DESARROLLO, IDENTIFICACION DE LOS PELIGROS, LOS CONTROLES

Evidence from pollen sequence and other biological material (molluscs and beetles) provides a detailed account of the climatic and vegetational changes within the early Middle to Late Pleistocene. In Northern Europe four pollen zones (numbered I - IV), which are sometimes subdivided into sub zones, have generally been recognised in each interglacial period (Turner and West 1968). Below is an outline of the vegetational succession observed in named each interglacial episode from the Cromerian to the Ipsvrichian.

As discussed above, the “Cromerian” and “Wolstonian” Stage names and the corresponding continental European stage names have been used as umbrella terms which, in reality, describe long periods of changing climatic conditions. Examination of the marine isotope curve, see Figure 2.1, shows that these periods contain several climatic cycles with even more complex cycles occurring vrithin each peak and trough (i.e. small changes in climate within the cold or temperate stage, relating to stadial or interstadial periods).

However, the allocation of stage numbers is not always possible because the current dating and correlation of some sites is insufficiency known to relate them to other, previously dated, sites. This is particularly important in the “Cromerian Complex” which represents at least four interglacial-glacial cycles. In the present study, where possible, sites are related to marine isotope stage numbers, but where this is not possible, they are grouped into stage names.

Cromerian

The pollen record from The Netherlands identifies at least four separate glacial - interglacial cycles (Zawijn 1996). The vegetational history of West Runton (the Cromerian type site), based on West 1980, is given below. It should be noted that the allocation of Roman numeral numbers here relates to pollen zones and are not related to

Cri - grass communities replaced by Betula and Pinus forest which increase throughout stage with the addition of Ulmus and Alms.

Crll - development of mixed temperate forest with increasing amounts of Quercus and reduced amounts of Pinus, The climatic optimum.

Crin - general mixed oak forest but showing a fall in Quercus, Ulmus and Alnus.

CrlV - development of boreal forest with Pinus and Betula.

Hoxnian

A complete pollen sequence for the Hoxnian was described from Marks Tey (the stratotype site of the Hoxnian) by Turner (1970) which provides a record from the late Anglian to the early “Wolstonian”. The sequence is summarised as follows:

Hoi - development of Betula forest with rapid increase of Pinus and Quercus at end of stage.

Hon - increase of temperate forest species. In HoHa Quercus is the dominant species while during Hollb it is replaced by Alnus and Corylus. Later in the stage a period of deforestation is seen with increased charcoal remains.

HoHl - development of mixed temperate forest once again.

HolV - shift to boreal forest with an increase in grass as forest opens out.

The marine isotope record shows that the Hoxnian (MIS 11, see section 2.2.1) consists of a series of warm pulses (Figure 2.1). These pulses, identified as lie , l ie and 11a, have been shown to have characteristic mammalian faunas (Schreve 1997). During sub­ stage l ie the fauna is dominated by forest adapted mammals while in sub-stage 11c a more open adapted mammal fauna is seen. This is coupled with an increase in the amount of non-arboreal pollen recovered from deposits attributed to sub-stage 11c, such

Post Hoxnian - Pre Ipswichian interglacials

As described in section 2.2.1, until analysis of the marine isotope record was carried out, many Middle - Late Pleistocene sites were, based on pollen records, placed in the

Ipswichian or Hoxnian interglacials. However, now two extra interglacial episodes are recognised, correlated with MIS 9 and MIS 7. The pollen records from sites attributed to these stages are very like those from Ipswichian and Stage 11 sites but, as discussed in section 2.2.1, lithostratigraphic and biostratigraphic work has shown them to be separate interglacial episodes.

• MIS 9: Pollen samples from Grays show this to have been a heavily wooded stage with little open vegetation (West 1969a; Hollin 1977).

• MIS 7: As discussed in section 2.2.1 the pollen records from Ilford and Aveley show this stage to have had two vegetational types (West 1969b) that can be divided into two sub-stages 7c and 7a. Sub-stage 7c is characterised by being more closed while the vegetation type of sub-stage 7a is one of a more open parkland environment with increased grassland and patchy woodland. These two vegetational types are coupled with a change from a browser-dominated fauna to one dominated by grazers (Schreve 1997).

Ipswichian (MIS 5e)

The stratotype for this interglacial is Bobbitshole, a site on the River Orwell south of Ipswich. West (1957) describes the pollen zones identified for the Ipswichian which are summarised as follows:

Ipl - development of park-tundra followed by increase in Betula woodland. Quercus and

Pinus become important towards the end of the stage.

IpH - increase in Pinus until the end of sub-stage Ha which is followed by increases in

Quercus, Corylus and Acer during sub-stage Ilb.

IpIV - Pinus and Betula are the dominant tree species while non-arboreal pollen increases throughout the stage.

As with MIS 9, the Ipswichian is characterised as having a heavily wooded environment throughout, with limited graze available.

Cold Stages

Remains of Palaeoloxodon antiquus in Northern Europe are mainly restricted to the interglacials described above. However, the cold stage site of Balderton, Lincolnshire, has yielded fossils that have been examined in this study. Balderton (Lister and Brandon

1991) has been attributed to an interstadial episode in MIS 6 or to the beginning or end of that cold stage. Direct evidence is not available but presumably this showed thermal warming but not reaching the temperatures observed in interglacials (Lowe and Walker

1997). There may or may not have been time for the immigration of trees.