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  • Elsevier  (7)
  • Deutsches Klima-Konsortium e. V. (DKK)
  • Frontiers Media
  • 2010-2014  (2)
  • 2005-2009  (5)
  • 1
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    Deep water formation, the subpolar gyre, and the meridional overturning circulation in the subpolar North Atlantic (2011)
    Elsevier
    In:  Deep Sea Research Part II: Topical Studies in Oceanography, 58 (17-18). pp. 1819-1832.
    Details
    Publication Date: 2020-08-05
    Description: On interannual to decadal times scales, model simulations suggest a strong relationship between anomalies in the deep water formation rate, the strength of the subpolar gyre, and the meridional overturning circulation in the North Atlantic. Whether this is valid, can only be confirmed by continuous, long observational time series. Several measurement components are already in place, but crucial arrays to obtain time series of the meridional volume and heat transport in the subpolar North Atlantic are still missing. Here we summarize the recent developments of the deep water formation rates and the subpolar gyre transports. We discuss how existing observational components in the subpolar North Atlantic could be supplemented to provide long-term monitoring of the meridional heat and volume transport. Through a combined analysis of observations and model results the temporal and spatial scales that had to be covered with instruments are discussed, together with the key regions with the highest variability in the velocity and temperature fields.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.dsr2.2010.10.061
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Rapid dispersal of a hydrothermal plume by turbulent mixing (2010)
    Elsevier
    In:  Deep Sea Research Part I: Oceanographic Research Papers, 57 (8). pp. 931-945.
    Details
    Publication Date: 2016-11-01
    Description: The water column imprint of the hydrothermal plume observed at the Nibelungen field (8 18'S 13 degrees 30'W) is highly variable in space and time. The off-axis location of the site, along the southern boundary of a non-transform ridge offset at the joint between two segments of the southern Mid-Atlantic Ridge, is characterized by complex, rugged topography, and thus favorable for the generation of internal tides, subsequent internal wave breaking, and associated vertical mixing in the water column. We have used towed transects and vertical profiles of stratification, turbidity, and direct current measurements to investigate the strength of turbulent mixing in the vicinity of the vent site and the adjacent rift valley, and its temporal and spatial variability in relation to the plume dispersal. Turbulent diffusivities K(rho) were calculated from temperature inversions via Thorpe scales. Heightened mixing (compared to open ocean values) was observed in the whole rift valley within an order of K(rho) around 10(-3) m(2) s(-1). The mixing close to the vent site was even more elevated, with an average of K(rho) = 4 x 10(-2) m(2) s(-1). The mixing, as well as the flow field, exhibited a strong tidal cycle, with strong currents and mixing at the non-buoyant plume level during ebb flow. Periods of strong mixing were associated with increased internal wave activity and frequent occurrence of turbulent overturns. Additional effects of mixing on plume dispersal include bifurcation of the particle plume, likely as a result of the interplay between the modulated mixing strength and current speed, as well as high frequency internal waves in the effluent plume layer, possibly triggered by the buoyant plume via nonlinear interaction with the elevated background turbulence or penetrative convection. (C) 2010 Elsevier Ltd. All rights reserved.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.dsr.2010.04.010
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Flux and dispersion of gases from the "Drachenschlund" hydrothermal vent at 8°18' S, 13°30'W on the Mid-Atlantic Ridge (2008)
    Elsevier
    In:  Earth and Planetary Science Letters, 270 . pp. 338-348.
    Details
    Publication Date: 2017-02-09
    Description: Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH4 to 3He in the neutrally buoyant plume is quite high, 4 × 108. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol s− 1 in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH4 calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of 3He is estimated to be between 0.14 and 1.2 nmol s− 1. In either case, the 3He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23 yr− 1, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m2m2 s− 1. These high values may be due to tidally driven internal waves across the promontory on which the vent is located.
    Type: Article , PeerReviewed
    Format: text
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    DOI: 10.1016/j.epsl.2008.03.054
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Methane and methane carbon isotopes in the Northeast Atlantic including the Mid-Atlantic Ridge (50°N) (2005)
    Elsevier
    In:  Deep Sea Research Part I: Oceanographic Research Papers, 52 (6). pp. 1043-1070.
    Details
    Publication Date: 2019-09-23
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.dsr.2004.12.006
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  • 5
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    Variability and propagation of Labrador Sea Water in the southern subpolar North Atlantic (2009)
    Elsevier
    In:  Deep Sea Research Part I: Oceanographic Research Papers, 56 . pp. 1656-1674.
    Details
    Publication Date: 2016-11-01
    Description: The variability of two modes of Labrador Sea Water (LSW) (upper and deep Labrador Sea Water) and their respective spreading in the interior North Atlantic Ocean are investigated by means of repeated ship surveys carried out along the zonal WOCE line A2/AR19 located at 43–48°N (1993–2007) and along the GOOS line at about 48–51°N (1997–2002). Hydrographic section data are complemented by temperature, salinity, and velocity time series recorded by two moorings. They have been deployed at the western flank of the Mid-Atlantic Ridge (MAR) in the Newfoundland Basin during 1996–2004. The analysis of hydrographic anomalies at various longitudes points to a gradual eastward propagation of LSW-related signals, which happens on time scales of 3–6 years from the formation region towards the MAR. Interactions of the North Atlantic Current (NAC) with the Deep Western Boundary Current (DWBC) close to Flemish Cap point to the NAC being the main distributor of the different types of LSW into the interior of the Newfoundland Basin. Comparisons between the ship data and the mooring records revealed that the mooring sites are located in a region affected by highly variable flow. The mooring time series demonstrate an elevated level of variability with eddy activity and variability associated with the NAC considerably influencing the LSW signals in this region. Hydrographic data taken from Argo profiles from the vicinity of the mooring sites turned out to mimic quite well the temporal evolution captured by the moorings. There is some indication of occasional southward flow in the LSW layer near the MAR. If this can be considered as a hint to an interior LSW-route, it is at least of minor importance in comparison to the DWBC. It acts as an important supplier for the interior North Atlantic, distributing older and recently formed LSW modes southward along the MAR.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.dsr.2009.05.010
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  • 6
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    Separation of 3He and CH4 signals on the Mid-Atlantic Ridge and 5°N and 51°N (2006)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 70 . pp. 5766-5778.
    Details
    Publication Date: 2019-09-23
    Description: Abiogenic methane may be produced in submarine hydrothermal systems by degassing of basalts or serpentinization of ultramafic outcrops. The latter process presumably releases little primordial helium and is therefore implicated by high CH4/3He ratios in vent fluids from the ultramafic-hosted Rainbow field and in methane plumes near ultramafic outcrops. In two segments of the Mid-Atlantic Ridge, at 5.4°N and 51°N, we have observed depth-separated CH4 and 3He plumes. In both cases, the helium plume was deeper, near the valley floor. It may be that the plumes issue from separate vents, where the helium is discharged near the volcanic axis and the methane is generated by serpentinization on the valley wall. However, at the present time the locations of the vents that produce these plumes are not known. Using a one-pass model, we investigated whether separate venting could arise from heat conduction from a primary, helium-carrying, hydrothermal circulation to a second, shallower fracture loop intersecting ultramafic rock. The model results indicate that the flow rate through the secondary loop would have to be relatively low in order for it to stay warm enough for serpentinization to proceed. In this case, some of the exothermic heat production is lost by conduction, and the temperature increase in the circulating fluid is only a fraction of that expected from a water/rock ratio of 1:1.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.gca.2006.06.005
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  • 7
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    Seasonal variability in the Deep Western Boundary Current around the eastern tip of Brazil (2005)
    Elsevier
    In:  Deep Sea Research Part II: Topical Studies in Oceanography, 52 . pp. 1414-1428.
    Details
    Publication Date: 2020-08-05
    Description: The spreading of recently ventilated North Atlantic Deep Water (NADW) from the formation region to the equatorial Atlantic occurs mainly in the Deep Western Boundary Current (DWBC). When crossing the equator between 44°W and 35°W, the DWBC is split in two velocity cores through a chain of seamounts around the Atoll das Rocas at 3.5°S. Further eastward the DWBC contributes to the zonally oriented equatorial current system. The circulation of the NADW in the crucial region around the eastern tip of Brazil is examined using 8 CTD and Chlorofluorocarbon (CFC) sections along 35°W and along 5°S, respectively, taken mainly in spring and fall in the years 1990–2002. As expected from the short direct flow path between the two sections, the CFC concentrations in the upper NADW (1400–2200 m) were similar at 35°W and 5°S during boreal autumn. In spring, however, a significant downstream CFC decrease was observed. If one attributes the decrease solely to the older age of water further downstream, the CFC concentration age difference between 35°W and 5°S in May 2002 would be 3–5 years. We interpret the aging to be caused by an eastward detour of the flow with the deep equatorial circulation before reaching 5°S in spring. Another conspicuous anomaly was found in the middle NADW (2200–3400 m) with downstream decreasing salinities in boreal spring, but not in autumn. This variability might also be caused by differences in the deep equatorial circulation, but in contrast to the uNADW, one cannot exclude enhanced mixing with water of South Atlantic origin in spring to be the cause of that variability. No seasonal difference was observed in the hydrography or the CFC concentrations for the lower NADW. The weaker CFC decrease along the equator compared with that in the DWBC downstream of 35°W, and the topographic features along the downstream path, point to a predominantly eastward flow of the deep lNADW core. The lNADW CFC core is no longer observed at 11°S. Repeated observations of CFCs in the DWBC highlight the diverse variability affecting the components of NADW.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.dsr.2005.03.004
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