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AN INTERHEMISPHERIC TRANSECT

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Author Topic: AN INTERHEMISPHERIC TRANSECT  (Read 785 times)
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Adam Hawthorne
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« on: July 15, 2007, 03:53:39 am »


This is common in glacial deposits in temperate/subtropical regions (cf. midwest of the U.S., Alpes, Andes, Kilmanjaro, and Himalaya), which is related to the proximity of fairly dense vegetation to the glaciers. The glacial deposits will also be dated with the now gradually refined TL/OSL method. It is also very likely that most of the islands Quaternary deposits will contain local/regional tephra layers. We therefore envisage to use them as well-defined (through micro-probe analysis) marker horizons, but also to date them with the Ar/Ar method (it has lately been shown to be useful not only Weichselian tephras but also early Holocene ones with some accuracy (G. Bond; pers. comm.)). 14C (or Ar/Ar) dated tephra horizons will be useful dating and correlation tools between sites and to local/regional marine cores. If old enough (>40.000 years) organic deposits are found, U-Th dating may also be attempted. However, this will depend much on the character of the deposits and amount of uranium in the sediments. More or less all of the datings will have to be paid for.

Biological analyses: Pollen analyses will be carried out on well-dated key-sequences for studies of vegetation dynamics during phases of climate change by mapping changes in time and space of the islands` often distinct vegetation zones (cf. Tristan with its alpine desert, heath, grassland, scrub, bush forest and lowland grass land zones). By altitudinal surface sample transects, the pollen characteristics of each zone will be defined. The sediments will also be analysed for content of biogenic silica. Samples for other type of biologic proxies, e.g. diatoms and chironomids, will be offered to colleagues with an interest in the project. Parts of the biogenic silica and pollen analyses will have to be paid for.

Chemical/physical analyses: Apart from the important sediment logging (often carried out in the field) and digital image analysis, the lake/peat sediments will also be characterised and analysed by a variety of chemi­cal/physical methods. Amount of C, N, and S will be analysed, stable isotopes,  13C and  18O, on different organic components and cellulose, respectively, will be measured, trace elements will be analysed with the ICP-MS technique, a large set of geomagnetic properties will be analysed, and microprobe analyses will be carried out on tephras. Through collaboration with different laboratories, mainly in Lund and Copenhagen, only parts of the analytical costs will have to be paid for.

Short account of possible paleoclimatic information from the methods planned

The methods briefly described above will be able to give information on several paleoenviron­mental and paleoclimatic aspects. Mapping, sedimentary analysis and dating of glacial deposits will be used to calculate and model changes in temperature (cf. lapse rates) and precipitation during phases of glacial activity on the islands. Fairly detailed climatic reconstructions can be obtained from peat stratigraphies by analysing C/N ratios, volumetric growth and acumulation rates, concentrations of N and C, and primary productivity of the peat, (cf. Björck et al. 1991). Pollen analyses will record changes in the islands` vegetation zonation, which, like glaciers, are mainly driven by temperature and precipitation. Sediment characteris­tics (incl. C content), biogenic silica and diatoms in lake sediments are good proxy records for aquatic productivity, and some of the geomagnetic properties and S content will provide information about depositional environment (e.g. ae­robic/anaerobic) while C/N ratios, S, 13C and some of the geomagnetic properties will be used for establishing lake sediment sources (e.g. autochtonous or allochtonous) and thereby i.a. erosion and humidity changes. 13C will also be used to detect changes between occurrence of C3 and C4 plants, which both occur on some of the islands, and may be a good climatic (humidity) indicator. Recent analyses of  18O on cellulose are very promising (T. Edwards; pers. comm.):  18O on aquatic cellulose is a good proxy for ambient lake water tempera­ture/precipitation-evaporation ratios, while  18O on peat cellulose is mainly a proxy for precipitation temperature. Since 16O/18O ratios often are species related, identification of the most common macrofossils and moss (peat) types will be necessary. We have recently started up trace element studies of lake sediments and peats, and our hope is to use these new exciting data, in combination with our other proxy records, to be able to use them as climate proxies. They will certainly be very useful in e.g. detecting periods with increased dust from the African continent as well as from the island itself, indicating periods with increased aridity. It will also be possible to detect periods with increased influence from marine spray (aerosols) into the lakes and peat bogs, indicating periods with increased storminess. Finally, it should be clarified that these varied and complex multistratigraphic data sets will be analysed by means of multivariate analysis techniques, which can e.g. be examplified by the Björck et al. (2000) study.

Project group members

A project with such a wide scope requires a multidisciplinary group of scientists, but there is not space for all in the application form! Apart from the PI, S. Björck, the main group consists of the following researchers. From the Dept. of Quat. Geol. at Lund University: D. Hammarlund (stable isotopes, paleoclimate), P. Möller (glacial geology, sedimentology), P. Sandgren and I. Snowball (geomagnetics, mineral magnetism). From the Geol. Inst. at Copenhagen University and GEUS: O. Bennike (macrofossils), B. Buchardt (stable isotopes, geochemistry), R. Frei (geochemistry, isotopes). From UNIS on Svalbard: O. Humlum (geomorpho­logy, gl­acial/climate modeling). From Earth. Sci. Centre at Göteborg University: O. Ingolfsson (glacial stratigraphy, paleoclimate). From Inst. of Quat. Res. at Stockholm University: S. Wastegård (tephra chemistry/stratigraphy).

PUBLICATION PLANS

We think that this type of project will result in many international publications; both in more specialised journals as well as in general paleoclimatically and stratigraphically oriented journals. A major and ultimate aim of a project like this is to describe climatic scenarios in very narrow time slices, and to describe and understand the transitions between scenarios. For the latter purpose, modelers should be engaged. The setting, geology and paleoclimate of these remote islands may certainly also be a topic of great interest for non-experts and will absolutely result in the dissemination of these islands´ special environment. Through e-mail contacts, the local islanders have also shown a big interest in the project; global change topics are important issues for island people.

Cited references

Bennett, K. D. et al. 1989: Pollen analyses of a Quaternary peat sequence on Gough Island, South Atlantic. New Phytologist 113 (3), 417-422.

Björck, S. et al. 1991: Stratigraphic and paleoclimatic studies of a 5500-year-old moss bank on Elephant Island, Antarctica. Arctic and Alpine Research 23, 361-374.

Fries, M. 1968: Organic sediments and radiocarbon dates from crater lakes in the Azores. GFF 90, 360-368.

Hafsten, U. 1951: A pollen analytical investigation of two peat deposits from Tristan de Cunha. Res. Norw. Sci. Exped. Tristan de Cunha 1937-1938, No 22, 42 pp.

Hafsten, U. 1960: Pleistocene development of vegetation and climate in Tristan de Cunha and Gough Island. Årbok Univ. Bergen, Mat.-Naturv. Serie 20, 45 pp.

Preece, R. C. et al. 1986: The Quaternary palaeobotany of Inaccessible Island (Tristan de Cunha Group. Journal of Biogeography 13, 1-33.

Wace, N. M. & Dickson, J. H. 1965: The terrestrial botany of the Tristan de Cunha islands. Phil. Trans. R. Soc. Lond. B, 249, 273-360.

NB Text somewhat shortened/John Ekwall February 22, 2003

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