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Miesner F.,University of Bremen | Miesner F.,FIELAX Gesellschaft fur wissenschaftliche | Lechleiter A.,University of Bremen | Muller C.,FIELAX Gesellschaft fur wissenschaftliche
Ocean Science | Year: 2015

Continuous monitoring of oceanic bottom water temperatures is a complicated task, even in relatively easy-to-access basins like the North or Baltic seas. Here, a method to determine annual bottom water temperature variations from inverse modeling of instantaneous measurements of temperatures and sediment thermal properties is presented. This concept is similar to climate reconstructions over several thousand years from deep borehole data. However, in contrast, the presented method aims at reconstructing the recent temperature history of the last year from sediment thermal properties and temperatures from only a few meters depth. For solving the heat equation, a commonly used forward model is introduced and analyzed: knowing the bottom water temperature variations for the preceding years and the thermal properties of the sediments, the forward model determines the sediment temperature field. The bottom water temperature variation is modeled as an annual cosine defined by the mean temperature, the amplitude and a phase shift. As the forward model operator is non-linear but low-dimensional, common inversion schemes such as the Newton algorithm can be utilized. The algorithms are tested for artificial data with different noise levels and for two measured data sets: from the North Sea and from the Davis Strait. Both algorithms used show stable and satisfying results with reconstruction errors in the same magnitude as the initial data error. In particular, the artificial data sets are reproduced with accuracy within the bounds of the artificial noise level. Furthermore, the results for the measured North Sea data show small variances and resemble the bottom water temperature variations recorded from a nearby monitoring site with relative errors smaller than 1 % in all parameters. © Author(s) 2015.

Muller H.,University of Bremen | von Dobeneck T.,University of Bremen | Nehmiz W.,FIELAX Gesellschaft fur wissenschaftliche | Hamer K.,University of Bremen
Geo-Marine Letters | Year: 2011

Submarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor. Resulting local anomalies in the morphology and physical properties of surface sediments are usually explored with seismo-acoustic imaging techniques. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility, hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for a first time with a high-resolution survey of freshwater seeps in Eckernförde Bay (SW Baltic Sea). We demonstrate that EM profiling, complemented and validated by acoustic as well as sample-based rock magnetic and geochemical methods, can create a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite, is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction (depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations. © 2010 Springer-Verlag.

Dillon M.,FIELAX Gesellschaft fur wissenschaftliche | Muller C.,FIELAX Gesellschaft fur wissenschaftliche | Usbeck R.,FIELAX Gesellschaft fur wissenschaftliche
Sea Technology | Year: 2012

The offshore market has recently increased demand for thermal conductivity values in shallow-water regions, which are important for estimating the dissipation mechanisms of thermal energy and the environmental impact of power cables. VibroHeat's heat-flow measuring system records temperature data as a function of time. A full measuring cycle starts with the penetration process. The probe is lowered to the seafloor and penetrates the sediment, causing the temperature to rise due to frictional heating. The mechanical design of the HeatFlowProbe follows the violin bow concept, where the sensor string is mounted parallel to a support. The data acquisition unit and the power supply unit are contained in Grade-S titanium pressure housings, which are placed in the head section of the HeatFlowProbe. After the stability criterion was adjusted, the subsequent measurements were performed without difficulty. It was possible to process all temperature data and derive thermal conductivities as a function of depth.

Gutt J.,Alfred Wegener Institute for Polar and Marine Research | Bohmer A.,Alfred Wegener Institute for Polar and Marine Research | Bohmer A.,Carl von Ossietzky University | Dimmler W.,FIELAX Gesellschaft fur wissenschaftliche
Marine Ecology Progress Series | Year: 2013

Sponge spicule sea-bed cover was analysed and related to the mega- and macroepibenthos along one video-recorded and one still image sea-bed transect in the southeastern Weddell Sea, Antarctica. The origin of the patterns of spicule mats and their associated fauna was conceptually reconstructed and interpreted to be a result of iceberg scouring as the main driver. Spicule beds were not necessarily correlated with a diverse fauna, which was shown by a comparison of sponge spicule cover and macrobenthic and megabenthic abundance and biodiversity. On the one hand, this result might reflect slow recolonisation processes, especially by the megabenthos. On the other hand, local maximum densities of adult sponges were found where spicule cover was highest. A simple numerical model revealed that biogenic silicon converted from living to dead material by iceberg scouring accounts for 0.69% of the global silicon flux to the deep sea, which originates from primary production. However, the sponge-derived silicon sequestration occurs over only 2.4% area of the global ocean. On the Antarctic continental shelf, flux rates of silicon that originated from primary production are similar to or twice as high as silicon sequestration due to iceberg-induced sponge mass mortality. © Inter-Research 2013.

Gutt J.,Alfred Wegener Institute for Polar and Marine Research | Cape M.,University of California at San Diego | Dimmler W.,FIELAX Gesellschaft fur wissenschaftliche | Fillinger L.,Alfred Wegener Institute for Polar and Marine Research | And 6 more authors.
Polar Biology | Year: 2013

The aim of this study was to contribute to a general understanding of the response of the Antarctic macrobenthos to environmental variability and climate-induced changes. The change in population size of selected macrobenthic organisms was investigated in the Larsen A area east of the Antarctic Peninsula in 2007 and 2011 using ROV-based imaging methods. The results were complemented by data from the Larsen B collected in 2007 to allow a conceptual reconstruction of the environment-driven changes before the period of investigation. Both Larsen areas are characterised by ice-shelf disintegration in 1995 and 2002, respectively, as well as high inter-annual variability in sea-ice cover and oceanographic conditions. In 2007 one ascidian species, Molgula pedunculata, was abundant north and south of the stripe of remaining ice shelf between Larsen A and B. Population densities decreased drastically in the Larsen A between 2007 and 2011, coincident with the decrease in Corella eumyota, another ascidian. Among the ophiuroids, the population of deposit feeders increased, while suspension feeders halved their abundance. Current measurements indicated a northward flow between the Larsen B and Larsen A, suggesting that a major physical forcing on benthic population development comes from the South. The results demonstrate that Antarctic macrobenthic populations can exhibit dramatic population dynamics. Analyses of sea-ice dynamics, salinity, temperature and surprisingly ice-shelf disintegration history, however, did not provide any clear evidence for environmental drivers underlying the apparent changes. © 2013 Springer-Verlag Berlin Heidelberg.

Sahling H.,Klagenfurter Str. | Romer M.,Klagenfurter Str. | Pape T.,Klagenfurter Str. | Berges B.,University of Southampton | And 10 more authors.
Biogeosciences | Year: 2014

We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m water depth. Flares from this depth have gained significant attention in the scientific community in recent years because they may be caused by bottom-water warming-induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m water depth, which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the Forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost. Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m water depth as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (> 99.70%) of microbial origin (average Î13C Combining double low line -55.7% V-PDB). Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single-beam echosounder data, bubble stream mapping using a ROV-mounted horizontally looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 106 mol methane were annually emitted at the two areas and allow for a large range of uncertainty due to our method (9 to 118 × 106 mol yr−1). First, these amounts show that gas emissions at the continental margin west of Svalbard were on the same order of magnitude as bubble emissions at other geological settings; second, they may be used to calibrate models predicting hydrate dissolution at present and in the future; and third, they may serve as a baseline (year 2012) estimate of the bubble flux that will potentially increase in the future due to ever-increasing global-warming-induced bottom water warming and hydrate dissociation. © Author(s) 2014.

Muller C.,FIELAX Gesellschaft fur wissenschaftliche | Usbeck R.,FIELAX Gesellschaft fur wissenschaftliche | Miesner F.,FIELAX Gesellschaft fur wissenschaftliche
Applied Thermal Engineering | Year: 2016

Analytical and 2D finite-element solutions of the heat transfer equation are used to model the temperature evolution in the upper 15 m of marine sediment deposits. These calculations comprise realistic, in-situ measured, thermal properties as well as natural (geothermal and sea bottom temperature) and man-made (e.g. high voltage, subsea power cables) heat sources. A main objective of this article is to focus on the influence of the thermal heterogeneity of sediments in the North and Baltic Seas. In combination with seasonal variations of bottom water temperatures, these result in large temperature differences in the upper 3–5 m of the sediments. Also, the impact of additional heat through power cables is considered. The modelling results show maximum cable temperatures as much as 16 K difference less compared to a conservative homogeneous scenario previously used from literature values. It can be shown that the 2 K-criterion is not a suitable instrument to monitor the man-made impact to the thermal conditions without knowing the actual thermal settings of a given location. © 2016 The Authors

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