Western Indian Shelf Research Articles

Interannual variability in particulate organic matter distribution and its carbon stable isotope signatures from the western Indian shelf waters

Particulate organic carbon and nitrogen (POC, PN, collectively particulate organic matter, POM) and the stable isotopic signature of POC (δ13CPOC) are important to delineate its sources and recycling in shelf water. The present study provides insights into the factors responsible for spatial and interannual variability in POM and δ13CPOC values along the western Indian shelf waters (8° N –21° N) during the southwest (SW) monsoon (August) 2017 and 2018. The dominance of phytoplankton-derived POM with a negligible terrestrial influence was evident from the positive correlation between POC and TChla contents, ratios of C: N, and δ13CPOC signatures. Prominent upwelling signatures [cold nutrient-rich water, higher POM, total Chlorophylla (TChla), and δ13CPOC values] were noted in the south (8–12° N), whereas low nutrient warm waters (lower values of POM, TChla, and δ13CPOC) were prevalent in the north (13–21° N). Phytoplankton biomass was significantly higher and matured in 2017 due to an early and stronger upwelling in the south. In 2018, delayed and weak upwelling (evident from Ekman offshore transport and pumping velocity) resulted in the late development of phytoplankton bloom and lower POM. Furthermore, considerably lower nutrient supply within the mixed layers in 2018 compared to 2017 was partially attributed to the enhanced spatial expansion of low salinity waters closer to the surface. In the north, in 2018, higher wind speeds enhanced vertical mixing resulting in increased nutrient supply and TChla compared to 2017. We conclude that monsoon wind speed in the northern shelf and strength as well as the timing of the upwelling, including freshwater flux in the south, can be the key factors in modulating the interannual variability in POM distribution and δ13CPOC signature in the western Indian Shelf waters.

The world’s largest coastal deoxygenation zone is not anthropogenically driven

The growing number of oxygen-deficient coastal zones around the world and their impacts on marine life has always been a controversial issue as their development is largely attributed to anthropogenic activities which can be mitigated by human actions. However, contrary to this prevailing understanding, we show here for the first time, using new coherent datasets from estuaries to coastal to offshore regions, that the world’s largest hypoxic-anoxic zone along the west coast of India is formed through a natural process, i.e. upwelling of deoxygenated waters during the summer monsoon. We further demonstrate that the persistence and extent of this coastal oxygen deficiency depend on the degree of deoxygenation of source waters for the upwelling. Consequently, the anoxia is confined only to the central shelf between 11° and 18° N, which is equivalent to almost half of the western Indian shelf, where upwelling brings suboxic waters from the core oxygen minimum zone in the Arabian Sea.

Contrasting phytoplankton and biogeochemical functioning in the eastern Arabian Sea shelf waters recorded by carbon isotopes (SW monsoon)

This study examines the relationship between the isotopic signature of phytoplankton-derived particulate organic matter (POM) (δ13CPOC) and estimates of growth rate and community composition along a north-south gradient in the western Indian shelf waters which possess contrasting biogeochemistry and mixed layers depths. The Eastern Arabian Sea/western Indian shelf turns highly productive due to the SW monsoon induced upwelling at its southern region and experiences drastic seasonal reversal. We have characterized the POM pool by quantifying Chla, particulate organic carbon (POC) and particulate nitrogen (PN), C:N ratios, along with δ13CPOC values during the SW Monsoon. The prevailing physicochemical features contrasted between the southern (8°N to 12°N) and northern (13°N to 21°N) stations. Close couplings between POC, PN, and Chla contents indicated the autochthonous nature of POM. Low temperature, shallow mixed layer depths (MLDs), high concentrations of nutrients, POM, and Chla, marked the upwelling signature in the south. Conversely, relatively higher temperatures, salinity, deeper mixed layers (MLs), lower concentrations of nutrients, POM, and Chla were evident at the northern stations. Five times higher POC concentrations were noticed in the south (65.5 ± 22.0 μmol L−1) than in the north (12.8± 5.4 μmol L−1) within the MLs. Phytoplankton community shift (based on marker pigment analysis) and the δ13CPOC values were closely coupled. The nutrient replete microphytoplankton (diatoms) dominated southern stations were associated with higher values of δ13CPOC (−23.0 ± 2.3‰) which was attributed to faster growth rate; conversely, the oligotrophic nitrogen-limited waters in the north dominated by pico and nanophytoplankton (haptophytes and cyanobacteria), were characterized by distinctly lower δ13CPOC values (−26.6 ± 0.9‰). Phytoplankton growth rates based on a model were consistent with this trend. Our results show that the autochthonous POC is the primary source of organic matter, and the contrasted biogeochemistry induced phytoplankton communities and their growth rates mostly governed the δ13CPOC variability in this region. However, the lower values in the subsurface waters at the south were likely to be influenced by light limitation and heterotrophy.

Submarine groundwater discharge and associated nutrient flux from southwest coast of India

The nutrient discharge associated with submarine groundwater discharge (SGD) into the Arabian Sea has been investigated for the first time using the Radon isotope (222Rn) mass balance model at three sites along the southwest coast of India. The SGD flux varied in the range of 49.48×104 - 335.84×104 m3/day, with high (low) discharge during the low (high) tide. SGD delivers a considerable amount of the nutrient into the Arabian Sea with 2.10×104 to 11.66×104 mol/day dissolved inorganic nitrogen (DIN), 1.23×102 to 56.31×102 mol/day dissolved inorganic phosphate (DIP), and 7.28×104 to 24.44×104 mol/day dissolved silicate (DSi). This significant nutrient input to the coastal waters through SGD is mainly attributed to the land-use practices like agricultural activities, improper waste disposal, and thickly populated coastal settlement zones. The increase in the nutrient discharge may lead to phytoplankton bloom in the nearshore environment and can accelerate seasonal coastal hypoxia over the western Indian shelf. This evidence of considerably high nutrient flux through SGD advocates the importance of understanding SGD associated flux along the southwest coast of India to maintain a sustainable ecological balance.

Upwelling intensity modulates N2O concentrations over the western Indian shelf

Upwelling intensity modulates N₂O concentrations over the western Indian shelf

Anaerobic Carbon Mineralisation Through Sulphate Reduction in the Inner Shelf Sediments of Eastern Arabian Sea

Monsoon-induced coastal upwelling, land run-off, benthic and atmospheric inputs make the western Indian shelf waters biologically productive that is expected to lead to high rates of mineralisation of organic matter (OM) in the sediments. Dissimilatory sulphate reduction (SR) is a major pathway of OM mineralisation in near-shore marine sediments owing to depletion of other energetically more profitable electron acceptors (O2, NO3−, Mn and Fe oxides) within few millimetres of the sediment-water interface. We carried out first ever study to quantify SR rates in the inner shelf sediments off Goa (central west coast of India) using the 35S radiotracer technique. The highest rates were recorded in the upper 10 cm of the sediment cores and decreased gradually thereafter below detection. Despite significant SR activity in the upper ∼12 to 21 cm at most of the sites, pore water sulphate concentrations generally did not show much variation with depth. The depth integrated SR rate (0.066–0.46 mol m−2 year−1) decreased with increasing water depth. Free sulphide was present in low concentrations (0–3 μM) in pore waters at shallow stations (depth <30 m). However, high build-up of sulphide (100–600 μM) in pore waters was observed at two deeper stations (depths 39 and 48 m), 7–11 cm below the sediment-water interface. The total iron content of the sediment decreased from ∼7 to 5 % from the shallowest to the deepest station. The high pyrite content indicates that the shelf sediments act as a sink for sulphide accounting for the low free sulphide levels in pore water. In the moderately organic rich (2–3.5 %) sediments off Goa, the measured SR rates are much lower than those reported from other upwelling areas, especially off Namibia and Peru. The amount of organic carbon remineralised via sulphate reduction was ∼0.52 mol m−2 year−1. With an estimated average organic carbon accumulation rate of ∼5.6 (±0.5) mol m−2 year−1, it appears that the bulk of organic matter gets preserved in sediments in the study region.

Lower Paleogene Tectonostratigraphy of Balochistan: Evidence for Time-Transgressive Late Paleocene-Early Eocene Uplift

Analysis of lithofacies, paleoflow directions, and sandstone petrography of upper Paleocene-lower Eocene paralic and continental sediments exposed along the transpressional suture zone of the western margin of the Indian plate indicate that the process of deformation and uplift of the carbonate shelf in this area had started by late Paleocene time. This tectonic uplift and deformation is documented by: (1) an overall shallowing upward synorogenic sequence of sediments, (2) proximal conglomerate facies (consisting of lower Paleocene and Mesozoic clasts) dominating in the western part of the study area and distal facies of sandstone and shale dominating in the eastern part of the study area, (3) the existence of an unconformity of late Paleocene-early Eocene age in the Quetta and Kalat regions, (4) paleocurrent directions in deltaic and fluvial deposits indicating southeastward flowing sediment dispersal paths during late Paleocene-early Eocene time, which is opposite to that found in the late Cretaceous, suggesting a reversal in the depositional slope of the Cretaceous shelf, and (5) petrographic study of sandstones indicating a collision suture/fold thrust belt provenance. This episode of uplift and deformation could be the result of India-Arabian transpression with associated ophiolite obduction or, more likely, to represent the local response to initial India-Asia contact. The unroofing pattern and uplift geometry of the western Indian shelf suggests that this tectonism first started in the southern part of the study area (Kalat-Khuzdar area) during the late Paleocene-early Eocene and proceeded northward in a time-transgressive fashion.

Meiofauna of the western continental shelf of India, Arabian Sea

Meiofaunal standing stock and nematode community structure were investigated in the western continental shelf of India by collecting samples from every degree square of the shelf during two cruises of the FORV ( Fishery and Oceanographic Research Vessel) Sagar Sampada, conducted in 1998 and 2001. Samples were collected from 30, 50, 100 and 200 m depths using a Smith Mc Intyre grab. Meiofaunal density ranged from 8 Ind. 10 cm −2 to 1208 Ind. 10 cm −2 and biomass from 0.07 mg 10 cm −2 to 6.11 mg 10 cm −2. Nematodes were the dominant meiofaunal group, contributing 88% of the density and 44% of the biomass. Harpacticoid copepods were the second important taxa, contributing 8% of both biomass and density. Altogether, 154 species of nematodes belonging to 28 families were recorded from the study area. Numerically, Desmodora spp., Dorylaimopsis sp. , Tricoma spp., Theristus spp. and Halalaimus spp. were the dominant species. In general, there was a decrease in biomass and density of meiofauna and species diversity of nematodes with increase in depth. There was a 67% drop in species number from 51 to 100 m (106 species) to the shelf edge (35 species). Species richness and diversity indices showed consistent decrease with depth. The species dominance index was higher below 150 m depth. ANOSIM (from PRIMER) showed a significant difference between the nematodes of the near shore and shelf edge. Latitudinal variation was observed only in the number of nematode species. Biomass and abundance of nematodes were found to increase from coarse to fine sediment, while copepods showed an opposite trend. Multivariate analyses of nematode communities did not reveal any latitudinal or substratum differences. Variables such as depth, latitude, organic matter (OM) and amount of clay were the most relevant parameters influencing the biomass and density of meiofauna, while depth and temperature were the important parameters explaining the distribution of the nematode communities along the western Indian shelf.

The oceanic fixed nitrogen and nitrous oxide budgets: Moving targets as we enter the anthropocene?

New data force us to raise previous estimates of oceanic denitrification. Our revised estimate of ~ 450 Tg N yr -1 (Tg = 10 12 g) produces an oceanic fixed N budget with a large deficit (~ 200 Tg N yr -1 ) that can be explained only by positing an ocean that has deviated far from a steady-state, the need for a major upwards revision of fixed N inputs, particularly nitrogen fixation, or both. Oceanic denitrification can be significantly altered by small re-distributions of carbon and dissolved oxygen. Since fixed N is a limiting nutrient, uncompensated changes in denitrification affect the ocean´s ability to sequester atmospheric CO 2 via the biological pump. We have also had to modify our concepts of the oceanic N 2 O regime to take better account of the extremely high N 2 O saturations that can arise in productive, low oxygen waters. Recent results from the western Indian Shelf during a period when hypoxic, suboxic and anoxic waters were present produced a maximum surface N 2 O saturation of > 8000%, a likely consequence of stop and go denitrification. The sensitivity of N 2 O production and consumption to small changes in the oceanic dissolved oxygen distribution and to the spin-up phase of denitrification suggests that the oceanic source term for N 2 O could change rapidly.

Source Areas and Migration Trends of Oil and Gas in Bombay Offshore Basin, India

The Bombay Offshore basin, located on the western Indian Shelf, is the southern extension of the onshore Cambay basin and contains oil and gas-bearing structures. Oil and gas have been generated mainly from the thick shale sequences deposited in local depressions within the basin. The Paleogene and Miocene prodelta muds deposited in one such major low, the Dahanu depression, appear to have been the main source beds for these hydrocarbons; the Paleogene and lower Miocene shale sequence in the eastern part of the Bombay-Ratnagiri Shelf, may have been a subsidiary source. The western clastic basin fringes the Bombay-Ratnagiri Shelf and may have provided some small oil and gas accumulations along the paleoshelf edge from local source areas. We postulate that the three main migration trends from the Dahanu depression are, (1) southwest toward the Bombay High and the H1-5 structure, (2) southward toward the Bassein-B-38 trend, and (3) northward toward South Tapti and North Tapti. Oil in the Anklesvar field of the onshore Cambay basin and in the Bombay High, Bassein, and B-38 structures contains a high wax content, suggesting that the prodelta muds of the Cambay Gulf Shelf and the Dahanu depression and its southern extension on the eastern part of the Bombay-Ratnagiri Shelf, may have provided for these genetically related oils a common system of source beds rich in terrestrial organic matter. Paleostructural analysis indicates that the early NNW-SSE-trending anticlinal structures, faulted down to the coast on their eastern flanks, adjoin the shale areas and were the main locales of oil and gas accumulation.

SEISMIC REFLECTION AND MAGNETIC STUDY OFF BOMBAY, INDIA

As part of the 1967 Global Expedition of the USC&amp;GS Ship OCEANOGRAPHER, approximately 1700 km (920 nm) of reconnaissance seismic reflection and total magnetic field intensity traverses were made between 17° and 21° N latitude, over the shelf and slope off Bombay, India. This investigation was designed to determine why the shelf area off Bombay is as much as three times wider than the rest of the western Indian shelf. Near the landward edge of the continental shelf, the seismic reflection profiles show strata dipping west from Bombay. To the west, these profiles indicate that the configuration of the continental slope is influenced by anticlinal structures, suggesting that the western Indian shelf in the study area is a sediment‐filled structural basin. The greater shelf width off Bombay is due to the location of the anticlinal trends. Within the basin, a buried north‐south trending anticlinal high is present approximately 75 km (40 nm) west of Bombay in water depths of about 50 m (165 ft). The structural basin and subsurface anticlinal trend off Bombay may continue northward into the Cambay Basin, where similar features are present. This continuation may be locally interrupted by the westward extension of a fault zone from the Narbada Valley into the southern Cambay Basin. The magnetic field is relatively flat, except for a zone of high frequency anomalies, which may be caused by dikes extending from the Bombay coast westward for approximately 40 km (20 nm).