Salinity Flux Research Articles

Messinian and Pliocene erosional surfaces in the Po Plain-Adriatic Basin: Insights from allostratigraphy and sequence stratigraphy in assessing play concepts related to accommodation and gateway turnarounds in tectonically active margins

The Messinian reservoirs of Po Plain-Adriatic Basin were controlled by the combination of Messinian Salinity Crisis (MSC) and morphostructural reshaping. Under these conditions, complex sedimentary architectures and unusual stratigraphic discontinuities would be overlooked using preconceived sequence-stratigraphic models. From this perspective, the establishment of a geological model constrained by a proper sequence-stratigraphic framework provided insights to develop new play concepts and better assess reservoir heterogeneities. More flexible criteria of sequence-stratigraphic analysis are discussed, focused on: (1) a wider range of erosional bounding surfaces (subaerial vs. subaqueous, regressive vs. transgressive); (2) the possible diachronism in accommodation and sediment fairways; (3) unusual relationships between stacking patterns, modes of sediment supply and sediment routing to deeper waters; (4) the relationships between 4th and 5th order sequences and their shoreline trajectories.Main results suggest an asymmetric MSC cycle related to a long overall relative base-level fall followed by a two-step accommodation increase. The total drop, not exceeding 900 m in the study area, occurred through distinct phases of evaporation drawdown, basin reshaping/differential subsidence and regional uplift/isostatic rebound. Subaerial and subaqueous erosional surfaces affected a stepped landscape–seascape, generating paleovalleys and canyons undergoing polyphasic reshaping throughout MSC. Latest Messinian low-accommodation, low salinity and high sediment flux favored the development of sustained hyperpycnites and shelfal lobes, providing upside potential in compartments onlapping basinward of main deltaic reservoir. Pliocene slope instability and sediment failure removed part of Messinian reservoirs owing to oversteepening generated by drowning unconformities, usually recorded by canyon reincision and retrogressive slump scars. This process left traps consisting of erosional remnants sealed by Pliocene muds, and in addition could feed, via surge-flows, backstepping intra-slope reservoirs.

Dynamic and Double-Diffusive Instabilities in a Weak Pycnocline. Part II: Direct Numerical Simulations and Flux Laws

Abstract This study examines the interaction of diffusive convection and shear through a series of 2D and 3D direct numerical simulations (DNS). The model employed is based on the Boussinesq equations of motion with oscillating shear represented by a forcing term in the momentum equation. This study calculates thermal diffusivities for a wide range of Froude numbers and density ratios and compares the results with those from the analysis of observational data gathered during a 2005 expedition to the eastern Weddell Sea. The patterns of layering and the strong dependence of thermal diffusivity on the density ratio described here are in agreement with observations. Additionally, the authors evaluate salinity fluxes that are inaccessible from field data and formulate a parameterization of buoyancy transport. The relative significance of double diffusion and shear is quantified through comparison of density fluxes, efficiency factor, and dissipation ratio for the regimes with/without diffusive convection. This study assesses the accuracy of the thermal production dissipation and turbulent kinetic energy balances, commonly used in microstructure-based observational studies, and quantifies the length of the averaging period required for reliable statistics and the spatial variability of heat flux.

Late quaternary glacial/interglacial cyclicity models of the Red Sea

The benthic foraminiferal faunas from samples of two piston cores retrieved along a north–south transect in the Red Sea were studied. The northern core was collected during Meteor cruise M 31/2, while the southern one was collected during the Sonne cruise 121. Benthic foraminiferal faunas from both sites exhibit large variability with respect to density, diversity, species composition and assemblages combined with stable oxygen and carbon isotope records of planktic and benthic foraminifera. One hundered thirty benthic foraminiferal species were identified in the investigated cores. The faunal data set of the northern core was reduced to five assemblages (factors) while the southern one was reduced to four assemblages. All assemblages were ranked according to their ecological significance. Besides, relative abundance of major benthic foraminiferal suborders [Textulariina (agglutinating foraminifera), Miliolina and Rotaliina], in addition to infaunal/epifaunal relative abundance were used as paleoenvironmental proxies allowing the reconstruction of past changes in deep-water salinity, ventilation and organic carbon fluxes at the sea-floor. Four distinct glacial/interglacial cycle during the past 380 Kyr have been recognized in the Red Sea. The identified four cycles reveal deviation in deep-sea ecosystem between the northern and southern Red Sea. In the northern Red Sea salinity fluctuations, productivity and deep-water ventilation and formation had the major impact on benthic foraminiferal pattern corresponding to glacial/interglacial cycles and glacio-eustatic sea level changes coupled with the impact of Mediterranean climate regime. On the otherhand, in the southern Red Sea region the oscillation trend of benthic foraminiferal pattern within the glacials and interglacials stages indicates a high frequency environmental alternation. This alternation is consistent with the extent of NE monsoonal wind that controls the intensity and extension of the productivity, which in turn determine organic matter fluxes and oxygen level at the sea floor.

High-resolution simulation of the circulation in the Bahamas and Turks and Caicos Archipelagos

We present the first high-resolution simulation of the Bahamas and Turks and Caicos archipelagos implemented with the Regional Oceanic Modeling System (ROMS). Our model resolves tides and our simulation consists of two nested grids of respectively 4 and 2km horizontal resolution. This oceanic region is constituted of the juxtaposition of very large and shallow banks, and very deep basins. Our modeling study shows that the dominant influence is the salinity from the sub-tropical gyre, which varies latitudinally with seasons. Less salty surface waters entered the archipelago from September to November from the north and through the Old Bahama (OBC) and the North West Providence (NWPC) channels as the western Atlantic saline waters moved south. Saltier Atlantic waters arrived from the south in December and moved north till the next fall.This salinity flux is mitigated by the atmospheric forcing on the banks as well as the water temperature, which varied diurnally, what modified the water density causing this water to be unique to the banks, yielding its own seasonality as seen in their SST. The banks were colder in winter and warmer in summer than their surrounding waters. Fronts were formed at every ebb tide in winter, spring, and fall. However the influence of bank waters on the surrounding waters was shown to be limited because of the dominant influence of the large-scale flow and of the local production of water masses by wind and cyclone driven upwelling. Bank waters were contained at the front where they mixed with the surrounding waters as shown by the short time presence of the fronts. Local wind, in addition to the tide was also shown to be a significant forcing which is responsible for some of the short scale variability 5–30days in Florida Current transport, the NWPC, the OBC, the Windward Passage (WP), and the Antilles Current (AC). Finally, the study also confirmed the mesoscale eddy influence at seasonal scales (100days) on the AC transport as well as on the FC, through a possible leakage of eddy flux through the NWPC. A similar influence was found south of the archipelago, where mesoscale eddy could also influence WP transport.

Analyzing the 2010–2011 La Niña signature in the tropical Pacific sea surface salinity using in situ data, SMOS observations, and a numerical simulation

AbstractThe tropical Pacific Ocean remained in a La Niña phase from mid‐2010 to mid‐2012. In this study, the 2010–2011 near‐surface salinity signature of ENSO (El Niño‐Southern Oscillation) is described and analyzed using a combination of numerical model output, in situ data, and SMOS satellite salinity products. Comparisons of all salinity products show a good agreement between them, with a RMS error of 0.2–0.3 between the thermosalinograph (TSG) and SMOS data and between the TSG and model data. The last 6 months of 2010 are characterized by an unusually strong tripolar anomaly captured by the three salinity products in the western half of the tropical Pacific. A positive SSS anomaly sits north of 10°S (>0.5), a negative tilted anomaly lies between 10°S and 20°S and a positive one south of 20°S. In 2011, anomalies shift south and amplify up to 0.8, except for the one south of 20°S. Equatorial SSS changes are mainly the result of anomalous zonal advection, resulting in negative anomalies during El Niño (early 2010), and positive ones thereafter during La Niña. The mean seasonal and interannual poleward drift exports those anomalies toward the south in the southern hemisphere, resulting in the aforementioned tripolar anomaly. The vertical salinity flux at the bottom of the mixed layer tends to resist the surface salinity changes. The observed basin‐scale La Niña SSS signal is then compared with the historical 1998–1999 La Niña event using both observations and modeling.

Cross shelf hydrographic and hydrochemical conditions and their short term variability at the northern Benguela during a normal upwelling season

Cross shelf hydrographic and hydrochemical conditions were investigated during the seasonal maximum of upwelling in the northern Benguela upwelling system. The study combines in situ observations, remotely sensed data and results of a regional 3-dimensional numerical model. In situ observations were recorded along a cross shelf transect off Namibia starting at 20°S 13°E, repeated five times during 16 August 2011 until 19 September 2011. Comparison of wind forcing and sea surface temperatures during the time of the expedition with long-term climatological data as well as the index of intensity of the Benguela upwelling indicates “normal” upwelling conditions in austral winter 2011 in the northern Benguela. Small scale temporal (days) and spatial (km) variability is high during the upwelling season, primarily caused by highly variable wind forcing and dynamics of mesoscale structures like eddies and filaments as found in remotely sensed data. This mesoscale dynamics impact the applicability of a conceptual 2-dimensional circulation model, i.e. a linear succession along the cross-shelf transect. Therefore, an age proxy for surface water was constructed based on oxygen and heat fluxes during the first aging period and on salinity and heat fluxes during the second phase. The application of an age proxy instead of distance to shore successfully validates the succession concept. Furthermore, the investigation of the upwelling strengths by analytical and circulation models verified their dependence on coastal- and curl driven upwelling processes with the onshore dominance of coastal upwelling. In the investigated time period, offshore, curl driven upwelling dominated with a maximum located on the shelf.

Modeling on the Aspects of Thermal Response of Bay of Bengal to Tropical Cyclone TC05B 1999 using Princeton Ocean Model (POM): Preliminary Results

Tropical cyclones (TC’s), while moving over the ocean bring about significant changes in ocean thermal structure and other ocean environment. These oceanic thermal responses provide vital information for understanding the air-sea interaction processes. This study used Princeton Ocean Model (POM) to investigate the changes in the oceanic thermal characteristics of Bay of Bengal (BOB) associated with TC05B (Orissa super cyclone) 1999. Model was forced with the wind and heat plus salinity fluxes as surface forcing in different experiments. In order to provide reasonable and realistic cyclonic winds to the model as forcing, the tropical cyclone wind model (TCWM) was developed to generate the synthetic cyclonic winds and superimposed with the analyzed blended QSCAT/NCEP wind field. Results show significant drop in sea surface temperature (SST) beneath the storm centre and to the right of the track confirming the earlier findings and are in qualitative agreement with the available Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) satellite SST imageries. Subsurface thermal structures also reflect significant impact of cyclonic vortex over BOB.

Decadal change of Antarctic Intermediate Water in the region of Brazil and Malvinas confluence

The Antarctic Intermediate Water (AAIW) exhibits a decadal variability during recent years, i.e., salinification before 1997 and freshening thereafter, with the maximum anomalies locating at the region of Brazil and Malvinas currents confluence. Our study proposed that the local mesoscale eddies may play an important role in triggering this decadal oscillation. The eddy activity intensification (weakening) leads to the increase (decrease) of poleward cross-frontal eddy salinity flux and upward eddy buoyancy flux, which results in the weakening (strengthening) of the subsurface stratification and potential vorticity (PV). The PV anomalies facilitate (block) the poleward transport of warm saline subtropical water, while the stratification weakening favors the further downward transmission of salinity anomalies by processes of eddy flux as well as mean-flow advection (the stratification strengthening inhibits the vertical transport), then initiates the decadal change of the AAIW property. The whole process of the eddy-related propagation of salinity anomalies takes about 4 to 6 years.

Forward osmosis pretreatment of seawater to thermal desalination: High temperature FO-MSF/MED hybrid system

Forward osmosis (FO) seawater pretreatment was proposed for the removal of scale ions from seawater to the thermal desalination plant. In the current study, previously developed models were applied to estimate the effectiveness of FO pretreatment in the removal of divalent ions from feed solution to MSF/MED at elevated temperatures. The simulation results showed that the water and salt flux across the FO membrane increased with increasing the seawater salinity. However, for given seawater salinity, the water and salt flux across the FO membrane decreased with increasing the FO recovery rate. It was found that the concentration of Ca2+, Mg2+ and SO42− ions increased with increasing the operating temperature in the thermal plant but decreased with increasing the recovery rate of the FO pretreatment. Additionally, an FO pretreatment-MED Scale Index (FMSI) was developed to determine the required FO recovery rate and avoids scale problems at different MED operating temperatures. Initially, Ryznar Scale Index (RSI) of the feed solution was calculated for different MED operating temperatures. Then, RSI was plotted against the FO recovery rates and the desirable FO recovery rate was determined from the plot based on the operating temperature of the MED plant. The scale index was also applied to determine the required mixing ratio of NF permeate-makeup water in the NF-MED desalination hybrid system. The application of the FO pretreatment-MED Scale Index has the potential to reduce the required time and resources to determine the desirable FO/NF pretreatment ratio of feed water to the MED plant.

Quantitative evaluation of turbulent mixing in the Central Equatorial Pacific

Turbulent mixing in the central equatorial Pacific has been quantitatively evaluated by analyzing data from microstructure measurements and conductivity temperature depth profiler (CTD) observations in a meridionally and vertically large region. The result that strong turbulent mixing with dissipation rate e (>O(10−7) W kg−1), continuing from sea-surface mixed layer to low Richardson number region below, in the area within 1° of the equator, shows that turbulent mixing has a close relationship to shear instability. e > O(10−7) W kg−1 and turbulent diffusivity K ρ > O(10−3) m2 s−1 were obtained from near-surface to 85 db at stations even southwardly beyond 3°S, where it is already far from the southern boundary (~2°S) of the Equatorial Undercurrent. Turbulence-induced heat flux and salinity flux were calculated, and both had their maxima in the equatorial upwelling region, though the former was downward and the latter was upward. Accordingly, vertical velocity in the upwelling region was estimated to be similar to the results derived by other methods. These fluxes and the vertical velocity suggest the critical importance of turbulent mixing in maintaining the well-mixed upper layer. Secondly, in the intermediate region (>500 db), turbulent eddies were investigated by applying Thorpe’s method to the CTD data. A large number of overturns were detected, with spatial-averaged K ρ (700–1,000 db) being 3.3 × 10−6 m2 s−1, and the corresponding K ρ-max reaching to O(10−4) m2 s−1 in the north (3°–13°N). The results suggest that, in the intermediate region, considerable turbulent mixing occurs and moderates the properties of the water masses.

Barrier Layers in the Tropical South Atlantic: Mean Dynamics and Submesoscale Effects*

Abstract Barrier layers are generated when the surface mixed layer is shallower than the layer where temperature is well mixed, in geographical regions where salinity plays a key role in setting up upper-ocean density stratification. In the tropical oceans, thick barrier layers are also found in a latitude range where spiraling trajectories from surface in situ drifters suggest the presence of predominantly cyclonic submesoscale-like vortices. The authors explore these dynamical processes and their interplay in the present paper, focusing on the tropical South Atlantic Ocean and using a high-resolution modeling approach. The objective is threefold: to investigate the mean dynamics contributing to barrier-layer formation in this region, to study the distribution and seasonality of submesoscale features, and to verify whether and how the submesoscale impacts barrier-layer thickness. The model used is the Regional Ocean Modeling System (ROMS) in its Adaptive Grid Refinement in Fortran (AGRIF) online-nested configuration with a horizontal resolution ranging between 9 and 1 km. The simulated circulation is first described in terms of mean and submesoscale dynamics, and the associated seasonal cycle. Mechanisms for barrier-layer formation are then investigated. The results confirm previous hypotheses by Mignot et al. on the relevance of enhanced winter mixing deepening the isothermal layer, whereas the salinity stratification is sustained by advection of surface fresh waters and subsurface salinity maxima. Finally, submesoscale effects on barrier-layer thickness are studied, quantifying their contribution to vertical fluxes of temperature and salinity. Submesoscale vortices associated with salinity fronts are found to have a significant effect, producing thicker barrier layers (by ~20%–35%) and a shallower mixed layer because of their restratifying effect on salinity.

Clarifying the link between surface salinity and freshwater fluxes on monthly to interannual time scales

Freshwater fluxes ( ) between the ocean and the atmosphere and land, comprised of evaporation, precipitation and terrestrial runoff, are an essential component of the Earth's climate system. However, direct observations of and its components are sparse and available estimates have substantial uncertainties. In this study we investigate if measurements of sea‐surface salinity ( ) can provide an alternative indirect method for estimating . We examine the relationship between , and oceanic fluxes from surface advection and mixing processes, on time scales from months to years, using a consistent estimate of the ocean/atmosphere state obtained from model/data synthesis produced by the ECCO (Estimating Circulation and Climate of the Ocean) consortium. ECCO salinity averaged over the mixed layer is used as an estimate of . Budget analysis shows that variability in tendencies can be attributed to both and oceanic fluxes, demonstrating the importance of the ocean's role in evolution of , for both local and global mean fields. Regression analysis of the 13 year long ECCO fields shows that there are only a few regions (e.g., subtropical gyres) where can be used as a proxy for using linear models, and only at monthly to annual time scales. Results are similar over a range of spatial scales from ∼100 to 2000 km. Findings are discussed in the context of the general sensitivities of to atmospheric and oceanic processes and the potential of satellite salinity measurements to constrain estimates of .

Southern Ocean nutrient trapping and the efficiency of the biological pump

We present a data‐assimilated model of the ocean's phosphorus cycle that is constrained by climatological phosphate, temperature, salinity, sea‐surface height, surface heat and freshwater fluxes, as well as chlorofluorocarbon‐11(CFC‐11) and natural Δ14C. Export production is estimated to be 5.8±2.0×1012 mol P/yr of which (26±6)% originates in the Southern Ocean (SO) south of 40°S. The biological pump efficiency, defined as the proportion of the ocean's phosphate inventory that is regenerated, is (39±7)%. Dividing the SO south of 40°S into a sub‐Antarctic zone (SANTZ) and an Antarctic zone (ANTZ) separated by the latitude of maximum Ekman divergence, we estimate that the SANTZ and ANTZ account, respectively, for (23±5)% and (3±1)% of global export production, (17±4)% and (3±1)% of the regenerated nutrient inventory, and (31±1)% and (43±5)% of the preformed nutrient inventory. Idealized SO nutrient depletion experiments reveal a large‐scale transfer of nutrients into circumpolar and deep waters and from the preformed to the regenerated pool. In accord with the concept of the biogeochemical divide, we find that nutrient drawdown in the ANTZ is more effective than in the SANTZ for increasing the efficiency of the biological pump, while having a smaller impact on production in regions north of 40°S. Complete SO nutrient drawdown would allow the biological pump to operate at 94% efficiency by short circuiting the transport of nutrients in northward Ekman currents, leading to a trapping of nutrients in circumpolar and deep waters that would decrease production outside the SO by approximately 44% while increasing it in the SO by more than 725%.

Eighteen Degree Water formation within the Gulf Stream during CLIMODE

Analysis of wintertime CLIMODE data for 2007 indicates that a substantial portion of new Eighteen Degree Water (EDW) is likely ventilated within the eastward flowing Gulf Stream (GS) between 67°W and 52°W longitudes, possibly exceeding that formed elsewhere in the northern Sargasso Sea. Use of some global air–sea interaction data sets applied to the study region for Feb/Mar of 2007 indicate that this winter may have been anomalously energetic in air–sea exchange compared to the mean of the prior 19yr. The largest heat and freshwater fluxes found directly over the meandering warm core of the Gulf Stream are capable of removing most of the subtropical heat anomaly of the GS, but cross-frontal fluxes of salinity are required to account for the observed regional salinity structure. An isopycnal diffusivity of ∼100m2s−1 is inferred from the salinity balance. This mixing would also account for the observation that EDW formed in the GS is slightly fresher than that formed in northern Sargasso Sea. The lateral flux of heat across the GS north wall also acts to cool the resulting EDW water, but the heat balance for EDW production is largely determined from GS advection and air–sea fluxes, in contrast to salinity. Based on oxygen saturation data, we estimate that 1.8–3.0Sv-yr of new EDW is formed in the GS for the winter of 2007. EDW originating from the GS is generated in a separate location from where it is accumulated in the northern Sargasso Sea. This manner of EDW formation will produce unique characteristics of EDW found in the northern Sargasso Sea: ones that differ in T/S properties from that formed south of the GS under the more traditional 1D, cooling-driven convection process.

A needle micro-osmometer for determination of glycosaminoglycan concentration in excised nucleus pulposus tissue

Aggrecan is one of the major macromolecular components of the intervertebral disc (IVD) and its loss is an early sign of degeneration. Restoration of aggrecan, and hence of biomechanical properties, is a major objective of biological therapies. At present, assessment of aggrecan concentration via its glycosaminoglycan (GAG) content is accomplished using biochemical and histological methods which require sacrifice of tissue. A minimally invasive method for assessing GAG, and hence aggrecan, which can avoid destruction of tissue, would be of benefit. We have developed a needle micro-osmometer that is capable of measuring flux of saline into excised human nucleus pulposus (NP) tissue. Using the isotropic osmotic stress technique to assess the swelling pressure of the excised NP tissue and assuming negligible collagen tensile stress, we were able to relate the flux to the tissue fixed charge density (FCD). GAG concentration is evaluated from its FCD via the radioactive tracer technique. Samples representing different ages (28-59 years) and degeneration grades (1-4) were analyzed. The flux is controlled by both the osmotic pressure difference across the probe's semi-permeable membrane and by the tissue permeability. A linear correlation was found between flux and the tissue FCD. The equation describing the linear fit is FCD/(total tissue hydration) = 1.97 × 10(-4) + 8283 × flux (R = 0.836, p < 10(-4)). Thus, by measuring saline flux, the concentration of GAG can be determined. Micro-osmometry provides a reliable and minimally invasive tool for assessing GAG content in excised NP tissue. This method may be usefully applied in tissue engineering applications. It may also be useful for in vivo measurements if the question of the degenerative effect of needle puncture can be overcome.

An assessment of the mixed layer salinity budget in the tropical Pacific Ocean. Observations and modelling (1990–2009)

This paper investigates mechanisms controlling the mixed-layer salinity (MLS) in the tropical Pacific during 1990–2009. We use monthly 1° × 1° gridded observations of salinity, horizontal current and fresh water flux, and a validated ocean general circulation model with no direct MLS relaxation in both its full resolution (0.25° and 5 days) and re-sampled as the observation time/space grid resolution. The present study shows that the mean spatial distribution of MLS results from a subtle balance between surface forcing (E − P, evaporation minus precipitation), horizontal advection (at low and high frequencies) and subsurface forcing (entrainment and mixing), all terms being of analogous importance. Large-scale seasonal MLS variability is found mainly in the Intertropical and South Pacific Convergence Zones due to changes in their meridional location (and related heavy P), in the North Equatorial Counter Currents, and partly in the subsurface forcing. Maximum interannual variability is found in the western Pacific warm pool and in both convergence zones, in relation to El Nino Southern Oscillation (ENSO) events. In the equatorial band, this later variability is due chiefly to the horizontal advection of low salinity waters from the western to the central-eastern basin during El Nino (and vice versa during La Nina), with contrasted evolution for the Eastern and Central Pacific ENSO types. Our findings reveal that all terms of the MLS equation, including high-frequency (<1 month) salinity advection, have to be considered to close the salinity budget, ruling out the use of MLS (or sea surface salinity) only to directly infer the mean, seasonal and/or interannual fresh water fluxes.

Influence of water flow on gas hydrate accumulation at cold vents

A cold vent is an area where methane-rich fluid seepage occurs. This seepage may alter the local temperature, salinity, and subsequent accumulation of the gas hydrate. Using a kinetic gas hydrate formation model and in situ measurement of temperature, salinity and fluid flux at the southern summit of Hydrate Ridge, we simulate the gas hydrate accumulation at three distinct fluid sites: clam, bacterial mat, and gas discharge sites. At the clam sites (pore water flux 0.8 mol kg−1) by the formation of gas hydrate causing the base of the hydrate stability zone to move gradually from ∼115 to ∼70 meters below seafloor (mbsf). The gas hydrate saturation at the clam sites is relatively high. The water flux at the bacterial mat sites ranges from 100 to 2500 kg m−2 yr−1. The water flow suppresses the increase in salinity resulting in a salinity close to or slightly higher than that of seawater (< 0.65 mol kg−1). Heat advection by water flow increases temperature significantly, shifting the base of the hydrate stability zone to above 50 or even 3 mbsf. The gas hydrate saturation is relatively low at the bacterial mat site. At the gas discharge sites, the pore water flux could reach 1010 kg m−2 yr−1, and the temperature could reach that of the source area in 9 min. There is no gas hydrate formation at the gas discharge sites. Our simulative analysis therefore reveals that a lower pore water flux would result in lower salinity, higher temperature, and a shallower base of the hydrate stability zone. This in turn induces a lower gas hydrate formation rate, lower hydrate saturation, and eventually less gas hydrate resources.

The Thiamine Content of Phytoplankton Cells Is Affected by Abiotic Stress and Growth Rate

Thiamine (vitamin B1) is produced by many plants, algae and bacteria, but by higher trophic levels, it must be acquired through the diet. We experimentally investigated how the thiamine content of six phytoplankton species belonging to five different phyla is affected by abiotic stress caused by changes in temperature, salinity and photon flux density. Correlations between growth rate and thiamine content per cell were negative for the five eukaryotic species, but not for the cyanobacterium Nodularia spumigena. We demonstrate a high variability in thiamine content among phytoplankton species, with the highest content in N. spumigena. Salinity was the factor with the strongest effect, followed by temperature and photon flux density, although the responses varied between the investigated phytoplankton species. Our results suggest that regime shifts in phytoplankton community composition through large-scale environmental changes has the potential to alter the thiamine availability for higher trophic levels. A decreased access to this essential vitamin may have serious consequences for aquatic food webs.

TAPHONOMIC CONTROLS ON THE DISTRIBUTION OF PALYNOMORPHS IN TIDALLY INFLUENCED COASTAL DELTAIC SETTINGS

Palynomorph concentrations for water-column and sediment-water interface samples are presented for two coastal deltaic systems influenced by the range of tidal regimes to assess whether any taphonomic bias exists in pollen assemblages. The localities are the microtidally influenced Mobile-Tensaw River Delta, Alabama, United States, and the meso- to macrotidally influenced Rajang River Delta, Sarawak, East Malaysia. Mobile-Tensaw Delta data represent collections over a 13-month period; Rajang River Delta data represent sampling during the 1992 dry season. Results are reported for: (1) pollen concentration in suspended sediment loads and bottom sediments of distributary channels, and (2) selected spore-and-pollen frequencies in channels influenced by meso- and macrotidal processes. An increase in pollen concentration/liter occurs in all delta distributaries at water-column depth, and a positive relationship exists between pollen concentration and suspension load. Concentrations at the sediment-water interface are highest in tidally influenced areas where Heterogenous Aggregate Organic Matter (OM) clasts (floccules) dominate palynofacies assemblages, and statistically significant for Mobile-Tensaw lower delta plain sites. Hence, microtidal influence at the interface between fresh and brackish waters affects palynomorph concentration in bottom sediment and may be a function of flocculation induced by salinity flux. In contrast, concentrations are highest in bottom sediments of Rajang River distributaries where little active sediment transport occurs and tidal range is highest, and is due to several factors including a high proportion of Heterogenous Aggregate OM clasts. Additionally, the magnitude of tidal displacement impacts palynomorph distribution in the Rajang Delta, with mangrove pollen found in alluvial plain samples, a distance of

The parameterisation of Mediterranean–Atlantic water exchange in the Hadley Centre model HadCM3, and its effect on modelled North Atlantic climate

Multiple palaeo-proxy and modelling studies suggest that Mediterranean Outflow Water (MOW) is an important driver of Atlantic Meridional Overturning Circulation (AMOC), particularly during periods of weak overturning. Here, we employ the HadCM3 ocean–atmosphere General Circulation Model (GCM) to investigate the effect of using different parameterisations of Mediterranean–Atlantic water exchange on global ocean circulation and climate. In HadCM3, simulating flow through the Gibraltar Straits with an ‘open seaway’ rather than a ‘diffusive pipe’ causes a shoaling and strengthening of the MOW plume. This reorganises shallow Atlantic circulation, producing regional surface air temperature anomalies of up to+11°C and −7.5°C. We conclude that when investigating the influence of MOW on modelled ocean circulation and climate, an accurate parameterisation of Mediterranean–Atlantic exchange is important and should match observed fresh water and salinity flux constraints. This probably cannot be achieved through a simple ‘diffusive pipe’ with depth invariant mixing coefficient.