F. Gasse

Abrupt post-glacial climate events in West Asia and North Africa monsoon domains

Regions beyond the present or past penetration of the Indian and African monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14C kyr. n nPollen and lake records from West Asia (Western Tibet and Rajasthan), East Africa (Ethiopia) and West Africa (Western Sahara, Sahel and subequatorial Africa) were selected on the basis of chronological control, sensitivity of both site and environmental indicators to climate change, the continuity of the record, and interdisciplinary control of the palaeoclimatic interpretation. n nConditions wetter than those of today prevailed during the early-mid-Holocene period, but major dry spells are recorded at all sites during the intervals ∼ 11.0–9.5 kyr BP, ∼ 8–7 kyr BP and 3–4 kyr BP. Several records also suggest dry events of minor amplitude around 6 kyr BP. Potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed. The solar radiation accounts for the general envelop of the post-glacial monsoon fluctuations, but explains neither the timing nor the amplitude of the short-term changes. In spite of apparent covariation between fluctuations in sea surface conditions in the North Atlantic and the monsoon record, no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength. Changes in tropical land surface conditions (soil moisture negative feedback, and changes in CH4 production from wetlands) provide a more satisfactory hypothesis for explaining abrupt reversal events.

Holocene environmental changes in Bangong Co basin (Western Tibet). Part 4: Discussion and conclusions

Abstract A 12.4 m core taken in Lake Bangong provides a continuous Holocene climatic record. We summarize information on changes in stable isotope and radiocarbon balances in the lake, hydrobiology and vegetal cover in the catchment, deduced from detailed analytical results given in the three preceding papers. The Bangong record is then compared with the environmental history of the neighbouring Lake Sumxi also constructed from multidisciplinary analyses. The two records show a major environmental change at ≈ 10-9.5 ka B.P., attributed to a rapid strengthening of the summer monsoonal circulation which led to wet-warm conditions. This event was followed by a long-term trend toward aridity which culminated around 4-3 ka B.P.. In Western Tibet, maximum monsoon rainfall seems to have occurred from ≈ 9.5 to 8.7 ka B.P. and from ≈ 7.2 to 6.3 ka B.P., as two wet pulses separated by a reversal event centered on 8.0–7.7 ka B.P. Our results broadly agree with the records from Lake Seling in Central Tibet, and Lake Qinghai at the plateaus northeastern margin, and with palaeoclimatic studies in western China which document conditions wetter and warmer than those of today during the early-middle Holocene. The environmental fluctuations recorded in western Tibet appear in phase with climatic changes recognized in tropical North Africa, suggesting that the 8.0–7.7 ka B.P. and the 4.0–3.0 ka B.P. dry events were caused by abrupt disequilibrium in the climatic system.

Holocene environmental changes in Lake Bangong basin (Western Tibet). Part 1: Chronology and stable isotopes of carbonates of a Holocene lacustrine core

Abstract A 12.4 m core collected from the eastern part of Lake Bangong is analyzed for mineralogy, radiometric chronology and stable isotope contents of authigenic, inorganic carbonate. Isotopic compositions of the modern lake waters and of its major tributaries are first presented. Radiocarbon activity of these waters show that equilibrium with the atmospheric CO2 is not reached, because of admixture of dead carbon in solution in the system. A model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing. The core provides a continuous Holocene record. Changes in mineralogy of bulk sediments, and in δ13C and δ18O contents of authigenic carbonates reveal extremely large changes in environmental conditions, induced by climate changes and/or local hydrological factors (e.g. closure/opening of the lake system controlled by the altitude of the outflow sill). The eastern basin of Lake Bangong was closed before ≈ 9.6 and after 1.2 ka B.P., in response to aridity on the catchment. Wet/warm conditions, attributed to the monsoon influence, were established suddenly at ≈ 9.6 ka B.P. and maintained until ≈ 6.2 ka B.P., although minor reversal events occurred between ≈ 8.6 and 7.7 ka B.P. The return towards aridity is non linear. Dry spells at ≈ 6.2, ≈ 3.9-3.2, and ≈ 1.3 ka B.P. are alternated with short-term periods with positive precipitation-evaporation balance.

Comparison of molecular and paleontological data in diatoms suggests a major gap in the fossil record

Diatoms, unicellular eukaryotic algae with a siliceous skeleton, offer the rare advantage of displaying both an extensive fossil record and numerous extant species, thus providing the opportunity of confronting molecular and paleontological data in a protist group. A portion of the 28S ribosomal RNA was sequenced from 5 diatoms, the divergence times of which are well known. The nucleotide substitution rate was estimated in these unicellular eukaryotes and compared with the rate of multicellular eukaryotes, using a broad data base comprising metazoans and metaphytes. When using fossil record derived divergence times, our results show that the nucleotide substitution rate is about 5 times faster in diatoms than in chordates. But, when using the relative rate test, it is observed that, over a long time period, the nucleotide substitution rate may in fact have been slightly slower in diatoms than in chordates. For this contradiction, two possible explanations are proposed: (i) a failure of the relative rate test, (ii) a gap in the pre‐Jurassic diatom fossil record. We have checked that our results concerning the relative rate test were valid. Thus, the second hypothesis, which implies pre‐Jurassic diatom evolution, in fact already suggested by some non‐molecular evidences, is favoured. Decoupling of morphological differentiation from genetic speciation also appears to have occurred and may account in part for the underestimation of the dates of recent cladogenesis events.

Stable isotope and radiocarbon balances of two Tibetan lakes (Sumxi Co, Longmu Co) from 13,000 BP

Hydrologic and climatic changes in western Tibet since 13,000 BP are inferred from mineralogical and isotopic analyses of lacustrine sediments from the Sumxi-Longmu Co basin. The interpretation is based on isotope data from the modern system. Carbon isotope chemistry is complex because of the presence of old limestones on the watershed. n nSediments contain detrital and authigenic calcites, the proportions of which are quantitatively estimated by X-ray diffractometry. Stable isotope contents of the authigenic calcite are reconstructed from mass-balance equations. The high 13C contents (up to +6‰) are attributed to an enhancement of calcite precipitation from water surface during freezing periods. The 18O content is related to residence time and evaporation. The validity of 14C dates obtained from different types of material is discussed. n nOur record, in good agreement with paleobiological data from the same material, shows the sudden establishment of wet/warm conditions at ≈10,000 BP which culminated at ≈7500-6000 BP after a brief reversal event at ≈7900 BP. Maximum aridity for the whole Holocene is recorded at ≈4300 BP.

PALHYDAF (Palaeohydrology in Africa) program: objectives, methods, major results

Abstract The PALHYDAF program aims to reconstruct the Late Quaternary (130 ka B.P. to Present) environmental and hydrological changes of northwest Africa, and to interpret them in terms of palaeoclimate for testing coupled atmospheric/ocean circulation models. Lacustrine sequences were obtained from closed basins which lie along two meridional transects: southern Tunisia-southern Niger, and Algeria-Mali. Particular attention is given to groundwater storage and periods of recharge of the aquifers. The chronological framework is based on 14C dates (conventional and AMS methods) and/or the Th/U disequilibrium method. In favourable cases (e.g. Mali), correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge. Various environmental markers (mineralogy, elemental chemistry, stable isotopes, biological remains) are used for palaeoenvironmental purposes. Quantitative estimates of palaeoclimatic variables (e.g. air humidity) may be attempted through the combination of geochemical and palaeobiological data. The major results already obtained are summarized for the following individual sectors: 1 — southern Tunisia. 1′ — northern Algeria. 2 — southern Algeria. 3 — northern Niger. 3′ — Mali. 4 — southern Niger. At the present stage of investigation, the following new inferences can be drown. • - One, or several humid episode(s) occurred in the northern Sahara between ≈ 150 and ≈ 75 ka B.P. Several factors suggest that these humid conditions also affected lower latitudes, although their southern extent cannot be given precisely at present. • - The previously postulated existence of large lakes at ≈20–35 ka B.P. is not confirmed. This time interval seems to coincide with raised water-tables, but not to a major humid phase. In all cases, the 14C chronologies obtained for the Late Pleistocene have to be checked by comparison with other dating methods (e.g. Th/U disequilibrium). • - After the arid period of the Last Glacial Maximum, the return of humid conditions occurred in two major steps, at ≥12 and ≈9.5-9 ka B.P. for both the southern and the northern Sahara.

Mineral-magnetic proxies of erosion/oxidation cycles in tropical maar-lake sediments (Lake Tritrivakely, Madagascar): paleoenvironmental implications

Abstract Mineral-magnetic measurements were performed on the upper 16xa0m of five piston-cored sedimentary sequences from Lake Tritrivakely, Madagascar. AMS 14 C dating and correlation of the five magnetic susceptibility records allowed establishment of a composite sedimentary record of the last 46 kyr. Our data suggest that mineral-magnetic changes result from changes in concentration of strongly ferrimagnetic terrigenous minerals and from preservation/dissolution cycles of detrital iron oxides (primarily high-coercivity Fe(III) phases such as hematite) in alternately oxic and anoxic depositional environments. High-coercivity iron oxide phases are restricted to stratigraphic intervals that are interpreted to represent dry periods with oxic conditions. Comparison with other paleoenvironmental proxies (pollen, total organic carbon, lithology) suggests that the sediments record relatively humid conditions before 22 kyr BP, and relatively dry conditions during the Last Glacial Maximum (LGM) and the early Holocene. Such results contrast with the previous observations of relatively dry glacial conditions and humid mid-Holocene conditions in the summer rainfall area of East and SE Africa.

Two high levels of continental waters in the southern Tunisian chotts at about 90 and 150 ka

Uranium and thorium content and activity ratios were measured on 20 samples (19 molluscs and 1 calcareous concretion) from the Great Chotts in southern Tunisia. Results were studied by the isochron method and by age frequency histograms. They suggest that two major flood episodes took place about 150 and 90 ka. Uranium activity ratios indicate a ground-water supply of continental origin, in agreement with biological indicators, which show large variations in salinity from fresh to marine-like conditions. The existence of a lacustrine phase, radiocarbon dated to 17-40 ka, as previously suggested for the northern Sahara, is highly questionable.

Freshwater Diatom Influx in Intertropical Atlantic: Relationships with Continental Records from Africa

Diatom assemblages from modern West African rivers and from lacustrine sediments subjected to deflation represent the present-day sources of continental diatoms to the sea. Diatom productivity in large rivers is high, especially for the genus Melosira. Windblown diatoms derive mainly from the central and northern Sahara (summer dust plume) where saline chloride-water assemblages are widespread, or from the southern edge of the Sahara (winter dust plume) where Melosira-rich assemblages of dilute water predominate. Freshwater diatom peaks in Atlantic cores may reflect (1) phases of increased river influx, correlated with humid episodes on the continent or (2) phases of enhanced deflation and wind transport during arid episodes (the single hypothesis for fine sediments from mid-ocean sites). Genus Melosira dominates the freshwater assemblages of many modern and fossil marine samples, whatever the transport agent is. Therefore, it is not an accurate paleoclimate indicator by itself, but associated taxa may provide information on the environmental and geographical origin of the displaced diatoms. This tentative approach shows that freshwater diatoms in Atlantic cores may be a good tool for reconstructing paleoclimates and for establishing continent-ocean correlations if species analyses are made and if the continental distribution of the taxa encountered is considered.

Holocene environmental changes in Bangong Co basin (Western Tibet). Part 3: Biogenic remains

Abstract A 12.4 m core collected from the eastern part of Lake Bangong provided a continuous Holocene record. This core was analyzed for diatoms, ostracods, charophytes, content in total organic carbon and identification of organic particles contained in the sediments. The record is remarkable for the abundance and diversity of its microorganisms. An oligo- to mesosaline marsh occupied the core site before 9.5 ka B.P. Bangong was most likely closed. A sudden filling of the lake basin by dilute, nutrient-poor water occurs around 9.4 ka B.P. Maximum lake extent seems to have occurred around 9.4–8.7 ka B.P. Generally freshwater, relatively deep conditions were maintained up to 6.3 ka B.P., although interrupted by short-term regressive events between 8.6 and 7.5 ka B.P. A trend towards palustral conditions begins after 6.3 ka B.P. and the interval 3.9-3.2 ka B.P., rich in detrital organic material, coincides with an extremely low lake level. A last lacustrine episode of minor amplitude is recorded from about 3 to 2 ka B.P., before the closure of the system leading again to slightly saline conditions. The resemblances and synchronism between environmental changes recorded at Bangong and in Lake Sumxi, situated 200 km northwards and 800 m higher in elevation, indicate that regional climatic changes have been the major driving factor on ecological and hydrobiological changes in lakes of western Tibet.