Scotian Slope Research Articles

Dispersal and Distribution of Thermophilic Endospores in Deep-Sea Ecosystems

The distance-decay relationship is a central concept in biogeography and spatial biodiversity, describing how two distinct entities decrease in similarity as the distance between them increases. The decay of community similarity with geographical distance is driven by multiple factors, such as gene drift, environmental selection, and the accumulation of mutations over time. While the distance-decay relationship has been recognized for several decades, there are certain circumstances where the biogeographical patterns of certain species and communities cannot be predicted by this relationship. The example addressed in this project is the case of thermophilic endospore-forming bacteria found in permanently cold deep ocean sediments. Thermophilic endospores (thermospores) are routinely found on the deep ocean floor, a permanently cold environment that does not support their metabolic activity (Hubert et al. 2009). Thermospores are metabolically dormant states developed by some thermophilic bacteria having optimal growth temperatures between 40°C and 70°C. There is evidence that these heat-loving bacteria originate from the deep subsurface and are transported upward to the deep ocean via geological features of the oceanic crust including geofluid fluxes in high-temperature axial systems and natural hydrocarbon seeps (Gittins et al. 2022). Due to the ability of endospores to stay viable for thousands of years and resist a wide range of physicochemical stressors, they can disperse over long distances while remaining unaffected by changing factors such as selection, drift, or mutation (Gittins et al. 2022; Fig. 1). Consequently, these thermospores have the potential to challenge the distance-decay relationship and exhibit unique biogeographical patterns. This work quantifies endospores in sediment cores at and around hydrocarbon seeps using cores from deep-sea Scotian Slope sediments. Given the challenge of quantifying specific groups of endospores (e.g., thermophiles but not mesophiles), high-temperature germination assays that allow tracking of an exponential increase in signal as spores in marine sediment samples germinate and grow can be used. Distinct exponential increases can be attributed to different populations of germinated thermospores in the post-germination growth phase and are being monitored through measurements of sulfate reduction rates (using radiolabelled 35SO4), strain-specific quantitative PCR (qPCR), and fluorescence in-situ hybridization (FISH) (Rezende et al. 2017). These measurements revealed growth dynamics enabling an estimation of the initial cell numbers using exponential functions. These approaches are being applied to samples obtained via push coring using a remotely operated vehicle at different distances from deep sea hydrocarbon seeps, to test for the presence of abundance gradients of different thermospores. Current results already show a difference in thermospore abundance between sites at different distances from the seep. Quantitative tracking of the dispersal of thermospores by oceanic currents in the deep sea provides an excellent opportunity to investigate their distribution and potential to colonize more remote habitats, improving the understanding of microbial biodiversity and biogeography on our planet. By investigating thermospores and their interactions with the geophysical features of the deep sea and deep subsurface, this study aims to challenge the canonical idea that speciation occurs with distance and presents new perspectives on the theme of microbial biogeography. The evidence supporting the subsurface origin of thermophilic endospores, along with the proposed model for their dispersal and distribution, will contribute toward improving an integrated understanding of ecology and geology and their intersection in the microbial realm.

Elevated bacterial endospores associated with thermogenic hydrocarbon seeps in deep sea sediments.

Introduction and approach Bacterial endospore distributions in marine sediments are influenced by geological conduits providing routes for subsurface to surface microbial dispersal. To examine this phenomenon in more detail, endospore abundance was determined by quantifying the biomarker 2,6-pyridine dicarboxylic acid (dipicolinic acid or DPA) in 16 deep sea sediment cores from hydrocarbon prospective areas in the NW Atlantic Ocean. DPA is specific to endospore-forming bacteria from the phylum Firmicutes and constitutes a significant percentage of endospore dry weight. DPA is therefore a potential biomarker for sediment dwelling endospores and geological conduits. Piston cores (10), gravity cores (3) and box cores (3) were collected during two expeditions to the Scotian Slope in the NW Atlantic Ocean off the east coast of Canada aboard the CCGS Hudson in 2016 and 2018 (Campbell (2016), Campbell and Normandeau (2018), Campbell and MacDonald. (2016)). Sampling sites were 1970 to 2791 m water depth, with piston cores (n=3) ranging from 344 to 953 cmbsf and gravity cores (n=10) ranging from 43 to 739 cmbsf, box coring captured the top 25 cmbsf. To address the efficacy of DPA biomarker analysis as a tool for hydrocarbon seep location we established a modified Tb3+ chelation method (Lomstein and Jørgensen (2012), Rattray (2021)). Sediment samples were extracted using acid hydrolysis, chelated with Tb3+ and analysed using HPLC fluorescence, measuring at 270 nm emission and 545 nm excitation. DPA concentrations were converted to Endospore numbers were calculated using 2.24 fmol DPA per endospore (Fichtel 2007), a conversion factor routinely used in other studies (Braun 2017, Gittins 2022, Heuer 2020, Lomstein 2012, Rattray 2022, Wörmer 2019, Lomstein and Jørgensen 2012). DPA concentrations were compared with measurements of over 250 different gaseous and liquid hydrocarbon compounds used to assess for the presence of thermogenic hydrocarbons. Results and discussion Samples and locations were assessed as being thermogenic hydrocarbon gas positive (stations 16-41, 18-07) or thermogenic hydrocarbon negative based on the abundance of C1-C5 hydrocarbons in sediments sampled from the same cores. Station 18-14 contained hydrocarbons from biogenic origin. Station 18-06 is the only site with higher endospore abundance but that was determined to be hydrocarbon negative. Deep water Scotian Slope sediment cores show high endospore abundance correlates with thermogenic hydrocarbon seeps (Fig. 1). Cores from locations lacking evidence for thermogenic hydrocarbons generally contained significantly lower endospore abundances, with the notable exception of site 18-06. This potential paleoenvironmental hydrocarbon seep site highlights the utility of a DPA proxy for potentially identifying ancient hydrocarbon seeps and investigating past geological systems. The association of high endospore abundances with thermogenic hydrocarbons and the quantity of gas expulsion points to an interesting new biological tool for understanding hydrocarbon seepage in the deep biosphere, based on DPA assays in marine sediments.

Multi-year seabed environmental baseline in deep-sea offshore oil prospective areas established using microbial biodiversity

Microorganisms are the ocean's first responders to marine pollution events, yet baseline studies rarely focus on microbial communities. Temporal and spatial microbial biodiversity baselines were established using bacterial 16S rRNA gene amplicon sequencing of seafloor sediments in a deep-water oil prospective area along the Scotian Slope off Canada's east coast sampled during 2015–2018. Bacterial diversity was generally similar in space and time, with members of the family Woeseiaceae detected consistently in >1 % relative abundance, similar to seabed sediments in other parts of the world. Anomalous biodiversity results at one site featured lower Woeseiaceae as well as higher levels of bacterial groups specifically associated with cold seeps such as Aminicenantes. This was unexpected given that site selection was based on sediment geochemistry not revealing any petroleum hydrocarbons in these locations. This finding highlights the sensitivity and specificity of microbial DNA sequencing in environmental monitoring. Microbiome assessments like this one represent an important strategy for incorporating microbial biodiversity as a new and useful metric for establishing robust environmental baselines that are necessary for understanding ecosystem responses to marine pollution.

Bacterial anomalies associated with deep sea hydrocarbon seepage along the Scotian Slope

Deep sea hydrocarbon seep detection relies predominantly on geochemical analyses of seabed marine sediment cores to identify the presence of gas or oil. The presence of seeping hydrocarbons in these locations alters local redox stratification and microbial community structure. Cultivation-independent biodiversity surveys of microbial communities can offer a proxy for seeping hydrocarbons, but this strategy has not been extensively investigated in deep water settings. In this study, 16S rRNA gene sequencing of bacterial communities was performed on sediment cores obtained in >2500 m water depth at 43 different locations in the Northwest Atlantic Ocean. Cores extending down to 10 m below seafloor (mbsf) were assessed for gas composition, gas isotopes and liquid hydrocarbons. Over 650 bacterial biodiversity libraries were constructed from different sediment depths at these locations. Sites with strong evidence for the presence of thermogenic or biogenic hydrocarbon seepage were reflected in bacterial population analyses that revealed significant differences between hydrocarbon seep and non-seep locations. Specific bacterial indicators were associated with different sediment depth intervals including Caldatribacteriota and Campilobacterota OTUs observed in high relative sequence abundance at 20-50 cmbsf in hydrocarbon seep sediments. These groups were differentially abundant between sites with thermogenic and biogenic hydrocarbon seepage.

Are submarine landslides an underestimated hazard on the western North Atlantic passive margin?

AbstractThe western North Atlantic passive margin is considered relatively stable, with few slope instabilities recognized during the Holocene. However, new multibeam bathymetry mapping and sediment core acquisition off eastern Canada indicate that previously unidentified, large, submarine landslide events occurred during the Late Holocene, between 4 and 1.5 ka. The recognition of these new gravitational events, in addition to the well-known C.E. 1929 Grand Banks earthquake-induced landslide, indicates that approximately one large landslide event per 1000 years has occurred offshore eastern Canada within the past 4000 years, a much shorter recurrence interval than hitherto reported. This Late Holocene recurrence rate is also similar to active margins around the world and is likely due to the under-consolidation and resultant instability of Scotian Slope sediments attributable to high glacial sedimentation rates. The discovery of these new Late Holocene landslides was made possible through detailed examination of cores recovered from the lower slope. These results demonstrate that submarine landslide hazard has been underestimated on the western North Atlantic margin—home to significant submarine infrastructure and proximal to a large coastal population.

Drivers of Regional Bacterial Community Structure and Diversity in the Northwest Atlantic Ocean.

The fundamental role of bacteria in global biogeochemical cycles warrants a thorough understanding of the factors controlling bacterial community structure. In this study, the integrated effect of seasonal differences and spatial distribution on bacterial community structure and diversity were investigated at the regional scale. We conducted a comprehensive bacterial survey, with 451 samples of the Scotian Shelf sector of the Northwest Atlantic Ocean during spring and fall of 2014 and 2016, to analyze the effects of physicochemical gradients on bacterial community structure. Throughout the region, Pelagibacteraceae and Rhodobacteraceae were the most common in the free-living fraction, while Flavobacteriia and Deltaproteobacteria were more abundant in the particle-associated fraction. Overall, there was strong covariation of the microbial community diversity from the two size fractions. This relationship existed despite the statistically significant difference in community structure between the free-living and particle-associated size fractions. In both size fractions, distribution patterns of bacterial taxa, and species within taxa, displayed temporal and spatial preferences. Distinct bacterial assemblages specific to season and depth in the water column were identified. These distinct assemblages, consistent for both 2014 and 2016, suggested replicable patterns in microbial communities for spring and fall in this region. Over all sites, temperature and oxygen values were highly correlated with community similarity, and salinity and oxygen values were the most strongly positively- and negatively correlated with alpha diversity, respectively. However, the strengths of these correlations depended on the depth and season sampled. The bathymetry of the Scotian Shelf, the abrupt shelf break to the Scotian Slope and the major ocean currents dominating in the region led to the formation of distinct on-shelf and off-shelf bacterial communities both in spring and fall. The highest species richness was observed at the shelf break, where water masses from the two major currents meet. Our study establishes the baseline for assessing future changes in the bacterial community of the Scotian Shelf waters, a rapidly changing sector of the Atlantic Ocean.

Observation of internal tides and sediment transport processes at the head of Logan Canyon on central Scotian Slope, eastern Canada

Abstract An instrumented seabed lander was deployed in the head of Logan Canyon in ~280 m water depth in 2005 to obtain the first observations of near-bed hydrodynamics and sediment transport processes in a shelf incising canyon on the glaciated Scotian Slope under strong influence of storms and ocean currents. The seafloor at the head of Logan Canyon is quite dynamic. Hourly mean current speeds can reach 35 cm/s while the maximum instantaneous currents are up to 70 cm/s. Under the forcing of semi-diurnal tides, the bottom currents are dominated by high frequency alternating up- and down-canyon current pulses of internal tides that cause periodic bedload and suspended load sediment transport. The current and sediment transport processes of the internal tides likely extend down to 800–1000 m depths along the canyon to contribute to the observed distribution of Holocene fine sand in the head and talweg of Logon Canyon. The high-frequency current and sediment concentration data show that the temporal changes of the current pulses are well correlated with the changes of temperature and sediment resuspension to demonstrate strong asymmetric patterns. The amplified currents of the internal tides cause the fine sands at the head of Logan Canyon to be transported as bedload in 2.1% of the time and in suspended-load in 1.2% of the time. Suspended-load transport is dominant over bedload transport. The maximum suspended sediment concentrations may reach ~100 mg/l in strong current events. As down-canyon transport is generally stronger and more frequent and the residual current is down-canyon, the net transport of sediment is in the down-canyon direction. Summer storms do not seem to directly impact the bottom currents at the head of Logan Canyon. However, sediments resuspended on the shelf during storms can be advected down slope into the canyon to cause events of substantially high sediment concentration enhancing sediment supply and transport at the head of Logan Canyon.

Assessing Submarine Slope Stability through Deterministic and Probabilistic Approaches: A Case Study on the West-Central Scotia Slope

A simplified geostatistical approach was adopted to assess the effect of spatial variability of soil properties on slope stability analysis in order to understand continental margin geologic processes and potential geohazards for an area of the central Scotian Slope, offshore Nova Scotia, Canada. The analyses are conducted on piston core samples, thus are restricted to ~12 m sub-seabed; however, the approach provides insight into the general effects of spatial and temporal variability. Data processing using geostatistics and assessment of spatial correlation are used to characterize the current dataset. A deterministic assessment was performed for both non-spatially averaged and spatially averaged core sections. The results indicate that the estimated factor of safety increased by about 30% when spatially averaged values were used. A probabilistic model is introduced to assess reliability of the slope. The approach makes use of estimates of both the mean and variance of input random variables (e.g., Su and γb). The model uses an exact probabilistic formulation for the total stress stability analysis and a Taylor series approximation for the effective stress stability analysis. In both cases, the mean and variance of the factor of safety are computed, leading to estimates of failure probability. The results suggest that the deterministic analysis is conservative with respect to slope reliability, although they do not lead to an estimate of the probability of failure. While these results indicate sediment instability is largely unlikely under static conditions, the reality is that many examples of submarine slope failure are observed in the geologic record. These results suggest that cyclic loading (earthquakes) or pre-conditioning factors (elevation of pore pressures) are critical for slope instability on the Scotian Slope.

Rethinking the Use of Seabed Sediment Temperature Profiles to Trace Submarine Groundwater Flow

AbstractSubmarine groundwater fluxes across the seafloor facilitate important hydrological and biogeochemical exchanges between oceans and seabed sediment, yet few studies have investigated spatially distributed groundwater fluxes in deep‐ocean environments such as continental slopes. Heat has been previously applied as a submarine groundwater tracer using an analytical solution to a heat flow equation assuming steady state conditions and homogeneous thermal conductivity. These assumptions are often violated in shallow seabeds due to ocean bottom temperature changes or sediment property variations. Here heat tracing analysis techniques recently developed for terrestrial settings are applied in concert to examine the influences of groundwater flow, ocean temperature changes, and seabed thermal conductivity variations on deep‐ocean sediment temperature profiles. Temperature observations from the sediment and bottom ocean water on the Scotian Slope off eastern Canada are used to demonstrate how simple thermal methods for tracing groundwater can be employed if more comprehensive techniques indicate that the simplifying assumptions are valid. The spatial distribution of the inferred groundwater fluxes on the slope suggests a downward groundwater flow system with recharge occurring over the upper‐middle slope and discharge on the lower slope. We speculate that the downward groundwater flow inferred on the Scotian Slope is due to density‐driven processes arising from underlying salt domes, in contrast with upward slope systems driven by geothermal convection. Improvements in the design of future submarine hydrogeological studies are proposed for thermal data collection and groundwater flow analysis, including new equations that quantify the minimum detectable flux magnitude for a given sensor accuracy and profile length.

Satellite remote-sensing observations for definitions of areas for marine conservation: Case study of the Scotian Slope, Eastern Canada

Characterizing offshore areas for marine conservation faces impediment from logistics and costs of in situ data collection. This study leveraged the high spatial and temporal coverage provided by remote-sensing ocean colour products, and the concept of biogeographical classification to delineate areas in the pelagic environment with application to marine conservation. Satellite-derived and modelled biogeophysical data were used to delineate an area of biological and physical homogeneity spatially consistent with the static boundaries of the Scotian Slope Ecologically and Biologically Significant Area (EBSA), off eastern Canada. We used iterative cluster analysis applied to an archive (2004–2014) that consisted of remotely-sensed Chlorophyll a, sea-surface temperature, and primary production derived from the Moderate resolution Imaging Spectroradiometer (MODIS), and simulated mixed layer depth to define “dynamic” boundaries of the EBSA at bi-weekly, seasonal and annual resolutions. The final cluster extended further east than the static EBSA in summer and fall, indicating that characteristics of the static EBSA environment persist beyond the current boundaries in these seasons. In winter and spring, the final areas derived by our analysis was smaller than the static EBSA, but again showed extension beyond the current eastern boundary. Both ice and cloud cover affecting remotely-sensed data and the extent of water column mixing were important in determining the size of the final cluster. The dynamic cluster east of the original static boundary incorporated an area of lower Chlorophyll a and water column primary production in the spring, but higher values in the autumn relative to the static area. Overall physical and biological characteristics of the static and dynamic EBSA considered in this research were similar within and across years. This methodology incorporates ocean-colour data and modelled estimates of multiple biological and physical characteristics to objectively refine areas of ecological interest at a spatial scale relevant for the management of marine conservation areas.

Singing through winter nights: Seasonal and diel occurrence of humpback whale (Megaptera novaeangliae) calls in and around the Gully MPA, offshore eastern Canada

AbstractHumpback whale use of areas off eastern Canada is poorly understood, a knowledge gap that could impact future conservation efforts. We describe the acoustic occurrence of humpback whales in and around the Gully Marine Protected Area (MPA), an eastern Scotian Shelf submarine canyon. Near‐continuous acoustic recordings sampling at 16 kHz were collected from the MPA and nearby slope areas from October 2012 to September 2014 using near‐bottom recorders. In an offshore region where humpbacks were thought to be rare, we observed calls from October to June with a peak in song and nonsong calls in December and January. This suggests that some individuals occur in Canadian waters in winter and the Gully region may be a North Atlantic humpback whale migratory corridor. Calls were predominantly songs indicating potential mating activities. Song and nonsong calls occurred more at sunset and during hours of darkness than during daylight. This study improves our understanding of the seasonal occurrence of humpback whales on the Scotian Slope and, more specifically, their use of an offshore protected area.

The thermal structure of the central Nova Scotia Slope (eastern Canada): seafloor heat flow and thermal maturation models

The thermal history and maturation potential of the central Scotian Slope is constrained using a combination of 47 recently acquired seafloor heat flow measurements, two-dimensional (2D) seismic reflection data, available well data, simple lithospheric rift models, and thermal and petroleum systems modelling. Consistent heat flow values of 41–46 mW·m−2 were measured seaward of the salt diapiric province and across the slope away from the influence of salt structures. Significant but highly variable increases in heat flow were measured for stations overlying salt diapiric structures, reaching values upwards of 72 mW·m−2. Simple models of conductive heat transfer with static salt geometries constrained from reflection profiles indicate that two of the four models fit the data, whereas two indicate much higher values suggestive of additional, convective effects. Dynamic 2D thermal models were developed to incorporate the effects of lithospheric rifting, crustal stretching, and radiogenic heat production in the sediment and basement. These models help constrain the hydrocarbon maturation potential of the central Scotian Slope, where deep borehole data are lacking. Our results suggest that a potential Late Jurassic source rock interval rests primarily within the late oil window and that salt structures act primarily to reduce maturation in the adjacent deep sediment layers.

Is there a north‐south phase shift in the surface Labrador Current transport on the interannual‐to‐decadal scale?

The transport of the Labrador Current has substantial interannual‐to‐decadal variability that may significantly impact the ocean climate and marine ecosystem off Atlantic Canada and Northeast United States. Nevertheless the alongshore coherence of the Labrador Current on the interannual‐to‐decadal scales has been a long standing question. Here we have used satellite altimetry data to study interannual‐to‐decadal variability of the unit‐depth surface Labrador Current in the past two decades, in the context of the subpolar gyre of the North Atlantic. The study for the first time reveals that the transport over the Labrador and northeastern Newfoundland Slope is out of phase with that over the Scotian Slope. The surface Labrador Current transport is positively and negatively correlated with the winter North Atlantic Oscillation (NAO) index (with the transport lagging by about 1 year) in the north (the Labrador and northeastern Newfoundland Slope) and in the south (the Scotian Slope), respectively. In addition, the surface Labrador Current transport declines over the Labrador Slope and northeastern Newfoundland Slope and increases over the Scotian Slope. The north‐south phase shift suggests two distinct regimes, which warrants further studies of underlying dynamic mechanisms and may have important implications for regional ecosystems and climate.

An Intercomparison of Acoustic Current Meter Measurements in Low to Moderate Flow Regions

Abstract Instrumented, subsurface moorings deployed in the Scotian shelf and slope regions of the North Atlantic provide data in low to moderate flows for a current meter intercomparison. The primary instruments being evaluated are two acoustic Doppler single-point current meters, the Aanderaa Seaguard (SG) and the Teledyne RD Instruments (RDI) Doppler volume sampler (DVS), which are compared against older-generation single-point current meters and acoustic Doppler current profilers. Analysis showed that the root-mean-square (RMS) of the speed difference between concurrent instrument combinations was in the range of 1.0–1.6 cm s−1, which is about 3%–6% of the upper limit of speeds observed at these sites. Best agreement was between the DVS and the nearby Seaguard (RMS speed difference of 1.2 cm s−1), during the shelf deployment, and between the Aanderaa recording current meter 11 (RCM11) and the nearby Seaguard (1.0 cm s−1), during the slope deployment. Speed differences larger than 4 cm s−1 were uncommon, occurring less than 1.5% of the time. Slight overspeeding of one of the Seaguards is traced to an intentional alteration in the instruments' sampling strategy. The DVS compass had a slight meandering tendency that caused it to routinely disagree with other instruments by as much as 15° for hours at a time. The disagreement was random in direction and had no impact on most of the comparisons, but it did produce a 15% smaller magnitude of mean current. Subsequent to this field test, Teledyne RDI redesigned the DVS and replaced the compass with a new sensor.

A reexamination of Pleistocene tunnel valley distribution on the central Scotian Shelf

Tunnel valleys are an erosional channel characterized by anastomosing, steep-sided channel systems formed by subglacial, confined meltwater flow. Our study uses two-dimensional (2D) industry reflection seismic on the Scotian Shelf near Sable Island to better constrain the geometry of tunnel valleys in the area. Here, U- to V-shaped buried channels north and west of Sable Island extensively branch, reconnect, and meander. They average 2–5 km wide and 150–700 m deep (below sea level) and have a dominantly north–south orientation. Immediately to the south and southwest of Sable Island, the channels become narrower (1–1.5 km), shallower (300–400 m maximum) and more widely spaced. Multiple tunnel valley generations may be present, but could not be separately mapped. There is a notable gap of 20–30 km between the ends of the detectable tunnel valleys and the shelf edge south of Sable Island. An exception is erosive features at the shelf edge at the head of Logan Canyon, but these channels are disconnected from the tunnel valley network further north and may not be tunnel valleys. The reason for the gap south of Sable Island is not clear; however, we do not believe it is due to data coverage or resolution issues. If the tunnel valleys genuinely end south of Sable Island, then the gap between them and the shelf edge means that between The Gully and Logan Canyon these subglacial meltwater channels did not empty directly at the shelf edge. This implies direct meltwater and sediment delivery by tunnel valleys to the Scotian Slope during the Pleistocene may have been limited in this area.

Variations in salt expulsion style within the Sable Canopy Complex, central Scotian margin1This article is one of a series of papers published in this CJES Special Issue on the theme ofMesozoic–Cenozoic geology of the Scotian Basin.

There are two allochthonous salt canopies within the central Scotian margin, one on the shelf and another in deeper waters on the Scotian slope. The Sable Shelf Canopy trends southwest–northeast and covers an area approximately 650 km2. Salt thickness within this canopy is generally <1000 m. Reloading of the salt has welded out the margins of the canopy, significantly reducing its extent. Salt tectonics related to this canopy produced many of the exploration targets drilled on the shelf. The Sable Slope Canopy is 50 km basinward on the Scotian slope. This canopy is significantly larger, covering 12 700 km2, and the allochthonous salt is as thick as 3500 m. Although the two canopies are separated by a significant distance, a common detachment surface connects them in multiple locations. Extensional systems detach in the Sable Shelf Canopy, displace strata downdip, and expel salt into the Sable Slope Canopy. The outer shelf and slope region separating these two canopies is dominated by detachment systems. One term, the Sable Canopy Complex, was developed to describe both salt canopies and the zone of detachment that separates them. Interpretation of two-dimensional and three-dimensional seismic data throughout the Sable Canopy Complex has resulted in a detailed allochthonous salt map and all related components including welded systems, salt stocks, faults, sutures, and minibasins. Four distinct expulsion styles are observed to contribute volumes of salt to the canopies: salt-based detachments, counterregional systems, hybrids, and salt stocks. Potential explanations for the variations in expulsion style range from varying volumes of sediment input to paleogeographic positions on the margin to salt sheet length.

2D waveform tomography applied to long-streamer MCS data from the Scotian Slope

Detailed velocity models of the earth’s subsurface can be obtained through waveform tomography. The accuracy of the long-wavelength component of such velocity models, which is the background velocity field, is particularly sensitive to modeling low-frequency refracted waves that have long paths through target structures. Thus, field examples primarily have focused on the analysis of long-offset wide-angle data sets collected using autonomous receivers, in which refractions arrive at significantly earlier times than reflections. Modern marine acquisition with long streamers now offers the ability to record refracted waves with high spatial density and uniform source, both in shallow and deep water. We used 2D multichannel seismic (MCS) data acquired with a 9-km-long streamer over the Scotian Slope in water depths of ∼1600 m. The refracted arrivals, although mostly restricted to far-offset receivers, provided sufficient information to successfully invert for a high-resolution background velocity field. Using a frequency-domain acoustic code over frequencies from 8 to 24 Hz on two crossing profiles, we found that the limited refracted waves can constrain the velocity field above the depth of the turning waves (∼1.5 km below seafloor). Several important features were resolved by the waveform velocity model that were not present in the initial traveltime model. In particular, a high-velocity layer at 300 m below the seafloor, interpreted as gas hydrates, was imaged even where a characteristic bottom-simulating reflector was not visible. At 750-m depth, a strong velocity increase of 300 m/s existed beneath a gently dipping reflector along which low-velocity zones, possibly related to gas, were present. Velocity models were highly consistent at the crossing point between the two profiles. The depth extent of the MCS waveform tomography constrained by refractions could be extended by even longer streamers (e.g., 15 km) or by joint inversion with data from ocean-bottom seismographs.

Phytoplankton phenology on the Scotian Shelf

AbstractZhai, L., Platt, T., Tang, C., Sathyendranath, S., and Hernández Walls, R. 2011. Phytoplankton phenology on the Scotian Shelf. – ICES Journal of Marine Science, 68: . The impact of physical forcing on seasonal and interannual phytoplankton dynamics was examined using SeaWiFS chlorophyll, AVHRR sea surface temperature (SST), nitrate, and other hydrographic measurements for the Scotian Shelf and Scotian Slope. The spring bloom was characterized by a shifted Gaussian function fitted to seasonal chlorophyll time-series. The background chlorophyll (a constant term in the Gaussian function) is a joint property of the stratification and bio-optics of the mixed layer. Rapid shoaling of the mixed-layer depth in spring promoted the early spring bloom on the middle Scotian Shelf and Slope, triggered when averaged light in the mixed layer reached 15 W m−2. The duration of the spring bloom was prolonged in slope water, resulting in a discontinuity in duration between the shelf and slope water masses. The position of the latitudinal discontinuity in duration was correlated with that of the shelf–slope front in SST. The amplitude of the spring bloom was correlated with the nitrate inventory in the surface layer at the end of winter. The rate of decrease (increase) in chlorophyll after the spring bloom was related to the depletion (resupply) of nitrate in summer. The position of the shelf–slope front influenced the interannual variability of bloom characteristics.

Tidal and non-tidal sea level variability along the coastal Nova Scotia from satellite altimetry

TOPEX/Poseidon (T/P) sea level data are used to investigate tidal and non-tidal sea level variability over the Scotian Shelf. Eight major diurnal and semi-diurnal tidal constituents are derived from the September 1992 to December 2001 T/P data. The present altimetric tides agree generally with in situ observations and show an error reduction of over 30° in phase over Han et al.’s [Han, G., Ikeda, M. and Smith, P.C., 1996, Oceanic tides on the Newfoundland and Scotian Shelves from TOPEX/POSEIDON altimetry. Atmosphere–Ocean, 34, pp. 589–604] results for the diurnal tides. De-tided T/P sea level data are used to provide geo-referenced seasonal-mean sea surface topography during the period September 1992 to July 2002. The T/P data are explored to estimate seasonal-mean sea level along the Nova Scotia coast. Comparisons with sea level measurements from coastal tide gauges are carried out to evaluate the potential of altimetry data for coastal sea level monitoring. Seasonally the sea level is highest along the coast in late autumn/early winter (associated with the peak of the Nova Scotian Current), and over the Scotian Slope in summer. On the interannual scale it was low in the early 1990s, high in the mid 1990s, and close to normal in the late 1990s and early 2000s.

Late Quaternary sedimentation on the southwestern Scotian Slope, eastern Canada: relationship to glaciationGeological Survey of Canada Contribution 20070176.

The sedimentary record on continental slopes has the potential to preserve a record of glacial retreat on the adjacent continental shelf. The glacial history of the southwestern part of the Scotian Shelf is poorly known. Air-gun and high-resolution sparker profiles and numerous sediment cores up to 10 m long have been used to determine the character of sedimentation on the southwestern Scotian Slope since the last glacial maximum (LGM). Seismic-reflection profiles show that glacial till was deposited at shallow depths on the upper continental slope, and correlation to dated piston cores farther downslope show that this till dates from the LGM. Slope sedimentation at this time was dominated by local ice and deposited as plume fallout and turbidites. Progressively increasing importance of red-brown sediment derived from glacial supply to Laurentian Channel indicates retreat of ice from the shelf edge and diminishing supply of proglacial sediment from the calving embayment in the mid-Scotian Shelf. With the termination of distal proglacial sediment supply, the sedimentation rate diminished rapidly and hemipelagic sedimentation prevailed through the Holocene.