Deep Central Basin Research Articles

Modern Sedimentation Patterns in Lake Issyk Kul (Kyrgyzstan), Derived From Surface Sediment and Inlet Stream Samples

AbstractThis study provides a descriptive characterization of the modern sedimentary processes in Lake Issyk Kul, Kyrgyzstan, important for the selection of a suitable coring or deep‐drilling site, interpretation of future core data and applicability of proxies. The quasi‐equidistant sampling grid of 66 sediment surface samples covers the entirety of the lake basin and is complemented by 10 samples from the major inflows. The methodological approach includes geochemical, granulometric, lipid biomarker, diatom, and statistical analyses. The quantitative and qualitative changes in sediment composition yield information on its generic origin and prevailing transport and depositional environments. The composition of the surface sediments in Issyk Kul is highly heterogenous. Nearshore deposition is mainly controlled by wave action and by fluvial sediment supply with highest quantities of detrital input coming from the high‐energetic, eastern tributaries. Sediments in the deep central basin are mainly produced in situ and dominated by authigenic calcite. Biogenic accumulation is overall low, except for the western extremity of the lake, where the nearshore, shallow‐water, and low‐energetic environment favors aquatic productivity and subsequent preservation of organic material and diatoms. Redeposition of sediments is a dominant process along the slopes across the southern and western basin floor, where run‐out distances of mass movement deposits are up to 5 km. Directional sediment transport by lake currents appears to be less important, except for the transport of very fine‐grained organic matter. Biomarker‐inferred temperature reconstructions suggest lake surface temperatures of ∼15°C in the western littoral zone and in Tyup Bay and a decrease to ∼13°C basinward.

Drivers of Marine CO2‐Carbonate Chemistry in the Northern Antarctic Peninsula

AbstractThe Bransfield Strait is a climate change hotspot at the tip of the northern Antarctic Peninsula (NAP). The region is marked by a mixture of relatively warm waters from the Bellingshausen Sea with cold shelf waters from the Weddell Sea. Additionally, its deep central basin (>800 m) preserves seawater properties from the north‐western Weddell Sea continental shelf. This study assessed long‐term changes in carbonate chemistry in the Bransfield Strait and found that the hydrographic setting (i.e., a mixture between modified‐Circumpolar Deep Water with Dense Shelf Water [DSW]) drives temporal variability of carbonate parameters. The western basin has experienced decreases in pH (seawater scale) over the last three decades (1996–2019), varying from −0.003 to −0.017 pH units yr−1, while Ωar decreased from −0.01 to −0.07 yr−1 throughout the water column. The central basin was characterized by a high contribution of DSW with high carbon dioxide (CO2) content and the decomposition of organic matter produced and transported into its deep layer. With lower variability for all carbonate system variables, the eastern basin was likely regulated by internal mixing. Overall, the entire strait is almost reaching a CO2‐saturated condition, highlighting how sensitive subpolar regions are to the effects of human‐induced climate change.

Water Exchange between Deep Basins of the Bransfield Strait

The Bransfield Strait is a relatively deep and narrow channel between the South Shetland Islands and the Antarctic Peninsula contributing to the water transport between the Pacific and Atlantic sectors of the Southern Ocean. The strait can be divided into three deep separate basins, namely, the western, central, and eastern basins. The sources of deep waters in the three basins are different, leading to differences in thermohaline properties and water density between the basins. The difference in water density should in turn cause intense deep currents from one basin to another through narrow passages over the sills separating the basins. However, there are still no works dedicated to such possible overflows in the Bransfield Strait. In this study, we report our new CTD and LADCP measurements performed in 2022 over the watersheds between the basins. Quasisimultaneous observations of the main circulation patterns carried out at several sections allowed us to analyze the evolution of thermohaline and kinematic structures along the Bransfield Strait. Volume transports of waters in the strait were estimated on the basis of direct velocity observations. These new data also indicate the existence of intense and variable deep current between the central and eastern basins of the strait. The analysis of historical data shows that the mean flow is directed from the central to the eastern basin. In addition, LADCP data suggest the intensification of the flow in the narrow part of the sill between the basins, and the possible mixing of deep waters at this location.

Potentially toxic elements in the Gusinoye Lake (Republic of Buryatia, Russia).

Enterprises of fuel and energy complex are the main sources of carcinogenic and mutagenic potentially toxic elements (PTEs). Their particulate and gaseous emissions, wastes, and effluents enforce migration processes of PTEs in atmosphere, soil, and then in water bodies, creating a danger to the functioning of aquatic ecosystems. Vulnerable objects include water bodies with slow water exchange, due to the accumulation of PTEs in the bottom sediments and the threat of secondary contamination. Among them it is necessary to note cooling ponds of thermal power plants (TPP). In this study, the ecological and geochemical state of components of "water-suspended sediments-bottom sediments" aquatic system is assessed on the example of the Gusinoye Lake (the Republic of Buryatia), which is used as cooling pond for the largest in the region TPP, burning brown coal from local deposits. The Gusinoye Lake is the second largest lake in Buryatia after the Baikal Lake and a valuable source of water supply, place of recreation, and fish breeding. To study the distribution of PTEs in the waters of the lake, a network of 19 monitoring stations was deployed in the summer of 2019. In contrast to the previous researches, detailed studies of the central deep-water part of the lake were carried out. As a result, data on the concentration of PTEs in the dissolved and suspended forms and also in bottom sediments was obtained. To study the balance of chemicals in the lake, its main tributaries were surveyed, as well as the drainage channel of TPP. For the first time, for the Lake Gusinoye, bulk concentrations of As, Cd, Pb, Zn, Co, Mo, Cu, Sb, Cr, V, Mn, Sr, and Ag were determined using the inductively coupled plasma mass spectrometry. The highest concentrations of dissolved PTEs in the lake water are confined to the mouths of tributaries in the northern part of the lake. Dissolved Sr, Mo, and Cu in the lake exceed maximum permissible concentrations (MPCs) for fishery. The maximum concentrations of Sr and Cu are almost 3 MPCs and that of Mo is 22 MPCs near the TPP. The suspended sediments of the Gusinoye Lake are depleted of all studied PTEs as compared to the regional clarkes (natural abundances) of the lithosphere. Mn has the highest concentrations in the suspension, and its maximum concentration was due to the surface runoff from the solid waste landfill and an abandoned coal mine. Concentrations of Cu, Zn, Sr, Mo, and Pb in bottom sediments are higher than in suspended sediments, thus indicating the accumulation of pollutants. The maximum intensity of accumulation is characteristic of the deep central basin of the lake and its eastern part, where Sr, Ag, and Mo accumulate most intensively near the coal field. The main sources of PTE pollution are concentrated in the northern and northeastern parts of the lake catchment area. Significant amount of dissolved Mo, Sr, and As enter the lake with the waters of quarries and mines of the coal fields, the solid waste landfill of the town of Gusinoozyorsk, and northern rivers-tributaries, draining the dumps of the large Zagustaj coal field.

Early taphonomy of benthic foraminifera in Storfjorden ‘sea‐ice factory’: the agglutinated/calcareous ratio as a proxy for brine persistence

The recurrent latent‐heat polynya characterizing Storfjorden (Svalbard, Norway) triggers seasonal formation of thin first‐year sea ice. This leads to the production of dense, salty, and corrosive brines that cascade towards the sea floor and mix with shelf waters. The bottom topography of the fjord is responsible for the retention of these dense waters in two central deep basins throughout the year. Recent studies show that living benthic foraminifera in Storfjorden are particularly affected by the persistence of brines on the sea floor, with a strong dominance of agglutinated (A) species and high degrees of dissolution of calcareous(C) faunas. Therefore, the A/C ratio, calculated on living faunas, was proposed as a proxy for brine persistence. In the present study we analyse the fossil faunas, found below the taphonomically active zone, to investigate the residual signal of the A/C proxy after the intense early taphonomic processes and challenge its applicability in sedimentary archives. Our results show that despite the generally high taphonomic loss inside the fjord, a high proportion of agglutinated species is still visible in fossil faunas at the stations experiencing regular and/or persistent presence of brine‐enriched shelf waters. These results support the application of the A/C ratio in historical records to reconstruct the persistence of brines and indirectly the first‐year sea ice formation in Storfjorden. This can be further applied to other Arctic fjords with similar settings and characterized by the production of brines during the winter–early spring season.

The foraging behavior of nonbreeding Adélie penguins in the western Antarctic Peninsula during the breeding season

AbstractInformation on marine predator at‐sea distributions is key to understanding ecosystem and community dynamics and an important component of spatial management frameworks that aim to identify regions important for conservation. Tracking data from seabirds are widely used to define priority areas for conservation, but such data are often restricted to the breeding population. This also applies to penguins in Antarctica, where identification of important habitat for nonbreeders has received limited attention. Nonbreeding penguins are expected to have larger foraging distributions than breeding conspecifics, which may alter their interactions with physical environmental factors, conspecifics, other marine predators, and threats. We studied the movement behavior of nonbreeding Adélie penguins tracked during the 2016/2017 breeding season at King George Island in the South Shetland Islands, Antarctica. We quantify how nonbreeding penguins' horizontal moment behavior varies in relation to environmental conditions and assess the extent of spatial overlap in the foraging ranges of nonbreeders and breeders, which were tracked over several years. Nonbreeders increased their prey search and area‐restricted foraging behavior as sea surface temperature and bottom depths decreased, and in response to increasing sea ice concentration. Nonbreeders tended to transit (high directional movement) over the relatively deep Central Basin of the Bransfield Strait. The majority of foraging behavior occurred within the colder, Weddell Sea–sourced water of the Antarctic Coastal Current (incubation) and in the Weddell Sea (crèche). The utilization distributions of breeders and nonbreeders overlapped in the central Bransfield Strait. Spatial segregation was greater during the crèche stage of breeding compared to incubation and brood, because chick provisioning still constrained the foraging range of breeders to a scale of a few tens of kilometers, while nonbreeders commenced with premolt foraging trips into the Weddell Sea. Our results show that breeding and nonbreeding penguins may not be impacted similarly by local environmental variability, given that their spatial and temporal scales of foraging differ during some part of the austral summer. Our study highlights the need to account for different life history stages when characterizing foraging behavior of marine predator populations. This is particularly important for “sentinel” species monitored as part of marine conservation and ecosystem‐based management programs.

Oriented-lake development in the context of late Quaternary landscape evolution, McKinley Bay Coastal Plain, western Arctic Canada

Oriented lakes—characterized by elongate forms, central basins and shallow littoral shelves—are common features of circum-arctic coastal lowlands. The environmental conditions, geological processes and chronology associated with the development of oriented lakes, however, are little known but essential for understanding how such Arctic lowlands evolve. Using combined techniques of field and drill-log stratigraphy and sedimentology, luminescence and radiocarbon dating methods and geomorphic mapping, we reconstruct the landscape evolution leading toward oriented-lake formation on the McKinley Bay Coastal Plain of western Arctic, Canada—a region with over 900 oriented lakes. Most lakes with deep central basins are inherited from a preglacial braidplain (ca. 73–27 ka) and alluvial braided-channel network that extended beyond the glacial limit (ca. 18.6–14.3 ka). Eolian erosion, active during the lateglacial and postglacial period (ca. 12.8–1.9 ka), reworked fluvial deposits. Eolian processes modified existing basins and created other shallow deflationary basins, as small barchanoid dunes migrated under cold, dry paraglacial conditions between about 12.8 and 10.7 ka. Vegetation cover developed at the onset of the early Holocene climatic optimum ca. 10.7 ka, and parabolic dunes were active between 9.6 and 4.6 ka. Thus, oriented lakes developed in basins conditioned by fluvial and eolian processes. In the absence of much near-surface ground ice, lateral expansion of deep-basin lakes and shallow stabilized deflationary basins predominated during the late Holocene through wind-induced wave and current processes. Overall, this sequence of oriented-lake formation does not support a thaw-lake cycle but, rather, small-basin evolution of a periglacial landscape.

A Role for Gravity Currents in Cross‐Sill Estuarine Exchange and Subsurface Inflow to the Southern Strait of Georgia

AbstractMultiyear current meter records from five sites are used to examine subsurface renewal processes in the southern Strait of Georgia. The lower layer inflow of dense oceanic‐derived water over the ~100‐m deep sill in Boundary Pass near the entrance to the strait reaches peak velocities during spring tides and rarely reverses direction. Corresponding changes in water density over the sill are inversely correlated with inflow velocity and, it is assumed, with temporal (t) variations in bottom layer thickness, h(t), arising from the fortnightly variations in tidal‐mixing. The dense bottom water crossing the sill transforms into a gravity current whose velocity is dependent on both h and sill depth, H. Gravity flow velocities, which peak at h(t)/H ~ 0.3 and weaken for h → 0 (neap tides) and h → H (spring tides), modulate the cross‐sill exchange with the strait. The density anomaly, Δρ, of the gravity flow determines its terminal depth and along‐strait extent. Lower‐density gravity flows generated during spring tides separate from the sloping bottom of the southern strait, but the higher‐density flows formed during neap tides can penetrate over 100 km northward to the deep central basin. The most extensive bottom intrusions occur monthly near neap tides in summer, when the underlying estuarine circulation delivers dense upwelled water from the coast to the sill. These deep inflows are accompanied by strong middepth outflow. There are typically three to five monthly replacement episodes each summer, but fewer during strong El Niño years due to reduced coastal upwelling.

231Pa and 230Th in the Arctic Ocean: Implications for boundary scavenging and 231Pa[sbnd]230Th fractionation in the Eurasian Basin

231Pa, 230Th and 232Th were analyzed in filtered seawater (n = 70) and suspended particles (n = 39) collected along a shelf-basin transect from the Barents shelf to the Makarov Basin in the Arctic Ocean during GEOTRACES section GN04 in 2015. The distribution of dissolved 231Pa and 230Th in the Arctic Ocean deviates from the linear increase expected from reversible scavenging. Higher 232Th concentrations were observed at the shelf, slope and in surface waters in the deep basin, pointing at lithogenic sources. Fractionation factors (FTh/Pa) observed at the Nansen margin were higher compared to FTh/Pa in the central Nansen Basin, possibly due to the residual occurrence of hydrothermal particles in the deep central Nansen Basin. Application of a boundary scavenging model quantitatively accounts for the dissolved and particulate 230Th distributions in the Nansen Basin. Modelled dissolved 231Pa distributions were largely overestimated, which was attributed to the absence of incorporation of water exchange with the Atlantic Ocean in the model. 231Pa/230Th ratios of the suspended particles of the Nansen Basin were below the 231Pa/230Th production ratio, but top-core sediments of the Nansen margin and slope have high 231Pa/230Th-ratios, suggesting that scavenging along the Nansen margin partly acts as a sink for the missing Arctic 231Pa.

A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century

Abstract. The anthropogenically forced expansion of coastal hypoxia is a major environmental problem affecting coastal ecosystems and biogeochemical cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea whose central deep basins have been highly prone to deoxygenation during its Holocene history, as shown previously by numerous paleoenvironmental studies. However, long-term data on past fluctuations in the intensity of hypoxia in the coastal zone of the Baltic Sea are largely lacking, despite the significant role of these areas in retaining nutrients derived from the catchment. Here we present a 1500-year multiproxy record of near-bottom water redox changes from the coastal zone of the northern Baltic Sea, encompassing the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warm Period (MoWP). Our reconstruction shows that although multicentennial climate variability has modulated the depositional conditions and delivery of organic matter (OM) to the basin the modern aggravation of coastal hypoxia is unprecedented and, in addition to gradual changes in the basin configuration, it must have been forced by excess human-induced nutrient loading. Alongside the anthropogenic nutrient input, the progressive deoxygenation since the beginning of the 1900s was fueled by the combined effects of gradual shoaling of the basin and warming climate, which amplified sediment focusing and increased the vulnerability to hypoxia. Importantly, the eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hypoxia in the 1950s. We find no evidence of similar anthropogenic forcing during the MCA. These results have implications for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geochemical proxies in order to robustly reconstruct subtle redox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality.

TECTONO-SEDIMENTARY STRUCTURE AND LATE QUATERNARY EVOLUTION OF THE NORTH EVIA GULF BASIN, CENTRAL GREECE: PRELIMINARY RESULTS

Systematic single channel seismic and 3.5 kHz profiling, gravity and box coring and Pb downcore analyses shed light to the structure and the Late Quaternary evolution of the North Evia Gulf graben. The latter is segmented along its axis in three distinct basins. The shallow, E-W striking western basin displays southward asymmetry. The NW-SE striking, deep central basin and the shallow southeastern one show NE-ward asymmetry and are controlled by the Aidipsos, Telethrion and Kandili fault zones. Sequence stratigraphie data and aragonite crystals in mud layers underlying Holocene deposits verifies that North Evia Gulf was a lake during the last glacial maximum with lake level at about 90m below the present sea level.

Causes of spatial distribution of subfossil diatom and chironomid assemblages in surface sediments of a remote deep island lake

Until recently, the distribution of diatom and chironomid assemblages and their attributes (species richness/diversity) in relation to water depth and sedimentary environments have been identified but not quantified. The influence of environmental variables on assemblage distribution and taxa richness in a deep, monomictic lake in Sao Miguel Island is assessed. Attention is given to community variation along a water-depth gradient. Sediment core samples were analysed for diatom content, chironomids, and grain-size clastic particles along three transects from the shoreline to the central deep basin of the lake at a resolution of 1 m water depth. Linear and unimodal regressions were used to test taxon richness, taxon diversity and taxon evenness versus water depth of each transect. A hump-shaped relationship between species richness and water depth was noted, with a peak occurring at mid-depth, meaning that samples located at that depth better represented the total subfossil assemblage living in lake Azul. Moreover, data indicate that both assemblages in Lake Azul, and taphonomic effects, were influenced by processes of clastic transport depending on the lake morphology. Results from this study allow us to access the spatial distribution of biological assemblages in clastic-dominated lakes with a high topographic gradient, and provide us with principal criteria that will allow us to determine coring locations that capture the true species diversity for studies in lakes.

Volcanism and Tectonics of the Central Deep Basin, Sea of Japan

The paper presents the results of a study on the geomorphic structure, tectonic setting, and volcanism of the volcanoes and volcanic ridges in the deep Central Basin of the Sea of Japan. The ridges rise 500–600 m above the acoustic basement of the basin. These ridges were formed on fragments of thinned continental crust along deep faults submeridionally crossing the Central Basin and the adjacent continental part of the Primorye. The morphostructures of the basin began to submerge below sea level in the Middle Miocene and reached their contemporary positions in the Pliocene. Volcanism in the Central Basin occurred mostly in the Middle Miocene–Pliocene and formed marginal-sea basaltoids with OIB (ocean island basalt) geochemical signatures indicating the lower-mantle plume origin of these rocks. The OIB signatures of basaltoids tend to be expressed better in the eastern part of the Central Basin, where juvenile oceanic crust has developed. The genesis of this crust is probably related to rising and melting of the Pacific superplume apophyse.

Export of calcium carbonate corrosive waters from the East Siberian Sea

Abstract. The Siberian shelf seas are areas of extensive biogeochemical transformation of organic matter, both of marine and terrestrial origin. This in combination with brine production from sea ice formation results in a cold bottom water of relative high salinity and partial pressure of carbon dioxide (pCO2). Data from the SWERUS-C3 expedition compiled on the icebreaker Oden in July to September 2014 show the distribution of such waters at the outer shelf, as well as their export into the deep central Arctic basins. Very high pCO2 water, up to ∼ 1000 µatm, was observed associated with high nutrients and low oxygen concentrations. Consequently, this water had low saturation state with respect to calcium carbonate down to less than 0.8 for calcite and 0.5 for aragonite. Waters undersaturated in aragonite were also observed in the surface in waters at equilibrium with atmospheric CO2; however, at these conditions the cause of under-saturation was low salinity from river runoff and/or sea ice melt. The calcium carbonate corrosive water was observed all along the continental margin and well out into the deep Makarov and Canada basins at a depth from about 50 m depth in the west to about 150 m in the east. These waters of low aragonite saturation state are traced in historic data to the Canada Basin and in the waters flowing out of the Arctic Ocean north of Greenland and in the western Fram Strait, thus potentially impacting the marine life in the North Atlantic Ocean.

Recent paleoseismicity record in Prince William Sound, Alaska, USA

Sedimentological and geochemical investigation of sediment cores collected in the deep (>400 m) central basin of Prince William Sound, along with geochemical fingerprinting of sediment source areas, are used to identify earthquake-generated sediment gravity flows. Prince William Sound receives sediment from two distinct sources: from offshore (primarily Copper River) through Hinchinbrook Inlet, and from sources within the Sound (primarily Columbia Glacier). These sources are found to have diagnostic elemental ratios indicative of provenance; Copper River Basin sediments were significantly higher in Sr/Pb and Cu/Pb, whereas Prince William Sound sediments were significantly higher in K/Ca and Rb/Sr. Within the past century, sediment gravity flows deposited within the deep central channel of Prince William Sound have robust geochemical (provenance) signatures that can be correlated with known moderate to large earthquakes in the region. Given the thick Holocene sequence in the Sound (~200 m) and correspondingly high sedimentation rates (>1 cm year−1), this relationship suggests that sediments within the central basin of Prince William Sound may contain an extraordinary high-resolution record of paleoseismicity in the region.

Shallow Gas Prospect Evaluation in Shahbazpur Structure Using Seismic Attributes Analysis - a Case Study for Bhola Island, Southern Bangladesh

Shahbazpur structure is located in the southern Part of the central deep basin in the Hatia trough, where lie all the largest Gas fields of Bangladesh. A method is established to delineate the structural mapping precisely by interpreting four 2D seismic lines that are acquired over Shahbazpur structure. Moreover direct hydrocarbon indicators (DHI) related attributes are analyzed for further confirmation for presence of hydrocarbon. To do this synthetic seismogram generation, seismic to well tie, velocity modelling and depth conversion are performed. A limited number of seismic attributes functions that are available in an academic version of Petrel software are applied to analyze attributes. Seismic attribute analyses that are used in this interpretation mainly are associated to bright spot detection. Presence of bright spots or high amplitude anomaly over the present Shahbazpur structure, reservoir zone are observed. This signature will play a very important role in next well planning on the same structure to test the shallow accumulation of hydrocarbon. For better understanding of this shallow reserve, it is suggested to acquire 3D seismic data over Shahbazpur structure which will help to evaluate the hydrocarbon accumulation and to identify gas migration pathways.
 Dhaka Univ. J. Sci. 64(2): 135-140, 2016 (July)

Microstructure observations in the upper layer of the South China Sea

A general pattern for turbulent mixing in the upper layer of the South China Sea (SCS) is presented based on TurboMAP measurements in April and May 2010. The turbulence level decreased significantly overall from north to south, and weakened from east to west in the northern SCS. The average dissipation rate north of 18°N reaches 1.69 × 10−8 W/kg, approximately six times larger than that south of 18°N. The mean mixing efficiency in the SCS is 0.2, with a maximum of 0.31 near the Luzon Strait. At one repeatedly occupied station located in the central deep basin, the dissipation rate varies diurnally in the mixed layer and pycnocline due to diurnal heating and cooling by solar radiation and local barotropic tide, respectively.

Fluxes of clay minerals in the South China Sea

In order to assess dominant settling processes that change the composition of the detrital clay fraction during transport from neighboring estuaries to a deep sea basin, we studied relative clay mineral abundances and absolute clay mineral fluxes of clay-sized sinking particulate matter collected by eight sediment trap systems deployed from shallow to deep water depth in the South China Sea. This is the first basin-wide study on recent sedimentation processes in the western Pacific marginal seas.Annual averages of relative clay mineral abundances at the shallow traps are temporally more variable and regionally more diverse, resembling those of surrounding drainage basins. In contrast, higher fluxes of material reach the deeper traps. Their characteristics trend temporally and spatially towards uniformity and are enriched with smectite in the entire deep basin.Sinking particulate matter that reaches the shallow traps spends less time in pelagic transport and is affected by monsoonal current reversals. The enrichment in smectite in the deeper traps is a result of longer duration in transport at low velocities, which may increase the effect of differential settling during transport. The trend is caused by lateral advection driven by the cyclonic deep circulation, and this is considered as the main transport process in the northern and central deep basin. The high fluxes in the south-western deep basin could be the result of laterally advected re-suspended sediments from the neighboring shelves.The effects on the composition of the detrital clay fraction caused by oceanographic control, which indirectly include those by differential settling, mask the climatic signal from surrounding drainage basins in the deep basin sediments. This strongly affects the interpretation of the clay mineralogical record in sediments deposited under recent conditions in the South China Sea deep basin.

Sea level rise projection in the South China Sea from CMIP5 models

Future potential sea level change in the South China Sea (SCS) is estimated by using 24 CMIP5 models under different representative concentration pathway (RCP) scenarios. By the end of the 21st century (2081–2100 relative to 1986–2005), the multimodel ensemble mean dynamic sea level (DSL) is projected to rise 0.9, 1.6, and 1.1 cm under RCP2.6, RCP4.5, and RCP8.5 scenarios, respectively, resulting in a total sea level rise (SLR) of 40.9, 48.6, and 64.1 cm in the SCS. It indicates that the SCS will experience a substantial SLR over the 21st century, and the rise is only marginal larger than the global mean SLR. During the same period, the steric sea level (SSL) rise is estimated to be 6.7, 10.0, and 15.3 cm under the three scenarios, respectively, which accounts only for 16%, 21% and 24% of the total SLR in this region. The changes of the SSL in the SCS are almost out of phase with those of the DSL for the three scenarios. The central deep basin has a slightly weak DSL rise, but a strong SSL rise during the 21st century, compared with the north and southwest shelves.

The isotopic signature and distribution of particulate iron in the North Atlantic Ocean

Iron (Fe) is a key micronutrient for life in the oceans. Particles play an important role in the marine biogeochemical cycling of Fe as a reservoir of marine Fe that may be directly accessible to phytoplankton, and as sources and sinks for seawater dissolved Fe. Here, we report the stable isotopic composition of Fe (δ56Fe) in suspended (0.8–51μm) particles from the US GEOTRACES GA03 North Atlantic zonal transect, in order to facilitate a better understanding of the marine biogeochemical cycling of Fe. Data are presented both for a total digestion of the particles, and for ‘ligand-leachable’ phases of Fe using a newly-developed pH 8 oxalate–EDTA leach. For total particle digests, the mean δ56Fe across the whole GA03 section was 0.08±0.09‰ (1 S.D.) which is equivalent to the isotope composition of known lithogenic Fe sources to the ocean. In contrast, ligand-leachable Fe was generally lighter than continental material with a mean δ56Fe of −0.30±0.17‰ (1 S.D.). Our data also provide valuable insight into Fe biogeochemical cycling in several key regions. In the deep ocean, but above the depths where near-sediment nephloid layers are present, ligand-leachable Fe is isotopically lighter in the deep Western Basin compared to the deep Central and Eastern Basins suggesting differences in particle surface chemistry between resuspended seafloor sediments, which may predominate in the west, and Saharan dust that predominates in the center and in the east. Within a nephloid layer above reduced continental margin sediments in the Eastern Basin, below the Mauritanian upwelling region, we report the lowest particulate δ56Fe values for both total and ligand-leachable Fe, suggesting a transfer of isotopically light dissolved porewater Fe2+ to the particulate phase. In contrast, δ56Fe values within a nephloid layer near Bermuda are similar to values higher in the water column. Within a hydrothermal plume sampled at the TAG hydrothermal field on the mid-Atlantic Ridge, decreasing Fe concentration and δ56Fe with distance from the vent indicate the water-column precipitation of Fe oxyhydroxides within the plume. Comparison between total and ligand-leachable particulate Fe concentrations throughout the transect also provides useful biogeochemical information. In the chlorophyll maximum, a high proportion of Fe in the labile phase demonstrates a relative lack of lithogenic material, while the high proportion of labile Fe observed between 2000 and 4000m depth to the west of the mid-Atlantic Ridge suggests that hydrothermal oxyhydroxide particles can be transported thousands of kilometers away from vent sites. Throughout the North Atlantic, measurements of total and ligand-leachable Fe concentration and δ56Fe are therefore useful tools for studying the biogeochemical cycling of particulate Fe.