A2.3EC3 Perspectives on Environmental Change

1. A2.3EC3Perspectives on Environmental Change LECTURE 3 The early Holocene

2. OVERVIEW • Introduction • Geographical changes • Early Holocene climates • European vegetational history • Soil development • Palaeolithic societies • The Mesolithic transition

3. INTRODUCTION

4. The early Holocene was a period of rapid environmental change in Europe and North Africa, following the northward migration of the polar oceanic front. • These changes were compounded by periodic outflows of cold, fresh water from the melting Laurentide ice sheet.

5. The principal changes were geographical (land-sea) and climatic. • From these followed changes in flora and soils. There were corresponding changes in faunas and in early human societies. • During the preceeding Lateglacial period there was a quarter cycle shift in the orbital precession cycle. This may have been the ultimate driver for these changes.

6. GEOGRAPHICAL CHANGES

7. During the early Holocene eustatic sea-level rose by around 40m. • In Britain, this rise was modified by isostatic effects. Relative sea-level fell by around 20m in southern areas. • In northern areas, relative sea-level fell by around 10m due to isostatic rebound. • These changes consequent movements of coastlines, especially in the North Sea basin.

8. Barbados coral curve

10. 10,000 BP

11. 9,000 BP

12. 8,000 BP

13. EARLY HOLOCENE CLIMATES

14. The shortest period orbital change is due to the Milankovitch precession component. • This relates the direction of the axial tilt to the long and short axes of the orbit. • This component changed by one quarter cycle during the Lateglacial (15,000-10,000 ka). This led to an increase in net solar radiation.

15. The increased solar radiation led to a reduction in ice and snow cover and to a strengthening of the Atlantic circulation. • This in turn increased the oceanic ventilation which led to an increase in atmospheric carbon dioxide. • The combined effect was a increase of ca. 10C in the mean annual temperature of northern Europe.

18. At the start of the Holocene the received solar heating in the northern hemisphere was thus about 8% greater than today. • This altered the hydrological balance over the hemisphere. • It is estimated that at 9,000 BP the available precipitation was around 10% greater than today.

19. While the Laurentide ice sheet remained in existence, there was a small number of outbursts of meltwater into the Atlantic ocean. • These led to climatic downturns lasting decades to centuries. • The most significant of these was the so-called 8,200 BP cold event.

23. The early Holocene period is also associated with widespread evidence of high water levels in many northern and central African lakes. • These presumably also reflect an earlier period of higher rainfall. • It was during this period that the major Saharan groundwater systems were last recharged.

26. EUROPEAN VEGETATIONAL HISTORY

27. Following the rapid warming in the early Holocene, temperate vegetation spread across Europe to produce the distribution seen today. • This can be traced in a series of maps based on a large-scale synthesis of pollen data.

32. The expansion occurred in a series of stages, dictated by: • the natural rate of species dispersal • the location of the ‘refuges’ for that species • This initially produced disequilibrium associations of species that are not seen today. • Natural competition then modified the associations to form the local climax communities that are found at the present day.

33. The initial phase was dominated by the arrival of wind-dispersed species such as birch. • These were associated with rapidly maturing species from local refugia such as pine and hazel.

35. Pinus sylvestris (Scots pine)

36. Betula pendula(birch)

37. Corylus avellana (hazel)

38. These species were followed by (in the south) closed canopy woodland, dominated at first by elm and later by oak and beech. • In the north the open canopy birch-pine assemblage persists to the present day.

39. Ulmus glabra (elm)

40. Quercus rubor (common oak)

41. Fagus sylvaticus (beech)

42. SOIL DEVELOPMENT

43. The vegetation succession is associated with a sequential soil development. • It is thought that the climax soil followed the climax vegetation, not vice versa. • There is a general sequence through • base rich Ca dominated soil • mature forest soil (brown earth) • acid leached soil (podzol)

45. The development of the vegetation and soils caused a corresponding development in freshwater lakes: • initial base-rich pH 8 condition • precipitation of iron leached from soils • reduction of pH due to acidic runoff from soils • final acidification to about pH 6 • This sequence governs the local oligotrophic to eutrophic hydrosere succession.

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47. Early human societies

48. In the early Holocene, human society was a stone-using nomadic culture of hunter-fisher-gatherer (h-f-g) type. • The culture and its period are usually known as the Palaeolithic (Old Stone Age). • This had existed since about 200,000 BP and had undergone a series of developments known from stone and bone tools of increasing sophistication.

49. Magdalenian (Upper Palaeolithic) flint implements

50. Magdalenian (Upper Palaeolithic) bone implements