ABSTRACT

    The stabilized sand dunefield of the middle São Francisco River represents one of the most important records of paleoenvironmental change, mainly paleoclimatic, during the Quaternary of northeastern Brazil. Such importance is not only a consequence of its large size (ca. 7,000 km²) and of the thickeness of its sediments (more than 100 m), but also because it contains a record of past climates, drier than present, which have strongly affected the evolution of its flora and fauna.
    Five geomorphological domains, which are representative of distinct eolian dune generations, have been recognized in the area through geomorphological and sedimentological studies. In addition, pollen analysis in peat bog sediments and sedimentological studies of sands indicate that tropical rain forest of Atlantic and Amazonian floristic affinities expanded in the Icatu River Valley during the Late Pleistocene. This forest expansion could explain the existence of present-day tropical forest enclaves with such affinities, within the modern phytogeographical domain of the northeastern brazilian caatinga.
    Therefore, the environmental protection and preservation of this important ecosystem is of paramount importance for the maintenance of the modern and rich biodiversity patterns found in the area, which in turn are consequence of past environmental change.

INTRODUCTION

    The inactive dunes of the middel Rio São Francisco River, presently stabilized by vegetation, are also referred to as fossil dunes. They represent a heritage of past climates in the area which were drier than present (Thomas & Shaw, 1991). They generally exhibit original forms and shapes that have been partially altered by weathering and erosion due to rainfall precipitation. Fossil dunes can be found in various parts of the world, in deserts or adjacent to them (Fig. 1), and represent a source of information on paleoclimates and paleowind patterns (Melton, 1940; Lowe & Walker, 1997).

    The inactive dunefield area presented in this study is located between the towns of Barra and Pilão Arcado, State of Bahia, have the São Francisco River and the Serra do Estreito mountain chain as its eastern and western geographical limits, respectively.
The barren landscape and consequently its low human population density are factors that favor and facilitate environmental protection measures in the area. In addition, the studied area is a suitable site for paleoenvironmental and paleoecological research that can contribute to a better understanding of its present day floristic and faunal biodiversity patterns.

LOCATION

    This stabilized dunefield, located 700 km away from the capital city of Salvador, occupies the NW portion of the State of Bahia, between 10⁰00’e 11⁰00’S latitude and 42⁰30’ e 43⁰20’W longitude (Fig. 2), thus belonging geographically to the municipalities of Barra, Pilão Arcado and Xique-Xique. The area can be reached by the BR-324 highway, from Salvador to Juazeiro, and later by the BR-235 highway to Pilão Arcado.
    The sandy nature of the terrain, the abscence of paved roads and the precarious infra-structure of the region restrict access to four-wheel or animal-traction vehicles.

HISTORY OF SCIENTIFIC STUDIES

    The sand dunefield was referred to as "the little Sahara along the São Francisco River"by Williams (1925). According to this author, the São Francisco River was the source for its sands, which were transported during dry climates by eolian activities and later stabilized by vegetation.
    Moraes Rego (1926) established the Vazantes Formation, which included the alluvial and eolian deposits of the dunefield bordering the São Francisco River.
    The first paleoclimatic considerations about the area are those of Domingues (1948), who attributed its origin to the supposedly pronounced aridity during the last glacial period of the northern hemisphere. According to this author, the São Francisco River would have acquired a senile phase in the Pleistocene characterized by intense sedimentation, synchronous with the formation of a wandering meander system due to its reduced capacity for sediment transport. The latter, was in turn, believed to be insufficient to carry all the sedimentary load of the river.
    According to King (1956), the eolian sands of the middle São Francisco River were the result of the post-Velhas erosive cycle, which formed the Plio-Pleistocene applanation. King believed that the main river had a NW flow into the Tocantins River, but it was brought to its modern position by fluvial piracy.
    This stabilized or inactive dunefield was interpreted as geomorphological evidence of a climate drier than present, which according to Tricart (1974) persisted during the Last Glacial Maximum ca. 18,000 years BP. He hypothesized that the region was characterized by endorheic drainagem pattern, which flowed into a huge lake and that the present exorheic pattern became established at the end of the last glaciation ca. 12,000 years BP.
    Goudie (1983), based on Tricart (op. cit.) included this inactive dunefield in a world-based map of areas subjected to eolian activities during the Last Glacial Maximum. Schobbenhaus et al. (1984) also accepted Tricart’s interpretation (op. cit.) and suggested that this area represents "the one and only example of dune formation during the desert-like environment of the Quaternary of Brazil".
    According to Costa (1984), the parabolic and longitudinal dunes reaching up to 50 m in height were deposited by SE and E winds and that its sands were originated from the São Francisco River and barred by the Serra do Estreito mountain chain, which acted as topographic barrier, thus limiting its westward expansion. Costa (op. cit.) also suggested that during the final stages of the last glacial cycle of the northern hemisphere, moist climates allowed the shift from an endo- to exorheic drainage pattern. Thus, the dunes would have been covered by the vegetation, thus becoming stabilized by a very unstable equilibrium. This environmental unstability prompted this author to suggest the incorporation of this area to the National System of Conservation, with assured permanent and compulsory preservation, which could be facilitated by the inexpressive pattern of human occupation, which is attributted to the local edaphic and hydrological constraints.
    Rodrigues (1991) studied the herpetofauna and identified new genera and species of lizards completely adapted to subterraneous life in the sands and similar to those reported for australian and south african deserts. According to this author, these animals had a common ancestral species whose distribution range was isolated by a geographical barrier provided by the São Francisco River, thus allowing the rise of three new species.
    Barreto & Suguio (1993) obtained sedimentological data that indicates that the São Francisco River had been the only source for the sands of dunefields, thus contradicting the ideas of Costa (1984). These authors compared the present sediment load of this river to the estimated volume of eolian sand that exists between the localities of Barra and Pilão Arcado and concluded that, at least, 100,000 years would have been necessary for its accumulation.

SITE DESCRIPTION

Climate

    According to the Köppen system the climate of the area can be classified as Bswh (semi-arid and hot with a 7-8 month long dry season). Annual average precipitation ranging from 400 to 800 mm is restricted mainly to the period of October to March. According to Nimer (1977, 1989), average annual temperature of coldest month is higher than 18⁰C and average of annual mean temperature is not higher than 27⁰C. The seasonal fluctuations of wind patterns are associated to the atlantic equatorial and to the atlantic tropical air masses in the winter, whereas in the summer they are associated to the continental equatorial air mass. Wind velocities, measured at the Meteorological Station at Barra and Remanso, between the years of 1928-1942 and 1972-1976 are relativelly low, ranging from 1.8 to 3. 1 m/s (Pessoa, 1979; Silva, 1974).

Vegetation

    The poorly developed sandy soils of the dunefield and the prevailing semi-arid climate is conducive to caatinga vegetation on the landscape (Jacomine et al., 1976).
    Caatinga plant formation can be subdivided into hyperxerophytic (shrub caatinga) and hypoxerophytic (arboreal caatinga) caatinga categories. Hyperxerophytic caatinga is preferentially well developed adjacent to the São Francisco RiverValley. Population density of shrubs is variable and imparts an open and scrub-like aspect to the landscape. Typical plant taxa of this vegetation type are Cnidoscolus phyllacanthus, Euphorbia and myrtaceaeous shrubs, spiny bromeliads and cacti (Opuntia inamoema). The hypoxerophytic caatinga is well represented on the eolian deposits and resembles a transitional vegetation type between the caatinga (sensu lato) and dry deciduous forest. This vegetation type is characterized by the presence of trees of Pseudobombax (Bombacaceae), Maytenus (Celastraceae), some arborescent cacti (Pilocereus sp., Cereus sp.).
    A gallery forest containing true caatinga elements such as the Copernicia cerifera and other palm species borders the São Francisco River. Also present in this vegetation type is the herbaceous Bromelia sartorum. On the margins of the few tributaries of the São Francisco River draining the dunefield area and on the interdune lowlands Mauritia vinifera palms form dense groves on the hydromorphic soils, on which the aquatic taxon Typha sp. becomes abundant.

Geomorphology and Geology

    The studied area is located in the peripherical depression of the middle São Francisco River, with elevation ranging from 400 to 800 m (IBGE, 1977).
    The features related to eolian sedimentation were analysed according to sedimentological and morphological characteristics, post-depositional modifications and paleowind patterns (Barreto, 1996). This study allowed the division of the area into five geomorphological domains (Table I, fig. 3,4,5).

Figure 3 – Simplified map of geomorphic domains (Barreto, 1996)

    Each geomorphological domain contains an area with similar morphological characteristics, originated at the time of sedimentation and of the post-depositional modifications which can contain more than one stage of dune generation. These premisses were considered and provided a guideline to the sampling of sands for sedimentological and thermoluminescence dating analyses.
    The height of the dunes is variable and ranges between 5 and 10m up to 50 to 60 m. Mean height varies between 15 and 25 m. Horizontal dimensions of dunes range from 1 to 3 km but some some parabolic dunes reach lengths greater than 10 km. According to the classification system proposed by Pye (1993) a large number of variable simple and compound parabolic dunes was found. The most common dune shape types found in this study are nested, echelon or rake-like, digitate and superimposed (Fig.6). This great diversity of shapes of parabolic dunes can be attributed to the variability of the paleowinds at a regional scale. The paleowinds responsible for the formation of these dunes are characterized by two main wind direction modes (NW and W), with a dispersion rate of 53⁰ (Barreto, 1996).

    The overwhelming size of the dunes can be attributed to the combined effect of high energy (velocity) of the winds, couple to the abundant supply of sands and to the local resistence to migration imposed by obstacles such as the vegetation and rock outcrops.
    The present state of knowledge on this system has generated various doubts regarding the relationship between paleowind patterns and the different dune generations. However, from the Late Pleistocene until Present, at least three main episodes can be distinguished:

a) From 28,000 to 15,000 years BP - During this time interval the main parabolic compound and nested dunes were generated. These were charaterized by shapes varying from "V" shaped elongated to simple elongated parabolic forms. The winds had a SE-NW direction, with single modal regimes and had a low dispersion rate.

b) From 9,000 to 4,000 years BP - During this period, compound and nested parabolic, as well as superimposed, digitated, rake-like and nested dunes with predominantly closed "U" shaped forms were generated. These reflect a E-SE to W-NW paleowind direction. Although, the latter had a single modal distribution, these winds favored higher dispersion rate of the sediments, which enhenced the occurrence of a variety of parabolic forms.

c) From 4,000 to 900 years BP - A new generation of dunes, superimposed on the older ones, was generated. These dunes were composed primarily of nested, elongated and asymetrical in shape. They were smaller than those of the previous generation and reflected paleowinds with SE to NW direction and lower dispersion rate.

    The three dune generations above mentioned are found superimposed on each other on the north central portion of geomorphological domain 4. The last two dune generations, mainly those found in the domain 3, along the São Francisco River, suggest that the main and almost only source of eolian sands to this system was the river, a hypothesis strongly supported by sedimentological analyses of the sediments.
    Grain size, morphoscopic and mineralogical studies have revealed high rates of compositional and textural maturity, thus suggesting them to be polyclic sands.
    Seven profiles of vertical electrical surveys, combined with the topographic caracteristics of the area, suggest that basement of the dunes is located at 140 m below the present base level, represented by the São Francisco River. These studies also revealed that eolian sand thickeness range from 50 to 150 m.
    Under the dunes there is an important aquifer, which has not yet been utilized due to the low human population density in the area but which represents a potential source of water supply for domestic and farming purposes.
    Thermoluminescence dating of 42 dune sand samples have indicated important phases of eolian activity from 28,000 to 900 years BP, which were intercalated by quiescence or periods of dune stabilization (Fig.7). The lack of dates ranging from 10,500 to 9,000 clearly suggest that, during the onset of the Holocene, eolian activity was reduced on the landscape. On the other hand, the high frequency of ages ranging from 4,500 to 1,700 years BP indicate an increase in eolian activity in the area. The thermoluminescence and radiocarbon dates were compared by using contiguous samples. The dates yielded by the two different methods were statistically similar.

Figure 7 – Graphic representation of the samples dated by Thermoluminescence.

    The majority of thermoluminescence dates from eolian sands are in close agreement with a paleoclimatic scenario of increasing aridity, as indicated by the rise of caatinga and cerrado vegetation in the last 4,000 years BP. This scenario is also supported by the occurrence of 12 sites, within an area of approximatelly 1,000 km², in which charcoal fragments were found dispersed in the dunes and dated by the ¹⁴C method (Barreto et al. 1996). Increase in aridity during the Late Holocene could possibly be related to climatic phenomena of long duration (decades to centuries) of the El Niño type as it has been suggested for other areas in Brazil (Meggers, 1994; Turcq et al., 1998).

Palynology and paleoclimates

    A palynological study was conducted in peat bog sediments of the Icatu River Valley, the main tributary of the São Francisco River in the study area. The studied locality known as Saquinho lies at 10⁰24’S and 43⁰13’W. The sediment core yield a basal age of 10.900 anos years BP (De Oliveira et al., 1997). This study, which represents the first palynological study in the semi-arid caatinga region of Brazil, allowed the authors to identify five distinct climatic and vegetational phases over the last 11,000 years.
    At the end of the Pleistocene and onset of the Holocene, more humid and colder climatic conditions permitted the expansion of gallery tropical forest in the floodplains of the dunefield system. Caatinga vegetation was less conspicuous on the landscape from 11.000 to 8,900 years BP but some of its botanical elements have always been present in the ecological profile of the sediments. This vegetation type became prevalent from 4,240 years BP to the present.
    The palynological study of De Oliveira et al. (1999) together with thermoluminescence data do not give any support to the simplistic hypothesis of Tricart (1974), who believed that this extensive dunefield system was formed during the Last Glacial Maximum, between 18,000 and 14,000 years BP. In the last 30,000 years, eolian reworking of the dunes appear to have been more intense in the Holocene rather than during the Late Pleistocene. Moreover, the great thickeness of the eolian sand deposits suggests that this system was probably formed ca. 2 million years ago or even in the Late Tertiary.
    Finally, the palynological analyses suggest that in the Late Pleistocene a large expansion of tropical rain forests with Atlantic and Amazonian floristic affinities were succesfully able to colonize the Icatu River Valley. Such event could explain the modern botanical affinity of the controversial tropical forest enclaves found presently within the caatinga phytogeographical and climatic domain. These forest enclaves have long been known to represent disjunct distribution of amazonian and atlantic forests in northeastern Brazil and to represent evidence of important paleoclimatic events (De Oliveira et al., 1999)

ENVIRONMENTAL PROTECTION

    The stabilized dunefield system of the middle São Francisco River, in the State of Bahia, represents a very important record of geomorphological, geological, climatic and biological evolutionary change, which are in large part is due to past paleoclimatic variability during the Quaternary in northeastern Brazil. On the other hand, faunal and floral diversities reflect the dynamic physical scenario found in this system.
    Due to high susceptibility to erosion of these sandy soils, human occupation in the area must be prevented for it may lead to a complete degradation and deterioration of these ancient sand dune systems. These inturn could lead to irreversible hydrological and biodiversity changes. There is absolutely no doubt that increased human densities would exacerbate the ongoing geological processes. The nutrient poor soils used by the small local human population of the area, the subsistence problems encountered by them and the difficulties in accessing the region favor the politics of permanent environmental preservation.
    Therefore it is essential that the vegetation, which protects the landscape against erosive forces and eolian activity, should be protected. Moreover, the native vegetation which controls the prevailing hydrological regime is not only a source of food to the caatinga animals but also provides ecological habitats for a very endemic and poorly still studied fauna.

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