Ocean Margin Research Articles

ИЗМЕНЕНИЯ ОТНОСИТЕЛЬНОГО УРОВНЯ МОРЯ - ПОВЕРХНОСТНОЕ ВЫРАЖЕНИЕ ГЛУБИННЫХ ПРОЦЕССОВ

The article presents an overview of current ideas about the possible causes and physical mechanisms of relative sea-level changes. The choice of the predominant factor can be demonstrated by the study of stratigraphic sequences formed at the oceanic basin margins or in the epeiric seas of cratonic interior areas. Such studies lead to the conclusion that high frequency fluctuations with a period of less than 1 Myr are caused primarily by the glacial episodes on the land. Fluctuations with a longer period of 1-3 Myr are controlled by intraplate tectonic processes, among which the prograde metamorphism at the crust-mantle boundary is the most important. Lateral stress of lithosphere with laterally variable thickness and the dynamic topography are also possible. Tectonic processes affecting the volume of ocean basins, such as accelerating or decelerating spreading, subduction appear to be in causal relationship with the long-period sea level changes of 10 to 100 Myr and over.

Northward subduction of the South Qilian ocean: Insights from early Paleozoic magmatism in the South-Central Qilian belts

The Qilian orogen consists of the North Qilian belt, the Central Qilian block and the South Qilian belt and is regarded as part of the northern margin of the Proto-Tethys Ocean. The tectonic mechanism responsible for the early Paleozoic magma generation in the Qilian orogen is crucial to understanding the ocean-continent configurations and the closure of the Proto-Tethys Ocean. This paper compiles the early Paleozoic mafic and granitic intrusions in the South Qilian belt and the Central Qilian block, to constrain the subduction and accretion of the South Qilian ocean. The early Paleozoic mafic intrusions in the Central Qilian block indicate that the influence of slab-derived fluids dominates their petrogenesis, while the mafic intrusions in the South Qilian belt were mostly affected by involvement of significant sediment-derived melts in their formation. They are supportive of a mélange model, in which melting of mélange diapirs originating at the slab-mantle interface generates fluid-metasomatised magma source in the deep mantle under the Central Qilian block and sediment-derived melts-metasomatised magma source under the Hualong block in the South Qilian belt, consistent with northward subduction of the South Qilian ocean. It is proposed that northward intra-oceanic subduction of the South Qilian Ocean occurred at 530–470 Ma in front of the Hualong block and continued at 470–446 Ma but was then accompanied by the development of a mature volcanic arc. Continuous subduction resulted in accretion of the Hualong block northwards onto the Central Qilian block, and this collision started at ca. 446 Ma and lasted to ca. 420 Ma. The final closure of the South Qilian ocean was marked by the onset of continental collision between the Qaidam-Quanji block and the amalgamated Hualong-Central Qilian block at ca. 440 Ma, and also the cause of the A-type granitoids after final collision occurred at ca. 420 Ma.

HBIs and Sterols in Surface Sediments Across the East Siberian Sea: Implications for Palaeo Sea‐Ice Reconstructions

AbstractHighly branched isoprenoids (HBIs) in marine sediments have emerged as promising semi‐quantitative proxies to reconstruct seasonal sea ice in polar oceans. In this work, we examine the distribution of sympagic HBIs (IP25 and HBI‐II), pelagic phytoplankton biomarkers (brassicasterol, dinosterol and HBI‐III) as well as terrestrial sterols (campesterol and β‐sitosterol) in the surface sediments of the East Siberian Sea (ESS) to test their reliability as sea‐ice proxies under continental run‐off influence. Our data suggest that dinosterol performs better than brassicasterol to assess sea ice across the ESS shelf, yet the correlation between PDIP25 and spring sea ice is relatively weak but improves when removing sites with salinity <25. Strongest relationship is found between PIIIIP25 and summer sea ice in regions remote from riverine influence. Overall, our results show that semi‐quantitative estimates of sea ice based on biomarkers can be problematic in Arctic Ocean margins because of biases induced by continental run‐off on biological productivity and sea‐ice production.

Enhanced hydrological cycle during Oceanic Anoxic Event 2 at southern high latitudes: New insights from IODP Site U1516

The Oceanic Anoxic Event 2 (OAE 2) represents one of the most profound global environmental disturbances of the Mesozoic Era, which was associated with a positive carbon isotope excursion due to widespread organic carbon burial. However, the character, evolutionary history, and exact driving mechanisms of OAE 2 are still much debated, exacerbated by the lack of records from climate-sensitive southern high latitudes. Here we present a multi-proxy record of a stratigraphically continuous succession spanning the late Cenomanian to early Turonian at the International Ocean Discovery Program Site U1516 in the Mentelle Basin off southwest Australia, which was located at southern high latitudes (~ 60–62°S) during the Late Cretaceous. Sedimentary records at Site U1516 allow the first detailed insights into source-to-sink processes in the Mentelle Basin and associated paleoenvironmental reconstruction in the southern Indian Ocean during OAE 2. Rare earth element abundances and clay-bound neodymium isotopes of the siliciclastic sediment fractions indicate that southwest Australia was the main detrital source of sediment at Site U1516 during OAE 2, with volcanogenic sediments representing only a minor component. A major provenance shift is observed across OAE 2, indicating an increased sediment contribution from distant sources (i.e., Yilgarn Craton and Albany-Fraser Orogen) relative to proximal sources (i.e., Perth Basin and Leeuwin Block) in southwest Australia, interpreted as reflecting a major reorganization of the drainage system due to enhanced fluvial runoff. Based on these findings, we infer that the OAE 2 interval showing the most prominent δ13C excursion was associated with an intensification of the hydrological cycle in the high-latitude southern hemisphere, with potential impacts on regional ocean chemistry and marine productivity. In particular, we suggest that enhanced terrigenous input and riverine nutrient export likely played a key role in controlling siliceous productivity at nearby ocean margins. As a consequence, the intense burial of organic matter (i.e., black shale) at Site U1516, which occurred episodically throughout OAE 2, possibly resulted from the combination of enhanced riverine-driven marine productivity and increased burial efficiency associated with the development of anoxic/euxinic bottom water conditions.

Non‐random distribution of ungulate salt licks relative to distance from North American oceanic margins

AbstractAimTerrestrial deposition of aerosol marine sodium declines with distance from coastlines. Salt deprivation in vertebrate herbivores and salt‐seeking behaviours should hence increase with distance inland. We analyse published geospatial data on ungulate‐patronized salt licks to test whether they are non‐randomly distributed relative to distance from oceans and elevation.LocationCanada, Alaska and the contiguous United States.TaxonCetartiodactyla (even‐toed ungulates).MethodsWe determined the land area and median elevation of 100 km increments from the North American coast. The null model of the expected number of licks within each interval was determined by the ratio of the interval's land area to the total land area, multiplied by the total number of licks. We asked whether the number of licks further from coastlines was significantly higher than chance. We also assessed whether licks occur disproportionately at higher elevations, comparing the median elevation of observed licks to the median elevation within each interval.ResultsWe found a strong positive relationship between salt lick patronage by ungulates and distance from the coast. Licks occurred significantly less often within, and more often beyond, 500 km inland, and at significantly higher elevations than would be expected by chance.Main conclusionsThese findings indicate that the patronage of salt licks is constrained geographically, and that the foraging behaviour of ungulates and other phytophagous vertebrate taxa may be influenced over large spatial scales by sodium availability. Salt‐seeking behaviour varies on a wide biogeographical scale across North America, with concomitant implications for vertebrate herbivore behaviour and ecology.

A vertical-mode extrapolation scheme applied to the Brazil Current domain: quasi-synoptic scenarios

The Brazil Current (BC) is a western boundary current, with an average vertical extension of 500 m, which occupies the southwestern margin of Atlantic Ocean. Recently, the South Brazil Bight - SBB (23º S - 28º S) has been intensively explored by the oil and gas industries, which brought a high demand in operational oceanography and ocean state monitoring. In this study we present a new technique based on the vertical dynamic modes of oscillation, to extrapolate velocity profiles in the SBB, with potential applications to data acquired by Vessel Mounted Acoustic Doppler Current Profilers (VMADCP). Using four quasi-synoptic crosssectional velocity data, acquired by Lowered Acoustic Doppler Current Profilers (LADCP), we observed that the mean circulation of this region can be characterized as a 2-dynamical mode system. This mean circulation is mainly dominated at surface by the BC and at intermediate levels by the Intermediate Western Boundary Current (IWBC), which flows northward. Furthermore, observed mesoscale vortical features in the SBB, such as eddies and meanders, are shown to be primarily barotropic-mode dominated. In our extrapolation methodology, the use of an independent set of velocity time series obtained by four moorings has shown to be a valid way for obtaining the relative contribution of each dynamical mode in the velocity field in the SBB. Normalized mean squared errors below 10% and radar diagrams show that the presented methodology was able to reproduce the main dynamic features observed by LADCP transects. Also, with this technique, we extrapolated three VMADCP sections and the results agreed with previous observations.

Organic carbon accumulation at the northern Gondwana paleomargin (Tunisia) during the Toarcian Oceanic Anoxic Event: Sedimentological and geochemical evidence

During the early Toarcian (Early Jurassic; ~183 Ma) global environmental changes promoted the expansion of oxygen-deficient conditions across vast shelf areas around the globe. Ocean margin deoxygenation favored the preservation of organic matter in shelf sediments and is considered to reflect a Toarcian Oceanic Anoxic Event (T-OAE). Higher-level global drivers controlled temperature fluctuations, sea level evolution, oceanic circulation patterns, marine primary productivity, and stable carbon isotope excursions. Lower-level factors, controlled by individual basin bathymetry and paleogeography, operated on local to regional scale and here regulated organic matter accumulation. Accordingly, the T-OAE has been described as global event with a strong regional character. During the Toarcian, preferentially organic matter-lean sediments have been deposited at the northern Gondwana paleomargin. Enhanced connectivity to the Tethys Ocean and low marine primary productivity are considered to have been major factors preventing the formation of oxygen-deficient conditions and organic matter preservation. We here report on early Toarcian sediments from the Châabet El Attaris Section (Tunisia), located at the northern Gondwana paleomargin that episodically became enriched in organic matter. Bio- and chemostratigraphy both confirm that organic matter-rich sediments preferentially formed during the T-OAE but also predate the T-OAE. Sedimentological and micropaleontological characterization in concert with bulk, molecular and stable isotope geochemical investigations reveal that accumulation of marine organic matter occurred under anoxic, but not under euxinic, conditions. Limitation in the supply of reactive iron regulated organic matter sulfurization indicating that during most of the depositional history the levels of free H 2 S in bottom waters were rather low but limited to the sediment pore water. We conclude that the chemocline was either situated within the sediment or close to the sediment water interface. Spatiotemporally-restricted occurrence of organic matter enriched sediments and anoxic pore waters was promoted by restriction in water column ventilation in a local graben structure. • Organic matter-enriched strata in Tunisia predates but maximizes in the T-OAE. • Organic matter preservation occurred under anoxic but non-euxinic conditions. • Formation of oxygen deficient conditions was promoted by local paleobathymetry and basin restriction.

Sedimentological study of Khalsi Formation of Ladakh "Implications for Paleogeographic reconstruction"

Carbonates of the Tethys Himalaya in the Ladakh region present a complex relation with rocks of the Indus group and subduction-related volcanics. The present study involves field recognition of the limestone units, collection of mega fossils, petrographic analysis, elemental and mineral composition of Khalsi Formation of Indus group. We present the results on depositional environment, field relation, paleogeographic conditions, and regional significance of Khalsi Formation. The field relation of Khalsi Formation suggests a complex interaction with the igneous intrusion, foredeep turbidite association, metamorphism, and tectonic deformation. The structural disturbance is evidenced by the folding in shale, imprints of sinistral movement, and localized compaction of limestone. The fossil assemblage collected during this study includes Auroradiolites biconvexus, rudists, crinoids, forams (Orbitolinid) corals, colonial bryozoans, belemnite, and some unidentified marine organisms. The faunal assemblage reflects a shallow-water depositional environment. The petrographic analysis of the samples of Khalsi Formation suggests bioclasts as a major component mainly enriched with the fragments of forams, micrite as cementing material, and pyrite and zircon crystals as noncarbonate constituents. The SEM-EDX data suggest significant calcite content in the samples. The SEM-EDX data suggest significant calcite content in the samples. We suggest the rudists, corals, forams (Orbitulina), crinoids, and bryozoans colonies were widely distributed as a reefal framework along tropical and subtropical tethyan ocean margin having a close affinity with the south Asian-Pacific faunal province. These fossils represent a shallow warm water environment of deposition for Khalsi Formation due to the uplifting of the ocean floor in this region after completion of subduction of the Indian plate beneath Eurasian.

Pacific ocean mega ecotone of Northern Eruasia as the belt of the origin of the modern continental biosphere

The Pacific Ocean margin of Eurasia includes the age row (at the geologic time scale) of geoecotone objects which can be imagined by certain nodal stages of the evolutionary trajectory of exogenous landscape genesis in the continental biosphere. The reported strategy of scientific research is aimed at the establishment of zonal-regional and local regularities of landscape organization of insular and marginal-continental land in different morphotectonic and macroclimatic sectors of the Pacific Ocean mega-ecotone of Northern Eurasia, which is considered as a natural laboratory for studying the modern stage of development of the geographical envelope The empirical statistical models of the island-arc stage of continental biosphere development in the North-West Pacific have been created by the example of experimental test ground of the active Mendeleev volcano on the Kunashir Island (the South Kuril Ridge). It was shown that on this initial stage local geomorphological and hydro-edaphic conditions created the centers of origin of diverse phytocoenological structures. An exceptionally high percentage of green mass in the structure of production results in the acceleration of the biological cycle as a factor of stability of forest community under unfavorable conditions of “cold” oceanicity. Geothermal energy induces rearrangement of the plant cover towards the increase in its flora- and phytocoenotic diversity to the detriment of biomass formation rate. The “climatically unjustified” but sufficiently stable sub-boreal forest ecosystems were formed, as well as the early stages of buffer forest communities. The multidimensional empirical-statistical modeling of landscape connections in the marginal-continental sector of the Pacific Ocean mega ecotone has been performed based on an experimental site in the Lower By-Amur Region. The ecology of the boreal-forest landscape of the continental margin characterizes the mature stage of evolution of the continental biosphere. The regularities and causal mechanisms of the formation of buffer forest communities typical of ecotone systems have been represented more integrally. Forests of this phenomenal buffer flora are distinguished by extremely high parameters of structural and functional development and have reached the state approaching the evolutionary climax. The previously advanced concepts of the Pacific ecotone of Northern Eurasia as a focus of evolutionary processes in the continental biosphere have been confirmed.

Petroleum hydrocarbons in Brazilian Northeast continental shelf waters: baseline values

Water quality on the continental shelf off the eastern Brazilian Northeast region was investigated in two oceanographic campaigns during winter and summer, when vertical profiles of salinity, temperature, turbidity, chlorophyll and dissolved oxygen were recorded. Dissolved / dispersed petroleum hydrocarbons (DDPH) were also analyzed in sub-superficial water samples using fixed wavelength fluorescence spectroscopy methods. Shelf waters are dominated by Tropical Waters, with high salinity (> 36.5 g kg-1) and temperature (> 26 °C) and little continental influence. Turbidity was higher during winter, which may be due to the higher continental contribution and/or higher wave action. Chlorophyll was mostly < 1 µg L-1 and dissolved oxygen saturation was predominantly high (> 90%), but innermost costal stations were influenced by the Capibaribe estuary. Median DDPH concentrations were as low as 0.07 and 0.04 µg L-1 Carmópolis oil equivalents during winter and summer respectively, and 0.02 µg L-1 chrysene equivalent for both periods. An exception was observed off Suape Harbor (0.35 µg L-1 Carmópolis oil equiv. and 0.13 µg L-1 chysene equiv.), suggesting navigation activities as the main local source. The overall DDPH median of 0.06 μg L-1 Carmópolis oil equivalents and 0.02 μg L-1 chrysene equivalents are proposed as the baseline concentration for non-polluted coastal water for the tropical western Atlantic Ocean margin.

Ophiolites and ocean plate stratigraphy (OPS) preserved across the Central Mongolian Microcontinent: A new mega-archive of data for the tectonic evolution of the Paleo-Asian Ocean

Following the breakup of Rodinia and the opening of the Paleo-Asian ocean the relative positions of geological units (now in Mongolia) remain uncertain, and in consequence relevant paleogeographic reconstructions are poorly construed. In this paper, we apply an integrated approach to the reconstruction of the Paleo-Asian ocean using preserved ophiolites and ocean plate stratigraphy (OPS) in the Mongolian Lake and Altay zones in western Mongolia, which surround the Precambrian Central Mongolian Microcontinent (CMM) in the center of Central Asia.The Lake Zone of Mongolia contains many sediments of Ocean Plate Stratigraphy (OPS) and their underlying basalt-dominated ophiolites, which occur in three main zones along the western margin of the Precambrian crystalline basement of the CMM. The ophiolites in all zones have similar isotopic ages of ca. 570 Ma. The sections of OPS, together with serpentinite mélanges and other volcanogenic-sedimentary assemblages, have been thrust in nappe sheets over the Zavkhan block of the CMM.In the Neoproterozoic microcontinents derived from Rodinia drifted towards Siberia, and a passive margin developed on the Zavkhan block on the eastern margin of the Paleo-Asian ocean. In mid-upper Neoproterozoic times the eastern side of the Paleo-Asian oceanic plate was thrust over the CMM, and in the early Cambrian the Khasagt and Tas Khairkhan ophiolites were obducted and a mid-ocean ridge (?) reached the CMM, preventing further obduction. In the Neoproterozoic the western margin of the Paleo-Asian ocean was subducted under the Rodinia-derived microcontinent and evolved into an active continental margin. In mid-Cambrian–early Ordovician times thick turbidites were deposited in the Altay. Marine sediments, which had accumulated in the Devonian to Carboniferous in the Paleo-Asian ocean are preserved in a remnant ocean in the Lake Zone. Locally in the Devonian, terrigenous – volcanogenic sequences were deposited in the Ulgey extensional superimposed basin, and later in Meso-Cenozoic time (?) some ophiolitic and OPS fragments (Tsagaan Nuur, Ulaankhus and Bodonch) were exposed along fault escarpments that were possibly re-activated during transpression related to the India-Eurasia collision.Our analysis of the Neoproterozoic–Middle Paleozoic tectonic development of western Mongolia leads to a new holistic model for the evolution of the western Altaids, according to which the ophiolites and OPS formed in the Mongolian Lake and Altay zones between two microcontinents within a new tectonic framework of the western Altaids.

Increasing River Alkalinity Slows Ocean Acidification in the Northern Gulf of Mexico

AbstractOcean acidification (OA) progression is affected by multiple factors, such as ocean warming, biological production, and river runoff. Here we used an ocean‐biogeochemical model to assess the impact of river runoff and climate variability on the spatiotemporal patterns of OA in the Gulf of Mexico (GoM) during 1981–2014. The model showed the expected pH and aragonite saturation state (ΩAr) decline, due to the increase in anthropogenic carbon, with trends close to values reported for the Subtropical North Atlantic. However, significant departures from the basin‐averaged pattern were obtained in part of the northern GoM shelf, where pH and ΩAr increased. Model sensitivity analyses showed that OA progression was counteracted by enhanced alkalinity from the Mississippi‐Atchafalaya River System. Our findings highlight that river alkalinity is a key driver of carbon system variability in river‐dominated ocean margins and emphasize the need to quantify riverine chemistry to properly assess acidification in coastal waters.

Contrasting Sensitivity of Weathering Proxies to Quaternary Climate and Sea‐Level Fluctuations on the Southern Slope of the South China Sea

AbstractTropical marginal seas host important sedimentary archives that may be exploited to reveal past changes in continental erosion, chemical weathering, and ocean dynamics. However, these records can be challenging to interpret due to the complex interactions between climate and particulate transport across ocean margins. For the southern South China Sea over the last 90 Kyr, we observe a contrasting temporal relationship between the deposition of clay minerals and magnetic minerals, which were associated with two different hydrodynamic modes. Fine‐grained clay minerals can be carried in suspension by ocean currents, leading to a rapid response to regional climate‐driven inputs. In contrast, changes in magnetic mineralogy were linked to glacial‐interglacial sea level variability, from which we infer a control by bedload transport and resuspension. Overall, this study indicates that the transfer pathways and mechanisms imparted by varying hydrodynamic conditions exert a substantial influence on the distribution of terrigenous material in continental margin sediments.

Biomarkers, stable carbon isotope, and trace element distribution of source rocks in the Orange Basin, South Africa: implications for paleoenvironmental reconstruction, provenance, and tectonic setting

Aptian to Campanian sediments from the Western offshore to Central Orange Basin were studied by integrating molecular geochemistry, inorganic and isotopic studies to recognize their geochemical characteristics via the reconstruction of the Orange basin’s paleoweathering, paleosalinity, paleovegetation, paleoclimate, and tectonic records. Molecular analyses of both aliphatic and aromatic compounds reveal an input dominantly from a marine source. The source rocks accumulated in a reduced, anoxic, saline water column. Based on various biomarker proxies and vitrinite reflectance data, some samples are thermally mature to produce petroleum, while others are not. According to the V/Ni ratio, samples from the Orange Basin in South Africa are mainly anoxic, with only a few samples ranging from suboxic to anoxic. This is congruent with biomarker and isotope analyses that further indicate the presence of marine-derived source rocks with some terrestrial remains generating hydrocarbons. The investigated sediments are made up of intermediate igneous rocks that have undergone moderate chemical weathering. Geochemical figures on tectonic setting discriminant function diagrams revealed a continental rift of passive margin settings. As a result, the extrapolated crustal processes are directly analogous to the genesis and evolution of the Orange Basin, demonstrating Gondwana’s breaking up and the opening of the Atlantic Ocean Margin.

Testing Methods for Reconstructing Glacial Antarctic Circumpolar Current Transport in an Isotope‐Enabled Climate Model

AbstractThe Antarctic Circumpolar Current (ACC) plays a vital role in the interbasin exchange of ocean properties. A robust method to reconstruct the ACC baroclinic transport is helpful to assess the ACC's sensitivity to a changed climate. Here we test the reconstruction methods at the Last Glacial Maximum (LGM; ∼20 ka) using end‐member water masses in a fully coupled, isotope‐enabled Community Earth System Model. Model results suggest that the density profile at the northern side of ocean margins across the ACC can be reconstructed well from end‐member water masses of Subtropical Surface Water (STSW), Antarctic Intermediate Water (AAIW), and Lower Circumpolar Deep Water. One additional pore fluid observation at 1,000 m can substantially improve transport reconstruction and is essential to constrain the sign of change in ACC transport during the LGM. Moreover, the uncertainty in transport calculation is large when salinities for STSW and AAIW are reconstructed independently based on the δ18Osw‐Salinity relationship of surface and intermediate waters in the South Indian Ocean. More direct measurements of LGM temperature and salinity may allow better transport reconstruction.

Elevated levels of radiocarbon in methane dissolved in seawater reveal likely local contamination from nuclear powered vessels

Measurements of the natural radiocarbon content of methane (14C-CH4) dissolved in seawater and freshwater have been used to investigate sources and dynamics of methane. However, during investigations along the Atlantic, Pacific, and Arctic Ocean Margins of the United States, as well as in the North American Great Lakes, some samples revealed highly elevated 14C-CH4 values, as much as 4–5 times above contemporary atmospheric 14C-CH4 levels. Natural production of the 14CH4 isotopologue is too low to cause these observations nor can it explain the variations in location and depth. Numerous lab and field validation tests and blanks, as well as the relatively small number of samples that display these elevated values, all suggest that these signals are not derived from an unknown procedural issue. Here we suggest that the byproducts of nuclear power generation include localized discharges of the 14CH4 isotopologue into marine and aquatic environments, severely altering the measured 14C-CH4 isotopic signals. Since several of our sample sites are distant from on-land nuclear powerplants, we conduct further calculations concluding that the most elevated anomalies in 14C-CH4 likely originate with discharge from nuclear-powered vessels.

Temporal trend of 240Pu/239Pu atom ratios in water columns in the Western North Pacific Ocean and its marginal seas

The 240Pu/239Pu atom ratios were determined by inductively coupled plasma mass spectrometry for seawater samples from the Okinawa Trough in the western North Pacific Ocean margin to identify their Pu sources and to elucidate the temporal variability in atom ratios in the water columns in the western North Pacific Ocean and its marginal seas. The 239Pu, 240Pu and 239+240Pu water column inventories were 12.2 ± 0.5, 10.5 ± 0.5 and 22.7 ± 0.7 Bq m−2, respectively. The observed 240Pu/239Pu ratios were clearly greater than the mean global fallout ratio of ∼0.18 with a mean value of 0.236 ± 0.014. The observed greater atom ratios confirmed the presence of close-in fallout Pu from nuclear testing at the Pacific Proving Grounds (PPG). The relative percentage of the PPG-derived Pu to global fallout Pu was estimated as 41 ± 10%. The 239+240Pu water column inventory originating from the PPG close-in fallout was also calculated as 9.2 ± 0.4 Bq m−2. The temporal variability in the mean 240Pu/239Pu atom ratios in the water columns in the western North Pacific Ocean and its marginal seas was obtained through comparison of the present study values and the previously reported values, and it had no significant temporal variation with a mean value of 0.237 ± 0.004 during the observation period of 1984–2015. The 239Pu and 240Pu derived from the two different sources of global fallout and the close-in fallout at the PPG were homogenized in the water masses in the studied areas.

Satellite retrieval of cloud base height and geometric thickness of low-level cloud based on CALIPSO

Abstract. Satellite-based cloud base and top height (CBH and CTH) and cloud geometrical thickness (CGT) are validated against ground-based lidar measurements and provide new scientific insights. The satellite measurements are done by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). The retrieval methodology is built on the 333 m resolution low-level water cloud data obtained from the Vertical Feature Mask product of CALIPSO. The methodology is based on the definition that CBH of boundary layer clouds is the lowest cloud base over an area of several tens of kilometers. This allows taking the CBH of the neighboring penetrable shallower cloud as having CBH representative for the entire cloud field. The methodology over the ocean was validated based on observations from two surface-based ceilometer measurements in the islands of Barbados and the Azores, with an error standard deviation of ±115 m. Validation over land was based on 4 years data of 138 terrestrial ceilometer sites with an error standard deviation of ±220 m. The unprecedented accurate CBH allows us to obtain CGT, which is an essential parameter in the understanding of the aerosol–cloud interaction. Based on this newly developed methodology, we retrieved the annual, seasonal, and diurnal distributions of global CBH, CTH, and CGT for two years and analyzed the variations of CBH and CTH over the ocean and land. Climatology of the annual mean cloud geometrical properties shows the following. (1) The lowest CBH occurs over the eastern margins of the subtropical oceans and increases westward from 300–400 to 800–900 m. The CGT increases from 300 to 1200 m, respectively. In the western part of the tropical oceans, CBH is 500–600 m and CGT is ∼ 1500 m. (2) A narrow band of lower CBH and CGT occurs over the Equator, especially over the eastern parts of the oceans. (3) CBH and CGT over the tropical rain forests (Amazon and Congo) are 1200 and 1500 m, respectively. CBH over the drier tropical land is 1500–2000 m, with CGT of 800–1000 m. (4) CBH decreases towards Antarctica in the Southern Ocean, while CGT deepens. (5) Seasonally, the mid-latitude global oceans have the lowest CBH (mostly below 500 m) and CGT in summer seasons and the highest values in winter. (6) There is just an obvious difference between day and night for the maximum CTH and CGT over the tropics. Over the ocean, there is no discernible difference in CBH, but during night CTH is higher by ∼ 300 m.

Detrital-zircon geochronology and provenance of Ediacaran–Silurian rocks of the central to northern Tajikistan traverse: Geodynamic implications for the evolution of the Tian Shan

The South and Middle Tian Shan form the marginal part of the Central Asian Orogenic Belt (CAOB) and comprise Paleozoic ocean margins, arcs, and Precambrian continental fragments. We use U-Pb dating of detrital zircons from the Ediacaran–Early Paleozoic sedimentary succession, across a south-north transect of the South Tian Shan (STS) fold belt, to show that the Karakum-Tajik Continent (KTC) of the south-western CAOB margin contains an Early to Late Proterozoic nucleus, overprinted by tectono-magmatic events of Ediacaran–Early Cambrian age. Accretionary-type crust formed between ca. 650–500 Ma, while marginal volcanism of the KTC and microcontinents inside the western STS represented the main provenance for clastic sediments deposited within the young Turkestan ocean basin. Abundant zircon grains with ages of 580–500 Ma are widely presented to the north of KTC suggesting a possible connection between the KTC and Baltica through the south-eastern Urals during the Late Ediacaran–Cambrian. The next major zircon cluster of ca. 460–430 Ma, found within sedimentary cover of the KTC, turbidites of its northern margin in the Kurama Range, as well as inside the STS, indicate that these sediments were derived from erosion of Late Ordovician–Early Silurian continental and/or island arcs at the northern margin of the KTC. Numerous accretionary events that occurred during early-middle Paleozoic in the STS were later overprinted by continental collision during the Carboniferous closure of the Turkestan Ocean. The Paleozoic sedimentary cover of the KTC was deformed into an imbricate thrust structure during the Visean, when the Ediacaran–Cambrian Yagnob turbidites were subducted under the KTC to form the matrix of the accretionary complex.

The Mid-Atlantic Bight Dissolved Inorganic Carbon System Observed in the March 1996 DOE Ocean Margins Program (OMP)—A Baseline Study

The United States Department of Energy (DOE)’s Ocean Margins Program (OMP) cruise EN279 in March 1996 provides an important baseline for assessing long-term changes in the carbon cycle and biogeochemistry in the Mid-Atlantic Bight (MAB) as climate and anthropogenic changes have been substantial in this region over the past two decades. The distributions of O2, nutrients, and marine inorganic carbon system parameters are influenced by coastal currents, temperature gradients, and biological production and respiration. On the cross-shelf direction, pH decreases seaward, but carbonate saturation state (ΩArag) does not exhibit a clear trend. In contrast, ΩArag increases from north to south, while pH has no clear spatial patterns in the along-shelf direction. In order to distinguish between the effects of physical mixing of various water masses and those of biological activities on the marine inorganic carbon system, we use the potential temperature-salinity diagram to identify water masses, and differences between observations and theoretical mixing concentrations to measure the non-conservative (primarily biological) effects. Our analysis clearly shows the degree to which ocean margin pH and ΩArag are regulated by biological activities in addition to water mass mixing, gas exchange, and temperature. The correlations among anomalies in dissolved inorganic carbon, phosphate, nitrate, and apparent oxygen utilization agree with known biological stoichiometry. Biological uptake is substantial in nearshore waters and in shelf-slope mixing areas. This work provides valuable baseline information to assess the more recent changes in the marine inorganic carbon system and the status of coastal ocean acidification.