古海洋学及古气候替代性指标.docx

1、古海洋学及古气候替代性指标PaleoceanographyIntroductionPaleoceanography is the study of the history of the oceans. It encompasses aspects of oceanography, climatology, biology, chemistry and geology. The main sources of information are biogenic and inorganic marine sediments, as well as corals. Biogenic sediment

2、includes planktonic and benthic fossils whereas inorganic sediment includes ice-rafted debris and dust. On land, paleo-shorelines and erosional features as well as outcroppings of paleomarine sediments are the principal sources of proxy data. Glaciological records can also give indirect information

3、about paleoceans. The oceans high heat capacity and its ability to transport energy and to sequester and release greenhouse gases give it an important role in helping to determine the state of the planets climate. Thus, paleoceanographic research is also intimately linked to paleoclimatology.Methods

4、The reconstruction of paleocean characteristics and dynamics requires climatic detective work. It involves the dating and interpretation of paleoclimatic records as well as the definition of physical and dynamical constraints which specify possible circulation patterns and characteristics.Reconstruc

5、tions (Proxy Data)Direct measurements of the quantities of interest to oceanographers extend only into the relatively recent past and in most cases do not go further back than the mid-nineteenth century. To study the ocean during periods for which there are no direct measurements one must rely on in

6、direct evidence. Historical documents can be used as sources of data. Ship logbooks and sailing times across frequently traveled routes have provided estimates of the directions and strengths of past prevailing winds (Brazdil et al., 2005). The frequency and intensity of El Nio events since the 1500

7、s have been reconstructed based partially on historical accounts of large floods and crop losses (Quinn and Neal, 1987). This type of analysis furnishes qualitative descriptions of the past.Quantitative reconstructions are possible by proxy, where a quantity which is preserved in a natural archive a

8、nd can be measured, stands as a surrogate of the parameter of interest. A basic requirement is that the relationship between the proxy parameter and the quantity of interest has to be known. When this is the case, the history of the proxy variable can be converted into the history of the variable of

9、 interest by the use of mathematical expressions of the type: (1)which state that the parameter of interest, Int, is a function of the proxy quantity, Prx. The t index refers to time. Equations of this kind are commonly called transfer functions. The confidence with which Int can be estimated will d

10、epend on a series of factors, starting with the quality of the proxy measurements. Also relevant, and a common source of uncertainty, is how well f represents the relationship between Prx and Int.In most cases, transfer functions are obtained empirically by comparing directly measured values of the

11、quantity of interest to a pertinent set of proxy data. A potential source of error is that the function obtained by this procedure might not be general, but in fact could represent a relationship between Prx and Int that is peculiar to the data sets used to generate f. This problem can be minimized

12、by expanding the spatial and temporal coverage of the data used to establish the transfer function. Still, even assuming that f is a perfect representation of how the proxy and the quantity of interest are connected to each other in the present, there is no guarantee that the relationship between th

13、em was the same in the past.Another source of error can be easily understood by re-writing Equation (1) so that it expresses the proxy quantity as a function of the variable of interest. It is reasonable to assume (and in many cases it has been demonstrated) that Int is not the only factor controlli

14、ng Prx, so that in fact we end up with: (2)where E 1, E 2, E n represent environmental parameters that also influence the proxy variable but are independent of the quantity of interest. An immediate conclusion is that reconstructions of Int based on Prx will be contaminated by other factors so that

15、part of the variability observed in the proxy quantity is not related to changes in the parameter of interest. Comprehensive analysis of the relationships between proxies and a series of observed parameters can offer some insight into how to remove part of the undesired influence of other factors fr

16、om the reconstruction.Given the complexity involved in developing skillful transfer functions as well as in identifying and correcting for potential sources of error, a common strategy is to reconstruct the same parameter of interest using different proxies. Such analyses are known as multi-proxy re

17、constructions (Fischer and Wefer, 1999).Types of proxies:The systematic use of proxies in quantitative reconstructions of past oceanic environments originated in the second half of the twentieth century. Since then, a large number of proxy techniques have been established and more are constantly bei

18、ng developed. Proxies can be grouped in six broad categories, based on the type of direct measurement (Fischer and Wefer, 1999). These are listed below, together with brief descriptions of the main variables of interest associated with each proxy. The following chapters on paleotemperature, paleopro

19、ductivity and paleocirculation present in more detail the proxies relevant to each of these fields. Comprehensive discussions of oceanographic proxies can be found in the literature (Bradley, 1999; Fischer and Wefer, 1999; Henderson, 2002). Microfossil assemblages. The relative abundance of plankton

20、ic and benthic species of foraminifera, coccoliths, radiolaria, diatoms and other organisms can be used to estimate past ocean temperature, productivity and sea ice distribution. This proxy type was used for the CLIMAP project, which produced the first global distribution of sea surface temperature

21、for the Last Glacial Maximum (CLIMAP - Project Members, 1976). Stable isotopesare based on the ratio between different isotopes of an element. The ratios are usually standardized by a reference value and named after the heavier isotope. The ratio between 18O and 16O, for example, is represented by 1

22、8O. Isotope readings are retrieved mainly from foraminifera skeletons (tests), organic matter or other sources (e.g., water molecules in continental ice sheets). The amount of 18O incorporated by organisms like foraminifera and corals increases as temperature decreases. Continental ice is relatively

23、 depleted in 18O compared to sea water. This makes 18O a proxy for both temperature and the extent of continental ice sheets. 11 B is used as a proxy for pH. Productivity, nutrient concentration and past circulation can be reconstructed from 15 N and 13 C (12C is taken up with slight preference to 1

24、3C during photosynthesis). Together with microfossil assemblages, 18O and 13 C are the paleoceanographic proxies with the most widespread use. Radiogenic isotopes. The different solubilities of uranium and two products of its naturally occurring decay, thorium (Th) and protactinium (Pa) can be used

25、to estimate the rate of deep water flow and the flux of particles from the water column to the sediments. This flux can also be used as a productivity estimate. 14C preserved in organic matter is used to estimate the age difference between near surface and deep waters and hence, ventilation rates. B

26、iogenic compounds. The concentrations of some compounds, mainly organic carbon, calcium carbonate and opal, are used as estimates of past productivity. Calcium carbonate is also an indicator of the calcite compensation depth. Alkenones are long chained organic molecules resistant to degradation. The

27、 alkenones produced by some coccolithophors can have two or three double bonds in their structure. The ratio between molecules with two and three double bonds reflect the temperature at the time of synthesis. Elements. The concentrations and ratios of certain elements in the sediment, organic remain

28、s, tests and corals are also used as proxies. The ratios of strontium to calcium (Sr/Ca) and magnesium to calcium (Mg/Ca) in biologically precipitated marine carbonates are temperature dependent. The cadmium to calcium (Cd/Ca) ratio is used for nutrient reconstructions. Barium concentration and the

29、Ba/Ca ratio are proxies for productivity and alkalinity, respectively. Sedimentology. Grain size distribution can provide qualitative information about bottom current speeds and act as an indicator of ice rafted debris. Information about past tides can be inferred from layered sediments called rhyth

30、mites. The mineralogy of the sediments can be used to establish source areas and direction of transport for both water and wind borne sediments.Conspicuously absent from the quantities of interest listed above is salinity, a fundamental parameter which influences many aspects of the ocean environmen

31、t. There is, at the moment, no independent proxy for this quantity. As salinity also influences 18O, an indirect measurement can be obtained by using independent estimates of temperature (alkenones, for example) to remove the temperature signal from existing 18O series. Attempts have also been made

32、to infer salinity from microfossil assemblages. Unfortunately, both approaches generate errors of 1 psu, very large compared to the range of salinity variability in the oceans (Fischer and Wefer, 1999).Reconstructions (Models)The theoretical approach to paleoceanography uses quantitative ocean and climate models to reconstruct paleoconditions of the ocean and interpret observations. A number of different ocean and coupled climate models have been used in the past and we will give a very short overview of the hierarchy and use of these models in the broad research area of paleoceanography.