| 1. |
- Baker, Andrea, et al.
(författare)
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Biomarker records of palaeoenvironmental variations in subtropical Southern Africa since the late Pleistocene : Evidences from a coastal peatland
- 2016
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier. - 0031-0182 .- 1872-616X. ; 451:1, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- Southern Africa's unique global position has given rise to a dynamic climate influenced by large sea surface temperature gradients and seasonal fluctuations in the Inter Tropical Convergence Zone. Due to the semi-arid climate of the region, terrestrial palaeorecords are rare and our understanding of the long-term sensitivity of Southern African terrestrial ecosystems to climatic drivers is ambiguous. A 810 cm continuous peat core was extracted from the Mfabeni peatland with a 14C basal age of c. 47 thousand years calibrated before present (kcal yr BP), positioning it as one of the oldest known sub-tropical coastal peatlands in Southern Africa. This peat core provides an opportunity to investigate palaeoenvironmental changes in subtropical Southern Africa since the late Pleistocene. Biomarker (n-alkane, n-alkanoic acid and n-alkanol) analysis, in conjunction with previously published bulk geochemical data, was employed to reconstruct organic matter (OM) sources, rates of OM remineralisation and peatland hydrology. Our results showed that the principal OM source into the peatland was emergent and terrestrial plants with exception of shallow lake conditions when submerged macrophytes dominated (c. 44.5–42.6, 29.7, 26.1–23.1, 16.7–7.1 and 2.2 kcal yr BP). n-Alkane proxies suggest that local plant assemblages were predominantly influenced by peatland hydrology. By incorporating temperature sensitive n-alkanoic acid and n-alkanol proxies, it was possible to disentangle the local temperature and precipitation changes. We report large variations in precipitation intensities, but subdued temperature fluctuations during the late Pleistocene. The Holocene period was characterised by overall elevated temperatures and precipitation compared to the preceding glacial period, interspersed with a millennial scale cooling event. A close link between the Mfabeni archive and adjacent Indian Ocean marine core records was observed, suggesting the regional ocean surface temperatures to be the dominant climate driver in this region since the late Pleistocene.
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| 2. |
- Baker, Andrea, et al.
(författare)
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Climatic variability in Mfabeni peatlands (South Africa) since the late Pleistocene
- 2017
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Ingår i: Quaternary Science Reviews. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0277-3791 .- 1873-457X. ; 160, s. 57-66
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Tidskriftsartikel (refereegranskat)abstract
- It has been postulated that a bipolar seesaw interhemispheric mechanism dominated the relationship between the Northern and Southern hemisphere climates since the late Pleistocene. A key test for this proposition would be to undertake palaeoenvironmental studies on terrestrial archives in climatically sensitive regions. Southern Africas contemporary C-3 and C-4 terrestrial plant distributions display a definitive geographical pattern dictated by different growing season rainfall and temperature zones; however, the region is generally archive poor due to its overall semi-arid climate and high relief topography. The Mfabeni peatland, with a basal age of c. 47 k yrs calibrated before present (kcal yr BP), is one of the oldest continuous coastal peat deposits in Southern Africa. Molecular leaf wax isotopes (delta C-13(wax)) were generated for a 810 cm long core, and combined with previously published bulk geochemical (delta C-13(bulk), %TOC), palynological, and stratigraphic data, to reconstruct the late Pleistocene and Holocene palaeoenvironments. We interpreted environmental shifts associated with the Heinrich 4, Last Glacial Maximum, deglacial and Holocene periods, which are consistent with adjacent Indian Ocean sea surface temperature records. However, the other shorter climate perturbations during the Heinrich 5, 3, 2, 1, Antarctic cold reversal and Younger Dryas, were muted, most likely due to local hydrological overprinting on the Mfabeni record. A general anti-phase sequence was observed between the Mfabeni record and better established Northern Hemisphere events, underpinning the bipolar seesaw interhemispheric mechanism proposed for global climate forcing since the Late Pleistocene. (C) 2017 Elsevier Ltd. All rights reserved.
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| 3. |
- Baker, Andrea, et al.
(författare)
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n-Alkan-2-one biomarkers as a proxy for palaeoclimate reconstruction in the Mfabeni fen, South Africa
- 2018
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Ingår i: Organic Geochemistry. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0146-6380 .- 1873-5290. ; 120, s. 75-85
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Tidskriftsartikel (refereegranskat)abstract
- The sub-tropical Mfabeni fen is the only continuous coastal peat deposit that documents glacial and interglacial palaeoenvironmental conditions since the late Pleistocene (ca. 47 cal kyr BP) in southern Africa. Published bulk geochemical, biomarker and leaf wax delta C-13 data, along with palynology and stratigraphic studies of the Mfabeni peat sequence, render it an ideal record for testing new palaeoreconstruction proxies. In this study, we aimed to establish the proxy potential of n-alkan-2-one (n-ket) compounds by tracing their source/origin and post-depositional diagenetic change, and if they preserve or not a robust palaeoenvironment signal that complements our understanding of palaeoclimatic variations. In the Mfabeni archive the most likely source for n-kets is via microbial decarboxylation of n + 1-alkanoic acids (n-FAs) and, to a lesser degree, oxidation of same chain length n-alkanes (n-alks). The n-ket average chain length (ACL(ket)) and n-C-23 and C(25)ket/precursor ratios displayed a statistical significant negative relationship with the n-alk aquatic plant proxy (P-aq), suggesting the source of n-kets to be submerged aquatic plants during waterlogged conditions that suppressed microbial activity during the ensuing anoxic conditions. Both the mid-chain and long chain n-ket/precursor ratios displayed predominant water level fluctuation controls, with temperature as a secondary regulator. By comparing the n-ket data with published environmental and climate reconstructions from the same core, and with geomorphology and palynological studies of the Mfabeni basin, we conclude that the n-kets show promise as a palaeoclimate proxy and can be used in conjunction with other biomarker proxies to reconstruct ancient hydrological changes in sub-tropical peatlands. (C) 2018 Elsevier Ltd. All rights reserved.
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| 4. |
- Baskar, Sushmitha, et al.
(författare)
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Evidences for Microbial Precipitation of Calcite in Speleothems from Krem Syndai in Jaintia Hills, Meghalaya, India
- 2016
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Ingår i: Geomicrobiology Journal. - : TAYLOR & FRANCIS INC. - 0149-0451 .- 1521-0529. ; 33:10, s. 906-933
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Forskningsöversikt (refereegranskat)abstract
- Speleothems from Krem Syndai, Meghalaya in Northeast India were studied for their microbial diversity using 16S rDNA-based phylogenetic approach and conventional microbiological techniques along with geochemistry, mineralogy and in vitro experiments to understand participation of microorganisms in CaCO3 precipitation. Speleothems imaged by scanning electron microscopy showed round coccoid-like, sporangia-like and spinose calcified structures, numerous broken cocci shells with spotted interiors inside a calcite crystal, honeycomb long reticulate, smooth, flat, twisted, ribbon-like, tubular, beaded, microbe-mineralized filaments and extracellular polymeric substances (EPS). Fourier spectroscopy indicated the presence of various organic compounds. C-13 and O-18 isotopic ratios of speleothems ranged from -4.65 to -7.34 parts per thousand and -3.06 to -6.80 parts per thousand, respectively. Total number of microbial cells using SYBR Gold was high. Fluorescence in situ hybridization (FISH) indicated approximately 3x 10(5) to 5x 10(5) cells g sed(-1) in the speleothems out of which the number of microbes belonging to Eubacteria ranged from 1.8x 10(5) to 3.6x 10(5) cells, g sed(-1). FISH showed approximate to 45% active microbial cells of the total cell number in samples. DNA-based high-throughput amplicon sequencing revealed 19 bacterial phyla in the speleothem. Approximately 42% of the sequences were similar to Proteobacteria (Alphaproteobacteria: 22.4%, Betaproteobacteria: 8.9%, Gammaproteobacteria: 8.6%). Sequences similar to Nitrospiraceae (22.8%) had the highest proportion of sequences belonging to a single family. Bacterial strains isolated from the speleothems raised alkalinity and precipitated calcite in the laboratory cultures which was confirmed by X-ray diffraction (XRD) analyses. These isolates belonged to Bacillus spp., Actinomycetes spp., Streptomyces spp., Pseudomonas spp., Micrococcus spp., Staphylococcus spp., Xanthobacter spp. and Arthrobacter spp. Overall, the results showed unequivocal evidence of bacterial fingerprints during CaCO3 precipitation in the cave.
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| 5. |
- Gayantha, Kasun, et al.
(författare)
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A multi-proxy reconstruction of the late Holocene climate evolution in Lake Bolgoda, Sri Lanka
- 2017
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : ELSEVIER SCIENCE BV. - 0031-0182 .- 1872-616X. ; 473, s. 16-25
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Tidskriftsartikel (refereegranskat)abstract
- Palaeoclimate investigations in Sri Lanka have been rarely attempted despite being located directly in the path of the Inter Tropical Convergence Zone. In this study, a 4.1-m undisturbed sediment core was retrieved from the Bolgoda Lake situated in the western coast of Sri Lanka, and influenced by the strong southwest monsoons. Mollusc shells in the core were dated, and the age-depth model indicated a depositional history extending from 2941 cal yr BP to the present. Grain size, major and trace elements, total organic C and N content and stable C and N isotopes were analysed in freeze-dried sediments to reconstruct the palaeoclimate changes. The multi proxy records in the core revealed four distinct zones that show distinct variations in physical and chemical conditions in the lake associated with climate change. Zone 1 (2941 to 2390 cal yr BP; 385-252 cm) indicated the climate to be warm and humid with intense precipitation. The resulting high lake level helped in organic matter preservation in bottom sediments. Zone 2 (2390 to 1782 cal yr BP; 252-140 cm) indicated an unstable dry period associated with weak precipitation. Consequently, low lake level and intense degradation of organic matter occurred in this zone. Zone 3 (1782 to 1299 cal yr BP; 140-60 cm) indicated a resurgence of intense monsoon along with warm and humid conditions. Zone 4 (1299 cal yr BP to present; 60-0 cm) indicated dry conditions with less intense monsoon, low lake level and extensive degradation of organic matter. Vascular plants were the predominant organic matter source into the lake during the late Holocene. In contrast, algal input was significant between 2390 cal yr BP and 2153 cal yr BP. The palaeoclimate evidences in this study showed an overall weakening trend of SW monsoon during the late Holocene, and this was consistent with changes happening in other locations as in southern and western India. (C) 2017 Elsevier B.V. All rights reserved.
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| 6. |
- Ghosh, Devanita, et al.
(författare)
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Characterization and microbial utilization of dissolved lipid organic fraction in arsenic impacted aquifers (India)
- 2015
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 527, s. 221-233
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Tidskriftsartikel (refereegranskat)abstract
- The coupled role of organic matter (OM) and microbial activity is widely acknowledged in arsenic (As) biogeochemical cycling in sedimentary environments. However, little is known about OM characteristics particularly the dissolved fraction in the Bengal Delta Plain aquifers – one of the worst As impacted regions in the world. Ongoing investigations in As-rich aquifers in Nadia district (West Bengal, India) indicate presence of arsenite As(III) oxidizing bacterial communities in the Grey Sand Aquifers (GSA), but absent in Brown Sand Aquifers (BSA). In this study, we investigate the key differences in dissolved organic carbon (DOC) characteristics and its relationship with differences in elemental concentrations, distribution of biomarkers, and utilization of DOC by in situ microbial communities in BSA and GSA. We demonstrate a new approach using ENVI™ C-18 DSK discs to pre-concentrate DOC from large volumes of water, and further extract the OM and separate it into different lipid fractions using the solid phase extraction technique. The aquifers show marked heterogeneity in terms of their DOC characteristics and elemental profiles irrespective of their grey or brown color. DOC indicates variable inputs of terrestrial derived OM sources, and OM derived from decomposition and/or microbial cellular components. DOC in the aquifers consist of predominantly n-alkanoic acids (∌80%) followed by n-alkanes and n-alcohols. The GSAs indicate high iron (Fe) and manganese (Mn) concentrations, and presence of mature petroleum derived hydrocarbons in DOC. BSA has comparatively lower concentrations of Fe and Mn, and shows absence of mature hydrocarbons in DOC. Experiments in presence of indigenous bacteria from groundwater with DOC lipid extracts as the sole carbon source indicate higher growth in the GSA samples implying preferential use of DOC. The potential availability of DOC in these aquifers can influence the community composition of indigenous heterotrophic microbial flora, which in turn can affect elemental cycles including that of As.
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| 7. |
- Ghosh, Devanita, 1990-, et al.
(författare)
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Coping with arsenic stress : Adaptations of arsenite-oxidizing bacterial membrane lipids to increasing arsenic levels
- 2018
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Ingår i: MicrobiologyOpen. - : John Wiley & Sons. - 2045-8827. ; 7:5
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Tidskriftsartikel (refereegranskat)abstract
- Abstract Elevated levels of arsenic (As) in aquifers of South East Asia have caused diverse health problems affecting millions of people who drink As-rich groundwater and consume various contaminated agriculture products. The biogeochemical cycling and mobilization/immobilization of As from its mineral-bound phase is controlled by pH, oxic/anoxic conditions, and different microbial processes. The increased As flux generated from ongoing biogeochemical processes in the subsurface in turn affects the in situ microbial communities. This study analyzes how the indigenous arsenite-oxidizing bacteria combat As stress by various biophysical alterations and self-adaptation mechanisms. Fifteen arsenite-oxidizing bacterial strains were isolated and identified using a polyphasic approach. The bacterial strains isolated from these aquifers belong predominantly to arsenite-oxidizing bacterial groups. Of these, the membrane-bound phospholipid fatty acids (PLFA) were characterized in seven selected bacterial isolates grown at different concentrations of As(III) in the medium. One of the significant findings of this study is how the increase in external stress can induce alteration of membrane PLFAs. The change in fatty acid saturation and alteration of their steric conformation suggests alteration of membrane fluidity due to change in As-related stress. However, different bacterial groups can have different degrees of alteration that can affect sustainability in As-rich aquifers of the Bengal Delta Plain.
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| 8. |
- Ghosh, Devanita
(författare)
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Distribution and Biogeochemical Cycling of Arsenic In Grey and Brown Sand Aquifers in the Bengal Delta Plains (India)
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An elevated level of Arsenic (As) in aquifers from India and Bangladesh affecting the human health has been widely reported since the late 1980s. The thesis aim is to investigate the present status of As contamination and biogeochemical cycling with direct role of diverse indigenous bacterial communities in As cycling in the Bengal Delta Plain (BDP) aquifers in Nadia district, West Bengal (India). The As(III) oxidizing bacterial communities were predominant in grey sand aquifers (GSA), but were characteristically absent in brown sand aquifers (BSA). Rainwater recharge containing inorganic and organic dissolved compounds played an important role in shaping the different groups of bacterial phenotypes. It included thearsenite-oxidizing bacteria as revealed by the aioA and 16S rRNA phylogeny. These bacterial communities in BDP groundwater were assumed to utilize the dissolved and sedimentary organic carbon (DOC and SOC) as the primary carbon source for respiration, and remobilization/immobilization of As involving reductive dissolution of iron oxyhydroxides. Hence, sediment and groundwater of these aquifer waters were characterized for their different inorganic constituents (metals) and organic compound classes. There were notable differences between the groundwater DOC and SOC pools. The only similarity between these carbon pools is presence of petroleum-derived hydrocarbons. The SOC in BSA has good correlation with the clay and silt-rich fraction. Notably, As formed complexes with iron, but not manganese. Biomarker characterization in sediments showed presence of terrigenous inputs along with petroleum-derived hydrocarbons. However, these hydrocarbons were absent in BSA sediments, and so were the arsenite oxidizing bacterial communities. Although DOC in groundwater plays an important role in sustaining the microorganisms, the contrasting character of SOC in BSA and GSA strongly influence the shaping of microbial community structure and biogeochemical cycling of As. This particularly affects the natural ‘safe’ drinking water capacity. Overall, the study gives a new directionfor long-term research on As biogeochemical cycling in the contaminated BDP aquifers.
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| 9. |
- Ghosh, Devanita, et al.
(författare)
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Elemental and biomarker characteristics in a Pleistocene aquifer vulnerable to arsenic contamination in the Bengal Delta Plain, India
- 2015
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Ingår i: Applied Geochemistry. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0883-2927 .- 1872-9134. ; 61, s. 87-98
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Tidskriftsartikel (refereegranskat)abstract
- An elevated level of arsenic (As) in the Indo-Gangetic delta plain aquifers has been reported since the 1990s. Organic matter (OM) present in groundwater and aquifer sediments supports the microbial communities in these aquifers. During installation of a drinking water well, 26 sediment intervals of 6 m each were retrieved up till 156 m from a Pleistocene brown sand aquifer (BSA). Grain size distribution, sequential extraction of metals and total extractable lipids were analyzed in each sample. These parameters were statistically correlated in order to establish relationship between the physical vs. inorganic and organic characteristics, and how these properties affected the distribution of As in BSAs. The aquifer sediments consisted of medium to coarse sand except the surface sediments and those at the bottom of the well, which had high clay and slit content. Arsenic (As) concentration in sediments ranged from 2 to 21 mg/kg and indicated a strong correlation with grain size. Arsenic was mostly associated with crystalline oxides and silicate-rich minerals. Arsenic showed significant correlation with Fe in all fractions, and suggests presence of pyrite bound As-bearing minerals in these sediments. The diagnostic sedimentary lipid biomarkers indicated presence of compounds derived from vascular plants and microbial cell wall. This inference was supported by various diagnostic lipid ratios. The biomarkers were abundant in surface and deeper layers, which had high clay and silt content. The BSA sediments indicated preferential preservation of n-alkanes over other functional compounds, which were more reactive and subject to degradation. The thick clay layer at 132-156 m contained visible plant fragments, and OM in this layer indicated preferential preservation of organic carbon most likely due to the absence of specific microbial communities that degraded these compounds and mobilized As. Statistical analyses indicated the influence of selective inorganic and organic components (As, Fe and fatty acids) controlling the co-distribution of various inorganic and organic components in the aquifer.
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| 10. |
- Ghosh, Devanita, 1990-, et al.
(författare)
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Sub-surface Biogeochemical Characteristics and Its Effect on Arsenic Cycling in the Holocene Gray Sand Aquifers of the Lower Bengal Basin
- 2017
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Ingår i: Frontiers in Environmental Science. - : Progressive Frontiers Press. - 2296-665X. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- High arsenic (As) content in the fertile delta plains of West Bengal has been widely reported since the 1990s. The shallow grey sand aquifers (GSA) deposited during the Holocene, are more commonly used as potable water sources, but they have high As levels. The release of As into groundwater is influenced by indigenous microbial communities metabolizing different organic carbon sources present in the GSA sediments. After pre-screening the groundwater for assessing their microbial phylogenetic diversity, two50-m deep boreholes were drilled in the GSAs, and 19 sediment samples were recovered from each core. In each of these samples, grain-size distribution, sequential extraction, and quantification of trace metals and total extractable lipids were analyzed. The aquifer sediments consisted of medium to fine micaceous sand with clay lenses in between them; a thick clay layer occurred on top of both boreholes. Arsenic concentration in these sediments varied from 1.80 to 41.0 mg/kg and was mostly associated with the oxide and silicate-rich crystalline minerals. Arsenic showed a significant correlation with Fe in all fractions, suggesting the presence of Fe-(oxy)-hydroxides bound As minerals. The diagnostic lipid biomarkers showed presence of compounds derived from higher plants (epicuticular waxes) and microbial inputs. The biomarkers were abundant in clay and silt-rich layers. The samples indicated preferential preservation of n-alkanes over other functional compounds (e.g. alcohols and fatty acids), that are more reactive, and hence subject to further degradation. Sediments recovered from the borehole indicated the presence of Eustigmatophytes and vascular plant waxes that are mostly surface-derived. The sedimentary lipids also indicated the presence of complex petroleum-derived hydrocarbons. These compounds provide organic substrates, and support the preferential survival of specific microbial communities in these sediments.
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