Unlike pollen analysis, few Eemian sediments have been examined for diatom remains throughout Europe. A majority of the Eemian diatom sequences investigated are fi-om Germany and Poland (see Tables 1.3.1 and 1.3.3). One of the most interesting studies was carried out by Benda (1974)
Chapter 1.3: The Eemian Interglacial____________________________________________ 45
who identified the seasonal succession of planktonic diatoms by analysing a laminated sediment (Table 1.3.1). In some sites, the temperature optimum during the Eemian corresponds to diatom assemblages largely dominated by periphytic taxa, in particular small benthic Fragilaria species (Table 1.3.3). These records were interpreted as low-lake levels (Miller, 1971; Kaczmarska, 1976; Kaczmarska, 1977; Marciniak, 1994).
In France, Eemian diatom sequences have only been studied from two sites (see Table 1.3.2), La Grande Pile in the Vosges Mountain (Bogaczewitz-Adamczak, 1998) and Lac du Bouchet in the M assif Central (Failles, 1989). In this study of Lac du Bouchet, only the end of the Eemian period was recorded in the sediment cores recovered. The diatom flora in these sediments was dominated by Cyclotella taxa, (C. kUtzingiana, C. kiitzingiana v. radiosa, C. comensis). This sequence will be discussed in more detail and compared with the Ribains sequence in chapter 6. Eemian deposits from coastal areas around the North Sea often record a marine transgression (Table 1.3.4). In the Netherlands, the Eemian diatom sequence from an Amsterdam borehole, located not far from Amersfoort where the type pollen sequence o f the Eemian was described, was analysed by De W olf & Cleveringa (1994). This sequence is typical of a coastal environment, the warm period (interglacial) being characterised by a sea level rise and marine transgression. Therefore, diatom species indicating a freshwater environment are recorded during the transition between the Saalian glacial and the Eemian interglacial and those indicating a brackish environment with strong marine influence are characteristic of the Eemian proper deposits.
From all these sequences, it appears that Cyclotella and/or Stephanodiscus are common features o f freshwater lake sediments deposited during the Eemian period.
1 3 .4 . Eemian sequences from marine sediments and ice cores
M arine sediment sequences provide both biological and isotopic data for the last interglacial period. Microfossil remains of several groups of organisms such as planktonic and benthic foraminifera, coccoliths, dinoflagellates and diatoms are used to quantitatively reconstruct sea surface conditions (such as salinity, temperature, sea-ice extent). However, it is isotopic records that provide the most relevant data for testing the astronomical theory (also called Milankovitch’s theory) o f the Pleistocene ice ages (Imbrie et a l , 1984). These isotopic records are essentially curves showing fluctuations in the ratio of oxygen isotopes to *^0 in tests of fossil foraminifera reported to an international standard as in parts per thousands (%o). 0^*0 in foraminifera varies with in the water from which their carbonate tests are deposited, but differs from the water value by an amount that is temperature dependent (Chappell and Shackleton, 1986).
Table 1.3.1: Summary of Eemian diatom sequences from eastern Europe dominated by planktonic species (in bold, mass-occurring species).
References: Site Location Diatom assemüages
Transition Saalian/Eemian
Start of Eemian Climatic optimum Transition Eemian/Würm
Krasske (1933) Oderberg-Bralitz (Germany)
Cyclotella ocellata, Cycotella kuetzingiana, Cyclotella comta, Stephanodiscus astraea, benthic Fragilaria spp.
Behre (1962) Liineburger Heide (Germany)
Cyclotella kuetzingiana Aulacoseira ambigua
Cyclotella comta C. kuetzingiana, C. comta
S. astraea v. minutula Cyclotella distinguenda
Stephanodiscus spp.
Stephanodiscus astraea S. astraea v. minutula A. Ambigua + A. granulata
Cyclotella spp.
Benda (1963) Luhetal (Germany) S. astraeav. minutula Aulacoseira ambigua
S. astraea, Synedra ulna Aulacoseira granulata
Cyclotella comta, A. ambigua S. astraea
Benda (1974) Lower Saxony (Germany)
Laminated deposits;
- Spring layers dominated by S, astraea (=S. neoastreal), S. astraea
V. minutula (=S. medius + S. minutulusl) and Synedra ulna - Autumn layers dominated by Aulacoseira granulata
Marciniak & Kowalski (1978)
Nidzica (Poland) Benthic Fragilaria spp. + epiphytic species
Cyclotella distinguenda
Cycotella kuetzingiana, Cyclotella comta, Stephanodiscus astraea Synedra ulna Cyclotella comta Aulacoseira ambigua Aulacoseira granulata Stephanodiscus hantzschii Benthic Fragilaria spp. Menke & Tynni
(1984) West-Holstein (Germany) Tabellaria fenestrata Ellerbeckia arenaria Cyclotella ocellata St. astraea v. minutula
S. astrea, S. astraeav. minutula
Aulacoseira italica v. tenuissima, Cyclotella kuetzingiana
Benthic Fragilaria spp. Aulacoseira italica Benthic Fragilaria spp. C. ocellata, C. kuetzingiana Stephanodiscus spp. U)
?
O STable 1.3.2: Summary of Eemian diatom sequences from France (in bold, mass-occurring species).
References: Site Location Diatom assemHages
Transition Saalian/Eemian
Start of Eemian Climatic optimum Transition Eemian/Würm
Pailles (1989) Lac du Bouchet (France) Cyclotella kuetzingiana (= C. krammeri, C. rossiil), Cyclotella comensis Bogaczewitz- Adamczak (1998) La Grande Pile (France) Cyclotella ocellata Cyclotella comensis
Cyclotella distinguenda v. unipunctata
Asterionella formosa Fragilaria nanoides Cyclotella stelligera Aulacoseira subarctica Tabellaria fenestrata T. flocculosa Aulacoseira distans + periphytic spp. typical of acid peat bogs
Table 1.3.3: Summary of Eem ian diatom sequences dominated by periphytic taxa (in bold, mass-occurring species).
References: Site Location Diatom assemWages
Transition Saalian/Eemian
Start of Eemian Climatic optimum Transition Eemian/Würm
Miller (1971) Leveaniemi (Finland) Benthic Fragilaria spp. Navicula oblonga Achnanthes spp. Benthic Fragilaria spp. Aulacoseira distans Aulacoseira valida
Terrestrial phase, diatom- poor deposits
Shallow lakes with benthic
Fragilaria spp. and Aulacoseira spp. Influence of acid-dystrophic water Kaczmarska (1976, 1977) Imbramowice (Poland) Benthic Fragilaria spp. Cymbella affinis, Gyrosigma attenuatum
Stephanodiscus astraea var. minutula (=S. mediusl)
Cyclotella kuetzingiana var. radiosa (=C. rossiil), Cyclotella ocellata C. distinguenda var. unipunctata
Benthic Fragilaria spp. Numerous Navicula spp., including N. Cincta, Minor percentages of Cyclotella spp. Benthic Fragilaria spp.,
Gyrosigma attenuatum and high proportions of Chrysophyte cysts
Marciniak (1994) Zbytki (Poland) Benthic Fragilaria spp.
Aulacoseira ambigua Cyclotella distinguenda Cyclotella ocellata Cyclotella kuetzingiana Stephanodiscus spp. Cyclotella meneghiniana Fragilaria brevistrita Diatom-poor deposits Benthic Fragilaria spp. Transition to a peat-bog r>
Table 1.3.4: Summary of Eemian diatom sequences from coastal areas recording marine transgression (in bold, mass-occurring species).
References: Site Location Diatom assemblages
Transition Saalian/Eemian
Start of Eemian Climatic optimum Transition Eemian/Würm
Brelie (1957) North S ^ioÉ e^a ocellata
(Germany) Stephanodiscus astraea
Marine transgression:
Diatom assemblages composed of brackish and marine species
Tynni (1971) Somero (Finland) Aulacoseira subarctica Marine transgression: assemblages of marine diatoms DeWolf & Cleveringa
(1984) Amsterdam (The Netherlands) Cyclotella kuetzingiana Stephanodiscus minutulus Aulacoseira italica
Benthic Fragilaria 5Dd. Marine transgression:
Lowering of lake-level Diatom assemblages composed of brackish and marine species
Gronlund (1991) Ostrobothnia (Finland) Aulacoseira islandica Aulacoseira lirata, Aulacoseira valida
Cyclotella kueztingiana S. astraea
Marine transgression: Lagoonal phase with
Eemian sea represented by an assemblage of littoral Campylodiscus clypeus
marine diatoms and Nitzschia scalaris
Robertsson et aL, (1997) Dellen region (Sweden) Eemian Sea characterised by an Eemian freshwater lake:
assemblage of marine diatoms: C. distinguenda var. unipunctata Chaetoceros spp., Grammatophora, C. ocellata + C. kuetzingiana
Paralia sulcata Aulacoseira italica
Aulacoseira distans % U)
?
r
S’Chapter 1.3: The Eemian Interglacial_____________________________________________ 49
The mean of the whole ocean varies with the quantity of isotopically light ice stored on the continents so that the record of foraminifera in a particular core is composed of global (ice volume) and local (temperature) components (Chappell and Shackleton, 1986), In other words, variations in 5^*0 down core reflect changes in oceanic isotopic composition, which are caused primarily by the waxing and waning of the Pleistocene ice sheets. The 0*^0 signal is therefore globally synchronous in the open oceans and thus provides a basic climatic stratigraphy for oceanic events (Imbrie et a l, 1984), which is referred to as the SPECM AP time scale. Global mean-sea-level records have also been obtained from studies of fluctuations in coral. In the SPECM AP record the last interglacial is characterised by low value o f (isotopically light) and show five substages (OIS 5a to 5e). It is widely thought that only the substage OIS 5e from the deep-ocean stratigraphy corresponds to the Eemian on land (Mangerud, 1989). However, this is still a subject of discussion (see later). It is worth noticing however, that there is considerable disagreement between sea level estimates deduced from geological evidence and from benthic foraminifera oxygen isotope records (Dansgaard and Duplessy, 1981). This is because tectonic movements including isostatic rebound are also important factors influencing the sea level record in shelf areas. However, most evidence shows that sea level stood several metres higher than present levels (Harmon et a l , 1981) and that the melting of the Greenland ice cap was largely responsible (Koemer, 1989).
Ice core data from Antarctica and Greenland provide evidence of both climate and climate forcings. The climate information derived from ice-cores includes an estimate of temperature changes at the atmospheric level where the snow formed and at the surface. The temperature record is reconstructed from the continuous deuterium (^H) profile measure along the core. Information on climate forcing includes data on the amount of aerosols in the atmosphere and the atmospheric chemical composition, especially gases such as CO2 and CH4 (Lorius et a l , 1990). Ice cores are indirectly dated with an ice-sheet flow model. The last interglacial as recorded in ice-cores shows marked differences with the one recorded in marine sediments (see following discussion).