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The processes controlling advance and retreat of outlet glaciers in fjords draining the Greenland Ice Sheet remain poorly known, undermining assessments of their dynamics and associated sea-level rise in a warming climate. Mass loss of the Greenland Ice Sheet has increased six-fold over the last four decades, with discharge and melt from outlet glaciers comprising key components of this loss. Here we acquired oceanographic data and multibeam bathymetry in the previously uncharted Sherard Osborn Fjord in northwest Greenland where Ryder Glacier drains into the Arctic Ocean. Our data show that warmer subsurface water of Atlantic origin enters the fjord, but Ryder Glacier’s floating tongue at its present location is partly protected from the inflow by a bathymetric sill located in the innermost fjord. This reduces under-ice melting of the glacier, providing insight into Ryder Glacier’s dynamics and its vulnerability to inflow of Atlantic warmer water. A bathymetric sill in Sherard Osborn Fjord, northwest Greenland shields Ryder Glacier from melting by warm Atlantic water found at the bottom of the fjord, according to high-resolution bathymetric mapping and oceanographic data.

Petermann Fjord is a deep (> 1000 m) fjord that incises the coastline of northwest Greenland and was carved by an expanded Petermann Glacier, one of the six largest outlet glaciers draining the modern Greenland Ice Sheet (GrIS). Between 5–70 m of unconsolidated glacigenic material infills in the fjord and adjacent Nares Strait, deposited as the Petermann and Nares Strait ice streams retreated through the area after the Last Glacial Maximum. We have investigated the deglacial deposits using seismic stratigraphic techniques and have correlated our results with high-resolution bathymetric data and core lithofacies. We identify six seismo-acoustic facies in more than 3500 line-km of sub-bottom and seismic-reflection profiles throughout the fjord, Hall Basin and Kennedy Channel. Seismo-acoustic facies relate to: bedrock or till surfaces (Facies I); subglacial deposition (Facies II); deposition from meltwater plumes and icebergs in quiescent glaciomarine conditions (Facies III, IV); deposition at grounded ice margins during stillstands in retreat (grounding-zone wedges; Facies V); and the redeposition of material down slopes (Facies IV). These sediment units represent the total volume of glacial sediment delivered to the mapped marine environment during retreat. We calculate a glacial sediment flux for the former Petermann Ice Stream as 1080–1420 m3 a−1 per meter of ice stream width and an average deglacial erosion rate for the basin of 0.29–0.34 mm a−1. Our deglacial erosion rates are consistent with results from Antarctic Peninsula fjord systems but are several times lower than values for other modern GrIS catchments. This difference is attributed to fact that large volumes of surface water do not access the bed in the Petermann system and we conclude that glacial erosion is limited to areas overridden by streaming ice in this large outlet glacier setting. Erosion rates are also presented for two phases of ice retreat and confirm that there is significant variation in these rates over a glacial-deglacial transition. Our new fluxes and erosion rates show that the Petermann Ice Stream was approximately as efficient as the palaeo-Jakobshavn Isbrae at eroding, transporting and delivering sediment to its margin during early deglaciation.

The ocean surface mixed layer is a nearly universal feature of the world oceans. Variations in the depth of the mixed layer (MLD) influences the exchange of heat, fresh water (through evaporation), and gases between the atmosphere and the ocean and constitutes one of the major factors controlling ocean primary production as it affects the vertical distribution of biological and chemical components in near-surface waters. Direct observations of the MLD are traditionally made by means of conductivity, temperature, and depth (CTD) casts. However, CTD instrument deployment limits the observation of temporal and spatial variability in the MLD. Here, we present an alternative method in which acoustic mapping of the MLD is done remotely by means of commercially available ship-mounted echo sounders. The method is shown to be highly accurate when the MLD is well defined and biological scattering does not dominate the acoustic returns. These prerequisites are often met in the open ocean and it is shown that the method is successful in 95 % of data collected in the central Arctic Ocean. The primary advantages of acoustically mapping the MLD over CTD measurements are (1) considerably higher temporal and horizontal resolutions and (2) potentially larger spatial coverage.

Seeps of free methane gas escaping the seabed can be found throughout the ocean basins. To understand the role of methane gas seeps in the global carbon cycle—including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume—it is important to quantify the amount of methane escaping the seabed. Few large-scale mapping projects of natural methane seeps have been undertaken, however, and even among these, quantitative estimates of flux are rare. Here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region (6000 km2) of the northern Gulf of Mexico and outline a general approach for methane seep mapping using a combination of multibeam and split-beam echo sounders. Using additional measurements collected with a remotely operated vehicle (ROV) together with the acoustic mapping results, we estimate the total gas flux within the 6000 km2 region to be between 0.0013 and 0.16 Tg/yr, or between 0.003 and 0.3% of the current estimates for global seabed methane seepage rates.

The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions >1-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (∼140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening., The development of pan-Arctic Ocean ice shelves during peak glacials was proposed in the 1970s, an idea that has been disputed due to lack of evidence. Here, the authors present geophysical mapping data supporting the presence of such an ice shelf during the peak of the penultimate glaciation ∼140–160 ka.

When in the form of free gas in the water column, methane seeps emanating from the seabed are strong acoustic targets that are often detectable from surface vessels using echo sounders. In addition to detecting that a seep is present at some location, it is also desirable to characterize the nature of the seep in terms of its morphology and flux rates. Here, we examine how much we can learn about seeps in the deep (> 1000 m) northern Gulf of Mexico using narrow-band split-beam echo sounders operating at fixed frequencies (18 kHz and 38 kHz). Methane seeps in this region are deeper than the methane hydrate stability zone, implying that bubbles of free gas form hydrate rinds that allow them to rise further in the water column than they otherwise would. While this behavior may aid in the classification of gas types in the seep, it is possible that the presence of hydrate rinds may also change the acoustic response of the bubbles and thereby make flux rate estimates more challenging. These and other aspects of seep characterization will be discussed.

In 2011 and 2012, measurements of acoustic backscatter from natural methane seeps were made in the northern Gulf of Mexico in water depths between 1000-2000 m. The measurements were made using a calibrated 18 kHz echo sounder with an 11 degree beamwidth in order to estimate the depth-dependent target strength (TS). The TS data indicate a wide variation in the rate of gas seepage from the seafloor. Several of these seeps were revisited with a remotely operated vehicle in order to optically assess the bubble size distribution and to estimate the rate at which gas bubbles were exiting the seafloor. The optical data show bubble sizes between 1-10 mm radius, and similar rates of gas seepage ranging from a few bubbles per second to several tens of bubbles per second. Together, these data help to suggest the requirements for acoustically estimating gas flux from the seafloor over large regions.

Underwater methane seeps support diverse biological communities on the seafloor and, in cases of bubble survival to the surface, contribute to the quantity of atmospheric methane. The National Oceanic and Atmospheric Administration (NOAA) ship Okeanos Explorer completed two research cruises for seep mapping and characterization in the northern Gulf of Mexico during August and September of 2011 and April of 2012. Seeps originating at depths of approximately 1500 m were observed during multiple transects with a 30-kHz Kongsberg EM 302 multibeam echosounder (MBES) and an 18-kHz Simrad EK60 split-beam scientific echosounder calibrated for backscatter. A methodology for determining vessel offsets for the EK60 using MBES seep observations as benchmarks is discussed as part of a larger framework for transformation of seep targets from the split-beam echosounder reference frame to the geographical reference frame. Utilizing sound speed and attitude data collected for the MBES, several EK60 observations of strong individual seeps are scrutinized for variability of seep position and target strength between 2011 and 2012.

Underwater methane seeps support diverse biological communities on the seafloor and, in cases of bubble survival to the surface, contribute to the quantity of atmospheric methane. The National Oceanic and Atmospheric Administration (NOAA) ship Okeanos Explorer completed two research cruises for seep mapping and characterization in the northern Gulf of Mexico during August and September of 2011 and April of 2012. A 30-kHz Kongsberg EM 302 multibeam echosounder (MBES) and an 18-kHz Simrad EK60 split-beam scientific echosounder were employed to detect and observe seeps during multiple transects over areas of known seep activity at depths of approximately 1500 m. This presentation includes analyses of EK60 data from both research cruises with emphasis on seep mapping in the water column and seep source positioning on the seafloor using EM 302 MBES observations of seeps as benchmarks. Uncertainty associated with interferometric principles employed by the EK60 and limits to midwater positioning capability impos...