260 research outputs found
Interaction between External and Internal Conditions in the Development of Photosynthetic Features in a Grass Leaf
Interaction between External and Internal Conditions in the Development of Photosynthetic Features in a Grass Leaf
Acclimation of Ribulose Bisphosphate Carboxylase and mRNAs to Changing Irradiance in Adult Tobacco Leaves
Influence of sample location and livestock numbers on Sporormiella concentrations and accumulation rates in surface sediments of Lake Allos, French Alps
International audienceSpores of coprophilous fungi, especially Sporormiella, are often well preserved in lake sediment cores. It has been hypothesized that such spores can be used to quantify past livestock abundance. The quantitative relationship between fungal spore abundance and livestock populations, however, is not well established, nor are the mechanisms of spore transport and deposition in lacustrine systems. Multiple cores from Lake Allos, a large high-elevation lake in the French Alps, were used to map the modern abundance of Sordaria and Sporormiella spores throughout the lake. We observed high spatial heterogeneity with respect to spore numbers. No correlation with the distance from shoreline was found. There was, however, a relation with distance from the two main lake inlets. These results were used to select two fungi-rich sediment cores to investigate grazing pressure over the last two centuries. Comparisons were made between spore influx and historic data on livestock densities in the catchment. A sharp decrease in Sporormiella influx ca. 1894-1895 was associated with a reported reduction in sheep in the Allos catchment at that time. Mean influx of Sporormiella decreased by a factor of three between the nineteenth and twentieth centuries, reflecting a reduction in the reported number of animals in the Lake Allos catchment, from 6,000 to 2,000. This study confirmed that Sporormiella spore abundance in lake sediments can be used as a proxy for catchment herbivore numbers in paleoecological reconstructions. Nevertheless, our data indicate that before spore accumulation can be used to infer past domestic herbivore density, one must understand the processes of coprophilous spore transfer from the catchment to the lake and the influence of core location on spore numbers in the sediment
Difficulties in dating Pleistocene marine levels using fossil mollusk shells: the Ouljian level on the High Atlas shore, Morocco
In order to comprehend the difficulty of dating
marine strata using samples of mollusks shells and in an attempt to establish criteria that help in judging
the validity of a 230Th /234U age determination, we present here the results of a radioactive element
analysis of some 80 samples of mollusk shells. The shells were taken from fossil marine beaches between
4 and 8 meters above current sea level. When compared with the Egyptian shore of the Red Sea
these levels are taken to be related to climate stage 5 dated as about 122,000 years ago. The ages
obtained are often rejuvenated regardless of site, the incidence of calcite, the amount of uranium and
the relationships of 234U/238U.
Unlike coral, mineralogical criteria cannot be used to select mollusk shells unaffected by recrystallization,
for present-day shells may be composed of aragonite, others are calcitic, and others contain both
minerals. In any case, the rules concerning their effects of composition on radioactivity are as yet poorly
understood. With these problems in mind, we made multiple analyses on several species of of mollusks,
always collected from the same sites, in order to understand progressive changes in the process
of age rejuvenation and to attempt eventually to determine systematic criteria providing information on
the validity of the calculated age. Possible mechanisms that may influence the incorporation of uranium
in rejuvenation have been conceived and are discussed.Afin de comprendre la difficulté de datation des niveaux marins à l'aide des échantillons de
coquilles de mollusques et d'essayer d'établir des outils méthodologiques pouvant aider à juger de la
validité d'un ùge 230Th /234U, nous confrontons 80 analyses radiochimiques d'échantillons de coquilles
de mollusques prélevés dans des plages marines fossiles situées entre 4 et 8 m par rapport au niveau
de l'eau actuel. les niveaux oĂč ont Ă©tĂ© faits les prĂ©lĂšvements, analogues Ă ceux de la cĂŽte Ă©gyptienne
de la Mer Rouge, sont sensés appartenir au stade climatique 5e daté à 122 ka environ. On note que les
ùges obtenus sont souvent rajeunis indépendamment du site, du taux de calcite, de la teneur en uranium
et du rapport 234U/238U.
Contrairement aux Ă©chantillons de coraux, le critĂšre minĂ©ralogique ne peut pas ĂȘtre utilisĂ© pour choisir
les échantillons non recristallisés, certaines coquilles de mollusques actuelles sont en aragonite, d'autres
sont en calcite et d'autres contiennent les deux formes sans que les rapports de proportion en
soient encore bien connus. Devant cette situation, nous avons multiplié les analyses sur des échantillons
de coquilles de mollusques appartenant Ă plusieurs espĂšces prĂ©levĂ©s aux mĂȘmes endroits afin de
comprendre le scénario de rajeunissement des ùges et d'essayer d'établir éventuellement des critÚres
méthodologiques pouvant nous renseigner sur la validité de l'ùge calculé. Des scénarios sur le mode
d'incorporation de l'uranium et son rÎle dans le rajeunissement de l'ùge ont été également imaginés et
discutés
Rajeunissement diagénétique des récifs émergés et précision des datations absolues. La contribution des récifs quaternaires de la Mer Rouge à la question de la fiabilité des datations par la méthode des déséquilibres radioactifs de la famille de l'uranium des terrasses récifales de référence du PléistocÚne moyen et supérieur
Une revue gĂ©nĂ©rale des datations de rĂ©cifs de la Mer Rouge, affleurant sur les cĂŽtes d'Ăgypte, de Jordanie, du Soudan, d'ĂrythrĂ©e, d'Arabie Saoudite et de Djibouti, est commentĂ©e en fonction des mĂ©thodes d'Ă©chantillonnage et de datation, par comparaison avec les nouvelles conceptions testĂ©es sur les rĂ©cifs Ă©gyptiens et divers dĂ©pĂŽts associĂ©s. Des processus de rajeunissement rĂ©vĂ©lĂ©s par les coraux les mieux prĂ©servĂ©s, attribuables Ă la diagenĂšse de la matiĂšre organique des bio-minĂ©raux, justifient une rĂ©vision de beaucoup de datations de coraux supposĂ©s plus rĂ©cents ou plus anciens que l'Ăąge admis pour le haut niveau marin du sous-stade isotopique (ÎŽ18O) MIS 5.5 (= 5e). Une baisse rapide du niveau de la mer, brĂšve et limitĂ©e Ă une dizaine de mĂštres, a Ă©tĂ© mise en Ă©vidence pendant cette culmination majeure du PlĂ©istocĂšne supĂ©rieur et interprĂ©tĂ©e en termes de glacio-eustatisme dont l'enregistrement se doit d'ĂȘtre global malgrĂ© sa briĂšvetĂ©. Une comparaison avec les chronologies rĂ©cifales les plus "classiques", de Nouvelle-GuinĂ©e, d'Australie occidentale et des CaraĂŻbes, plus ou moins dĂ©calĂ©es vis-Ă -vis des courbes globales (isotopiques) du niveau de la mer remet en question plusieurs reconstitutions rĂ©gionales et appelle un rĂ©examen du fondement chronologique des courbes de rĂ©fĂ©rence qui en ont rĂ©sultĂ©.This paper is a general review of the dating of reefs on the coasts of the Red Sea, including those of Egypt, Jordan, Sudan, Eritrea, Saudi Arabia and Djibouti. New methods of sampling and dating (U/Th) already tested on the reefs and associate deposits of the African coast of Egypt have demonstrated that processes of rejuvenation shown to exist in the best-preserved corals are probably attributable to the diagenesis of the organic material in their bio-minerals, thus justifying a revision of a great many datings of corals supposedly younger or older than the age assigned to the high-level isotopic substage (ÎŽ18O) MIS 5.5 (= 5e). During this late Pleistocene substage, a rapid lowering of sea level, short and limited to about ten meters, was detected and associated with a glacio-eustatic episode of global influence. A comparison of these Middle East reef chronologies with those of New Guinea, Australia and the western Atlantic that are referred only with difficulty to the ÎŽ18O global sea-level curves, casts doubt on the reliability of many regional reconstructions. Moreover the most "classic" reef chronologies, more or less out-of-phase with global isotopic records calls for a reexamination of the chronologic basis of the reference curves derived from marine isotopic data
Ăvolution biosĂ©dimentaire du dĂ©pĂŽt quaternaire de la lagune de l'Ariana, Tunisie (une zone humide du Maghreb Nord)
La sebkha de lâAriana est une lagune fermĂ©e qui fait partie des zones humides du Maghreb. Elle a une superficie de 30 Ă 35 km2et est situĂ©e sur la bordure nord-ouest du golfe de Tunis, au sud de lâoued Medjerda. Vers 190 000 ans BP, elle a Ă©tĂ© occupĂ©e par la mer MĂ©diterranĂ©e. Actuellement, elle en est sĂ©parĂ©e par un cordon littoral. Ce dernier sâest mis en place lors du comblement dâun golfe peu profond sous lâimpulsion dâimportantes quantitĂ©s dâalluvions dĂ©posĂ©es par le fleuve de la Medjerda sur sa plaine deltaĂŻque dont elle fait partie. Cette Ă©tude de lâĂ©volution et de la dynamique de la sebkha de lâAriana, ainsi que des changements environnementaux rĂ©cents qui gouvernent la sĂ©dimentation, est basĂ©e sur une approche sĂ©dimentologique, palĂ©ontologique et isotopique. Six carottes longues de 20 m Ă 50 m, prĂ©levĂ©es dans les dĂ©pĂŽts alluviaux de la sebkha, ont fait lâobjet dâune Ă©tude dĂ©taillĂ©e qui a permis de suivre la rĂ©partition spatio-temporelle du stock sĂ©dimentaire â formĂ© essentiellement de silts argileux vaseux Ă la base, surmontĂ©s de sables â et de mettre en Ă©vidence des associations faunistiques particuliĂšres traduisant des modifications environnementales : environnement marin (PlĂ©istocĂšne moyen), lagune ouverte (HolocĂšne) et sebkha (Actuel). Cette Ă©volution dâun milieu marin franc vers un environnement typiquement Ă©vaporitique (sebkha) pratiquement azoĂŻque, est probablement en rapport avec le changement climatique quaternaire.The Sebkha of Ariana, covering 30 to 35 km2is a closed lagoon, located south of the Medjerda and considered to be part of the Maghreb humid zones. It belongs to the Mediterranean coast and is positioned on the N-W margin of the Gulf of Tunis. The Sebkha is situated on Quaternary lowland resulting from the filling-up of a shallow gulf by alluvial deposits sourced from the Medjerda delta. Historically at - 190 ka, the lagoon was related to the sea but at the present time, being bunged up from the seaside by a beach barrier, is shaped to the sediments of the Madjerda low valley. The study of the evolution and the dynamics of the Ariana Sebkha as well as the recent environmental changes which have governed sedimentation are based on a sedimentological, palaeontological and isotopic approach. For that, six cores (from 20 to 50 m) were realised from four sites of the Sebkha respectively. Furthermore, correlations between the different cores throughout the lagoon have allowed the reconstruction of the spatialâtemporal distribution of sediments. The granulometric analyses revealed an alluvial sedimentation mainly composed of argillaceous silts at the base, becoming sandy towards the top. On the other hand, the benthonic fauna determinations of the six cores show a distinct evolution from an open-marine environment to a typically lagoonal environment where fauna are rare to absent. Moreover, the palaeontological study yielded particular benthonic foraminifera associations suggesting important palaeonvironmental changes which in turn are related to Quaternary climatic changes
QTLs and candidate genes for desiccation and abscisic acid content in maize kernels
<p>Abstract</p> <p>Background</p> <p>Kernel moisture at harvest is an important trait since a low value is required to prevent unexpected early germination and ensure seed preservation. It is also well known that early germination occurs in viviparous mutants, which are impaired in abscisic acid (ABA) biosynthesis. To provide some insight into the genetic determinism of kernel desiccation in maize, quantitative trait loci (QTLs) were detected for traits related to kernel moisture and ABA content in both embryo and endosperm during kernel desiccation. In parallel, the expression and mapping of genes involved in kernel desiccation and ABA biosynthesis, were examined to detect candidate genes.</p> <p>Results</p> <p>The use of an intermated recombinant inbred line population allowed for precise QTL mapping. For 29 traits examined in an unreplicated time course trial of days after pollination, a total of 78 QTLs were detected, 43 being related to kernel desiccation, 15 to kernel weight and 20 to ABA content. Multi QTL models explained 35 to 50% of the phenotypic variation for traits related to water status, indicating a large genetic control amenable to breeding. Ten of the 20 loci controlling ABA content colocated with previously detected QTLs controlling water status and ABA content in water stressed leaves. Mapping of candidate genes associated with kernel desiccation and ABA biosynthesis revealed several colocations between genes with putative functions and QTLs. Parallel investigation via RT-PCR experiments showed that the expression patterns of the ABA-responsive <it>Rab17 </it>and <it>Rab28 </it>genes as well as the late embryogenesis abundant <it>Emb5 </it>and aquaporin genes were related to desiccation rate and parental allele effect. Database searches led to the identification and mapping of two <it>zeaxanthin epoxidase </it>(<it>ZEP</it>) and five novel <it>9-cis-epoxycarotenoid dioxygenase </it>(<it>NCED</it>) related genes, both gene families being involved in ABA biosynthesis. The expression of these genes appeared independent in the embryo and endosperm and not correlated with ABA content in either tissue.</p> <p>Conclusions</p> <p>A high resolution QTL map for kernel desiccation and ABA content in embryo and endosperm showed several precise colocations between desiccation and ABA traits. Five new members of the maize <it>NCED </it>gene family and another maize <it>ZEP </it>gene were identified and mapped. Among all the identified candidates, aquaporins and members of the <it>Responsive to ABA </it>gene family appeared better candidates than <it>NCEDs </it>and <it>ZEPs</it>.</p
Upper Pleistocene comparativeOSL, U/Th and 14C datings of sedimentary sequences and correlative morphodynamical implications in the South-Western Anti-Atlas (Oued Noun, 29° N, Morocco)
The lower Oued Noun valley, in the arid region of the Atlantic SW Anti-Atlas, contains an extensive Soltanian (= Upper Pleistocene pro-parte) terrace. The paper presents dates for these deposits and an interpretation of their fluvial dynamic and morpho-climatic geneses in this distal part of the valley.The Soltanian terrace, about 30 m thick at Fort Oued Noun, consists of 3 units: a basal deposit of coarse gravel buried by slope deposits and travertine (Unit 1); a main unit, more than 20 m thick (Unit 2) that consists of at least 7 repeated sequences each comprising basal fine gravels and sands, associated with detrital travertine, overlain by stromatolithic tufa and finally by a thick accumulation of sandy silts. These silts were deposited by suspension and decantation in shallow non-turbulent water bodies. The gravels, sands and travertines are more prominent at the bottom and in the upper parts of Unit 2 whilst silts dominate the mid members. Middle Paleolithic artefacts and bones of large mammals are found throughout this Unit.Unit 3, at the top of the formation, comprises red silts that differ from those of Unit 2, containing more aeolian grains and more palygorskyte and being spatially associated with adjacent tributaries fans.Radiometric dates were obtained on travertine (U/Th), on quartz grains extracted from sediments (OSL) and on Melanopsis and Charcoal (14C). U/Th results show three travertine constructions at ca 90, 55-50 and 25-20 ka B.P. The 90 ka dates, however, are not in correct stratigraphic position. The OSL dates suggest that the period of silt aggradation of Unit 2 occured between ca. 50 and 25 ka B.P., the main part of them being deposited between 40 and 30 ka B.P. 14C dates from the upper part of Unit 2 and the base of Unit 3, range between ca 28 and 18 ka B.P. These dates, together with geomorphological and sedimentological analyses, indicate that the valley floor had been lowered to its present depth before ca. 90 ka B.P. However, the slope deposits, older travertines and the basal gravel (Unit 1), classic fluvial and colluvial deposits, are not yet securely dated but they possibly correspond with wetter episodes in O.I.S. stages 5b, 5a and 4, or even later. The silts that form the bulk of Unit 2 were deposited into shallow swamps during biostasic episodes of O.I.S. 3 and were associated with high groundwater levels. Then, large mammals found grass and water along the valley and were hunted by Middle Paleolithic people. The water table remained high after 30 ka B.P. and the gravel-bed channels of the local tributaries were active during the 30-20 ka B.P. period. After ca. 20 ka, sediments suggest more varied conditions in this part of the valley, vegetation disappeared and soils were reworked into local colluvial fans, with concomitant aeolian deposits (Unit 3, O.I.S. 2). Finally, deep linear incision occured dissecting the Soltanian aggraded valley floor during the early Holocene.La basse vallĂ©e de l'oued Noun, aujourd'hui situĂ©e dans le domaine aride du versant sud de l'Anti-Atlas, montre une terrasse soltanienne (PlĂ©istocĂšne supĂ©rieur pro-parte) bien dĂ©veloppĂ©e. Les objectifs de cet article sont de dĂ©terminer avec prĂ©cision les Ăąges des sĂ©diments et de comprendre les changements de la dynamique fluviale, avec ses phases successives dâaggradation et dâĂ©rosion, Ă la fin du PlĂ©istocĂšne supĂ©rieur.La formation soltanienne, Ă©paisse de 30 m Ă Fort Oued Noun, se compose de 3 unitĂ©s : un dĂ©pĂŽt conglomĂ©ratique de base surmontĂ© par des dĂ©pĂŽts de pente et Ă des travertins (UnitĂ©Â 1) ; un dĂ©pĂŽt principal de plus de 20 m (UnitĂ©Â 2), qui consiste en au moins 7 sĂ©quences rĂ©pĂ©titives montrant chacune Ă la base des petits galets et des sables associĂ©s Ă des travertins dĂ©tritiques, surmontĂ©s de travertins stromatolithiques construits finalement recouverts d'accumulations Ă©paisses de limons. Ces limons ont Ă©tĂ© dĂ©posĂ©s par suspension et dĂ©cantation dans des eaux calmes et peu profondes. Les galets, sables et travertins sont surtout frĂ©quents Ă la base et au sommet de lâUnitĂ©Â 2, tandis que les limons dominent dans sa partie moyenne. Dans toute lâUnitĂ©Â 2, on trouve des ossements de grands mammifĂšres et des outillages lithiques du PalĂ©olithique moyen. Au sommet de la formation, des limons rouges supĂ©rieurs (UnitĂ©Â 3) diffĂšrent des prĂ©cĂ©dents et appartiennent Ă des cĂŽnes dĂ©posĂ©s par les affluents locaux, avec davantage de grains Ă©oliens et de palygorskyte.Les datations radiomĂ©triques ont Ă©tĂ© obtenues pour des travertins (U/Th), des grains de quartz extraits des sĂ©diments (OSL), des charbons et Melanopsis (14C). Les datations U/Th montrent trois pĂ©riodes de construction de travertins autour de 90, 55-50 et 25-20 ka B.P. Les rĂ©sultats Ă 90 ka ne sont toutefois pas en bon accord avec la stratigraphie. D'aprĂšs les rĂ©sultats OSL, la pĂ©riode d'aggradation des silts de l'UnitĂ© 2 se situe entre environ 50 et 25 ka B.P., la majeure partie de ces silts se dĂ©posant entre 40 et 30 ka B.P. Les datations 14C se rangent entre 28 et 18 ka B.P. Elles concernent la partie supĂ©rieure de l'UnitĂ©Â 2 et la partie basale de l'UnitĂ©Â 3.Ces rĂ©sultats, associĂ©s aux observations gĂ©omorphologiques et sĂ©dimentologiques, montrent que la vallĂ©e Ă©tait dĂ©jĂ creusĂ©e autour de 90 ka B.P. Les dĂ©pĂŽts de pente, les plus anciens travertins et le dĂ©pĂŽt basal graveleux, ayant tous une signature de dĂ©pĂŽts colluviaux ou alluviaux classiques, ne sont pas datĂ©s avec sĂ»retĂ©Â ; ils peuvent correspondre Ă des Ă©pisodes humides des stages isotopiques 5b, 5a et 4, mais peuvent aussi ĂȘtre plus rĂ©cents. Les silts qui forment la masse de l'UnitĂ©Â 2 ne peuvent ĂȘtre reliĂ©s uniquement Ă une activitĂ© fluviale et ont Ă©tĂ© dĂ©posĂ©s dans des Ă©tendues d'eaux calmes liĂ©es Ă une Ă©lĂ©vation concomitante du niveau des nappes phrĂ©atiques durant des Ă©pisodes biostasiques du stage isotopique 3. De grands mammifĂšres ont alors pu trouver de l'eau et de l'herbe dans la vallĂ©e et ĂȘtre chassĂ©s par les hommes du PalĂ©olithique moyen. La nappe phrĂ©atique s'est maintenue Ă un niveau Ă©levĂ© aprĂšs 30 ka B.P et les chenaux Ă graviers des affluents locaux sont restĂ©s actifs durant la pĂ©riode 30-20 ka B.P. AprĂšs cette date, lâinstabilitĂ© sâest accrue ; dans cette partie de la vallĂ©e, la vĂ©gĂ©tation s'est rarĂ©fiĂ©e, les sols ont Ă©tĂ© remaniĂ©s, Ă©difiant des cĂŽnes locaux oĂč s'observent aussi des influences Ă©oliennes (UnitĂ©Â 3, stage isotopique 2). Enfin, une forte incision linĂ©aire sâest produite Ă lâHolocĂšne infĂ©rieur, dissĂ©quant l'accumulation soltanienne
- âŠ