Fe isotope and Fe speciation study of water column redox dynamics during Eastern Mediterranean sapropel events S5 and S7

2020 ◽  
Author(s):  
Alan Matthews ◽  
Ayelet Benkovitz ◽  
Nadya Teutsch ◽  
Simon Poulton ◽  
Miryam Bar-Matthews ◽  
...  
Keyword(s):  
Mediterranean Sea ◽  
Water Column ◽  
Black Sea ◽  
Eastern Mediterranean ◽  
The Black Sea ◽  
Eastern Mediterranean Sea ◽  
Positive Balance ◽  
Fe Speciation ◽  
Bottom Waters ◽  
Oxide Minerals

<p>Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea  during peak interglacial periods MIS5e and MIS7a, respectively. This study investigates the dynamics of  water column redox change during their formation, through Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967 south of Cyprus. Both sapropels show an inverse correlation between δ<sup>56</sup>Fe and Fe<sub>T</sub>/Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in highly euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak stages of S7, where the negative δ<sup>56</sup>Fe - Fe<sub>T</sub>/Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (Fe<sub>HR</sub>) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. Fe<sub>HR</sub>/Fe<sub>T</sub> plots show increased strengthening of anoxic water conditions during the transformation from pre-sapropel sediment into the sapropel. Nevertheless, despite the evidence for highly euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropels, Fe<sub>py</sub>/Fe<sub>HR</sub> ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters.  Thus, the operationally defined ferruginous/euxinic boundary for Eastern Mediterranean Sea sapropels is better placed at Fe<sub>py</sub>/Fe<sub>HR</sub> = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the δ<sup>56</sup>Fe - Fe<sub>T</sub>/Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile into the Eastern Mediterranean basin. The euxinic water column conditions that developed in sapropels S5 and S7 are interpreted to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of Fe (oxyhydr)oxide minerals. Both of these parameters in turn depend on the extent to which water overturn times are reduced during sapropel formation. Water overturn rate is therefore considered to define the strength of euxinic water column conditions during these periods of organic carbon-rich sedimentation.</p>

scholarly journals On the Circulation and Thermohaline Properties of the Eastern Mediterranean Sea

2021 ◽  
Vol 8 ◽  
Author(s):  
Milena Menna ◽  
Riccardo Gerin ◽  
Giulio Notarstefano ◽  
Elena Mauri ◽  
Antonio Bussani ◽  
...  
Keyword(s):  
Mediterranean Sea ◽  
Water Column ◽  
Mixed Layer ◽  
Eastern Mediterranean ◽  
Mixed Layer Depth ◽  
Eastern Mediterranean Sea ◽  
Salt Fingers ◽  
Main Circulation ◽  
Satellite Altimetry Data

The circulation of the Eastern Mediterranean Sea is characterized by numerous recurrent or permanent anticyclonic structures, which modulate the pathway of the main currents and the exchange of the water masses in the basin. This work aims to describe the main circulation structures and thermohaline properties of the Eastern Mediterranean with particular focus on two anticyclones, the Pelops and the Cyprus gyres, using in-situ (drifters and Argo floats) and satellite (altimetry) data. The Pelops gyre is involved in the circulation and exchange of Levantine origin surface and intermediate waters and in their flow toward the Ionian and the Adriatic Sea. The Cyprus Gyre presents a marked interannual variability related to the presence/absence of waters of Atlantic origin in its interior. These anticyclones are characterized by double diffusive instability and winter mixing phenomena driven by salty surface waters of Levantine origin. Conditions for the salt finger regime occur steadily and dominantly within the Eastern Mediterranean anticyclones. The winter mixing is usually observed in December–January, characterized by instability conditions in the water column, a gradual deepening of the mixed layer depth and the consequent downward doming of the isohalines. The mixing generally involves the first 200 m of the water column (but occasionally can affect also the intermediate layer) forming a water mass with well-defined thermohaline characteristics. Conditions for salt fingers also occur during mixing events in the layer below the mixed layer.

scholarly journals Thalassospira sp. Isolated from the Oligotrophic Eastern Mediterranean Sea Exhibits Chemotaxis toward Inorganic Phosphate during Starvation

2011 ◽  
Vol 77 (13) ◽  
pp. 4412-4421 ◽  
Author(s):  
Annemarie Hütz ◽  
Karin Schubert ◽  
Jörg Overmann
Keyword(s):  
Mediterranean Sea ◽  
Water Column ◽  
Inorganic Phosphate ◽  
Phosphate Uptake ◽  
Eastern Mediterranean ◽  
Phosphate Limitation ◽  
Soluble Phosphate ◽  
Free Living ◽  
Eastern Mediterranean Sea ◽  
Content Type

ABSTRACTThe eastern Mediterranean Sea represents an ultraoligotrophic environment where soluble phosphate limits the growth of bacterioplankton. Correspondingly, genes coding for high-affinity phosphate uptake systems and for organophosphonate utilization are highly prevalent in the plankton metagenome. Chemotaxis toward inorganic phosphate constitutes an alternative strategy to cope with phosphate limitation, but so far has only been demonstrated for two bacterial pathogens and an archaeon, and not in any free-living planktonic bacterium. In the present study, bacteria affiliated with the genusThalassospirawere found to constitute a regular, low-abundance member of the bacterioplankton that can be detected throughout the water column of the eastern Mediterranean Sea. A representative (strain EM) was isolated in pure culture and exhibited a strong positive chemotaxis toward inorganic phosphate that was induced exclusively in phosphate-starved cultures. Phosphate-depleted cells were 2-fold larger than in exponentially growing cultures, and 43% of the cells retained their motility even during prolonged starvation over 10 days. In addition,Thalassospirasp. strain EM was chemotactically attracted by complex substrates (yeast extract and peptone), amino acids, and 2-aminoethylphosphonate but not by sugar monomers. Similarly to the isolate from the eastern Mediterranean, chemotaxis toward phosphate was observed in starved cultures of the other two available isolates of the genus,T. lucentensisDSM 14000TandT. profundimarisWP0211T. AlthoughThalassospirasp. represents only up to 1.2% of the total bacterioplankton community in the water column of the eastern Mediterranean Sea, its chemotactic behavior potentially leads to an acceleration of nutrient cycling and may also explain the persistence of marine copiotrophs in this extremely nutrient-limited environment.

scholarly journals The effects of decomposing invasive jellyfish on biogeochemical fluxes and microbial dynamics in an ultraoligotrophic sea

2020 ◽  
Author(s):  
Tamar Guy-Haim ◽  
Maxim Rubin-Blum ◽  
Eyal Rahav ◽  
Natalia Belkin ◽  
Jacob Silverman ◽  
...  
Keyword(s):  
Mediterranean Sea ◽  
Water Column ◽  
Nutrient Dynamics ◽  
Eastern Mediterranean ◽  
Oxygen Demand ◽  
Bacterial Biomass ◽  
Ecosystem Functions ◽  
Water Interface ◽  
Overlying Water ◽  
Eastern Mediterranean Sea

Abstract. Over the past several decades, jellyfish blooms have intensified spatially and temporally, affecting functions and services of ecosystems worldwide. At the demise of a bloom, an enormous amount of jellyfish biomass sinks to the seabed and decomposes. This process entails reciprocal microbial and biogeochemical changes, typically enriching the water column and seabed with large amounts of organic and inorganic nutrients. Jellyfish decomposition was hypothesized to be particularly important in nutrient-impoverished ecosystems, such as the Eastern Mediterranean Sea – one of the most oligotrophic marine regions in the world. Since the 1970s, this region is experiencing the proliferation of a notorious invasive scyphozoan jellyfish, Rhopilema nomadica. In this study, we estimated the short-term decomposition effects of R. nomadica on nutrient dynamics at the sediment-water interface. Our results show that the degradation of R. nomadica has led to increased oxygen demand and acidification of overlying water as well as high rates of dissolved organic nitrogen and phosphate production. These conditions favored heterotrophic microbial activity, bacterial biomass accumulation, and triggered a shift towards heterotrophic bio-degrading bacterial communities, whereas autotrophic pico-phytoplankton abundance was moderately affected or reduced. This shift may further decrease primary production in the water column of the Eastern Mediterranean Sea. Deoxygenation, acidification, nutrient enrichment and microbial community shifts at the sediment-water interface may have a detrimental impact on macrobenthic communities. Based on these findings we suggest that jelly-falls and their decay may facilitate an additional decline in ecosystem functions and services.

scholarly journals Coastal hypoxia/anoxia as a source of CH<sub>4</sub> and N<sub>2</sub>O

2009 ◽  
Vol 6 (5) ◽  
pp. 9455-9523 ◽  
Author(s):  
S. W. A. Naqvi ◽  
H. W. Bange ◽  
L. Farías ◽  
P. M. S. Monteiro ◽  
M. I. Scranton ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
Black Sea ◽  
Coastal Upwelling ◽  
Dead Zone ◽  
Nitrous Oxide Emissions ◽  
Quantitative Prediction ◽  
The Black Sea ◽  
N2o Production ◽  
Bottom Waters

Abstract. We review here available information on distributions of methane (CH4) and nitrous oxide (N2O) from major, mostly coastal, oxygen (O2)-deficient zones produced due to both natural processes and human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O2 levels. In the case of CH4, bottom-water O2 content probably affects emission from sediments, believed to be the main source of water-column CH4, as well as its oxidative loss in water itself. Highest CH4 accumulation (several μM) occurs in silled basins having anoxic deep waters such as the Black Sea and the Cariaco Basin. One to two orders of magnitude smaller, but still significant, accumulation also occurs in bottom waters of open margins experiencing anoxia and in silled basins containing suboxic/severely hypoxic waters. In highly eutrophic waters over open continental shelves (such as the upwelling zone off Namibia and the "dead zone" in the Gulf of Mexico) high CH4 concentrations (several hundred nM) may occur in non-sulphidic waters as well, but in these regions it is difficult to differentiate the hypoxia-induced enhancement from in situ production of CH4 in the water column and, sometimes, large inputs of CH4 associated with freshwater runoff or seepage from sediments. Despite the observed CH4 build-up in low-O2 bottom waters, methanotrophic activity severely restricts its emission from the ocean. As a result, an intensification or expansion of coastal hypoxic zones will probably not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH4 budget. The situation is different for N2O, the production of which is greatly enhanced in severely hypoxic waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of semi-enclosed/land-locked anoxic basins such as the Black Sea. Most O2-deficient systems serve as strong net sources of N2O to the atmosphere. This is especially true for regions of coastal upwelling with shallow oxygen minimum zones where a dramatic increase in N2O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N2O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N2O as well as insufficient data from some key coastal regions.

scholarly journals Behaviour of Temperature Variations in Subsurface Layers in the South-Eastern Mediterranean Sea

2020 ◽  
Vol 8 (1) ◽  
pp. 9-22
Author(s):  
Tarek M. El-Geziry ◽  
Ibrahim A. Maiyza ◽  
Mohamed S. Kamel
Keyword(s):  
Mediterranean Sea ◽  
Water Column ◽  
Eastern Mediterranean ◽  
Seawater Temperature ◽  
Eastern Mediterranean Region ◽  
Mean Annual Temperature ◽  
The South ◽  
Eastern Mediterranean Sea ◽  
South Eastern ◽  
Subsurface Layers

The present work is a qualitative research, which aims at drawing the general behaviour of variations in the subsurface seawater temperatures within three distinctive subsurface layers in the south-eastern Mediterranean region. The work is based on hydrographic data collected over 65 years (1948–2012). The investigated layers are the subsurface, the intermediate and the deep water layers. The general trend of the mean annual temperature anomaly (MATA) was examined using the linear and quadratic regressions. Results revealed that the MATA over the water column in the south-eastern Mediterranean has the same trend, regardless of the place (layer) of investigation. Linearly, all MATA have increasing trends with different rates, with the exception of the 75 m level, which has a decreasing trend following that previously concluded for the surface water in the region. Also, the quadratic approach reflects the same trend of MATA over the water column, with different years of minimum occurrence. These same trends from the surface to deep can be attributed to the vertical convection processes in this region, and to the expanded impact of solar radiation which may reach more than 30 m depth. Keywords: South-eastern Mediterranean Sea, seawater temperature, linear regression, quadratic regression, cyclic trend

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