María Ángeles Bárcena

Impact of iceberg melting on Mediterranean thermohaline circulation during Heinrich events

Down-core samples of planktonic and benthic foraminifera were analyzed for oxygen and carbon isotopes in International Marine Past Global Changes Study (IMAGES) core MD99-2343 in order to study the interactions between climate change in the Northern Hemisphere and the western Mediterranean thermohaline circulation at times of Heinrich events (HE). Our results confirm the antiphase correlation between enhanced North Atlantic Deep Water formation and low ventilation in the Mediterranean. However, this study reveals that this antiphase relationship in deepwater formation between the North Atlantic and Mediterranean was interrupted during times of HE when the injection of large volumes of water from melting icebergs reached the entrance to the Mediterranean. These events, which lasted less than 1000 years, are represented by pronounced decreases in both planktonic d18O and benthic d13C signals. Lower salinities of Mediterranean surface water resulted in a slowdown of western Mediterranean deepwater overturn even though cold sea surface temperatures and drier climate should have resulted in enhanced deepwater formation.

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.

Evidence of a high-productivity area off the coast of Málaga from studies of diatoms in surface sediments

Abstract The assemblages of diatoms contained in nineteen surface sediment samples recovered off the coast of Malaga province (Spain) between 5°W and 4°38′W have been analysed. The variations observed in the diatom assemblage point to the presence of nutrient-enriched waters, transported laterally from the centre of an upwelling that occurs to the west of the study area. The utility of species such as Paralia sulcata, Thalassionema nitzschioides and the spores of Chaetoceros spp. in the reconstruction of physico-chemical and biological characteristics and of the spatial structure of the upwelling is confirmed.

Paleoproductivity variations related to climatic conditions in the Alboran Sea (western Mediterranean) during the last glacial–interglacial transition: the diatom record

The present study addresses the analysis of changes in marine primary productivity derived from climatic variability over the last 30,000 yr. The study area is the Alboran Sea; this is the westernmost basin of the Mediterranean Sea, and was a very sensitive area to changes in sea levels and atmospheric configuration during the last glacial‐interglacial transition. We report the results from four sediment cores recovered in both the western and eastern Alboran basins. This allowed us to monitor the W‐E evolution of the hydrological structures. The study is mainly based on the diatom record, although total organic carbon (TOC), isotopes and UK 0 37-SST were also used to gain further information. Paleoceanographic conditions favoured high paleoproductivity rates during the Last Glacial Maximum, although this occurred at Termination 1a and during the Younger Dryas event (YD), increasing towards the easternmost sites. During these periods, meteorological conditions, with increased westerlies, intensified the flow to the east, driving an eastward migration of the Western Anticyclonic Gyre. A possible displacement of the North Alboran upwelling system is also considered. The major component of the diatom assemblage during the YD are the resting spores of Leptocylindrus danicus, which suggest the presence of cooler and less saline surface waters. The opposite behaviour between planktonic and benthic proxies in addition to the high TOC values allows us to infer a phase of reduced deep water renewal between 16.5 and 9 cal ky BP. Fresh-water diatoms and opal phytoliths were used as indicators of aridity and humid land-conditions, respectively. During the glacial period and until 14 cal ky BP we observed a gradual climatic deterioration, accompanied by a loss of grass belts. Climatic conditions were warmer and moister during the Bolling‐Allerod while the YD was dry. The re-establishment of vegetation belts and replenishment of lakes was observed at 8 cal ky BP. q 2001 Elsevier Science B.V. All rights reserved.

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.

A shift in the biogenic silica of sediment in the Larsen B continental shelf, off the Eastern Antarctic Peninsula, resulting from climate change.

In 2002, section B of the Larsen ice shelf, off of the Eastern Antarctic Peninsula, collapsed and created the opportunity to study whether the changes at the sea surface left evidence in the sedimentary record. Biogenic silica is major constituent of Antarctic marine sediment, and its presence in the sediment column is associated with diatom production in the euphotic zone. The abundance of diatom valves and the number of sponge spicules in the biogenic silica was analyzed to determine how the origin of the biogenic silica in the upper layers of the sediment column responded to recent environmental changes. Diatom valves were present only in the upper 2 cm of sediment, which roughly corresponds to the period after the collapse of the ice shelf. In contrast, sponge spicules, a more robust form of biogenic silica, were also found below the upper 2 cm layer of the sediment column. Our results indicate that in this region most of the biogenic silica in the sedimentary record originated from sponge spicules rather than diatoms during the time when the sea surface was covered by the Larsen ice shelf. Since the collapse of the ice shelf, the development of phytoplankton blooms and the consequent influx of diatom debris to the seabed have shifted the biogenic silica record to one dominated by diatom debris, as occurs in most of the Antarctic marine sediment. This shift provides further evidence of the anthropogenic changes to the benthic habitats of the Antarctic and will improve the interpretation of the sedimentary record in Polar Regions where these events occur.

High‐latitude forcing of diatom productivity in the southern Agulhas Plateau during the past 350 kyr

The hydrography of the Indian-Atlantic Ocean gateway has been connected to high-latitude climate dynamics by oceanic and atmospheric teleconnections on orbital and suborbital timescales. A wealth of sedimentary records aiming at reconstructing the late Pleistocene paleoceanography around the southern African continent has been devoted to understanding these linkages. Most of the records are, however, clustered close to the southern South African tip, with comparatively less attention devoted to areas under the direct influence of frontal zones of the Southern Ocean/South Atlantic. Here we present data of the composition and concentration of the diatom assemblage together with bulk biogenic content and the alkenone-based sea surface temperature (SST) variations for the past 350 kyr in the marine sediment core MD02-2588 (approximately 41°S, 26°E) recovered from the southern Agulhas Plateau. Variations in biosiliceous productivity show a varying degree of coupling with Southern Hemisphere paleoclimate records following a glacial-interglacial cyclicity. Ecologically well-constrained groups of diatoms record the glacial-interglacial changes in water masses dynamics, nutrient availability, and stratification of the upper ocean. The good match between the glacial maxima of total diatoms concentration, Chaetoceros spores abundance, and opal content with the maximum seasonal cover of Antarctic ice and the atmospheric dust records points to a dominant Southern Hemisphere forcing of diatom production. Suborbital variability of SST suggests rapid latitudinal migrations of the Subtropical Front and associated water masses over the southern Agulhas Plateau, following millennial contractions and expansions of the subtropical gyres. Warmings of the upper ocean over site MD02-2588 during terminations IV to I occurred earlier than that in the Antarctic Vostok, which is indicative of a Northern Hemisphere lead. Our multiparameter reconstruction highlights how high-latitude atmospheric and hydrographic processes modulated orbital highs and lows in primary production and SST as triggered by northward transport of Si, eolian dust input, and latitudinal migrations of frontal zones.

Quantitative comparison of taxa and taxon concepts in the diatom genus Fragilariopsis: a case study on using slide scanning, multiexpert image annotation, and image analysis in taxonomy

Semiautomated methods for microscopic image acquisition, image analysis, and taxonomic identification have repeatedly received attention in diatom analysis. Less well studied is the question whether and how such methods might prove useful for clarifying the delimitation of species that are difficult to separate for human taxonomists. To try to answer this question, three very similar Fragilariopsis species endemic to the Southern Ocean were targeted in this study: F. obliquecostata, F. ritscheri, and F. sublinearis. A set of 501 extended focus depth specimen images were obtained using a standardized, semiautomated microscopic procedure. Twelve diatomists independently identified these specimen images in order to reconcile taxonomic opinions and agree upon a taxonomic gold standard. Using image analyses, we then extracted morphometric features representing taxonomic characters of the target taxa. The discriminating ability of individual morphometric features was tested visually and statistically, and multivariate classification experiments were performed to test the agreement of the quantitatively defined taxa assignments with expert consensus opinion. Beyond an updated differential diagnosis of the studied taxa, our study also shows that automated imaging and image analysis procedures for diatoms are coming close to reaching a broad applicability for routine use.

LOW-LATITUDE MIOCENE CALCAREOUS AND SILICEOUS MICROFOSSIL BIOSTRATIGRAPHY FROM NW OF SOUTH AMERICA: LADRILLEROS-JUANCHACO SECTION, COLOMBIAN PACIFIC --- Uncorrected Proofs doi:10.5710/AMGH.11.08.2016.2978

Abstract. Fragmentary anuran remains in the Ameghino Collection in the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” were recently considered as belonging in Ceratophryidae (the clade of the South American horned frogs) and coming from middle—late Oligocene sediments exposed south of Lake Colhue Huapi, Chubut, Argentina. However, both the taxonomic placement and the geographic and stratigraphic origin attributed to this material are problematic. A new study of these remains show they possess several features that allow their attribution to the extant genus Ceratophrys Wied-Neuwied. The available information on the collection locality of this material is dubious —namely Gran Barranca, where rocks range from the middle Eocene to early Miocene in age. Additionally, details of the history of this collection and the specimen itself suggest that its provenance should better be considered unknown.The Neogene sedimentary basins from the Colombian Pacific coasts were formed in forearc basins associated with the uplift of the Andes. Some marine sequences in this area remain unexplored and lack of biostratigraphic information. In addition, the available high-resolution biostratigraphic and biochronologic data from this region are limited almost exclusively to research in the open sea by the Integrated Ocean Drilling Programs (IODP). Ladrilleros-Juanchaco section is a continuous and well-exposed marine terrigenous sedimentary sequence with abundant Miocene microfossils. In this section, we carried out biostratigraphic and biochronologic studies through calcareous nannofossils, planktonic foraminifera, and diatoms identifying seventeen astronomically calibrated and standard calcareous microfossil biohorizons and two tropical diatom biozones. This information allowed the establishment of a chronologic framework between the Base of Praeorbulina glomerosa and Catinaster coalitus Base, corresponding to 16.27 and 10.79 Ma, respectively (Burdigalian–Tortonian). Low-latitude planktonic foraminifera and calcareous nannofossil bioevents show a high correlation, whereas variations between diatom biozones and calcareous nannofossil biohorizons were observed since the lowest occurrence of diatoms at ~12.2 Ma. Furthermore, using an interpolated age between the Sphenolithus heteromorphus Top and Fohsella praefohsi Base bioevents, it is interpreted that the Reticulofenestra pseudoumbilicus (>7μm) Base common took place around 13.6 Ma, which is correlated with other sites revealing that it can be considered as a good bioevent for the base of the Serravallian in these tropical areas.

Stratigraphic Transect of Northwestern Colombia: a Key to Understanding the Origin of the Panamanian Isthmus

We present new data on the stratigraphy of the Miocene in northwestern Colombia, in the Pacific Basin. The sedimentological and biostratigraphic study, based on an analysis of calcareous nannofossils, foraminifera, palynomorphs, and diatoms, has enabled a new framework to be constructed that allows the closing process of the Isthmus of Panama to be monitored and palaeoceanographic implications to be made.