Rosa Utrilla

Oxygen and sulphur isotope compositions as indicators of the origin of Mesozoic and Cenozoic evaporites from Spain

Abstract The general investigation of oxygen and sulphur isotope compositions of the main evaporitic formations of Mesozoic and Cenozoic ages from Spain, for primary purposes, had to differentiate between marine and continental sulphate evaporites. Furthermore, the isotope data would also make it possible to determine the source of sulphate of the continental evaporites. The δ-values measured for the marine evaporites of Spain of Triassic, Lias, Cretaceous and Eocene age, are in good agreement with the general oxygen and sulphur isotopic curves of the oceanic sulphate. The continental evaporites deposited in the Ebro, Tajo, Calatayud and Teruel basins, since Paleocene up to Miocene times, exhibit wide ranges of δ-values both through time and space. The main source of sulphate reworked by continental waters was Triassic evaporitic sulphate, although another one was locally Cretaceous evaporite. In addition, in continental formations the local conditions of drainage and of in situ bacterial activity acted to modify the isotope composition of the sulphate source, thus imprinting specificity of the isotopic signature of each basin.

Messinian pre-evaporite sapropels and precession-induced oscillations in western Mediterranean climate

Abstract Cyclical fluctuations in planktic foraminiferal assemblages have been recognized in the pre-evaporitic Messinian in a marginal basin of the western Mediterranean. The fluctuations coincide with a dominantly precession-controlled sedimentary cyclicity (sapropels). During sapropel deposition, high planktic foraminiferal diversities are indicative of relatively stable marine conditions, while during homogeneous marl deposition low diversities seem to indicate the presence of unfavourable, more saline surface water conditions. The dominance of a precession-related signal indicates that regional climate oscillations rather than (obliquity-related) glacio-eustatically controlled influxes of Atlantic and/or Mediterranean waters are responsible for the faunal fluctuations and sedimentary cyclicity. Our scenario links the persistence of normal marine conditions during sapropel formation with increased rainfall and run-off along the western Mediterranean at times that perihelion occurred in Northern Hemisphere summer. Less favourable, highly saline surface water conditions prevailed during periods of drier climate induced by opposite precessional extremes. The cyclical oceanographic fluctuations could also have governed periodic reef growth along the margins.

Palaeoenvironmental reconstruction of a Pleistocene lacustrine sequence from faunal assemblages and ostracode shell geochemistry, Baza Basin, SE Spain

Abstract We have reconstructed the Early Pleistocene palaeoenvironmental record for a shallow lacustrine sequence from the Baza Basin (Southern Spain), using faunal assemblages and the shell chemistry of two ostracode species (Candona sp. and Cyprideis torosa). In the NE basin sector (Orce area), a Lower Pleistocene lacustrine sequence, up to 10 m thick, contains alternating phases of two fossil assemblages which differ in their salinity requirements. The faunal assemblages record phases of (1) slightly saline, Ca+bicarbonate-rich water, when freshwater organisms predominated, and (2) saline, NaCl-dominant water in which marine-like organisms lived. The ostracode shells from intervals with a saline fauna have higher δ18O values than those from intervals with a freshwater fauna. This feature corresponds to a parallel alternation of positive and negative hydrologic balances. δ18O and δ13C values from ostracode calcite display a covariant trend which indicates that the ostracodes lived in a closed lacustrine system. The isotopic data show a better agreement with the inferences from faunal assemblages than with trace element trends. The trace element data, although locally fitting well with the isotope and faunal interpretations, suggest overall that trace element contents in ostracodes are not consistent indicators of salinity and/or temperature variations, but depend on a set of complex factors, which may not be easily deciphered. The alternation of concentration/dilution phases recorded in the studied section can be correlated with climatic cycles described from synchronous ocean basin records from the Late Matuyama chron.

Orbitally-controlled oscillations in planktic communities and cyclic changes in western Mediterranean hydrography during the Messinian

We use quantitative analyses of the planktic foraminiferal assemblages and stable isotope analyses of the Sorbas section in the western Mediterranean to reconstruct the cyclical changes in surface and deep water hydrographic conditions during the Late Miocene prior to the Messinian salinity crisis. Oscillations in winter and summer temperatures linked to cyclical fluctuations in the hydrographic conditions and nutrient availability were the main mechanisms driving the cyclical changes in the planktic foraminiferal assemblages during the Messinian. The winter intensification and southward expansion of the northern cool and dry winds during precession maxima lowered sea surface temperatures (below≃14–15°C), favoring water convection and the upward mixing of nutrients with the consequent increase in the proportion of cold, eutrophic water foraminifera. The low summer temperatures (below 24°C) inhibited the growth of warm, oligotrophic water foraminifera, as occurs today in the Mediterranean. By contrast, at times of precession minima the low influence of the high latitude air masses caused winter temperatures to remain relatively high which, combined with the lower surface salinities, led to the formation of a permanent pycnocline. Winter conditions prevented the growth of cold, eutrophic water foraminifera, while high summer temperatures and the formation of a permanent pycnocline stimulated the growth of warm, oligotrophic water foraminifera. The prevalence of stable water stratification during winter due to the large density gradient between the surface and intermediate waters prevented deep water formation and slowed down the rates of oxygen supply to the bottom, resulting in the formation of the sapropels. A sharp decrease in δ13C of benthic and planktic foraminifera that occurred between 6.8 and 6.7 Myr is related to an increase in the residence time of Mediterranean waters. Because similar changes have been observed in other regions of the Mediterranean, we conclude that this change records a significant reduction in the Atlantic–Mediterranean water exchange at that time. Mediterranean climatic amplification increased throughout the Messinian as the Atlantic–Mediterranean water exchange was progressively more limited and oscillations in the ratio of planktic/benthic foraminifera and of warm-oligotrophic/cold-eutrophic species are good records of this increasing amplification, which culminated at 5.95 Myr with the deposition of gypsum–pelitic layers that mark the onset of the Messinian salinity crisis.

A sediment–nutrient–oxygen feedback responsible for productivity variations in Late Miocene sapropel sequences of the western Mediterranean

Abstract Cyclic sediments observed throughout the Mediterranean during the Late Miocene have been related to precessional forcing of ocean stratification. Individual couplets, typically 2-m-thick sequences of sapropels and diatom-rich marls, can be reliably traced from western Spain to Crete, and were formed in restricted marginal basins. Micropaleontological evidence indicates paradoxically that the organic carbon-rich sapropels were formed under low productivity conditions marked by surface water stratification and deep anoxia, whereas the diatom-rich marls were formed under high productivity conditions marked by upwelling. Here we present geochemical evidence, mainly from detailed phosphorus determinations and paleo-redox proxies, indicating that a sediment–nutrient–oxygen feedback (herein dubbed the SNO Effect) is in part responsible for driving the observed productivity variations. During stratification, anoxic conditions in these basins cause the release of the limiting nutrient phosphorus from reducible oxide phases in the sediments. Basin stagnation causes the buildup of phosphorus below the photic zone. Subsequent overturn driven by precessionally-driven winds injects phosphorus-rich bottom waters into the photic zone. The biotic response to this overturn is high productivity in marls directly overlying the sapropels culminating in diatom mat formation. Exhaustion of the stored excess phosphorus results in lower productivity marls, which grade back into sapropels due to lower wind stresses and the return of stagnant basin conditions. These findings indicate that the SNO Effect may in part be responsible for precessional-scale productivity variations observed in parts of the Mediterranean, and perhaps present in other restricted ocean basins, like the Miocene Monterey Formation and the modern Santa Barbara Basin.

Mineralogy and Sr-Mg geochemistry of charophyte carbonates: A new tool for paleolimnological research

Abstract The carbonate biominerals formed in the oogonial cells and on stems of charophytes reveal intra-specific and species-to-species variations within the genus Chara . Calcite, high-magnesian calcite (HMC) and aragonite may be present in both oogonium and stem calcifications, and for the first time evidence is presented of polymineral calcification (HMC+aragonite) within single oogonial cells. The data indicate strong specific differences in the biological mechanisms of mineralization within this group of plants. Among the various species of the genus Chara studied, there is no correlation between the charophyte carbonate mineralogy and the salinity of water where they formed: calcite, HMC and aragonite may all be present in charophyte calcifications formed in low-salinity waters. On the contrary, there is an overall relationship between the mineralogy of these carbonates and the Mg/Ca ratio of the water in which they formed. Sr content in charophyte calcite and high Mg–calcite reflect directly the Sr/Ca ratio of the host water, whereas Mg content in these carbonates probably depends on the temperature and the Mg/Ca ratio of the water. Partitioning coefficients for Mg and Sr for some widespread species are discussed. Our results indicate that charophyte carbonates may be used as paleohydrochemical proxies and imply that special attention must be paid to the mineralogical control of charophyte carbonates when they are to be used in isotopic studies.

Western versus eastern Mediterranean paleoceanographic response to astronomical forcing: a high-resolution microplankton study of precession-controlled sedimentary cycles during the Messinian

Abstract Planktic foraminifera and stable isotopes were analyzed in samples from four precession-controlled sedimentary cycles from southeastern Spain and the island of Gavdos (Greece) to reconstruct the paleobiologic and paleoceanographic response of the western and eastern Mediterranean to astronomically driven climate variability during the Messinian. Although the lithological succession in the western Mediterranean cycles (sapropel–homogeneous marl–diatomite–homogeneous marl) is different from that in the eastern Mediterranean (sapropel–diatomite–homogeneous marl), the pelagic biological succession is similar. Four stages were recognized in both environments, characterized by the dominance of warm-oligotrophic foraminifera (stage 1), Neogloboquadrinids and Globorotaliids (stage 2), Globigerina bulloides (stage 3) and Globigerinita glutinata (stage 4). The first stage occurs during deposition of the upper part of the sapropels in Spain, and the sapropels and part of the diatomites on Gavdos. This stage is inferred to relate to high summer insolation and strong surface water stratification with winter temperatures not high enough to allow eutrophic foraminifera growth. Stage 2 occurs during the deposition of the lower homogeneous marls in Spain and the middle–upper part of the diatomites in Greece, most likely indicating stratification, with colder winter temperatures and a nutricline above the euphotic layer. The end of surface water stratification is indicated during stage 3 by the replacement of the Neogloboquadrinids by G. bulloides, which occurs at the base of the diatomites in Spain and near the top of the diatomites in Greece. We link this event to the appearance of dryer climates in the Mediterranean at times of relatively low summer insolation in the Northern Hemisphere which would result in higher surface salinities, deep water convection, and deep water ventilation. Stage 4, defined by the dominance of G. glutinata and low concentrations of planktic foraminifera, appears linked to the base of the sapropels both in the western and eastern Mediterranean. The onset of sapropel deposition occurred at a time of relatively high surface water salinities, as suggested by the heavy δ18O in Orbulina universa, and prior to the development of surface water stratification evident in the subsequent increase in warm-oligotrophic foraminifera. Gavdos diatomites were deposited under anoxic conditions, whereas Sorbas diatomites were formed in an oxygenated environment. The difference between western and eastern Mediterranean diatomites would be due to differences in hydrographic conditions between the two studied settings.

Use of charophyte carbonates as proxy indicators of subtle hydrological and chemical changes in marl lakes: example from the Miocene Bicorb Basin, eastern Spain

Abstract The lower, alluvial unit in the Miocene Bicorb Basin contains several metric-scale limestone intervals which record episodic shallow lacustrine environments in an alluvial setting developed during the early stage of the basins evolution. Five main carbonate facies have been differentiated in the lacustrine limestones, although calcite charophyte incrustations predominate and constitute the most striking features of these deposits. The thinnest limestone intervals correspond to deposits from charophyte meadows in ponded shallow depressions in floodplains. The thickest limestone intervals are mainly formed by banded limestones and usually correspond to diverse types of regressive sequences that have been interpreted as resulting from the infill of shallow lakes. The sedimentological features and sequences show noticeable differences in the gradient of the littoral zones and the amount of palustrine deposits with models proposed for marl lakes. Charophytic carbonates from the best-preserved facies show similar microtextures to those from recent charophyte incrustations. The variations in stable isotopes ( δ 13 C, δ 18 O) for these primary carbonates occur in parallel with luminescence variations and correspond to hydrological changes and variations in solute composition and Eh–pH status in the lake waters. The carbonates that display moderate to strong diagenetic modifications show a diverse degree of compaction, aggrading neomorphism, strong cementation and nodulization. The isotopic values for these are arranged in diverse clusters. There is a correlation between the degree of luminescence and the δ 13 C. This suggests that hydrological and hydrochemical variations both in the lacustrine and diagenetic environments are being recorded in parallel. We emphasize the need for further comparative studies between recent and ancient charophytic carbonates. As these carbonates have been used in palaeoenvironmental reconstructions, special attention must be paid to the diagenetic changes in ancient charophytic marls.

Paleohydrology of the Upper Miocene Bicorb Lake (eastern Spain) as inferred from stable isotopic data from inorganic carbonates

Abstract The hydrological evolution of the Upper Miocene Bicorb lake (eastern Spain) has been established based on sedimentology, carbonate mineralogy and stable isotopic data from inorganic carbonates from the upper lacustrine unit. Three main subunits have been recognized. In subunit A, the sedimentological features suggest a hydrologically closed lake although the isotopic data are not conclusive. The isotopic compositions of primary carbonates from subunit B display a distribution typical for a hydrologically closed lake, and allow the differentiation of four developmental stages. The first stage was defined by the deposition of gypsum, LMC, dolomite and aragonite, indicating large variations in the water Mg/Ca ratio. During the second and third stages only LMC was deposited suggesting only minor Mg/Ca fluctuations. The δ13C and δ18O pattern of the second stage shows a well-defined covariance (r=0.8), while in the third stage the isotopic data display a negative covariant relationship (r=−0.8). Changes in dissolved inorganic carbon composition are considered the main factor to explain the variations from positive to negative covariance. In the fourth stage the lake waters underwent large Mg/Ca fluctuations, as indicated by the deposition of LMC, HMC and aragonite. The isotopic trend is also covariant. Finally, subunit C is characterized by LMC and minor amounts of HMC, suggesting minor changes in Mg/Ca ratio. The isotopic features of this subunit are more congruent with a hydrologically open lake. The lowest δ13C and δ18O values are very similar in the three subunits, reflecting the most dilute conditions of the lake water, and a uniform isotopic composition of the local meteoric water during these periods. The changes in water balance and solute inputs recorded in the mineralogy and isotopic signatures of carbonates were controlled by the diapiric evolution and changes in the E/P balance.

Lacustrine sedimentation in the diapir-controlled Miocene Bicorb Basin, Eastern Spain

Abstract The Miocene Bicorb Basin is a small elongated basin developed by normal faulting of a thick Jurassic–Cretaceous carbonate succession and subsequent diapirism of Upper Triassic mudstones and evaporites. The basin fill comprises a sequence over 650 m thick formed by two units. The lower, alluvial unit consists of a complex alternation of conglomerates, sandstones, mudstones, and minor lacustrine limestones. The upper unit comprises marginal alluvial and lacustrine deposits in which seven lacustrine facies associations have been distinguished. The inner lacustrine deposits comprise mudstones and carbonates with minor evaporitic deposits. At the northeast and southwest basin edges, alluvial inputs led to the development of delta and fan-delta environments where sandstone and conglomerate deposits dominate. The latter stages of the lake evolution are represented by an expansive thick limestone sequence which close to the NW fault-bounded margin pass laterally to breccias. The hierarchical arrangement of the upper unit shows five different orders of sequences. The first-order sequence defines a major vertical trend of lake expansion whereas three second-order sequences are linked to major flooding-expansion episodes. All these sequences are related to the tectonic evolution of the diapir. Metre-scale, third- and fourth-order sequences and fifth-order sequences, which consist of finely laminated rhythmite deposits probably record climatically forced processes. The diapir–graben system evolution and the climatic variations have exerted major controls on the sequential arrangement and evolution of the lacustrine system. Both factors have also strongly influenced the changes in the gastropod assemblages.