ALBERT

Description: The long-term data sets of total alkalinity (TA) (1929–2002 A.D.) and δ18O (1966–2002 A.D.) are used to investigate freshwater and brine distributions in the Arctic Ocean. Fractions of sea ice meltwater and other freshwaters (OF) (precipitation, river runoff, and freshwater carried by Pacific water implied as salinity deficit) are calculated on the basis of salinity-TA and salinity-δ18O relationships. Rejected brine during sea ice growth resides in surface water in the central Arctic Ocean, while net melting is found along the surface flow of water from the Pacific and Atlantic oceans. Distribution of OF at 10 m water depth suggests that Russian runoff leaves the shelf mainly west of the Mendeleyev Ridge, enters into the deep basin, and exits from the ocean through the western part of Fram Strait. The influence of Mackenzie River water is limited in the region and in depth. Accumulation of freshwater in the Canadian Basin is caused by deep penetration of OF with brine, indicating the transport of freshwater by shelf-derived water. The major origin of shelf-derived water entering into the upper halocline layer in the Canadian Basin should be the Chukchi and East Siberian Sea shelves, and the main freshwater sources are the salinity deficit of Pacific water and/or Russian runoff. An increase in OF inventory accompanied by an increase in brine content may suggest an increase of the shelf-derived water supply into the western Canadian Basin in anticyclonic years.

Description: Zero-age basalts dredged from the Kolbeinsey Ridge directly north of Iceland are mafic quartz tholeiites (MgO 6-10 wt. %), strongly depleted in incompatible elements. Fractionation-corrected Na2O contents ('Na(sub 8)') are amongst the lowest found on the global ridge system, implying that the degree of partial melting at Kolbeinsey is amongst the highest for all mid-ocean ridge basalt (MORB). In contrast, the basalts show large ranges of incompatible-element ratios (e.g., K2O/TiO2 of 0.01 to 0.12 and Nd/Sm of 2.1 to 2.9) not related to variations in radiogenic isotope ratios; this suggests recent enrichment/depletion events associated with small-degree partial melting as their cause, rather than long-lived source heterogeneity. Tholeiitic MORB from many regions globally show similar or more extreme variations in K2O/TiO2. Dynamic melting of an adiabatically upwelling source can reconcile these conflicting indications of the degree of melting. Through dynamic melting, the incompatible elements are partially separated into different melt fractions based on their bulk partition coefficients, more incompatible elements being concentrated in deeper, smaller-degree partial melts. The final erupted magma is a mix of melts from all depths in the melting column. The concentration of highly incompatible elements in the mix will be very sensitive to the physical processes allowing the deep melts to separate and migrate to the site of mixing, and small fluctuations in the efficiency of the separation process can account for the large range of trace element ratios seen at Kolbeinsey. The major element chemistry of the erupted mix (and Na(sub 8) is much more robust, depending mainly on the integrated total amount of melting. The large variations of incompatible element ratios seen at Kolbeinsey, and in MORB in general, therefore give no information about the total degree of melting occuring beneath the ridge, nor do they require a heterogeneous source.

Description: We numerically study the dynamics of coherent anticyclonic eddies in the ocean interior. For the hydrostatic, rotating, stably stratified turbulence we use a high-resolution primitive equation model forced by small-scale winds in an idealized configuration. Many properties of the horizontal motions are found to be similar to those of two-dimensional and quasi-geostrophic turbulence. Major differences are a strong cyclone-anticyclone asymmetry linked to the straining field exerted by vortex Rossby waves, which is also found in shallow water flows, and the complex structure of the vertical velocity field, which we analyze in detail. Locally, the motion can become strongly ageostrophic, and vertical velocities associated with vortices can reach magnitudes and levels of spatial complexity akin to those reported for frontal regions. Transport and mixing properties of the flow field are further investigated by analyzing Lagrangian trajectories. Particles released in the pycnocline undergo large vertical excursions because of the vertical velocities associated to the vortices, with potentially important consequences for marine ecosystem dynamics.

Description: The Denmark Strait overflow provides about half of the total dense water overflow from the Nordic Seas into the North Atlantic Ocean. The velocity of the overflow has been monitored in the Strait with two moored Acoustic Doppler Current Profilers since 1996 with several interruptions due to mooring losses or instrument failure. So far, overflow transports were only calculated when data from both moorings were available. In this work, we introduce a linear model to fill gaps in the time series when data from only one instrument is available. The mean overflow transport is 3.4 Sv and exhibits a variance of 2.0 Sv2. No significant trend was detected in the time series. The highest variability in the transport is associated with the passage of mesoscale eddies with time scales of 2–10 days (associated with a variance of 1.5 Sv2). Seasonal variability is weak and explains less than 5% of the variance in all time series, which is in contrast to the strong seasonal cycle found in high resolution model simulations. Interannual variability is on the order of 10% of the mean. A relation to atmospheric forcing such as the local wind stress curl, as well as to larger scale phenomena, e.g. the North Atlantic Oscillation, is not detected. Since 2005 data from moored temperature, conductivity and pressure recorders have been available as well, monitoring the hydrographic variability at the bottom of Denmark Strait. In recent years the temperature time series of the Denmark Strait overflow revealed a cooling, while the salinity stayed nearly constant.

Description: This study presents aspects of the spatial and temporal variability of abyssal water masses in the Ionian Sea, as derived from recent temperature, salinity, dissolved oxygen and velocity observations and from comparisons between these and former observations. Previous studies showed how in the Southern Adriatic Sea the Adriatic Deep Water (AdDW) became fresher (ΔS ≈ −0.08) and colder (ΔT ≈ −0.1°C) after experiencing warming and salinification between 2003 and 2007. Our data, collected from October 2009 to July 2010 from two bottom moorings, one within the Strait of Otranto and the other in the northern Ionian, confirm this tendency: a bottom vein of southward-flowing AdDW, whose temperature and salinity continuously decreased during the observation time, was detected there. Typically, the vein travel time between the two stations ranged between 45 and 50 days. This gave us a temporal estimate for AdDW anomaly propagation towards the Ionian abyss from their Adriatic generation region. The density excess of the observed vein was always enough to enable its existence as a bottom-arrested current. This evidence confirms that, at that time (2009 and 2010), the Adriatic Sea was greatly contributing to the formation of Eastern Mediterranean Deep Water (EMDW), the bottom water of the Eastern Mediterranean. Hence, based on these results and on the evidence that, from 2003 to 2009, abyssal Ionian waters became saltier and warmer under the time-lagged influence of AdDW, possible future changes in the EMDW characteristics, as a response to Adriatic variability, are discussed.

Description: A chlorophyll a hindcast in the Madeira Basin from 1871 to 2008 was used to analyze the long-term variability in the oligotrophic, subtropical gyres in relation to the climate change of the last century. The deep chlorophyll maximum (DCM), as dominant pattern of the chlorophyll a field, showed a fast decrease in its strength in the 1940s. An absolute minimum was reached between 1967 and 1973 when no DCM established with a recovering to the end of the time series. Long-term variability of the DCM was related to the North Atlantic Oscillation with a time delay of 9 years. The marked decrease in the 1940s was correlated to the drop of the solar radiation in transition from early brightening to global dimming. Caused by the influence of the solar radiation and maybe related to increasing global temperatures in the last century, the integrated chlorophyll a concentration decreased by about 0.7 mg m−2 in 2008 compared to 1871. The high-resolved chlorophyll a hindcast allowed an estimation of the carbon uptake by the ocean due to primary production in the euphotic zone. A rough calculation over the area of the global subtropical oceans showed 700 megaton less carbon uptake in 2008.

Description: Helium isotope measurements in six major basins in the Gulf of California show that the deep Guaymas Basin has 3He/4He 65–70% higher than atmospheric helium, clear evidence of mantle helium injection. Smaller 3He excesses observed in the Carmen and Farallon basins may be derived from this Guaymas Basin anomaly. The 3He concentrations in the Mazatlan Basin in the mouth of the Gulf of California are similar to average eastern Pacific values, indicating that the Gulf does not provide a significant flux of 3He into the general Pacific circulation. On the basis of temperature and salinity measurements an upper limit of 0.28°C can be placed on the amount of geothermal heating observed in any of the basins. The isotopic ratio of the injected Guaymas Basin helium is found to be 3He/4He = (1.10±0.06) × 10−5, almost identical to the helium signature observed at the Galapagos Rift but somewhat lower than the average ratio in oceanic basalt glasses.

Description: The best place to seek evidence of the style of past magma flow through a conduit is in the country rock. Heat flow has been studied in country rock adjacent to two Tertiary dolerite sills intruding the Caledonian schists and quartzites, on the Isle of Mull, Scotland. Radiogenic 40Ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the Ultra-Violet Laser Ablation Micro-Probe, to discriminate between a history of prolonged magma flow, a history of conductive cooling following laminar flow, and instantaneous emplacement of the intrusions. The 40Ar/39Ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3–5 months resulted in extensive argon loss from the micas, country rock melting, and mineral breakdown adjacent to a 6-m sill. These features were absent from the wall rocks of a smaller 2.7-m-thick sill, which exhibited even less argon loss than might have been predicted for an instantaneous intrusion. If the heat loss from the 6-m sill observed in one locality had been repeated along its length, it would have formed an important magma conduit to the Mull volcano, but dolerite is not a common flow composition on Mull. If on the other hand, the heat loss from the sill varies along strike, it constitutes strong evidence for channelling and heterogeneous flow within the sill.

Description: The Water and Global Change (WATCH) forcing datasets have been created to support the use of hydrological and land surface models for the assessment of the water cycle within climate change studies. They are based on 40-yr ECMWF Re-Analysis (ERA-40) or ECMWF interim reanalysis (ERA-Interim) with temperatures (among other variables) adjusted such that their monthly means match the monthly temperature dataset from the Climatic Research Unit. To this end, daily minimum, maximum, and mean temperatures within one calendar month have been subjected to a correction involving monthly means of the respective month. As these corrections can be largely different for adjacent months, this procedure potentially leads to implausible differences in daily temperatures across the boundaries of calendar months. We analyze day-to-day temperature fluctuations within and across months and find that across-months differences are significantly larger, mostly in the tropics and frigid zones. Average across-months differences in daily mean temperature are typically between 10% and 40% larger than their corresponding within-months average temperature differences. However, regions with differences up to 200% can be found in tropical Africa. Particularly in regions where snowmelt is a relevant player for hydrology, a few degrees Celsius difference can be decisive for triggering this process. Daily maximum and minimum temperatures are affected in the same regions, but in a less severe way.

Description: A high-resolution near-bottom survey has been conducted of the Clipperton transform fault and adjoining segments of the East Pacific Rise (EPR), using the Sea MARC I side-looking sonar system and the Lamont-Doherty Geological Observatory Olympus-based camera system. The transform fault zone (TFZ) is a narrow, well-defined belt of transform-parallel lineaments, which varies along strike from a single, sharp-edged notch to a complex band of subparallel lineaments up to 1 km wide. The TFZ is set within a 5-km-wide band of unusually fine-grained side scan texture, which could indicate nonbasaltic seafloor and/or pervasively sheared and mass-wasted basaltic crust The fine-grained swath is surrounded by constructional volcanic terrain with no hint of strike-slip motion; this observation puts an upper limit of 5 km on the extent of lateral migration of the TFZ in the last 1.5 m.y. Both ridge transform intersections (RTIs) are dominated by bathymetric highs located on the old plate opposite the spreading center. A mantling of fresh-looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism; thermal expansion may also play a part. The EPR south of Clipperton has recently experienced extrusion of high effusion rate basalts, burial of faults and fissures by lava flows, and development of vigorous hydrothermal circulation. On the EPR north of Clipperton, the axial zone of faults and fissures tapers toward the transform fault; this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault.