ALBERT

Beschreibung: A unique set of ferromanganese crusts and nodules collected from Shatsky Rise (SR), NW Pacific, were analyzed for mineralogical and chemical compositions, and dated using Be isotopes and cobalt chronometry. The composition of these midlatitude, deep-water deposits is markedly different from northwest-equatorial Pacific (PCZ) crusts, where most studies have been conducted. Crusts and nodules on SR formed in close proximity and some nodule deposits were cemented and overgrown by crusts, forming amalgamated deposits. The deep-water SR crusts are high in Cu, Li, and Th and low in Co, Te, and Tl concentrations compared to PCZ crusts. Thorium concentrations (ppm) are especially striking with a high of 152 (mean 56), compared to PCZ crusts (mean 11). The deep-water SR crusts show a diagenetic chemical signal, but not a diagenetic mineralogy, which together constrain the redox conditions to early oxic diagenesis. Diagenetic input to crusts is rare, but unequivocal in these deep-water crusts. Copper, Ni, and Li are strongly enriched in SR deep-water deposits, but only in layers older than about 3.4 Ma. Diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals. The highest concentrations of Li are in crust layers that formed near the calcite compensation depth. The onset of Ni, Cu, and Li enrichment in the middle Miocene and cessation at about 3.4 Ma were accompanied by changes in the deep-water environment, especially composition and flow rates of water masses, and location of the carbonate compensation depth. Key Points - Fe-Mn crusts can have a diagenetic component - Mid-latitude N. Pacific deep-water Fe-Mn crusts are uniquely enriched in Cu, Th, Li - Temporal changes in deep-ocean geochemical processes are recorded

Beschreibung: The goal of the Chemistry-Climate Model Validation (CCMVal) activity is to improve understanding of chemistry-climate models (CCMs) through process-oriented evaluation and to provide reliable projections of stratospheric ozone and its impact on climate. An appreciation of the details of model formulations is essential for understanding how models respond to the changing external forcings of greenhouse gases and ozone-depleting substances, and hence for understanding the ozone and climate forecasts produced by the models participating in this activity. Here we introduce and review the models used for the second round (CCMVal-2) of this intercomparison, regarding the implementation of chemical, transport, radiative, and dynamical processes in these models. In particular, we review the advantages and problems associated with approaches used to model processes of relevance to stratospheric dynamics and chemistry. Furthermore, we state the definitions of the reference simulations performed, and describe the forcing data used in these simulations. We identify some developments in chemistry-climate modeling that make models more physically based or more comprehensive, including the introduction of an interactive ocean, online photolysis, troposphere-stratosphere chemistry, and non-orographic gravity-wave deposition as linked to tropospheric convection. The relatively new developments indicate that stratospheric CCM modeling is becoming more consistent with our physically based understanding of the atmosphere.

Beschreibung: We present results of three-dimensional joint inversion of seismic, magnetotelluric, and gravity data over a marine salt dome. Such structures are difficult to image with a single method, and our results demonstrate how combining different techniques can yield improved results. More importantly, we examine the reliability of velocity-conductivity relationships derived from structure-coupled joint inversion approaches. Comparison with a seismic reflection section shows that our models match the upper limit of the salt. Furthermore, velocity and resistivity logs from a borehole drilled into the salt dome's flank match, within error, those recovered by the inversion. The good match suggests that the difference in length scale does not have a significant effect in this case. This provides a strong incentive to incorporate borehole data into the joint inversion in the future and substantiates approaches that use the relationships derived from joint inversion models for lithological classification.

Beschreibung: Multichannel seismic reflection data were used to determine the evolutionary history of the forearc region of the central Aleutian Ridge. Since at least late Miocene time this sector of the ridge has been obliquely underthrust 30° west of orthogonal convergence by the northwestward converging Pacific plate at a rate of 80–90 km/m.y. Our data indicate that prior to late Eocene time the forearc region was composed of rocks of the arc massif thinly mantled by slope deposits; the forearc region probably lacked both major depositional basins and a tectonically attached accretionary prism of offscraped oceanic deposits. Beginning in latest Miocene or earliest Pliocene time, a zone of outer-arc structural highs and a forearc basin began to form. Formation of these companion intraarc structures may be linked to the late Neogene growth of an accretionary wedge that formed as the result of the deposition of a thick turbidite wedge in the Aleutian Trench. Initial structures of the zone of outer-arc highs formed as the thickening wedge underran, compressively deformed, and uplifted the seaward edge of the arc massif above a landward dipping backstop thrust. Forearc basin strata ponded arcward of the elevating zone of outer-arc highs. However, most younger structures of the zone of outer-arc highs cannot be ascribed simply to the orthogonal effects of an underrunning wedge. Oblique convergence created a major right-lateral shear zone (the Hawley Ridge shear zone) that longitudinally disrupted the zone of outer-arc highs, truncating the seaward flank of the forearc basin and shearing the southern limb of Hawley Ridge, an exceptionally large antiformal outer-arc high structure. Slivers of forearc basement rocks and overlying strata have been transported along the shear zone that is flanked by differentially elevated structures attributed to localized transpressive and transtensional processes. Uplift of Hawley Ridge may be related to the thickening of the arc massif by westward directed basement duplexes. In addition, the forearc is disrupted by structures transverse to the margin that occur where unusually high-stress accumulations have resulted in the rupture of repeated great earthquakes. It is likely that many ancient active margins evolved in tectonic and depositional settings similar to those of the central Aleutian Ridge. Great structural complexity, including the close juxtaposition of coeval structures recording compression, extension, differential vertical movements, and strike-slip displacement, should be expected, even within areas of generally kindred tectonostratigraphic terranes.

Beschreibung: An examination of 311 intraplate earthquakes in the Australian plate portion of the Pacific Ocean basin reported from 1918 to 1990 reveals that only 113 events are reliably intraplate, with most of the rest relocating to active trenches and transforms. The non-random distribution of the reliably intraplate events gives insight into the tectonic stresses present. The central Tasman Sea is mostly aseismic except for a swarm of activity at the predicted site of the Tasmantid hot spot. To the north, the broad regions of the Coral Sea, South Fiji Basin and Lord Howe Rise show very little intraplate seismicity, yet the narrow Norfolk Ridge and Three Kings Rise, caught between the double convergence of the New Hebrides and Tonga subduction zones, support many more earthquakes. High levels of intraplate seismicity in the southern Tasman Sea adjacent to the Macquarie Ridge Complex (MRC) indicate that this region may be undergoing internal deformation due to the unusual nature of the Australia-Pacific plate boundary. Additional support exists in the form of intraplate focal mechanisms similar to those at the plate boundary and a set of parallel gravity rolls which are observed in recent geoid maps. Some aftershocks of the Mw = 8.2 Macquarie Ridge earthquake of 1989 occurred in a fracture zone west of the Macquarie Ridge Complex [Das, 1992], but we have found several earthquakes from as early as 1924 which relocate to this feature, suggesting that its reactivation may be more significant than previously thought. This reactivation of a fossil fracture zone may be the result of the increasing amount of oblique convergence between the Australia and Pacific plates at the Macquarie Ridge Complex, formerly a spreading center, and the stresses associated with subducting recently formed Australian ocean crust beneath the older Pacific plate.

Beschreibung: The Mauritanian coastal area is one of the most biologically productive upwelling regions in the world ocean. Shipboard observations carried out during maximum upwelling season and short-term moored observations are used to investigate diapycnal mixing processes and to quantify diapycnal fluxes of nutrients. The observations indicate strong tide-topography interactions that are favored by near-critical angles occurring on large parts of the continental slope. Moored velocity observations reveal the existence of highly nonlinear internal waves and bores and levels of internal wave spectra are strongly elevated near the buoyancy frequency. Dissipation rates of turbulent kinetic energy at the slope and shelf determined from microstructure measurements in the upper 200 m averages to ɛ = 5 × 10−8 W kg−1. Particularly elevated dissipation rates were found at the continental slope close to the shelf break, being enhanced by a factor of 100 to 1000 compared to dissipation rates farther offshore. Vertically integrated dissipation rates per unit volume are strongest at the upper continental slope reaching values of up to 30 mW m−2. A comparison of fine-scale parameterizations of turbulent dissipation rates for shelf regions and the open ocean to the measured dissipation rates indicates deficiencies in reproducing the observations. Diapycnal nitrate fluxes above the continental slope at the base of the mixed layer yielding a mean value of 12 × 10−2 μmol m−2 s−1 are amongst the largest published to date. However, they seem to only represent a minor contribution (10% to 25%) to the net community production in the upwelling region.

Beschreibung: Recently seismic reflection methods have been successfully applied to oceanographic issues. Here, we present a new approach, combining XBT and CTD surveys with seismic observations, to visualize long sections with a resolution down to a few meters. The challenge to a full investigation of mixing processes has been the tremendous span of spatial scales ranging from hundreds of kilometers to centimeters. Traditional hydrographic observations could only resolve the large scale effects by measuring temperature and salinity profiles at discrete locations typically several kilometers apart, whereas dedicated localized measurements allowed investigation of the ocean fine structure at the other end of the spatial spectrum. The intermediate scales have in contrast been difficult to observe systematically. Here we present temperature and salinity data inverted from seismic observations that cover the intermediate scales and provide a new approach to image mesoscale processes and allow the investigation of their dynamics at unprecedented resolution.

Beschreibung: Reconstructions of the spatial pattern of recent multi-decadal sea level trends in the Indian Ocean (IO) indicate a zonally-extended band in the southern tropics where sea level has substantially fallen between the 1960s and 1990s; the decline is consistent with the observed subsurface cooling associated with a shoaling thermocline in this region. Here the origin and spatio-temporal characteristics of these trends are elucidated by a sequence of ocean model simulations. Whereas interannual variability in the southwestern tropical IO appears mainly governed by IO atmospheric forcing, longer term changes in the south tropical IO involve a strong contribution from the western Pacific via wave transmission of thermocline anomalies through the Indonesian Archipelago, and their subsequent westward propagation by baroclinic Rossby waves. The late 20th-century IO subsurface cooling trend reversed in the 1990s, reflecting the major regime shift in the tropical Pacific easterlies associated with the Pacific Decadal Oscillation.

Beschreibung: The Mid-Atlantic Ridge south of the equator is a key region for many aspects of spreading axis studies, from biogeography to ridge-hotspot interaction. Despite this, the ridge axis had, until 2004, seen little systematic study. Repeated trips to the area since then have mapped and explored some 900 km of ridge length, from 2° to 14°S. The result is complete bathymetric and side-scan coverage of the axial region and the discovery and characterization of the first hydrothermal vents south of the equator. Such multisegment detailed and interdisciplinary coverage allows us to formulate a general model for the interplay between volcanism, tectonics, and hydrothermalism on a slow spreading ridge. The model defines three basic types of ridge morphology with specific hydrothermal characteristics: (a) a deep, tectonically dominated rift valley where hydrothermalism is seldom associated with volcanism and much more likely confined to long-lived bounding faults; (b) a shallower, segment-center bulge where a combination of repeated magmatic activity and tectonism results in repeated, possibly temporally overlapping periods of hydrothermal activity on the ridge axis; and (c) a very shallow axis beneath which temperatures in all but the uppermost crust are so high that deformation is ductile, inhibiting the formation of high-porosity deep fractures and severely depressing hydrothermal circulation. This model is used together with satellitederived predicted bathymetry to provide forecasts of the best places to look for hydrothermal sites in the remaining unexplored regions of the South Atlantic.

Beschreibung: Near the western boundary of the tropical North Atlantic, where the North Brazil Current (NBC) retroflects into the North Equatorial Countercurrent, large anticyclonic rings are shed. After separating from the retroflection region, the so-called NBC rings travel northwestward along the Brazilian coast, until they reach the island chain of the Lesser Antilles and disintegrate. These rings contribute substantially to the upper limb return flow of the Atlantic Meridional Overturning Circulation by carrying South Atlantic Water into the northern subtropical gyre. Their relevance for the northward transport of South Atlantic Water depends on the frequency of their generation as well as on their horizontal and vertical structure. The ring shedding and propagation and the complex interaction of the rings with the Lesser Antilles are investigated in the inline equation Family of Linked Atlantic Model Experiments (FLAME) model. The ring properties simulated in FLAME reach the upper limit of the observed rings in diameter and agree with recent observations on seasonal variability, which indicates a maximum shedding during the first half of the year. When the rings reach the shallow topography of the Lesser Antilles, they are trapped by the island triangle of St. Lucia, Barbados and Tobago and interact with the island chain. The model provides a resolution that is capable of resolving the complex topographic conditions at the islands and illuminates various possible fates for the water contained in the rings. It also reproduces laboratory experiments that indicate that both cyclones and anticyclones are formed after a ring passes through a topographic gap. Trajectories of artificial floats, which were inserted into the modeled velocity field, are used to investigate the pathways of the ring cores and their fate after they encounter the Lesser Antilles. The majority of the floats entered the Caribbean, while the northward Atlantic pathway was found to be of minor importance. No prominent pathway was found east of Barbados, where a ring could avoid the interaction with the islands and migrate toward the northern Lesser Antilles undisturbed.