III.    Cabon Cycle and Climatic Variations

(Papaer presented at Nouakchott Meeting, Jan., 1997)

A. VELICHKO¹⁾O, O. BORISOVA¹⁾, H. FAURE²⁾ L. FAURE NARD³⁾
C. KREMENETSKI¹⁾, T. MOROSOVA¹⁾, V. NECHAEV¹⁾, E. ZELIKSON¹⁾.

1)Institute of Geography, Russian Academy of Sciences Staromonetny lane 29, 109017
Moscow Russia Tel: 7-095-238 0298 Fax: 7-095-230 2090 E-rnail: paleo@glas.apc.org
2) LGQICNRSICEREGE, Case 907, Faculte des Sciences de Luminy, 13288 Marseille
Cedex09 - France Tel: 33-0491269952 Fax: 33-0491413879
E-rnail:faure@riou.univ-mrs.fr
3)CEREGE, BP 80,13545 Aix en Provence Cedex 04 - France

Spatial reconstructions of climates and landscapes for the Northern Hemisphere based on proxy data show a drastic change in temperature and precipitation as well as in the global structure of ecosystem belts (Atlas of palaeoclimates and palaeoenvironments..., 1992). Under global climate changes variations of climate parameters were different in high and low latitudes. In low latitudes they were manifested as phases of humid and arid climate, which changed seriously the biomass balance and so influensed the carbon cycle.

Data on Eurasia and Africa proved that under global warming + 2 (optimum of Eemian interglacial, phase 5e) the most serious increase in temperature occurred in high latitudes, where annual temperature was 6-8 higher than now. Locally the increase in winter temperature was up to 12. Southward positive temperature deviations became lower. At latitudes below 30-35⁰ N the increase in temperature was very low, and in some parts there was a small decrease in temperature (up to 1). The intertropical zone was characterized by high increase in precipitation, including hot deserts area, where according to the several estimations precipitation increased up to 300-400%.

In high latitudes increase in precipitation was within 10-50%. Estimations in evaporation shift demonstrated that in boreal forest and tundra belts an increase in summer temperature up to 6-8 provoked a relative dissecadon, which was favorable for the biomass balance.

In Eastern Europe and Siberia northern treeline moved north up to 500 km to north as compared with its present day position. Southern tree-line moved south up to 300 km. The volume of biomass seriously increased. In the East European plain estimation in polygon between 25-30⁰ E and 55⁰ E and between 46⁰ N and 65⁰N 125,000 BP terrestrial vegetation biomass was 8ll0⁹ tons (36.5l0⁹ tons of carbon) i.e. 154% as compared with the presentday pre-agricultural volume (Velichko et al., 1995).

In lower latitudes during the last interglacial the very considerable increase in the biomasse volume occurred in marginal areas of hot deserts due to northward expansion on savannas belt (Faure et al., 1993). It is also probable that in low latitudes maximal increase in humidity was within 140,000-130,000 BP (Vemet, 1994), i.e. possibly earlier than the optimal wet phase in high latitudes.

Tne similar quasi- asynchroncous position of wet phases is probable also for the Holocene. As a whole during the Holocene optimum the mean global temperature was 0.8- 1.0 higher than the moden one. General trends of meridional shifts in temperature and precipitation was similar to that one of the Eemian interglacial. 6000-5500 BP in latitudes over 60⁰N the positive deviation of temperature was +3 - 40, and positive deviation of precipitation - 25-30%. In lower latitudes, as it was during the last interglacial the positive temperature deviations vanished.

Distribution of the precipitation deviation was of more complicate character, and it was to some extent different from that one of the Eemian interglacial. In the temperate belt there are regions with some aridisation ( decrease of the annual precipitation up to 25%). In lower latitudes the amount of precipitation increased. The most serious increase in precipitation occurred in the intertropical belt, including deserts (Faure, 1969; Petit-Maire, 1992). Nevertheless the main stage of humid climate occurred in hot deserts in early Holocene around 8500 BP, and in the middle Holocene (6000-5,500 BP) the amount of precipitation slightly decreased (Vemet, 1994), which is rather syncronous to the decrease in precipitation in the middle latitudes.

As a whole reconstruction of vegetation and soil covers demonstrate considerable changes in the biomass volume. Ca 6000 BP in the East European plain the vegetation biomass volume was 61.4l0⁹ tons (27.6l0⁹ tons of carbon). The increase in the biomass volume as compared to the LGM 18,000 Bil was 2000% (116% as compared with the present-day pre-agricultural volume). Africa probably stored an extra 150 10⁹ tons of organic matter about at the same time.

During the last glaciation period sharp changes in the landscape structure occurred. Transition to the glacial environment was gradual with serious climatic oscillations. In high and middle latitudes a sharp decrease in temperature (especially the winter temperature) up to 20-30 maximum and was accompanied by the strong reduction of precipitation for more than 50% (Velichko, 1984). During the Last Glaciation Maximum ca 18,000 BP tree vegetation belts degraded. The structure of landscape zonality was represented mainly by different types of open landscapes (periglacial steppes, semi-deserts, deserts). Southern limit of permafrost reached 45⁰-47⁰ N. The volume of biomass seriously decreased. It was the most spectacular in former forest belt of Eurasia. Periglacial vegetation belt with sporadic occurrence of trees in river valleys expanded. Desert species (like Ephedra , Salsola etc.) progressed far to the north in East European plain. Cold and dry "tundra-steppes" were spread over large spaces of Eurasia. Great area of permafrost together with loess accumulation area formed large spaces which were covered with dust-cryodeserts and semideserts. Dust deserts expanded mainly in area under domination of anticyclones and of glacial winds. Desert -like soils were widespread. An expansion of dust eolian processes took its maximal expansion also in tropical deserts (Rognon, Coude-Gaussen, 1992).

Very serious reduction of biomass occurred. In the East European plain in a rectangle between 25-30^oE and 55^o E and between 46 ^o N and 65 ^o N 18,000 BP terrestrial vegetation biomass was only 3.l10⁹ tons (about 1.4l0⁹ tons of carbon).The volume of biomass and carbon were only 4% of the values during Eemian interglacial (6% as compared with the present-day pre-agricultural volume).

In Late glacial, and especially during the Younger Dryas period, phases of aridization restored. In central part of East European plain sand deserts with active dunes expanded. That processes reflected active atmosphere dynamics.

Atlas of palaeoclimates and palaeoenvironment of the Northern Hemisphere (Late
Pleistocene Holocene). 1992. B. Frenzel, M. Pecsi, A.A. Velichko (Eds.). Stuttgart,
Budapest. 149 maps, 146 p.

Faure, H., 1969: Lacs quatemaires du Sahara. Mitt. Internat. Vereinig. Limnol., 17: 131-
146. Faure, H., Branchu, Ph. & Ambrosi, J.P., 1993: Contribution de I'Afrique au cycle
global du carbone depuis 18.000 a-ns. Wurzburger Geographische Arbeiten 87: 443-463

Petit-Maire N., 1992: Environnements et climats de la ceinture tropicale nord-africaine
depuis 140,000 ans. Mem. Soc. Geol. Fr., n.s., n ^o 160: 27-14.

Rognon, P., Coud6-Gaussen, G., 1992: Reconstitution des circulations atmospheriques du Pleistocene terminal et de I'Holocene au large de l'Afrique entre 15 et 35 ^o N. In:
K.Heine (Ed.) Palaeoecology of Africa and the surrounding islands. 23: 1-25.

Velichko, A.A., 1984: Late Pleistocene spatial paleoclimatic reconstructions. In: A.A.
Velichko Ed. "Late Quaternary environments of the Soviet Union", University of
Minnesota press, 261-286.

Velichko, A.A., Borisova, O.K., Zelikson, E.M., Nechaev, V.P. Permafrost and vegetation response to global warming in North Eurasia. In: G.M. Woodwell, F.T. Mackenzie
(Eds.) Biotic feedbacks in the global climatic system. Will the warming feed the warming? Oxford University Press, N-Y, Oxford. P. 134-156.

Vemet, R. 1993:Prehistoire de la Mauritanie. Nouakchott-Mauritanie.427 p.

Vemet, R. 1994: Les paleoenvironnements du nord de l’Afrique depuis 600,000 ans.

CNRS, Meudon. 131 p.

Paper presented at Nouakchott , Jan., 1997
Myriam NAMRI¹⁾ & Dominique SCHWARTZ²⁾

1) etudiante maitrise de Geographie, ULP/CEREG, 3 rue de l’Argonne, 67000 Strasbourg
2) ORSTOM, CEREG, 3 rue de l’Argonne, 67000 Strasbourg

(Paper presented at Nouakchott meeting, Jan., 1997)
Hugues FAURE, Liliane FAURE-DENARD

Today Mauritania represents an area of more than one million square kilometres of the Saharan geobiome (ecosystem). Except in the extreme south, vegetation is absent, rare, or sparse. Lithosoils are dominant between large linear sand dunes and organic carbon is low to absent in the soils. The total vegetation biomass (phytomass) and soil organic matter probably does not exceed one or two gigatonnes (10 ¹² kg) of carbon. Mean annual rainfall is less than 100 mm on most of the country and only the extreme south receives more than 300 mm.

The Mauritania environment has undergone several changes during Quaternary climatic variations. The main geographic modifications proceed as rapid changes between two opposite extreme situations which we can simply call "wet" and "dry".

During Quaternary "wet" intervals Atlantic ocean water which extended up to 100km inland as large gulfs on the flat low coastal lands, producing a very irregular coast line that is recorded by shell sand limestone deposits. This transgressive phase occurred several times: isotope stage 7 (Inchirien C, or Aioujien), stage 5 (Inchirien B), stage 3 (Inchirien A; below present sea level), and during Mid-Holocene time (Nouakchottien). These marine beds covered the "Continental Terminal" (sandstones, sandy clays, iron dunricrust probably Oligo/Mio/Pliocene), and contain material reworked from kaolinitic weathering on land at a time when the climate of Mauritania was humid tropical. During the wet interval (high transgressing sea level, corresponding to interglacial conditions in the temperate zone), inland sand dunes were inactive, flattened, covered with Sudanian to Sahelian vegetation, and altered by soil development. The soils, morphology, and associated rich prehistoric sites suggest at least 3 or 4 (or more) generations of sand dune reactivation, separated by wet periods with artifacts(Neo-lithic during Holocene, Aterian during stage 3 and possibly 5d, and different Acheulean also related to Wadi terraces in Adrar). Evidence of the main recent maximum wet phases are soft, locally laminated diatomites and lacustrine limestone dated mainly 9-6 ka and 20-30(?) ka. Older limestone has large mammal footprints and is associated with Acheulean industries. The typical Holocene lacustrine molluscan fauna was a Melania tuberculate association in sediment that is usually soft and easily wind eroded. It is estimated that less than 1% of the lacustrine deposits are preserved. In deep inter-dune depressions (especially along the coast and in sebkhas) some unoxidized peaty organic material is preserved below the water table. However, most of the lacustrine deposits in the Sahara have been blown into the Atlantic ocean and as far as the Caribbean area.

The paleohydrologic conditions that prevailed during the wet phases in the Sahara are characterized by a high groundwater level. In the former dune area, high porosity favours lateral renewal of the water and permits fresh water fauna to live for several thousand years in some lakes. Increasing evaporation alters the water budget, transforming the lakes into sebkha-type depressions and increasing the groundwater salt concentration. During wet episodes, Mauritania was a country where vegetation, fauna, soils were widely expanding even in its most remote parts and humans have left their marks everywhere. During these periods five to ten times (and possibly more) more carbon than at present was probably stored in the total biomass.

The wet phases were separated by very dry phases during which strong winds produced two environments. Pre-existing soils were stripped away along wide deflation corridors with a stony pavement (reg, regolith), where the former drainage pattern is clearly visible as wadis and their terraces. the stones and associated prehistoric tools show different patinas which locally are very dark in the older Acheulean. The other environment was wide stripes of sand in various dune forms aligned with the dominant ENE wind. During lowered sea level (corresponding to the glacial maximum in northern latitudes) the sand stripes extended onto the continental shelf. The sand was thus introduced to the marine environment, both directly by wind transportation and coastal redistribution, as well as indirectly by turbidites which moved it into the submarine canyons. Acheulean tools under the older dunes are well preserved and have no patina.

Approximate dates on the arid periods are provided by marine and lacustrine deposits and by soils and artifacts above and below the dune systems. It appears that the sand stripes are being repeatedly reshaped from an older stock of sand. The last great and general sand dune reactivation was at LGM and probably until 14--11ka (1ocally called "Ogolien"A), other periods (Ogolien B, Ogolien C, etc.) are believed to correspond to isotopic stages 4, 6 and probably older (8, 10?). There were several important reactivations during the Holocene and another during the last twenty years with a strong anthropogenic component. With such complex mixtures and reworkings of the sand, dating of the dunes (as for the groundwater) must take in consideration the "fluid" behaviour of the system at the millennium time scale.

During the dry periods groundwater level was probably very low, (even lower than at present when it may be several hundred meters below the surface). Moreover, during lowered sea levels, the freshwater table at tile coast adjusted its slope to the low sea level and water was drained toward the sea. Probably this produced an oasis mechanism whereby freshwater arose as springs in the depressions unaffected by sea water pressure. The shelf was probably a refugium for flora, animals and human beings (for instance during the Aterien at stage 3/4/5a). Inlarld, in depressions when lakes dried up, similar springs appeared, juxtaposing fresh water and salt that accumulated by evaporation. During these intervening arid phases life was probably concentrated in a few points and the total biomass was much reduced from its present levels.

A feature of the changing environment is that the events occur at all time scales (2-3 y, 20-30 y, a few centuries, 2-3, 20, and 100 ka) depending on the individual periodicities of internal mechanisms or external forcings such the biologic,hydrologic, oceanic, climatic, and astronomic dynamics. It appears that there are no long lasting "stable" conditions. This constantly fluctuating behaviour of the environment is apparently bounded until now between the two extremes of wetness and aridity. Recovery of the living environment takes longer than its destruction and, because the cycles or quasi-cycles are not symmetric, all these changes are gradually trending toward aridificadon at the million-year time scale. For this reason, the terrestrial carbon that was stored in Mauritania during the early Quaternary was probably 10 times larger than at present.

Short selected list of References on "Mauritania" and Sahara environment

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BALOUT L. - 1952 - Pluviaux interglaciaires et Prdhistoire saharienne. Travaux de 1’I.R.S.t. VIII, pp. 9-21.

BARBEY C. - 1989 -Etude chronologique de la sedimentation eolienne dans le Sud-Ouest de la Mauritanic et dans le Nord du Senegal. - Bulletin de la Societe Geologique de France (8), 5/1 : 21-24.

BARBEY C. - Les ergs du Sud-Ouest de la Mauritanie et du Nord du Senegal s.d. - Universite Paris VII (FR).

BIBERSON P. - 1964 - Nouvelles decouvertes d’industries du Paleolithique inferieur in situ dans les formations quatenaires de I'Adrar mauritanien. C.R.A.S., Paris, t. 258, pp. 3074-3076.

BIBERSON P. - 1965 a - Recherches sur le Paleolithique inferieur de I'Adrar de Mauritanie. Actes du veme Congres Panafricain de Prihistoire, Santa Cruz de Tenerife. 1963 (1965) pp. 173-189.

BIBERSON P. - 1965 b - Essai sur I'evolution du Paleolithique inferieur de I'Adrar de Mauritanie. Quaternaria, t. VII, Rome, pp. 59-78.

BRANCHU P., FAURE H., AMBROSI J.P., VAN Z. BAKKER E.M. & FAURE-DENARD L. - 1993 - Africa as a source and sink for atmospheric carbon dioxide. - In: Faure H., Faure-Dcnard L. & LIU Tungsheng (Eds.) Quaternary Earth System Changes: (Global and Planetary Change, 7/1-3) 4 I -49 - Amsterdam (NL): Elsevier.

CARUBA R. & DAKS R. (Eds.) - Geologie de la Mauritunie,. Nice (FR): Universite & Centre Regional de Documentation pedagogique / Nouakchott (MR): Institut Superieur Scientifique.

CHAMARD P. - 1972 - Les lacs holocenes de I'Adrar de Mauritanie et leurs peuplements prehistoriques. - Notes. africaines, 133: 1-8.

CHAMARD P. - 1973 -Monographic d'unc scbkha continentale du Sud-Ouest saharien de Chemchane. - Bulletin de l’Instiiut Fondamental d’Afrique Noire (A) . 35 (2): 207-249.

COMMELIN D., GARCEA E.A.A. & SEBASTIANI R. - 1992 - A review of the archaeological material froni Tintan and Chami (Atlantic coast of Mauritania). - Quaternaria Nova, 11. 111 - 159.

DAVEAU S. - 1964 - Faconnement des versants de I'Adrar mauritanien. Ann. de Geomorph., Suppl. Band 5, PP. 118-130.

DAVEAU S. - 1966 - Le relief du Baten d'Atar (Adrar mauritanien). Mem. et Doc. du Centre de Docum. Geogr. du CNRS, nelle ser., vol. 11.

DAVEAU S. & BIBERSON P. - 1967 - Livret-Guide. CoIloque sur le terrain en Mauritanie -IIeme partie. Le Quaternaire et le Paleolithique de I'Adrar mauritanien. Congrres Panafricain de Prehistoire el de l’Etude du Quaternaire. 11 pages.

DAVEAU S., MOUSINHO R. & TOUPE'I' Ch. - 1967 - Grandes depressions fermees de I'Adrar mauritanien, Sebkha de Chemchane et Richat. Bull. dt, l’IFAN, Dakar, XXIX, A. n° 2, pp. 413-446.

DEBENAY J.P. - 1991 -Benthic foraminifera used as indicators of a gradient of marine influences in paralic environments of Western Africa. - Journal of African Earth Sciences, 12/1-2 : 335-340.

DEMAREE G.R. - 1990 - An indication of an abrupt climatic change as seen from the rainfall data of' a Mauritanian station. In: Paepe R. et al. (Eds.) -Greenhouse effect, sea level and drought, 369-

381.-s.l. (NL): Kluwer.

DEYNOUX M., KOCUREK G., BENAN C.A.A., CRABAUGH M., HAVHOLM K. & PION J.C..- 1993 Stratigraphie sequentielle en milieu desertique. Exemple de 1'erg Akchar en Mauritanie occidentale (Afrique de l'Ouest). Sedimentologie/Sedimentology, C.R. Acad. Scl. Paris, t. 317, Serie II, p. 1199-1205.

Values of d ¹³C in carbonates close to 0 are commonly interpreted as the result of' precipitation of those carbonates in solutions in which HCO ₃^- issued from the dissolution of atmospheric CO₂ in equilibrium with the atmospheric P_CO2.

An example of such a situation, in ponds at die surface of reef islands (French Polynesia) , is given in figure 1.

This result must be linked with the increasing acknowledgment of microbial facies in ancient series. It is tentatively proposed that this intense CO₂ absorption at the surface of microbial sediments can have acted as a sink for CO₂ during periods of growth of large microbial conununities in the past (Baud and Cirilli, 1996; Merz-Preiss and Zankl, 1996).