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 indicate a shallow water system that predominantly preserved long chain n -alkanes derived from vascular plant leaf waxes and macrophytes. The capacity of macrophyte to biosythesize long chain n-alkanes most likely reflects adaptation of these organisms to partial exposure to the atmosphere. The occurrence in moderate abundance of C 17 and C 19 n -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 was associated with barite hydrolysis and acidification accompanying mining activity. Our results have demonstrated that long chain n -alkanes of terrestrial and submerged/floating macrophyte origins are better preserved under conditions of salinity and acidification in inland shallow lakes than those derived from algae/bacteria.
Research Article
Stable carbon isotope and n-alkane distributions in sediment cores from saline and freshwater Gabu lakes, Southeast Nigeria: Environmental implications
https://doi.org/10.21203/rs.3.rs-1950140/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 24 Aug, 2023
Read the published version in Environmental Earth Sciences →
You are reading this latest preprint version
Acidification
Barite mining
Fate
Lacustrine environment. Source
Inland alkaline salt basins are ecospheres with well-defined physical demarcations that enable the study of unique microbiological and chemical interactions. They are utmost domains that offer the opportunity to investigate distinctive aspects of biogeochemistry. For instance, these systems may represent modern analogs for states under which life might have evolved on Earth and potentially elsewhere (Zettler et al. 2002; Joeckel and Ang Clement, 2005).
Stable isotope investigations of organic preservation in non-saline lacustrine systems are well established (Brocks et al., 2003; Fang et al., 2014). Primary production and associated biosynthetic processes fractionate isotopes in expected ways which can be used to distinguish sources of OM to lacustrine systems (O`Donnell et al., 2003). Bulk organic δ13C and total organic carbon/total nitrogen (TOC/TN) have been extensively utilized to determine the source and fate of OM in various aquatic systems (Harmelin–Vivien et al. 2008). Elemental C/N and δ13C values of total OM can retain paleoenvironmental information across million-year time scales (Meyers, 1994). Biomarker analyses can be used in conjunction with bulk identifiers of OM as they convey more useful and source-specific information about the sources and fate of OM in freshwater systems (Meyers 2003).
Due to n-alkanes high preservation potential, most terrestrial biomarker studies have primarily focused on leaf wax lipids (e.g., Cranwell 1981; Diefendorf et al. 2009). These compounds have high resistance to microbial degradation in relation to other types of OM, making n-alkanes generally robust recorders of the origins of OM in lake sediments. Relative abundances of n-alkane biomarkers found in sediments have been used in reconstructing paleoenvironmental records of lake systems (Meyers 1995). Mid-chain n-alkanes (n-C21-n-C25) are sourced from submerged macrophytes (Ficken et al. 2000), while long-chain n-alkanes (n-C27-n-C33) with an odd over -even carbon number predominance are of terrestrial higher-plant origin (Oyo-Ita et al. 2017).
The paucity of data about elemental, isotopic and biomarker compositions of any kind of organisms that are prospective contributors of OM to any subaquatic sediments has been a source of exasperation to organic geochemists for decades. While an increasing number of studies of lake sediments have been undertaken worldwide, no study has reported the effects of salinity and barite mining on the distribution of n-alkanes in salt and freshwater lakes, respectively. Here, we examine the effects of natural (salt deposit) and land use (barite mining) changes on the distribution profiles of n-alkanes in the two lakes, respectively.
Our main objectives were to: (i) use bulk sediments and biomarkers to quantify the sources of OM, (ii) evaluate the effects of barite mining on n-alkane dispersion in the freshwater lake and, (iii) assess the influence of salinity on the distribution of n-alkanes in the saline lake.
1.1 Study area
Two small remote oligotrophic Gabu Lakes were studied (Fig. 1), (1) a closed basin saline lake located 69 m elevation above sea level (EASL), and (2) an adjoining semi-enclosed freshwater lake located 44 m EASL. The lakes are in a sub-humid climate, located in the Gabu community within Yala, Local Government Area of Cross River State, in Southeast Nigeria (8.450E; 6.250N). The saline lake has a surface area of ~ 600 m2 and a greatest depth of 6 m at the center and an approximate water volume of 3000 m3 fed by an underground spring. The freshwater lake has a surface area of ~ 500 m2 and a maximum depth of 5 m at the center and an approximate water volume of 2200 m3. An underground spring provides water to the lake. In addition, an ephemeral stream serves as an occasional outflow to a semi-enclosed lake, especially during the intense rainfall in the wet season (Ene and Okogbue, 2013). Currently, barite mining activity takes place at a site approximately 100 m from the freshwater lake.
The vegetation of the region is mainly mangrove forest overlapping with a rain forest belt identified by disperse trees and low covering grassland and shrubs. It is officially categorized as grassland towards the north and rain forest type towards the south (Ene and Okogbue, 2013). The lake catchment is farmland cultivated by indigenous subsistence farmers. Intermittent clearing of shrub and tree logging take place during farmland preparation.
The region is drained by the Konshisha, Oruaba, Okpauku and Kpa. rivers and their tributaries which show dendritic and occasional trellis drainage motifs that empty into the Cross River drainage system. While some of the streams are perennial, others are ephemeral and are regularly sourced as surface flows from springs that notch the relatively higher altitudes from the north towards the southern part of the region. These rivers and their tributaries influence the ground water dynamics around the study area (Ene and Okogbue, 2013).
The geology of the study area falls within the southern Benue Trough. The Benue Trough is an elongated intra-cratonic basin basal by a thick progression of Cretaceous sedimentary rocks deposited on an undulating basement and punctuated by economic ore veins, volcanic and intrusive rocks. The basement is bared to the south-eastern and north-western boundaries of the trough where their peaks normally match with the major hydrologic extremities. The Cross River plain reveals a moderate rise from the plains in the south to the uplands in the central north, while northern part of the area is identified by lightly sloping uplands that are between 300 m to 500 m EASL. The landscape in the southern part of the region is characterized by lowlands that are well below 300 m EASL (Ene and Okogbue, 2013).
The districts inhabitants are mainly subsistence farmers. Mining of barite and salt (the major solid minerals in the area) is assumed to predate the period of occupation of the British (i.e. late 18th century) and peaked in the 1980s. About two decades ago, people were barred from drinking, bathing, fishing, and making salt from the saline lake by the local authorities. This activity involves occasional drainage of the saline lake and excavation of the sediments to reach the depocenter, causing redistribution of salts in the overlying sediment layer. Barite mining activity taking place around the low-lying area is expected to influence the population of organisms within the freshwater lake and along its catchment but not the hypersaline lake owing to its higher elevation.
2.1 Samples
Two sediment cores (40 cm long each) were collected from two different locations within the enclosed saline lake (one near the shoreline-D and the other from the open-water M) using a stainless steel box corer. Another core of same size was collected from a semi-enclosed freshwater lake lying at a lower altitude, tagged F. Each core was sectioned into 8 parts of 5 cm, giving a total of 24 subsamples (Table 1). Subsamples were wrapped with pre-cleaned aluminum foil, tagged, kept in an ice contained cooler to inhibit microbial action, transferred to the laboratory and preserved in a deep freeze at -20oC until further protocols. The freeze–dried samples were sieved to pass through < 65µm mesh and stored in a deep freeze (-20oC) previous to extraction and clean up protocols.
| Sample ID | Depth (cm) | Water column (m) | Salinity (PSU) | pH | Coordinate/ Elevation |
| D1 | 35–40 | 2 | 1.63 | 5.86 | 6.52N 8.45E/ 69 m |
| D2 | 30–35 | 1.68 | 6.53 | ||
| D3 | 25–30 | 1.81 | 6.45 | ||
| D4 | 20–25 | 1.73 | 6.41 | ||
| D5 | 15–20 | 2.44 | 6.20 | ||
| D6 | 10–15 | 3.58 | 6.34 | ||
| D7 | 5–10 | 1.12 | 6.78 | ||
| D8 | 0–5 | 1.85 | 6.88 | ||
| Mean/STD | 1.98 ± 1.04 | 5.10 ± 1.24 | |||
| M1 | 35–40 | 6 | 3 .75 | 4.48 | 6.25N 8.4E/69 m |
| M2 | 30–35 | 3.25 | 6.73 | ||
| M3 | 25–30 | 2.07 | 6.65 | ||
| M4 | 20–25 | 2.66 | 6.63 | ||
| M5 | 15–20 | 4.72 | 6.46 | ||
| M6 | 10–15 | 6.57 | 6.55 | ||
| M7 | 5–10 | 2.45 | 6.48 | ||
| M8 | 0–5 | 2.52 | 6.89 | ||
| Mean /STD | 3.50 ± 1.43 | 6.59 ± 0.13 | |||
| F1 | 35–40 | 5 | 0.00 | 3.40 | 6.52N 8.45E/ 44 m |
| F2 | 30–35 | 0.00 | 3.56 | ||
| F3 | 25–30 | 0.00 | 3.45 | ||
| F4 | 20–25 | 0.00 | 3.45 | ||
| F5 | 15–20 | 0.00 | 3.58 | ||
| F6 | 10–15 | 0.00 | 3.52 | ||
| F7 | 5–10 | 0.00 | 3.61 | ||
| F8 | 0–5 | 0.00 | 3.80 | ||
| Mean/STD | 0.00 ± 0.00 | 3.52 ± 0.11 |
2.2 Grain size, bulk density, pH and salinity determinations
Analysis of sediment grain sizes was performed using a laser particle sizer (LS 200, Beckmann-Coulter; 0.375–1000 µm). Details is as optimized in Strauss et al., 2015.
Bulk density (BD) measurements were carried out by quantifying the volume of frozen samples with the Archimedes Principle involving the determining water displaced in a water-filled glass beaker using a balance (FCB 8K0.1, Kern; Arnaud et al., 2006). BD was estimated as follows:
BD [103 kgm− 3] = sample dry weight [103 kg]/sample volume [m3].
The pH of sediment samples was potentiometrically measured in the supernatant suspension of a 1:5 (sediment: solution) (w/v) mixture using a pH meter. The solution was made up of a 0.01 mol/l calcium chloride in distilled water. Buffer solution of pH 4 and 7 was used in the calibration of the pH-meter. The probe of the pH-meter was inserted into a thoroughly shaken/settled suspension and reading obtained after stabilization of the system (International Organization for Standardization, 2006).
Sediment salinity was estimated using an electrical conductivity (EC) meter by blending 1-part sediment with 5 parts deionized water. After mixing the sample and allowing the sediment to settle, the EC of the solution was tested. The estimated value (a measure of the amount of salt in sediment) was multiplied by an appropriate factor related to the sediment texture.
2.3 Elemental and δ13CTOC determination
Prior to organic carbon and nitrogen analysis, an aliquot of the dry sediment was analyzed on a Costech CNS elemental analyzer coupled with a Thermo Scientific Delta V Advantage isotope-ratio mass spectrometer (Contreras et al. 2016; Oyo-Ita et al. 2016) for molecular carbon and nitrogen concentrations. Isotope compositions were normalized to the VPDB scale, and represented in conventional delta notation as follows:
| δ13C= [(13C/12C) sample/(13C/12C) standard)) – 1] x 1000] |
2.4 Lipid extraction and quantification
Lipids were extracted from 2 g of freeze-dried sediment via sonication with DCM: methanol; 2:1 v/v. The total lipid extracts (TLE) were reduced to near-dryness under a gentle stream of nitrogen, and the neutral and acidic lipid fractions were isolated with an aminopropyl solid phase extraction column. Neutral lipids were eluted from the column using 2:1 (v/v) DCM:IPA, and free fatty acids were eluted with 4% acetic acid in ethyl acetate. Neutral lipids were further isolated on the bases of polarity, into classes of compound, by column chromatography, using 5% deactivated silica gel, adapted from Contreras et al. (2016). About 4.5 mL of 9:1 n-Hexane:DCM was used to elute hydrocarbons from the silica gel column.
Analysis of the hydrocarbon fraction was conducted on a Thermo Scientific Trace 1310 gas chromatograph with an Agilent DB-5 column, interfaced to a Thermo Scientific ISQ single quadrupole mass spectrometer with electron ionization. For the hydrocarbons, the inlet was operated in splitless mode at 280°C. The column flow rate was set to 2.0 mL min-1 and the oven was programmed to an initial temperature of 60°C and held for 1 minute, then ramped to 320°C at 6°C/min and held for 20 minutes. Compound assignment was performed based on the comparison of the retention time and ion fragmentation pattern of the target compounds with that of authentic standards. Quantification was on the premise of the calibration curves produced from the external standards peak areas (C7-C40) with concentrations range 5-250 µg mL. Quantification was performed with selected ion monitoring of m/z 57.
3.1 Sediment bulk properties
3.1.1. Grain size, bulk density, pH and salinity distributions
The proportion of sand fraction in the salt and freshwater lakes was higher in the top layers with a mean of 26 ± 2%, 25 ± 4%, and 28 ± 3% for shoreline D, open-water M, and fresh water F cores, respectively. For D, the proportion of silt ranged from 39% at 5–10 cm to 44% at 35–40 cm and 20–25 cm. However, the proportion of silt for M ranged from 38% at the top layer to 44% at 25–35 cm, while the silt fractions for F were in the range 40–44%, minimizing at 35–40 cm and 25–30 cm and a maximum value at the top layer. The proportion of clay, on the other hand, was relatively low in the two lakes ranging from 27–37% with a minimum at the top layer and a maximum at the middle (Fig. 2).
Bulk density values for the saline lake were in the range 0.8–1.20 g/cm3, minimizing at the bottom and near bottom layers of shoreline D (0–5, 5–10 cm) and maximizing at the top layer of the open-water M (0–5 cm) with a mean of 0.93 ± 0.08 g/cm3. Similarly, the freshwater lake F showed uniform BD (1.00–1.10 g/cm3) down core except at the top layer (1.40 g/cm3) with a mean of 1.15 ± 0.07 g/cm3 (Fig. 2).
Values of pH recorded for D ranged between 5.86 and 6.88 (mean = 5.10 ± 1.24) with a minimum and maximum at the bottom and top layers, respectively, while values for M ranged between 4.48 and 6.89, minimizing at the bottom layer (35–40 cm) and maximizing at the top layer (0–5 cm) with a mean of 6.59 ± 0.13. In contrast, the freshwater lake exhibited lower and regular pH values ranging from 3.40 at the bottom layer (35–40 cm) to 3.80 at the top layer (0–5 cm) with a mean of 3.52 ± 0.11 (Table 1).
Sediment salinity values recorded for the saline lake ranged between 1.63 and 3.58 PSU, minimizing at the bottom layer (35–40 cm) and maximizing at the near top layer (10–15 cm) of the shoreline D (mean = 1.98 ± 1.04 PSU), while higher values in the range; 2.07–6.575 PSU were recorded for the open-water M, minimizing at the middle layer (25–30 cm) and maximizing at the near-top layer (10–15 cm) with a mean of 3.50 ± 1.43. However, the freshwater lake exhibited 0.00 PSU values throughout (Table 1).
3.1.2. TOC, TN, C/N and δ13CTOC distributions
Total organic carbon (TOC) content for the saline lake ranged from 1.9% at 35–40 cm to 2.9% at 15–20 cm with a mean of 2.5 ± 0.3% in the shoreline D core, while the open-water M exhibited TOC values that ranged from 1.5% at 35–40 cm to 2.8% at 15–20 cm with a mean of 2.3 ± 0.4%. Relatively lower TOC content was found for the freshwater lake, ranging from 0.03% at 25–30 cm to 0.51% at the near top layer (5–10 cm) with a mean of 0.23 ± 0.2% (SI-I).
Total nitrogen (TN) values for the saline lake were generally low and the shoreline D exhibited values in the range 0.01–0.11%, minimizing at the bottom layer (35–40 cm) and maximizing at the top layer (0–5 cm) with a mean of 0.08 ± 0.03%, while those for the open-water M ranged from 0.01% at 10–15 cm to 0.09% at the near bottom (30–35 cm), middle (20–25 cm) and near top (5–10 cm) layers with a mean of 0.45 ± 0.04%. However, TN content for the freshwater lake was below detection (SI-I).
Atomic Corg/NTotal values for the saline lake were in the range 25 − 184 with a minimum at 0–5 cm and a maximum at the bottom layer (35–40 cm; mean = 50.4 ± 5.5) in the case of the shoreline D core, while the open-water M exhibited C/N values that ranged from 28 at the near top layer (5–10 cm) to 174 at 10–15 cm with a mean of 45.2 ± 1.8. Calculations of Corg/NTotal ratios for the freshwater lake were not possible due to non-detection of N (SI-I).
Bulk stable carbon isotope values (δ13CTOC) for the saline lake ranged from − 29.7‰ at the middle layer (15–20 cm) to -25.1‰ at the near top layer (5–10 cm; mean = -26.2 ± 1.5‰) in the case of the shoreline D. For the open-water M, the values ranged from − 25.9‰ at the near top layer (10–15 cm) to -25.4‰ at the middle (20–25 cm) with a mean of -25.8 ± 0.2‰. A similar trend was found for the freshwater lake with δ13C values ranging from − 25.5‰ at the near top layer (5–10 cm) to -22.1‰ at the near bottom (30–35 cm) with a mean of -24.5 ± 1.3‰. The value recorded at 10–15 cm for freshwater lake sample was below detection (Fig. 3). A plot of Corg/NTotal ratio versus δ13CTOC for saline lake samples only is shown in Fig. 4.
3.2 Vertical distributions of n-alkanes
Generally, the n-alkane abundance in the open-water M core was higher than that recorded for the shoreline D core, while the n-alkane abundance found for the freshwater F lake was relatively lower than that recorded for the open-water M (Fig. 5a, b and c). The n-alkane distribution profiles for the saline and freshwater lakes consisted mainly of the C21–C35 homologs with minor-to-moderate C17 and C19 members detected in M and upper layers (0–25 cm) of D only. All three cores exhibited a general trend of odd/even carbon number predominance (Fig. 5a, b and c). Besides the detection of unresolved complex mixture (UCM) (Fig. 5c), another supportive evident of petroleum products utilization at the near bottom layer of fresh water core is shown in Fig. 6a and b.
Recent studies have demonstrated that long chain n-alkanes (eg. C27 and C29) in sediments from fresh and saline shallow water depth lakes may also be produced by submerged/floating macrophyte (e.g. Potamogeton, Myriophyllum and chara) due to their adaptation to partial exposure to the atmosphere while certain species of algae are also known to synthesize mid-chain n-alkanes (C21 and C23) (e.g. Cladophora) (Aichner et al., 2010a; Liu and Liu, 2016). On this basis, we grouped the n-alkanes according to their primary OM source as follows: higher plant leaf waxes (∑C29 + C31 + C33), submerged/floating macrophytes (∑C23 + C25 + C27) and algae/photosynthetic bacteria (∑C17 + C19 + C21).
For D, leaf wax n-alkane concentrations were in the range 19.6–1288 ng/g dry weight (dw), with the lowest values at the bottom layer (35–40 cm) and highest in the middle (10–15 cm) with a mean of 779 ± 411 ng/g dw. A range of 16.5–1058 ng/g dw with a minimum at 35–40 cm and a maximum at the middle layer (10–15 cm; mean = 590 ± 308) was found for submerged/floating macrophyte n-alkanes, while the concentrations of algal/bacterial n-alkanes ranged from 1.1 ng/g dw at 35–40 cm to 45.2 ng/g dw at the middle layer (25–30 cm) with a mean of 26.5 ± 12.8 ng/g (Fig. 7a). In core M, concentrations of leaf wax n-alkanes were in the range 0.1–788 ng/g dw, with a minimum at the middle layer (20–25 cm) and a maximum at 10–15 cm; mean = 436 ± 220 ng/g. A range 0.1–570 ng/g dw was found for submerged/floating macrophyte with a minimum at 20–25 cm and a maximum at the near bottom layer (25–30 cm; mean = 326 ± 158 ng/g). The concentrations of algae/bacteria n-alkanes ranged from 0.02 ng/g dw at the middle layer (20–25 cm) to 38.3ng/g dw at the near top layer (5–10 cm) with a mean of 24.9 ± 11.6 ng/g (Fig. 7b). For the freshwater lake, concentrations of leaf wax n-alkanes were in the range 239–1457 ng/g dw, minimizing at the middle layer (10–15 cm) and maximizing at the top layer (0–5 cm) with a mean of 832 ± 402. A range 490 − 1149 ng/g dw was found for submerged/floating macrophytes with a minimum at the bottom layer (35–40 cm) and a maximum at 0–5 cm (mean = 642 ± 340 ng/g), while concentration of algae/bacteria n-alkanes ranged from 4.80 ng/g dw at the near top layer (5–10 cm) to 29.4 ng/g dw at the near bottom layer (30–35 cm) with a mean of 15.4 ± 9.5 ng/g (Fig. 7c).
3.3 Molecular proxies and Multi Linear Regression (MLR)
Carbon preference index (CPIC20 − C35) was calculated as:
for the saline lake exhibited values in the range 4.89–9.0, with a minimum at the near top layer (5–10 cm) and a maximum at the bottom (35–40 cm), with a mean of 6.7 ± 1.2 in the case of D, and a range from 2.12 at the middle layer (15–20 cm) to 7.8 at 10–15 cm with a mean of 6.1 ± 1.7 was found for M. For the freshwater lake, CPIC20 − C35 ranged from 1.6 to 15.6 with a minimum at the bottom (35–40 cm) and near-bottom layers (30–35 cm) and a maximum at the top layer (0–5 cm) with a mean of 5.9 ± 4.7 (SI-II).
Another proxy parameter that provides supportive proof of the occurrence of biogenic OM in the sediment is the n-alkane chain length with the greatest abundance, called the carbon maximum (Cmax). All the sediment layers for the two lakes exhibited Cmax at 27, 29 and 31 (Fig. 5a, b and c). However, in all the layers, Cmax 29 dominated.
Following the traditional and widely applied ratio of hydrocarbons for terrigenous and aquatic sources (TAR = C 27+ C29 + C31/ C15 + C17 + C19; eg. Oyo-Ita et al., 2017), here modified for the shallow lake (TARSL = nC29 + n C31 + nC33/ nC17 + nC19 + nC21 + nC23 + nC25 + nC27). In our case study, we added C31 to the numerator since C31 has not been reported to occur in high abundance in submerged/floating macrophyte and because C27 and C29 are produced from both higher plant leaf waxes and macrophyte, we assigned C29 to higher plant leaf waxes (i.e. numerator) and C27 to submerged/floating macrophyte (i.e. denominator) in addition to C23 and C25. TARSL values recorded for D ranged from 1.07 at the near top layer (5–10 cm) to 1.47 at the near bottom layer (30–35 cm), with a mean of 1.22 ± 0.28, while a range 1.18–1.72 was found for M with a minimum value at the near top layer (5–10 cm) and a maximum at the middle (20–25 cm; mean 1.18 ± 0.25). For the freshwater lake, values of TARSL ranged from 1.23 at the bottom and near bottom layers (30–40 cm) to 1.70 at the middle layer (15–20 cm) with a mean of 1.42 ± 0.19) (SI-II).
To further assess the source of the long chain n-alkanes so as to determine the relative proportions of submerged/floating macrophyte and higher plant inputs, the traditional and widely applied parameter (Paq = C23 + C25/C23 + C25 + C27 + C29; Ficken et al ; 2000) (here modified as Paq(SL)) was calculated according to the expression: (C23 + C25 + C27)/(C23 + C25 + C27+C29+C31+C33) and the values for the saline lake were in the range 0. 14–0.23, minimizing at 10–15 cm and maximizing at - top layer ( 0–5 cm) with a mean of 0.18 ± 0.0 4 in the case of D. Paq(SL) values for M ranged from 0.14 to 0.23 with a minimum at 10–15 cm and a maximum at the near- top layer (5–10 cm) ( mean = 0. 17.±0.03), while those calculated for the freshwater lake were in the range 0.56 − 0.76, minimizing at the middle layer (20–25 cm) and maximizing at the top layer (0–5 cm) with a mean of 0.65 ± 0.05 ((SI-II).
The extracted n-alkane concentrations were subjected to multiple linear regression (MLR) analysis to quantify their relative contributions to the lakes. In D, n-alkanes of vascular plant leaf waxes, submerged/floating macrophyte and algae/bacteria origins contributed 56%, 42% and 2%, respectively, while 55%, 42% and 3% were the contributions from leaf waxes, submerged/floating macrophyte and algae/bacteria to M, respectively. In the case of the freshwater lake, leaf waxes, submerged/floating macrophyte and algae/bacteria contributed 59%, 39% and 2%, respectively (Fig. 8a, b and c).
4.1. Bulk properties
4.1.1 Grain size, bulk density, pH and salinity distributions
Grain size distributions were more homogenous in the saline lake than in the freshwater system. However, the proportion of sand fraction was higher in the top layer of the freshwater lake than in the salt lake, most likely related to the recent outflow of water from Cameroonian dams, which triggered an unprecedented flood in 2012 and devastated properties and livelihoods of nearby inhabitants as well as the low-lying relief and semi-enclosed nature of the freshwater lake.
Mean bulk density (BD) values of 1 g/cm3 in the saline lake (D and M cores) suggests that land use change involving farmland and mining activity might not have played a significant role in the flux of sediments to the lake, most likely due to its relatively high elevation.
Hydrolysis of barite (barium sulphate) can result in acid enhancement in the environment. Thus, the low pH (< 4) found in the freshwater lake is attributed to hydrolysis of barite deposits associated with the mining activity taking place in the catchment. Relatively higher salinity found at the near -top layer (10–15 cm) of D may be attributed to current salt mining venture prior to its prohibition. Salt mining requires the lake to be drained and excavated which would result in the redistribution of salt into adjacent sedimentary layers. Higher salinity levels recorded all through the M core is not unusual due to its propinquity to the center of deposition.
4.1.2. TOC, TN, C/N and δ13CTOC distributions
TOC contents of the saline and freshwater lakes reveal both allochthonous inputs from eroded soils and leaf litter and autochthonous production. The slightly higher TOC content found in the shoreline saline core relative to open-water saline core likely reflects higher contribution from allochthonous OM over autochthonous production due to its proximity to the shoreline. The relatively lower TOC content of the freshwater lake may be attributed to a higher fractional abundance of sand with low adsorption capacity for organic carbon (OC) or dilution from sand inputs as well as low primary productivity (Oyo-Ita et al 2016). Despite its semi-enclosed and relatively low-lying terrain, it appears that the role of stream outflow occasioned by intense rainfall during the wet season in the transport of terrigenous material to the freshwater lake was not significant. These results suggest greater capacity of enclosed lake systems in preserving OM. The observation that TOC concentration in the two saline cores climaxed at the same layer (15–20 cm) indicate inputs from similar sources of OM at both sites, with higher delivery to nearshore areas. Generally, TOC contents of the cores were low relative to other sedimentary environments such as Yungui Plateau Lake, China; TOC = 27.16%; Huang et al., 2017) and higher than Zabuye Salt Lake; TOC = 0.3%; Wang et al; 2004). Unlu and Alpa (2006), reported that TOC concentrations generally increase as sediment grain size decreases. A fairly good relationship was observed between clay/silt (%) and TOC (%) for the two saline cores (D- r2 = 0.3791 and M- r2 = 0.3502). In the case of the freshwater lake, however, the poor relationship existing between these paired variables (F- r2 = 0.1764) may be linked to its coarser sediment distribution.
TN contents in lacustrine sediments may indicate higher influx of aquatic flora, especially in an environment where fertilizer utilization and bed rock mineralization are absent (Gonzales-vila et al., 2003) as at our study sites. The low TN content in the saline cores indicates a slight input from aquatic flora to the OM content. Fresh OM from lake algae, which are protein abundant and cellulose deficient, has atomic C/N values between 4 and 10, whereas vascular land plants, which are protein deficient and cellulose abundant, produce OM with C/N values of 20 and above (Meyers, 2003). OM produced by C3 land plant has an average δ13CTOC value of ~ -27‰, while C4 land plant-derived OM has an average δ13CTOC value of ~ -14‰ (O’Leary, 1988; Farquhar et al., 1989). Thus, a plot of C/N versus δ13CTOC values allows for the identification of OM sources in lake sediments (Meyers, 1994). In this study, a similar plot revealed lake systems dominated by C3 plant preservation (Fig. 4). The high C/N values found at our study sites indicate greater contribution from vascular land plants and macrophytes to the sedimentary OM pool and/or greater preservation of terrigenous OM relative to aquatic OM. While we cannot calculate C/N ratios in the freshwater lake due to low N content, the range of δ13CTOC values in all three lakes reflect a significant input from C3 plants, with a slight input from algal-derived OM. Furthermore, higher level of terrestrial OM was found at the upper layer samples of the shoreline saline core relative to the open-water saline core M. The observed single point plotted in Fig. 4 supports our earlier suggestion of greater wash-in of C3 woody material (lignin) arising from occasional tree logging for farmland preparation.
4.2 Vertical distributions of n-alkanes.
Generally, the observed higher LMW n-alkane abundances in the open-water saline core relative to the shoreline core suggests higher OM input from aquatic organisms in the center of the lake. The observed lower n-alkane abundance found for the freshwater lake relative to the saline lake may in part be a consequence of higher OM degradation, a conclusion supported by a slight unresolved complex mixture (UCM) peak found in the freshwater lake (Fig. 5c). This interpretation is also consistent with the downcore trend of decreasing CPI. An UCM is an attribute often observed in gas chromatograms of crude oil and extracts from organisms exposed to oil (Donkin et al. 2003) and can also indicate microbial degradation of OM (Meyers 2003). Therefore, the detection of a minor UCM near the bottom layer of freshwater lake suggests not only the occurrence of microbial degradation of OM at this site but also an input from petroleum products utilization derived from deltaic origin during certain geologic time frame evident by detection of very low levels of hopanes, oleananes and steranes (Fig. 6a and b).
Additionally, the observed odd/even carbon numbered predominance in the study area was attributed to input from fresh OM, most likely of terrestrial higher plant, submerged/floating macrophyte and algae/photosynthetic bacteria origins (Liu and Liu 2016). This is in contrast to other sedimentary environments elsewhere which exhibited even/odd carbon numbered predominance, reported to be inundated by aquatic microorganisms (i.e, non-photosynthetic bacteria; Elias et al. 1997; Ekpo et al. 2005; Oyo-Ita et al. 2006).
Although some n-alkanes come in via wind, slightly higher levels of leaf wax n-alkane in the shoreline saline core D relative to open-water saline core M was not surprising as the former received slightly more terrigenous materials due to its proximity to the shoreline. Interestingly, despite its relatively lower TOC content, the higher plant leaf wax n-alkane concentration recorded in the freshwater lake was significantly higher than in the two saline cores. This could be attributed to greater preservation of long chain alkanes, but could also be due to preferential degradation of shorter (algal-derived) n-alkanes. Algae/photosynthetic bacteria-derived n-alkane concentrations were slightly higher in the open-water core M, suggesting that halo-tolerant microorganisms thrived more in the central lake relative to the littoral zone, although we cannot rule out sediment focusing.
The occurrence in low abundances of low molecular weight n-alkanes in both saline lake cores may be linked to the influence of salt stress, which probably limited the growth of most microorganisms (e.g. algae/bacteria). Earlier studies demonstrated that higher salinity concentration of lake sediments correlated with higher proportion of long-chain n-alkanes and that microbial alkane degradation could be potentially influenced by salinity (Wang et al., 2019). Other studies showed that salinity can significantly decrease microbial growth and metabolic activities in saline and hypersaline lakes due to energetic constraints (Lozupone and Knight, 2007; Oren, 2011), and thus greater salinity would reduce the degradation capacity of alkane degrading bacteria (Liu et al., 2009). According to Wang et al., (2019), short-chain n-alkane-producers (microorganisms) may be abundant and active in the lakes with low salinity, but may be absent and/or inactive in high-salinity lakes. It appears therefore that microbial degradation potentials differed among lakes of different salinity. The implication for this study is that higher n-alkane degradation occurs in lower salinity systems (Wang et al., 2019). Therefore, the difference in composition of n-alkanes between the two saline lake cores may be ascribed to the fact that microorganisms bring to bear dissimilar impact on the n-alkanes across a salinity gradient within the lake.
The absence of LMW (nC12 – nC19) n-alkanes in the freshwater lake is interpreted to be a result of acidification attributable to barite mining activity inhibiting primary productivity, though we cannot rule out preferential degradation of short chain n-alkanes. Our recent report on natural and anthropogenic biomarkers in sediment cores from Refome Lake (pH 4.3–4.7) showed higher abundances of LMW n-alkanes related to input from algae/bacteria (Oyo-Ita et al., 2017), suggesting pH of 4 as a threshold for survival of certain species of algae and photosynthetic bacteria.
It therefore appears that long chain n-alkanes derived from terrestrial and/or submerged/floating macrophyte in these shallow lakes was not the only OM preserved under salinity, OM derived from certain specific halotolerant aquatic organisms (e.g. algae and bacteria) can also contribute to the sediments. However, human-induced activity associated with barite mining limited primary production of aquatic OM in the freshwater lake. Thus, anthropogenic activity appears to alter ecosystem balance more than natural-induced process in these systems.
Comparing data to those reported elsewhere, Silliman and Schelske (2003) reported a sharp drop in high molecular weight n-alkanes over low molecular weight counterpart that corresponded to a shift from macrophyte or higher plants to algal dominance of OM production in Lake Apopka, Florida. According to these authors, this trophic shift was linked to increase in nutrient delivery to the lake arising from agricultural development of the watershed in the mid-1940s. On the contrary, OM production in some lakes has been reported to be significantly impacted by submerged and floating macrophytes, rather than algae (e.g. Ficken et al., 2000). In this study, however, we found a predominance of long-chain, high molecular weight n-alkanes, indicating that terrestrial plants and macrophytes were the primary contributors of leaf waxes in each lake. Lower abundances of short-chain, low molecular weight n-alkanes in both systems suggest that autochthonous primary productivity was either limited or severely curtailed by saline and acidic conditions. We cannot rule out preferential degradation of the short-chain n-alkanes, but we infer from their low abundances in the top of each core that algal and photosynthetic bacterial populations were low or not present in these lacustrine systems.
4.3 Molecular proxies
n-Alkanes of lacustrine sediments frequently have multiple sources, including terrestrial plants, submerged/floating and emergent macrophytes, and algae/bacteria (Rao et al., 2014). Thus, it is a necessity to comprehend the contribution from different OM sources before undertaking reconstruction of paleoenvironmental conditions (Liu et al., 2016). Carbon preference index (CPI) values above 3 are generally taken as indicating fresh/un-degraded OM, whereas lower values are indicative of degradation and/or petroleum inputs (Weihai et al., 2007; Pisani et al., 2013). The high CPI values recorded in the two saline cores indicate significant contribution from fresh/un-degraded OM to the lakes, arising from varying inputs of submerged/floating macrophyte and terrestrial plants. The highest CPI value found at the top layer of the freshwater lake may be linked to its low-lying terrain prone to higher terrigenous input run-off occasioned by the flood episode of 2012 and/or its shallower water depth. The decreasing CPI values down core is a reflection of extensive degradation of OM in sediments during diagenesis.
Differences in Cmax at 27, 29 and 31 recorded in all layers of the saline and freshwater lakes showed that our studied lakes are primarily documenting changes in the terrestrial landscape and may also arise in part as a result of adaptation of submerged/floating macrophyte to partial exposure to the atmosphere (Liu et al., 2016). Occasional tree logging/grass clearing for farmland preparation may cause enhanced delivery of higher plant litter to the lakes relative to submerged/floating macrophyte. However, a recent study has shown that most of the Cmax terrestrial plant usage have been on the bases of a narrow set of data that cannot address intra- and inter-plant variability (Bush and Mcinerney, 2013). A comprehensive analysis of data on trees and grasses of various species from different locations of the world show that C27, C29 and C31 are highly variable within plant groups, such that differentiation between woody plants and grasses are not easy to make based on n-alkane abundances (Bush and Mcinerney, 2013). More recently, n-alkane distributions by plant functional type (PFT) have been measured and analyzed (Diefendorf et al., 2011). According to these authors, the dominant n-alkane chain lengths in both gymnosperms and angiosperms vary among PFTs and taxonomic groups. n-Alkane abundances are highest in the evergreen angiosperms with a Cmax at n-C31, followed by the deciduous angiosperms with Cmax at n-C29. Evergreen gymnosperms have a higher Cmax at n-C33 but with much lower abundances. Although Cmax values of 27, 29 and 31 were most common in our study sites, a predominance of C29 suggests that the sedimentary n-alkanes are of deciduous angiosperm origin supported by the type of vegetation grown in the region. This assertion does not preclude the contribution of C27 and C29 from submerged/floating macrophytes to the shallow lakes.
Generally, high TAR (TAR > 1) values suggest increased external supply of OM to the lake, sourced from terrestrial vegetation in the surrounding watershed, whereas lower TAR values indicate an internal source of OM to the lake from aquatic algae and microbes (Meyers, 1997). Our TARSL mean values are all above 1, suggesting mostly terrestrial OM input, consistent with our C/N ratios.
According to Ficken et al., (2000), Paq values ranging from 0.01 to 0.23 are attributed to terrestrial higher plant waxes, while those in the range 0.48–0.94 are linked to submerged/floating species of macrophyte. The present study demonstrated a major contribution of long chain n-alkanes from terrestrial OM to the lakes.
In this work, bulk organic stable carbon isotope ratios and n-alkane distributions in saline and freshwater sediment cores from two adjacent Gabu lakes in Southeast Nigeria were evaluated to characterize OM sources and assess the effects of natural (salinity) and anthropogenic (barite mining induced acidity) influences had on their distributions. The poor relationship observed between TOC and sediment grain size distribution for the freshwater lake was associated with the coarser particles with low OM adsorption capacity. The relatively higher bulk density found for the top layer of the freshwater lake may be associated to the flood episode of 2012 occasioned by outflow of water from the Cameroon dams.
Utilization of atomic C/N, stable carbon isotope ratios and n-alkane molecular proxies (eg. CPI, Cmax, TARSL and Paq(SL)) as well as MLR data allowed OM source characterization to be made. The results revealed a scenario with an admixture predominated by vascular plant leaf waxes with a considerable proportion of submerged/floating macrophyte and a minor contribution from algae/photosynthetic bacteria. The scenario here indicated a shallow water lake system that predominantly preserved long chain n-alkanes derived from vascular plant leaf waxes and macrophytes, and that, submerged/floating macrophyte also contributed considerably to long chain n-alkanes flux relative to OM delivery from vascular plant leaf waxes.
The low abundances of LMW n-alkanes in the saline lake were interpreted to be a consequence of the effects of salt stress, while the absence of LMW n-alkanes in the freshwater lake was attributed to acidification arising from barite mining, inhibiting the growth of micro-organisms (e.g. algae/bacteria) and biodegradation. Our results have demonstrated that long chain n-alkanes of terrestrial and submerged/floating macrophyte origins are better preserved under conditions of salinity and acidification in the inland basins than those derived from algae/bacteria.
Acknowledgement
This study was funded in part by the Management of the University of Calabar Travel Research grant, 2018. We are grateful for the assistance and facilities provided by the Department of Geology and Environmental Sciences, University of Pittsburgh, Pennsylvania, USA in the analysis of our samples at no cost. The cooperation received from members of the Geochemistry research team of the host Institute is also highly appreciated.
Statements and Declarations
I would like to submit our manuscript entitled “Stable carbon isotope and n-alkane distributions in sediment cores from saline and freshwater Gabu lakes, Southeast Nigeria: Environmental implications” for possible publication as a Research Paper in the Environmental Earth Science Journal.
All authors contributed to the study conception and design. Inyang Oyo-ita: Conceptualization, Investigation, Visualization, Writing to original draft preparation, Funding acquisition Edidiong Sam.: Data curation, Validation, Formal analysis, writing to original draft preparation Thomas Elliot: Methodology, Resources, Supervision, software, Project Administration. David Inyang: Data curation, Field work Advisory. Josef Werne: Methodology, Resources, Supervision, Funding acquisition: Orok Oyo-ita: Conceptualization, Investigation, Visualization, Writing to original draft preparation. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
We confirm that this manuscript is not under consideration for publication elsewhere, that its publication in Organic Geochemistry Journal is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out, and that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder. The authors declare no conflicts of interest associated with this manuscript.
- Aichner, B., Herzschuh, U., Wilkes, H., 2010. Influence of Aquatic macrophytes on stable carbonisotopic signatures of sedimentary organic matter in lakes on the Tibetan Plateau. Organic Geochemistry 41, 706-718.
- Arnaud, F., Magand, O.,Chapron, E.,Bertrand, S.,Boe¨s, X.,Charlet, M. A.,Me´lie`res, F., 2006. Radionuclide dating (210Pb,137Cs,241Am) of recent lake sediments in a highly active geodynamic setting (Lakes PuyehueandIcalma-Chilean Lake District). Sci. Tot. Environ. 366, 837,
- Boroujeni, H.S., Faith-Moghadam, M., Shafaei-Bejestan, M. (2009). Investigation on Bulk Density of Deposited Sediments in Dez Reservoir. Trends in Applied Science Research 4, 148-157.
- Bouillon, S., Moens, T., Koedam, N., Dahdouh-Guebas, F., Baeyens, W., Dehairs, F. (2004). Variability in the Origin of Carbon Substrates for Bacterial Communities in Mangrove Sediments. FEMS Microbiology Ecology 49:171-179.
- Bourbonniere, R.A., Meyers, P.A. (1996). Anthropogenic Influences on Hydrocarbon Contents of Sediments deposited in Eastern Lake Ontario Since1800. Environmental Geology 28, 22-28.
- Brocks, J.J., Buick, R., Summons, R.E., Logan, G.A. (2003). A Reconstruction of Archean Biological Diversity Based on Molecular Fossils from the 2.78-2.45-Billion-Year-Old Mount Bruck Super group. Hamersley Basin, Western Australia. Geochimica Et Cosmochimica Acta 67, 4321-4335.
- Bush, R.T., Mcinerney, F.A. (2013). Leaf Wax N-Alkane Distributions in and Across Modern Plants: Implications for Paleoecology and Chemotaxonomy. Geochimica et Cosmochimica Acta 117, 161 – 179.
- Contreras, S., Werne, J. P., Erik T. Brown, E. T., Scott Anderson R., Fawcett, P. J. (2016). A molecular isotope record of climate variability and vegetation response in southwestern North America during mid-Pleistocene glacial/interglacial cycles. Palaeogeography, Palaeoclimatology, Palaeoecology 459: 338-347.
- Cranwell, P.A., Eglinton, G. & Robinson, N. (1987). Lipids of Aquatic Organisms as Potential Contributors to Lacustrine Sediments. Organic Geochemistry 11: 513 – 527.
- Diefendorf, A. F., Freeman, K. H., Wing, S. L. & Graham, H. V. (2011). Production of n- alkyl lipids in living plants and implication for the geologic past. Geochimica et Cosmochimica Acta 75, 7472-7485.
- Donkin, P., Smith, E.L. & Rowland, S.J. (2003). Toxic Effects of Unresolved complex Mixtures of Aromatic Hydrocarbons Accumulated by Mussels, Mytilusedulis, from contaminated Fields sites. Environmental Science & Technology 37(21), 4825-4830.
- Ekpo, B. O., Oyo-Ita, O. E., Wehner, H., (2005). Even n-alkanes/alkenes predominances in surface sediments from Calabar River, S.E. Niger Delta, Nigeria. NaturWissenshaften 92,341.
- Elias, V.O., Simoneit, B.R.T. & Cardoso, J.N. (1997). Even n-alkane predominances on the Amazon Shelf and a Northeast Pacific Hydrothermal System. Naturwissenchaften, 84, 415-420.
- Ene, E.G., Okogbue, C. O. (2013). Groundwater Dynamics in Gabu-Alifokpa-Osina Barite Fields, South Eastern Benue Trough, Nigeria. Journal of Mining and Geology Vol. 49(1) 2013, Pp 65 -79.
- Fang, J.D., Wu, F. C., Xiong, Y. Q., Li, F.S., Du, X.M., An, D., Wang, L.F. (2014). Source Characterization of Sedimentary Organic Matter Using Molecular and Stable Carbon Isotopic Composition of N-Alkanes and Fatty Acids in Sediment Core from Lake Dianchi, China. Science Total Environment, 473 – 474, 410 – 421.
- Farquhar GD, Ehleringer JR, Hubick KT. (1989) Carbon isotope discrimination and photosynthesis. Annu. Rev. Plant Physiol. 40:503–37
- Ficken, K.J., Li, B., Swain, D. L. &Eglinton, G. (2000). An n-Alkane Proxy for the Sedimentary Input of Submerged/Floating Freshwater Aquatic Macrophytes. Organic Geochemistry 31: 745 – 749.
- Gonza’les-Vila F.J., Povillo, O., Bosky, T., Moura, D., Andres De, J.R. (2003). Biomarker Patterns in a Time Resolved Holocene/Terminal Pleistocene Sedimentary Sequence from the Gurdiana River Estuarine Area (SW Portugal/Spain Border). Organic Geochemistry 34, 1601 – 1613.
- Harmelin-Vivien, M., Le Dereach, L., Bayle-Sempere, J., Charbonnel, E., Garcia-Charton, J.A., Ody, D., Perez-Ruzafa, A., Renones, O., Sanchez-Jerez, P., Valle, C. (2008). Gradients of abundance and biomass across reserve boundaries in six Mediterranean marine protected areas: Evidence of fish spillover. Biological conservation 141, 1829- 1839.
- Huang, C., Yao, L., Zhang, Haung, T., Zhang, M., Zhu, A., Yang, H. (2017). Spatial and Temporal Variation in Autochthonous and Allochthonous Contributors to Increased Organic Carbon and Nitrogen Burial in a Plateau Lake. Science of the Total Environment 604, 390-400.
- Jahnke, L., Embaye, T., Hope, J.E., Turk, K.A., Zuilen, M.V., Des Marais, D.J., Farmer, J.D., Summons, R. E. (2004). Lipid Biomarker and Isotope Signatures for Stromatolite- Forming, Microbial Mat Communities and Pharmidium Cultures from Yellowstone National Park. Geobiology 2, 31 - 47.
- Joeckel, R.M. & Ang Clement, B.J. (2005). Soil, Surficial geology and Geomicrobiology of Saline-Sodic Wetlands, North Platte River Valley, Nebraska, USA Catena 61(1): 63 – 101.
- Kanzari, F., Syakti, A.D., Asia, L., Malleret, L., Mille, G., Jamoussi, B., Abderraba, M., Doumenq, P. (2012). Hydrocarbons, Polycyclic Aromatic Hydrocarbons, Polychlorinated Biphenyls, Organ Chlorine and Organophosphorus Pesticides In: Surface Sediments F from Arc River And The Berre Lagoon, France. Environmental Science and Pollution Research 19: 559 –576.
- Liu, C., Chang, W., Liu, H., 2009. Bioremediation of n-alkanes and the formation of biofloccules by Rhodococcus erythropolis NTU-1 under various saline conditions and sea water. Biochem. Eng. J. 45, 69–75.
- Liu, H. and Liu, W. (2016). n-alkane distributions and concentrations in algae, submerged plants and terrestrial plants from the Qinghai-Tibetan Plateau. Organic Geochemistry 82: 32-41.
- Lozupone, C.A., Knight, R., 2007. Global patterns in bacterial diversity. Proc. Natl. Acad.
- Sci. 104, 11436–11440 USA.
- Meyers P. A., (1994). Preservation of elemental and isotopic source identification sedimentary organic matter. Chemical Geology 114: 289-302.
- Meyers, P. A., Ishiwatari, R. (1995). Organic Matter Accumulation Records in Lake Sediments. In: Physics and Chemistry of Lakes, Lerman, A., Imboden D. M., Gat, J. R. (Eds.), Springer, Berlin PP 279 – 328.
- Meyers, P. A. 1997. Organic Geochemical Proxies of Paleoceanographic, Paleolimnologic and Paleoclimatic Processes. Organic Geochemistry 27: 213 – 250.
- Meyers, P.A. (2003). Applications of Organic Geochemistry to Paleolimnological Reconstructions: A Summary of Examples from Laurentian Great Lakes. Organic Geochemistry 34: 261 – 289.
- O`Donnell, M.J., Ianowski, J.P., Linton, S.M., Rheault, M.R. (2003). Inorganic and Organic Anion Transport by Insect Renal Epithelia. Biochimica Biophysica. Acta 1618 (2): 194 - 206.
- O'Leary, Marion H. (1988). "Carbon Isotopes in Photosynthesis". BioScience. 38 (5): 328–336.
- Oren, A., 2011. Thermodynamic limits to microbial life at high salt concentrations. Environ. Microbiol. 13, 1908–1923.
- Oyo-lta, O.E., Ekpo, B.O., Umana, U.S., & Simoneit, B.R.T. (2006). Predominance of n-docosane/docosene as molecular indicators of microbial and recent biogenic organic matter incorporation into surface sediments of Cross River Estuary, S.E Niger Delta, Nigeria. Global Journal of Environmental Science, 5, 43-48.
- Oyo-Ita, O.E., Ekpo, B.O. & Orosa, D. R. (2010). Distributions and Sources of Aliphatic Hydrocarbons and Ketones in Surface Sediments from The Cross River Estuary, S. E. Niger Delta, Nigeria. Journal of Applied Science in Environmental Sanitation 5:1 – 11.
- Oyo-Ita, I. O., Oyo-Ita, O. E., Dosunmu, M. I.,Dominquez, C., Bayona, J. M., Albaiges, J., 2016. Sources and distribution of petroleum hydrocarbons in recent sediment of the Imo River, SE Nigeria. Arch. Environ.Contam.Toxicol. 70, 372.
- Oyo-ita, O.E., Oyo-ita, I.O., Sam, E.S., Ikip, E.O., Ugim, U.S (2017). Natural and Anthropogenic Biomarkers in Recent Dated Sediment Cores from Refome Lake SE Nigeria: Environmental Implications. Environmental Earth Science 75: 1449 – 1463.
- Pisani, O., Oros, D.H., Oyo-Ita, O.E., Ekpo, B.O., Laffe, R., Simoneit, B.R.T (2013). Biomarkers in Surface Sediments of Cross River and Estuarine System, SE Nigerian Assessment of Organic Matter Sources of Natural and Anthropogenic Origins. Applied Geochemistry 31: 239 – 250.
- Ratheesh-Kumar, C.S., Renjith, K.R., Joseph, M.M., Salas, P.M., Resmi, p., Chandramohanakumar, N. (2019). Inventory of Aliphatic Hydrocarbons in a Tropical Mangrove Estuary: A Biomarker Approach. Environmental Forensics.
- Sigman, D.M., Altabet, M.A., Francois, R., McCorkle D.C., Gaillard, J.F. (1999). The Isotopic Composition of Diatom-Bound Nitrogen in Southern Ocean Sediments. Paleoceanography14, 118 – 134.
- Strauss, J., Schirrmeister, L., Mangelsdorf, K., Eichhorn, L., Wetterich, S.,Herzschuh,
- U., 2015, Organic-matter quality of deep permafrost carbon – a study from Arctic Siberia.Biogeosci.12, 2227.
- Unlu, S., Alpa, B. (2006). Distribution and Sources of Hydrocarbons in Surface Sediments of Gemlike Bay (Marmeara Sea. Turkey). Chemophere 64: 764 – 777.
- Wang, R.L., Brassell, S.C., Scarpitta, S.C., Zheng, M.P., Zhang, S.C., Hayde, P.R., Muench, L.M. (2004). Steroids in Zabuye Salt Lake, Western Tibet: Diagenetic, Ecologic and Climatic Signals. Organic Geochemistry 35, 157-168.
- Wang, B., Yang, J., Jiang, H., Zhang G., Dong H. (2019). Chemical composition of n-alkanes and microbially mediated n-alkane degradation potential differ in the sediments of Qinghai-Tibetan lakes with different salinity. Chemical Geology 524, 37–48.
- Weihai X. Wen Y. Zhong C. (2007). Characteristics of Organic Biomarkers in Surface Sediments from the Northern South China Sea and Their Implications for Gas Hydrate Deposit. Chinese Academy of Sciences, Guangzhou, P 510301.
- Zettler, L.A; Laatsch, A; Keenan, B; Sogin, M (2002). A Molecular survey of eukaryotes from Nebraska`s Alkaline lakes. In: second Astrobiology Science Conference, NASA AMES Research Centre, April 7- 11, 2002.
No competing interests reported.