Total Carbon Analysis Research Articles

Water‐soluble organic carbon, dicarboxylic acids, ketoacids, and α‐dicarbonyls in the tropical Indian aerosols

Tropical aerosol (PM10) samples (n = 49) collected from southeast coast of India were studied for water‐soluble dicarboxylic acids (C2–C12), ketocarboxylic acids (C2–C9), and α‐dicarbonyls (glyoxal and methylglyoxal), together with analyses of total carbon (TC) and water‐soluble organic carbon (WSOC). Their distributions were characterized by a predominance of oxalic acid followed by terephthalic (t‐Ph), malonic, and succinic acids. Total concentrations of diacids (227–1030 ng m−3), ketoacids (16–105 ng m−3), and dicarbonyls (4–23 ng m−3) are comparative to those from other Asian megacities such as Tokyo and Hong Kong. t‐Ph acid was found as the second most abundant diacid in the Chennai aerosols. This feature has not been reported previously in atmospheric aerosols. t‐Ph acid is most likely derived from the field burning of plastics. Water‐soluble diacids were found to contribute 0.4%–3% of TC and 4%–11% of WSOC. Based on molecular distributions and backward air mass trajectories, we found that diacids and related compounds in coastal South Indian aerosols are influenced by South Asian and Indian Ocean monsoons. Organic aerosols are also suggested to be significantly transported long distances from North India and the Middle East in early winter and from Southeast Asia in late winter, but some originate from photochemical reactions over the Bay of Bengal. In contrast, the Arabian Sea, Indian Ocean, and South Indian continent are suggested as major source regions in summer. We also found daytime maxima of most diacids, except for C9 and t‐Ph acids, which showed nighttime maxima in summer. Emissions from marine and terrestrial plants, combined with land/sea breezes and in situ photochemical oxidation, are suggested especially in summer as an important factor that controls the composition of water‐soluble organic aerosols over the southeast coast of India. Regional emissions from anthropogenic sources are also important in megacity Chennai, but their influence is weakened due to the dispersion caused by dynamic land/sea breeze on the coast.

Total carbon analysis may overestimate organic carbon content of fresh waters in the presence of high dissolved inorganic carbon

Automated carbon analyzers often are configured to provide estimates of both total organic carbon (TOC) and nonpurgeable organic carbon (NPOC). We show there can be an overestimation of total carbon in the presence of moderate to large quantities of dissolved inorganic carbon. This leads to overestimates of TOC, which is measured as the difference between total carbon and inorganic carbon. Water samples were analyzed as both TOC and NPOC on a Shimadzu TC 5050 Carbon Analyzer. The difference between TOC and NPOC increased as a function of concentrations of dissolved inorganic carbon (DIC). Water samples spiked with DIC ranging from 0 to 100 mg DIC/L also reported increased TOC as large as 8 mg C/L. Our data suggest that the Shimadzu 5050 analyzer (and by analogy other instruments that estimate TOC by difference between TC and IC) overestimates total carbon (TC) when calibrated with an organic standard as recommended by the manufacturer. The magnitude of the overestimation varies both with the amount of DIC present in the sample and the extent to which measurement efficiency of the analyzer is less than 100%. The consequences will be most severe in analysis of samples from systems spanning a large range in DIC. Time series from individual systems are less likely to be affected because the necessary large change in DIC would be detected as changes in pH or other attributes well before any change in DOC. Systems with high DIC will, however, be susceptible to even small variations in measurement efficiency.

AZO-DYE ORANGE II DEGRADATION IN PLASMA TORCH WITH GERDIEN ARC

There is increasing interest for the degradation of organic compounds in contaminated and polluted waters. This paper deals with utilization of Gerdien arc for this purpose. The arc is generated in a plasma torch and is hybridly stabilized by water and argon. Gerdien arc features very high plasma temperatures and thus high intensity of short wave ultraviolet radiation. Water (for the arc stabilization) flows at the flow rate of 20 l.min−1 in a thin layer around the arc column. Thus, all water flowing through the torch passes the positions with high level of ultraviolet radiation. Azo-dye Orange II was used as a model organic compound. The certain amount of the dye was dissolved in a 40 l water system tank before every experiment. Degradation of the dye took place with high efficiency so that the fast decrease of a dye concentration during first minutes of experiments was observed. Measurements were done by means of UV/Vis spectroscopy and analyses of total carbon, total organic carbon, and total inorganic carbon.

The removal of phenol from aqueous solutions by adsorption using surfactant-modified bentonite and kaolinite

The natural bentonite (BC) and kaolinite (KC) were modified with two surfactant of hexadecyltrimethylammonium bromide (HDTMA) and phenyltrimethylammonium bromide (PTMA) to form four kinds of organic-modified clays, i.e., HDTMA–bentonite (BHM), HDTMA–kaolinite (KHM), PTMA–bentonite (KPM) and PTMA–kaolinite (KPM). The modified minerals were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD) and FT-IR spectroscopy. The surface areas were determined using methylene blue adsorption method. Cation-exchange capacity (CEC) was estimated using an ethylenediamine complex of copper method and the modifier loading was calculated from the total carbon analysis. The ability of raw and organo-modified clays to remove phenol from aqueous solutions has been carried out as a function of contact time, pH and temperatures using a batch technique. The removal of phenol from aqueous solutions by modified clays seems to be more effective than unmodified samples. The adsorption capacity was found to increase with increasing temperature indication that the adsorptions were endothermic. The adsorption of phenol onto these clays was found to be increased by increasing of pH value and the adsorption patterns data are correlated well by Langmuir and Freundlich isotherm models and that the adsorption is physical in nature. The experimental data fitted very well with the pseudo-second-order kinetic model. The thermodynamic study of adsorption process showed that the adsorption of phenol with these six adsorbents was carried out spontaneously, and the process was endothermic in nature.

14C, δ 13C and total C content in soils around a Brazilian PWR nuclear power plant

Nuclear power plants release 14C during routine operation mainly as airborne gaseous effluents. Because of the long half-life (5730 years) and biological importance of this radionuclide (it is incorporated in plant tissue by photosynthesis), several countries have monitoring programs in order to quantify and control these emissions. This paper compares the activity of 14C in soils taken within 1 km from a Brazilian nuclear power plant with soils taken within a reference area located 50 km away from the reactor site. Analyses of total carbon, δ 13C and 137Cs were also performed in order to understand the local soil dynamics. Except for one of the profiles, the isotopic composition of soil organic carbon reflected the actual forest vegetation present in both areas. The 137Cs data show that the soils from the base of hills are probably allocthonous. The 14C measurements showed that there is no accumulation due to the operation of the nuclear facility, although excess 14C was found in the litter taken in the area close to power plant. This indicates that the anthropogenic signal observed in the litter fall has not been transferred yet to the soil. This study is part of an extensive research programme in which other samples including air, vegetation and gaseous effluents (taken in the vent stack of the Brazilian nuclear power reactors Angra I and II) were also analyzed. The present paper aimed to evaluate how 14C emissions from the nuclear power plant are transferred and stored by soils present in the surroundings of the reactor site. This is the first study concerning anthropogenic 14C in soils in Brazil.

Catalysts and inhibitors for MEA oxidation

Aqueous monoethanolamine (MEA) was subjected to oxidation by O2/CO2 at 55 ∘C. Hydroxyethyl-formamide (HEF) and hydroxyethylimidazole (HEI) are the major oxidation products of MEA. Dissolved metals catalyze oxidation in the order copper > chromium/nickel > iron > vanadium. Inhibitors A, B and ethylenediaminetetracetic acid (EDTA) are effective degradation inhibitors. The addition of the expected inhibitors formaldehyde, formate or sodium sulfite had unintended effects on MEA losses. Total carbon and nitrogen analysis shows a greater than 90% closure of the material balance.

Younger Dryas to mid-Holocene environmental history of the lowlands of NW Transylvania, Romania

Pollen, micro-charcoal and total carbon analyses on sediments from the Turbuta palaeolake, in the Transylvanian Basin of NW Romania, reveal Younger Dryas to mid-Holocene environmental changes. The chronostratigraphy relies on AMS 14C measurements on organic matter and U/Th TIMS datings of snail shells. Results indicate the presence of Pinus and Betula open woodlands with small populations of Picea, Ulmus, Alnus and Salix before 12,000 cal yr BP. A fairly abrupt replacement of Pinus and Betula by Ulmus-dominated woodlands at ca. 11,900 cal. yr BP likely represents competition effects of vegetation driven by climate warming at the onset of the Holocene. By 11,000 cal yr BP, the woodlands were increasingly diverse and dense with the expansion of Quercus, Fraxinus and Tilia, the establishment of Corylus and the decline of upland herbaceous and shrubs taxa. The marked expansion of Quercus accompanied by Tilia between 10,500 and 8000 cal yr BP could be the result of low effective moisture associated with both low elevation of the site and with regional change towards a drier climate. At 10,000 cal yr BP, Corylus spread across the region, and by 8000 cal yr BP it replaced Quercus as a dominant forest constituent, with only little representation of Picea abies. Carpinus became established around 5500 cal yr BP, but it was only a minor constituent in local woodlands until ca. 5000 cal yr BP. Results from this study also indicate that the woodlands in the lowlands of Turbuta were never closed.

Indirect Effects of Antibiotics in the Aquatic Environment: A Laboratory Study on Detritivore Food Selection Behavior

ABSTRACT With regard to possible detrimental effects of human and veterinary antibiotics in the aquatic environment, most research in this field assesses direct impacts of pharmaceuticals on vertebrate or invertebrate test organisms. Another related area of concern is the possible development of antibiotic-resistant bacteria by introducing antimicrobials into the aquatic compartment. However, indirect effects of antibacterials on the trophic cascade have rarely been investigated. This study contributes with an example of how indirect effects of antibiotics on leaf litter decay can be measured and to what extent shredder organisms might be affected. Results from food-selection experiments using Gammarus pulex (Amphipoda) demonstrated clear preferences for leaves conditioned in the absence versus those conditioned in the presence of two antibiotics, oxytetracycline and sulfadiazine. Although this result suggested that microbial and fungal colonisation during leaf litter conditioning might be adversely affected in the antibiotic-treated groups, analyses of total carbon and nitrogen content of conditioned leaf discs did not reveal differences among the treatments.

Seasonal characteristics of the physicochemical properties of North Atlantic marine atmospheric aerosols

Seasonal physicochemical characteristics of North Atlantic marine aerosols are presented for the period from January 2002 to June 2004. The aerosol size distribution modal diameters show seasonal variations, 0.031 μm in winter and 0.049 μm in summer for the Aitken mode and 0.103 μm in winter and 0.177 μm in summer for the accumulation mode. The accumulation mode mass also showed a seasonal variation, minimum in winter and maximum in summer. A supermicron sized particle mode was found at 2 μm for all seasons showing 30% higher mass concentration during winter than summer resulting from higher wind speed conditions. Chemical analysis showed that the concentration of sea salt has a seasonal pattern, minimum in summer and maximum in winter because of a dependency of sea‐salt load on wind speeds. By contrast, the non‐sea‐salt (nss) sulphate concentration in fine mode particles exhibited lower values during winter and higher values during midsummer. The water soluble organic carbon (WSOC) and total carbon (TC) analysis also showed a distinctive seasonal pattern. The WSOC concentration during the high biological activity period peaked at 0.2 μgC m−3, while it was lower than 0.05 μgC m−3 during the low biological activity period. The aerosol light scattering coefficient showed a minimum value of 5.5 Mm−1 in August and a maximum of 21 Mm−1 in February. This seasonal variation was due to the higher contribution of sea salt in the MBL during North Atlantic winter. By contrast, aerosols during late spring and summer exhibited larger angstrom parameters than winter, indicating a large contribution of the biogenically driven fine or accumulation modes. Seasonal characteristics of North Atlantic marine aerosols suggest an important link between marine aerosols and biological activity through primary production of marine aerosols.

Biodegradation of dibromoneopentyl glycol by a bacterial consortium

Dibromoneopentyl glycol (DBNPG) is a brominated flame retardant that is used as an additive during the manufacture of plastic polymers and as a chemical intermediate for other flame retardants. It is classified as not readily biodegradable and based on experimental studies in animals is believed to be a carcinogen. We have demonstrated, to the best of our knowledge for the first time, the complete biodegradation of DBNPG under aerobic conditions. Total organic carbon (TOC) analysis indicates the complete mineralization of DBNPG. DBNPG biodegradation was accompanied by the release of bromide into the medium, probably due to a biological debromination reaction by bacterial consortia. A denaturing gradient gel electrophoresis (DGGE) analysis of PCR amplified 16S rRNA gene was used, to characterize the bacterial consortia involved in DBNPG biodegradation. At least seven bacterial species were found to be involved in this process, among them species with similarity to strains that are known for their dehalogenating ability.

Catalytic sterilization of Escherichia coli K 12 on Ag/Al 2O 3 surface

Bactericidal action of Al 2O 3, Ag/Al 2O 3 and AgCl/Al 2O 3 on pure culture of Escherichia coli K 12 was studied. Ag/Al 2O 3 and AgCl/Al 2O 3 demonstrated a stronger bactericidal activity than Al 2O 3. The colony-forming ability of E. coli was completely lost in 0.5 min on both of Ag/Al 2O 3 and AgCl/Al 2O 3 at room temperature in air. The configuration of the bacteria on the catalyst surface was observed using scanning electron microscopy (SEM). Reactive oxygen species (ROS) play an important role in the expression of the bactericidal activity on the surface of catalysts by assay with O 2/N 2 bubbling and scavenger for ROS. Furthermore, the formation of CO 2 as an oxidation product could be detected by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and be deduced by total carbon analysis. These results strongly support that the bactericidal process on the surface of Ag/Al 2O 3 and AgCl/Al 2O 3 was caused by the catalytic oxidation.

Soil carbon sequestration under different exotic tree species in the southwestern highlands of Ethiopia

At Belete forest in southwestern Ethiopia (7°33′N, 36°35′E), tree plantations were established on abandoned farmland, which was previously mainly used for maize cultivation. Total carbon and 13C analyses were used to evaluate the changes in soil organic carbon (SOC) pools associated with land use changes using a comparative approach. Forest clearing followed by continuous cultivation of crops caused a loss of 43% (75.4 Mg ha − 1 ) total SOC and 73% (128.4 Mg ha − 1 ) forest derived SOC after nearly 75 years. The net loss of SOC was lower because of addition of 53.0 Mg ha − 1 of SOC of C 4 crop origin (mainly maize) to the farmland. On the other hand, afforestation of farmland led to a net accretion of SOC of 69.6 and 29.3 Mg ha − 1 after 20 years under Cupressus lusitanica and Pinus patula stands, respectively . The SOC accretion of plantation origin amounted to 63.3 Mg ha − 1 under C. lusitanica and 24.2 Mg ha − 1 under P. patula. Contrary to the results obtained in some other studies, the SOC of C 4 origin did not decline in these stands. This could be attributed to pasture grasses of C 4 origin that took over after land abandonment and continued to grow under the tree canopies. The grasses could thus have compensated for the SOC loss. SOC might also have been close to a steady state under the pre-plantation period. Based on the SOC amount found in a reference stand of native forest, afforestation with Eucalyptus grandis during 20 years, preceded by 20 years of cultivation and 35 years of pasture, returned the total SOC to nearly pre-deforestation levels. SOC accumulation rates of 1–3.2 Mg ha − 1 y − 1 are apparently possible 20 years after afforestation of an abandoned farmland but the accumulation rate is species dependent.

Evaluating sedimentary geochemical lake-level tracers in Walker Lake, Nevada, over the last 200 years

Walker Lake, a hydrologically closed, saline, alkaline lake located along the western margin of the Great Basin of western United States, has experienced a 77% reduction in volume and commitment drop in lake level as a result of anthropogenic perturbations and climatic fluctuations over the last century. The history of lake-level change in Walker Lake has been recorded instrumentally since 1860. A high-resolution multi-proxy sediment core record from Walker Lake has been generated through analysis of total inorganic carbon (TIC), total organic carbon (TOC), and oxygen and carbon isotope ratios (δ18O and δ13 C) of both downcore bulk TIC and ostracods over the last 200 yr. This allows us to examine how these sediment indices respond to actual changes in this lake’s hydrologic balance at interannual to decadal timescales. In Walker Lake sediments, changes in %TIC, %TOC, and δ13C and δ18O of TIC and ostracods are all associated to varying degrees with changes in the lake’s hydrologic balance, with δ18O of the TIC fraction (δ18OTIC) being the most highly correlated and the most effective hydrologic indicator in this closed-basin lake. The δ18OTIC record from Walker Lake nearly parallels the instrumental lake-level record back to 1860. However, comparison with sporadic lake-water δ18O and dissolved inorganic carbon δ13C (δ13CDIC) results spanning the last several decades suggests that the isotopic values of downcore carbonate sediments may not be readily translated into absolute or even relative values of corresponding lake-water δ18O and δ13CDIC. Changes in the lake’s hydrologic balance usually lead to changes in isotopic composition of lake waters and downcore sediments, but not all the variations in downcore isotopic composition are necessarily caused by hydrologic changes.

English

Twelve specimens of intrusive rocks, ranging from granitic to ultramafic, were ground and subjected to the static-test neutralization-potential (NP) protocol so that the results could be compared with those computed by using the NP values previously obtained by Sobek tests of the constituent minerals. The quantitative mineralogy of the rocks was determined by Rietveld refinements of X-ray powder diffraction data, and was supplemented by optical microscopy, fizz tests, and analyses of total carbon to determine the presence of carbonate minerals. Despite the igneous nature of the suite, most samples were found to be carbonate-bearing; optical microscopy and fizz tests of the coarse (minus 6 mm) fractions were observed to be more sensitive to the presence of carbonates than was the minus 60-mesh fraction that is used in the Sobek protocol. For some minerals, notably olivine and serpentine, the acid-digestion period in the Sobek test has a pronounced effect on the resulting NP, and this part of the test protocol is in need of new standardization. Mineralogical prediction of the NP values is sensitive to the composition of the plagioclase because these feldspars are typically a major component of igneous rocks and the NP of the calcic end- member, anorthite, is about 12 times that of the sodic end-member, albite.

Sampling and analysis of geothermal fluids

AbstractSampling of geothermal fluids presents some problems not encountered when sampling surface and nonthermal ground waters. Specific collection techniques are required to obtain representative samples because of the elevated temperature and boiling of these fluids, the effect of exposing them to the atmosphere and cooling of the samples. Sample treatment during collection depends on the analytical method to be used. When sampling wet‐steam wells, both the liquid and the vapour fractions should be collected at the same fluid separation pressure. When sampling fumarole steam, maximum information is obtained if the total discharge is collected into a single container without separating the gas and the steam condensate fractions. Silica polymerization affects the solution pH. The only way to obtain reliable pH measurement of a water sample supersaturated with respect to amorphous silica is to measure it on site, before the onset of polymerization. This paper provides an outline of the geothermal sampling techniques and analytical methods currently in use in Iceland. Sampling of hot‐water and wet‐steam wells is described, as is sampling of hot springs, fumaroles and gas bubbling through hot‐spring waters. Detailed procedures are given for the analysis of total carbonate carbon and total sulphide sulphur in geothermal water and steam condensate samples.

Aggregate and soil organic carbon dynamics in South Chilean Andisols

Abstract. Extreme sensitivity of soil organic carbon (SOC) to climate and land use change warrants further research in different terrestrial ecosystems. The aim of this study was to investigate the link between aggregate and SOC dynamics in a chronosequence of three different land uses of a south Chilean Andisol: a second growth Nothofagus obliqua forest (SGFOR), a grassland (GRASS) and a Pinus radiata plantation (PINUS). Total carbon content of the 0-10cm soil layer was higher for GRASS (6.7 kg C m-2) than for PINUS (4.3 kg C m-2, while TC content of SGFOR (5.8 kg C m-2) was not significantly different from either one. High extractable oxalate and pyrophosphate Al concentrations (varying from 20.3-24.4 g kg-1, and 3.9-11.1 g kg-1, respectively) were found in all sites. In this study, SOC and aggregate dynamics were studied using size and density fractionation experiments of the SOC, δ13C and total carbon analysis of the different SOC fractions, and C mineralization experiments. The results showed that electrostatic sorption between and among amorphous Al components and clay minerals is mainly responsible for the formation of metal-humus-clay complexes and the stabilization of soil aggregates. The process of ligand exchange between SOC and Al would be of minor importance resulting in the absence of aggregate hierarchy in this soil type. Whole soil C mineralization rate constants were highest for SGFOR and PINUS, followed by GRASS (respectively 0.495, 0.266 and 0.196 g CO2-Cm-2d-1 for the top soil layer). In contrast, incubation experiments of isolated macro organic matter fractions gave opposite results, showing that the recalcitrance of the SOC decreased in another order: PINUS>SGFOR>GRASS. We deduced that electrostatic sorption processes and physical protection of SOC in soil aggregates were the main processes determining SOC stabilization. As a result, high aggregate carbon concentrations, varying from 148 till 48 g kg-1, were encountered for all land use sites. Al availability and electrostatic charges are dependent on pH, resulting in an important influence of soil pH on aggregate stability. Recalcitrance of the SOC did not appear to largely affect SOC stabilization. Statistical correlations between extractable amorphous Al contents, aggregate stability and C mineralization rate constants were encountered, supporting this hypothesis. Land use changes affected SOC dynamics and aggregate stability by modifying soil pH (and thus electrostatic charges and available Al content), root SOC input and management practices (such as ploughing and accompanying drying of the soil).

Effects of ambient versus reduced UV‐B radiation on high arctic Salix arctica assessed by measurements and calculations of chlorophyll a fluorescence parameters from fluorescence transients

A UV‐B exclusion‐experiment was conducted in the high Arctic Zackenberg, NE Greenland, in which Salix arctica leaves during most of the growing season were fixed perpendicular to the solar zenith angle, thereby receiving maximal solar radiation. Covered with Teflon and Mylar foil, the leaves received approximately 90 and 40% of the ambient UV‐B irradiance, respectively. The effects were examined through recordings of chlorophyll a fluorescence transients, determination of biomass and analysis of total carbon and nitrogen content and amount of soluble flavonoids in the leaves. The processing of light was analysed by means of the chlorophyll a fluorescence transient, using the so‐called JIP test, as evolved by Reto J. Strasser and his coworkers. Reduction of the UV‐B irradiance caused a rise in many of the fluorescence parameters during July, but not in August (late season). Thus increases in the efficiency that an absorbed photon will be trapped by the PSII reaction centre with the resultant reduction of QA to QA– (ET0/ABS = FV/FM) and the efficiency that an electron residing on QA– will enter the intersystem electron transport chain (ET0/TR0) were observed in reduced UV‐B. Moreover, estimated per cross‐section of leaf sample, the number of active PSII reaction centres (RC/CSM) and electron transport rate (ETM/CSM) and all performance indexes (PIABS, PICSo and PICSm) were increased in reduced UV‐B. The total soluble flavonoid content was highest in ambient UV‐B. The treatment effects on fluorescence parameters that were directly measured (e.g. F0 and FM) and those that were derived (e.g. quantum efficiencies, parameters per PSII reaction centres and per cross‐section of leaf sample) are discussed in relation to one another, in relation to daily and seasonal variation, and from the perspective of evaluating the relative importance of UV‐B of donor and acceptor side capacity in Photosystem II. In conclusion, the experimental set‐up and non‐invasive measurements proved to be a sensitive method to screen for effects of UV‐B stress.

A Simple Roller-Mill Grinding Procedure for Plant and Soil Samples#

Obtaining finely ground, homogeneous plant and soil samples for chemical analysis without cross contamination is a major concern when using direct combustion procedures for determination of total nitrogen (N), total carbon (C), and isotope-ratio analysis. A roller-milling device, using square glass bottles instead of round grinding bottles, was evaluated. Rotation of the square bottles on the roller mechanism increased the milling action by the stainless steel rods inside the bottles. The roller-milling device with square grinding bottles resulted in a sample with smaller particles (99.2% <100 mesh) than with a ball-milling device (83% <100 mesh). The roller-mill provided acceptable results (soil and plant samples ranging from 1.6–43.8% C, 0.5–2.3% N, and 0.3777–1.038 atom% 15 N (isotopic ratio analysis) had coefficients of variation ranging from 0.44–1.78, to 0.97–1.60, and 0.09–1.43, respectively) while providing an economical approach to grinding large numbers of samples to achieve finely ground, homogeneous samples with minimal labor and equipment costs. #This article is a joint contribution of the US Department of Agriculture and University of Nebraska, Lincoln. Published as Journal Series Number 13961.

Upper Pleistocene and Holocene paleosalinity and trophic state changes in relation to sea level variation in Rocha Lagoon, southern Uruguay

Paleolimnological data are presented on trophic development in relation to sea level variation in Rocha Lagoon, a 72 km2 coastal lagoon in southern Uruguay. Using sediment cores that extended to ∼20,000 yr BP, analyses of grain size, organic matter, carbonate, total carbon, nutrients, and diatoms allowed us to infer changes in trophic state and paleosalinities, which were closely related to Holocene relative sea level variation. Higher trophic states were observed during regressive events, most probably due to increases in runoff and erosion as regressions progressed. Diatom Association Zones (DAZ) were identified in both cores. Those DAZ corresponding to transgressive events were dominated by marine/brackish taxa and relatively low organic matter and nutrient values, while those DAZ corresponding to regressive events showed increases in brackish/freshwater diatoms and both organic matter and nutrients. Although the lagoon formed after the first Holocene marine transgression, our data indicate the existence of a marine/brackish aquatic system during upper Pleistocene (i.e., before 15,000 yr BP), but by ∼20,000 yr BP, the system was still likely to be a semi-arid terrestrial system.

Holocene trophic state changes in relation to sea level variation in Lake Blanca, SE Uruguay

Paleolimnological data are presented relating trophic development to sea level variation in Lake Blanca, a small (0.6 km2), coastal fresh waterbody in southern Uruguay. Using a sediment core that extended to 7300 years BP, analyses of grain size, thin sections, organic matter, carbonate, total carbon, nutrients, diatoms and palynomorphs, allowed us to infer changes in trophic state and paleosalinities, which were closely related to Holocene sea level variation. Higher trophic states were observed during regressive events, most probably due to increases in runoff and erosion as regression progressed. Four diatom association zones (DAZ) were identified in the sediment core. The basal core section pre-dated the first Holocene marine transgression, contained no diatoms, chrysophyte cysts or non-siliceous microalgae, and showed C/N ratios values higher than 20. Thus, it is likely that the system exhibited terrestrial characteristics. In the second section (∼6500–2200 years BP, following the first Holocene transgression), there was dominance of marine/brackish diatom species. The lowest trophic states of the core were observed in this section. The third section (2200–1100 years BP), represented the system as it became separate from the Atlantic Ocean, and showed a dominance of brackish/freshwater species and increases in trophic state were observed. In the last section (after 1100 years BP), the system became fully freshwater as no marine or brackish diatom species were found, but a trend to oligotrophication was observed, probably due to nutrient depletion. However, after ∼1967 AD, eutrophication intensified because of forestry and soil fertilization in the catchment. Pollen association zones (PAZ) allowed us to identify four sections. Below 250 cm (2200 years BP), the core contained no pollen grains as redox potential and pH values were not conducive for pollen preservation. After 2200 years BP (when the system started to separate from the ocean), xerophilic taxa typical of coastal dunes colonized the catchment. Only after ∼1100 years BP (after fully freshwater conditions established) pollen grains of trees were observed.