RESEARCH HIGHLIGHTS 2008

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1 RESEARCH HIGHLIGHTS 2008 In This Issue: Institut für Umweltphysik (IUP) - Department of Oceanography 2008 The Two-Ships Expedition SF6 - A promising tracer to infer water mass formation rates About us The Department of Oceanography ( AG Ozeanographie ) is a member of the Institute of Environmental Physics (IUP) at the University of Bremen which is located at the Research Division 1 "Physics/Electrical Engineering". Research Topics Deep Water Formation and the Subpolar Gyre What is the rate of deep water formation and how are changes connected to the strength of the subnpolar gyre? Funding Our research is part of national and international programmes and is mainly funded by the German Federal Ministry of Education and Research (BMBF), the German Science Foundation (DFG), and the European Union (EU). Atlantic inventory of anthropogenic carbon Modulation of inflow into the Caribbean Sea by North Brazil rings Upwelling pathways in the tropical Atlantic The global meridional overturning circulation (MOC) plays an important role in distributing the heat received from the sun. It is thus important for climate and climate change. Whether and how the global warming will affect the circulation and how this will feed back on the climate is one of our key issues of marine research. Our activities comprise the sampling and analysis of oceanic observational data. In the framework of our scientific projects we address the inner-oceanic circulation which is important for the oceanic heat budget and thus for the evolution of the global climate. Climate relevant warm water transport How do North Brazil Current rings influence the warm water flow? What is the role of eddies for the upwelling pathways? Storage of man-made CO 2 Where and how much anthropogenic carbon is stored in the Atlantic Ocean? Key regions of the Atlantic Ocean investigated by the Dep. of Oceanography. Excellence Initiative of the German Federal and State Governments In October 2006 the University of Bremen received funding to establish a graduate school which aims at supporting young scientists in the marine sciences: the Bremen International Graduate School for Marine Sciences GLOMAR (''Global Change in the Marine Realm''). The AG Ozeanographie is one of the partners of GLOMAR. In the framework of the Excellence Initiative GLO- MAR will be funded with one million Euro per year for the next five years. Within a second round of the Excellence Initiative, the MA- RUM-Research Center "Ocean Margins" was enhanced in October 2007 to become a 'Cluster of Excellence'. For the coming five years funding of this cluster will be enlarged by 7.5 millions Euro. The Department of Oceanography is embedded in this concept "The Ocean in the Earth System" via a particular sub-project. This aims at extending existing expertise with respect to variations in the deep water formation in the North Atlantic by a comparison of observational data and model analyses from the Nordic Seas and from the subpolar North Atlantic. Institut für Umweltphysik (IUP) Dep. of Oceanography

2 IUP - Dep. of Oceanography Page 2 The Two-Ships Expedition In July and August the Department of Oceanography carried out the so-called Two-Ships - Expedition to study the importance of the North Atlantic Ocean within the Earth's climate system. On July 23 rd 2008 research cruise MSM-09/1 began, and R/V MARIA S. MERIAN left Bremen to sail across the subpolar North Atlantic. The cruise ended on August 18 th in St. John's, Newfoundland, but scientific work was resumed again with the French R/V THALASSA. Cruise SUBPOLAR-08 led from St. John's to Brest, France, where it ended on September 14 th In total, Bremen scientists and technicians covered a distance of almost km, visited the Mid-Atlantic Ridge in the central North Atlantic, the Newfoundland Basin as well as the Labrador and Irminger Seas. Overall, 155 top-to-bottom stations have been made to measure the oceanic parameter distributions and currents (Figure 1). to return to the south. For ten years a decline of deep water formation in the Labrador Sea has been observed. Investigation of the causes and potential interrelation to changes in the strength of the Gulf Stream-North Atlantic Current system was one of the research topics of the Two-Ships expedition. Figure 2. Schematic showing the set of instruments involved to infer time series of absolute transports of the North Atlantic Current while entering the eastern basin of the subpolar North Atlantic. Figure 1. Station net of the Two-Ships expedition, conducted during July-September 2008 with R/V MARIA S. MERIAN (cruise MSM-09/1, green dots) and R/V THALASSA (cruise SUBPOLAR-08, red dots). Since more than a decade our team investigates the variability of deep water formation in the Labrador Sea. This process is of great importance for the meridional overturning circulation (MOC). In the course of this circulation the Gulf Stream and North Atlantic Current transport warm and saline water from the tropics into the higher latitudes of the northern hemisphere. There, heat is released from the ocean into the atmosphere. Under certain conditions the transformed water starts localized to sink to greater depths and returns as part of the deep and cold branch of the MOC During cruise MSM-09/1, it was possible for the first time to survey the changes in the strength of the North Atlantic Current when it flows across the Mid-Atlantic Ridge and enters the eastern North Atlantic basin. This was done by recovering data recorded for two years by bottom-mounted inverted echo sounders equipped with pressure sensors (PIES). Since summer 2006 four of these instruments form a line along the western flank of the Mid-Atlantic Ridge. They allow the reconstruction of oceanic transports across the Mid-Atlantic Ridge from measurements of the travel time of an acoustic signal. The acoustic signal is sent regularly from the PIES located at the sea bottom towards the sea surface where it is reflected and returned again to the PIES. The bottom sensors log the arrival time of these signals and store them as time series on internal hard disk. To receive the logged data, it is either necessary to entirely recover the bottom-mounted PIES or to read out the hard disk information via advanced telemetry techniques. Since the travel time is dependent on temperature and salinity it is possible to reconstruct time series of transports. At present, the data evaluation for the period is in progress, but will reveal soon the amplitude of transport changes, while the North Atlantic Current enters the eastern basin.

3 2008 Page 3 SF6 - A promising tracer to infer water mass formation rates A multitude of natural and anthropogenic gases enter the oceanic environment via airsea gas exchange. Winterly deep convection in key regions of the North Atlantic (e.g. the Labrador Sea) acts as a gateway for these gases to enter the deep ocean. At depth, the entering gases are distributed basin-wide via the deep oceanic current system. Their measured concentrations can provide estimates concerning the origin and age of the water masses which carry these tracers. Also the oceanic storage capability with respect to the greenhouse gas carbon dioxide (CO2) can be investigated from such tracers. Among these gases that enter the deep ocean, changes in the oceanic inventories of chlorofluorocarbons (CFC) are commonly investigated to estimate the formation rate of deep water components, especially Labrador Sea Water (LSW). However, atmospheric CFC concentrations peaked during the past decade and since then began to decline. In addition, dissolved CFC concentrations in LSW have almost reached saturation equilibrium with the atmosphere during this period. For this reason, it will probably become more and more difficult in future to detect changes in LSW production when using the CFC inventory technique. A CFC-increase in the deep ocean is probably no longer detectable without ambiguity. In contrast to the declining CFC trends, man-made sulphur hexafluoride (SF6) shows almost linearly increasing concentrations in the atmosphere for the past several decades (Figure 3). This behavior makes SF6 a promising parameter for future estimations of deep water formation rates. SF6 will have a greater temporal sensitivity to changes in formation rate making it highly suitable for this task. Figure 3. Atmospheric history of the anthropogenic trace gases CFC-11, CFC-12, and SF6. The German Sciences Foundation (DFG) funded the construction of a sea-going analysis system for simultaneous measurements of CFC-12 and SF6 in seawater. The successful transition from a CFC-only to a 'dual-tracer'- analysis system and the joint measurement of both tracers from one water sample allows the continuation of time series of water mass formation rates. The analysis system consists of an extraction unit to degas the water samples, a gas chromatograph with capillary columns to separate SF6 and CFC-12, and an electron capture detector necessary for the quantitative analysis (Figure 4). The measurements are calibrated by means of a standard gas provided by the United States Department of Commerce, NOAA, Earth System Research Laboratory. Figure 4. Installation of the dual tracer analysis system during cruise MSM- 09/1, R/V MARIA S. MERIAN. In 2007, the sea-going analysis system came into operation for the first time during R/V MARIA S. MERIAN cruise MSM-05/1. Due to technical problems of the vessel, cruise time had to be shortened considerably by 21 days, and the system could not be tested appropriately. In 2008, the first basin-wide SF6 measurement campaign by the Department of Oceanography was carried out during two cruises to the subpolar North Atlantic. In the framework of an extensive two-ships expedition, simultaneous CFC and SF6 measurements were conducted during R/V MARIA S. MERIAN cruise MSM-09/1 in the Newfoundland Basin and during R/V THALASSA cruise SUBPOLAR-08 in the Labrador Sea and Irminger Sea. 680 samples during MSM-09/1 and 800 samples during SUBPOLAR-08 were analyzed at very high precision. The overall sample throughput was unexpectedly high, making it possible to analyze up to 19 water samples per station. Tracer offline-samples from 2007 together with direct measurements in 2008 will now allow the estimation of the water mass formation rate.

4 IUP - Dep. of Oceanography Page 4 Atlantic inventory of anthropogenic carbon The German research vessel MARIA S. MERIAN, the major platform for shipboard observations in the North Atlantic. The greenhouse gas CO2 is an important driving agent for global warming. In order to recommend appropriate reduction rates for CO2 emissions, it is necessary to know the amount and relative importance of the oceanic sources and sinks under actual and future environmental conditions. The oceanic carbon sinks can either be measured directly, or the oceanic inventory of anthropogenic carbon can be determined. A large oceanic carbon inventory implies a large uptake of atmospheric CO2 by the oceans and thus a mitigation of the greenhouse effect. method. This estimation is based on CFC data, the parameter for the TTDs are inferred from the observed CFC values, and then CANT is derived from the TTDs. In the North Atlantic, all CFC data have been collected after the year South of the equator, also data from the 1990s (the WOCE era) are included due to a lack of recent measurements. In this case, the inventory estimate is based on the assumption of a steady state oceanic circulation. The total inventory of CANT south of 65 N amounts to 57± 16 gigatons of carbon (Gt C). This is roughly 40% of the CANT content of the global ocean, whereby the surface area of the Atlantic makes up less than 30%. Figure 6. Specific (mean) column inventory of anthropogenic carbon in 2003 for the western (red) and the eastern Atlantic (green), calculated in latitudinal belts of 10. Figure 5. Column inventory of anthropogenic carbon in the Atlantic Ocean. Concentrations of anthropogenic carbon (CANT) in the ocean cannot be measured directly, as they represent an (unknown) portion of the concentration of total carbon. Several methods exists, which allow to infer anthropogenic carbon from other quantities (total carbon, oxygen, nutrients, alkalinity, transient tracers). Here, the CANT inventory of the Atlantic is calculated for the year 2003 by applying the Transit Time Distribution (TTD) The column inventories are high in the northern subpolar and the northern west subtropical Atlantic (Figure 5). This finding demonstrates the uptake and storage of anthropogenic carbon by North Atlantic Deep Water. This newly formed deep water carries a large burden of anthropogenic carbon and is spreading southward along the American coast, which leads to higher inventories of anthropogenic carbon in the western Atlantic compared to the east and also to the Indian and Pacific Ocean. The important role of North Atlantic Deep Water for the storage of anthropogenic carbon is also highlighted when the mean column inventories (specific inventory) for the eastern and western basin are compared (Figure 6). References Steinfeldt, R., M. Rhein, J. L. Bullister, and T. Tanhua, Inventory changes in anthropogenic carbon from in the Atlantic Ocean between 20 S and 65 N, Global Biogeochem. Cycl., in revision, 2009.

5 2008 Page 5 Modulation of the Inflow into the Caribbean Sea by North Brazil Current Rings The circulation of the ocean is often divided into a wind-driven and a thermohaline component. Wind is forcing the basin-wide ocean gyres and the Antarctic Circumpolar Current, while the meridional overturning circulation (MOC) is driven by heat- and freshwater (thermohaline) fluxes at the sea surface. The MOC carries the oceanic heat transport, at maximum in the subtropical latitudes of the North Atlantic, by transporting warm water northward at the surface, and cold water southward at depth. A major component of the upper ocean flow within the MOC is the inflow into the Caribbean Sea. The Caribbean is separated from the Atlantic by the Antilles island arc, where narrow passages between South America and Cuba allow an exchange of water with the open ocean. The transport through the channels of the south eastern Lesser Antilles is sustained by both water from the North and South Atlantic. This water continues through the Yucatan Channel into the Gulf of Mexico and ultimately returns into the Atlantic within the Florida Current, the only continuous outflow from the Caribbean. The transport of water from the South Atlantic (SAW) towards the Antilles is mainly carried by large eddies, North Brazil Current rings, that propagate northward along the continental margin. The rings are formed close to 5 N at the retroflection of the North Brazil Current (NBC) and enclose large amounts of SAW. A two year long record from a triangular array of moorings between the Lesser Antilles islands Tobago, Barbados, and St. Lucia was used to investigate the inflow into the Caribbean Sea, the amount of SAW carried with the inflow, and the role of NBC rings in the observed variability. The observations consist of time series of temperature, salinity, and current velocity, as well as measurements by bottom mounted inverted echo sounders. The survey was complemented by shipboard, satellite, and autonomous profiler recordings. The acoustic travel time measurements of the inverted echo sounders and the conductivity/ temperature time series were used for continuous estimation of dynamic height and geostrophic currents as well as the amount of SAW found at the mooring positions. The observations show a domination of intraseasonal variability between 0 and 15 Sv, superimposed on long term fluctuations. With time scales of one to three months, these represent the signature of the NBC rings. The transport time series shows nine periods of increased variability, indicative of Figure 7. Tracks of the surface signature (sea surface heigth, SSH, from satellite altimetry) of NBC rings coinciding with ring-associated variability at the moorings recorded during the observational period. a) Event i; b) Event ii; c) Event iii; d) Event iv; e) Event vi; f) Event vii; g) Event viii; h) Event ix. No SSH signal was observed during event v. (Figure from Mertens et al., 2009). the rings interacting with the Lesser Antilles island arc. With the exception of one, these periods were associated with corresponding sea surface height anomalies from satellite altimetry (Figure 7). No marked seasonality was observed in the transport variability or the ring frequency. The arrival of individual rings leads to a weakening of the inflow into the Caribbean. Nevertheless, the rings carry large amounts of SAW into the area, and the immediate increase of the transport towards the end of ring events suggests a subsequent flow of SAW rich water into the Caribbean. The average transport of SAW into the Caribbean south of St. Lucia during the observations (Figure 8) amounted to 5.5 Sv, with no significant seasonal cycle, but a small positive trend in SAW fraction as well as in SAW transport of about 15% and 1 Sv, respectively. The trends are most prominent in the range of the Intermediate Water ( m depth), a corresponding trend in volume transport was not observed. Although not significant in such a short time series, the trends could be a sign Deployment of a bottommounted inverted echosounder which is equipped with a pressure sensor (PIES).

6 IUP - Dep. of Oceanography Figure 8. Time series of the volume transport of South Atlantic Water (SAW) into the Caribbean south of St. Lucia. Upper 100 m (green), below 100 m (red) and sum of both (black). Periods with NBC rings present in the moored record are shaded in light gray. (Figure from Mertens et al., 2009) of a long term MOC fluctuation. Page 6 Much more prominent are the large fluctuations caused by the arrival of North Brazil Current rings. The analysis of satellite altimetry showed 13 surface rings during the mooring deployment; of these, 10 were associated with periods of increased transport variability in the time series. Only one of the events identified in the moored record had no corresponding surface expression, indicative of a subsurface intensified ring. Consistent with earlier studies, the surface signals could be classified into rings that disintegrate in the island triangle, rings that were reflected to the northwest, and rings that entered the Caribbean (Figure 7). The latter two types are associated with modulations of the SAW transport into the Caribbean, a decrease of transport for the reflected rings and an increase for those entering. Several periods of increased SAW were observed at Barbados without covarying inflow modulations. These are signs of NBC rings passing the island arc offshore and transporting SAW directly northward without entering the Caribbean, and corresponds to previous observations at 16 N. Earlier studies have shown that NBC rings are generated continually at the retroflection of the NBC with a frequency of 7 to 8 rings per year. Most of these rings pass close to Barbados, and only less then one per year enters into the Caribbean. In both years of our observations, we found 4 to 5 rings per year interacting with the southern part of the Lesser Antilles island arc in the moored time series and 6 to 7 in the altimetry. Both records showed no marked seasonality, except that the two strongest events in the transport time series occurred both during spring. References Kirchner, K., M. Rhein, S. Hüttl Kabus, and C. W. Böning, On the spreading of South Atlantic Water into the northern hemisphere. J. Geophys. Res., in press, Kirchner, K., M. Rhein, C. Mertens, C. W. Böning, and S. Hüttl, Observed and modeled meridional overturning circulation related flow into the Caribbean. J. Geophys. Res., 113:C03028, Mertens, C., M. Rhein, M. Walter and K. Kirchner, Modulation of the Inflow into the Caribbean Sea by North Brazil Current Rings. Deep-Sea Res. I, accepted, Rhein, M., K. Kirchner, C. Mertens, R. Steinfeldt, M. Walter, and U. Fleischmann- Wischnath, Transport of South Atlantic water through the passages south of Guadeloupe and across 16 N, Deep Sea Res. I, 52: , Stramma, L., M. Rhein, P. Brandt, M. Dengler, C. Böning, and M. Walter, Upper ocean circulation in the western tropical Atlantic in boreal fall Deep Sea Res. I, 52: , Upwelling pathways in the tropical Atlantic Figure 9. Schematic of the subsurface circulation (50-300m) in the tropical Atlantic with currents contributing to the upwelling. Undercurrents are marked in red, all other currents in gray. The upwelling regions of the eastern tropical oceans are areas of strong interaction between different components of the climate system. The cold upwelling waters not only influence the heat contents of the atmosphere and ocean but are also essential for the marine biological activity by providing recently ventilated waters. Upwelling regions in the Atlantic are found along the equator, off northwest Africa in the Guinea Dome and off the coast of Mauritania as well as off southwest Africa in the Angola Gyre/Dome. The upwelling regions are supplied by waters from the subtropics as revealed by examinations of salinities and tracers in the tropical Atlantic. The currents contributing to the upwelling are sketched in Fig. 9. The detailed watermass pathways toward the upwelling regions are complicated by the equatorial system of zonal currents flowing alternately east- and westward between 10 N and approximately 6 S. The Equatorial Undercurrent (EUC), flowing eastward along the

7 2008 Page 7 equator feeds the equatorial upwelling. As a source of the off-equatorial upwelling regions subsurface eastward flows (50-300m depth) centered around 4 N/S, the North/South Equatorial Undercurrent (NEUC/SEUC), are under discussion. An assumption of a link between the Guinea Dome (8 N-14 N, 28 W -20 W) and the NEUC as well as between the Angola Dome (5 S-15 S, 5 W-5 E) and the SEUC was first suggested by Voituriez (1981). More recent observations and modelling studies support the idea of such a connection. However, observational evidence of the fate of the undercurrents and their links to the doming regions is still very limited for both the tropical Pacific and the Atlantic oceans. Analyses of ocean models which are capable to resolve a nearly realistic tropical current system can contribute to the understanding of water mass pathways into the upwelling regions. The FLAME model developed by IfM-GEOMAR in Kiel shows a very realistic tropical current system and is used to investigate if and how the NEUC and SEUC feed the upwelling regions of the Guinea and Angola Dome. In order to trace the water masses of both currents, trajectories of drifting buoys (floats) have been calculated using the model data. Figure 11. Upwelling location of NEUC floats. The black dots denote the position where trajectories reach a depth of shallower than 30m the first time (Figure from Hüttl-Kabus and Böning, 2008). The circulation pathways (see Fig. 10) obtained from the calculations were quite surprising. Instead of a clear, prevailing watermass path into the upwelling regions, a large part of the trajectories showed very strong recirculation loops in the Atlantic. The trajectories of the floats (NEUC trajectories are in red colors, SEUC in blue) revealed to a large part westward pathways into the northern and southern subtropics instead of eastward pathways into the upwelling regions. Only 12% of the waters from the NEUC reach the upwelling region along the Guinea Dome/African coast and nearly 20% contributed to the equatorial upwelling. The locations of the NEUC-related upwelling are given in Fig. 11. The contribution of the SEUC to the Angola Dome upwelling was found to be even less than 10% of the SEUC transport. As an explanation for this unexpected circulation paths tropical instabilities vortices, i.e. westward propagating eddies, have been identified. These eddies spin off from tropical instability waves and mix the waters between the various zonal current bands. For the connection of the NEUC/SEUC and the upwelling in the Guinea/Angola-Dome regions this means a strong detrainment of water from the undercurrent cores and only a weak supply of waters flowing eastward to feed the upwelling in the doming areas. The connection between the NEUC/SEUC and the doming regions thus exists, however, multiple recirculations cause long residence times in the tropics and thus offer an explanation for the relative oxygen minima going along the dome circulations. Figure 10. Pathways of modelled floats following the NEUC (red trajecories) and SEUC (blue trajectories). The black lines indicate the start position of the floats. References: Hüttl-Kabus, S. and C. W. Böning, Sources and fate of the off-equatorial undercurrents, J. Geophys. Res., C10018, doi: / 2007JC004700, Stramma, L., S. Hüttl, and J. Schafstall, Water masses and currents in the upper tropical northeast Atlantic off northwest Africa, J. Geophys. Res., Vol. 110, No. C12, C12006, doi: /2005JC002939, Voituriez, B., Northern and southern equatorial undercurrents and the formation of tropical thermal domes, Oceanol. Acta, 4(4), , 1981.

8 IUP - Dep. of Oceanography Page 8 Publications 2008 Cisewski, B., V. H. Strass, M. Losch, and H. Prandke (2008), Mixed layer analysis of a mesoscale eddy in the Antarctic Polar Front Zone. J. Geophys. Res., 113, C05017, doi: /2007jc Fine, R. A., W. M. Smethie Jr., J. L. Bullister, D. H. Min, M. J. Warner, M. Rhein, A. Poisson, and R. F. Weiss (2008), Decadal ventilation and mixing of Indian Ocean water. Deep-Sea Res. I, 55(1), Haine, T., C. W. Böning, P. Brandt, J. Fischer, A. Funk, D. Kieke, E. Kvaleberg, and M. Rhein (2008), North Atlantic Deep Water Transformation in the Labrador Sea, Recirculation through the Subpolar Gyre, and Discharge to the Subtropics. In: Arctic- Subarctic Ocean Fluxes - Defining the Role of the Northern Seas in Climate, R. R. Dickson, J. Meincke, P. Rhines (Eds.), Springer, X, chap. 26. Hüttl-Kabus, S. and C. W. Böning (2008), Pathways and variability of the off-equatorial undercurrents in the Atlantic Ocean. J. Geophys. Res., 113, C10018, doi: /2007jc Huhn, O., W. Roether, and R. Steinfeldt (2008), Age spectra in North Atlantic Deep Water along the South American continental slope, 10 N - 30 S, based on tracer observations, Deep-Sea Res. I, 55 (10), Huhn, O., H. H. Hellmer, M. Rhein, W. Roether, C. Rodehacke, M. Schodlok, and M. Schröder (2008), Evidence of deep and bottom water formation in the western Weddell Sea. Deep-Sea Res. II, 55(8-9), Keir, R. S., O. Schmale, M. Walter, J. Sültenfuß, R. Seifert and M. Rhein (2008), Flux and dispersion of gases from the 'Drachenschlund' hydrothermal vent at 8 18'S, 13 30'W. Earth Planet. Sci. Lett., 270(3-4), Kirchner, K., M. Rhein, C. Mertens, C. W. Böning, and S. Hüttl (2008), Observed and modeled MOC related flow into the Caribbean. J. Geophys. Res., 113, C03028, doi: /2007jc LeBel, D. A., W. M. Smethie Jr., M. Rhein, D. Kieke, R. A. Fine, J. L. Bullister, D.-H. Min, W. Roether, R. F. Weiss, C. Andrie, D. Smythe-Wright, and E. P. Jones (2008), The distribution of CFC-11 in the North Atlantic during WOCE: Inventories and calculated water mass formation rates. Deep-Sea Res. I, 55(8), Massmann, G., J. Sültenfuß, U. Dünnbier, A. Knappe, T. Traute, and A. Pekdeger (2008), Investigation of groundwater residence times during blank filtration in Berlin - a multi-tracer approach. Hydrological Processes, 22(6), Massmann, G. and J. Sültenfuß (2008), Identification of processes affecting excess air formation during natural bank filtration and managed aquifer recharge. J. Hydrology, 359, Melchert, B., C. W. Devey, C. R. German, K. S. Laschkewitz, R. Seifert, M. Walter, C. Mertens, D. R. Yoerger, E. T. Baker, H. Paulick, and K. Nakamura (2008), First evidence for high-temperature off-axis venting of deep crustal/mantle heat: The Nibelungen hydrothermal field, southern Mid-Atlantic Ridge. Earth Planet. Sci. Lett., 275(1-2), Michels, J., G. S. Dieckmann, D. N. Thomas, S. B. Schnack-Schiel, A. Krell, P. Assmy, H. Kennedy, S. Papadimitriou, and B. Cisewski (2008), Short-term biogenic particle flux under late spring sea ice in the western Weddell Sea. Deep-Sea Res. II, 55(8-9), Osenbrück, K., S. Stadler, J. Sültenfuß, A. O. Suckow, and S. M. Weise (2008), Impact of recharge variations on water quality as indicated by excess air in groundwater of the Kalahari, Botswana. Geochim. Cosmochim. Acta, doi: / j.gca Stadler, S., K. Osenbrück, K. Knöller, A. Suckow, J. Sültenfuß, H. Oster, T. Himmelsbach, and H. Hötzl (2008), Understanding the origin and fate of nitrate in groundwater of semi-arid environments. J. Arid Environments, 72(10), Stöber, U., M. Walter, C. Mertens, and M. Rhein (2008), Mixing estimates from hydrographic measurements in the Deep Western Boundary Current of the North Atlantic. Deep-Sea Res. I, 55(6), Institut für Umweltphysik (IUP) Universität Bremen Otto-Hahn-Allee D Bremen Telefon: (secretariat) Fax: ocean@uni-bremen.de