Joan Villanueva

Variability in the Benguela Current upwelling system over the past 70,000 years

Abstract This study was designed to see if the intensity and location of upwelling in the Benguela Current Upwelling System off Namibia changed significantly during the last 70,000 years. Most of the analytical work focused on geochemical, micropalaeontological and stable isotopic analyses of a 6.5m long combined pilot and piston core, PGPC12, from 1017m on the continental slope close to Walvis Bay. The slope sediments are rich in organic matter. Most of it is thought to represent deposition beneath a productive shelf edge upwelling system, but some is supplied by downslope nearbottom flow of material probably resuspended on the outer continental shelf. Temporal changes in upwelling intensity as represented by fluctuations in the accumulation of organic matter do not show the simple ‘classical’ pattern of less upwelling and lower productivity in interglacials and more upwelling and higher productivity in glacials, but instead show a pattern of higher frequency fluctuations. The broad changes in organic carbon accumulation reach maxima at times when the earth-sun distance was greatest, indicating that this accumulation responded to changes in the precession index; at these times monsoons would have been weakest and Trade Winds strongest. Maximum accumulation of organic matter on the slope occurred in the last interstadial (isotope stage 3), and coincided with coldest sea surface temperatures as recorded by alkenone data (U k 37 ), and by nannofossil assemblages. It is attributed largely to increased productivity in situ , rather than the lateral supply of material eroded from older organic rich deposits exposed by the lowering of sealevel at that time. The enhanced productivity is attributed to a strengthening of upwelling-favourable winds in this area in response to the minimal solar insolation typical of this period. Diatoms generally are not abundant in these sediments, so appear to be unreliable indicators of productivity over the continental slope. When sealevel was lowest (isotope stages 2 and 4) organic matter previously deposited on the continental shelf was eroded and dumped on the continental slope; this reworked material constitutes up to 43% of the flux of organic matter to the slope at these times. This process did not affect the slope in stage 3, when sealevel fell by only 50m. The accumulation of terrigenous material was highest in stages 2 and 4. The available data suggest that the terrigenous influx at those times was primarily aeolian. We interpret this to mean that more of the winds then came from the east (‘Berg’ winds), bringing an influx of aeolian dust from the hinterland; these easterlies were less favourable for upwelling than were the more southerly Trade Winds that dominated during stage 3. Carbonate accumulation was least in stages 2 and 4, largely in response to dissolution induced by CO 2 -rich bottom waters.

A biomarker approach to the organic matter deposited in the North Atlantic during the last climatic cycle

Abstract The study of the composition of total organic carbon (TOC), C37 alkenones, and C23C33 n-alkanes in the North Atlantic Ocean (cores SU90/08 and SU90/39) has allowed the development of a model for the differentiation of marine and terrigenous TOC. This model gives rise to results in good agreement with inorganic markers such as magnetic susceptibility (MS) and non-carbonate content. According to this model, the terrigenous TOC accounts for most of the organic matter in the glacial sediments. Thus, the higher TOC of the glacial periods (0.1–0.45% vs. 0.05–0.15% in the interglacials) is due to the increase in terrigenous TOC. The changes in marine TOC (those associated to sea-surface productivity) are independent of the glacial-interglacial evolution. The terrigenous TOC is more important at higher latitudes, probably due to higher terrigenous detrital inputs associated with iceberg transport. In this respect, the correlation between n-alkanes and MS strongly suggests that the main source of these hydrocarbons are ice-rafted materials and that aeolian inputs only represent minor contributions. The four peaks of reworked n-alkanes in the SU90/08 core that are coincident with the Heinrich Layers (H1, H2, H4, and 115) are in agreement with this hypothesis. On the other hand, the abrupt marine TOC peaks of SU90/08 show a 23 ka periodicity, indicating that the marine productivity at 43°N in the North Atlantic Ocean was precessionally driven. The productivity maxima in this core correlate with low latitude oceanographic processes, such as periods of enhanced trade wind intensity and equatorial upwelling which are also tuned to precession and independent of the global glacial-interglacial evolution.

Clean-up procedures for the unbiased estimation of C37 alkenone sea surface temperatures and terrigenous n-alkane inputs in paleoceanography

Abstract Sample preparation procedure for the quick determination of C25C33 n-alkanes and the U37k index in small sediment sample amounts (ca. 1 g dry weight) have been developed. Alkaline hydrolysis and silica column fractionation are the two clean-up steps considered. The former eliminates the interferences from the sedimentary wax esters which dominate the lipid composition in high latitude Atlantic Ocean sediments. The latter has been designed to collect both lipid groups (C25C33 n-alkanes and C37 alkenones) in one single fraction avoiding the interferences from highly polar compounds which affect, in particular, the quantitative determination of the n-alkanes. The whole procedure has been shown not to affect the U37k measurements, even at low C37 ketone concentrations such as 10 ng which is the minimum threshold of total C37 alkenones in the gas chromatographic system for avoidance of adsorption-related interferences [1]. As shown with real case examples the usefulness of these clean up methods is highly dependent on the amounts of interfering lipid compounds present in the sediment extracts and their application may not be necessary for the open sea sediments present in most low latitude areas.

New insights into the glacial latitudinal temperature gradients in the North Atlantic. Results from UK′37 sea surface temperatures and terrigenous inputs

Abstract Sea surface temperatures (SST) and input of continental materials have been reconstructed from the study of the long-chain alkenones and n-alkanes, respectively, in a core located in the western side of the Mid-Atlantic Ridge (MD952037, 37°05′N 32°02′W, 2630 m depth). Both the long- and short-term variability recorded by the temperatures and the planktonic δ18O are basically the same over the last 280 kyr, showing a clear glacial/interglacial evolution. Comparison with core SU90/08, located only 6° north and directly influenced by the cold polar waters associated with the polar front during glacial times, revealed different climatic conditions during glacial periods at both locations. Whereas core MD952037 recorded similar SST values during the last two glacial periods (ca. 14–15°C), the northern core displayed colder conditions during isotopic stage 2 (8–10°C) than in stage 6 (13–15°C). These results indicate the existence of a well-developed steep north–south gradient between 37 and 43°N during the last glacial period but not during stage 6, which suggests a southern expansion of the polar front during the last glacial maximum.

Precessional forcing of productivity in the North Atlantic Ocean

The sedimentary C37 alkenone fluxes in core SU90/08 situated in the North Atlantic (43°N) showed that export productivity related to some Haptophyta was driven by precession (1/23 kyr−1) in the last 260 kyr during the glacial periods. In the interglacial stages these alkenones were strongly depleted. This different behaviour is attributed to the displacement of Ekman pumping to lower latitudes and to thermocline stability decreases in the glacial periods. The alkenones were coherent and nearly phased with boreal winter insolation, suggesting a link between productivity and the strength of the westerlies. They were also coherent with the precessionally driven nutriocline minima and winter sea surface temperature in the equatorial Atlantic Ocean. The combination of the measurements at these different latitudes point to a strong coupling in the dynamics of the Hadley and Ferrel cells and to a dominant North Atlantic modulation of marine productivity to precession during the glacial stages.

Sources and transformations of chlorophylls and carotenoids in a monomictic sulphate-rich karstic lake environment

Abstract A study of the pigment composition in the water column particulates and bottom sediment column of an anoxic lake, Lake Ciso (Catalonia, Spain), has been carried out. The depth-dependencies of the carotenoid and chlorophyll distributions in the water column during stratification and holomixis, as well as during the diurnal cycles, have been determined. In the sediment, the composition of the pigments has been studied over the top 25 cm. The analyses were carried out by high performance liquid chromatography (HPLC) and HPLC coupled to mass spectrometry. The distributions of carotenoids, bacteriochlorophylls and chlorophylls in the water particulates closely follow the population dynamics of the photosynthetic organisms during stratification and holomixis. Among these, only chlorophyll a transformation products are found in the water column, namely phaeophytin a , pyrophaeophytin a and pyrophaeophorbide a . These derivatives also occur in the sediment, where they are found together with bacteriophaeophytin a and pyrobacteriophaeophytin a . This compartment-dependent occurrence corresponds to a higher degree of preservation of the bacterial vs algal pigments. The former occur in the anoxic hypolimnion or mixed water column during holomixis, and the latter in the oxic epilimnion. The presence of these compounds and the very low abundance of phaeophorbide a and absence of bacteriophaeophorbide a point to a transformation pathway where, in the absence of zooplankton grazing or high abundances of the enzyme chlorophyllase, the loss of the phytyl chain occurs predominantly after loss of the C-13 2 carbomethoxyl group. Okenone, alloxanthin, α-carotene and zeaxanthin/lutein are the major water column carotenoids. Their relative abundances contrast with their distribution in the sediments where okenone is the predominant compound and zeaxanthin/lutein are below detection limit. This effect may be in part a consequence of a more uniform okenone concentration in the water particulates over the year, leading to a higher annual sedimentation flux. Again, it may also be influenced by the higher degree of preservation of the bacterial vs algal carotenoids. A similar situation is observed for the relative abundances of chlorophyll a and bacteriochlorophyll a in water particulates and sediments.

Chlorophyll and carotenoid pigments in solar saltern microbial mats

The distributions of carotenoids, chlorophylls, and their degradation products have been studied in two microbial mat systems developed in the calcite and calcite/gypsum evaporite domains of a solar saltern system. Phormidium valderianum and Microcoleus chthonoplastes are the dominant cyanobacterial species, respectively, and large amounts of Chloroflexus-like bacteria occur in the carbonate/gypsum mat. In both systems, the major pigments are chlorophyll a, zeaxanthin, β-carotene and myxoxanthophyll, which originate from these mat-building cyanobacteria. This common feature contrasts with differences in other pigments that are specific for each mat community. Thus, chlorophyll c and fucoxanthin, reflecting diatom inputs, are only found in the calcite mat, whereas the calcite/gypsum mat contains high concentrations of bacteriochlorophylls c produced by the multicellular green filamentous bacteria. In both cases, the depth concentration profiles (0–30 and 0–40 mm) show a relatively good preservation of the cyanobacterial carotenoids, zeaxanthin, β-carotene, myxoxanthophyll, and echinenone. This contrasts with the extensive biodegradation of cyanobacterial remains observed microscopically. Fucoxanthin in the calcite mat is also transformed at a faster rate than the cyanobacterial carotenoids. Chlorophyll a, the major pigment in both mats, exhibits different transformation pathways. In the calcite/gypsum mat, it is transformed via C-132 carbomethoxy defunctionalization prior to loss of the phytyl chain, leading to the formation of pyrophaeophytin a and, subsequently, pyrophaeophorbide a. On the other hand, the occurrence of the enzyme chlorophyllase, attributed to diatoms in the calcite mat, gives rise to extensive phytyl hydrolysis, with the formation of chlorophyllide a, pyrophaeophorbide a and, in minor proportion, phaeophorbide a. Studies of the sources of the photosynthetic pigments and of their transformation pathways in such simplified ecosystems provide a basis for the understanding of the distribution patterns of these compounds in more complex aquatic environments.

Assessment of sea surface temperature variations in the central North Atlantic using the alkenone unsaturation index (U37k

Abstract A high resolution record of U37k′ -derived SST estimations has been obtained in a core from the central North Atlantic Ocean (SU90/08, 43°30′N, 30°24′W) spanning a time period of 280 ky. The general trend of the U37k′ profile parallels closely the δ180 signal and represent an independent confirmation that the SST variations in the glacial-interglacial time scales are correlated with the northern ice sheets evolution. In contrast to the CLIMAP (1984) conclusions based on the foraminiferal transfer function technique, the SST during the last interglacial period (LIP, isotopic stage 5.5) are 2–3°C warmer than during the Holocene. This result is in good agreement with many studies based in coastal and continental records that strongly suggest warmer climatic conditions over North America and Europe. Finally, the SST estimates during the last glacial maximum (LGM, 18 ky bp ) where 4.5°C cooler than during the previous glacial period (PGP, stage 6). We argue that this difference is caused by a northward position of the polar front during the PGP in the Central North Atlantic.

Isotope dilution mass spectrometry for absolute quantification in proteomics: Concepts and strategies

Isotope dilution mass spectrometry is a reference technique for quantitative analysis, given that it combines the sensitivity and selectivity of MS instruments with the precision and accuracy associated with the use of internal standards. Isotope-labeled proteins are the optimal internal standards for quantitative proteomics as they closely mimic the behavior of their natural counterparts during the analytical process. A major complication of isotope dilution mass spectrometry proteomics is the technical difficulty of obtaining these internal standards, especially in studies where a high number of proteins have to be quantified simultaneously. In this paper, we review some of the characteristics of the isotope dilution mass spectrometry approach, its benefits in terms of reliability and quality control in targeted proteomic analysis and the different strategies developed for its application in proteomics.

Pitfalls in the chromatographic determination of the alkenone U37k index for paleotemperature estimation

Abstract Co-elution and irreversible adsorption are major error sources in the gas chromatographic determination of paleotemperatures from the analysis of C 37 di- and triunsaturated ketones. The former are related to the interference of methyl hexatriaconta-(7 E , 14 E , 21 E )-trienoate with heptatriaconta-(15 E , 22 E )-dien-2-one. These compounds can be resolved using capillary columns with a thin coating ( ca. 0.1 μm) of 100% dimethylsiloxane. Irreversible adsorption concerns both the di- and triunsaturated species and has significant effects (deviations of temperature measurements higher than 0.5°C) when the absolute total amounts of ketones introduced into the system are smaller than 10 ng. The higher adsorption of the C 37:3 species gives rise to deviations of the U 37 k measurements towards higher temperatures that become more important the lower is the proportion of this species in the mixture. This adsorption effect also increases the dispersion of the results as a higher standard deviation is observed the lower is the total amount of ketones and the proportion of the triunsaturated species.