeline Sorenson and Knox, 1974 . more extensive boreal forest zone show an additional warming of 1 C in summer and 4 C in spring. This was the result of higher net radiation due to lower albedo...
The Climate System
Although it is common to consider climate as simply a function of the atmospheric circulation over a period of time, to do so overlooks the complexity of factors that determine the climate of a particular region. Climate is the end-product of a multitude of interactions between several different subsystems the atmosphere, oceans, biosphere, land surface, and cryosphere which collectively make up the climate system. Each subsystem is coupled in some way to the others Fig. 2.3 such that Changes...
Info Czs
FIGURE 6.14 Schematic diagram illustrating the relationship between orbital forcing and the signal that is eventually preserved in the sedimentary record. then to examine the coherency spectrum of the tuned record with the spectrum of orbital forcing at frequencies other than those used in the tuning procedure. In the tuned records obtained by Imbrie et al., 1984 , coherency was not only very high in the tuning bands but also in the -100 ka eccentricity band, which was not used in tuning at...
How Rapidly Does Vegetation Respond To Changes In Climate
An important question that arises in studies of high resolution e.g., varved sediments is What is the lag response of vegetation to climate change Can the pollen record provide information on short period, large amplitude changes in climate, or to put the question more generally What are the frequency response characteristics FIGURE 9.3 Generalized vegetation map of eastern North America and isopoll maps of selected taxa and groups of taxa at 500 yr B.P. Contours shown are 5 and 10 for forbs,...
Paleomagnetism
Variations in the Earth's magnetic field, as recorded by magnetic particles in rocks and sediments, may be used as a means of stratigraphie correlation. Major reversals of the Earth's magnetic field are now well known and have been independently dated in many localities throughout the world. Consequently, the record of these reversals in sediments can be used as time markers or chronostratigraphic horizons. In effect, the reversal is used to date the material by correlation with reversals dated...
Oxygen Isotope Studies Of Calcareous Marine Fauna
If calcium carbonate is crystallized slowly in water, lsO is slightly concentrated in the calcium carbonate relative to that in the water. The process is temperature-dependent, with the concentrating effect diminishing as temperature increases. In a nutshell, this is the basis for a very important branch of paleoclimatic research the analysis of oxygen isotopes in the calcareous tests of marine microfauna princi- pally foraminifera, but also coccoliths . The approach was first enunciated by...
Paleoclimatic Reconstruction From Long Quaternary Pollen Records
There are now numerous palynological records that span the late Quaternary and, in some cases, extend back continuously into the Pliocene. Dating is, of course, problematic beyond the range of radiocarbon dating 40,000 yr in most cases and often the records are simply assumed to extend back to the last interglacial, and beyond, based on the character of the pollen record itself. In some cases, correlation with marine oxygen isotope stratigraphy, either directly in marine sedimentary records or...
Isotope Dendroclimatology
Many studies have demonstrated empirically that variations in the isotopic content of tree rings 813C, 8lsO, and 82H are in some way related to climate.33 Early studies used whole wood samples but some studies now suggest that latewood may need to be isolated from earlywood to get a clear signal of climatic conditions in the growth year Switsur et al., 1995 Robertson et al., 1995 . In mid- and high latitudes, there is a positive relationship sensu lato between the oxygen and hydrogen isotopic...
Fundamentals Of Dendroclimatology
A cross section of most temperate forest trees will show an alternation of lighter and darker bands, each of which is usually continuous around the tree circumference. These are seasonal growth increments produced by meristematic tissues in the tree's cambium. When viewed in detail Fig. 10.1 it is clear that they are made up of sequences of large, thin-walled cells earlywood and more densely packed, thick-walled cells latewood . Collectively, each couplet of earlywood and latewood comprises an...
A I Calculation of Radiocarbon Age and Standardization Procedure
Although it is not necessary for a user of radiocarbon dates to know, in detail, how the actual value is arrived at, some understanding of the procedure is enlightening, particularly when considering adjustments for 14C fractionation effects see Section 3.2.1.4c . The whole subject is a complex sequence of calibrations, adjustments, and corrections. The following brief explanation is offered for the adventurous. In order to make dates from different laboratories comparable, a standard material...
Dendroclimatic Reconstructions
The following sections provide selected examples of how tree rings have been used to reconstruct climatic parameters in different regions of the world. This is not by any means an exhaustive review and for further information the reader should examine the sections on dendroclimatology in Bradley and Jones 1995 , Jones et al. 1996 , and Dean et al. 1996 . 10.3.1 Temperature Reconstruction from Trees at the Northern Treeline Many studies have shown that tree growth at the northern treeline is...
Acknowledgments
As in the first edition, I have benefited greatly from discussions with colleagues who are often far more deeply immersed in a particular aspect of paleoclimatology than I am. In particular, I have had the great luxury of meeting frequently with some of the leaders of the field while I was a member of the Scientific Steering Committee of the IGBP PAGES Project. Discussions with members of this committee were always stimulating. Special thanks go to those individuals who took the time to read...
Info Mdb
FIGURE 5.40 The fiiaOATM record in the GISP2 andVostok ice cores top derived from gas bubbles in each core.The strong correlation has enabled the SD and 8I80 records in the ice lower two series to be aligned chronologically, though the absolute timescale remains uncertain.The two records show that interstadi-als in the Greenland record can also be identified in Antarctica if they lasted more than -2 kyr Bender et al., 1994 . FIGURE 5.41 Isotopic and gas records from Byrd and Vostok Antarctica...
Paleoclimatic Information From Biological Material In Ocean Cores
Paleoclimatic inferences from biogenic material in ocean sediments derive from assemblages of dead organisms thanatocoenoses , which make up the bulk of all but the deepest of deep-sea sediments biogenic ooze . However, thanatocoenoses are generally not directly representative of the biocoenoses assemblages of living organisms in the overlying water column. For example, selective dissolution of thin-walled specimens at depth see Section 6.6 , differential removal of easily transported species...
Radioisotopic Methods
spontaneous radioactive decay by the loss of nuclear particles a or 3 particles and, as a result, they may transmute into a new element.5 For example, 14C decays to 14N, and 40K decays to 40Ar and 40Ca. Furthermore, the decay rate is invariant so that a given quantity of the radioactive isotope will decay to its daughter product in a known interval of time this is the basis of radioisotopic dating methods. Providing that the radioisotope clock is started close to the stratigraphically relevant...
Biological Dating Methods
Biological dating methods generally use the size of an individual species of plant as an index of the age of the substrate on which it is growing. They may be used to provide minimum age estimates only, as there is inevitably a delay between the time a substrate is exposed and the time it is colonized by plants, particularly if the surface is unstable e.g., in an ice-cored moraine . Fortunately this delay may be short and not significant, particularly if the objective is simply establishing a...
Info Bhx
FIGURE 7.20 Relationship between mean annual runoff and mean annual precipitation for areas with different mean annual temperatures in degrees Celsius .Temperatures are weighted by dividing the sum of the products of monthly precipitation and temperature by the annual precipitation.The quotient gives a mean annual temperature in which the temperature of each month is weighted in accordance with the precipitation during that month. A weighted mean annual temperature greater than the mean, which...
Paleoclimatic Reconstruction From Ice Cores
Ice cores have revolutionized our understanding of Quaternary paleoclimatology by providing high resolution records of many different parameters, recorded simultaneously at each location. Here, we highlight the main results from the northern and southern hemispheres and show how these records are related to each other, and to changes in forcing. 5.4.1 Ice-core Records from Antarctica A number of long ice-core records are available from Antarctica but the crown jewel of Antarctic ice cores is...
Dating Ice Cores
One of the most important problems in any ice-core study is determining the age-depth relationship. Many different approaches have been used and it is now clear that very accurate timescales can generally be developed for at least the last 10,00012,000 yr if accumulation rates are high enough. Prior to that, there is increasing uncertainty about the age of ice, but new approaches are constantly improving age estimates, allowing comparisons with other proxy records to be made with more...
The Basis Of Pollen Analysis
Paleoclimatic reconstruction by pollen analysis is possible thanks to four basic attributes of pollen grains 1 they possess morphological characteristics that are specific to a particular genus or species of plant 2 they are produced in vast quantities by wind-pollinated plants, and are distributed widely from their sources 3 they are extremely resistant to decay in certain sedimentary environments and 4 they reflect the natural vegetation at the time of pollen deposition, which if viewed at...
Periglacial Features
The use of fossil periglacial phenomena as an index of former climatic conditions is limited by two basic problems. First, dating periglacial features directly is often difficult, if not impossible generally they are dated by reference to the deposits within which they are found, thereby obtaining only a maximum age for the features. Secondly, although regions of modern periglacial activity can be circumscribed by particular isotherms, the occurrence of similar activity in the past can only...