Western Continental Margin Research Articles

Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE Arabian Sea during the Holocene and Younger Dryas: A multi-proxy approach

A sediment core from NE Arabian Sea was studied to reveal the variations in shelf environment of western continental margin of India. A multi-proxy analysis based on planktonic foraminifera abundance (PFA), benthic foraminifera abundance (BFA), planktonic-benthic foraminiferal ratio (P/B), CaCO3 percentage (wt%), Oxygen (δ18OG. ruber) and Carbon (δ18OG. ruber) isotopes of foraminifera as well as sediment X-ray studies were carried out on 3.4m long core SK-240/485 recovered from offshore Saurashtra, NE Arabian Sea. The study aims to understand the monsoon variability in the NE Arabian Sea offshore Saurashtra since the late Pleistocene-Holocene transition and how monsoon climate coupled with sea-level change influenced paleo-productivity pattern. Six AMS radiocarbon dates place the core in the time interval 12–1Ka. Our study reveals that there have been three periods of major changes in the monsoonal climate, productivity and sea level; viz. 12–8Ka (Younger Dryas and early Holocene); 8–4Ka (middle Holocene) and 4–1Ka (late Holocene). The studied time period was also marked by weakened south west monsoon (SWM) during the cold events such as Younger Dryas (YD), Abrupt Event (AE) centring around 9.8Ka as well as 8.2 and 4Ka cold events. The Younger Dryas and early Holocene shows the low sea level stand and less Indus river discharge. Thereafter during middle and late Holocene the monsoon strengthened which resulted in raised sea level and increased Indus river discharge. The proxy records broadly suggest relatively low foraminiferal productivity at the core location during the Younger Dryas and early Holocene. We record a moderate foraminiferal productivity during the middle Holocene between 8 and 4Ka, which suggest the mesotrophic condition. The foraminiferal productivity however was relatively high since 4Ka suggesting the eutrophic conditions at the core site.

Basement tectonics and flexural subsidence along western continental margin of India

The Paleocene-recent post-rift subsidence history recorded in the Mumbai Offshore Basin off western continental margin of India is examined. Results obtained through 2-D flexural backstripping modelling of new seismic data reveal considerable thermo-tectonic subsidence over last ca. 56 Myr. Reverse post-rift subsidence modelling with variable β stretching factor predicts residual topography of ca. 2000 m to the west of Shelf Margin Basin and fails to restore late Paleocene horizon and the underlying igneous basement to the sea level. This potentially implies that: (1) either the igneous basement formed during the late Cretaceous was emplaced under open marine environs; or (2) a laterally varying cumulative subsidence occurred within Mumbai Offshore Basin (MOB) during ca. 68 to ca. 56 Ma. Pre-depositional topographic variations at ca. 56 Ma across the basin could be attributed to the extensional processes such as varied lower crustal underplating along Western Continental Margin of India (WCMI). Investigations about basement tectonics after unroofing of sediments since late Paleocene from this region support a transitional and heavily stretched nature of crust with high to very high β factors. Computations of past sediment accumulation rates show that the basin sedimentation peaked during late Miocene concurrently with uplift of Himalayan–Tibetan Plateau and intensification of Indian monsoon system. Results from basin subsidence modelling presented here may have significant implications for further studies attempting to explore tectono–climatic interactions in Asia.

Cambrian–Ordovician conodonts from slump deposits of the Argentine Precordillera: new insights into its passive margin development

AbstractThe Los Sombreros Formation represents the western continental margin slope deposits of the Argentine Precordillera, a sub-terrane accreted to Gondwana as part of the Cuyania Terrane in early Palaeozoic times. The age of these gravity-driven deposits is controversial and, therefore, a precise biostratigraphic scheme is essential to reveal the evolution of the continental margin. New conodont samplings along with sedimentological and structural analysis carried out in the Los Sombreros Formation in the La Invernada Range provide clues to its depositional framework. The sedimentary succession is made up of dominantly calciturbidites, carbonate breccias and conglomerates, along with mudstones that represent the pelagic/hemipelagic background sedimentation. It displays hectometric to outcrop-scale slump folds with variable hinge-line orientations and pinch-and-swell structures, evidencing soft-sediment deformation, consistent with a slope to base-of-slope setting. Three limestone samples from this succession include conodonts referable to the pandemicHirsutodontus simplexSubzone of theCordylodus intermediusZone (upper Furongian, Cambrian) and from theMacerodus dianaeZone (upper Tremadocian, Ordovician), implying that a slope connected the shallow-water shelf with a deep-water (oceanic) basin at least since late Cambrian times. The conodont faunas show affinities to coeval assemblages from outer shelf and slope environments around Laurentia yet they are not conclusive to postulate a geographic origin for the Precordillera. The thermal alteration of the conodonts is consistent with sedimentary burial and nappe stacking in this sector of the Precordillera.

Subsidence around oceanic ridges along passive margins: NE Arabian Sea

Abstract The northern part of the western continental margin of India formed due to the separation of the Seychelles from India at c. 63 Ma. This produced offshore tectonic elements such as the Gop Rift, the Saurashtra Volcanic Platform (SVP) and the Laxmi Ridge, as well as numerous seamounts, e.g. the Raman and Panikkar seamounts. The Laxmi Ridge and the Laxmi Basin have been studied using high-resolution 2D reflection seismic data and well data. Patch and pinnacle carbonate reefs, indicating shallow waters, are common in the north, whereas large, isolated platforms are usually noted in the south. Palaeo-depth estimates are made from well biostratigraphy. Subsidence studies of the SVP suggest that the burial history is consistent with the anomalously hot Réunion plume. We have performed a subsidence analysis south of the SVP on the Laxmi Ridge and Laxmi Basin. The sediment-unloaded basement depths, estimated using using flexural isostasy with effective elastic thicknesses of 10–40 km have been found to be 2000–4000 m in areas where carbonates exist. These carbonates indicate <200 m bathymetry at c. 65 Ma, and the subsidence discrepancy is thus due to thermal cooling or anomalous heating due to the Deccan plume. Patch and pinnacle reefs in the north suggests that either the rise in sea-level or the rate of subsidence of the basement were fast. The presence of large platforms in the south indicates otherwise. This is possibly due to a greater influence from the Indus Fan sediments towards the north. In addition, the Laxmi Ridge is a spreading centre that remained emergent near or above sea-level due to plume support, which was also greater in the south due to proximity to the plume. When the plume support discontinued, the ridge subsided quickly to present-day depths, which matches the subsidence expected for 60–70 Myr old oceanic crust. Supplementary material: A table is available at https://doi.org/10.6084/m9.figshare.c.3470751

Multifractal Detrended Fluctuation Analysis to Compare Coral Bank and Seafloor Seepage Area-Related Characterization Along the Central Western Continental Margin of India

In this letter, the characterization of two coral banks is initiated employing the multifractal detrended fluctuation analysis (MFDFA) for multibeam bathymetry and associated backscatter data. The seafloor roughness was estimated using the Hurst exponent of a second-order moment. The two coral banks are located around the buried channels off Malpe, western continental margin of India (WCMI). The Gaveshani bank is lying on a submerged headland with high backscatter strength. The presence of greater seafloor roughness and medium multifractality in the backscatter data series indicates less heterogeneity of the involved process. In an evolutionary process, the Gaveshani bank may not be affected by the paleochannels. Contrary to the Gaveshani bank, another unnamed coral bank located on lowland and with lower backscatter strength shows lower seafloor roughness and greater multifractality, comparatively. This smoothness is due to the considerable heterogeneity of the seafloor process of a paleochannel system. During the Holocene period, the waves and the current action have further modified the morphology of the coral banks. To support the present analysis, we have juxtaposed the multifractal data analysis of seepage and nonseepage seafloor off Goa, WCMI, along with the coral banks.

Hydrocarbon accumulation in the deep waters of South China Sea controlled by the tectonic cycles of marginal sea basins

AbstractThe tectonic cycle of the marginal sea basins in South China Sea (SCS) includes two cycles, i.e., the formation and contraction of Palaeo-SCS and Neo-SCS. The northern part of SCS is a rift basin on a passive continental margin, while the the Nansha Block is a drifting rift basin. The southern part is a compound compressional basin on an active continental margin; the western part is a shear-extensional basin on a transform continental margin; the eastern part is an accretionary wedge basin on a subduction continental margin. The deep-water basins are mainly distributed on the continental slope and the the Nansha Block. There are three sets of source rocks in the deep-water areas of the northern continental margin in SCS, i.e., Eocene terrestrial facies, early Oligocene transitional facies and late Oligocene marine facies. A set of Late Cretaceous-Early Oligocene terrestrial marine facies source rocks are developed in the drift-rift basin of SCS. Three sets of Oligocene, early Miocene and Mid-Miocene marine-terrestrial transitional facies source rocks are developed in the deep-water areas of both the southern and western continental margins of SCS. Four sets of reservoirs developed in the northern deep waters of SCS are dominated by deep sea fans. Two sets of reservoirs developed in the the Nansha Block are dominated by delta and biogenic reef. The southern part of SCS is dominated by deep sea fan and biogenic reef. Reservoirs of large channels and other clastic facies were developed in front of the estuaries, while biogenic reef bank was formed in the uplift zone. The hydrocarbon accumulation assemblages are mainly presented as Oligocene-Pliocene in the deep waters on the northern continental margin of SCS, Eocene-Lower Oligocene in the the Nansha Block, Oligocene-Pliocene and Oligocene-Miocene in the deep waters on the southern and western continental margin of SCS, respectively. The major hydrocarbon reservoir types in the deep waters of SCS are related to structural traps, deep water fans and biogenic reefs. The formation of basin, hydrocarbon and reservoir in the deep waters of SCS are controlled by the tectonic cycles of the marginal sea basins, revealing a great potential for hydrocarbon exploration.

Geophysical evidence and inferred triggering factors of submarine landslides on the western continental margin of the Ulleung Basin, East Sea

Submarine landslides form very complex depositional and erosional features on the seafloor, and their dynamics and triggering processes are yet to be understood completely. Numerous studies are being undertaken both because of the scientific significance but also for their potential harm to seafloor infrastructure and coastal areas. This study investigates the styles and causes of landsliding along the western margin of the Ulleung Basin in the East Sea, based on multiple sparker, subbottom profiler, multibeam echosounder and sediment core datasets collected in 2015. The bathymetric analyses indicate that the southern slope of the Ulleung Basin has experienced at least seven submarine failures. These failures left clear arcuate-shaped scarps that initiated at water depths of ~600 m. The observed headwall scarps have heights that exceed 60 m and appear to be the result of retrogressive-type failures. Seismic reflection data clearly image the basal sliding surface that is characterized by a prominent high-amplitude reflector. Chaotic-to-transparent seismic facies occur immediately downslope of the headwall scarps; these represent ~20 m thick landslide deposits. Gravity cores taken from areas adjacent to the scars suggest that these slides are older than ca. 97 ka. Interpretation of the present data shows that faults appear to cut recent sediments upslope of scarps, and that the slope may still be in an active phase of failure. Seismic data also image various overpressurized gases and/or gas fluids, as evidenced by the occurrence of pockmarks and seismic chimneys in upslope or adjacent areas of the scarps. Hence, earthquakes associated with tectonic activity and development of fluid overpressure may have acted as the main conditioning factor for destabilizing the slope sediments. Geotechnical stability analyses indicate that the sampled slope sediments are exceptionally stable under present-day conditions, even under seismic loading. This finding points to additional forces such as excess pore pressure caused by gas fluids at the times of slide emplacement.

Effects of bottom water dissolved oxygen variability on copper and lead fractionation in the sediments across the oxygen minimum zone, western continental margin of India

This study describes the effect of varying bottom-water oxygen concentration on geochemical fractionation (operational speciation) of Cu and Pb in the underneath sediments across the oxygen minimum zone (Arabian Sea) in the west coast of India. Both, Cu and Pb were redistributed among the different binding phases of the sediments with changing dissolved oxygen level (from oxic to hypoxic and close to suboxic) in the bottom water. The average lability of Cu-sediment complexes gradually decreased (i.e., stability increased) with the decreasing dissolved oxygen concentrations of the bottom water. Decreasing bottom-water oxygen concentration increased Cu association with sedimentary organic matter. However, Pb association with Fe/Mn-oxyhydroxide phases in the sediments gradually decreased with the decreasing dissolved oxygen concentration of the overlying bottom water (due to dissolution of Fe/Mn oxyhydroxide phase). The lability of Pb-sediment complexes increased with the decreasing bottom-water oxygen concentration. This study suggests that bottom-water oxygen concentration is one of the key factors governing stability and lability of Cu and Pb complexes in the underneath sediment. Sedimentary organic matter and Fe/Mn oxyhydroxide binding phases were the major hosting phases for Cu and Pb respectively in the study area. Increasing lability of Pb-complexes in bottom sediments may lead to positive benthic fluxes of Pb at low oxygen environment.

The chronology and tectonic style of landscape evolution along the elevated Atlantic continental margin of South Africa resolved by joint apatite fission track and (U‐Th‐Sm)/He thermochronology

AbstractAtlantic‐type continental margins have long been considered “passive” tectonic settings throughout the entire postrift phase. Recent studies question the long‐term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin's evolution. The Namaqualand sector of the western continental margin of South Africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity. Here we present the first systematic study using joint apatite fission track and apatite (U‐Th‐Sm)/He thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region. Apatite fission track ages range from 58.3 ± 2.6 to 132.2 ± 3.6 Ma, with mean track lengths between 10.9 ± 0.19 and 14.35 ± 0.22 µm, and mean (U‐Th‐Sm)/He sample ages range from 55.8 ± 31.3 to 120.6 ± 31.4 Ma. Joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150–130 Ma and 110–90 Ma with limited cooling during the Cenozoic. Estimates of denudation based on these thermal histories predict approximately 1–3 km of denudation coinciding with two major tectonic events. The first event, during the Early Cretaceous, was driven by continental rifting and the development and removal of synrift topography. The second event, during the Late Cretaceous, includes localized reactivation of basement structures as well as regional mantle‐driven uplift. Relative tectonic stability prevailed during the Cenozoic, and regional denudation over this time is constrained to be less than 1 km.

Structure and function of nematode communities across the Indian western continental margin and its oxygen minimum zone

Abstract. We studied patterns of nematode distribution along the western Indian continental margin to determine the influence of habitat heterogeneity and low oxygen levels on the community's taxonomic and functional structure. A single transect, perpendicular to the coast at 14° N latitude was sampled from 34 to 2546 m depth for biological and environmental variables during August 2007. The oxygen minimum zone extended from 102 to 1001 m. Nematodes (described and undescribed) were identified to species and classified according to biological and functional traits. A total of 110 nematode species belonging to 24 families were found along the transect. Three depth zones were identified: the shelf (depth range: 34–102 m; highest nematode mean density: 176.6 ± 37 ind 10 cm−2), the slope (525–1524 m; 124.3 ± 16 ind 10 cm−2), and the basin (2001–2546 m; 62.9 ± 2 ind 10 cm−2). Across the entire study area, the dominant species were Terschellingia longicaudata, Desmodora sp. 1, Sphaerolaimus gracilis, and Theristus ensifer; their maximum density was at shelf stations. Nematode communities in different zones differed in species composition. Chromadorita sp. 2 (2.78 %) and Sphaerolaimus gracilis (2.21 %) were dominant on the shelf, whereas Terschellingia longicaudata (4.73 %) and Desmodora sp. 1 (4.42 %) were dominant on the slope, but in the basin, Halalaimus sp. 1(1.11 %) and Acantholaimus elegans (1.11 %) were dominant. The information in a particular functional group was not a simple reflection of the information in species abundance. Ecological information captured by adult length, adult shape, and life-history strategy was less site-specific and thus differed notably from information contained in other taxonomic groups. The functional composition of nematodes was strongly linked to the organic-carbon and dissolved-oxygen concentration. Seven species were found exclusively in the oxygen minimum zone: Pselionema sp. 1, Choanolaimus sp. 2, Halichoanolaimus sp. 1, Cobbia dentata, Daptonema sp. 1, Trissonchulus sp. 1, and Minolaimus sp. 1. Correlation with a number of environmental variables indicated that food quantity (measured as the organic-carbon content and chlorophyll content) and oxygen level were the major factors that influenced nematode community structure and function.

Influence of the upwelling events on the δ13C and δ18O of the benthic bivalve shells of the South Western Continental Margin of India

In this study, the influence of the spatial and temporal variability of upwelling intensity and the associated biological productivity observed during different phases of summer monsoon along the southwestern continental margin of India (SWCMI) on the δ13C and δ18O of the inorganic biogenic carbonate shells was investigated. Multispecies benthic bivalve shells (1–5 mm) separated from ten surface sediment samples of SWCMI (off 12°N, 10°N and 9°N) collected during the onset (OSM) and peak (PSM) phase of the summer monsoon of 2009 were analysed for δ13C and δ18O. Sea surface temperature along the study region indicates prominent upwelling in PSM than in OSM. A comparison of analytical and predicted values for δ18O in the bivalve shells confirmed their in situ origin during both the sampling periods. During PSM, the δ13C values in the benthic bivalve shells were more depleted in 13C than during OSM which recorded lower values of δ13C in dissolved inorganic carbon of bottom waters expected in the study region in PSM due to the upwelled waters, high surface productivity and the associated high degradation of the organic matter in the subsurface and bottom waters. However, this depletion of δ13C was not observed in benthic bivalve shells obtained from 10°N, since it is influenced by high export fluxes of carbon from the Cochin estuary since early monsoon months.

Barra Fan: a major glacial depocentre on the western continental margin of the British Isles

The Barra Fan is a large prograding sedimentary wedge of Pliocene–Pleistocene age that has built out into the deep-water basin of the Rockall Trough west of Britain (Holmes et al. 1998). Together with the Donegal Fan, considered to be part of the same fan complex, it covers an area of about 7000 km2 (Armishaw et al. 2000) and locally approaches 700 m in thickness. The bulk of the sediments are thought to be glacially derived from the British Ice Sheet, with ice flowing from northern Ireland and western Scotland converging to reach the shelf edge between 56° N and 57° N via several cross-shelf troughs, most recently at c. 29–27 ka cal BP (Dunlop et al. 2010). Located on the western British continental margin between the Hebrides Shelf and the Rockall Trough, a large sedimentary depocentre with outward-bulging contours is found in water depths between c. 200 m at the shelf edge and c. 2500 m in Rockall Trough (Fig. 1a, b). Gradients commonly reach 7.5° on the upper slope, reducing to 1.5° on the …

Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia)

Subduction of buoyant continental lithosphere is one of the least understood plate-tectonic processes. Yet under the Pamir–Hindu Kush, at the northwestern margin of the India–Asia collision zone, unusual deep earthquakes and seismic velocity anomalies suggest subduction of Asian and Indian lithosphere. Here, we report new precise earthquake hypocenters, detailed tomographic images and earthquake source mechanisms, which allow distinguishing a narrow sliver of Indian lithosphere beneath the deepest Hindu Kush earthquakes and a broad, arcuate slab of Asian lithosphere beneath the Pamir. We suggest that this double subduction zone arises by contrasting modes of convergence under the Pamir and Hindu Kush, imposed by the different mechanical properties of the three types of lithosphere involved. While the buoyant northwestern salient of Cratonic India bulldozes into Cratonic Asia, forcing delamination and rollback of its lithosphere, India's thinned western continental margin separates from Cratonic India and subducts beneath Asia. This torn-off narrow plate sliver forms a prominent high-velocity anomaly down to the mantle transition zone. Our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab, providing a rare glimpse at the ephemeral process of slab break-off.

Mechanism of crustal extension in the Laxmi Basin, Arabian Sea

Continental rifting and magmatism has been extensively studied worldwide as it is believed that continental rifting, break up of continents and associated magmatism lead to genesis of new oceanic crust. However, various regions of the world show that these processes may lead to genesis of other types of crust than the oceanic crust. Laxmi Basin in the western continental margin of the India is one such region with an enigmatic crust. Due to its extreme strategic significance for the palaeogeographic reconstruction of continents during Cretaceous continental breakup of India, this basin has attracted various workers for more than two decades. However, still the issue of nature of crust in the basin remains controversial. In this contribution, in order to identify nature of crust, mechanism of continental extension in the Laxmi Basin has been studied for the first time through newly acquired seismic data from the basin. Here, we propose a plausible mechanism of crustal extension in the Laxmi Basin which eventually constrains the nature of crust of the Laxmi Basin. We have demonstrated that the crust in the Laxmi Basin can be categorised in two zones of stretched and transitional crust. In the stretched zone several fault bounded horst and graben structures are identified which preserve syn- and post-rift sediments along with different periods of hiatus in sedimentations as unconformities. These faults are identified as listric faults in the upper crust which sole out in the detachment faults. Detachment faults decouples the upper brittle and lower ductile crust. The transitional crust is identified as heavily intruded by sills and basaltic volcanic which were emplaced due to melting of subcontinental mantle (SCM) after hyper-stretching of crust and serpentinisation of the SCM. Panikkar Ridge is proposed to be one such basaltic volcanic body derived from melting of lower part of the SCM.

Late Pleistocene-Holocene events on the continental slope of the Laptev Sea: Evidence from benthic and planktonic foraminiferal assemblages

This work is dedicated to the study of benthic and planktonic foraminifers and is a contribution to the multidisciplinary investigations of Core PS51/154-11 from the Laptev Sea. The paleoecological analysis of foraminiferal assemblages makes it possible to reconstruct in detail environmental changes on the western continental margin of the Laptev Sea during the Late Pleistocene and Holocene. The examined core dated by the AMS radiocarbon method is divided into intervals that reflect main stages in the regional evolution for the last 17.6 k.y.: early deglaciation, Bolling–Allerod warming, Younger Dryas cooling, transition to the Interglacial, Holocene climatic optimum, Middle-Late Holocene. The presence of subpolar planktonic foraminifers and benthic species Cassidulina neoteretis (Tappan) provides grounds to reconstruct for the continental slope area stages of the enhanced activity of subsurface Atlantic-derived water in the intervals of 12.0–14.7 and 0.6–5.4 ka. The benthic assemblage reflects changes in depositional environments related to the postglacial transgression and also climatic change impacts affecting bioproductivity. The events defined on the basis of foraminifers are correlated with climatic oscillations and changes in circulation of water masses.

In this issue of Weather

Following on from the article in July's issue on the understanding of climate change, demonstrated by n‐gram phrases in books published since 1850, Ifor Williams opens this issue with an analysis of the possible public confusion about the subject caused by news reports in Climate change confusion – who is responsible? This poignant Viewpoint clearly shows how our perception can be affected by these news reports, even though they are often ill‐conceived in trying to make the slow rate of global warming something easily understood, or stating that, because change is slow and somewhat irregular, it might not be real.Putting this into perspective, our next article is the annual review, Global and regional climate in 2014 by John Kennedy, Colin Morice and David Parker of the Met Office Hadley Centre. Here is shown clearly the continuing change of climate, last year having been almost certainly the joint warmest on record, repeating the global warmth of 2010. The situation is, as always, complex, although most of the world had a warm year, only some central areas of continents experiencing a cold year, overall. Precipitation anomalies were yet more complex, but the signal for anthropogenic climate change is clear.Our need to get reliable data on river levels and the associated likelihood of flooding are laid out in Colin Clark's A 247‐year flood record on the upper Stour, Dorset, UK. The relatively short records we have for the levels of many rivers in the UK makes planning and preparation for flooding difficult, but this paper addresses the issue, using the long data record from the Stour, carefully describing the difficulties associated with any lack of full understanding of the response of rivers to rainfall.It is a well‐established tenet of climatology that the mid‐latitude westerlies bring warmer, wetter conditions than might otherwise be expected in the countries of the western continental margins. The warmth is related to the temperature of the sea surface, so variability of sea‐surface temperatures in the North Atlantic are a significant influence on the climate of Ireland, as described by G. D. McCarthy, E. Gleeson and S. Walsh in The influence of ocean variations on the climate of Ireland.In his letter: The narrow cold‐frontal rainband of 22/23 November 2013 C. Kidd adds to the information presented by Martin Young in his paper in the October 2014 issue: An unusual case of line convection – the birth of a classical cold front over the UK, showing how good an example of ‘linear convection’ this was.

Microbial iron reduction and methane oxidation in subsurface sediments of the Arabian Sea

Arabian Sea is one of the most productive regions of the world's ocean with seasonal upwelling and a characteristic oxygen minimum zone. It receives a continuous input of windborne iron-rich dust which possibly stimulates phytoplankton productivity. A sediment core from this area, which is on the western continental margin of India in the Kerala-Konkan basin was studied from the surface to 63 m below seafloor, to establish the co-occurrence of microbial iron reducing activity and methane oxidation. Total bacterial abundance (TC), viable aerobic counts accounting for aerobic and facultative anaerobes (TVCa), viable anaerobic counts (TVCan), methane, sulfate, ferrous [Fe(II], hydroxylamine extractable iron and HCl extractable iron concentrations were measured. Average TC was 52.6 ± 29.8 × 105 cells g−1 sediment while TVCa and TVCan were an order less. Methane and sulfate concentrations were 1.3 ppm and 23.2 mM, respectively. Average Fe(II) concentration, hydroxylamine and HCl extractable Fe was 0.32 ± 0.26, 0.86 ± 1.44 and 1.90 ± 2.47 mmol g−1 sediment, respectively. Interestingly, Fe(II) significantly correlated with TVCan (r = 0.66, p ≤ 0.001).In the experimental tubes, the build-up of Fe(II) was accompanied by the disappearance of methane. Average methane concentration and Fe(II) concentration in the experimental sediment slurries was 0.56 ± 0.76 ppb and 0.50 ± 0.27 mmol g−1 sediment respectively. The microcosm experiment showed that Fe(II) determined was significantly higher (ANOVA, F = 6.74, p ≤ 0.014) after 5 days of incubation, thus implying microbial iron reducing activity. A strong negative linear correlation (p ≤ 0.001) of methane concentration with Fe(II) suggests that increasing activity of microbial iron reduction caused suppression in methane production. This is also indicative of the co-occurrence of iron reduction and methane oxidation in these sediments.

Erratum to: Characterizing slope morphology using multifractal technique: a study from the western continental margin of India

Erratum to: Characterizing slope morphology using multifractal technique: a study from the western continental margin of India

Glacial-interglacial productivity contrasts along the eastern Arabian Sea: Dominance of convective mixing over upwelling

The western continental margin of India is one of the highly productive regions in the global ocean. Primary productivity is induced by upwelling and convective mixing during the southwest and northeast monsoons respectively. Realizing the importance of high primary productivity, a sediment core was collected below the current oxygen minimum zone (OMZ) from the southwestern continental margin of India. This was dated by AMS radiocarbon and as many as 60 paleoclimate/paceoceanographic proxies, such as particle size, biogenic components, major, trace and rare earth elements (REEs) which were measured for the first time to determine sources of sediment, biogeochemical processes operating in the water column and their variations since the last glacial cycle. R-mode factor analysis of comprehensive data indicates that the dominant regulator of paleoproductivity is the southwest monsoon wind induced upwelling. Other paleoproductivity related factors identified are the marine biogenic component and biogenic detritus (as an exported component from the water column added to the bottom sediment). All paleoproductivity components increased significantly during the marine isotope stage-1 (MIS-1) compared to those accumulated from MIS-4 to MIS-2. The second group of factors identified are the terrigenous sediments with heavy minerals like zircon and ilmenite. The terrigenous sediment, in particular, increased during MIS-2 when the sea-level was lower; however, the heavy mineral component fluctuated over time implying pulsed inputs of sediment. The diagenetic fraction and reducing component are the third group of factors identified which varied with time with increased accumulation during the MIS transitions.The primary productivity along the southwestern continental margin of India seems to have been controlled principally by the upwelling during the southwest monsoon season that was weaker from MIS-4 to MIS-2, as relative to that during the MIS-1. In contrast, increased glacial productivity noticed in sediments deposited below the current oxygen minimum zone (OMZ) along the north of the study area that can be linked to entrainment of nutrients through the intensified convective mixing of surface water during the northeast monsoon. The sequestration of greenhouse gases by the western continental margin of India was higher during glacial than interglacial cycles.

Application of Hybrid Techniques (Self-Organizing Map and Fuzzy Algorithm) Using Backscatter Data for Segmentation and Fine-Scale Roughness Characterization of Seepage-Related Seafloor Along the Western Continental Margin of India

<?Pub Dtl=""?> In this paper, a seafloor characterization technique to unravel the number of data classes using multibeam echo-sounding backscatter data has been demonstrated. An application of self-organizing maps (SOMs) to backscatter profile data has been developed to determine the number of classes. Consequently, the fuzzy C-means (FCM) method is employed thereafter, using the number of class information of the backscatter profiles segmentation. The use of the soft-computational technique for seafloor backscatter data facilitates in achieving stationary profile data sets suitable for seafloor roughness model application. The power spectral density (PSD) function of the segmented profiles provides the power law parameters ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\beta$</tex> </formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$a'$</tex> </formula> ) through curve fitting, using power law expression. The data acquired from western continental margin of India (WCMI), off Goa, reveal five distinct classes of backscatter strength having different segment lengths. The estimated roughness parameters ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\beta$</tex> </formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$a'$</tex> </formula> ) of the segmented profiles provide quantitative information about the area seafloor roughness. A gridded map of the estimated roughness parameter <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\beta)$</tex></formula> is generated using the “krigging” method. The gridded map and the class of the segmented profiles overlaid on the backscatter map have been presented, which is a first time application relevant for the understanding of the seafloor. The present study underscores a combination of soft-computational (SOM and FCM) and numerical techniques (power spectral density at short and fine scales) to effectively recognize the seafloor processes and the associated sedimentological dynamics in a complex geographical environment (including the pockmarks and faulted structures), which is subjected to strong bottom currents and seasonal upwelling.