Henry F. Lamb

Vegetational response to Holocene climatic change : pollen and palaeolimnological data from the Middle Atlas, Morocco

Pollen data from a radiocarbon-dated lake-sediment core from the Middle Atlas of Morocco show that forests of evergreen and winter-deciduous oaks (Quercus rotundifolia, Q. canariensis), present from the start of the Holocene, were invaded at 6200 BP by Cedrus atlantica in low numbers. Two thousand years later, after a short period characterized by higher deciduous oak frequency, Cedrus increased to its present abundance. These changes may have been in response to increasing effective moisture as summer temperatures decreased with declining northern-hemisphere seasonality, under the influence of precessional forcing. Palaeolimnological data from the same core show that the lake level fell sharply in five 200-400 yr-long arid intervals, but the pollen data show little or no evidence of a vegetational response. This suggests that summer (growing-season) rainfall remained adequate during the dry intervals, whereas depletion of the groundwater aquifer was the result of reduced winter precipitation. Anthropogenic exploitation since c. 1300 BP has had a greater effect on the forest ecosystem than any of the Holocene arid intervals.

Human Impact On the Vegetation of the Middle Atlas, Morocco, During the Last 5000 Years

Pollen and magnetic mineralogical analyses of sediment cores from lakes in the Middle Atlas region of Morocco provide evidence for vegetational disturbance, almost certainly anthropogenic, since about 5000 BP. Localized periodic clearances of pine are recorded at Dayat Iffir before a major episode of pine clearance and soil erosion at about 2000 BP. Catchment disturbance at this and two other sites intensified about 1500 years ago, as arable farming increased in a predominantly pastoral economy. Because evergreen oak resprouts after cutting or burning, it has become the dominant forest tree in much of the region. Deciduous oak, once abundant, has become rare. Locally, the abundance of cedar has decreased. Overall, the forests have become less diverse, more open, and scrub-dominated as a result of human exploitation.

Lake — Groundwater Relationships, Oxygen Isotope Balance and Climate Sensitivity of the Bishoftu Crater Lakes, Ethiopia

Oxygen isotope data from groundwater, streams and crater lakes in central Ethiopia provide a basis for modelling lake hydrological and isotopic budgets. The environmental parameters δ A(isotopic composition of vapor) and e* (equilibrium fractionation factor) were determined by a semi-empirical approach using one lake as a terminal index lake. The models show that lake oxygen-isotope composition is more sensitive to rainfall-related parameters (humidity and δ A) than to the isotopic composition of inflow (δ in). The isotopic composition of the lacustrine sedimentary carbonates should therefore provide a record of past rainfall variation.

The Age and Causes of Mid-Late Holocene Environmental Change in Southwest Turkey

Proxy records such as lake sediment sequences provide important data on abrupt environmental changes in the past, but establishing their specific causes from the palaeoenvironmental record can be problematic. Pollen diagrams from southwest Turkey show a mid-late Holocene pollen assemblage zone, designated as the Beysehir Occupation phase, the onset of which has been 14C dated to ca. 3000 BP (ca. 1250 BC). A second millennium BC date for the start of the Beysehir Occupation phase can now be confirmed as a result of the discovery of volcanic tephra from the Minoan eruption of Santorini (Thera) in lake sediment cores from the region. Palaeoecological analyses on sediment cores from Golhisar golu, a shallow montane lake, indicate that tephra deposition was followed by a sustained response in the aquatic ecosystem, in the form of increased algal productivity. The onset of pollen changes marking the beginning of the Beysehir Occupation phase was not, on the other hand, precisely coincident with the tephra layer, but rather occurred at least a century later at this site. Despite the paucity of archaeological evidence for Late Bronze Age settlement in the Oro-Mediterranean region of southwest Turkey, it would appear that the second millennium BC saw the start of a period of major human impact on the landscape which continued until the late first millennium AD. The Santorini ash represents an important time-synchronous, stratigraphic marker horizon, but does not appear to have been the immediate cause of the onset of the Beys ehir Occupation phase.