In this paper, we conduct a comparative study on the mineralogy and geochemistry of metalliferous sediment collected near the active hydrothermal site (Wocan-1) and inactive hydrothermal site (Wocan-2) from Wocan Hydrothermal Field, on the Carlsberg Ridge (CR), northwest Indian Ocean. We aim to understand the spatial variations in the primary and post-depositional conditions and the intensity of hydrothermal circulations in the Wocan hydrothermal systems. Sediment samples were collected from six stations which includes TVG-07, TVG-08 (Wocan-1), TVG-05, TVG-10 (Wocan-2), TVG-12 and TVG-13 (ridge flanks). The mineralogical investigations show that sediment samples from Wocan-1 and Wocan-2 are composed of chalcopyrite, pyrite, sphalerite, barite, gypsum, amorphous silica, altered volcanic glass, Fe-oxides, and hydroxides. The ridge flank sediments are dominated by biogenic calcite and foraminifera assemblages. The bulk sediment samples of Wocan-1 have an elevated Fe/Mn ratio (up to ~1545), with lower U contents (<7.4 ppm) and U/Fe ratio (<~1.8 × 10−5). The sulfide separates (chalcopyrite, pyrite, and sphalerite) are enriched in Se, Co, As, Sb, and Pb. The calculated sphalerite precipitation temperature (Sph.PT) yields ~278 °C. The sulfur isotope (δ34S) analysis returned a light value of 3.0–3.6‰. The bulk sediment samples of Wocan-2 have a lower Fe/Mn ratio (<~523), with high U contents (up to 19.6 ppm) and U/Fe ratio (up to ~6.2 × 10−5). The sulfide separates are enriched in Zn, Cu, Tl, and Sn. The calculated Sph.PT is ~233 °C. The δ34S returned significant values of 4.1–4.3‰ and 6.4–8.7‰ in stations TVG-10 and TVG-05, respectively. The geochemical signatures (e.g., Fe/Mn and U/Fe ratio, mineral chemistry of sulfides separates, and S-isotopes and Sph.PT) suggest that sediment samples from Wocan-1 are located near intermediate–high temperature hydrothermal discharge environments. Additionally, relatively low δ34S values exhibit a lower proportion (less than 20%) of seawater-derived components. The geochemical signatures suggest that sediment samples from Wocan-2 has undergone moderate–extensive oxidation and secondary alterations by seawater in a low–intermediate temperature hydrothermal environments. Additionally, the significant δ34S values of station TVG-05 exhibit a higher estimated proportion (up to 41%) of seawater-derived components. Our results showed pervasive hydrothermal contributions into station TVG-08 relative to TVG-07, it further showed the increased process of seafloor weathering at TVG-05 relative to TVG-10.
The heavy metals and physicochemical properties of Lagos coastal waters and sediments, Southwestern Nigeria were investigated in 5 stations (station 1-5), to determine their levels, distributions and pollution index. The observed ranges of physicochemical characteristics from the study area are: dissolved oxygen (DO): 2.15-11.3 mg/L; pH: 6-7.7; temperature (Temp): 28.05-29.25°C; biochemical oxygen demand (BOD): 1.6-110.1 mg/L; electrical conductivity (EC): 1.8-29.8 mS/cm; salinity (SAL): 0.02-2.05ppt; total dissolved solids (TDS): 211.8-1210.2 mg/L and total suspended solids (TSS): 12.13-117.95 mg/L. The analyzed physicochemical properties from all the stations are within the desired values for healthy marine ecosystems when compared with Federal ministry of environment (FMENV) Nigeria and World Health organization permissible limits (DO:>5 mg/L; pH:6.5-9; TEMP:<40°C; BOD: 50 mg/L; EC:900 mS/cm; TDS:1000mg/L and TSS:600 mg/L), with the exception of low DO at station 1 and station 5; and high BOD at; station 2 and station 3 respectively. The concentrations of heavy metals present in the water samples followed the sequence: Fe > Pb > Cu > Cr > Zn > Cd, and showed high values for Pb, Cd and Cr, relative to the standard permissible limits according to FMENV (Pb 0.001; Zn 50 mg/L; Cd < 0.0018; Cu 2-4 mg/L and Cr 0.02-2.0 mg/L). The calculated contamination factor (CF) in the sediment samples showed low-moderate contamination for the analyzed heavy metals (Cr, Zn, Cu and Pb), except for cadmium (Cd) that showed extremely severe contamination (CF > 6) values across the sampling stations. The calculated Nemerow pollution index (NPI) and ecological risk index (RI) showed heavily polluted (NPI > 10) values and very high ecological risk (RI > 380) values at Apapa (location 1), Iddo (location 2) and Makoko (location 3). The potential ecological risk factor (Eir) confirmed Cd as the highest polluted heavy metals (Eir >320), with the greatest potential threat to the marine ecosystems in the Lagos coastal waters. The study highlights the impacts of anthropogenic pollutions on the coastal ecosystems and human health.
The pollution status of Lagos Lagoon was investigated to determine some physical and chemical parameters, in order to further understand its water quality. Five locations were selected according to the anthropogenic activities taking place in the areas. The measured parameters are water temperature, salinity, conductivity, pH, dissolved oxygen (DO) and alkalinity. The mean values ranged from 28.50-30.15°C for temperature, 7.86-8.50 for pH, ND-10.60‰ for salinity, 0.18-15.20µS/cm for electrical conductivity, 12.00-14.00 mg/l for alkalinity and 1.60-6.40 mg/l for dissolved oxygen. The statistical analysis of the measured physicochemical parameters revealed non-significant difference (p>0.05) across all stations in the physico-chemical parameters. Generally, the values obtained suggest the influence of anthropogenic activities especially the depletion of DO in most of the stations. Therefore, point source and non-point source pollution into the Lagos Lagoon should be controlled for the health and safety of the aquatic ecosystems. Generally, all the measured parameters were either below or within the permissible limits obtainable in the marine environment.
We have studied morphology, mineralogy and geochemical characteristics of Fe-oxyhydroxide deposits from metal-enriched sediments of the active (Wocan-1) and inactive (Wocan-2) hydrothermal sites (Carlsberg Ridge, Northwest Indian Ocean). Fe-oxyhydroxide deposits on the Wocan-1 site are reddish-brownish, amorphous and subangular. They occur in association with sulfides (e.g., pyrite, chalcopyrite and sphalerite) and sulfate minerals (e.g., gypsum and barite). The geochemical composition shows enrichment in transition metals (Ʃ (Cu + Co + Zn + Ni) = ~1.19 wt. %) and low (<0.4 wt. %) values of Al/(Al + Fe + Mn) ratio. The Wocan-2 samples show poorly crystallized reddish brown and yellowish Fe-oxyhydroxide, with minor peaks of goethite and manganese oxide minerals. The mineral assemblage includes sulfide and sulfate phases. The geochemical compositions show two distinct types (type-1 and type-2). The type-1 Fe-oxyhydroxides are enriched in transition metals (up to ~1.23 wt. %), with low values of Fe/Ti vs. Al/(Al + Fe + Mn) ratio similar to the Wocan-1 Fe-oxyhydroxides. The type-2 Fe-oxyhydroxides are depleted in transition metals, with Al/(Al + Fe + Mn) ratio of 0.003–0.58 (mean value, 0.04). The ridge flank oxyhydroxides exhibit an extremely low (mean value ~ 0.01) Fe/Mn ratio and a depleted concentration of transition metals. Our results revealed that the Wocan-1 Fe-oxyhydroxides and type-1 Fe-oxyhydroxides of the Wocan-2 site are in the range of Fe-oxyhydroxides deposits that are precipitated by mass wasting and corrosion of pre-existing sulfides. The type-2 Fe-oxyhydroxides are precipitated from sulfide alteration by seawater in an oxygenated environment relative to type-1. The association of biogenic detritus with the oxyhydroxides of the ridge flanks and the low Fe/Mn ratio suggests hydrogenous/biogenic processes of formation and masked hydrothermal signatures with distance away from the Wocan hydrothermal field.
The current study deals with the evaluation of physicochemical characteristics, nutrient status, and variation in the height of surface water from the commodore channel, and its comparison with the hydrochemical characteristics of the monitoring borehole from the NIOMR Jetty, Lagos, Nigeria. We aim to further improve our understanding of the interrelationship of climate change and hydrological cycle, and its impacts in the Lagos coastal environments. The observed ranges of physicochemical characteristics of the surface water are dissolved oxygen (DO) 3.07-6.05mg/L; pH: 7.99-8.48; temperature 26.97-30.34°C, electrical conductivity (EC) 9680-47800mS/cm; salinity (Sal) 6.51-28.05‰ and water level 1.24-1.79m. The concentration of nutrient range from nitrite 10-36mg/l, nitrate 28-44mg/l, sulphate 38-90mg/l and phosphate 2-78mg/l. The analyzed hydrochemical characteristics from all the stations are within the desired values for healthy marine ecosystems when compared with Federal Ministry of Environment (FMENV) Nigeria permissible limits which are: DO >5 mg/l, pH 6.5-9, temperature <40°C, nitrite 1mg/l, nitrate 20mg/l, phosphate 5mg/l and sulphate 500mg/l; except low DO (<4.5mg/l) in January, November and December, and elevated nitrite, nitrate and phosphate values across the season. The low DO values may be attributed to enhanced transportation of non-biodegradable organic loads that utilize the available DO for chemical oxidation and microbial decomposition. While the enriched nitrite, nitrate and phosphate concentrations suggest enhanced productivity and influx of nutrients rich flood water from the anthropogenic source. Our study showed a coincidence in the highest brackish water height of the surface water (1.24m) from the commodore channel with the highest groundwater level (1.58m) in October. The lowest height of water levels was recorded in February and March at 2.23m and 1.79m for groundwater and surface water respectively. The similarities in the height of water levels were recorded with contrasting hydrochemical variables (e.g., high Ec vs. low Ec; and high Sal vs. low Sal), which confirmed the effect of rainfall/freshwater incursion on the surface water and simultaneous occurrence of subsurface pollution arising from seawater intrusion on the groundwater samples in October. This study had shown that a long term monitoring of rise in water levels and hydrochemical variables of surface and groundwater can be used to understand climate change and early warning predictions of flooding in coastal environments.
The nutrients (nitrate, nitrite, phosphate and silicate) of thirty-two stations in Ilaje, coastal waters and estuaries of Ondo state, Nigeria were studied in the month of August (during the wet season). The stations consist of two estuaries; Awoye (A1-A6), Abereke (AB1-AB7), anthropogenic impacted stations (I1-I12) and control stations (C1-C7). The aim of this study is to evaluate the eutrophication status of Ilaje coastal waters and estuaries in response to the stress caused by anthropogenic inputs from land use changes, agriculture, and industrial effluents. Our study reveal highest nitrate (3.20-6.98 mg/l) and phosphate (11.59-17.48mg/L) concentrations at the Abereke Estuaries. The significant correlation of nitrate with chlorophyll-a (p<0.01, r= +0.886) and the low N/P (<0.25) ratio suggest that nitrate is the primary limiting nutrient for phytoplankton production, and accounted for the high chlorophyll-a concentrations in the study area. The strong correlation of nitrite and silicate from the statistical bi-plots at the impacted stations, and the accompanied lowest dissolved oxygen (<5 mg/L) and acidic pH (<6) values suggest nitrite and silicate are sourced from anthropogenic effluents. The mean nitrite concentrations of 0.13mg/L at the impacted stations above the maximum permissible limit of nitrite (0.06 mg/L) for Tilapia production suggest an unfavorable environment for the Tilapia growth at the impacted stations. The chlorophyll-a concentrations scale further showed that the study area is characterized as bad eutrophication status (> 2.21 µg/L) an indication of the presence of heterogeneous phytoplankton community during the wet season. The study highlights the use of nutrients and chlorophyll-a scaling to determine the eutrophication status of coastal area.
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