Fulgurites as Paleoclimatic Indicators - New Approaches on Holocene Climatic Change in the Sahara
by
Sponholz, Barbara
Geographical Institut, University of Wuerzburg/Germany

Abstract Most fulgurites are formed by lightning strikes to sandy ground. The paper describes the occurence of fulgurites in the southern and central Sahara and their palaeoclimatic relevance as well as new methods in the proof of fulgurite fragments in sand samples. All the studied fulgurite fragments were found near to palaeolake sediments in midslope positions of interdune depressions. The mineralogical composition (Lechatelierite, Cristobalite, Chalcedony, Opal) of the fulgurites is related to the palaeo-environmental conditions of the semi-(arid) regions and to the melting conditions during the fulgurite forming lightning strike to the ground.

Résumé La plupart de fulgurites est crée par des coups de foudre dans des sables. L'article décrit la repartition des fulgurites au Sahara méridional et central et leur rôle paléoclimatique aussi que des nouvelles méthodes pour la preuve des fragments de fulgurites dans des sables. Tous les fragments étudiés étaient échantillonés auprès des dépôts paléolacustres en position mi-versant des dépressions interdunaires. La minéralogie des fulgurites (Lechatelierite, Cristobalite, Chalcedony, Opal) est liée aux conditions paléo-environnementales des régions (semi-)arides et aux conditions de fonte pendant le coup de foudre au sol.

Introduction Fulgurites (latin: "fulgur" = lightning) are exclusively formed by lightning strikes to the ground. Therefore fossil fulgurites indicate former lightning and thunderstorm activity as well as thunderstorm-related rainfall. Their general value for palaeoclimatic reconstruction has been pointed out by SPONHOLZ et al. (1993).

In areas where lightning hits mostly quartz sands (e.g. in dunes, river terraces, etc...), melting of the quartz sands will take place at very high temperatures (up to 3,000 K; after FELDMANN 1988) followed by immediate cooling. These processes cause the melted quartz to transform itself into an amorphous substance, the socalled "Lechatelierit”. Also Cristobalite may occur as one of the high-temperature modifications of SiO2 in the external parts of the fulgurites. Chalcedony and Opal are accessorial minerals not strictly related to the fulgurite formation.

As melting occurs along the lightning path through the ground, the fulgurites are more or less cylindrical. Their total thickness is up to several centimeters, the glassy wall around the central tubular void; the former lightning channel; being up to a maximum of some millimeters thick (Fig. 1). This form makes fulgurites very fragile and susceptible to mechanical stress.

Fig. 1: Longitudinal section of a fulgurite (schematic) (after SPONHOLZ et al. 1993):

1: central tubular void, diameter 1-2 cm (A) 2: glass bridge (Lechatelierite) 3: Lechatelierite matrix 4: flow structures 5: former quartz grains, melted with preserved contours 6: quartz grains, partially transformed into Cristobalite 7 and 8: air bubbles 9: longitudinal axis B: thickness of the fulgurite crust: 3-4 mm maximum In the Saharan sites, we found only fulgurite fragments of about 20 cm maximum length, but in fluvial sediments of the Elbe River (Germany), a fulgurite of several meters length was collected. Its shape is similar to a root system, perhaps by alignment of the lightning path through the ground along plant roots. (Fig. 2) Fig. 2: Fulgurite formed in fluvial deposits of the Elbe River (Germany), Staatl.

Sammlung für Naturkunde, Dresden. Regional and topographic setting A very important concentration of fulgurite fragments up to 20 cm long found in the central and the southern Sahara of Eastern Niger (south of 18°N) was interpreted by SPONHOLZ et al. (1993) for palaeoclimatic purposes. Very similar fulgurite sites are known from northern Nigeria (pers. comm.. THIEMEYER) and Mauritania (SPONHOLZ in print).

The former studies on fulgurites were focussed on the southern central Sahara of eastern Niger, an area under today arid, even hyperarid conditions. Towards South, related probably to an increase of rainfall also under Holocene paléoclimatic conditions, the frequency of fulgurite sites and also the number of specimen per site increases (Fig. 3).

Fig. 3: Distribution of studied fulgurite sites in eastern Niger, showing an increase in site frequency towards South. Also the increase in fulgurite fragments per site is shown by the lightning symbols.

(after SPONHOLZ et al. 1993) All studied fragments have been found close to Holocene palaeolake sediments in midslope positions of interdune depressions. This is explained by the presence of dominant electrical fields in the midslope position during thunderstorm events. The reason for the strong electrical field formation is the hydrological situation during fulgurite formation: the waterlevel of the lake-filled depressions is linked to the groundwater level inside the surrounding dunes (Fig. 4).

Fig. 4: The topographic position of fulgurite sites in interdune depressions (schematic) (after SPONHOLZ et al. 1993) Between the water-saturated dune sands at the base, and the overlying dry dune sands, the electric tension is strong enough to direct the lightning strikes to the midslope position. The geomorphological, sedimentological and archaeological characteristics (see SPONHOLZ et al. 1993) of the studied fulgurite sites indicate a main period of fulgurite formation during the middle and the upper Holocene, becoming more recent from north to south in the central Sahara. This is associated with the development of the (palaeo-)monsoon during the Holocene and the progressive southward movement of the northermost monsoon thunderstorm limit. The preservation of the fulgurites around the palaeolakes/interdune depressions, however, is restricted by the fact that in such positions an important destruction of fossil fulgurites is probable because of trampling by both man and animals near these former waterpoints.

Another reason for considerable mechanical fulgurite destruction in the Sahara is corrasion by the strong winds, e.g. in the southern foreland of the Tibesti Mountains west of Faya/Tchad (SPONHOLZ in print).

Analytical studies In order to support the palaeoclimatic interpretation of the fulgurite distribution, mapping and statistical analyses were carried out. As many of the fulgurites should have been completely fragmentized by the above mentioned mechanical stresses since their formation in the Mid-Holocene, even the evidence of very small fragments (sand size) is helpful.

Related to the total volume of a dune body, the share of fulgurites does not exceed several %o. Therefore, the presence of sand size fulgurite fragments in several samples taken from adjacent sampling sites gives the sure information of former fulgurite presence at the same place. Even a short-distance transport of the fulgurite fragments would minimize the very small fulgurite share of the complete dune body so much that adjacent sampling sites would not necessarily contain fulgurite fragments any more.

So how can we get proof of sand size fulgurite fragments? Both silica variations, Lechatelierit and Cristobalite, are characterized by their crystallographic structure: The quartz crystal with regular extinction under polarised light changes to a similar image in Cristobalite, but with "bag-like" disturbances all over the crystal (PICHLER & SCHMITT-RIEHGRAF 1987). The most important part of the fulgurites, however, is composed of Lechatelierit. This mineral does not show any organised crystallographic structure under polarized light, but it looks exactly like the sample bearing glass slide, i.e. a perfect isotropic substance (Fig. 5). This makes the Lechatelierit clearly different from the original quartz sands. In addition, from amorphous to microcrystalline silica the Lechatelierit is easy to distinguish. Opal and Chalcedony occur in the foreland of the Tibesti Mountains by volcanic influence or in other parts of the southern Sahara as surface formation, e.g. in plant silicifications. While these types of amorphous to microcrystalline silica contain a certain amount of water and display at least a minimal stage of crystallographic orientation (Fig. 6), the Lechatelierit does not have these properties.

By thin section analysis of sand samples under polarized light it is possible to distinguish fulgurite bearing and non-fulgurite bearing sand samples (Fig. 7). The preparation of thin sections of sand samples (after "natural" sampling in the field or artificial fragmentation of defined minerals) does not present a difficulty. This method provides a reliable and easy way to prove (former) fulgurite existence e.g. in dune areas or in fluvial (quartz rich) sediments. In regions where fulgurites have formed under palaeoclimatic conditions, at least several grains of lechatelierit are present in the sand samples even if the larger fulgurite fragments have been completely destroyed. With the increasing fragmentation the regional information gets somewhat less precise because of a certain influence of sand transport. On the other hand, this method allows a denser network of mineralogical analyses on fulgurite distribution over a certain area than only macroscopic fulgurite mapping does.

Fig. 5: Fulgurite/Lechatelierit, artificially fragmentised in the Specks mill (thin section under polarised light). The regular dark spots are lechatelierit, the strucural matrix is formed by the resin preparation.

Fig. 6: Opal and chalcedony, partially quarztine, artificially fragmentised in the Specks mill (thin section under polarised light). Fig. 7: Quartz sand from the Bahr el Ghazal/Tchad (thin section under polarised light)..

Fig. 5-7 show 1,3 mm wide details of thin sections Perspectives Fulgurite sites in today arid to semi-arid regions as important paléoclimatic indicators:

o Macroscopic field mapping of fulgurite sites o Microscopic analyses of sand samples in order to identify also minor fulgurite remains Fulgurites as indicators of paléo-monsoon o Absolute dating of fulgurite formation (in progress in 2004) Recent fulgurite formation - comparison of fulgurite formation sites in various climatic and paleoclimatic zones.

The characteristic of fulgurites: being a direct and monogenetic (paleo-)climatic indicator for thunderstorm activity, makes them very helpfull in any reconstruction of climatic dynamics. With the above shown possibility of fulgurite mapping by the help of (easy and quick) sand sample analyses from wide areas, their distribution and their frequency is one essential input into paleoclimatic reconstruction models.

References Feldmann, V. 1988. Comparative Characteristics of Impactite, Tektite and Fulgurite Glasses. In Konta, J. (editor), International Conference on Natural Glasses, Prague 1987, 215-220.

Pichler, H. & C. Schmitt-Riehgraf 1987. Gesteinsbildende Minerale im Dünnschliff. Enke, Stuttgart, 230 p.

Sponholz, B., Baumhauer, R. & P. Felix-Henningsen 1993. Fulgurites in the southern Central Sahara, Republic of Niger, and their palaeoenvironmental significance. The Holocene, 3,2, 97-104.

Sponholz, B (in print): Fulgurites as palaeoclimatic indicators — the proof of fulgurite fragments in sand samples.

Date received: November 30, 2003