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AN INTERHEMISPHERIC TRANSECT


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Author Topic: AN INTERHEMISPHERIC TRANSECT  (Read 548 times)
Adam Hawthorne
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« on: July 15, 2007, 03:50:13 am »

AN INTERHEMISPHERIC TRANSECT

PALEOCLIMATIC ANALYSES OF LATE QUATER­NARY LACUSTRINE AND TERRESTRIAL RECORDS OF ATLANTIC ISLANDS (ATLANTIS)

By Svante Bjorck

SCIENTIFIC OBJECTIVES AND AIMS

The overall objective of this Nordic collaboration project is to retrieve and analyse fairly unique and previously almost unexplored pal­eoclimatic records in lakes, peat bogs and glacial deposits of the last glacial-interglacial cycle along an inter­hemispheric transect of islands in the Atlantic Ocean. We will thereby obtain terrestrial data, recording atmospheric changes, but in a complete ocean setting. A key to success will be the establishment of a few, but extremely well-dated and thoroughly in­vestigated, pal­eoclimatic records from each island group. By paleoclimatic analyses of, and correlations between, these new (detailed) records, and by correlations and comparisons to surrounding continental and marine records an overarching aim could be reached: increased understand­ing of the atmospheric and marine cou­plings between the hemispheres and to detect changes in atmospheric and marine circulation patterns during phases of climate change. Ultimately, these type of unique interhemispheric records could be another step towards an increased comprehension of how the global climate system functions during different climate modes.

INTRODUCTION AND BACKGROUND

The heat advection from the equatorial regions to the higher latitudes is a key process in the global "climate machine". In this respect the Atlantic ocean plays one of the key-roles, distributing the heat/energy surplus from southern and lower latitudes to the high north by means of the North Atlantic Current. It is postulated that even smaller changes in the efficiency of this general heat conveyor may lead to global climate change. Therefore numerous marine geological studies have been published during the last decades about the role and dynamics of the Atlantic Ocean, especially the North Atlantic, during the "flip-flop" behaviour of the climate system over the last glacial-interglacial cycle.

                      The temperature of the tropics/subtropics, incl. the ocean, during glacial time is one very important and still unresolved anomaly between different types of data; marine records imply a very moderate cooling, while terrestrial data indicate a cooling comparable to what occurred in more northern regions. Since the high amount of incoming solar radiation to the lower latitudes is an important heat source for northern regions and the temperature gradient between low and high latitudes is a key parameter for global climates, this anomaly between marine and terrestrial proxy data must be resolved to be able to understand, and ultimately model, the glacial-interglacial "climate machine".

                      Another much debated, paleoclimatic topic is the conflicting evidence on the existence/non-existence of an anti-phase relationship between the two hemispheres, and the lead and lag relationships between the hemipheres. These discussions mainly originate from correlations between different deep polar ice cores (correlated with the methane signal) and interpretations of different modeling attempts. For example, the Vostok ice core data from Antarctica show periods with a clear anti-phase relationship with the Greenland Summit ice cores (GRIP/GISP2). This means that e.g. certain warmings over Greenland correspond to coolings in Antarctica, and vice versa. This type of climate situation has also been predicted by a 2.5D ocean-atmosphere climate model for the Younger Dryas cooling. However, there are also ice core records from Antarctica, e.g. the Taylor Dome, which seem to fit well to the GRIP/GISP2 records. In fact, there are also indications that some events may be synchronous and in phase, while others are in anti-phase.

                      The climatic relationships between the two hemispheres and the dynamics behind the meridional heat transport from south to north as well as any phase lags in this complex system are obvious key topics within the paleoclimatic community, but also among climatologists. This is also shown by the large interest from these groups for the three PEP (Pole-Equator-Pole) continental transects within the PAGES programme of IGBP, N America-S America-Antarctica (PEP I), Asia-Australia (PEP II), and N Europe-S Africa (PEP III).

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Adam Hawthorne
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« Reply #1 on: July 15, 2007, 03:51:25 am »


PROJECT DESCRIPTION

The short background presented above shows that increased understanding of the climatic relationships and mechanisms between the two hemispheres during events of global as well as regional climate change, are necessary to understand underlying processes behind climate variability of both glacial and interglacial scenarios. We therefore propose to start an Atlantic palaeoclimatic project focused on: 1) the last glacial-interglacial period, 2) interhemispheric comparisons and correlations, and 3) direct comparisons between marine and atmospherically influenced climate records.

                      We believe that these aims can be best achieved by analysing environmental/climatic variables, mainly from lacustrine records and peat bogs, as well as glacial records, on islands along a north-south transect in the Atlantic Ocean.

AN ATLANTIC TRANSECT

Parts of, or complete late-glacial and Holocene palaeoclimatic reconstructions, mainly based on lake and peat deposits, and geomorphic features, do already exist or are under progress from eastern Greenland (70-730N), Iceland (c. 660N), the Faroe Islands (620N), and southernmost Greenland (600N), and the principal investigator (PI) together with some of the project members are responsible or co-responsible for several of these studies (see PI`s CV and ref. list), as well as for similar studies around the Antarctic Peninsula (CV and ref. list). The transect is therefore tightly anchored in the north as well as in the very south. Furthermore, O. Ingolfsson plans to anchor the transect in the very north, Svalbard, in a future application.

The Azores

This island group of 2335 km2 is situated at the Azores High, in the central part of the N Atlantic (c. 380N), and consists of 9 islands, of which eight are of volcanic origin. Annual temperatures vary between 12-260C, with a fairly humid climate. Lakes of different types, but usually of volcanic origin, occur on all islands. The first attempts to retrieve cores from some of these were made in 1965. Samples were collected from five lakes and four samples were 14C dated, with an oldest age of 3.9 14C ka BP at a sediment depth of 150 cm in a crater lake on São Miguel (Fries, 1968). This was, however, not the deepest retrieved sediment. In the mid-90`s B. Ammann and H. E. Wright cored the same lake and the corer stopped in fairly coarse material at about 4.0 14C ka BP (Ammann & Wright; pers. comm.). Sediment slumping and related processes are possibly big problems in some of these steep-sided crater lakes, and may explain the difficulties to penetrate older sediments. No stratigraphic details have come out from those studies. However, the PI visited the two larger islands São Miguel and Terceira in January 1999 and found that there is a larger potential for lake corings in some non-crater lake basins as well as in some older, less steep craters. Fries (1968) also indicated more than 20 "promising" lakes on six of the islands, incl. information on size, altitude and often also water depths. Furthermore, the summit on the island of Pico, surrounded by some very promising lake basins, is situated at 2351 m.a.s.l. and is completely snow-covered in winter. This summit area has most likely been glaciated, and may thus hold glacigenic deposits.

Logistics: regular flights from Lisboa and good infrastructure on and between the islands.

St. Helena

In contrast to Cape Verde and Ascension, the climate of the island of St. Helena (120 km2) at 160S, situated between the Intratropical Convergence zone and the South Atlantic High (SAH), is humid enough for the formation of peaty and muddy deposits in some of the valleys on the northern part of the island. At least 5 m thick “mud” deposits in Fisher`s Valley as well as some thick open peat exposures in Rupert`s Valley have been reported to us (Q. Cronk; pers. comm.). At the arrival of the first (Portugese) settlers in 1504 the island was to a large extent covered by lush vegetation, with at least 50 endemic species, but the southern part was, and still is, desert-like. This also shows that the local climate is sensitive for changes in atmospheric circulation, i.e. the position of the southeast trade wind. The highest point on St. Helena reaches 820 m.a.s.l. and the most promising areas for organic rich Quaternary deposits are situated in some of the deeper valleys around 500 m.a.s.l., where also the richest vegetation of bushes and semi-tropical trees can be found. Mean summer temperatures vary from 320 (at sea level) and 210C, and winter temperatures vary between 26-150C, but with 50 lower temperatures in the central hills. Precipitation varies between 150-1000 mm/year, depending on altitude.

Logistics: regular connecton with Cardiff, Tenerife or Cape Town by the ship RMS St. Helena. Good infrastructure on the island with its 6000 inhabitants.

Tristan de Cunha and Nightingale Island

The Tristan de Cunha Island Group is situated at 370S, where Tristan de Cunha (78 km2) is the largest island with 300 inhabitants. It is built up around an impressive volcano, reaching 2060 m.a.s.l., which is snow-covered in winter. Annual temperatures vary between 4-260C in this southern part of the SAH, and precipitation is up to 1700 mm/yr. Areas below the volcano, situated at 600-900 m.a.s.l., are covered by thick peat deposits, e.g. at Soggy Plains in the south, where Hafsten (1960) reports at least 3.75 m peat, but peat bogs also occur close to the settlement of Edinburgh, e.g. at Jenny`s Waterun (Hafsten 1960). Furthermore, three smaller lakes, The Ponds, occur on the northeastern part of the island, only some 4 km ESE of Edin­burgh, and a small lake region is also found on Nightingale Island, where Hafsten (1951) pollen analysed a 3.5 m thick undated core, showing some very significant pollen stratigraphic changes. Furthermore, Wace and Dickson (1965) and Preece et al. (1986) report 14C ages of 8.3, 10.8, 11.3, 39.2, and >40 ka BP from lake sediments and peat on Tristan and Inaccessible Island. In contrast to the latter, Nightingale Island, which is extensively covered by peat, is fairly accessible with smaller paths around the island (B. Baldwin, Tristan de Cunha manager; pers. comm.). Altogether this shows that there is a good potential for studies of both peat and lake sediments, as well as possible glacigenic deposits from past glacial activity on Tristan de Cunha.

Logistics: Tristan de Cunha is connected with Cape Town and St. Helena by RMS St. Helena. Bad infra-structure (apart from two guest bungalows and roads stretching a few kms from Edinburgh) necessitates travel by foot and by local boat to a few landing places. Nightingale I is reachable from Tristan and we are able to rent some of the many huts on the island, owned by Tristanians (B. Baldwin; pers, comm.).

Gough Island

Gough Island (50 km2), which has a meteorological station with a staff of 7 people, is situated at 400S, 350 km SSE of Tristan de Cunha, in the “Roaring Fourties”, at the border between the SAH and the southern westerlies. The island`s climate is sensitive to Polar Front changes. Annual precipitation is >2500 mm and mean annual temperature at sea level is 110C, with extremes between -30 and 250C. Its highest peak reaches 910 m.a.s.l. and the island has a rugged, “wild” topography, bounded by 150-300 m high cliffs. The volcanic activity ceased long time ago and the landscape is main­ly shaped by erosion. The forest zone extends up to 300 m.a.s.l. According to the survey map of Heaney and Holdgate (1957) at least six small lakes occur on the island, of which five are situated on the Tarn Moss plateau 3-5 km from the only harbouring place (Quest Bay), and can be reached through the island`s largest valley, The Glen. Flatter areas inside and above the coastal cliffs at 600-700 m.a.s.l. are covered by thick peat deposits. At least 4.5 m thick peat was penetrated on the Albatross Plain (Hafsten 1960) and a 14C dating at 2.15 m resulted in an age of 4.7 14C ka BP. Furthermore, Bennett et al. (1989) pollen analysed and dated well-bedded organic-rich sediments at the meteorologi­cal station­. The age of the bottom-most retrieved sediments at 2.9 m was >43.000 14C years old, overlain by a hiatus. However, the total thickness of these deposits were estimated to 9.1 m. Studies of other Quaternary deposits were not carried out during the different Gough Island expeditions, but photographs of the landscape morphology together with the presence of snow on the higher parts of the island in winter, show that the higher areas may have been influenced by glacier activity. In general, the previous studies from Tristan, Nightingale and Gough I did not result in any stratigraphic details, but they clearly point out the potential of these island records.

Logistics: Some boat connections with S Africa and possibilities for hiring local boats on Tristan. Bad infrastructure (apart from the meteorological station) will necessitate boat travel to Quest Bay (see above) and travel by foot.

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Adam Hawthorne
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« Reply #2 on: July 15, 2007, 03:52:25 am »

METHODS

Field work

Together with available information, including the sparse previous scientific documentation, each island will be carefully studied for optimal planning of the field work.

                      Mainly three types of field work are planned for the proposed project, 1) coring and sampling of lake sediments and peat deposits, 2) mapping and description of glacigenic features and opening up sections for logging and sampling, and 3) sampling of plants for macrofossil and pollen reference collections, sediment surface and peat samples, water samples, as well as local rock samples for the understanding of lake sediment composition and its sources.

Laboratory analyses

Dating: The type of laboratory work will clearly vary between the different types of deposits, but a key component for all will be the age control. The 14C method will be the main dating tool, and in order to create reliable and detailed enough chronologies large sets of 14C dates will be necessary. 14C dates on peat are usually reliable, as is the case with lake sediments in regions with volcanic rocks, so we envisage that the dating of organic sediments will be fairly unproblematic. We do, however, also hope that any glacial deposits found on the Azores, Tristan de Cunha/Nightingale Island and Gough Island may contain dateable organic debris.

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Adam Hawthorne
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« Reply #3 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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