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Studies on the utilization of molecular and isotopic proxies for the characterization of organic matter (OM) sources and environmental conditions in lakes have been well-documented globally. Nevertheless, inland lacustrine salt basins remain less well-studied, particularly in tropical sub-Saharan Africa. In this study, we quantified OM sources and evaluated the effects of salt deposit and barite mining on the distribution of n -alkanes in saline and freshwater Gabu lakes sediment cores, southeast Nigeria using elemental, bulk carbon isotope ratios and biomarker distributions. Our results indicate a system inundated with comparable proportions of vascular plant leaf waxes (mean = 56.6%) and submerged/floating macrophytes (mean = 41.1%) with minor contribution from algae/photosynthetic bacteria (mean = 2.3%). The scenario indicates a shallow water system that predominantly preserved long chain n-alkanes derived from vascular plant leaf waxes and macrophytes. The capacity of macrophyte to biosynthesize long chain n-alkanes most likely reflects adaptation of these organisms to partial exposure to the atmosphere. The occurrence in moderate abundance of C17 and C19n-alkanes and the near absence of other low molecular weight (LMW) n-alkanes in the saline lake was linked to the effect of salt stress. The absence of LMW n-alkanes in freshwater lake may be associated with slow barite hydrolysis likely to cause acidification. Our results have demonstrated that long chain n -alkanes of terrestrial and submerged/floating macrophyte origins are better preserved under conditions of low salinity and acidification in inland shallow lakes than those derived from algae/bacteria.

The distribution of aliphatic and aromatic hydrocarbons in surface sediments of the lower course of the Imo River (Nigeria) was investigated to determine the sources and fate of these compounds. The aliphatic fraction is characterized by a widespread contribution of highly weathered/biodegraded hydrocarbon residues (reflected in the absence of prominent n-alkane peaks coupled with the presence of 17α(H),21β(H)-25-norhopane, an indicator of heavy hydrocarbon biodegradation) of Nigerian crude oils (confirmed by the occurrence of 18α(H)-oleanane, a compound characteristic of oils of deltaic origin). The concentrations of polycyclic aromatic hydrocarbons (PAHs) ranging from 48 to 117 ng/g dry weight (dw; ∑13PAHs) indicate a moderate pollution, possibly lowered by the sandy lithology and low organic carbon (OC) content of the sediments. Concentrations slightly decrease towards the estuary of the river, probably due to the fact that these stations are affected by tidal flushing of pollutants adsorbed on sediment particles and carried away by occasional storm to the Atlantic Ocean. A number of PAH ratios, including parent/alkylated and isomeric compounds, indicates a predominance of petrogenic sources, with a low contribution of pyrolytic inputs, particularly of fossil fuel combustion. On the basis of OC/ON (>10) and Per/ΣPAHₚₑₙₜₐ₋ (>10) values, a diagenetic terrigenous OC was proposed as a source of perylene to the river.

Shrimp species ( Macrobrachium felicinum ) collected from estuarine mangrove area of the Imo River is an important route of exposure to polycyclic aromatic hydrocarbons (PAHs). The estuarine associated sediment (EAS) composited sample showed higher TPAH, ΣAlkyl, ΣPAH carc and ΣPAH EPA concentrations (550.84, 172.36, 413.17 and 482.11 ng/g dry weight—dw) than their mean concentrations in shrimp samples (509.39 ± 354.21, 31.38 ± 18.49, 52.10 ± 1.35 and 460.06 ± 330.76 ng/g wet weight—ww), respectively. Among the individual PAHs congeners, phenanthrene was the dominant species detected in the EAS accounting for 21.02 % of the total PAH load and the decreasing order of 3- > 2- > 5- > 4- > 6-ring contamination was found. A different pattern predominated by naphthalene was observed for the shrimp species, suggesting that the organisms have different selectivity for a range of PAHs congeners. These variations may be attributed to different degree of bioavailability of these compounds, characteristic sandy lithology of the EAS and the protective capacity of soot particles associated with liquid fossil fuel combustion masking the uptake of high molecular weight PAHs by the organisms. Cancer risk associated with consumption of shrimps in the region was assessed using estimated daily intake (EDI) and compared with standards. The EDI values for naphthalene, benzo(a)pyrene and ∑PAH carc were lower than the USEPA benchmarks and EFSA levels of concern values for adult and children population, suggesting low probability of developing cancer.

Polycyclic aromatic hydrocarbons (PAHs) released from diverse sources passing through water column carry information into the sediment where they can be used to assess the environmental status of an ecosystem over specified geologic time frame. The vertical distributions of PAHs in two recent sediment cores (RS and RC, 30 cm long) from Refome Lake, South–South Nigeria, were investigated using gas chromatography–mass spectrometry in order to evaluate their sources and historical trends of deposition over the last ca. century. The total PAHs (TPAHs—sum of parent and retene) concentrations ranged from 66.99 ng/g dry weight (dw) at the middle layer of RC core (RC3 10–15 cm) to 182.24 ng/g dw at the near-bottom layer of RS core (RS5 20–25 cm) with a mean of 102.21 ± 24.32 ng/g. The elevated TPAH level at the near-bottom layer of the RS core, corresponding to geologic time-frame ca. 1930–1947, coincided with the period of inhabitation of the European settlers along the lake’s catchments when utilization of coal and/or coal products for domestic/recreational activity was at its peak. A decline in TPAH levels up-cores thereafter reflected the periods of gradual evacuation of inhabitants of the lake area hinterland following the departure of the White after the Nigerian independence in 1960. Evaluation of PAH category according to ring size coupled with data from specific molecular ratios revealed inputs dominated by wood/coal combustion with a moderate contribution from petrochemical/liquid fossil fuel exhaust emissions and a minor diagenetic sources. Principal component analysis result not only distinctively separated RS from RC core samples but also revealed that the RS samples were more impacted by wood/coal combustion emissions than the RC, while liquid fossil fuel exhaust emission dominated the RC over the RS samples. Although short-range eolian transport did play a role in the delivery of PAHs to the lake, localization of source contamination was more important.

Polycyclic aromatic hydrocarbon (PAH) analyses of surface sediments from the Cross River estuary by gas chromatography–mass spectrometry indicated natural diagenetically derived PAHs in the upper estuary, with minor and variable amounts of petrogenic and combustion-derived PAHs from human activities (lower estuary). The occurrence of significant amounts of perylene (average 23% of all PAHs) with the diagenetic natural PAHs in the middle estuary bordered by mangrove forests supports its origin from terrestrial organic matter. The natural PAHs represent the highest percentage (average 76%) of the total PAHs in this tropical environment. The traditional geochemical parameters, including the petrogenic PAHs, confirmed trace petroleum contamination in the estuary. Specific PAH ratios such as Fl/Py and Fl/(Fl+Py) also support this source contribution.

Pentacyclic triterpenol methyl ethers (PTMEs), germanicol methyl ether (miliacin), 3-methoxyfern-9(11)-ene (arundoin), β-amyrin methyl ether (iso-sawamilletin), and 3-methoxytaraxer-14-ene (sawamilletin or crusgallin) were characterized in surface sediments of the Cross-River system using gas chromatography-mass spectrometry (GC-MS). Triterpenol esters (mainly α- and β-amyrinyl acetates and hexanoates, and lupeyl acetate and hexanoate) were also found. These distinct compounds are useful for assessing diagenesis that can occur during river transport of organic detritus. Poaceae, mainly Gramineae and Elaeis guineensis higher plant species, are proposed as primary sources for the PTMEs and esters in the sediments. PTMEs are biomarkers of specific higher plant subspecies, while the triterpenol esters are indicators of early diagenetic alteration of higher plant detritus.