Organic carbon in sediments of the Southwestern margin of India: influence of productivity and monsoon variability during the late Quaternary

The texture, organic carbon (OC), CaC03 and Rock-Eval parameters of the sediments from two gravity cores collected at depths below the oxygen minimum zone (OMZ) of the southwestern margin of India are presented and compared the results with those within the OMZ. Clayey silt/ silty clays are the characteristic sediments. The OC in the core top sediments between Cape Comorin and Mangalore is higher below the OMZ than those from the OMZ. However, it is higher within the OMZ than those below the OMZ in the sediments between Mangalore and Goa. The down-core variations of OC are identical in these cores. In both the cores, relatively high OC content and low sedimentation rates correspond to the intervals of late Holocene and Last Glacial Maximum (LGM) and, low OC and high sedimentation rates to the early Holocene sediments. The CaC03 follows sand content in a core off Cape Comorin, with low values at the core top, increase marginally in the early Holocene and LGM and then decrease in the late Pleistocene sediments. The CaC03 values in a core off Mangalore are higher in the intervals of the late Holocene and early deglaciation than m early Holocene and LGM intervals. Rock-Eval parameters distinguish the sources of organic matter only at high OC concentrations. The high OC during the LGM may be related to the productivity, associated with convective mixing occurring during the NE monsoon. The low OC/CaC03 and high clay content during the early Holocene may be the consequences of the intensified SW monsoon that results in stronger near-surface stratification leading to low productivity High OC and low CaC03 during the late Holocene suggest increased productivity and early diagenesis in the near surface sediments. We suggest that the variations in productivity and downslope transport of sediment controlled the OC enrichment

Paleoclimatic and diagenetic history of the Late Quaternary sediments in a core from the Southeastern Arabian Sea: geochemical and magnetic signals

Geochemical and rock-magnetic investigations were carried out on a sediment core collected from the SE Arabian Sea at 1420 m depth in oxygenated waters below the present-day oxygen minimum zone. The top 250 cm of the core sediments represent the last 35 kaBP. The δ18O values of Globigerinoides ruber are heaviest during the Last Glacial Maximum (LGM) and appear unaffected by low-saline waters transported from the Bay of Bengal by the strong northeast monsoon and West Indian coastal current. The signatures of Bølling-Allerød and Younger Dryas events are distinct in the records of magnetic susceptibility, organic carbon (OC) and δ 18O. Glacial sediments show higher OC, CaCO3, Ba, Mo, U and Cd, while the early-to-late Holocene sediments show increasing concentrations of OC, CaCO3, Ba, Cu, Ni and Zn and decreasing concentrations of Mo, U and Cd. Productivity induced low-oxygenated bottom waters and reducing sedimentary conditions during glaciation, and productivity and oxygenated bottom waters in the Holocene are responsible for their variation. The core exhibits different stages of diagenesis at different sediment intervals. The occurrence of fine-grained, low-coercivity, ferrimagnetic mineral during glacial periods is indicative of its formation in organic-rich, anoxic sediments, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea and Japan Sea. The glacial sediments exhibiting reductive diagenesis with anoxic sedimentary environment in this core correspond to reductive diagenesis and intermittent bioturbation (oxygenation) reported in another core in the vicinity. This suggests that the poorly oxygenated bottom water conditions during glacial times should not be generalized, but are influenced locally by productivity, sedimentation rates and sediment reworking

The nature and distribution of particulate matter in the Mandovi estuary, Central West coast of India

Systematic seasonal variations of suspended particulate matter (SPM) along a 44-km transect of the Mandovi estuary reveal that the concentrations of SPM are low at river-end stations, increase generally seaward, and are highest at sea-end stations of the estuary. An estuarine turbidity maximum (ETM) occurs at sea-end stations during June-September when river discharge is high and also in February-May when river discharge is low. These are the two windiest times of year, the former associated with the southwest monsoon and the latter characterized by a persistent sea breeze. The salinity vs. SPM plot shows that high SPM is a seaward deposit and skewed landward. Suspended matter comprised of floccules, fecal pellets, and aggregates that consist of clay and biogenic particles occur everywhere in the estuary. Diatoms are the most common and are of marine type at the sea-end and freshwater-dominated at river-end stations of the estuary. SPM is characterized by kaolinite- and smectite-rich clay mineral suites at the river- and sea-end stations, respectively. Smectite concentrations increase seawards with the increase in SPM content and are not influenced by salinity. Wind-driven waves and currents and biogeochemical processes at the mouth of estuary likely play an important role in the formation of ETM in resuspension and transformation of SPM into floccules and aggregates and in their upkeep or removal

Rock magnetic and geochemical record in a sediment core from the Eastern Arabian Sea: diagenetic and environmental implications during the late Quaternary

Rock magnetic concentration, grain size and mineralogy parameters together with organic carbon, calcium carbonate, redox-sensitive elements, δ18O of Globigerinoides ruber and radiocarbon dating were carried out on a 445 cm long sediment core collected at 1380 m depth off Mangalore, southwestern margin of India. The top 290 cm sediments of the core correspond to the last 18 kaBP. The δ18O and magnetic records exhibit major events at ∼ 16 kaBP, 14.5 kaBP, 11.5 kaBP and 9.8/8.6 kaBP related to start and intensity of the summer monsoon and climate change, and are synchronous with that of the western Arabian Sea and North Atlantic. The sediments with high magnetic susceptibility correlate with high sedimentation rates. The sediments are dominated by fine-grained magnetite, but intervals of 1.2-3.8 kaBP and 10-13.5 kaBP were subjected to diagenetic changes, resulting in the dissolution of fine-grained magnetites and enrichment of redox-sensitive trace elements (Cu, Ni, Zn, V, Mo and U). The sediments between 290 cm and 445 cm correspond to 18-27 kaBP and are characterized by distinct decrease in magnetic concentration, grain size and mineralogy parameters, high organic carbon, low concentrations of redox-sensitive trace elements and abundant pyritized tubules. The reductive diagenetic conditions indicated by rock magnetic properties are in contrast with the weak sub-oxic conditions revealed by low concentrations of trace elements in the sediments. The seasonal organic matter flux produced during the winter monsoon and moderate sedimentation rates favoured reductive diagenesis in the sediments at and below the last glacial maximum (LGM). Intermittent bioturbation, however, allowed oxidants to penetrate into the sediments, remobilized redox-sensitive trace elements into the water column and modified the primary geochemical signal of the sedimentary environment

Wind-driven estuarine turbidity maxima in Mandovi Estuary, central West coast of India

Systematic studies on the suspended particulate matter (SPM) measured on a seasonal cycle in the Mandovi Estuary, Goa indicate that the average concentrations of SPM at the regular station are ~20mg/l, 5mg/l, 19mg/l and 5mg/l for June–September, October–January, February–April and May, respectively. SPM exhibits low-to-moderate correlation with rainfall indicating that SPM is also influenced by other processes. Transect stations reveal that the SPM at sea-end stations of the estuary are at least two orders of magnitude greater than those at the river-end during the monsoon. Estuarine turbidity maximum (ETM) of nearly similar magnitude occurs at the same location in two periods, interrupted by a period with very low SPM concentrations. The ETM occurring in June–September is associated with low salinities; its formation is attributed to the interactions between strong southwesterly winds (5.1–5.6ms-1) and wind-induced waves and tidal currents and, dominant easterly river flow at the mouth of the estuary. The ETM occurring in February–April is associated with high salinity and is conspicuous. The strong NW and SW winds (3.2–3.7ms-1) and wind-driven waves and currents seem to have acted effectively at the mouth of the estuary in developing turbidity maximum. The impact of sea breeze appears nearly same as that of trade winds and cannot be underestimated in sediment resuspension and deposition

Origin of Cretaceous phosphorites from the onshore of Tamil Nadu, India

Cretaceous phosphorites from the onshore of Tamil Nadu have been investigated for their origin and compared with those in the offshore. Cretaceous phosphorites occur as light brown to yellowish brown or white nodules in Karai Shale of the Uttatur Group in the onshore Cauvery basin. Nodules exhibit phosphatic nucleus encrusted by a chalky shell of carbonate. The nucleus of the nodules consists of light and dark coloured laminae, phosphate peloids/coated grains and detrital particles interspersed between the laminae. Scanning electron microscope (SEM) studies reveal trapping and binding activity of microbial filaments. A mat structure with linearly arranged microbial filaments and hollow, cell-based coccoid cyanobacterial mat are present. Nodules contain abundant carbonate fluorapatite, followed by minor calcite, quartz and feldspar. The P2O5 content of the phosphorites ranges from 18 to 26%. The CaO/P2O5, Sr and F contents are higher than that of pure carbonate fluorapatite. Concentrations of Si, Al, K, Fe, and Ti are low. We suggest that the nuclei of the nodules represent phosphate clasts related to phosphate stromatolites formed at intertidal conditions. At high energy levels the microbial mats were disintegrated into phosphate clasts, coated with carbonate and then reworked into Karai Shale. On the other hand, Quaternary phosphorites occur as irregular to rounded, grey coloured phosphate clasts at water depths between 180 and 320m on the continental shelf of Tamil Nadu. They exhibit grain-supported texture. Despite Quaternary in age, they also resemble phosphate stromatolites of intertidal origin and reworked as phosphate clasts onto the shelf margin depressions. Benthic microbial mats probably supplied high phosphorus to the sediments. Availability of excess phosphorus seems to be a pre-requisite for the formation of phosphate stromatolites

Fluctuations in productivity and denitrification in the Southeastern Arabian Sea during the Late Quaternary

Sedimentological and stable isotopic characteristics of sediments have been studied in a core from the southeastern Arabian Sea containing records of the past 70 ka. Palaeoproductivity proxies such as organic carbon (Corg), total nitrogen (TN) and calcium carbonate (CaCO3) contents, show high values at the core top and during the Last Glacial Maximum (LGM) and marine isotope stage (MIS) 4, suggesting high productivity, whereas low Corg and CaCO3 contents are associated with the MIS ½ and mid-MIS 3, indicating reduced productivity. The δ18O values in planktonic foraminifera range between - 2.7% and - 0.1%, with a large glacial-interglacial amplitude Δδ18O of ∼2.6%, suggesting changes related to monsoonal precipitation/ runoff. The δ15N values fluctuate between 5.4% and 7.3%, signifying variation in denitrification intensity. The δ15N indicates an overall increase in denitrification intensity during MIS 1 and MIS 3 and, reduced intensity during MIS ½, LGM and mid-MIS 3. Higher primary productivity and reduced denitrification intensity during LGM and MIS 4 might be due to convective winter mixing and more oxygenated subsurface waters. Reduced primary productivity during MIS ½ and mid-MIS 3 might be the effect of enhanced precipitation associated with the intensified southwest monsoon fortifying near-surface stratification

Suspended sediment dynamics on a seasonal scale in the Mandovi and Zuari estuaries, Central West coast of India

Suspended particulate matter (SPM) collected at regular stations from the Mandovi and Zuari estuaries indicates that the peaks of high SPM coincide with peaks of high rainfall and low salinity and also with peaks of moderate/low rainfall coupled with high salinity during the monsoon. The estuarine turbidity maximum (ETM) is a characteristic feature, it occurs in the channel accompanying spring tide during the monsoon and pre-monsoon, and shifts to the bay on neap tide during post-monsoon. ETM remains at the same position in the Mandovi River, both during the monsoon and pre-monsoon, whereas in Zuari it stretched upstream during monsoon and migrates seaward of the channel during pre-monsoon. The ETM coincides with the freshwater-seawater interface during the monsoon and is formed by the interaction between tidal currents and river flows. The ETM during pre-monsoon is associated with high salinities and is generated by tidal and wind-induced currents. The turbidity maximum on neap tide during post-monsoon may be due to the erosion and resuspension of sediments from the emergent tidal flats and transport of these turbid waters into the bay. Funneling effect of the narrowing bay in the Zuari estuary and associated physical processes effectively enhance the magnitude of the currents and transports sediments to the channel. SPM retention percentage indicates that the estuarine channel is prone to siltation

Miocene phosphorites from the Murray Ridge, Northwestern Arabian Sea

Phosphorites from the Murray Ridge, NW Arabian Sea comprise nodules, bioclasts, and bone fragments. The nodules are made up of a homogeneous, light-colored phosphate nucleus consisting of Rivulariacean filamentous cyanobacteria and a thin dark-grey colored phosphate cortex showing abundant microbial filaments and microborings. The bioclasts comprise of ∼14-14.5 Ma old planktonic foraminifers, accepted as the time of deposition. Spherical to ovoid-shaped apatite microparticles resembling fossil bacteria are distinct components in the bioclasts. Bone fragments exhibit apatite fillings. The nodules and bone fragments consist entirely of carbonate fluorapatite (CFA) with low Al, K, and Th concentrations suggesting absence of continental detritus. Shale-normalized REE patterns of the samples support a seawater-derived composition. The highly uniform initial εNd values of -4.8 to -5.1 are interpreted as the seawater value at the onset of phosphatization ∼14 Ma ago. In contrast, 87Sr/86Sr ratios show a large range of 0.709055 to 0.709124 corresponding to unusually young stratigraphic ages of ∼1 to 3 Ma. The data are interpreted as evidence for post-depositional Sr exchange of the recrystallizing phosphorites with fluids isotopically not much different from modern seawater. It is concluded that the phosphorites formed under oxic, shallow-water conditions where microbial populations assimilated phosphorus primarily from seawater and mediated precipitation of CFA during early diagenesis at the sediment-water interface on different substrates

Variations in Denitrification and Ventilation Within the Arabian Sea Oxygen Minimum Zone During the Holocene

The continental slope of India is exposed to an intense perennial oxygen minimum zone (OMZ) supporting pelagic denitrification. Sediments that are presently in contact with the lower boundary of the denitrification zone indicate marked changes in the intermediate and bottom waters ventilation of OMZ during the past 9,500 years. The δ15N of sediment suggests that the OMZ waters were less ventilated during the early Holocene (between 9.5 and 8.5 ka BP) resulting in intensified denitrifying conditions with an average δ15N value of 7.8‰, while at the same time stable Mo isotope composition (average δ98Mo of -0.02‰) indicates that the bottom waters that were in contact with the sediments were better oxygenated. By the mid-Holocene OMZ became more oxygenated suppressing denitrification (average δ15N of 6.2‰), while bottom waters gradually became less oxygenated (average δ98Mo of 1.7‰). The mid-Holocene reduction in denitrification coincided with a global decrease in atmospheric N2O as inferred from ice core records, which is consistent with a decreased contribution from the Arabian Sea. Since ~5.5 ka BP OMZ waters have again been undergoing progressive deoxygenation accompanied by increasing denitrification