Respiration and denitrification in permeable continental shelf deposits on the South Atlantic Bight: Rates of carbon and nitrogen cycling from sediment column experiments

Nitrogen (N) cycling and respiration rates were measured in sediment columns packed with southeastern United States continental shelf sands, with high permeability (4.66×10⁻¹¹ m²) and low organic carbon (0.05%) and nitrogen (0.008%). To simulate porewater advection, natural shelf seawater was pumped through columns of different lengths to achieve fluid residence times of approximately 3, 6, and 12 h. Experiments were conducted seasonally at in situ temperature. Fluid flow was uniform in nearly all columns, with minimal dead zones and channeling. Significant respiration (O₂ consumption and ∑CO₂ production) occurred in all columns, with highest respiration rates in summer. Most (78–100%) remineralized N was released as N₂ in the majority of cases, including columns with oxic porewater throughout, with only a small fraction released as NO₃⁻ from some oxic columns. A rate of 0.84–4.83×10¹⁰ mol N yr⁻¹, equivalent to 1.06–6.09×10⁻⁶ mmol N cm⁻² h⁻¹, was calculated for benthic N₂ production in the South Atlantic Bight, which can account for a large fraction of new N inputs to this shelf region. Metal and sulfate reduction occurred in long residence time columns with anoxic outflow in summer and fall, when respiration rates were highest. Because permeable sediments dominate continental shelves, N₂ production in high permeability coastal sediments may play an important role in the global N cycle.     

Organic carbon and nutrient fluxes to the coastal zone from the Sepik River outflow

The Sepik River is a major contributor of water, sediment and associated organic loads to the coastal waters of northern New Guinea and the Bismarck Sea. We compare dissolved and particulate organic carbon data from September 1997 during an extremely dry El Nino year with those from 1996, 1999 and 2000 during La Nina wet season discharges. Estimated Sepik River flux of DOC is 3.2×10¹⁰ mol yr⁻¹ and POC is 1.1×10¹¹ mol yr⁻¹. The estimates for total river nutrient fluxes to the sea are 1.1×10¹⁰ mol yr⁻¹ for nitrogen and 4.6×10⁸ mol yr⁻¹ for phosphorus. The Sepik DOC flux is about equal to that combined from all four major rivers that enter the Gulf of Papua on the south coast of PNG. The Sepik inorganic PIC flux is low (1.4×10⁸ mol yr⁻¹) as the river does not drain carbonate soils. With a narrow continental shelf, and strong coastal currents, much of this exported material is available for long distance transport into the coastal Bismarck Sea and beyond.     

Sediment delivery,resuspension, and transport in two contrasting canyon environments in the southwest Gulf of Lions

Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35 km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5 mg/l. In Cap de Creus canyon head (located ∼14 km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20 mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists.     

Variability in the annual cycle of vertical particulate organic carbon export on Arctic shelves: Contrasting the Laptev Sea,Northern Baffin Bay and the Beaufort Sea

The ongoing regression of sea ice cover is expected to significantly affect the fate of organic carbon over the Arctic continental shelves. Long-term moored sediment traps were deployed in 2005–2006 in the Beaufort Sea, Northern Baffin Bay and the Laptev Sea to compare the annual variability of POC fluxes and to evaluate the factors regulating the annual cycle of carbon export over these continental shelves. Annual POC fluxes at 200 m ranged from 1.6 to 5.9 g C m⁻² yr⁻¹ with the highest export in Northern Baffin Bay and the lowest export over the Mackenzie Shelf in the Beaufort Sea. Each annual cycle exhibited an increase in POC export a few weeks before, during, or immediately following sea ice melt, but showed different patterns over the remainder of the cycle. Enhanced primary production, discharge of the Lena River, and resuspension events contributed to periods of elevated POC export over the Laptev Sea slope. High POC fluxes in Northern Baffin Bay reflected periods of elevated primary production in the North Water polynya. In the Beaufort Sea sediment resuspension contributed to most of the large export events. Our results suggest that the outer shelf of the Laptev Sea will likely sustain the largest increase in POC export in the next few years due to the large reduction in ice cover and the possible increase in the Lena River discharge. The large differences in forcing among the regions investigated reinforce the importance of monitoring POC fluxes in the different oceanographic regimes that characterize the Arctic shelves to assess the response of the Arctic Ocean carbon cycle to interannual variability and climate change.     

Air-sea CO2 fluxes in the southern Yellow Sea: An examination of the continental shelf pump hypothesis

The southern Yellow Sea (SYS), located to the north of the East China Sea (ECS), was considered part of the ECS when Tsunogai et al. (1999) proposed the “continental shelf pump” (CSP) hypothesis. However, the original CSP carbon dioxide (CO₂) uptake flux (2.9 mol C m⁻² yr⁻¹) appears to have been overestimated, primarily due to the differences between the SYS and the ECS in terms of their CO₂ system. In this paper, we estimated air-sea CO₂ fluxes in the SYS using the surface water partial pressure of CO₂ (pCO₂) measured in winter, spring, and summer, as well as that estimated in fall via the relationship of pCO₂ with salinity, temperature, and chlorophyll a. The results indicate that overall, the entire investigated area was a net source of atmospheric CO₂ during summer, winter, and fall, whereas it was a net sink during spring. Spatially, the nearshore area was almost a permanent CO₂ source, while the central SYS shifted from being a CO₂ sink in spring to a source in the other seasons of the year. Overall, the SYS is a net source of atmospheric CO₂ on an annual scale, releasing ∼7.38 Tg C (1 Tg=10¹² g) to the atmosphere annually. Thus, the updated CO₂ uptake flux in the combined SYS and ECS is reduced to ∼0.86 mol C m⁻² yr⁻¹. If this value is extrapolated globally following Tsunogai et al. (1999), the global continental shelf would be a sink of ∼0.29 Pg C yr⁻¹, instead of 1 Pg C yr⁻¹ (1 Pg=10¹⁵ g).The SYS as a net annual source of atmospheric CO₂ is in sharp contrast to most mid- and high-latitude continental shelves, which are CO₂ sinks. We argue that unlike the ECS and the North Sea where carbon on the shelf could be exported to the open ocean, the SYS lacks the physical conditions required by the CSP to transport carbon off the shelf effectively. The global validity of the CSP theory is thus questionable.     

Ross ice shelf cavity circulation,residence time,and melting: Results from a model of oceanic chlorofluorocarbons

Despite their harmful effects in the upper atmosphere, anthropogenic chlorofluorocarbons dissolved in seawater are extremely useful for studying ocean circulation and ventilation, particularly in remote locations. Because they behave as a passive tracer in seawater, and their atmospheric concentrations are well-mixed, well-known, and have changed over time, they are ideal for gaining insight into the oceanographic characteristics of the isolated cavities found under Antarctic ice shelves, where direct observations are difficult to obtain. Here we present results from a modeling study of air–sea chlorofluorocarbon exchange and ocean circulation in the Ross Sea, Antarctica. We compare our model estimates of oceanic CFC-12 concentrations along an ice shelf edge transect to field data collected during three cruises spanning 16 yr. Our model produces chlorofluorocarbon concentrations that are quite similar to those measured in the field, both in magnitude and distribution, showing high values near the surface, decreasing with depth, and increasing over time. After validating modeled circulation and air–sea gas exchange through comparison of modeled temperature, salinity, and chlorofluorocarbons with field data, we estimate that the residence time of water in the Ross Ice Shelf cavity is approximately 2.2 yr and that basal melt rates for the ice shelf average 10 cm yr⁻¹. The model predicts a seasonal signature to basal melting, with highest melt rates in the spring and also the fall.     

Benthic oxygen fluxes and sediment properties on the northeastern New Zealand continental shelf

We examined spatial variations in benthic remineralisation (measured as sediment oxygen consumption (SOC)) and sediment properties on the northeastern New Zealand continental shelf and slope to assess the importance of benthic mineralisation in this ecosystem and to provide data for more complete global carbon budgets. SOC measured in dark incubations conducted in early summer ranged from 128 μmol m⁻² h⁻¹ at the deepest (360 m) to 1222 μmol m⁻² h⁻¹ at the shallowest (4.2 m) site and decreased significantly with water depth (p<0.001, r²=0.78, SOC=1222.8−456.3×log₁₀[water depth], n=14 sites). These rates were in the range found on continental shelves elsewhere (64–1750 μmol m⁻² h⁻¹, n=30 studies) and had a very similar distribution with water depth. SOC was also measured in light incubations at seven sites (4.2–35 m water depth) to examine the effects of microphytobenthos and accounted for 42–106% of rates measured in the dark. Measurements of near-bed light intensities suggested that microphytobenthos production was not solely regulated by light intensity but evidently influenced by other factors. A two-dimensional PCA ordination of surface sediment properties accounted for 83.3% of the total variance in the data and divided the study area into three clusters that corresponded well to its spatial division into the shallow (<30 m) Firth of Thames, the Hauraki Gulf (30–50 m) and the northern shelf-slope region. In the Firth of Thames sediments were very fine-grained with low CaCO₃ and high total organic matter and pigment content, and low C:N ratios. The northern shelf-slope sediments showed the opposite trends to the Firth of Thames and those in the Hauraki Gulf had mostly intermediate values. Dark SOC was significantly correlated with sediment organic matter, carbon, nitrogen, pigments and silt/clay content (p<0.05, r=0.55–0.85) but a multiple linear regression revealed that water depth was the only significant predictor. Calculations suggest that approximately 13%, 10% and 34% of primary production is remineralised in the sediments of the northern shelf-slope region, Hauraki Gulf and Firth of Thames, respectively, indicating a strong benthic–pelagic coupling on the northeastern New Zealand continental shelf that was particularly pronounced in the Firth of Thames due to its shallow depth and significant terrestrial and riverine inputs.     

Winter variability of physical processes and sediment-transport events on the Eel River shelf,northern California

A 5-yr data set of near-bed current and suspended-sediment concentration measured within 2 m of the seabed in 60-m water depth has been analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River continental shelf, northern California. This data set encompasses a wide range of shelf conditions with winter events characterized as: Major Flood (1996/97), strong El Niño (1997/98), strong La Niña (1998/99), and Major Storm (1999/00). Data were collected at a site located 25 km north of the Eel River mouth, on the landward edge of the mid-shelf mud deposit. During the winter months sediment resuspension is forced primarily by near-bed oscillatory flows, and sediment transport occurs both as suspended load and as gravity-driven (fluid-mud) flows. Winter conditions that caused periods of increased sediment transport existed on average for 142 d yr⁻¹ over the total record, ranging between 89 d in the Major Flood year (1996/97) and 171 d in the La Niña year (1998/99). Hourly averaged values of significant wave height varied between 0.5 and 10.7 m and hourly averaged values of near-bed orbital velocities ranged between 0 and 125 cm s⁻¹. During the five winters, sediment threshold conditions were exceeded an average of 35% of the time, ranging from 19% in the Major Flood year (1996/97) to 52% in the La Niña year (1998/99). Mean concentration of suspended sediment, measured at 30 cmab, ranged from values close to 0–8 g l⁻¹. Among winters, major sediment flux events exhibited different patterns due to varying combinations of physical processes including river floods, waves, and shelf circulation. Within winters, the major period of sediment flux varied from a 3-d fluid mud event (Major Flood winter) to a 50-d period of persistent southerlies (El Niño winter) and a winter of continuous storm cycles (La Niña winter). Winter-averaged suspended-sediment concentration appeared to vary in response to river discharge, while total sediment flux responded to storm intensity. The net sediment flux appeared to depend on timing of river discharge and shelf conditions. On the Eel River shelf, the mid-shelf mud deposit apparently is not emplaced by deposition from the river plume, but by secondary processes from the inner shelf including off-shelf transport of sediment suspensions and gravity-driven fluid-mud flows. Thus, these inner-shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that forms and nourishes the mid-shelf deposit. Large-scale shelf circulation patterns and interannual variability of the physical forcing are also important in determining the locus of the mid-shelf deposit, and both are influenced by climate variations. Post-depositional alteration of the deposit also depends on the subsequent shelf conditions following major floods.     

A quantitative examination of modern sedimentary lithofacies formation on the glacially influenced Gulf of Alaska continental shelf

Continental-shelf lithofacies are described from a series of cores collected in the northern Gulf of Alaska, a high-energy paraglacial shelf experiencing rapid rates of sediment accumulation. Short-lived tracers (²³⁴Th and chlorophyll-a) indicate that during the annual peak in fluvial sediment input (summer), biologic sediment mixing coefficients in the surficial seabed are generally lower than other coastal environments (<20 cm² yr⁻¹) and mixing extends downward <10 cm.²¹⁰Pb geochronology indicates that sediment accumulation rates (time scales of 10–100 yr) are 0.1–3 cm yr⁻¹. The measured bioturbation and accumulation rates lead to predictions of moderate to bioturbated lithofacies, as observed. Primary depositional fabric is preferentially preserved where sediment accumulation rates >2 cm yr⁻¹ and non-steady sediment deposition occurs. Depositional fabric is also observed in strata at 50–100 m water depths and is similar in appearance to beds that may form through deposition of wave-induced fluid-mud flows, which have been observed forming on other shelves with moderate to high wave energy. Five general lithofacies can be identified for the study area: inner-shelf sand facies, interbedded sandy mud facies, moderate-to-well-bioturbated mud facies, gravelly mud facies, and Tertiary bedrock facies. The moderate-to-well-bioturbated mud facies is areally dominant, representing over 50% of the shelf area, although roughly equal volumes (∼0.4 km³) of strata with some preservation of primary fabric are annually accumulating. Lithofacies on this paraglacial shelf generally resemble mid- and low-latitude allochthonous shelf strata to a much greater degree than Holocene glacimarine strata formed on shelves dominated by icebergs and floating ice shelves. Paraglacial strata may be differentiated from non-glacial shelf strata by lower organic carbon concentrations, a relatively lower degree of bioturbation, and increased preservation of primary depositional fabric.     

River water and nutrient discharges in the Northern Adriatic Sea: Current importance and long term changes

Runoff and nutrient transport by rivers were analysed in the Northern Adriatic continental shelf, in order to evaluate their interannual and multidecal variability, as well as their current contribution to determine freshwater and nutrient budgets in this marine region. During the years 2004-2007, the runoff in the basin (34.1-64.6 km³ yr⁻¹) was highly imbalanced, being 84% of freshwater discharged along the western coast, because of the contributions of Po, Adige and Brenta rivers. In the northern and eastern sections of the coast, freshwater discharge by rivers was less important (10 and 6%, respectively), but not negligible in determining the oceanographic properties at sub-regional scales. The oscillations of the transport of biogenic elements (124-262×10³ t N yr⁻¹ for TN, 72-136×10³ t N yr⁻¹ for DIN, 4.5-11.1×10³t P yr⁻¹ for TP, 2.2-3.5×10³ t P yr⁻¹ for PO₄ and 104-196×10³ t Si yr⁻¹ for SiO₂) were strictly dependant to the differences in the annual runoff. A strong excess of N load in comparison to P load characterised all rivers, both in inorganic nutrient (DIN/PO4=37-418) and total (TN/TP=48-208) pools, particularly in the northern and eastern areas of the basin.The annual runoff showed significant oscillations for Po on multidecadal time scale, whereas a general decrease (−33%) was observed for the other N Adriatic rivers as the recent discharges were compared to those before the 1980s. During the dry years 2005-2007, a strong reduction of river water flows and nutrient loads was experienced by the N Adriatic ecosystem with respect to years characterised by medium-high regimes. An increased frequency of similar drought periods, due to ongoing climate changes or to a larger human usage of continental waters, would be easily able to significantly change the biogeochemistry of this basin.     

Phytoplankton biomass and primary production responses to physico-chemical forcing across the northeastern New Zealand continental shelf

Phytoplankton biomass and primary production were monitored in the Hauraki Gulf and on the northeastern continental shelf, New Zealand - using ship surveys, moored instruments and satellite observations (1998-2001) - capturing variability across a range of space and time scales. A depth-integrated primary production model (DIM) was used to predict integrated productivity from surface parameters, enabling regional-specific estimates from satellite data. The shelf site was dominated by pico-phytoplankton, with low chlorophyll-a (<1 mg m⁻³) and annual production (136 g C m⁻² yr⁻¹). In contrast, the gulf contained a micro/nano-phytoplankton-dominated community, with relatively high chlorophyll-a (>1 mg m⁻³) and annual production (178 g C m⁻² yr⁻¹). Biomass and productivity responded to physico-chemical factors; a combination of light, critical mixing depths and/or nutrient limitation—particularly new nitrate-N. Relatively low biomass and production was observed during 1999. This coincided with inter-annual variability in the timing and extent of upwelling- and downwelling-favourable along-shelf wind-stress, influencing the fluxes of new nitrate-N to the shelf and gulf. Relationships with the Southern Oscillation Index are also discussed. Our multi-scaled sampling highlighted details associated with stratification and de-stratification events, and deep sub-surface chlorophyll-a not visible to satellite sensors. This study demonstrates the importance of multi-scaled sampling in gaining estimates of regional production and its responses to physico-chemical forcing.     

Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic—A model study,Part II: Carbon budget and fluxes

The 3-d coupled physical–biogeochemical model ECOHAM (version 3) was applied to the Northwest-European Shelf (47°41′–63°53′N, 15°5′W–13°55′E) for the years 1993–1996. Carbon fluxes were calculated for the years 1995 and 1996 for the inner shelf region, the North Sea (511,725 km²). This period was chosen because it corresponds to a shift from a very high winter-time North Atlantic Oscillation Index (NAOI) in 1994/1995, to an extremely low one in 1995/1996, with consequences for the North Sea physics and biogeochemistry. During the first half of 1996, the observed mean SST was about 1 °C lower than in 1995; in the southern part of the North Sea the difference was even larger (up to 3 °C). Due to a different wind regime, the normally prevailing anti-clockwise circulation, as found in winter 1995, was replaced by more complicated circulation patterns in winter 1996. Decreased precipitation over the drainage area of the continental rivers led to a reduction in the total (inorganic and organic) riverine carbon load to the North Sea from 476 Gmol C yr⁻¹ in 1995 to 340 Gmol C yr⁻¹ in 1996. In addition, the North Sea took up 503 Gmol C yr⁻¹ of CO₂ from the atmosphere. According to our calculations, the North Sea was a sink for atmospheric CO₂, at a rate of 0.98 mol C m⁻² yr⁻¹, for both years. The North Sea is divided into two sub-systems: the shallow southern North Sea (SNS; 190,765 km²) and the deeper northern North Sea (NNS; 320,960 km²). According to our findings the SNS is a net-autotrophic system (net ecosystem production NEP>0) but released CO₂ to the atmosphere: 159 Gmol C yr⁻¹ in 1995 and 59 Gmol C yr⁻¹ in 1996. There, the temperature-driven release of CO₂ outcompetes the biological CO₂ drawdown. In the NNS, where respiratory processes prevail (NEP<0), 662 and 562 Gmol C yr⁻¹ were taken up from the atmosphere in 1995 and 1996, respectively. Stratification separates the productive, upper layer from the deeper layers of the water column where respiration/remineralization takes place. Duration and stability of the stratification are determined by the meteorological conditions, in relation to the NAO. Our results suggest that this mechanism controlling the nutrient supply to the upper layer in the northern and central North Sea has a larger impact on the carbon fluxes than changes in lateral transport due to NAOI variations. The North Sea as a whole imports organic carbon and exports inorganic carbon across the outer boundaries, and was found to be net-heterotrophic, more markedly in 1996 than in 1995.     

Fluvial incision and coarse gravel redistribution across the modern Dead Sea shelf as a result of base-level fall

Global eustatic lowstands can expose vast areas of continental shelves, and occasionally the shelf edge and the continental slope. The degree of fluvial connectivity to receding shores influences the redistribution of sediments across these emerging landscapes. Shelf and slope emergence in the Dead Sea since the middle of the 20th century, offers a rare opportunity to examine evolution of stream connectivity in response to continuous base-level decline. We characterize the connectivity evolution of two streams, using high-resolution time series of aerial imagery and elevation models, field mapping, and grain-size analyses. Our rich spatiotemporal dataset of evolving channel geomorphology, sediment transport conditions, and sediment redistribution, allows calculating potential coarse sediment mobility in response to base level decline. Following shelf emergence, alluvial fans first prograded onto the low-gradient shelf under unfavourable conditions for transporting coarse sediment to the regressing shoreline. Then, with shelf and slope emergence, the two adjacent streams evolved differently. The smaller, more arid watershed still maintains its highstand delta progradation on the shelf and is practically disconnected from the receding lake. The larger catchment, heading in wetter environments and having a narrower shelf, has incised the shelf and renewed and gradually intensified the sediment transport from the highstand to the lowstand delta. Sediment mobilization to lowstand shorelines is controlled by the evolution of the channel profile and by the average speed of gravel transport (10s-100s m yr^-1). These findings from the Dead Sea are relevant to fluvial processes operating on continental shelves during glacial maxima. Streams would have commonly stored high proportions of their coarse sediment on the continental shelves rather than efficiently connecting with the lowstand level. Additionally, differences in sediment routing patterns should exist among nearby streams, primarily due to continental margin geometry and watershed hydrology. © 2019 John Wiley & Sons, Ltd.     

Dispersal of Mississippi and Atchafalaya sediment on the Texas-Louisiana shelf: Model estimates for the year 1993

A three-dimensional coupled hydrodynamic-sediment transport model for the Texas-Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed.At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20-40 km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (<10 m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the “Dead Zone” (low-oxygen area) within a seasonal-annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and “stepwise-like” compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave-current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf.     

Satellite observation of Brazil Current inshore thermal front in the SW South Atlantic: Space/time variability and sea surface temperatures

A data set of 199 sea surface temperature maps derived from the Advanced Very High Resolution Radiometer for the period 2000–2002 was processed to derive the position of the surface inshore thermal front of the Brazil Current (BCIF) in the SE Brazilian coastal and oceanic area. After the derivation of the position of the BC front for each image, the ensemble of digital frontal vectors was processed using the algorithm of frontal density (FD). For each 5′×5′ cell in the domain the calculated FD provided an index expressing the presence and persistence of the front in the area or the probability of finding the front in the region. In the paper we present the results of the FD analysis to get a better view of the space and time variability of the BC front in the region. The highest values of FD were in general observed close to or at the shelf break zone (between 200 and 1000 m isobaths). From 20°S to 23°S there is a tendency of BCIF to be positioned over the outer shelf, inshore of the 200 m isobaths. SE of Cape Sao Tome and S of Cape Frio it was observed a bimodal spatial distribution of highest FD caused by the presence of two semi-permanent frontal eddies. After moving offshore near Cape Frio, the BCIF tends to return to the shelf break zone south of 24°S probably due to a potential vorticity conservation mechanism. The position of the highest FD values calculated for different seasons confirms previous studies in that BCIF is closer to the coast during the summer and furthest offshore in the winter. Statistical analysis of the SST data gave for the BCIF an average SST gradient of 0.31°C km⁻¹ with a standard deviation of 0.15°C km⁻¹. A mean frontal width of 6 km was inferred from the average SST gradient and typical temperatures near the front at both sides, at outer shelf and in the BC itself. A Weibull probability density function can be fitted to describe the BCIF SST gradients with scale factor c=0.3460°C km⁻¹ and shape factor k=2.1737. The BCIF SST gradient showed a seasonal variability with the smallest gradients in summer (~0.24 °C km⁻¹) and the highest in autumn (~0.33 °C km⁻¹). Using a three harmonic Fourier fit for the SST field near the BCIF, at the outer shelf and at interior of BC, it was possible to derive an analytical model for the time variability of the SST gradient of BCIF.     

Seasonal variations in the inorganic carbon system in the Pearl River (Zhujiang) estuary

Seasonal variations in the inorganic carbon system in the Pearl River estuary are examined based on data from five surveys during the spring, summer, fall, and winter seasons. Both total dissolved inorganic carbon (DIC) and total alkalinity (TAlk) values in the freshwater end-members are high in the dry season (>2700 μmol kg⁻¹ for DIC and >2400 μmol kg⁻¹ for TAlk) and substantially lower in the wet season (DIC and TAlk were ∼1000 and 700 μmol kg⁻¹, respectively). Riverine DIC flux and drainage basin weathering rates, however, are significantly higher in the wet season (611×10⁹ mol yr⁻¹ and 13.6×10⁵ mol km⁻² yr⁻¹) than in the dry season (237×10⁹ mol yr⁻¹ and 5.3×10⁵ mol km⁻² yr⁻¹).     

Interannual (1999-2005) morphodynamic evolution of macro-tidal salt marshes in Mont-Saint-Michel Bay (France)

This paper provides a detailed study on the sedimentation patterns and the recent morphodynamic evolution affecting the macro-tidal salt marshes located west of the Mont-Saint-Michel (France). Twenty-two stations along three transects on the marshes were seasonally monitored for marsh surface level variations from 1999 to 2005, using a sediment erosion bar. The corresponding erosion/accretion rates were obtained together with data on topography, vegetation cover, and grain size of surface sediment. To examine the mechanisms contributing to the salt marsh sedimentation, the data and their evolution were treated with respect to tides, relative mean regional sea level, and wind speed/frequency variations.From 1999 to 2005, the marsh was globally accreting (from 3.45 to 38.11 mm yr⁻¹ in the low marsh, up to 4.91 mm yr⁻¹ in the middle marsh, and up to 1.35 mm yr⁻¹ in the high marsh), while the study was conducted during a window of decreasing trend in mean regional sea level (−2.45 mm yr⁻¹ according to regional-averaged time series). These sedimentation rates are one of the highest recorded worldwide; however, the sedimentation was not found to be continuous over the period in question. This pattern is illustrated by the strong extension of the marshes from 1999 to 2002, and the relative stability observed from 2003 to 2005. The imported and reworked sediments are trapped and fixed by the dense vegetation (Puccinellia maritima, Halimione portulacoides), inducing the general seaward extension of the marshes. The processes governing sediment budget (accretion/erosion) show annual, seasonal, and spatial variability on the marsh. Spatial variations display contrasted patterns of erosion/sedimentation between the low, middle, and high marsh, and between the different transects. These patterns are a result of distance from sediment sources, strong heterogeneity in vegetation cover (human induced or not), and contrasting topographic and micro-topographic characteristics. The higher accretion rates are observed in distal settings in the low marsh, and strongly decrease toward the middle and high marsh. This evolution results from a decrease in accommodation space/water column thickness, and frequency of inundation coupled with an increase in station elevation, but also from the cumulated effects of vegetation cover and micro-topography. The vegetation cover of the low and middle marsh enhance the settling and fixing of fine sediments imported through tides or dispersed by flood and ebb currents.The seasonal evolution of the marshes is marked by contrasting effects of water storage in the sediment. The overall seasonal sediment budget is controlled by the variation of the frequency of inundation relative to tidal range and marshes topography. Autumns are influenced by the tide (equinoxes), relative mean regional sea level, and variations in wind speed/frequency. Winter wind speed and frequency in relation with tidal variations appear to be the main parameters regulating winter marsh evolution. Summers are predominantly under the influence of local variations in water storage (desiccation) while external parameters generally display a low influence. Although it is not governed by any one parameter, springtime sediment budget seems to result from strong interaction between the above-cited parameters, despite the significant frequency of inundation (equinoxes).     

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi,Pearl, and Rhône rivers

We examined the occurrence of seasonal hypoxia (O₂<2 mg l⁻¹) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m⁻² d⁻¹), and lower off the Rhône River (<1 g C m⁻² d⁻¹), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia.     

On sediment accumulation rates and stratigraphic completeness: Lessons from Holocene ocean margins

Stratigraphic completeness is a fundamental consideration when deciphering the mass accumulation history of sediments and the geologic record of earth and ocean processes. In this study, stratigraphic completeness was examined in the context of late Holocene sedimentary successions using published sediment accumulation rates for five ocean margin systems (Amazon shelf, Hudson estuary, northern California shelf, Mid-Atlantic slope, Santa Monica Bay). Plots of mass accumulation rate versus time span of averaging were used to determine how rates scale with measurement period, and to estimate levels of stratigraphic completeness for comparison within and among margin systems. Statistically significant inverse correlations between accumulation rate and time span of averaging are indicated for all but one of these systems—most of the sedimentary records examined are stratigraphically incomplete. At the 10³-yr level of resolution, completeness is 20–48% for strongly tidal estuarine (Hudson estuary) and deltaic shelf (Amazon shelf) sites, 51–91% for accretionary shelves (northern California shelf) and slopes (Mid-Atlantic slope), and 85–100% for a sediment-starved slope (Santa Monica bay). Mass accumulation rates converge to a relatively narrow range (0.01–0.1 g cm⁻² yr⁻¹) at the 10⁴-yr level of resolution, consistent with the notion that there are universal controls on sediment accumulation rate, i.e., rate of sea-level rise and sediment supply. Among sites on the Amazon and northern California shelves, within-system completeness varies by ∼10–20% on account of site-specific sedimentary processes that preferentially trap or disperse suspended sediment. Overall, stratigraphic completeness increases with water depth shelf-to-slope, yet depth is not a robust predictor of completeness in general owing to differences in strata-forming processes among shallow-marine environments. Significantly, completeness varies inversely with instantaneous deposition rate as the most sediment-rich systems tend to exhibit the most incomplete sedimentary records. The findings of this study emphasize the importance of considering time span and fidelity when interpreting the accumulation history of modern and Holocene sedimentary strata.