Eastern Shelf Research Articles

Spatiotemporal variability of the nitrogen deficit in bottom waters on the eastern Bering Sea shelf

As water flows from the North Pacific Ocean to the Arctic Ocean, it passes through the shallow eastern shelf of the Bering Sea which serves as a major sink of inorganic nitrogen. This study explores the physical and biological factors that influence the spatiotemporal variability of this sink. A regional relationship of dissolved inorganic nitrogen to inorganic phosphorus (DIN:P) was established for waters entering the shelf. Residuals from this relationship (termed N∗∗) are a measure of the nitrogen deficit and were determined for bottom waters on the shelf using nutrient data collected on 52 hydrographic cruises spanning 2003 – 2018. Spatial variability in N∗∗ was related to advection, cross-shelf and vertical mixing, and residence time (using simulated ages of bottom water over the middle shelf). On average, this deficit accounted for approximately one-third of the inorganic nitrogen that enters the shelf, and the highest deficits (>8 ​μM DIN) were observed on the middle shelf between 60°N and St. Lawrence Island (63°N). Temporal variability in N∗∗ was examined over the middle shelf, and higher nitrogen deficits that occurred in colder years were hypothesized to result from weaker flow and increased export of organic matter in the presence of sea ice. On the southern middle shelf, the volume integrated (40 ​m to bottom) seasonal change in N∗∗ was equivalent to a denitrification rate of 0.7 ​± ​0.3 ​mmol ​N m-2 d-1. Rates of nitrogen loss were also estimated by combining N∗∗ with the simulated residence time of water on the shelf and found to be 0.20 ​± ​0.02 ​mmol ​N m-2 d-1. These rates were comparable to prior measurements of denitrification/anammox reported on the shelf. The nitrogen deficit could not be wholly ascribed to denitrification/anammox as the N:P stoichiometric ratio in particulate matter is known to be lower at higher latitudes, and a lower ratio was observed when dissolved organic matter was measured in a small number of samples. It remains unclear how future reductions in sea ice might impact the extent of nitrogen loss in the Bering Sea.

Geomechanical Measurements and Machine Learning Help Predict Trouble Stages

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 201699, “Predicting Trouble Stages With Geomechanical Measurements and Machine Learning: A Case Study of Southern Midland Basin Horizontal Completions,” by Eric Romberg, SPE, Keban Engineering; Aaron Fisher, Tracker Resources; and Joel Mazza, SPE, Fracture ID, et al., prepared for the 2020 SPE Annual Technical Conference and Exhibition, originally scheduled to be held in Denver, 5–7 October. The paper has not been peer reviewed. Unexpected problems during completion create costs that can cause a well to be outside its planned authorization for expenditure, even uneconomic. These problems range from experiencing abnormally high pressures during treatment to casing failures. The authors of the complete paper use machine-learning methods combined with geomechanical, wellbore-trajectory, and completion data sets to develop models that predict which stages will experience difficulties during completion. Field Modeling and Well Planning The operator’s acreage is in the southeastern portion of the Midland Basin. In this area of the basin, the Wolfcamp B and C intervals often contain a significant amount of slope sediments and carbonate debris flows because of the proximity of the eastern shelf. These intervals cause significant drilling and completion issues. During the past 5 years, the operator acquired and licensed approximately 130 sq mile of 3D seismic data. In addition, the operator cored three wells, drilled six pilot wells with complete log suites, licensed 40 wells with a triple/quad combination, acquired data and surveys on 112 existing horizontal wells, and has 347 vertical wells with formation tops for depth control. This rich data set yielded a robust 3D reservoir model that was used to map a sequence of stacked, high-quality landing targets. Model-Aided Well-Completion Strategy. The operator often encountered difficult stages in the form of high breakdown pressures, high pumping pressures, and the inability to place proppant. On a few occasions, drilling out all plugs was not possible because of casing obstructions possibly related to fault activation during the stimulation. The operator began analyzing curvature and similarity volumes for potential fault/fracture identification near the difficult completion stages and compromised casing intervals. Drillbit geomechanics data collection was planned for all lateral wells. The geomechanical properties recorded were used to reduce risks during completions further by informing the plug and perforation stage design. Stages were planned to reduce variation in minimum horizontal stress (Shmin) within each stage. The geomechanical data also identified carbonate debris flows within the well path, allowing completion engineers to bypass rock considered unproductive. Completion Issues and Other Factors Contributing to Casing Deformation. From February 2017 through November 2019, the operator drilled and completed 28 Wolfcamp horizontal wells. The plug-and-perforation completion technique was used on all 28 wells. While drilling out composite fracturing plugs, casing obstruction was encountered in six of 28 wells. These obstructions limited the working internal diameter of the production casing and either prevented or inhibited access beyond the obstruction. In two of the Phase 1 wells, conventional drillout assemblies were not able to pass the obstructions.

Tectonic controls on the evolution of mixed carbonate‐siliciclastic systems: Insights from the late Palaeozoic Ouachita‐Marathon Foreland, United States

AbstractSea level is thought to be the primary driver of alternating deposition of carbonate and siliciclastic sediment in shelf settings, with carbonates dominating during transgressive/highstands and siliciclastics during lowstands. Although sediment supply is critically important for shelf‐margin growth in siliciclastic systems, few studies demonstrate its impact on mixed carbonate‐siliciclastic systems. The Permian Basin in Texas, United States, provides an opportunity to investigate the basin evolution regarding the source, sediment routing and particularly shelf/slope growth from syn‐ to postorogenic phases during alternating carbonate and siliciclastic sedimentation. Published detrital zircon data show that the proportion of orogen‐related sources decreased significantly from an earliest Permian synorogenic phase (ca. 298 Ma) to a Leonardian (ca. 280–271 Ma) postorogenic phase, in concert with a grain‐size change from fine‐ to medium‐grained sand to silt. Although along‐strike lateral variabilities exist on the shelf margin, the shelf‐margin evolution characteristics show a significant difference among the Northern Shelf, Eastern Shelf and Central Basin Platform. The synorogenic Eastern Shelf exhibits a significant higher progradation rate than does the postorogenic Northern Shelf. The progradation and aggradation ratio of siliciclastic‐rich intervals in the Eastern Shelf is significantly higher than those of carbonate‐rich intervals in the Eastern Shelf and carbonate‐ or siliciclastic‐rich intervals in the Northern Shelf. In contrast, the Central Basin Platform, with no siliciclastic sediment supply, records almost no progradation regardless of orogenic phases. There is an increase in slope gradient with decreasing sediment supply during this second‐order sequence from the Permian Cisuralian Series to the end of the Guadalupian Series. This study demonstrates that tectonically driven siliciclastic sediment supply was the main mechanism controlling the shelf and slope evolution in alternating siliciclastic and carbonate deposition.

Variability of the food potential of the Kerch strait and the black sea pre-strait zone as a prospective area for the development of mollusks Mariculture

Based on the data of integrated observations, the variability of phytoplankton, the amount of suspended organic matter and zooplankton of the pelagic zone of the Kerch Strait and the pre-strait zone of the Black Sea is considered. The algal flora of the entire Black Sea, the Kerch Strait and the pre-strait is characterized by the predominance of diatoms over pyridineas (pyrrophytes or dinoflagellates). The study of the species range, number and biomass of the phytoplankton community during several periods of the year showed that its dynamics is largely dependent on seasonal weather changes (climate). In the process of phytoplankton development, several phases of autogenic succession were revealed. It is expressed in a sequential change in phytoplankton forms, changes in its abundance and biomass typical of the eastern shelf of the Black Sea. The zooplankton community of the Kerch Strait is represented by micro-, meso- and macro- zooplankton typical of the Black Sea. Microzooplankton is represented by zooflagellates and protozoa, as well as by early larval stages of planktonic crustaceans, apendicularia and mollusks. Its total biomass averages about 80 mg/m3.The above analysis of the collected material allows us to conclude that the distribution of suspended matter and larvae is related primarily to the peculiarities of water circulation in the strait. In the case of low concentrations of larvae, the dynamic factor can be decisive in the process of sedimentation intensity. The amounts of suspended matter can reach 20 mg/m3, which is 2-3 times higher than on average for the Black Sea. It further demonstrates the high productivity of waters and the prospects of the latter to be the area for commercial growing of mussels.

Sedimentology of the Upper Pennsylvanian organic‐rich Cline Shale, Midland Basin: From gravity flows to pelagic suspension fallout

AbstractAlthough deep‐water fine‐grained sedimentary rocks comprise approximately two‐thirds of the stratigraphic record, the transportation and depositional processes are poorly understood compared with their shelf counterparts. This study reports the range of fine‐grained sedimentary rock lithofacies, transport, and depositional processes and cyclicities recorded in deep‐water deposits on the basis of three continuous cored wells from the Upper Pennsylvanian Cline Shale, Midland Basin, USA, with the goal of elucidating general principles that can inform synthesis depositional models for deep‐water fine‐grained sedimentary systems. A combination of sedimentological, petrographic and bulk geochemical analysis has defined seven lithofacies that stack in a repeated pattern to constitute ca 8 to 20 m thick composite cycle sets. The lower unit of each composite cycle set is characterized by basal siliciclastic‐rich lithofacies interpreted to record dilute, low‐density turbidity currents, potentially derived from hyperpycnal input which grade upward into carbonate‐rich lithofacies interpreted as debris‐flow deposits or pelagic suspension deposits. The upper unit of each composite cycle set is characterized by basal carbonate‐rich lithofacies interpreted as debris‐flow deposits or pelagic suspension deposits which grades upward into siliciclastic‐rich lithofacies interpreted to record dilute, low‐density turbidity currents, potentially derived from hyperpycnal input. The cyclicities recorded in the Cline Shale are believed to be controlled by high‐amplitude glacioeustatic sea‐level fluctuation (mean: ca 100 m). The siliciclastic‐rich lithofacies, potentially derived from hyperpycnal turbidity flows, were deposited during sea‐level lowstand, when siliciclastic sediment‐transport systems extended across the wide Eastern Shelf and deltas developed at the shelf margin. Carbonate‐rich lithofacies are interpreted to be deposited from debris flows or pelagic suspension fallout during sea‐level highstand when carbonate platforms were developed on the surrounding shelves. The prevalence of sediment density flow deposits, even in distal basin floor environments, challenges the conventional model that deep‐water fine‐grained sedimentary rocks are dominantly background sedimentation.

Extreme oceanographic conditions in the northwestern Bering Sea in 2017–2018

Anomalous oceanographic conditions were observed in the northwestern Bering Sea in 2017–2018, including warmer air and water temperatures and reduced ice cover. Their effects on the water structure and circulation are considered. These conditions were formed on the background of warming tendency known in the Bering Sea since 2014, but the year 2018 was exceptional even relative to this background. In this year, deflections of some properties from their mean values exceeded two standard deviations (>2σ). The main reason for such abnormal conditions was southerly winds prevailed over the entire sea in winter because of a westward shift of the Aleutian Low; another factor was the strengthened advection of oceanic waters that still were abnormally warm after the period of extreme warming in the North-East Pacific in 2014–2016. As the result, the ice extent in winter and spring was below its mean value (more than twice lower in 2018), the air and water temperatures were above their mean values through the year (up to 15 and 3 °C higher, respectively, in 2018), the cold water mass was not formed at the shelf bottom in winter and was absent in summer. Under this considerable redistribution of water density, the water circulation changed, the Navarin Current weakened and was even absent in fall 2018/ and the northward water transport toward the Bering Strait was realized through the eastern shelf and delivered the coastal waters from Alaska instead of the waters from the deep-water basins, as has been usual. The oceanographic conditions in 2018 were statistical outliers, atypical even for recent period of warming, but such conditions could be expected if the warming continues. Thus, the phenomenon of 2017–2018 could be useful for understanding and predicting the reconstruction of the oceanographic regime in the near future.

Prostranstvennaya struktura i vnutrigodovaya izmenchivost' fronta morya Ueddella po dannym reanaliza NOAA OISST

Based on the NOAA OISST reanalysis data, the spatial structure of the Weddell Sea Front in the climatic field of the sea surface temperature was analyzed and the seasonal variability of front’s characteristics was estimated. The spatial position of the frontal zone in the Weddell Sea was analyzed using distributions of the total horizontal temperature gradient. The characteristics of the front (the position of the gradients' extrema corresponding to the front, their magnitude and temperature on the front axis) were determined for each month on the profiles of meridional and zonal temperature gradients along meridians and parallels with a discreteness of 2.5° of longitude and 0.25° of latitude. It is shown that the interaction of Weddell Sea cold waters, which are transported by currents northward along the Antarctic Peninsula coasts, with the warmer waters of the eastern shelf of the Antarctic Peninsula and the Bransfield Strait surface water causes formation of two branches of the Weddell Sea Front. These branches round from a vast shelf at the Antarctic Peninsula tip and the Joinville archipelago the south and north and are traced further east along the boundaries of the bottom rise located approximately between 62.5S and 64.5S. To the south of the South Orkney Islands shelf, the two branches merge into one front, which follows to the east along the depth dump of the relative shallow between the South Orkney and South Sandwich Islands. In the seasonal cycle of the Weddell Sea Front intensity, a time lag was revealed of the front intensification period in the direction from west to east. In Bransfield Strait the front is most intense in February, between the Antarctic Peninsula tip and the South Orkney Islands – in March, east of the South Orkney Islands – in April. The branch of the Weddell Sea Front off the northeastern of the Antarctic Peninsula coasts intensifies in November – January, in the western part of the water area east of the James Ross and Snow Hill Islands – in January – February.

Evidence for increasing anthropogenic Pb concentrations in Indian shelf sediments during the last century

India is industrializing rapidly and with this there comes higher releases of contaminants into the environment. Change in Pb deposition over the last century on the eastern (off Andhra Pradesh) and western (off Karnataka) shelves of India was investigated based on the data extracted from two sediment cores covering the past ~114 and ~145 yrs. The variations of the total Pb content, its enrichment factor, and concentrations of non-residual Pb in both the sediment cores document that there was a gradual increase in anthropogenic Pb input into the coastal sediments of India over the last century. Sediment leachates were used to monitor the increase in anthropogenic Pb input and its Pb isotope composition. The anthropogenic end member composition of the western shelf sediment location (206Pb/207Pb: 1.105; 206Pb/208Pb: 2.149) was significantly less radiogenic than the eastern shelf isotopic composition (206Pb/207Pb: 1.145; 206Pb/208Pb:2.120). A binary mixing model suggests that Pb emitted from the heavy industries (e.g., ore mining, Pb processing and smelting plants) of India has been the major source of anthropogenic Pb to the sediments of western continental shelf. In contrast, the isotopic signatures suggest that coal combustion is responsible for elevated anthropogenic Pb levels in the sediments from the eastern shelf of India.

Environmental impacts on walleye pollock (Gadus chalcogrammus) distribution across the Bering Sea shelf

Adult and juvenile (age-1) walleye pollock (Gadus chalcogrammus) were sampled by the US NOAA Alaska Fisheries Science Center summer bottom trawl survey in 2010, 2017, 2018, and 2019 in the northeastern and southeastern Bering Sea, with profiles of temperature collected concurrently. Similarly, the Russian Research Institute of Fisheries and Oceanography, Pacific branch, collected adult and juvenile pollock and temperature profiles on summer bottom trawl surveys in the northwestern Bering Sea. Results from these surveys show that adult pollock abundance in recent years (2017, 2018, 2019) has increased in northern regions of the Bering Sea shelf in both the US and Russian sectors. Lower abundances, compared to historic means, were observed in southern regions of the shelf, suggesting the pollock moved directionally from the south to the north. We relate changes in pollock distribution in recent intermediate (2017) and warm, low-ice years (2018–2019) to a prior cold, high-ice year (2010) and describe how these observations relate to our longer time series. We link temperature data from bottom trawl surveys (US and Russian), sea-ice indices (retreat timing and extent), as well as model-based estimates of ocean circulation to changes in pollock distribution and examine potential environmental factors driving the observed changes. Changes in sea-ice and bottom temperature (e.g., reductions in ice extent and shrinking of the cold pool), and changes in circulation (stronger northward currents over the northeastern shelf in warmer years, particularly in 2018) led to changes in distributions of adult and age-1 pollock. Adult pollock were concentrated north of St. Lawrence Island and had larger longitudinal distributions in warm years, 2017–2019; whereas they had a more southerly and narrow distribution over the outer shelf in the cold year, 2010. Age-1 pollock had higher densities over the inner eastern shelf in 2017–2019 compared to 2010. Northward flow around St. Lawrence Island (particularly in the spring) alternated between stronger flow on the west side of the island in 2010 and 2017 and stronger flow on the east side of the island in 2018 and 2019; variations in flow may have impacted the location of prey and movement of feeding pollock to the Chukchi Sea. Size structure comparisons between NW, NE and SE sections of the Bering Sea shelf suggest that movement of fish between US and Russian waters may have been highest in 2019, one of the two warmest years, and lowest in 2010, the coldest year. Spatial comparisons of distributions and size structure across the Bering Sea help provide a comprehensive view of factors affecting the movement of this highly important commercial fish species.

Estimating particulate organic carbon flux in a highly dynamic estuary using satellite data and numerical modeling

Quantitative estimates of the flux, transportation, and burial of riverine particulate organic carbon (POC) in estuaries and shelf areas are crucial for understanding the carbon cycle in marginal seas. The complexity of the hydrodynamic environment and estuarine processes in the Changjiang River estuary (CRE) results in a POC flux with high spatiotemporal variability. In this study, we proposed a new method for time series monitoring of riverine POC flux by combining satellite ocean color and numerical modeling data. We used 500-m resolution geostationary satellite data from the Geostationary Ocean Color Imager (GOCI) to retrieve surface POC concentrations in the CRE and used the numerical Finite Volume Community Ocean Model (FVCOM) to simulate three-dimensional (3D) current and sediment distribution. Diurnal, seasonal, and annual variations in POC flux through the lower reaches (Datong Hydrological Station), upper estuary (Xuliujing Hydrological Station), and outlets of the CRE, as well as transportation and deposition outside the CRE, were examined from July 2011 to June 2018. The annual mean POC fluxes through the Datong, Xuliujing, and outlet sections were 1.16 ± 0.16 Tg C/yr, 1.29 ± 0.11 Tg C/yr, and 1.17 ± 0.11 Tg C/yr, respectively. After passing through the estuary, 35.2% of POC was deposited in the sandbar outside the outlets, 52.9% was transported to the south, and a small amount entered the eastern shelf. Thus, these results indicated that POC flux through the river and estuary sections differed from the effective riverine POC flux into the sea. The proposed method combining high spatiotemporal-resolution satellite data and numerical modeling not only makes full use of their advantages, but also reduces the uncertainty of their individual estimates. This approach also supports long-term monitoring of riverine fluxes to the sea and helps clarify the effects of terrestrial inputs on marginal seas under multiple stresses from human activities and climate change.

Clay minerals, Sr-Nd isotopes and provenance of sediments in the northwestern South China Sea

The clay mineralogy and Sr-Nd isotopic composition of sediments collected in the northwestern South China Sea (SCS) were investigated to understand sediment provenance and changes in the SCS since the Late Pleistocene. Our results show that the clay mineral assemblages of the surface sediments are characterized by high amounts of illite and smectite, and low amounts of kaolinite and chlorite. The 87Sr/86Sr ratios of the surface sediments vary from 0.709617 to 0.734416, and εNd values range from −11.6 to −8.3. The clay minerals and Sr-Nd isotope records suggest that sediments in the northwestern SCS are derived from multiple sources. The sediments deposited in the eastern shelf offshore of Hainan originate predominantly from Hainan Island and South China, whereas sediments in the western shelf offshore of Hainan are derived mainly from Indochina and Hainan Island. However, sediment source and transport mechanisms in the northwestern slope of the SCS are more complicated. Our results suggest that input from the west may be the major sediment source, but regions in the east (Taiwan and Luzon) also contribute part of the terrigenous materials. Downcore variations of Sr-Nd isotopic composition in core sediments show clear glacial–interglacial fluctuations. Higher 87Sr/86Sr ratios occur during glacial period, indicating that the input of terrigenous materials from old continental rocks becomes one of the important sediment sources during these periods. The exposure of continental shelf and southward migration of the coastline during the glacial sea-level lowstand led to an increase of terrigenous supply to the northwestern slope of the SCS. The paleo-Red River and Indochina are also potentially important contributors.

Hourly maximum individual wave height in the Indian shelf seas—its spatial and temporal variations in the recent 40 years

The spatial and temporal variability of the high individual wave heights, which pose a hazard in India’s shelf seas, has not been investigated. In this study, the spatial and temporal variation of hourly maximum individual wave height (Hmax) in the Indian shelf seas is examined based on 40 years ERA5 Re-Analysis data from European Centre for Medium-Range Weather Forecasts. Waverider buoy data from five locations are used to assess ERA5 Hmax. The variability of Hmax in different temporal scales; daily, monthly and interannual and their linkages with major ocean-atmosphere coupled interaction in the Pacific and the Indian Ocean are presented. Wave climatology throughout India’s western shelf seas shows average Hmax exceeding 2 m during most of the year. Maximum Hmax in the western shelf seas is during May–July, whereas in the southern tip and the eastern shelf seas are during the tropical cyclones and are either in October–December or May. Maximum Hmax is 17.94 m, and it occurred at the northern Bay of Bengal on 25 May 2009 during the passage of tropical cyclone Aila. Spatial variations in Hmax between locations spaced at 1.5° latitude are less than 10% in the northwestern and southeastern shelf seas, but large variations (~ 28%) are in the southwestern and northeastern shelf seas. Annual mean Hmax shows contrasting trends when data for 1979 to 2018 and 1992 to 2018 is considered. Indian Ocean Dipole influence on the Hmax suggests negative correlations in the western shelf seas and positive correlations in the eastern shelf seas. The correlation with climate indices is similar for both significant wave height and Hmax.

Tidal Effect on Water Export Rate in the Eastern Shelf Seas of China

AbstractWater export rate of shelf seas is a pivotal factor impacting the global carbon cycle. Tides have important impacts on shelf hydrodynamics but are excluded in many climate models. To assess the effect of tides on export rates of shelf water, this study used a regional hydrodynamic model and a water residence time (WRT) adjoint model and examined model runs with and without tides for the eastern shelf seas of China. The results show that the average WRTs in the Bohai, Yellow, and East China seas were 11.60, 4.95, and 0.39 years, respectively. When tides were excluded, the WRTs decreased by >70% in the Bohai and Yellow seas and by ~10% in the East China Sea, indicating a significant acceleration in the shelf water export due to the absence of tides. The tidal effect has spatial variability associated with the water depth. Sensitivity experiments suggest that the tidal effect on the mean WRT was stronger than the effect of other dynamical factors (winds, rivers, and boundary currents). In the model with tides, tides weakened the wind‐driven coastal current by intensifying the bottom resistance and thus slowed water export in the inner and middle portions of the shelf, compared to the model without tides. Parameterization of the tidal bottom friction in the model without tides could significantly improve the WRT result. This study highlights the crucial role of tides on the long‐term transport of shelf seas and the significance of parameterizing the effect of tidal friction in climate models.

Current State and Economic Value of Astacofauna in Certain Areas of the Eastern Shelf of the Caspian Sea

Aim. The purpose of this work was to assess the current state of crayfish populations in their habitats on the eastern shelf of the Caspian Sea (Alexander Bekovich‐Cherkassky Bay).Material and Methods. Fieldwork was based on underwater research methods using light diving equipment. Collection of research material was carried out according to an established crayfish survey protocol using fixed grid sites of 100 square metres. Collected crayfish were identified by species, measured and weighed, fecundity of females and the condition of crayfish shells were recorded. Crayfish were then released back into the sea. Calculation of numbers was carried out on the basis of determination of cluster density (ind/m2) with a further assessment of useful biotype areas. Calculation of commercial stocks based on crayfish length. The research synthesized information derived from comparative analysis of modern research results and archival data from the 1970s to 1990s.Results. Analysis showed that significant changes in the composition of populations had occurred. The dominant species recorded was Caspiastacus pachypus Rathke which has substantially displaced from the biotopes Pontastacus eichwald Воtt. which was previously the dominant species here.Conclusion. Comparative analysis of the modern state of the astacofauna on the eastern shelf and historical data indicates significant changes in the structure of populations of Caspian crayfish. The resilient C. pachypus has become the dominant species. Its small commercial size has significantly reduced the economic value of crayfish stocks. In general, however, commercial stocks of these crayfish make it possible to develop these marine biological resources.

Imprints of volcanic, erosional, depositional, tectonic and mass-wasting processes in the morphology of Santa Maria insular shelf (Azores)

The present-day morphology of volcanic island shelves is the result of several factors, namely the original shape and structure of the volcanic edifice, the nature and age of the materials that compose its flanks, and the erosional, volcanic, tectonic and sedimentary processes that acted to form these shelves. By combining detailed seafloor mapping with onshore studies it is possible to gain a unique insight onto the evolution of volcanic island edifices and the processes that helped to shape their shelves. Here we look at Santa Maria Island in the Azores Archipelago, which is a 6 Ma old ocean-island volcano surrounded by a 135 km2 shelf, characterized by an history of subsidence since its formation c. 6 Ma ago, followed by uplift from 3.5 Ma onwards. By analyzing the geomorphic parameters of the insular shelf (such as the shelf width and the erosive edge depth), the island's history of vertical movements, and by relating the morphology of the outcrops found on the shelf with those onshore, we contribute to a better knowledge of the evolutionary history of Santa Maria with particular focus on its earlier stages of growth. We infer that the outer part of the northern shelf is carved on units older than the oldest volcanic sequences exposed on land; the western and southern shelves are almost entirely carved in the products of the oldest shield volcano (Anjos Volcanic Complex), whilst the eastern shelf is the youngest in formation, being related to units of the Pico Alto volcanic complex. We also discuss the distribution of sediments on the shelf. High wave energy concomitant with sea level drops contributed to the erosion and transport of older sequences offshore, which were ultimately lost to the slopes of the island, resulting in a shelf that is sediment-stripped for the most part. Presently, wide shelves further contribute to wave-energy dissipation, resulting in a significantly diminished sediment production. The shelf edges around Santa Maria exhibit several erosive scars, interpreted as the head of a submarine drainage network carved on the slopes of the island. On the western, southern and eastern shelves, these mass-wasting features are mostly controlled by tectonics since the extension of faults can be followed from onshore to the shelf edge. On the northern shelf mass-wasting appears to be mostly correlated with presence of unstable sediments on the shelf edge that triggered landsliding. Overall, this work shows how detailed mapping of the shelves surrounding old volcanic islands can still provide important complementary information about the evolution of these islands, which could not be acquired solely by the study of their subaerial and deeper submarine parts.

Low dissolved oxygen in the Pearl River estuary in summer: Long-term spatio-temporal patterns, trends, and regulating factors

Several large semi-enclosed seas and coastal bodies have formed seasonal hypoxic water masses over large areas. The dominant cause for such formations is believed to be the increasing inflow of nutrients from watersheds and urban areas into estuaries and coastal waters. Several studies have reported hypoxic events in the Pearl River estuary (PRE). However, hypoxia events appear to be episodic, which are neither seasonal nor estuary-wide. The reasons for such occurrences are not understood fully. The objectives of this study are to use 24-year time series data obtained during 1988–2011 to analyze the long-term variability in dissolved oxygen (DO) in summer over the entire estuary from urban areas to estuarine coastal waters and examine regulating factors for the occurrences of hypoxia. The results showed that various regulating factors were responsible for the variability of DO in different regions of the PRE. DO decreased in the urban reach, upstream and main estuarine zones of the PRE from about 2000. It was hypoxic (DO <2.0 mg L−1) in the urban reach zone and periodic low DO (DO <3.5 mg L−1) and episodic hypoxic in the upstream zone after 2000, mainly influenced by the increasing sewage discharge. The main estuarine zone experienced episodic low DO and sporadic hypoxia after 2000. The ecosystem buffering capacity was characterized by turbidity, well vertical mixing and short residence time which diluted the pressure for the formation of hypoxia. The eastern shelf appeared to be more vulnerable to hypoxia because of its deeper topography, plume front position, and stratification of the water column compared with the western shelf. Its bottom waters experienced seasonal low DO but seldom hypoxia. In comparison, low DO rarely occurred in the western shelf because of its shallow topography and short residence time. In brief, 24-year series data captured from 1988 to 2011 showed a lack of seasonal and estuary-wide hypoxia in the PRE.

Discovery of Afifi, the shallowest and southernmost brine pool reported in the Red Sea

The previously uncharted Afifi brine pool was discovered in the eastern shelf of the southern Red Sea. It is the shallowest brine basin yet reported in the Red Sea (depth range: 353.0 to 400.5 m). It presents a highly saline (228 g/L), thalassohaline, cold (23.3 °C), anoxic brine, inhabited by the bacterial classes KB1, Bacteroidia and Clostridia and the archaeal classes Methanobacteria and Deep Sea Euryarcheota Group. Functional assignments deduced from the taxonomy indicate methanogenesis and sulfur respiration to be important metabolic processes in this environment. The Afifi brine was remarkably enriched in dissolved inorganic carbon due to microbial respiration and in dissolved nitrogen, derived from anammox processes and denitrification, according to high δ15N values (+6.88‰, AIR). The Afifi brine show a linear increase in δ18O and δD relative to seawater that differs from the others Red Sea brine pools, indicating a non-hydrothermal origin, compatible with enrichment in evaporitic environments. Afifi brine was probably formed by venting of fossil connate waters from the evaporitic sediments beneath the seafloor, with a possible contribution from the dehydration of gypsum to anhydrite. Such origin is unique among the known Red Sea brine pools.

Benthic communities of the Filchner Region (Weddell Sea, Antarctica)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 628:37-54 (2019) - DOI: https://doi.org/10.3354/meps13093 Benthic communities of the Filchner Region (Weddell Sea, Antarctica) Santiago E. A. Pineda-Metz1,2,*, Enrique Isla3, Dieter Gerdes1 1Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, 27568 Bremerhaven, Germany 2Fachbereich 2 Biologie/Chemie, Universität Bremen, 28334 Bremen, Germany 3Institut de Ciències del Mar - CSIC, 08003 Barcelona, Spain *Corresponding author: santiago.pineda.metz@gmail.com ABSTRACT: Due to extreme pack ice, the Filchner Region in the southern Weddell Sea is one of the least studied regions on the planet. Here, we provide a detailed description of the benthic communities of this high-Antarctic ecosystem, and assess the relationship between environmental factors and benthic distribution patterns. Fieldwork was performed in the austral summers of 2013-14 and 2015-16 during the R/V ‘Polarstern’ cruises PS82 and PS96. Using a combination of multibox corer (MBC) and seabed image data from 37 stations (water depths 243-1217 m), we differentiated 6 station groups. While 1 of these groups was comprised of a single station, the other 5 groups represented distinct benthic communities. Three of these correspond to the previously described Eastern Shelf, Southern Shelf, and Southern Trench communities. However, we found distribution shifts and MBC abundance and biomass reductions when comparing our results with earlier studies. The other 2 groups have novel characteristics and are presented here as an Ice/Ice Shelf Water-related community and a Continental slope community. Water depth in combination with 2 or 3 other environmental variables (out of 7 available) explained <30% of the benthic distribution and composition. We found a tighter relationship between water mass circulation and spatial distribution of the communities; we suggest using water-mass-related characteristics (e.g. productivity regimes, water currents) to better explain benthic spatial distribution patterns. KEY WORDS: Southern Ocean · Infaunal benthos · Epifaunal benthos · Mega- and macrofauna · Zoobenthic distribution patterns · Sediment cores · Seabed images · Environment-benthic relationships Full text in pdf format Supplementary material PreviousNextCite this article as: Pineda-Metz SEA, Isla E, Gerdes D (2019) Benthic communities of the Filchner Region (Weddell Sea, Antarctica). Mar Ecol Prog Ser 628:37-54. https://doi.org/10.3354/meps13093 Export citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 628. Online publication date: October 10, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

BERING SEA: 2018 AS THE EXTREME LOW-ICE AND WARM YEA

Phenomenon of anomalous oceanographic conditions in the Bering Sea in 2018 is considered, with heightened air and water temperature and very low ice cover, as well as its consequences for the water structure, circulation, and chemical properties. These extreme conditions were formed on the background of warming tendency observed in the Bering Sea since 2014, but they were exceptional even relative to this background: deflections of some parameters from their normal values exceeded the standard deviations more than twice (&gt; 2s). The main reason for such conditions was the southern winds prevalence over the entire Sea in winter, in opposite to usual regime when the southern winds blew over its southeastern part only; another factor was strengthened advection of the oceanic waters, still abnormally warm after the period of extreme warming in the North-East Pacific in 2014–2016. As the result, the ice cover in winter and spring was more than twice lower the normal values, the air and water temperatures through the year were above their normal values in 15 and 5 degrees Celsius, respectively. The winter convection was weakened that caused deepening of the cold subsurface layer core and prevented forming of the cold near-bottom water masses on the shelf that was usual for the Bering Sea. Under this considerable redistribution of the water density coupled with unusual wind regime, the water circulation changed, in particular in the northern Bering Sea where the Navarin Current were weakened or even absent and the northward water transport was realized through the eastern shelf delivering to the northern shelf and Bering Strait the coastal waters from Alaska instead of the deep-water basins waters, as usually. Strong advection of the oceanic waters through the Aleutian Straits promoted growth of nutrients concentrations and lowering of oxygen content in the intermediate layer of the Bering Sea. These extreme oceanographic conditions in the Bering Sea in 2018 were statistical outliers, atypical even for recent period of warming, but such conditions would be expected frequently if the warming will continue. Thus, phenomenon of the year 2018 could be useful for understanding and prediction of the oceanographic regime reconstruction in the nearest future.

&amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; variability in the surface waters of the eastern Gulf of Cádiz (SW Iberian Peninsula)

Abstract. Spatio-temporal variations in the partial pressure of CO2 (pCO2) were studied during eight oceanographic cruises conducted between March 2014 and February 2016 in surface waters of the eastern shelf of the Gulf of Cádiz (SW Iberian Peninsula) between the Guadalquivir river and Cape Trafalgar. pCO2 presents a range of variation between 320.6 and 513.6 µatm with highest values during summer and autumn and lowest during spring and winter. For the whole study, pCO2 shows a linear dependence with temperature, and spatially there is a general decrease from coastal to offshore stations associated with continental inputs and an increase in the zones deeper than 400 m related to the influence of the eastward branch of the Azores Current. The study area acts as a source of CO2 to the atmosphere during summer and autumn and as a sink in spring and winter with a mean value for the study period of -0.18±1.32 mmol m−2 d−1. In the Guadalquivir and Sancti Petri transects, the CO2 fluxes decrease towards offshore, whereas in the Trafalgar transect fluxes increase due to the presence of an upwelling. The annual uptake capacity of CO2 in the Gulf of Cádiz is 4.1 Gg C yr−1.