|
1. Introduction to Climate Reconstruction 2. Stalagmite sampling
strategy 3. Tree-core sampling
strategy |
The technique of climate reconstruction, that is the application of technology to understand the long-term variation in Ethiopia's climate, can place Ethiopia into a specific climatic context. To an extent, it could produce a model for climate prediction in Ethiopia.
At this preliminary stage, EV is entering into a collaboration with Addis Ababa University (AAU) to attempt a reconstruction of Ethiopia's past climate. The Disaster Prevention and Preparedness Commission (DPPC) have endorsed the collaboration. The aspiration of this collaboration is to facilitate a climate reconstruction laboratory inside AAU within the next 2 - 3 years under a funding and time framework yet to be established.
Scientific knowledge of climate variation has
been determined by climate reconstruction of past events. For
instance, 30,000 years ago, climate deteriorated and the world
entered the last major ice age. As a result of this, the Sahara
reached up to the Ethiopian Highlands. Central Africa's mountain
ranges were covered by ice flow. The River Nile, North of Khartoum,
disappeared.
12,000 years ago, it was climate change that brought 500 years of persistent, heavy rains and transformed the Nile from a sluggish flow into a wild river with gorges one mile deep.
5,000 years ago, a major recession occurred, since when dry conditions have prevailed. Especially in Ethiopia in the last 200 years.
The accumulation of evidence for climate reconstruction has come from a variety of proxy indicators such as sea surface temperatures, vegetation and deep-sea cores. Although much remains to be understood, analysis of samples from each has helped to fill in the details of past climate change in Africa and enhanced scientists' understanding of how global variations affect individual regions.
Stalagmites and tree-cores
are also proxy indicators of
climate variation. Both can contain high-resolution records of annual
rainfall. Although there is a vast difference in life expectancy of
both, they are reliable indicators in themselves and their combined
significance lies in being retrievable from the same watershed.
Positive information from either sample type can corroborate the
value of the other. Stalagmite 'luminescence' uses laser and optical
equipment to digitally reconstruct past events from the sample's
laminae growth bands. Tree cores contain annual growth rings, the
size of which are directly attributable to climatic variation.
Dendrochronology analyses how climate variation has dictated the size
and pattern of growth rings.
Simply put:
|
|
|
|
|
|
It is from these techniques that EV aims to reconstruct past climate in Ethiopia, in detail. How far back we can go depends on the quality of our sampling. But as an indication, a stalagmite the size of a hand, on average, contains 10,000 years of detailed information. Although an indigenous tree may only be 600 years old by comparison, a tree core plays an important role in qualifying the information provided by stalagmites. Especially if they are co-located. More than one proxy indicator increases the chance of establishing a start date for reconstruction.
Stalagmites are found in limestone caves and can properly yield the following information:
1. High-resolution records of rainfall and
atmospheric temperatures.
2. The relationship between the sun and the earth (solar terrestrial
cycles).
3. The effects of pollution and land use changes.
Limestone regions can contain subsurface solutions resulting in the formation of Karst scenery, of which caves and stalagmites (speleotherms) within such caves, are evidence.
"Stalagmites contain naturally occurring chemicals and compounds that are washed onto them by drip water. The drip water becomes trapped within their calcite. These include soil organic acids, metals such as strontium and magnesium that occur naturally in limestone, as well as larger particles such as sediment and pollen grains. Every stalagmite has a different response to climate variation and some of these substances can provide information about surface climate, in theory for as long as the stalagmite has been growing. The difference in response is due to the complexities of water flow within limestone, with waters taking different routes at different times of year." (Stump Cross Stalagmite Science, Gordon Hanley, 1998)
Some factors controlling growth:
"Dendro-climatic research has proven potential for providing extended reconstructions of rainfall-related, particularly river flow, records in diverse regions of the world. Such records are quantified and provide a picture of long-term changes in high frequency climate variability. To date, however, dendro-climatology has proven very difficult to utilise in Africa and most efforts have failed because of an inability to positively identify and date true annual rings." (Exploring the potential for dendroclimatic analysis in northern Ethiopia, D.Conway, March 1997)
When calibrated with the information extracted from climatic-responsive stalagmite samples, adequate tree-ring samples from the same region as the stalagmites, serve to strengthen the reconstruction. When calibrated together this process can only extend to the age of the oldest sample. When compared with meteorological results a three way calibration will only be effective as far back as the first record taken, but can assist with reliability of longer term reconstruction. The adequacy of tree-core samples cannot be determined before laboratory analysis. Samples are taken with best knowledge of indigenous trees in a particular area of study. Analysis of the annual growth rings found in tree-cores can give an accurate record of annual rainfall.
In Ethiopia, juniper
trees, that is tid, appear to have the greatest potential for
containing clear annual rings, seen under a microscope. See below for
justification.
The stalagmite and tree-core results, when calibrated with the existing meteorological data available in Ethiopia, provide the basis for climate reconstruction. We hope to have these results, that is the third phase of Ethiopian Venture, by the end of 2000. We can then conduct a feasibility study for a climate reconstruction laboratory at AAU.
An application to the British government for Ethiopian students to study the analytical techniques in the UK will be submitted. The establishment of a laboratory and the skills that would be brought back to AAU will allow further fieldwork in different regions of Ethiopia, thus fully exploiting the data held in limestone caves and old juniper trees.
If successful, a climate reconstruction model will allow scientists to determine where Ethiopia is within the present climatic cycle, and possibly what the future holds. How this information is exploited is in the hands of the Ministries and Institutions of Ethiopia.
see geological map
Limestone outcrops in the following regions of Ethiopia: Bale, Hararge, Blue Nile gorge, Tigray and Danakil. Only three of these regions have documented limestone cave potential: Tigray, Bale and Hararge. Several early explorers visited the largest cave system of Sof Omar, Bale, but the first published accounts are those of Causer who visited Zayei Cave in Tigray (1962) and Robson who describes a visit to Sof Omar. These regions were explored and mapped most extensively by the British Speleological Expedition (BSEE) to Ethiopia (1972) and two expeditions by the Huddersfield University Caving Club: Cave Ethiopia '95 and CE '96. BSEE reports the following of the Mesozoic period of cave bearing formations:
"Taken as a whole, the Antolo limestones in Ethiopia tend to be more thinly bedded and with more marl and shale bands than the Carboniferous limestones of the English Pennines. In many parts of Ethiopia, including most of the region northwest of the Rift Valley, the limestones are comparable with the English Jurassic limestones. Subsurface solution undoubtedly occurs, and the large deposits of calcareous tufa are found around small springs and also along river courses in Tigre, but well developed karstic features, including notable caves, are lacking. South of the Rift Valley a more promising situation is seen. The limestones of Bale and Harrar provinces include many thickly bedded horizons and the proportion of marls and shales is lower. Surface karstic features are common in many areas…"
Of the Precambrian cave bearing formations:
"Steeply inclined [marble…locally grading into little altered limestones] beds up to several hundred metres thick are found west and northwest of the Antolo limestone area around Mekele. Near Wukro, due North of Mekele, several shallow caves occur in a dark Precambrian limestone which has developed clints and grikes on the hilltops. Similar karstic features are developed elsewhere in northern Ethiopia on such limestones, but no caves… have been reported by Survey geologists… Nevertheless these areas warrant further investigation."
BSEE discovered 6 km of new passage in Sof Omar, Bale. They also discovered vertical pits in Hararge. In Tigre, despite producing a sturdy account of limestone scenery, they only discovered a few small caves.
CE '95 and '96 report the following: "There are over 100,000 square kilometres of carbonate rock in Ethiopia but very little is known about the extent of karst terrain. The total length of known limestone caves is less than 30 km giving what must be one of the highest ratios of limestone outcrop to cave passage of any country… All cave passage has developed within the Antolo limestone, which is upper Jurassic age, by exploiting faulting and fracturing generated by Rift Valley tectonics… Ethiopia is a fascinating and under-explored country, and as a result information for planning expeditions is difficult to find…"
Indeed detailed accounts of the caves explored so far at Sof Omar and in Hararge lay testament to the potential of further areas of Antolo limestone which remain unprobed.
The first two regions (Bale and Hararge) are well documented by BSEE and CE '95/'96 and their potential is summarised in their reports.
The third region, around Mekele, Tigray is described by BSEE: "the Antolo limestones outcrop over a large area and attain a thickness of about 800m, the thickest well-exposed succession in Ethiopia. Much of it is marly limestone, however, with many clay and shale layers."
Further, the Afar Depression is described to be strongly faulted and relatively arid in the Danakil. For this reason the likelihood of caves in the Danakil is described by BSEE to be 'not promising' as well as a 'very inaccessible' area.
This only leaves the gorges of the Blue Nile. EV has found very little documentation of the region's potential. Perhaps because it is very unlikely that any caves exist, or that there is any degree of Karst terrain. Only three factors mitigate this presumption:
(i) It is a relatively difficult area to
access, due to water level fluctuations and extremely steep sided
gorges.
(ii) It has been a politically sensitive region, as a headwater of
the Nile.
(iii) There are greater areas to be explored further of definite
potential.
BSEE describe the area: "Northwest of the Rift Valley the Antolo limestones are again encountered in the gorges of the Blue Nile and its tributaries. In this region the limestone members of the Antolo formation contain a high proportion of marl and clay layers, pure limestones not exceeding 50 metres in thickness in the better known easily accessible sections. Where the main Addis Ababa-Gonder road crosses the Blue Nile gorge, pure limestones do not exceed 25 metres in thickness."
It is the regions south of the Rift Valley (where "Surface karstic features are common…") that are well documented with exploration potential but in the region north of the Rift Valley (where "well developed karstic features, including notable caves, are lacking…") that EV intends to explore.
(a) Blue Nile Gorge: Along the route of EV, the Antolo limestone region extends to either side of the Muger tributary of the Blue Nile leading from Addis Ababa to Dejen. The Blue Nile is in a basin of Antolo until the river crosses 11°N roughly opposite the town of Mota. A region of 'geologically possible' cave entrances are estimated graphically by Dr Baker, Newcastle University, UK, based on a research model from the Yorkshire Dales. Baker estimates that caves exist at a distance approximately between 150km and 330km from Lake Tana. They all fall within the limestones, sandstones and shales (200 million years old) section of the Blue Nile. i.e the section from Lake Tana to 350km along the Addis Road and between 1700m/1200m above sea level. Terrain restrictions, lack of roads and administrative boundaries limited us to approaching the gorge from two woredas (sub-regions): Dejen 038°10'E/10°10'N and Bichena 038°12E/10°26'N.
Dr Baker's theory was proved correct at Location #1, where a limestone cave was located.
(b) Tigray Region: As in the Blue Nile Gorge, the limestone outcrop in Tigray is clearly small in comparison with Bale and Hararge, but not without considerable potential. 16 sites are reported by BSEE (1972) in the Tigray limestone region. The caves are mentioned as containing karstic scenery. "The Geological survey have subdivided the Antolo limestone into 5 units, 'Jta-Jte', the whole comprising about 750m of yellow marls and limestones. The only massive limestone occurs in 'Jta' and 'Jtd' and outcrops as cliffs." EV intended to further survey this region and examine stalagmite potential.
The BSEE report was unfortunately misleading on the two occasions that EV pursued their groundwork. EV was only able to visit 3 out of 13 sites deemed to hold potential. Two of these yielded results.
a. Completion of comment on all Antolo regions
of Ethiopia.
b. Increased potential for climatic reconstruction of northern
Ethiopia when combined with dendro-climatological potential of the
same region.
c. Complementary value to 'Famine Food Survey'
d. Field test of the Baker theory.
As stated earlier we were working under theoretical guidance with no prior knowledge of caves / stalagmites in the Blue Nile Gorge and limited details of stalagmite-yielding caves in Tigray. Our methodology relied almost totally on local knowledge. For as many times as the below table describes success, there were equally days when long treks would end with dismal disappointment.
Provided village authorities are persuaded of your legitimacy and can read your letters of permission/introduction, it will not be long until a local may be introduced to assist with his knowledge of the area. From this point on, the results are unpredictable. Communication with such individuals should be as informed as possible. Local men were willing to accompany us on every trek, provide mules and arrange a night in a village if necessary. We carried sufficient provisions and purification kit to camp over if necessary.
Example 1- which did not work…
EV: "Are there any caves in the vicinity"
Response: "Yes, we used them to shelter from bombs during the
war."
(Analysis: Sounds promising, let's give it a
go.
Result: 6 hours trek on difficult ground to look at a granite
overhang.)
Example 2 - a more successful method...
EV: "What are the names of the largest caves in
this area?"
Response: "Enda Abab Cave" (dismissive wave in a general
direction)
EV: "Which of these photographs does it most resemble? Is there water
running through or near it? What colour is the rock?"
Response: (According to photograph recognition)
EV: " How far is it to walk?"
Response: "Far. Maybe half a day. Women cannot enter."
(Analysis: He seems to know what we are after
and others confirm this.
Result: Much more predictable and promising.)
1 x large metal mallet; several stone masons
chisels (varying sizes).
Scoring around the outline of the sample and chiselling carefully
around the edges will remove the sample intact.
Once back in the vehicle, the samples can be wrapped in sponge foam, taped up with masking tape and labelled.
Blue Nile Section:
1. A thorough investigation of the Jb and Jt Antolo limestone outcrop at the Blue Nile Gorge and its immediate tributaries from 10°00'N - 10°45'N and between 1300-2000 metres above sea level.
2. An investigation of similar limestone features on the River Uolaka and tributaries from 038°30E/10°22N to 038°58'E/10°20N and 038°50'E/10°32N.
Tigray Region:
1. Remaining sites listed in BSEE report.
2. Specific investigation of sites where a river runs through bedrock changing from Antolo limestone to Adigrat Sandstone. (e.g. site 3, Hadmet Cave).
"During sampling it was apparent that two species possessed good ring structure (Juniperus procera and Ekebergia capensis) and so sampling was concentrated on these two species." Numerous samples were taken by Conway in 1996: 354 cores from 188 trees of 18 species, but of all he concentrated on measuring and cross-dating the Juniperus cores.
A major comment made by Conway is the ambiguity in identifying annual (as opposed to intra-annual) ring boundaries and the occurrence of partially missing rings. The report recommends a larger sample base in order to achieve more consistent identification of false rings.
1. Inherent difficulties in cross-dating trees. It is not possible to guarantee that all rings are true annual growth rings.
2. Despite a well-defined dry season in Northern Ethiopia (4 months under 30mm) it is possible that rainfall or moisture supply during some years is sufficient during the dry season to maintain continuous growth and prevent annual growth rings.
3. Changes in wet season structure, such as early rains followed by a dry period, leads to false formation of rings.
Dr Conway outlined several areas as holding potential for collections of juniper. These centred on protected forests in the Simien Mountains and Taragedem forest, Gojam. Dr Sue Edwards, National Herbarium, AAU told us of 'dying' forests at Agula, Tigray and around Lake Ashange, Tigray. Our remit was to identify areas as close as possible to cave sites. We achieved this at cave sites 1, 2 & 3. Other juniper sites were found during the course of the fieldwork and samples taken to satisfy the first three justifications. Indeed, juniper is found in sparse collections in all regions of northern Ethiopia and is a common feature of areas with protected status. This category is mainly comprised of church grounds. Our risk analysis had limited us to locating sites other than at churches due to the sensitivity attached to sacred items on holy ground.
1. Locate more samples of the tree types that Dr. Conway had proved most climatically-responsive during a field study in 1996 i.e. supplement the general Juniper data already possessed by Dr Conway.
2. Facilitate a more detailed analysis of the Juniper ring structure to improve the overall Juniper ring-width series and ability to assess climate variation.
3. Assess the wider distribution and quantity of mature Juniper in northern Ethiopia.
4. Sample in conjunction with 'Baker' project where possible to allow the most efficient cross-calibration of the three variables (stalagmites, tree-cores and meteorological figures for all regions concerned).
Realistic sampling was achievable with reference to potential for dendro-climatic analysis in northern Ethiopia conducted by Dr Declan Conway, 1996. With basic tree recognition skills we were able to easily identify juniper, either as part of a forest, as isolated trees and indeed, inside church grounds.
Areas of juniper ('tid') are relatively easy to locate despite being found in isolated groups. Local co-operation was forthcoming in all areas visited. Access to samples inside churchyards was granted at sites 2, 3 and 5 by priests who were extremely keen to assist. (The priest at site 2 however voiced his displeasure at the ordination of women priests in Britain.)
As with the stalagmite sampling an informed and illustrated delivery of intentions assisted in access and compliance.
2 x tree-corers.
The technique is simple and can be learnt in ½ an hour with a
practitioner.
Corrugated cardboard is the most suitable shape to protect the cores, which are on average 10cm long. Plenty of clear tape is needed to secure the cores. The card can be rolled and placed in a poster tube for protection.
There is unlimited potential for this style of research. Many 'tid' sites are found in church grounds. Areas previously forested also retain a few of the original species. Forests are largely immature or in small protected pockets, but individual examples are not rare.
|
|
|
|
|
|
|
|
|
|
|
|