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Past environmental and climate changes in northern Tanzania: Vegetation and lake level variability in Empakaai Crater
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents palaeoenvironmental data from equatorial Africa covering two important time intervals; i) the warming period forming the Pleistocene/Holocene transition and ii) the last millennium. The Empakaai Crater, in northern Tanzania contains a lake from where sediment cores, spanning two time-slices 14.8-9.3 ka and 800-2000 AD, have been studied. Palaeoecological and palaeohydrological reconstruction is based on a multitude of proxies from the sediments, representing both catchment environment and the lakes aquatic ecosystem response. Between 14.8 and 10 ka the catchment vegetation and lake hydrology responded to both regional climate changes and local environment, but with different amplitude and frequency, reflecting temporal and spatial lags between the two systems. However, at c 10 ka both lake conditions and catchment vegetation showed drastic changes towards drier conditions. The record covering the last millennium reveals environmental changes related to climate and human activities. The catchment’s vegetation was affected by frequent fires, most probably human induced, while near shore vegetation responded to lake level fluctuation associated with rainfall variability. About 15 km from Empakaai Crater is an extensive abandoned irrigation system, the Engaruka complex, which was in active use between c 1400 AD and 1840 AD. By comparing a number of social and environmental factors potentially influencing the societal development at Engaruka it is shown that wet climate conditions have had positive effects on the societal development but also that dry climate conditions were not always disastrous to the society. The resemblance of the pollen taxa present is strong between the two time slices and pollen representing catchment conditions respond in similar manner in both records. The lake conditions are however very different between the two periods Thus the lake responds to both long and short term changes of variable amplitude, while the catchment vegetation seems to responds to high amplitude, low frequency changes.

Place, publisher, year, edition, pages
Stockholm: Institutionen för naturgeografi och kvartärgeologi , 2007. , 102 p.
Series
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 5
Keyword [en]
palaeoecology, palaeolimnology, physical and societal factors, Holocene, Tanzania, Lake Emakat, Engaruka
National Category
Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-6835ISBN: 978-91-7155-455-0 (print)OAI: oai:DiVA.org:su-6835DiVA: diva2:197167
Public defence
2007-06-01, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 13:00
Opponent
Supervisors
Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2010-10-07Bibliographically approved
List of papers
1. Vegetation changes in Empakaai Crater, northern Tanzania, at 14,800-9300 cal yr BP
Open this publication in new window or tab >>Vegetation changes in Empakaai Crater, northern Tanzania, at 14,800-9300 cal yr BP
2006 In: Review of Palaeobotany and Palynology, ISSN 0034-6667, Vol. 140, no 3-4, 163-174 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24290 (URN)
Note
Part of urn:nbn:se:su:diva-6835Available from: 2007-05-10 Created: 2007-05-10Bibliographically approved
2. Climatic and hydrological instability in semi-arid equatorial eastern Africa during the late Glacial to Holocene transition:  a multiproxy reconstruction of aquatic ecosystem response in northern Tanzania
Open this publication in new window or tab >>Climatic and hydrological instability in semi-arid equatorial eastern Africa during the late Glacial to Holocene transition:  a multiproxy reconstruction of aquatic ecosystem response in northern Tanzania
2007 (English)In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, Vol. 248, no 3-4, 440-458 p.Article in journal (Refereed) Published
Abstract [en]

This paper reports new multi-proxy palaeoenvironmental data on the late Glacial-Holocene transition (c. 14.8–9.3 ka) in equatorial East Africa, in the form of microfossil assemblages (chironomids, diatoms and ostracods) recovered from the sediment record of Lake Emakat, Empakaai Crater, northern Tanzania. In the context of available palaeoclimatic and palaeoecological information from the region and previously published fossil pollen and carbon and nitrogen isotopic data for the same sediment sequence, we here reconstruct local lake-system response to regional climatic and hydrological instability during the period of post-glacial warming. The aquatic biological proxy indicators suggest that the water level and chemistry of Lake Emakat evolved, first from a shallow freshwater body at 14.8 ka to a deeper freshwater phase between c.14.4 and 10.3 ka and then to a markedly shallower, alkaline-saline environment after c.10.3 ka. The lake appears to have been deepest between 13.2 and 12.0 ka, at a time of climatic drying when moist montane forest vegetation within the lake's crater catchment was being replaced by open wood-and scrubland. Some palaeohydrological changes reconstructed for Lake Emakat are in phase with lake evolution elsewhere in the region and thus apparently track broad-scale climate changes, but some are not. Collectively these multi-proxy paleolimnological data indicate a complex adjustment of the local aquatic ecosystem to temporal variations both in total annual effective precipitation and its seasonal distribution. The lake's hydrological response was further conditioned by local factors, notably its geological and topographic setting.

Place, publisher, year, edition, pages
Elsevier B.V, 2007
Keyword
Climate change; Crater lake; Chironomids; Diatoms; East Africa; Empakaai; Multi-proxy reconstruction; Ostracods
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-24291 (URN)10.1016/j.palaeo.2006.12.014 (DOI)000247250200009 ()
Note
Part of urn:nbn:se:su:diva-6835Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2010-12-21Bibliographically approved
3. A record of vegetation dynamics and lake level changes from Lake Emakat, northern Tanzania, during the last c. 1200 years
Open this publication in new window or tab >>A record of vegetation dynamics and lake level changes from Lake Emakat, northern Tanzania, during the last c. 1200 years
2007 (English)In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 40, no 2, 583-601 p.Article in journal (Refereed) Published
Abstract [en]

Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (14C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS 14C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.

Place, publisher, year, edition, pages
Springer, 2007
Keyword
Africa - Climate change - Palaeoecology - Lake level change - Tanzania
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-24292 (URN)10.1007/s10933-007-9184-0 (DOI)
Note
Part of urn:nbn:se:su:diva-6835Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2017-12-13Bibliographically approved
4. The development of the ancient irrigation system at Engaruka, northern Tanzania: Physical and societal factors
Open this publication in new window or tab >>The development of the ancient irrigation system at Engaruka, northern Tanzania: Physical and societal factors
Show others...
2010 (English)In: Geographical Journal, ISSN 0016-7398, E-ISSN 1475-4959, Vol. 176, no 4, 304-318 p.Article in journal (Refereed) Published
Abstract [en]

Climate data from Empakaai Crater in northern Tanzania, covering the last 1200 years, are related to the establishment, development and decline of the ancient irrigation system at Engaruka. New dates for the system are linked to reconstructed climatic variations and historical data on long-distance and regional trade and migration patterns. A shift from a comparatively humid climate to drier conditions in the 1400s prompted the establishment of irrigated agriculture at Engaruka, and a flourishing long-distance trade increased its value as a water and food source for passing caravans. Once established, the land-use system at Engaruka was sufficiently resilient to survive and even intensify during much drier climate from c. 1500 to 1670 CE (Common Era) and during the decline of caravan trade between c. 1550 and 1750. The ancient land-use system probably reached its maximum extension during the humid conditions between 1670 and 1740, and was deserted in the early to mid 1800s, presumably as a result of the added effects of climate deterioration, the Maasai expansion, and change of livelihood strategies as agriculturalists became pastoralists. Towards the end of the 1800s irrigated agriculture was again established at Engaruka, in part driven by the transfer from pastoral to agricultural livelihoods caused by the Rinderpest.

Keyword
Engaruka, Tanzania, pre-colonial irrigation, climate variability, incremental change, Maasai
National Category
Physical Geography Human Geography
Identifiers
urn:nbn:se:su:diva-24293 (URN)10.1111/j.1475-4959.2010.00370.x (DOI)000283727600003 ()
Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2017-12-13Bibliographically approved

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