Continental Shelf Environments Research Articles

Acoustic measurements of marine sediments with pebbles and cobbles

ABSTRACTThe majority of sediment acoustics research has focused to date on sediments with sand‐sized particles or smaller; measurements for sediments containing cobbles (6–26 cm) are rare. This paper presents the first measurements (to the authors’ knowledge) over a wide frequency range of compressional‐wave velocity and bulk density for a sediment with cobbles. The in situ velocity from inversion from wide‐angle reflection‐coefficient data at 0.4–2 kHz for cobbles suspended in sand is found here to be 1800 m/s with 95% credibility interval bounds of [1787–1828] m/s and for cobbles suspended in clay 1532 [1528–1536] m/s. Measured core velocities at 50 kHz and 200 kHz are lower for each sediment due to multiple scattering. The in situ bulk density for cobbles suspended in sand is 2.25 [2.21–2.28] g/cm3 and for cobbles suspended in clay 1.83 [1.78–1.87] g/cm3. Though cobbles in the upper few metres of sediment may be considered unusual on the mid‐shelf at mid‐latitudes, they appear to be present over tens of square kilometres on the Malta Plateau in a several metre thick layer starting at about 1 m below seafloor. In fact, geologic process considerations suggest that cobbles may be generally more common in mid to outer shelf environments than the paucity of measurements would suggest. Increased interest in the Arctic continental shelf environment, where pebbles and cobbles are expected to be prevalent, provides an additional motivation for this work.

Improvements in signal processing for the estimation of environmental parameters with machine learning using a compact tetrahedral array and sources of opportunity

In previous work, we showed that we could localize sound sources using a compact tetrahedral hydrophone array in a continental shelf environment south of Block Island, Rhode Island. The tetrahedral array of phones, 0.5 m on a side, was deployed to monitor the construction and operation of the first offshore wind farm in the United States. Directions of arrival (DOAs) for a number of ships were computed using a time difference of arrival technique. Given the DOAs, ranges are estimated using supervised machine learning techniques. We extended that work to estimate a number of environmental parameters including water depth and sediment composition. Here, we report on results using new spectrogram processing techniques based on high resolution PE modeling. These results include inversions for sediment parameters with estimates of error. With this new higher resolution spectrogram processing, we also report on the impact of the sediment parameters on range estimation. We generalize the technique to generic continental shelf environments. [Work supported by the Office of Naval Research.]

Laboratory measurements and simulations of reflections from a water/clay interface during the diffusion of salt.

Estuarine, riverine, and certain continental shelf environments experience significant temperature and salinity variability near the ocean bottom that can produce significant changes in how sound interacts with fine-grained sediments, presenting challenges in applications including shallow water sonar and bottom surveys. To begin to understand the effects of this variability on acoustic reflection, reflection measurements in the laboratory near 1 MHz were obtained from a water-clay interface while varying the salinity of the bottom water. At certain angles of incidence, salinity variations caused changes in bottom loss up to 15 dB and 180-degree phase shifts in the reflected signal, and induced changes in the reflectivity of the clay through the diffusion process, thereby leading to complicated, coupled interactions at the water-clay interface. By modeling the reflectivity of clay during molecular diffusion of salt, the diffusion coefficients were experimentally inferred and simulations at lower frequencies and longer timescales were performed. Derived characteristic length scales associated with the molecular diffusion of salt are compared with acoustic wavelengths to identify frequency regimes that are sensitive to salinity fluctuations. Results indicate that the dynamic nature of the bottom water can cause measurable and significant effects in reflectivity at and below frequencies applicable to sonar.

Carbonate rocks of offshore northern Zealandia

ABSTRACTThirty-eight non-tropical carbonate rock samples with subtropical affinities, from northern Zealandia, were obtained from oceanographic dredging expeditions in the Reinga-Aotea Basin (2013), and the Norfolk, Loyalty, and Three Kings ridges (2015). Samples range in age from Pleistocene to Paleocene and were petrographically classified into five depositional and one authigenic petrofacies: (1) coarse, bioclastic grainstone to packstone, (2) coarse, bioclastic packstone, (3) coated grains, (4) fine, bioclastic packstone, (5) wackestone, and, and (6) dolomite. The coarse grainstone, packstone and coated-grains petrofacies are dominated by >mm-scale bioclastic components indicating deposition in high energy continental shelf environments. These petrofacies show similarities to non-tropical carbonates such as the onshore New Zealand Oligocene Te Kuiti Group limestones. Fine bioclastic packstone and wackestone petrofacies, consistent with bathyal deposition, comprise primarily foraminfera and micrite, similar to Eocene fine-grained carbonates outcropping in New Zealand. The dolomite petrofacies comprises diagenetic dolomite similar to Paleocene and Eocene rocks of the Mead Hill and Amuri Limestone formations. Other post-depositional features are also directly comparable to onshore New Zealand limestones. Despite no clear relationship between age, lithology, or geographic location, this study shows that non-tropical shelf carbonates with subtropical affinities extend north beyond New Zealand (southern Zealandia) to at least 23.7°S latitude.

Variability in Shelf Sedimentation in Response to Fluvial Sediment Supply and Coastal Erosion Over the Past 1,000 Years in Monterey Bay, CA, United States

Continental shelf environments are uniquely situated to capture some of the most dynamic processes on Earth including climatic variability and anthropogenic modifications to coastal systems. Understanding how these processes have affected sediment delivery and accumulation on the shelf in the past may provide insight into potential changes in the future. To address this, we investigated shelf sedimentation within Monterey Bay, California. Sediment cores were collected from four locations throughout the bay to capture both the modern and late Holocene sedimentological record using grain size analysis, and sediment chronologies determined from 210Pb, 137Cs, and 14C. From the grain size results we focused on the total percent sand, and established a Littoral Sand Fraction (LSF) index to assess sediment contribution from the littoral zone as a result coastal erosion. Grain size results from the multicores consistently showed an increase in sand over the past several decades (post 1970s). For the cores located within the bay proximal to three major rivers, the increase in sand corresponded to a general increase in the LSF over the same period. We attributed these trends to increased sediment contributions to the shelf due to accelerated coastal erosion in the region. This accelerated coastal erosion was likely the combined result of dam construction in the mid-twentieth century that limited fluvial supply to the coast, and a shift in climate toward wetter, stormier period. Applying these sediment characteristics back over the past ~1,000 years we found that dry climatic periods resulted in deposits that were limited in total sand but enriched in littoral material suggesting elevated coastal erosion. During wet periods deposits were enriched in total sand but limited in littoral sand suggesting elevated fluvial supply and low erosion. Compared to the late Holocene record, the previous several decades represent a shift to a new regime, uncharacteristic of deposits over the past millennia, highlighting the impact humans have had on shelf sedimentation.

Hot Spots of Carbon and Alkalinity Cycling in the Coastal Oceans

Ocean calcium carbonate (CaCO3) production and preservation play a key role in the global carbon cycle. Coastal and continental shelf (neritic) environments account for more than half of global CaCO3 accumulation. Previous neritic CaCO3 budgets have been limited in both spatial resolution and ability to project responses to environmental change. Here, a 1° spatially explicit budget for neritic CaCO3 accumulation is developed. Globally gridded satellite and benthic community area data are used to estimate community CaCO3 production. Accumulation rates (PgC yr−1) of four neritic environments are calculated: coral reefs/banks (0.084), seagrass-dominated embayments (0.043), and carbonate rich (0.037) and poor (0.0002) shelves. This analysis refines previous neritic CaCO3 accumulation estimates (~0.16) and shows almost all coastal carbonate accumulation occurs in the tropics, >50% of coral reef accumulation occurs in the Western Pacific Ocean, and 80% of coral reef, 63% of carbonate shelf, and 58% of bay accumulation occur within three global carbonate hot spots: the Western Pacific Ocean, Eastern Indian Ocean, and Caribbean Sea. These algorithms are amenable for incorporation into Earth System Models that represent open ocean pelagic CaCO3 production and deep-sea preservation and assess impacts and feedbacks of environmental change.

Estimation of environmental parameters with machine learning using a compact tetrahedral array and sources of opportunity

In a previous paper, we showed that we could localize sound sources using a compact tetrahedral hydrophone array in a continental shelf environment south of Block Island, Rhode Island. The tetrahedral array of phones, 0.5 m on a side, was deployed to monitor the construction and operation of the first offshore wind farm in the United States. Directions of arrival (DOAs) for a number of ships were computed using a time difference of arrival technique. Given the DOAs, ranges are estimated using supervised machine learning techniques. Here, we extend this work to estimate a number of environmental parameters including water depth and sediment composition. Training sets of range-dependent ocean waveguides and sediment sound speeds were generated using a propagation model for a neural network. Data from the tetrahedral array were processed by the neural network, which provided estimates of the water depth and sediment parameters such as sound speed and density. These estimates are compared to bathymetric data and core data collected as part of the site characterization for the wind farm. [Work supported by the Office of Naval Research and the Bureau of Ocean Energy Management.]

Stratigraphy and Microfacies Analyses of The Al Faidiyah and Benghazi Formations at Al Fatiah, Al Huarry and Al Abyar Quarries, NE Libya

Three quarries named Al Fatiah Cement, Al Huarry Cement, and Al Abyar are located in the northeastern part of Libya and consist of carbonate rocks of Miocene age. The main objectives of this study based on microfacies analyses are to construct the depositional environment with a comprehensive stratigraphical correlation of the study areas. The Al Fatiah Quarry has been divided into three microfacies which are operculinid packstone microfacies, algal packstone microfacies, and moluskan packstone grading to grainstone microfacies, one species of large benthic foraminifera is found to be abundant (Operculina complanata). Accordingly, the Al Faidiyah Formation was deposited in a continental shelf (outer to middle neritic zones) environment. Al Huarry Cement Quarry has been divided into five microfacies which are algal boundstone microfacies, amphisteginid - algae packstone microfacies, amphisteginid grainstone microfacies, algal - bryozan packstone microfacies and amphisteginiid - bryozoan - algal boundstone microfacies, one species of large benthic foraminifera is found to be abundant and well preserved (Amphestegina cf. lessonii). Accordingly, the Benghazi Formation was deposited in an inner self to open a marine shelf environment. On the other hand, the Al Abyar Quarry has been divided into two microfacies which are pelloidal packstone microfacies and miliolid wackestone–packstone microfacies, suggesting that the sedimentation was of low energy under very shallow marine to the restricted environment.

Diet and trophic interactions of a circumglobally significant gelatinous marine zooplankter, Dolioletta gegenbauri (Uljanin, 1884).

Gelatinous zooplankton play a crucial role in marine planktonic food webs. However, primarily due to methodological challenges, the in situ diet of zooplankton remains poorly investigated and little is known about their trophic interactions including feeding behaviour, prey selection and in situ feeding rates. This is particularly true for gelatinous zooplankton including the marine pelagic tunicate, Dolioletta gegenbauri. In this study, we applied an 18S rRNA amplicon metabarcoding approach to identify the diet of captive-fed and wild-caught D.gegenbauri on the midcontinental shelf of the South Atlantic Bight, USA. Sequencing-based approaches were complimented with targeted quantitative real-time polymerase chain reaction (PCR) analyses. Captive-fed D.gegenbauri gut content was dominated by pico-, nano- and micro-plankton including pico-dinoflagellates (picozoa) and diatoms. These results suggested that diatoms were concentrated by D.gegenbauri relative to their concentration in the water column. Analysis of wild-caught doliolids by quantitative real-time PCR utilizing a group-specific diatom primer set confirmed that diatoms were concentrated by D.gegenbauri, particularly by the gonozooid life stage associated with actively developing blooms. Sequences derived from larger metazoans were frequently observed in wild-caught animals but not in captive-fed animals suggesting experimental bias associated with captive feeding. These studies revealed that the diet of D.gegenbauri is considerably more diverse than previously described, that parasites are common in wild populations, and that prey quality, quantity and parasites are likely all important factors in regulating doliolid population dynamics in continental shelf environments.

The effect of pH on the acute toxicity of phenanthrene in a marine microalgae Chlorella salina

Phenanthrene is one of the most abundant polycyclic aromatic hydrocarbons (PAHs) found in continental shelf environment of China and is on the EPA’s Priority Pollutant list. In this study, the effects of phenanthrene on marine algal growth rate were determined after 96-h exposure at pH 6.0, 7.0, 8.0, 9.0, and 10.0 in seawater of salinity 35. Two measuring techniques to assess growth inhibition were also compared using prompt fluorescence and microscopic cell count. The results showed that the toxicity of phenanthrene increased significantly (p < 0.05) with decreasing pH, with the nominal concentration required to inhibit growth rate by 50%, EC50, decreasing from 1.893 to 0.237 mg L−1 as pH decreased from 9.0 to 6.0, with a decrease higher than 55% from 10.0 to 9.0. In addition, the nominal EC50 values calculated in this study were at the same range of some environmental concentrations of phenanthrene close to areas of crude oil exploration. Based on the two measuring techniques, the results showed that cell count and fluorescence measurement were significantly different (p < 0.05), and the nominal EC50 values calculated with cell count measurement were significantly higher than fluorescence measurement at pH 8.0, 9.0 and 10.0. In conclusion, the present studies confirmed that acidification of seawater could affect the toxicity of phenanthrene to this species of microalgae, and which encouraged further studies involving responses of marine organisms to ocean acidification.

Source localization using a compact tetrahedral array

We localized sound sources collected on a compact tetrahedral hydrophone array in a continental shelf environment south of Block Island, Rhode Island. The tetrahedral array of phones, 0.5 m on a side, was deployed to monitor the underwater sound of construction and operation of the first offshore wind farm in the United States. Signals from shipping and marine mammals, including fin whales, humpback whales, and right whales, were detected on the array. Directions of arrival (DOAs) for a number of signals were computed using a time difference of arrival technique. Given the DOAs, ranges were estimated using supervised machine learning techniques outlined by Niu et al. (JASA, 2017). The approach was tested using simulated data from Kraken assuming environmental information consistent with this continental shelf environment. Performance on signals from individual ships and marine mammals is presented. Ship localizations are compared to Automated Identification System (AIS) fixes. An error analysis is also presented. [Work supported by the Office of Naval Research and the Bureau of Ocean Energy Management.]

Late Holocene paleoceanography in the Chukchi and Beaufort Seas, Arctic Ocean, based on benthic foraminifera and ostracodes

Calcareous microfossil assemblages in late Holocene sediments from the western Arctic continental shelf provide an important baseline for evaluating the impacts of today’s changing Arctic oceanography. This study compares 14C-dated late Holocene microfaunal assemblages of sediment cores SWERUS-L2-2-PC1, 2-MC4 and 2-KL1 (57 mwd), which record the last 4200 years in the Herald Canyon (Chukchi Sea shelf), and HLY1302-JPC-32, GGC-30, MC-29 (60 mwd), which record the last 3000 years in the Beaufort Sea shelf off the coast of Canada. Foraminiferal and ostracode assemblages are typical of Arctic continental shelf environments with annual sea-ice cover and show relatively small changes in terms of variability of dominant species. Important microfaunal changes in the Beaufort site include a spike in Spiroplectammina biformis coinciding with a decrease in Cassidulina reniforme in the last few centuries suggesting an increase of Pacific Water influence and decreased sea-ice. There is low-amplitude centennial-scale variability in proportions of benthic foraminiferal species, such as C. reniforme. In addition to these species, Cassidulina teretis s.l., Elphidium excavatum clavatum and Stainforthia feylingi are also common at this site. At the Herald Canyon site in the last few centuries, C. reniforme peaks around 150 years BP and then decreases while Spiroplectammina earlandi spikes and Acanthocythereis dunelmensis decreases also suggesting an increase in Pacific Water influence and decreased sea-ice at this site. This site also includes Buccella spp. and Elphidium excavatum clavatum. Differences in benthic foraminifera and ostracode species dominance between the two sites may be due to a greater influence of Pacific Water in the Chukchi shelf, compared to the more distal Beaufort shelf, which is also affected by the Beaufort Gyre and the Mackenzie River.

The Rio Doce Group revisited: An Ediacaran arc-related volcano-sedimentary basin, Araçuaí orogen (SE Brazil)

The Araçuaí orogen developed between the São Francisco and Congo cratons, during the Western Gondwana amalgamation in Late Neoproterozoic time. Located in the eastern Araçuaí orogen, the Ediacaran Rio Doce magmatic arc includes a large amount of plutonic rocks (the G1 supersuite, ca. 630-580 Ma) spatially associated with a supracrustal succession, the Rio Doce Group (RDG). This group, poorly studied until recently, encompasses four formations (from base to top): Palmital do Sul, Tumiritinga, São Tomé, and João Pinto. The Palmital do Sul and Tumiritinga formations mostly include pelitic schists to paragneisses, interbedded with metavolcanic and meta-volcaniclastic rocks, representing proximal deposits in relation to the volcanic arc front. Pyroclastic deposits represented by lapilli-rich meta-tuffs with scattered volcanic bombs denote explosive volcanism of dacitic to rhyolitic composition. Zircon U-Pb data provide magmatic crystallization ages of 595 ± 13 Ma and ca. 584 ± 5 Ma for the volcanic protoliths. The São Tomé Formation mainly includes psammitic to pelitic meta-turbidites with lenses of calcsilicate rocks, indicating relatively distal marine deposits with a maximum depositional age of ca. 594 Ma and a striking arc-related lithochemical signature. The João Pinto Formation mostly includes quartz sandstones and micaceous to feldspathic sandstones, with a maximum sedimentation age of ca. 620 Ma, representing fluvial to continental shelf environments related to the late stages of the Rio Doce arc development. The Rio Doce Group can be correlated with other orogenic supracrustal successions (i.e., parts of the Andrelândia Group) found in the region covered by the Rio Doce arc, connecting the Araçuaí and Ribeira orogens.

Ancient DNA analysis of Indigenous rockfish use on the Pacific Coast: Implications for marine conservation areas and fisheries management

Rockfish (Sebastes spp.) are a common marine fish in nearshore and continental shelf environments in the North Pacific Ocean. They are frequently identified in coastal archaeological sites in western North America; however, the morphological similarity of rockfish species limits conventional zooarchaeological identifications to the genus level. This study applies ancient DNA analysis to 96 archaeological rockfish specimens from four sites on separate islands in an archipelago on western Vancouver Island, British Columbia, Canada. Two of the archaeological sites are located within a marine protected area specifically designed to facilitate the recovery of inshore rockfish populations; two sites are located outside this boundary and remain subject to considerable fishing pressure. Using mitochondrial 16S and control region DNA sequences, we identify at least twelve different rockfish species utilized during the past 2,500 years. Identification of rockfish at closely spaced and contemporaneously occupied sites confirms that a variety of Sebastes species were consistently exploited at each site, with more exposed areas having a higher number of species present. Identification results indicate that four of the twelve species did not occur within the conservation area boundary and, instead, were found in sites where commercial and recreational fishing continues to be permitted. This study demonstrates that ancient DNA identifications of archaeological assemblages can complement and expand perspective on modern day fisheries conservation and management in this National Park Reserve and First Nations ancestral territory.

Generalized ray method for three-dimensional propagation in a penetrable wedge

A generalized ray solution is presented for the three-dimensional (3-D) acoustic field produced by a transient point source in a rigid-bottom, fast-speed fluid-bottom, and fast-speed elastic-bottom wedge modeling a continental shelf environment. The effect of diffraction (scattering) at the wedge apex is ignored in the solution, but nevertheless the solution is exact and complete for the image component of the field, expressed by a sum of partial waves including a pulse emitted from the source plus a finite number of pulses reflected off the wedge boundaries. Time records of the acoustic pressure illustrating 3-D acoustical propagation effects are evaluated at five receivers located at equal radial range in the horizontal from the source, but different orientation to the source, measured by the azimuthal angle assuming values: 0^{circ } (down-slope), 45^{circ } (obliquely down-slope), 90^{circ } (cross-slope), 135^{circ } (obliquely up-slope), and 180^{circ } (up-slope). The arrival times of the spherical and head waves propagating along various paths are evaluated at each receiver for each bottom. It was found that the time interval between the first arrival and the ultimate arrival diminished, and the pulses became more peaked, as the azimuthal angle of the receiver increased. For the rigid-bottom wedge, we have found that all backscattered pulses are significant at the up-slope receiver. For the fluid-bottom wedge, we have found that: at each receiver, the source-pulse is preceded by the ground wave which is much weaker than the water wave; and, at the up-slope receiver, the backscattered pulses are insignificant. For the elastic-bottom wedge, we have found that: at each receiver, the ground-wave-response begins earlier than that in the fluid-bottom wedge, and thus the record separates out into distinct ground-wave and water-wave phases; and, at the up-slope receiver, the backscattered pulses are significant.

Surficial sediments of the Paraná Inner Continental Shelf, Southern Brazil

This study examines surficial sediment distribution patterns, slope and morphological characteristics of the Parana inner continental shelf. Parana inner continental shelf is located on the southern Brazilian continental shelf, a wide and low gradient shelf, in the vicinity of two large unfilled estuaries and the Serra do Mar mountain range. It is characterized as a wave/storm dominated continental shelf with microtidal, semidiurnal regime and a small supply of recent continental sediments. We analyzed grain-size parameters and sediment composition of 875 grab samples and investigated the inner continental shelf morphology and slope through a digital bathymetric model built from regional nautical charts. We combined and evaluated the characteristics of grain-size curves, the spatial distribution of sediments, and the morphology and slope of the inner continental shelf to identify different sedimentary facies. Sediment distribution patterns change from the coast seaward and between three defined sectors along the coastline: south, central and north inner continental shelf. This variation seems to be related to recent sediment sources and to the presence of deposits from ancient coastal plain and inner continental shelf environments formed before the last Holocene sea-level highstand. Sediments with mean diameter of fine to very fine sands prevail at all depths. Deposits of sediments with mean diameter of silt and medium to coarse sands are significant in particular areas, generally associated with steep slopes, troughs, and sand ridges. Possible causes of sedimentary patchiness are the outcropping of medium to coarse sand and the supply of recent estuarine muddy sediments provided by large estuarine complexes to the Parana inner continental shelf. The results obtained contribute to the knowledge about the sediment dynamics of Parana inner continental shelf.

Morphological features of the gorgonian Paramuricea macrospina on the continental shelf and shelf edge (Menorca Channel, Western Mediterranean Sea)

ABSTRACTMorphological variability in gorgonians is frequent and commonly associated with habitat variability, often resulting in segregated morphotypes. Paramuricea macrospina is an endemic Mediterranean gorgonian species found on rocky bottoms between 40 and 160 m depth and has recently been reported as one of the most abundant species in continental shelf and shelf edge environments. Three different chromatic forms of P. macrospina were observed in the Menorca Channel: a yellow form and a light purple form occurring on maërl beds of the continental shelf, and a dark purple form occurring on rocky substrates of the shelf edge. The objective of the present work is to verify if these P. macrospina forms may represent distinct taxonomic units by analysing differences in colony morphology and sclerite size and shape of the three chromatic forms. No significant differences were found in colony shape, suggesting that environmental variability between the continental shelf and the shelf edge is not influential enough to significantly alter colony morphology. Significant differences in sclerite size and shape were found amongst all forms, suggesting that sclerites may be influenced by environmental conditions. However, the co-occurrence of the yellow and light purple forms side by side on the continental shelf may indicate a certain degree of genetic differentiation.

Modelling Fine Sediment Dynamics: Towards a Common Erosion Law for Fine Sand, Mud and Mixtures

This study describes the building of a common erosion law for fine sand and mud, mixed or not, in the case of a typical continental shelf environment, the Bay of Biscay shelf, characterized by slightly energetic conditions and a seabed mainly composed of fine sand and muddy sediments. A 3D realistic hydro-sedimentary model was used to assess the influence of the erosion law setting on sediment dynamics (turbidity, seabed evolution). A pure sand erosion law was applied when the mud fraction in the surficial sediment was lower than a first critical value, and a pure mud erosion law above a second critical value. Both sand and mud erosion laws are formulated similarly, with different parameters (erodibility parameter, critical shear stress and power of the excess shear stress). Several transition trends (linear or exponential) describing variations in these erosion-related parameters between the two critical mud fractions were tested. Suspended sediment concentrations obtained from simulations were compared to measurements taken on the Bay of Biscay shelf with an acoustic profiler over the entire water column. On the one hand, results show that defining an abrupt exponential transition improves model results regarding measurements. On the other hand, they underline the need to define a first critical mud fraction of 10 to 20%, corresponding to a critical clay content of 3–6%, below which pure sand erosion should be prescribed. Both conclusions agree with results of experimental studies reported in the literature mentioning a drastic change in erosion mode above a critical clay content of 2–10% in the mixture. Results also provide evidence for the importance of considering advection in this kind of validation with in situ observations, which is likely to considerably influence both water column and seabed sediment dynamics.

Sources, cycling and export of nitrogen on the Greenland Ice Sheet

Abstract. Fjord and continental shelf environments in the polar regions are host to some of the planet's most productive ecosystems and support economically important fisheries. Their productivity, however, is often critically dependent upon nutrient supply from upstream terrestrial environments delivered via river systems. In glacially fed coastal ecosystems, riverine nutrients are largely sourced from melting snow and ice. The largest and most extensive glacially fed coastal ecosystem in the Arctic is that bordering the Greenland Ice Sheet. The future primary productivity of this ecosystem, however, is uncertain. A potential increase in primary productivity driven by reduced sea ice extent and associated increased light levels may be curtailed by insufficient nutrient supply, and specifically nitrogen. Research on small valley glaciers indicates that glaciers are important sources of nitrogen to downstream environments. However, no data exist from ice sheet systems such as Greenland. Time series of nitrogen concentrations in runoff are documented from a large Greenland glacier, demonstrating seasonally elevated fluxes to the ocean. Fluxes are highest in mid-summer, when nitrogen limitation is commonly reported in coastal waters. It is estimated that approximately half of the glacially exported nitrogen is sourced from microbial activity within glacial sediments at the surface and bed of the ice sheet, doubling nitrogen fluxes in runoff. Summer dissolved inorganic nitrogen fluxes from the Greenland Ice Sheet (30–40 Gg) are a similar order of magnitude to those from a large Arctic river (Holmes et al., 2012). Nitrogen yields from the ice sheet (236 kg TDN km−2 a−1), however, are approximately double those from Arctic riverine catchments. We assert that this ice sheet nitrogen subsidy to Arctic coastal ecosystems may be important for understanding coastal biodiversity, productivity and fisheries and should be considered in future biogeochemical modelling studies of coastal marine productivity in the Arctic regions.

Laboratory measurements of a phase shift in the reflection from a water-mud interface due to water column variability

Harbor basins and certain continental shelf environments experience significant variability due to tides, surface heating, currents, and other oceanographic processes. Dynamic processes such as these can cause substantial sound speed and density fluctuations in the water column over short time periods, which are often unknown to sonar users. Such variations are not critical over high-velocity bottoms such as sand, but can produce significant changes in how sound interacts with low-velocity fine-grained sediments. High levels of variability and a low-velocity bottom, such as mud, can therefore present challenges in applications including mine detection, port protection and shallow water sonar. At certain penetration angles, temporal variations in the water-to-sediment sound speed ratio can cause 180 degree phase shifts in the reflected field, which in turn produces waveguide propagation and target scattering field variability. To begin to understand these processes, laboratory measurements of plane wave reflection were obtained from a water-mud interface while varying salinity and temperature of the water. Results indicate that the dynamic nature of sound speed ratio at the ocean bottom can cause significant effects in shallow water environments with muddy bottoms. [Work supported by ONR.]