Methane in waters and bottom sediments of the North (Severnaya) Dvina mouth area in 2004-2006

The mouth area of the North (Severnaya) Dvina River is characterized by a high concentrations of methane in water (from 1.0 to 165.4 µl/l) and bottom sediments (from 14 to 65000 µl/kg), being quite comparable to productive mouth areas of rivers from the temperate zone. Maximum methane concentrations in water and sediments were registered in the delta in segments of channels and branches with low rates of tidal and runoff currents, where domestic and industrial wastewaters are supplied. In the riverine and marine water mixing zone with its upper boundary, locating far into the delta and moving depending on a phase of the tidal cycle, decrease of methane concentration with salinity increase was observed. The prevailing role in formation of the methane concentration level in water of the mouth area pertains to bottom sediments, which is indicated by close correlation between gas concentrations in these two media. Existence of periodicity in variations of methane concentration in river water downstream caused by tidal effects was found.

Composition of suspended matter and bottom sediments from the North Dvina mouth area

Grain-size, mineral and chemical compositions of suspended particulate matter (SPM) from waters of the Severnaya (North) Dvina River mouth area during the spring flood in May 2004 is studied. Data published on composition of riverine SPM in the White Sea basin are very poor. The spring flood period when more than half of annual runoff is supplied from the river to the sea in during short time is understood more poorly. The paper considers comparison results of the grain size compositions of SPM and bottom sediments. Data of laser and hydraulic techniques of grain size analysis are compared. Short-period variations of SPM concentration and composition representing two diurnal peaks of the tide level are studied. It is found that SPM is mainly transferred during the spring flood as mineral aggregates up to 40 µm diameter. Sandy-silty fraction of riverine SPM settles in delta branches and channels, and bulk of clay-size material is supplied to the sea. Mineral and chemical compositions of SPM from the North Dvina River are determined by supply of material from the drainage basin. This material is subjected to intense mechanic separation during transfer to the sea. Key regularities of formation of mineral composition of SPM during the flood time are revealed. Effect of SPM grain size composition on distribution of minerals and chemical elements in study in the dynamic system of the river mouth area are characterized.

Grain-size distribution, sedimentation rate and x-ray fluorescence data of four sediment cores off the Senegal River estuary

Fine-grained sediment depocenters on continental shelves are of increased scientific interest since they record environmental changes sensitively. A north-south elongated mud depocenter extends along the Senegalese coast in mid-shelf position. Shallow-acoustic profiling was carried out to determine extent, geometry and internal structures of this sedimentary body. In addition, four sediment cores were retrieved with the main aim to identify how paleoclimatic signals and coastal changes have controlled the formation of this mud depocenter. A general paleoclimatic pattern in terms of fluvial input appears to be recorded in this depositional archive. Intervals characterized by high terrigenous input, high sedimentation rates and fine grain sizes occur roughly contemporaneously in all cores and are interpreted as corresponding to intensified river discharge related to more humid conditions in the hinterland. From 2750 to 1900 and from 1000 to 700 cal a BP, wetter conditions are recorded off Senegal, an observation which is in accordance with other records from NW-Africa. Nevertheless, the three employed proxies (sedimentation rate, grain size and elemental distribution) do not always display consistent inter-core patterns. Major differences between the individual core records are attributed to sediment remobilization which was linked to local hydrographic variations as well as reorganizations of the coastal system. The Senegal mud belt is a layered inhomogeneous sedimentary body deposited on an irregular erosive surface. Early Holocene deceleration in the rate of the sea-level rise could have enabled initial mud deposition on the shelf. These favorable conditions for mud deposition occur coevally with a humid period over NW-Africa, thus, high river discharge. Sedimentation started preferentially in the northern areas of the mud belt. During mid-Holocene, a marine incursion led to the formation of an embayment. Afterwards, sedimentation in the north was interrupted in association with a remarkable southward shift in the location of the active depocenter as it is reflected by the sedimentary architecture and confirmed by radiocarbon dates. These sub-recent shifts in depocenters location are caused by migrations of the Senegal River mouth. During late Holocene times, the weakening of river discharge allowed the longshore currents to build up a chain of beach barriers which have forced the river mouth to shift southwards.

Geochemical and physical sediment properties of sediment cores from the Portuguese shelf

Marine sediments from the Portuguese shelf are influenced by environmental changes in the surrounding continental and marine environment. These are largely controlled by the North Atlantic Oscillation, but additional impacts may arise from episodic tsunamis. In order to investigate these influences, a high resolution multi-proxy study has been carried out on a 5.4 m long gravity core and five box cores from the Tagus prodelta on the western Portuguese margin, incorporating geochemical (Corg/Ntotal ratios, d13Corg, d15N, d18O, Corg and CaCO3 content) and physical sediment properties (magnetic susceptibility, grain-size). Subsurface data of the five box cores indicate no major effect of early postdepositional alteration. Surface data show a higher fraction of terrigenous organic material close to the river mouth and in the southern prodelta. Gravity core GeoB 8903 covers the last 3.2 kyrs with a temporal resolution of at least 0.1 cm/yr. Very high sedimentation rates between 69 and 140 cm core depth indicate a possible disturbance of the record by the AD1755 tsunami, although no evidence for a disturbance is observed in the data. Sea surface temperature and salinity on the prodelta, the local budget of marine NO3- as well as the provenance of organic matter remained virtually constant during the past 3.2 kyrs. A positive correlation between magnetic susceptibility (MS) and North Atlantic Oscillation (NAO) is evident for the past 250 years, coinciding with a negative correlation between mean grain-size and NAO. This is assigned to a constant riverine supply of fine material with high MS, which is diluted by the riverine input of a coarser, low-MS component during NAO negative, high-precipitation phases. End-member modelling of the lithic grain-size spectrum supports this, revealing a third, coarse lithic component. The high abundance of this coarse end-member prior to 2 kyr BP is interpreted as the result of stronger bottom currents, concentrating the coarse sediment fraction by winnowing. As continental climate was more arid prior to 2 kyr BP (Subboreal), the coarse end-member may also consist of dust from local sources. A decrease in grain-size and CaCO3 content after 2 kyr BP is interpreted as a result of decreasing wind strength. The onset of a fining trend and a further decrease in CaCO3 around AD900 occurs simultaneous to climatic variations, reconstructed from eastern North Atlantic records. A strong increase in MS between AD1400 and AD1500 indicates higher lithic terrigenous input, caused by deforestation in the hinterland.

(Table 1) Insoluble aerosol concentrations and vertical fluxes at the eastern shelf of the Black Sea during breeze circulation in October 1987

Amounts of aerosols transported to the shelf surface were calculated on the basis of in situ measurements of concentrations of eolian matter (insoluble aerosol fraction) and vertical fluxes of settling dust in five areas of the Black Sea shelf from the Danube delta to the Inguri River mouth. More than 8.3 mln t of eolian matter are annually transported from the land over the shelf of the former USSR. At the same time more than 5.4 mln t are supplied to the northwestern shelf area, 1.7 mln t are supplied to the Crimean area, about 0.8 mln t are supplied to the Kerch-Taman' area, and about 0.45 mln t are supplied to the Caucasian area.

Stable isotope ratios from foraminfera in Kara Sea sediments

Ostracods secrete their valve calcite within a few hours or days, therefore, its isotopic composition records ambient environmental conditions of only a short time span. Hydrographic changes between the calcification of individuals lead to a corresponding range (max.-min.) in the isotope values when measuring several (>=5) single valves from a specific sediment sample. Analyses of living (stained) ostracods from the Kara Sea sediment surface revealed high ranges of >2per mil of d18O and d13C at low absolute levels (d18O: <3per mil, d13C: <-3per mil) near the river estuaries of Ob and Yenisei and low ranges of not, vert, similar1per mil at higher absolute levels (d18O: 2-5.4per mil, d13C: -3 per mil to -1.5per mil) on the shelf and in submarine paleo-river channels. Comparison with a hydrographic data base and isotope measurements of bottom water samples shows that the average and the span of the ostracod-based isotope ranges closely mirror the long-term means and variabilities (standard deviation) of bottom water temperature and salinity. The bottom hydrography in the southern part of the Kara Sea shows strong response to the river discharge and its extreme seasonal and interannual variability. Less variable hydrographic conditions are indicative for deeper shelf areas to the north, but also for areas near the river estuaries along submarine paleo-river channels, which act as corridors for southward flowing cold and saline bottom water. Isotope analyses on up to five single ostracod valves per sample in the lower section (8-7 cal. ka BP) of a sediment core north of Yenisei estuary revealed d18O and d13C values which on average are lower by 0.6? in both, d18O and d13C, than in the upper core section (<5 cal. ka BP). The isotope shifts illustrate the decreasing influence of isotopically light river water at the bottom as a result of the southward retreat of the Yenisei river mouth from the coring site due to global sea level rise. However, the ranges (max.-min.) in the single-valve d18O and d13C data of the individual core samples are similar in the upper and in the lower core section, although a higher hydrographic variability is expected prior to 7 cal. ka BP due to river proximity. This lack of variability indicates the southward flow of cold, saline water along a submarine paleo-river channel, formerly existing at the core location. Despite shallowing of the site due to sediment filling of the channel and isostatic uplift of the area, the hydrographic variability at the core location remained low during the Late Holocene, because the shallowing proceeded synchronously with the retreat of the river mouth due to the global sea level rise

Reconstruction of the provenance and distribution of terrigenous sediments off NE South America over the last 30ka

We investigate the redistribution of terrigenous materials in the northeastern (NE) South American continental margin during slowdown events of the Atlantic Meridional Overturning Circulation (AMOC). The compilation of stratigraphic data from 108 marine sediment cores collected across the western tropical Atlantic shows an extreme rise in sedimentation rates off the Parnaíba River mouth (about 2°S) during Heinrich Stadial 1 (HS1, 18-15 ka). Sediment core GeoB16206-1, raised offshore the Parnaíba River mouth, documents relatively constant 143Nd/144Nd values (expressed as epsilonNd(0)) throughout the last 30 ka. Whereas the homogeneous epsilonNd(0) data support the input of fluvial sediments by the Parnaíba River from the same source area directly onshore, the increases in Fe/Ca, Al/Si and Rb/Sr during HS1 indicate a marked intensification of fluvial erosion in the Parnaíba River drainage basin. In contrast, the epsilonNd(0) values from sediment core GeoB16224-1 collected off French Guiana (about 7°N) suggest Amazon-sourced materials within the last 30 ka. We attribute the extremely high volume of terrigenous sediments deposited offshore the Parnaíba River mouth during HS1 to (i) an enhanced precipitation in the catchment region and (ii) a reduced North Brazil Current, which are both associated with a weakened AMOC.

Phytoplankton in the Nhatrang Bay, South China Sea

Species composition, phytoplankton abundance, and relative yield of variable fluorescence (F_v/F_m) were determined in the mesotrophic Nhatrang Bay in October-November 2004. Species diversity (250 taxonomic units) and heterogeneity of the phytoplankton structure were high. With respect to number of species and their abundance, diatoms prevailed. In selected parts of the bay, dinoflagellates dominated. Average biomass in the water column under 1 m**2 (Bt) varied from 2.3 to 64.4 mg C/m**3 (av. 31.0 mg C/m**3). Bt values were the lowest at stations nearest to the river mouth. Seaward, Bt increased. Bt values increased with depth at some stations and decreased at others. In surface layers biomass was lower than that in the underlying waters. F_v/F_m values ranged from 0.10 to 0.64 (av. 0.49). The lowest F_v/F_m values were observed in the area close to the seaport. Over greater part of the bay, F_v/F_m values were higher than 0.47. Such values are indicative of relatively high potential of photosynthetic activity of phytoplankton. Abundance and species diversity were higher than those in the dry season (March-April).

Updated bathymetric chart of the East Aral Sea

The Aral Sea is located in an arid region with much sand and salt deposits in the surrounding barren open land. Hence, significant displacements of sediments into the lakebed by the action of wind, water, gravity, or snow are likely. The bathymetry of the lake was last observed in the 1960s, and within the last half century, the structure of the lakebed has changed. Based on satellite observations of the temporal changes of shoreline (Landsat optical remote sensing) and water level (multi-mission satellite altimetry) over more than one decade an updated bathymetric chart for the East Basin of the Aral Sea has been generated. During this time, the geometry of the shallow East Basin experienced strong fluctuations due to the occurrence of periods of drying and strong inflow. By the year 2014 the East Basin fell dry. The dynamic behavior of the basin allowed for estimating the lake's bathymetry from a series of satellite-based information. The river mouth made its impression in the present East Aral Sea, because its shrinking led to the inflow of much sediment into the lake's interior. In addition, salt deposits along the shorelines increased the corresponding elevation, a phenomenon that is more pronounced in the reduced lakebed because of increased salinity. It must be noted that height estimates from satellite altimetry were only possible down to a minimum elevation of 27 m above sea level due to a lack of reliable altimetry data over the largely reduced water surface. In order to construct a complete bathymetric chart of the lakebed of the East Aral Sea heights below 27 m were obtained solely from Landsat optical images following the SRB (Selected Region Boundary) approach as described by Singh et al. (2015).

Benthos investigations in the Lena Delta

According to monitoring data gained between 1982-1992, macrobenthos in the Tiksi Bay is characterized by low indices of the total abundance, biomass and taxonomic diversity. 30 macrobenthic species have been recorded in the Tiksi Bay. The bottom biocenoses within the estuarine-arctic water mass consist of widespread eurybiontic boreal-arctic and brackish-water species. The maximal number of species was observed at a depth of 8.5 m. The maximum biomass was recorded on muddy grounds. The studied bottom fauna is characterized by a high population density (from 1160-600 ind/m**2) and low biomass of 15.5-22.4 g/m**2. The predominant benthic animals of the main Lena River channel 4.7 km upstream Stolb Island are Chironomidae, Plecoptera and Oligochaeta. In total, 48 species of macrobenthos were registered here. In spring the average density of macrozoobenthos in the channel is 680, in summer 770, in autumn 720 and in winter 380 ind/m**2, with the average biomass varying between 2.9 g/m**2 in spring, 7.06 in summer, 4.4 in autumn, and 2.6 in winter.

Stable isotope ratios measured on carbonate tests from the Kara Sea

River discharge of Ob and Yenisei to the Kara Sea is highly variable on seasonal and interannual time scales. River water dominates the shallow bottom water near the river mouths, making it warmer and less saline but seasonally and interannually more changeable than bottom water on the deeper shelf. This hydrographic pattern shows up in measurements and modelling, and in stable isotope records (delta18O, delta13C) along the growth axis of bivalve shells and in multiple analyses of single benthic foraminiferal shells. Average isotope ratios increase, but sample-internal variability decreases with water depth and distance from river mouths. However, isotope records of bivalves and foraminifera of a sediment core from a former submarine channel of Yenisei River reveal a different pattern. The retreat of the river mouth from this site due to early Holocene sea level rise led to increasing average isotope values up core, but not to the expected decrease of the in-sample isotope variability. Southward advection of cold saline water along the palaeo-river channel probably obscured the hydrographic variability during the early Holocene. Later, when sediment filled the channel, the hydrographic variability at the core location remained low, because the shallowing proceeded synchronously with the retreat of the river mouth.