We investigate the velocity field of the Larsen C ice shelf, Antarctic Peninsula, over the periods 2002–2006 and 2006–2009 based on repeat optical satellite data. The velocity field of the entire ice ...shelf is measured using repeat low resolution MODIS data (250 m spatial resolution). The measurements are validated for two ice shelf sections against repeat medium resolution Landsat 7 ETM + pan data (15 m spatial resolution). Horizontal surface velocities are obtained through image matching using both orientation correlation operated in the frequency domain and normalized crosscorrelation operated in the spatial domain, and the two methods compared. The uncertainty in the displacement measurements turns out to be about one fourth of the pixel size for the MODIS derived data, and about one pixel for the Landsat derived data. The difference between MODIS and Landsat based speeds is −15.4 m a−1 and 13.0 m a−1, respectively, for the first period for the two different validation sections on the ice shelf, and −26.7 m a−1 and 27.9 m a−1 for the second period for the same sections. This leads us to conclude that repeat MODIS images are well suited to measure ice shelf velocity fields and monitor their changes over time. Orientation correlation seems better suited for this purpose because it produces fewer mismatches, is able to match images with regular noise and data voids, and is faster. Since it can match images with regular data voids it is possible to match Landsat 7 ETM+ images even after the 2003 failure of the Scan Line Corrector (SLC off) that leaves significant image stripes with no data. Image matching based on the original 12-bit radiometric resolution MODIS data produced slightly better results than using the 8-bit version of the same images. Streamline interpolation from the obtained surface velocity field on Larsen C indicates ice travel times of up to 450 to 550 years between the inland boundary and the ice shelf edge. In a second step of the study we test our method successfully on 10 other ice shelves around Antarctica demonstrating that the approach presented could in fact be used for large scale monitoring of ice shelf dynamics.
We have constructed 23‐year (1994–2016) time series of Antarctic Peninsula (AP) ice‐shelf height change using data from four satellite radar altimeters (ERS‐1, ERS‐2, Envisat, and CryoSat‐2). ...Combining these time series with output from atmospheric and firn models, we partitioned the total height‐change signal into contributions from varying surface mass balance, firn state, ice dynamics, and basal mass balance. On the Bellingshausen coast of the AP, ice shelves lost 84 ± 34 Gt a−1 to basal melting, compared to contributions of 50 ± 7 Gt a−1 from surface mass balance and ice dynamics. Net basal melting on the Weddell coast was 51 ± 71 Gt a−1. Recent changes in ice‐shelf height include increases over major AP ice shelves driven by changes in firn state. Basal melt rates near Bawden Ice Rise, a major pinning point of Larsen C Ice Shelf, showed large increases, potentially leading to substantial loss of buttressing if sustained.
Plain Language Summary
Antarctica's ice shelves regulate the flow of grounded ice into the ocean, and enhancing our processes that control how ice shelves respond to atmospheric and oceanic drivers will ultimately improve estimates of sea level change. We have generated a 23‐year record of ice‐shelf height changes for the Antarctic Peninsula region using four satellite altimeters. We focussed our analysis on the height variability rather than just the long‐term trends so that we can better separate the contributions of the ocean and the atmosphere. Most ice shelves on the Peninsula showed height increases since 2009, at least temporarily reversing previously reported trends of declining height. At Larsen C Ice Shelf on the eastern coast of the Peninsula, this reversal was caused by reduced summertime surface melting. The resulting increase in surface‐snow air content may reduce the susceptibility of this ice shelf to the surface‐melt‐driven collapse previously seen on nearby ice shelves. We also found large variations in how fast the ocean is eroding the base of this ice shelf near a pinning point crucial to its stability. On the western coast, while their surface heights have also recently increased, the ice shelves have continued to lose mass from excess ocean melting.
Key Points
New, 23‐year (1994–2016) records of Antarctic Peninsula ice shelf heights reveal recent increases, reversing previously reported declines
Spatial and temporal variations of ice shelf height across the Antarctic Peninsula are linked to specific oceanic and atmospheric forcings
Temporal variability of basal melt rates under Larsen C Ice Shelf provides insight into processes that will lead to changes in buttressing
Modern shelf morphology is the result of the interplay between short and long term sedimentary processes. The relation between rates of sediment supply/carbonate growth and accommodation space ...creation will not only control coastal transgression and regression, but will also define the shelf sedimentary regimes acting to shape the seabed. Herein, shelf morphology and sedimentology are investigated in order to discuss how these characteristics can be representative of distinct sedimentary regimes. The study area is the eastern Brazilian shelf where coastal transgression and regression coexist with the most important coral reef system of the South Atlantic. A compilation of existing published and unpublished data was carried out in order to produce morphological and faciological maps and compare the mapped features with high-resolution seismic and sonographic data. The results show three major regions or morphological compartments: Abrolhos Shelf, Doce River Shelf and the Paleovalleys Shelf. In terms of shelf sedimentary domain, rhodolith beds predominate over the outer shelf along the entire area, coralline reefs are present along the northern Abrolhos inner shelf and a significant terrigenous mud deposit is observed associated to the Doce River adjacent inner shelf beds. The rest of the shelf is composed by bioclastic or terrigenous mud sand and gravel. Terrigenous sedimentation is always restricted to the shoreface or inner shelf shallower areas and carbonate sands and gravels are predominant elsewhere. The Abrolhos shelf shows two distinct sectors; the northern area is a typical mixed sediment environment that has a supply regime along the coast/shoreface, mainly due to longshore transport and a carbonate regime along the inner and outer shelf. The southern shelf morphology and sedimentation are controlled by the antecedent topography and is typically a accommodation regime shelf with associated rhodolith beds. The Doce river shelf is a supply regime environment with the formation of a 5–8 m thick regressive deposit with downlapping clinoforms. Southward from the Doce river shelf, a significant shift in sedimentary regime is observed as the morphology becomes very irregular with associated hardbottoms and unfilled paleovalleys. This sector of the shelf (Paleovalley shelf) is characterized by an accommodation regime. The interpretation shows that the entire study area can be defined as a mixed sedimentation shelf, showing supply and accommodation regimes. Shelf morphology worked as an indicator of these changes. Carbonate/terrigenous deposition during a highstand/regressive phase coeval along the eastern Brazilian shelf, either laterally and across shelf. This lateral/along coast variation in sediment supply and carbonate production leads to distinct lateral facies and geometry. These spatial changes in morphology and facies, with coexistence of carbonate and siliciclastic sedimentation, are very important for the correlation and interpretation of the geological record, especially stratigraphic surfaces and sequence units.
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•Eastern Brazilian shelf morphology indicates distinct sedimentary regimes.•Coeval carbonate/terrigenous deposition during shelf highstand/regressive phase.•Along and across shelf sedimentation changes from terrigenous to carbonate deposits.•In mixed sedimentation shelf, accommodation regime.
The Filchner‐Ronne Ice Shelf (FRIS) is characterized by moderate basal melt rates due to the near‐freezing waters that dominate the wide southern Weddell Sea continental shelf. We revisited the ...region in austral summer 2018 with detailed hydrographic and noble gas surveys along FRIS. The FRIS front was characterized by High Salinity Shelf Water (HSSW) in Ronne Depression, Ice Shelf Water (ISW) on its eastern flank, and an inflow of modified Warm Deep Water (mWDW) entering through Central Trough. Filchner Trough was dominated by Ronne HSSW‐sourced ISW, likely forced by a recently intensified circulation beneath FRIS due to enhanced sea ice production in the Ronne polynya since 2015. Glacial meltwater fractions and tracer‐based water mass dating indicate two separate ISW outflow cores, one hugging the Berkner slope after a two‐year travel time, and the other located in the central Filchner Trough following a ∼six year‐long transit through the FRIS cavity. Historical measurements indicate the presence of two distinct modes, in which water masses in Filchner Trough were dominated by either Ronne HSSW‐derived ISW (Ronne‐mode) or more locally derived Berkner‐HSSW (Berkner‐mode). While the dominance of these modes has alternated on interannual time scales, ocean densities in Filchner Trough have remained remarkably stable since the first surveys in 1980. Indeed, geostrophic velocities indicated outflowing ISW‐cores along the trough's western flank and onto Berkner Bank, which suggests that Ronne‐ISW preconditions Berkner‐HSSW production. The negligible density difference between Berkner‐ and Ronne‐mode waters indicates that each contributes cold dense shelf waters to protect FRIS against inflowing mWDW.
Plain Language Summary
We visited the largest floating Antarctic ice shelf in the southern Weddell Sea in 2018 with an icebreaker expedition, and measured ocean temperature, salinity, meltwater content, and other parameters in front of the FRIS. We found that the ocean conditions were still dominated by the very cold and dense waters needed to protect the ice shelf from inflowing warm waters from the deep ocean. We compared the 2018 conditions with earlier surveys since the 1980s and concluded that, in spite of climate change and in contrast to other Antarctic regions, the water masses on the southern Weddell Sea shelf remained relatively stable overall. We found that most of the stations we visited near the Filchner Ice Shelf edge were dominated by cold ISW, which forms when water masses interact with the underside of the shelf ice. Our measurements helped improve our understanding regarding the currents and water masses on the southern Weddell Sea continental shelf.
Key Points
Hydrographic status update with the first comprehensive CTD survey along the entire FRIS front since 1995
Strong and stable presence of High Salinity Shelf Water in Ronne Depression over decades
Dominance of Ronne‐sourced Ice Shelf Water in Filchner Trough in 2018 points to intensified sub‐FRIS circulation
Ice shelves in the Amundsen Sea Embayment have thinned, accelerating the seaward flow of ice sheets upstream over recent decades. This imbalance is caused by an increase in the ocean‐driven melting ...of the ice shelves. Observations and models show that the ocean heat content reaching the ice shelves is sensitive to the depth of thermocline, which separates the cool, fresh surface waters from warm, salty waters. Yet the processes controlling the variability of thermocline depth remain poorly constrained. Here we quantify the oceanic conditions and ocean‐driven melting of Cosgrove, Pine Island Glacier (PIG), Thwaites, Crosson, and Dotson ice shelves in the Amundsen Sea Embayment from 1991 to 2014 using a general circulation model. Ice‐shelf melting is coupled to variability in the wind field and the sea‐ice motions over the continental shelf break and associated onshore advection of warm waters in deep troughs. The layer of warm, salty waters at the calving front of PIG and Thwaites is thicker in austral spring (June–October) than in austral summer (December–March), whereas the seasonal cycle at the calving front of Dotson is reversed. Furthermore, the ocean‐driven melting in PIG is enhanced by an asymmetric response to changes in ocean heat transport anomalies at the continental shelf break: melting responds more rapidly to increases in ocean heat transport than to decreases. This asymmetry is caused by the inland deepening of bathymetry and the glacial meltwater circulation around the ice shelf.
Key Points
The onshore heat transport is coupled to the offshore zonal wind stress
The seasonal cycle of ice‐shelf melting depends on its thickness distribution
Asymmetric response to change in ocean heat transport enhances ice‐shelf melting
This article discusses the results of a suite of numerical simulations of the oceanic circulation in the Southwestern Atlantic Shelf region that are aimed to characterize its mean circulation and ...seasonal variability and to determine the dynamical mechanisms controlling them. Our experiments indicate that south of 40°S the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf, which is controlled by the discharges from the Magellan Straits, tidal mixing, wind forcing, and the offshore influence of the Malvinas Current farther north. The region from 40°S to 33°S presents the highest seasonal variability, with intrusions of cold sub‐Antarctic waters and the northward expansion of mixtures of the Río de la Plata waters in late fall and a slower retraction of the plume during spring‐summer. Wind stress variability seems to be the primarily forcing mechanism for the plume dynamics. These model results are in reasonable agreement with observations and previous model results. The present solutions also reveal important additional features of the shelf response. The along‐shelf circulation, for example, is largely driven by the western boundary currents in the middle and outer shelf, with induced transports that are 3 times larger than in experiments forced by winds and tides. The analysis also indicates that the upstream influence of the Malvinas Current is felt well beyond its retroflection point in the form of a northward middle‐shelf current and that the interaction of the Brazil Current with the Brazilian shelf topography is primarily responsible for inducing steady shelf break upwelling.
A major challenge in sequence stratigraphy is objectively identifying stratigraphic surfaces and sequences across multiple scales of observation. Identification is commonly dependent on the ...resolution of the data used (i.e., seismic vs. well data), its dimension (i.e., 1D vs. 2D vs. 3D) or the criteria chosen to select sequence boundaries. Through shelf-edge trajectory analysis, the clinothem (i.e., highest order seismic sequence identified on seismic data) can constitute the elementary building block of an observation-based and data-driven quantitative workflow to develop sequence stratigraphic frameworks across different orders and ranks of hierarchy. Here, we use high-quality 3D seismic data to interpret a Late Tithonian–Early Cretaceous shelf margin, the Lower Barrow Group (LBG), developed in the Northern Carnarvon Basin on the North West Shelf of Australia. Based on full-volume seismic interpretation techniques that integrate the 3D variability of the data when identifying seismic unconformities, a high-resolution seismic stratigraphic framework was built (73 interpreted clinothems with an average time duration of ~63,000 yrs). The computation of high-frequency shelf-edge trajectory angle (Tse) curves on selected seismic cross-sections is used to objectively pick sequence stratigraphic surfaces based on the accommodation succession method, thereby highlighting small changes in trajectory and proposing a method reproducible by interpreters based on the same quantitative data. Within the D. lobispinosum interval (142.3–140.9 Ma), the definition of stratigraphic sequences and composite stratigraphic sequences through this workflow is used to discriminate the controls at high and low temporal frequency on the vertical and lateral variability (which is here quantified) of this shelf-slope-basin system. The results show that the high-frequency interplay between short-term glacio-eustasy (i.e., Milankovitch eccentricity cycles of ~100,000 yrs) and sediment supply (locus of fluvial input along the margin) impacted the three-dimensional stratigraphic architecture of the LBG. In contrast, tectonic subsidence had a significant impact on the stratigraphic architecture of the LBG within the main depocentre at lower temporal frequency by overprinting the eustatic signal and accelerating/decelerating the rates of accommodation creation. However, identification of long-term glacio-eustatic Milankovitch cycles (~400,000 yrs) outside the main depocentre, where the rates of accommodation creation due to rift-related subsidence are moderate, also suggests low-frequency eustatic control. Therefore, the vertical and lateral variability of the LBG results from variations in sediment supply and subsidence regime under local (i.e., process regime, currents), regional (i.e., tectonics) and global (i.e., eustasy, climate) forcing parameters interplaying across timescales. In contrast to standard sequence stratigraphic workflows that are based on model-dependent choices to select sequence boundaries, quantitative 3D seismic stratigraphy constitutes an improved method to interpret 3D seismic data in shelf-margin depositional systems within a sequence stratigraphic framework, which provides an observation-based and model-independent tool allowing the definition of stratigraphic sequences with results that are reproducible across multiple stratigraphers. This work highlights the need for developing new sequence stratigraphic tools and methods that integrate the 4D variability of depositional systems and moves beyond the two-dimensionality inherent to current sequence stratigraphic methods. Quantitative 3D seismic stratigraphy represents a first step towards the creation of 3D sequence stratigraphic workflows that could improve the prediction of stratigraphic patterns and facies relationships (source, reservoir, seal distribution) across shelf margins.
•A high-resolution seismic stratigraphic framework was built using 3D seismic data.•Shelf-edge trajectory angle curves are used to pick sequence stratigraphic surfaces.•Lateral and vertical variability of the clinothems is quantified.•Controls are identified at high and low temporal frequency.•Quantitative 3D seismic stratigraphy represents a first step towards 3D sequence stratigraphy.
Oxidative processes and meat spoilage bacteria are major contributors to decreasing the shelf‐life of meat and meat products. Oxidative processes occur during processing, storage, and light exposure, ...lowering the nutritional and sensory value and acceptability of meat and generating toxic compounds for humans. Polyphenols inhibit oxidative processes in 3 ways: as reactive species scavengers, lipoxygenase inhibitors, and reducing agents for metmyoglobin. Thus, polyphenols are candidate antioxidants for meat and meat products. The cross‐contamination of meat with spoilage and pathogenic microorganisms can occur in production lines and result in economic losses. The ability of polyphenols to interact with bacterial cell wall components and the bacterial cell membrane can prevent and control biofilm formation, as well as inhibit microbial enzymes, interfere in protein regulation, and deprive bacterial cell enzymes of substrates and metal ions. Thus, polyphenols are candidate antimicrobial agents for use with meat and meat products. Commercially available polyphenols can decrease primary and secondary lipid peroxidation levels, inhibit lipoxygenase activity, improve meat color stability, minimize the degradation of salt‐soluble myofibrillar protein and sulfhydryl groups, and retard bacterial growth. Further studies are now needed to clarify the synergistic/antagonistic action of various polyphenols, and to identify the best polyphenol classes, concentrations, and conditions of use.
The present knowledge of Holocene continental shelf deposits in relation to the processes for their formation, from the prospective of marine sediment dynamics, is examined. Over the last 50years, ...various innovative techniques have been developed to measure and calculate currents, waves and suspended sediment concentrations. Thus, sediment transport rates can be defined by in situ observations, or numerical modeling. At the same time, sediment dynamics has been applied to morphodynamics, such that deposition rate and seabed morphological change can be predicted. On the continental shelf, tide–wave action, residual circulation and sediment gravity flow are the major sediment dynamic forcing mechanisms. The tidal current is often responsible for landward transport, waves can cause landward transport of sand and gravel but seaward transport of fine-grained sediment, and shelf circulation and sediment gravity flow favor the dispersal of suspended sediment towards offshore or even across the shelf. For the various types of the shelf, wide or narrow, sediment starved or supply abundant, prediction on the basis of processes is consistent with the observed spatial distribution of Holocene sedimentary systems, e.g., river deltas, beaches, barrier islands, lagoons, tidal ridges, tidal inlets, tidal flats and shelf muds. The sedimentary records associated with these systems often consist of high-resolution slices, i.e., each record tends to have a resolution within 101years, but covers only a relatively short period of the Holocene. Nevertheless, if different records are connected, according to their chronological order, then the overall period covered by the records will be extended. In order to achieve this objective, a modeling approach to the formation of sedimentary records should be developed. Based upon an understanding of process–product relationships, simulation can be undertaken for: the formation of early Holocene reworking-induced deposits, during sea level rise; event beds due to extreme events; the preservation potential of the sedimentary sequences; the post-depositional modifications to the sedimentary records; the environmental settings during sedimentation; and the long-term effects of material budgeting processes on the sediment system behavior.
•Holocene continental shelf deposits are related to sediment dynamic processes.•The sedimentary records associated with shelf deposits are high-resolution slices.•Modeling approaches to the formation of sedimentary records can be developed.
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