ALBERT

Description: Lake Superior has exhibited a continuous, century-long increase in nitrate whereas phosphate remains at very low levels. Increasing nitrate and low phosphate has led to a present-day severe stoichiometric imbalance; Lake Superior's deepwater NO3−:PO43− molar ratio is 10,000, more than 600 times the mean requirement ratio for primary producers. We examine the rate of [NO3−] increase relative to budgets for NO3− and fixed N. Nitrate in Lake Superior has continued to rise since 1980, though possibly at a reduced rate. We constructed whole-lake NO3− and N budgets and found that NO3− must be generated in the lake at significant rates. Stable O isotope results indicate that most NO3− in the lake originated by in-lake oxidation. Nitrate in the lake is responding not just to NO3− loading but also to oxidation of reduced forms of nitrogen delivered to the lake. The increasing [NO3−]:[PO43−] stoichiometric imbalance in this large lake is largely determined by these in-situ processes.

Description: The Pacific Subtropical Cell (STC) circulation is being analyzed from transport time series across 9°S and 9°N, obtained from the German ECCO (GECCO) assimilation results for the period 1952–2002. In this estimate, the interior Pacific STC convergence shows significantly less decadal slowdown from the 1960's to the 1990's (∼5Sv), than in previous estimates based on hydrographic sections. In the GECCO results, about half of this STC convergence decrease is compensated by an increase in the equatorward transport of the western boundary currents. Overall, the STC varies primarily on interannual time scale, with relatively short time lags between STC convergence and transport variations of the Equatorial Undercurrent at 140°W.

Description: A Holocene Gulf of Guinea record of riverine runoff, based on Ba/Ca in tests of a shallow-dwelling planktic foraminifer, and sea surface temperature (SST), based on Mg/Ca, reveals centennial-scale instabilities in West African monsoon (WAM) precipitation and eastern equatorial Atlantic (EEA) thermal conditions. The long-term Holocene climate trend is characterized by a warm and wet early-mid Holocene and gradual drying and cooling during the late Holocene. Superimposed on this trend are numerous centennial scale drops in precipitation during the early-mid Holocene. The greatest declines in early Holocene monsoon precipitation were accompanied by significant SST cooling in the EEA and correlate with drops in air temperature over Greenland and fresh water outbursts into the North Atlantic (NA). This observation suggests that early Holocene climate instabilities in the NA were closely linked to changes in the WAM. The strong imprint of NA events in summer monsoon precipitation suggests that these events were not confined to winter-time. The late Holocene does not show large amplitude changes in riverine runoff at the centennial level. The relatively stable late Holocene conditions likely reflect a weakening and stabilization of the monsoon system, probably due to diminished influence of the NA region due to a reduction in ice sheet.

Description: We present a new dust source area map for the Sahara and Sahel region, derived from the spatiotemporal variability of composite images of Meteosat Second Generation (MSG) using the 8.7, 10.8 and 12.0 μm wavelength channels for March 2006–February 2007. Detected dust events have been compared to measured aerosol optical thickness (AOT) and horizontal visibility observations. Furthermore the monthly source area map has been compared with the Ozone Monitoring Instrument aerosol index (AI). A spatial shift of the derived frequency patterns and the local maxima of AI-values can be explained by wind-transport of airborne dust implicitly included in the AI signal. To illustrate the sensitivity of a regional model using the new dust source mask, we present a case study analysis that shows an improvement in reproducing aerosol optical thickness in comparison to the original dust source parameterization.

Description: [1] On the basis of the detailed sedimentological record of the key-core PS66/309-1 and a review of open literature, we present an assessment of the paleoenvironmental conditions as well as trigger mechanism of the Hinlopen/Yermak Megaslide north of Spitsbergen. The Svalbard archipelago is characterized by strong inflow of Atlantic water accompanied by rapidly falling sea level, rapidly growing Svalbard-Barents Sea-Ice Sheet, and associated increasing glaciotectonic activity during the time window around 30 calendar kyr B. P. of this catastrophic failure event. Thus the potential trigger mechanisms include sediment buoyancy and excess pore pressure, hydrate stability, and tectonic/glaciotectonic processes. While the common scenarios seem to fail to explain this unique submarine megaslide, we focus on glacial processes and their consequences for the regional tectonic framework. We conclude that the Hinlopen/Yermak Megaslide has been the consequence of the rapid onset of Late Weichselian glaciation resulting in a drastic sea level drop, asymmetrical ice loading, and a forebulge development leading to enhanced tectonic movements along the Hinlopen fault zone. As the final trigger we assume a strong earthquake positioned below or close to the SE Sophia Basin.

Description: Multibeam sonar surveys have been conducted since their invention in the 1970s; however, mainly reflections from the seafloor were considered so far. More recently, water column imaging with multibeam is becoming of increasing interest for fisheries, buoy, mooring, or gas detection in the water column. Using ELAC SEABEAM 1000 data, we propose a technique to detect gas bubbles (flares) although this system is originally not designed to record water column data. The described data processing represents a case study and can be easily adapted to other multibeam systems. Multibeam data sets from the Black Sea and the North Sea show reflections of gas bubbles that form flares in the water column. At least for reasonably intense gas escape the detection of bubbles is feasible. The multibeam technique yields exact determination of the source position and information about the dimension of the gas cloud in the water. Compared to conventional flare imaging by single-beam echo sounders, the wide swath angle of multibeam systems allows the mapping of large areas in much shorter time.

Description: Measurements of CH4 concentrations in the bottom water during two discrete sampling periods in subsequent years above different cold seeps at the Pacific margin off Costa Rica indicate large-scale variations of CH4 release. CH4 is emitted from mud extrusions and a slide scar at 1000–2300 m water depth. Maximum CH4 concentrations were found to be lower above all investigated sites in autumn 2003 than in autumn 2002 although seep sites are up to 300 km apart. Tidal and current changes were observed but found to apply only to individual seep sites. Increased seismic activity connected to the moment magnitude (M W ) 6.4 earthquake offshore Costa Rica in June 2002 could have had an impact on all seep sites and thereby caused an increase in CH4 emission. This is supported by the largest variations of CH4 concentration found above mud extrusions located above faults likely more strongly affected by tectonic movements. Even though our data indicate a relation between seismicity and CH4 seepage, the relation is not proven, and future work is needed to comprehensively test this hypothesis.

Description: We assessed interrelations between climatic, meteorological and hydrological factors in the Balearic Sea, and identified drivers of the abundance variability of planktonic copepods over the period 1994–2003. Temporal variability in the total abundance of copepods appears to be indicative of the southern spreading of the Northern Current, and therefore of meridional transport of water masses in the Balearic Sea. The results show that the large-scale atmospheric variability in the North Atlantic Ocean acts as a main driver of regional meteorological variations and hydrographic patterns in the Balearic Sea area. The clearly most novel insight from this study is that the effects of the North Atlantic climate on hydrographic variability and the abundance of copepods in the studied area are noticeable at monthly scales. Thus, the North Atlantic climate influence on the structure and biogeochemical fluxes in the Western Mediterranean pelagic ecosystems deserve more attention that they have been received so far.

Description: Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three-dimensional (3-D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3-D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS-06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low-velocity anomaly in the crust, just north of the volcanic arc (Merapi-Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low-velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.

Description: In 2005 an amphibious seismic network was deployed on the Chilean forearc between 41.75°S and 43.25°S. 364 local events were observed in a 11-month period. A subset of the P and S arrival times were inverted for hypocentral coordinates, 1-D velocity structure and station delays. Main seismic activity occurred predominantly in a belt parallel to the coast of Chiloé Island in a depth range of 12–30 km presumably related to the plate interface. The 30° inclination of the shallow part of the Wadati-Benioff zone is similar to observations further north indicating that oceanic plate age is not controlling the subduction angle of the shallower part for the Chilean subduction zone. The down-dip termination of abundant intermediate depth seismicity at approximately 70 km depth seems to be related to the young age (and high temperature) of the oceanic plate. Crustal seismicity is associated with the Liquiñe-Ofqui fault zone and active volcanoes.