Shallow Hypersaline Research Articles

Hydrologic Versus Biogeochemical Control of Nutrient Dynamics in a Shallow Hypersaline Coastal Lagoon: Insight From a Coupled Hydrodynamic‐Water Quality Model

AbstractShallow coastal lagoons with restricted connection to the ocean are often productive but can also be sensitive to nutrient enrichment and hydrologic changes. Resolving nutrient dynamics is important for their sustainable management, yet being able to accurately resolve nutrient budgets has remained a challenge due to their complex hydrological regimes and habitat heterogeneity. In this study, we undertake a systematic nutrient budget of a large shallow hypersaline lagoon (Coorong, South Australia), with assistance of a high‐resolution coupled hydrodynamic‐biogeochemical model, to demonstrate the conditions that lead to nutrient retention. Under current conditions, high rates of evapo‐concentration and limited water connectivity have led to a persistent accumulation of nutrients and poor water quality in substantial areas of the lagoon. The interplay between hydrological drivers and biogeochemical processes was quantified using an adjusted Damköhler number, comparing the timescales of nutrient flushing versus processing. This showed a general transition from hydrologic control to biogeochemical control with increasing distance from the main ocean connection, modified by episodes of increased flows and external loads. Whilst water age was a useful indicator of the factors controlling rates of nutrient retention, interannual variability in retention between areas of the lagoon was explained based on river flows and changes in mean sea level. As the system has been affected by reduced flows over past decades, the results provide evidence that increasing river flow to the lagoon would reduce the nutrient retention, and we discuss the potential for net nutrient export to the ocean under sustained high flows.

Analysis of Depths Derived by Airborne Lidar and Satellite Imaging to Support Bathymetric Mapping Efforts with Varying Environmental Conditions: Lower Laguna Madre, Gulf of Mexico

In 2017, Bureau of Economic Geology (BEG) researchers at the University of Texas at Austin (UT Austin) conducted an airborne lidar survey campaign, collecting topographic and bathymetric data over Lower Laguna Madre, which is a shallow hypersaline lagoon in south Texas. Researchers acquired 60 hours of lidar data, covering an area of 1600 km2 with varying environmental conditions influencing water quality and surface heights. In the southernmost parts of the lagoon, in-situ measurements were collected from a boat to quantify turbidity, water transparency, and depths. Data analysis included processing of Sentinel-2 L1C satellite imagery pixel reflectance to classify locations with intermittent turbidity. Lidar measurements were compared to sonar recordings, and results revealed height differences of 5–25 cm where the lagoon was shallower than 3.35 m. Further, researchers analyzed satellite bathymetry at relatively transparent lagoon locations, and the results produced height agreement within 13 cm. The study concluded that bathymetric efforts with airborne lidar and optical satellite imaging have practical limitations and comparable results in large and dynamic shallow coastal estuaries, where in-situ measurements and tide adjustments are essential for height comparisons.

Carbonate Sedimentation in High-Mineralized Lake Bolshoi Bagan (South of West Siberia): Dependence on Holocene Climate Changes

Abstract ––We present the results of comprehensive studies of Holocene bottom sediments from the shallow hypersaline (mineralization up to 282 g/L) Lake Bolshoi Bagan, located in the East Baraba lowland (south of Western Siberia). The research methods include X-ray diffractometry (XRD), IR spectroscopy, laser granulometry, scanning electron microscopy, elemental analysis of sediments and pore water, radiocarbon (14C AMS) dating. It has been found that during the Holocene an intensive authigenic mineral formation took place in the lake basin; gypsum, halite and carbonates of calcite-dolomite series dominate among the newly formed mineral phases. Mg-calcites with different Mg contents, excess-Ca dolomites, aragonite and occasionally Mg-siderite have been found in the assemblage of carbonate minerals by mathematical modeling of complex XRD patterns. Mineralogical and crystallochemical studies, supplemented by the results of geochemical analyses, allowed us to identify four stages of the evolution of Lake Bolshoi Bagan in the Holocene, due to regional climate variations. The boundaries of the stages in general correspond to the boundaries of the climatostratigraphic phases by the Blytt–Sernander system: Stage I (the end of the Boreal) – the lake formation, humid climate; Stage II (Atlantic) – climate aridization, shallowing of the lake; Stage III (Subboreal) – unstable climate, frequent change of conditions; Stage IV (Subatlantic) – moderately cool and dry climate.

Hydrocarbon accumulation characteristics in the inter-salt shale oil reservoir in the Eocene Qianjiang Depression, Hubei Province, China

Research on the migration and accumulation of hydrocarbons in shale oil reservoirs composed solely of organic-rich mudstones has not received adequate attention. The Qianjiang Formation provides an opportunity for research on the hydrocarbon accumulation mechanism due to the presence of hydrocarbon migration characteristics in its inter-salt organic-rich fine-grained rocks. In this study, the inter-salt stratum of the tenth rhythmite can be classified into three sections: lower, middle, and upper. The middle section is mostly composed of laminated marlstones and laminated calcite-bearing argillaceous dolomicrites, whereas, the lower and upper sections mostly contain bedded muddy dolomicrites and glauberite. Geochemical analysis was conducted to study the organic matter enrichment mechanism and shale oil accumulation characteristics. The results indicate that the middle section has the highest organic matter abundance, with poor organic matter abundance displayed by the other sections. The C27/C29 sterane and extended tricyclic terpane ratios suggest that the organic matter types in different sections of the inter-salt stratum differ, and the middle section has more aquatic organic matter than other sections. The sedimentary structures and salinity-related parameters indicate that the middle section was developed in a deep stratified saline lake and that the other sections were formed in a shallow hypersaline lake. This study shows that high biological productivity and stratified lake water facilitated the enrichment of organic matter, resulting in high organic matter content in the middle section. The oil saturation index and hydrocarbon composition suggest that hydrocarbons were differentially accumulated in the inter-salt stratum of the tenth rhythmite. The maturity-related biomarkers further revealed the possible migration paths of hydrocarbons in the inter-salt stratum. Based on the aforementioned analyses, this study established an inter-salt shale oil accumulation model. This study suggests that hydrocarbons were released from the mature source rocks in the deep depression with differential migration laterally and vertically within the inter-salt stratum. The study of the Qianjiang Depression shows that organic-rich fine-grained rocks cannot only form in situ sweet spots but may also possess migrated hydrocarbons inside them, which are characterized by high organic matter abundance and oil saturation. This research provides a new case for further understanding hydrocarbon accumulation inside source rocks.

Mineralogy and Geochemistry of Sediments from Lake Tuz

Lake Tuz is in a closed basin, in the center of the Anatolia (Turkey). The lake has shallow hypersaline water. In this study, mineralogical and geochemical features of the lake sediments sampled by core drillings were investigated. Halite, polyhalite, calcite, magnesite, dolomite, huntite, quartz, and albite minerals were found in bulk sample and montmorillonite and vermiculite minerals were found in the clay fraction XRD analyzes. In geostatistical evaluations, elements are divided into four clusters. These clusters are named as Clastic, Hydrothermal, Evaporite-carbonate and Evaporite-sulphate. Trace elements included in the clastic cluster were used to interpret provenance. The Light Rare Earth Element (LREE) enriched chondrite normalized average REE pattern suggests a cratonic provenance for the lake sediments except for the low negative Eu anomaly. Trace element ratios of La/Sc, La/Co, Th/Sc, Th/Co, Zr/Sc, Zr/Co, Ba/Sc, and Ba/Co, which are critical for provenance, show a provenance in a “felsic-intermediate magmatic” composition. According to the La-Th-Sc diagram, the tectonic setting of the source area was found as "Continental Island Arc".

Distribution patterns and seasonal variations in phytoplankton communities of the hypersaline Pulicat lagoon, India.

Phytoplankton structure and patterns are key components to forecast the net result of the gain and loss process that outline the resilience of the lagoon ecosystem. In order to understand the phytoplankton community structure and its relationship with the environmental variables in the shallow hypersaline Pulicat lagoon, east coast of India, observations were carried out during August 2018-January 2019 covering the three seasons: premonsoon (PrM), monsoon (M), and postmonsoon (PoM). The salinity of the lagoon varied with a minimum of 12.1 for the M and a maximum of 81.65 during the PoM. The clustering analysis performed on the phytoplankton abundance data separated the lagoon into three sectors: north sector (NS), central sector (CS), and south sector (SS). A total of 59 taxa/morphotypes from four taxonomic classes (Bacillariophyceae, Chlorophyceae, Cyanophyceae, and Dinophyceae) were recorded during the study period. The class Bacillariophyceae was dominant in the lagoon both spatially and temporally by 44.06% with Chaetoceros borealis as dominant species. Presence of characteristic species like Dunaliella sp. was observed in the higher salinity, whereas Pediastrum duplex and Scenedesmus sp. were dominant in the freshwater influx areas. The individual-based functional approach allowed grouping these taxa into 11 functional entities based on the derived 4 functional trait values (cell size, trophic regime, mobility, and coloniality). Formation of algal blooms of Protoperidinium sp. (3.3×105ind L-1) and Odentella sp. (2.8×105ind L-1) was observed in the SS during PoM as a result of reduced water exchange in the lagoon. During the same period, toxin-producing strains like Anabaena sp. and Nostoc sp. of Cyanophyceae were also recorded. Correlating the three sectors of the lake (NS, CS, and SS), it is observed that the physical, chemical, and biological properties of the lake varied continuously depending on the season and freshwater availability. Seasonal nutrient stoichiometry played a significant role in regulating the community structure and distribution pattern of phytoplankton communities of the lagoon.

Deciphering cryptic methane cycling: Coupling of methylotrophic methanogenesis and anaerobic oxidation of methane in hypersaline coastal wetland sediment

Methanogenesis has recently been shown to fuel anaerobic oxidation of methane (AOM) within the sulfate-reducing zone of marine sediments, coining the term “cryptic methane cycle”. Here we present research on the relationship between methanogenesis and AOM in a shallow hypersaline pool (∼130 PSU) within a southern California coastal wetland. Sediment (top 20 cm) was subjected to geochemical analyses, in-vitro slurry experiments, and radiotracer incubations using 35S-SO42−, 14C-mono-methylamine, and 14C-CH4, to study sulfate reduction, methylotrophic methanogenesis, and AOM. An adapted radioisotope method was used to follow cryptic methane cycling in 14C-mono-methylamine labeling incubations with increasing incubation times (1 hour to three weeks). Results showed peaks in AOM (max 13 nmol cm−3 d−1) and sulfate reduction activity (max 728 nmol cm−3 d−1) within the top 6 cm. Below 6 cm, AOM activity continued (max 15 nmol cm−3 d−1), while sulfate reduction was absent despite 67 mM sulfate, suggesting AOM was coupled to the reduction of iron. Methane concentrations were low (<50 nM) throughout the sediment. Batch sediment slurry incubations with methylated substrates (mono-methylamine and methanol) stimulated methanogenesis, pointing to the presence of methylotrophic methanogens. Incubations with 14C-mono-methylamine revealed the simultaneous activity of methanogenesis and coupled AOM through the stepwise transfer of 14C from mono-methylamine to CO2 via methane. Our results suggest that AOM is a crucial process in coastal wetland sediments to prevent the buildup of methane in the sulfate-reducing zone. We propose that cryptic methane cycling has been largely overlooked in coastal wetlands resulting in incomplete understanding of carbon cycling in this environment.

Sedimentology and stratigraphy of a modern halite sequence formed under Dead Sea level fall

AbstractHalite sequences in the geological record accumulated in deep hypersaline basins. However, such halite sequences are interpreted based on modern analogues of halite deposition in shallow hypersaline environments. Recently, halite deposition in the deep, hypersaline Dead Sea has been studied together with its coeval environmental and limnogeological forcing. This is the closest modern analogue for deep environments. Therefore, stratigraphy, sedimentology and petrography of a well‐dated, high‐resolution modern Dead Sea halite sequence are explored. The sequence was deposited during a ca 30 m lake‐level decline since the onset of modern halite deposition in 1980, and was compared with sub‐annual lake levels, precipitation and flood records. The sedimentology of the sequence documents the trend of shallowing water depth, including individual floods. The sequence base is composed of alternating bottom growth‐cumulate halite annual couplets, typical of deep hypolimnetic water deposition. Up‐sequence, the annual couplets disappear and towards its top are composed of cumulate layers with dissolution features, typical of shallow epilimnetic water deposition. Halite deposition rate is reduced by 60% at the shallow lakefloor compared with the deep lakefloor, mainly due to the summer undersaturation that leads to depocentre ‘halite focusing'. The top of the sequence contains shoreline deposits, halolites (halite ooids) and polygonal surface cracks, indicating subaerial exposure. This study shows petrographic indicators for summer thermal dissolution (partially dissolved crystals), which are distinct from dissolution features by winter floods that generate a regional truncation surface. Spatial variations in halite thickness and facies, indicating much thinner and spatially limited halite units compared to modelled halite units based on mass balance considerations were also observed. These observations provide criteria for: (i) recognizing water depths and shallowing lake‐level trends from halite sequences throughout the geological record; and (ii) interpreting palaeolimnology, water column structure and the relations between stratigraphic horizons and corresponding shorelines.

Palynomorphs in southern Western Australian lake sediments: evidence of climate change and hypersalinity during the Cenozoic

This study focuses on five cores in and around three of the hundreds of shallow ephemeral hypersaline lakes distributed in chains along traces of ancient drainages that dominate the southern Western Australian landscape. Cores within and on the shore of Lake Aerodrome (LA1-09, LA2-09), Prado Lake (PL1-09, PL2-09), and Gastropod Lake (GLE1-09) in the Cowan Paleodrainage were drilled as part of a multidisciplinary study to understand the evolution of the lakes. Lithological and palynomorph data provide insights into the region’s the depositional history, and floristic and climate evolution of the region. The nearly 60 m deep LA2-09 core provides the most comprehensive data. The basal ∼15 m interval of this core comprises two distinct lignite units with a clay interbed that preserve a rich palynomorph assemblage of the Werillup Formation. This assemblage is characterized by the first appearance datum (FAD) and last appearance datum (LAD) of key late Eocene taxa that correlate with the Middle Nothofagidies asperus Zone, and is indicative of freshwater swamp surrounded by subtropical to temperate rainforest. Above the upper lignite (except the topmost sample), evaporitic-siliciclastic units preserve a depauperate palynomorph assemblage comprising mostly long-ranging sclerophyllic-xerophilous-halophilic taxa. The pollen Myrtaceidites lipsis constrains the age of the upper ∼23 m interval as Pliocene to Recent. This younger assemblage also preserves reworked palynomorphs from the Werillup Formation and older units, and is identified in the other four cores. The presence of the halophilic green alga Dunaliella in the younger assemblage is used as a proxy for aridity and hypersaline conditions during post-Eocene deposition in southern Western Australia. Its absence in the neutral-alkaline lake water and uppermost core samples of GLE1-09 is either due to predation by the acidophobic gastropod Coxiella residing in the lake, or the fact that the type of Dunaliella preserved in the core samples is acidophilic.

Composition and distribution of planktonic ciliates with indications to water quality in a shallow hypersaline lagoon (Pulicat Lake, India)

Planktonic ciliate composition and distribution together with physicochemical variables were investigated in a shallow hypersaline lagoon, Pulicat, India, during three seasons, i.e., pre-monsoon (PRM), monsoon (MON), and post-monsoon (POM). The low freshwater inflow, evaporation, and closure of the lake mouth were the main factors for the hypersaline conditions in Pulicat Lake. The average depth and salinity were 1.8 ± 0.12m (0.8 to 2.8m) and 35.3 ± 1.68 (12.5 to 61), respectively. A total of 29 ciliate taxa belonging to 18 genera and five classes were identified. Strombidium conicum (24%) was the dominant species followed by Euplotes sp. (10.7%) and Stenosomella sp. (7.02%). Spirotrichea (84%) was the dominant class followed by Oligohymenophorea (9.6%) and Heterotrichea (5.8%). Fabrea salina, a typical species in hypersaline systems, was abundant at locations where the salinity was more than 35. Multivariate analysis using the Bray-Curtis similarity, followed by SIMPROF (Similarity Percentage Analysis), on ciliate abundance data revealed three ciliate assemblages characterizing south, central, and north of the lake at 40% similarity (SIMPROF, cophenetic correlation = 0.622, P = 5%). Both ciliate abundance and chlorophyll-a were positively correlated with salinity. Species richness and evenness were higher in the south sector when compared with those in the other two sectors. Biotic-environmental interaction through canonical correspondence analysis (CCA) inferred that the combined effects of salinity, chlorophyll-a, and nutrient levels are the key factors responsible for the distribution of the ciliate species, suggesting that ciliates can be considered to be potential bioindicators of water quality.

Abnormal test growth in Larger Benthic Foraminifera from hypersaline coastal ponds of the United Arab Emirates

The larger benthic foraminifera (LBF) assemblage from shallow hypersaline coastal ponds located in the intertidal area of theUnitedArab EmirateWestern Region was investigated. The studied coastal ponds are located between a lagoonal areawith carbonate sedimentation, and a supratidal, evaporite-dominated, sabkha. Epiphytic larger benthic foraminifera, mostly belonging to the genus Peneroplis, dominate the benthic foraminifera assemblage. At all the sampled locations, the larger benthic foraminifera assemblage is characterised by high percentages of tests with abnormal growth. Dissolution and microboring are also common on the majority of LBF tests. The high percentage of abnormal tests reflects natural environmental stress caused by the instability of physical parameters (particularly high and variable salinity and temperature) in this transitional marine environment. The unique presence of epiphytic species in some of the ponds suggests that epiphytic foraminifera are transported into the ponds attached to floating seagrass and subsequently continue to live in the stressed pond environment.

Late-Holocene sedimentation and sodium carbonate deposition in hypersaline, alkaline Nasikie Engida, southern Kenya Rift Valley

Continental rift systems are often characterized by geothermal activity and associated discharge of hot groundwater, which can substantially impact the water, solute and sediment budgets of rift-valley lakes. Hot-spring inflow can result in complex lake hydro- and geochemistry, but also buffers against the desiccation of closed-basin lakes in dry climate regimes. Consequently, hydrothermally fed lakes can potentially provide continuous sedimentary records from regions where other types of paleoenvironmental archives are lacking. This is illustrated by Nasikie Engida, a shallow hypersaline and alkaline (soda) lake in the semiarid Rift Valley of southern Kenya. Here, inflow of hot-spring water has maintained a shallow but permanent water body and continuous deposition of sediments through past climate episodes when many other lakes in the eastern (Gregory) branch of the East African Rift System stood dry. We present the first data on late-Holocene sedimentation in this remarkable lacustrine system, typified by authigenic nahcolite [NaH(CO3)] formation during part of its recorded history. Our data include measurements of bulk-sediment and mineralogical composition, clastic-mineral grain size and magnetic susceptibility. Analytical issues related to the large amount of salts within the sediments, both as crystals and in solution in the pore water, are discussed. We also present exploratory time series of the stable-isotope composition of bulk organic matter and authigenic nahcolite. Core lithostratigraphy and preliminary radiocarbon dating indicate that Nasikie Engida has accumulated finely laminated sediments continuously since ca. 2850 cal year BP, which is remarkable given its current maximum depth of only 1.6 m. Synsedimentary nahcolite appears abruptly ~ 2260 cal year BP and since then has been deposited regularly, in the form of distinct pure layers up to several cm thick. Its formation has been enabled by high pCO2, supplied principally from geothermal sources, accumulating in a high-density brine. Although Nasikie Engida has likely been saline over the entire time span covered by the studied sediment sequence, variation in nahcolite deposition and bulk-sediment composition suggests multi-decadal to centennial oscillations in water-column stability and stream inflow driven by variation in climatic moisture balance. Further analyses of this paleoenvironmental archive may thus produce the first continuous climate-proxy record from a vast dry region of equatorial East Africa. Moreover, Nasikie Engida’s extraordinary setting makes it a unique modern analogue for the interpretation of ancient nahcolite-bearing salt-lake deposits.

Efficient recycling of nutrients in modern and past hypersaline environments

The biogeochemistry of hypersaline environments is strongly influenced by changes in biological processes and physicochemical parameters. Although massive evaporation events have occurred repeatedly throughout Earth history, their biogeochemical cycles and global impact remain poorly understood. Here, we provide the first nitrogen isotopic data for nutrients and chloropigments from modern shallow hypersaline environments (solar salterns, Trapani, Italy) and apply the obtained insights to δ15N signatures of the Messinian salinity crisis (MSC) in the late Miocene. Concentrations and δ15N of chlorophyll a, bacteriochlorophyll a, nitrate, and ammonium in benthic microbial mats indicate that inhibition of nitrification suppresses denitrification and anammox, resulting in efficient ammonium recycling within the mats and high primary productivity. We also suggest that the release of 15N-depleted NH3(gas) with increasing salinity enriches ammonium 15N in surface brine (≈34.0‰). Such elevated δ15N is also recorded in geoporphyrins isolated from sediments of the MSC peak (≈20‰), reflecting ammonium supply sufficient for sustaining phototrophic primary production. We propose that efficient nutrient supply combined with frequent bottom-water anoxia and capping of organic-rich sediments by evaporites of the Mediterranean MSC could have contributed to atmospheric CO2 reduction during the late Miocene.

Temporal dynamics of monthly evaporation in Lake Urmia

As a UNESCO biosphere, Lake Urmia is a shallow hypersaline lake which is facing a rapid water surface degradation. Evaporation from the surface of the Lake, as a physical process which accelerates the Lake’s degradation, was evaluated using chaos theory. Seven hydrometeorological stations scattered around the Lake were selected, and a 40-year time span between October 1974 and September 2014 was used at each station. Missing data in time series was removed and the whole time series was tested for consistency, randomness, and presence of trend. Since evaporation at each station was measured by means of class A evaporation pan, time series at each station was multiplied by a pan coefficient to incorporate the effect of saline water and free water surface environment simultaneously. Measurement errors arising from assumption of zero evaporation in winter were removed from the time series using locally weighted scatterplot smoothing method after which unification of time series into a single time series is achieved. Results of the data transformation and information loss were monitored by means of auto-correlation, partial-auto-correlation, mutual information, power spectrum, false nearest neighbor, and correlation dimension. A local prediction method is then used to capture the temporal dynamics of the evaporation with consideration of an appropriate time delay and embedding dimension. Finally, the representative model was projected on a 3-dimensional phase space to evaluate the temporal dynamics of the evaporation. Results indicate that the chaotic approach shows accurate predictions in advance.

Copepoda in the shallow hypersaline Bardawil coastal lake (Egypt): Are there long-term changes in composition and abundance?

Abstract Coastal Lake Bardawil (Egypt) is one of the largest hypersaline lakes in the world. In 2009–2010, the authors studied composition, distribution and seasonal dynamics of copepods at 12 sites. A total of 10 species of copepods were recorded in zooplankton during the study period, including 5 Calanoida, 2 Cyclopoida and 3 Harpacticoida. Oithona nana was the most common and most abundant species. All copepods in the lake can be divided into three groups: 1) planktic species that form stable populations, 2) species of Mediterranean plankton incidentally entering the lake from the adjacent sea area, 3) benthic Cyclopoida and Harpacticoida that can be abundant in plankton. Two species – Acartia tonsa and A. danae were recorded here for the first time. The total abundance of copepods in the lake was significantly higher (90 times on average) compared to that observed in 2008–2009 in the waters of the Egyptian Mediterranean Sea. Since 1967, the complex of common and dominant copepod species in the lake has changed significantly. The total average annual copepod abundance varied: in 2002 – it was about 4000 ind. m−3, in 2004 – 152 000 ind. m−3, in 2005 – about 25 300 ind. m−3, and in 2009–2010 – about 56 000 ind. m−3.

Effect of salinity on diazotrophic activity and microbial composition of phototrophic communities from Bitter-1 soda lake (Kulunda Steppe, Russia).

Bitter-1 is a shallow hypersaline soda lake in Kulunda Steppe (Altai region, Russia). During a study period between 2005 and 2016, the salinity in the littoral area of the lake fluctuated within therange from 85 to 400g/L (in July of each year). Light-dependent nitrogen fixation occurred in this lake up to the salt-saturating conditions. The rates increased with a decrease in salinity, both under environmental conditions and in laboratory simulations. The salinities below 100g/L were favorable for light-dependent nitrogen fixation, while the process was dramatically inhibited above 200g/L salts. The analysis of nifH genes in environmental samples and in enrichment cultures of diazotrophic phototrophs suggested that anaerobic fermenting and sulfate-reducing bacteria could participate in the dark nitrogen fixation process up to soda-saturating conditions. However, we cannot exclude the possibility that haloalkaliphilic nonheterocystous cyanobacteria (Euhalothece sp. and Geitlerinema sp.) and anoxygenic purple sulfur bacteria (Ectothiorhodospira sp.) might also play a role in the process at light conditions. The heterocystous cyanobacterium Nodularia sp. develops at low salinity (below 80g/L) that is not characteristic for Bitter-1 Lake and thus does not make a significant contribution to the nitrogen fixation in this lake.

Compressional and shear in situ measurements in the Lower Laguna Madre

This presentation describes the apparatus, procedure, and results from a field experiment conducted in July 2017 in the seagrass beds of the Lower Laguna Madre. The Laguna Madre is a shallow hypersaline lagoon on the western coast of the Gulf of Mexico. The relatively flat bathymetry is covered by a thick meadow of Thalassia testudinum. Acoustic piezoelectric transducer probes measured compressional wave speed and attenuation in the seagrass canopy as well as geoacoustic properties (compressional and shear wave speed and attenuation) of the underlying sediment. Geoacoustic parameters were measured at depths between 5 cm and 20 cm in 5 cm increments. Compressional wave measurements were made over a frequency range of 20 kHz to 100 kHz, and shear wave measurements were made at 500 Hz and 1 kHz. The compressional wave data from this experiment indicates dispersion within the seagrass canopy and sediment. This dispersion is likely due to the high presence of gas bubbles in both the seagrass and supporting sed...

Shallow hypersaline lakes as paleoclimate archives: A case study from the Laguna Salada, Málaga province, southern Spain

Although numerous studies concerning the Holocene climate of the southern Iberian Peninsula were accomplished within the last few decades, the climate history of this region is still poorly understood. Various studies deal with a combination of proxies, which are neither easy to compare nor is their connection easy to explain, e.g., due to spatial patterns and time transgression. Within this study, the suitability of the lacustrine sediments from the Laguna Salada (Andalucía region, southern Spain) as a paleoclimate archive is investigated. The lake sediments were evaluated using a multi-proxy approach including sedimentological, mineralogical, geochemical and biological analyses. The sediments reflect the evolution of the lake from pre-Medieval times onwards and Characeae as well as Ostracod analyses give an indication of paleosalinity. Moreover does the geochemical composition provide profound information concerning changes of elemental and mineralogical composition. Nevertheless, a robust, high-resolution chronology could not be achieved owed to the scarcity of material available for radiocarbon dating and contamination problems. Furthermore, poor preservation of pollen restricted the reconstruction of vegetation history, which could have complemented important information concerning climatic changes and human activity.

Gypsum stromatolites from Sawda Nathil: relicts from a southern coastline of Qatar

Sawda Nathil is one of a series of inland depressions (inland sabkhas and salinas) that extend nearly continuously along the Southern Qatar border with Saudi Arabia. Six to eight thousand years ago, these depressions were marine embayments that separated the peninsula of Qatar from the mainland. These embayments in-filled rapidly with marine sediments and dune sands, blown southeastward across Qatar. Over time, they have become progressively more evaporitic. Inland depressions like Sawda Nathil host a set of unique environments within Qatar. Most depressions are close to or below sea level, bringing water near the surface in the driest parts of Qatar. Evaporation to salt saturation creates thick gypsum and halite crusts (sabkhas), as well as shallow hypersaline lakes/ponds (salinas) with spectacular domal microbial gypsum stromatolites. The present ground surface is a mosaic of relict marine facies, deflated dune sands, and inland sabkhas and salinas. Sediment of four short push cores was photographed, described, and sampled for petrographic thin-section and X-ray diffraction (XRD) analyses. Radiocarbon (AMS) as well as optically stimulated luminescence (OSL) age-dating were carried out on three samples. Scanning electron microscopy (SEM), carried out on samples from a gypsum stromatolite, was used to resolve the microbial-influence on gypsum precipitation. Radiocarbon dating of marine shells provides ages of approximately 6600 un-calibrated 14C years before present (year BP), coinciding with a well-documented sea-level highstand, approximately 2–4 m higher than present. During that time, Qatar was mostly an island, only connected in the south to the Arabian Peninsula by narrow land bridges (isthmuses). SEM examinations of gypsum stromatolites show gypsum crystals developing in close spatial association with microbial biofilms (filamentous structures). Whether this is purely a passive microbial-influenced gypsum mineralization process or an example where microorganisms actively control the gypsum crystal morphology to obtain ecological advantages, remains to be evaluated.

Biological and physical modification of carbonate system parameters along the salinity gradient in shallow hypersaline solar salterns in Trapani, Italy

We investigated changes in the chemical characteristics of evaporating seawater under the influence of microbial activity by conducting geochemical analyses of the brines and evaporite sediments collected from solar salterns in Trapani, Italy. The microbial activity had a substantial effect on the carbonate system parameters. Dissolved inorganic carbon (DIC) was substantially removed from the brine during the course of evaporation from the seawater to the point where calcium carbonate precipitates, with an accompanying decrease in its carbon isotopic composition (δ13CDIC) to as low as −10.6‰. Although the removal of DIC was due to calcium carbonate precipitation, photosynthesis, and the degassing of CO2(aq) induced by evaporation, the presence of 13C-depleted δ13CDIC in ponds where calcium carbonate precipitates can be attributed to the dissolution of atmospheric CO2 because of intensive CO2(aq) uptake by photosynthesis, and/or mineralization of organic matter by sulfate reduction. In contrast, δ13CDIC increased up to 7.2‰ in the salinity range where halite precipitates, which can be ascribed to the domination of the effect of degassing of CO2(aq) under conditions with reduced microbial activity. A gradual decrease in microbial activity was also reflected in compound-specific δ13C of photosynthetic pigments; isotopic fractionation associated with DIC assimilation increased linearly as the evaporation proceeded, indicating DIC-limited conditions within the microbial mats and gypsum crusts because of restricted DIC diffusion from the overlying brine and/or suppression of primary production at higher salinity.