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High-resolution seismic and ground-penetrating-radar (GPR) data have been acquired over Lake Nikolay in the western Lena Delta in order to study the uppermost basin fill and the bordering frozen margins. GPR (100 MHz antenna pair) measurements were completed on the frozen lake and its permafrost margins, while high-resolution seismic data were acquired from the lake during open-water conditions in summer using a 1.5-11.5 kHz Chirp profiler. The combined use of the two profiling systems allows stratigraphic profiling in both frozen and unfrozen parts of the lake. Shallow seismic reflection images of the uppermost 4 to 5 m of sediments are compared to GPR sections, which have approximately the same horizontal and vertical resolution. Short sediment cores aid calibrate the geophysical data.

Submarine channels are the primary conduits for terrestrial sediment, organic carbon, and pollutant transport to the deep sea. Submarine channels are far more difficult to monitor than rivers, and thus less well understood. Here we present 9 years of time-lapse mapping of an active submarine channel along its full length in Bute Inlet, Canada. Past studies suggested that meander-bend migration, levee-deposition, or migration of (supercritical-flow) bedforms controls the evolution of submarine channels. We show for the first time how rapid (100–450 m/year) upstream migration of 5-to-30 m high knickpoints can control submarine channel evolution. Knickpoint migration-related changes include deep (>25 m) erosion, and lateral migration of the channel. Knickpoints in rivers are created by external factors, such as tectonics, or base-level change. However, the knickpoints in Bute Inlet appear internally generated. Similar knickpoints are found in several submarine channels worldwide, and are thus globally important for how channels operate.

Seismic quality factor has the potential to characterize sediment properties but seldom is used by the industry for offshore site investigations because of practical difficulties with reflection seismology ͑e.g., restricted bandwidth͒ and because of uncertainties in rock-physics models. A spectral-ratio analysis of highresolution marine seismic data can determine a quality factor to within a 95% confidence of ‫01ע‬ within the uppermost 30 m of unconsolidated marine sediments. Our spectral-ratio technique does not require assumptions on how attenuation scales with frequency. Emphasis is placed on interpretation of spectral signatures before applying an iteratively reweighted robust leastsquares regression to subdue the effects of noise and local heterogeneities when determining the quality factor of a sediment package. We combined data from boomer and chirp sources to examine attenuation over four octaves of frequency ͑0.5-8.0 kHz͒ and to demonstrate that expanding the frequency range improves the precision and accuracy of quality-factor fits. We obtain frequency-independent quality factors with 95% confidence intervals of 135 ‫;21ם͑‬ ‫͒01מ‬ and 107 ‫;6ם͑‬ ‫͒5מ‬ for silty clays with mean grain sizes of 7.7 and 6.9 phi, respectively, and 63 ‫;01ם͑‬ ‫͒7מ‬ for a modern sand deposit with mean grain size 2.5 phi, from the Solent ͑U. K.͒. Sediments with higher quality factors require more independent observations to achieve a desirable 95% confidence. We required only 45 traces over sands and 1250 traces over the lowest attenuating silty clays. By constructing an empirical model of quality factor against mean grain size from published sediment studies, the mean grain sizes of our Solent sediments can be located, and we find that quality factor can be used to distinguish between coarse grain-dominated and claydominated sediments.

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[1] Remote prediction of gassy marine sediment properties is important for geohazard assessment. Gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity-frequency and attenuation-frequency relationships that depend on gas bubble size, gas content, and sediment elastic properties. An acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at Dibden Bay, Southampton, United Kingdom. A vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed. A miniboomer in the seabed above the array was used to generate broadband (600 Hz to 3000 Hz) acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m (subaerial exposure) at low tide and 2.35 m at high tide. The calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum (over 200 dB/m) that shifts in frequency from 1050 Hz at low tide to 1250 Hz at high tide, thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments. Modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum, supported by X-ray computed tomography scan observations on a pressure core (which also indicate that bubble shape depends on sediment type). Modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum, implying that gas diffusion and nonspherical bubbles are significant.

A high-resolution record of pollution is preserved in recent sediments from Windermere, the largest lake in the English Lake District. Data derived from X-ray core scanning (validated against wavelength dispersive X-ray fluorescence), radiochronological techniques ((210)Pb and (137)Cs) and ultrahigh precision, double-spike mass spectrometry for lead isotopes are combined to decipher the anthropogenic inputs to the lake. The sediment record suggests that while most element concentrations have been stable, there has been a significant increase in lead, zinc, and copper concentrations since the 1930s. Lead isotope down-core variations identify three major contributory sources of anthropogenic (industrial) lead, comprising gasoline lead, coal combustion lead (most likely source is coal-fired steam ships), and lead derived from Carboniferous Pb-Zn mineralization (mining activities). Periods of metal workings do not correlate with peaks in heavy metals due to the trapping efficiency of up-system lakes in the catchment. Heavy metal increases could be due to flood-induced metal inwash after the cessation of mining and the weathering of bedrock in the catchment. The combination of sediment analysis techniques used provides new insights into the pollutant depositional history of Windermere and could be similarly applied to other lake systems to determine the timing and scale of anthropogenic inputs.

This paper describes the detection and classification of targets against clutter by distinguishing between linear and nonlinear scatterers and, further, by distinguishing those nonlinear targets that scatter energy at the even-powered harmonics from those that scatter in the odd-powered harmonics. This is done using twin inverted pulse sonar (TWIPS), which can also, in some manifestations, require no range correction (and therefore does not require the a priori knowledge of the environment needed for most remote detection technologies). The method applies, in principle, to a range of sensor technologies, including the use of radar to distinguish between circuitry, metal and soil; Light Detection and Ranging (LIDAR) to detect combustion products; and Magnetic Resonance Imaging (MRI). A sonar application is demonstrated, detecting objects in bubbly water (including in the wake of a ship of 3953 gross register tonnage). A manmade sonar that can operate in bubbly water is relevant: Cold War sonar is not optimized for the shallow coastal waters that typify many current operations. The US Navy use dolphins in such waters. TWIPS arose as a demonstration that echolocation was possible in bubbly water in response to a video showing dolphins generating bubble nets when hunting: if echolocation were impossible in these nets, then during this hunt, the dolphins would have blinded their sonar.