Red Sea Sediments Research Articles

Monsoon-driven changes in aeolian and fluvial sediment input to the central Red Sea recorded throughout the last 200 000 years

Abstract. Climatic and associated hydrological changes controlled the transport processes and composition of the sediments in the central Red Sea during the last ca. 200 kyr. Three different source areas for mineral dust are identified. The dominant source is located in the eastern Sahara (Sudan and southernmost Egypt). We identify its imprint on Red Sea sediments by high smectite and Ti contents, low 87Sr / 86Sr, and high εNd. The availability of deflatable sediments was controlled by the intensity of tropical rainfall and vegetation cover over North Africa linked to the African monsoon. Intense dust input to the Red Sea occurred during arid phases, and low input occurred during humid phases. A second, less significant source indicated by palygorskite input is probably located on the eastern Arabian Peninsula and/or Mesopotamia, while the presence of kaolinite suggests an additional minor dust source in northern Egypt. Our grain size data reflect episodes of fluvial sediment discharge to the central Red Sea and document the variable strength in response to all of the precession-paced insolation maxima during our study interval including both those that were strong enough to trigger sapropel formation in the eastern Mediterranean Sea and those that were not. The African humid period most strongly expressed in our Red Sea record was the one during the Eemian last interglacial at ca. 125 ka (when the Baraka River was far more active than today), followed by those at 198, 108, 84, and 6 ka.

Declining nutrient availability and metal pollution in the Red Sea

Reconstructing sediment accumulation rates reveals historical environmental shifts. We analyzed 15 element concentrations and accumulation rates in two Red Sea sediment cores over 500 years. Post-1870s, the South Red Sea experienced a significant drop in element accumulation rates, with a noticeable decline in nutrients like magnesium (−8.52%), organic carbon (−14.9%), nitrogen (−14.0%), phosphorus (−16.4%), sulfur (−17.2%), and calcium (−17.8%). This suggests a potential reduction in nutrient inflow from the Indian Ocean, possibly due to warming-induced ocean stratification. Conversely, the North Red Sea saw an increase in all element accumulation rates after the 1870s, highlighting a rise in trace elements such as iron (4.56%), cadmium (8.69%), vanadium (12.6%), zinc (13.8%), copper (14.4%), chromium (17.6%), and nickel (19.5%), indicative of increased anthropogenic coastal activities. We introduce the term “Cai-Agusti Marine Crisis Conflux” to encapsulate the escalating thermal stress, nutrient depletion, and elemental pollution in the Red Sea, underscoring potential risks to its ecosystems and global implications.

Young Red Sea sediments: formation processes, engineering properties, and implications

Young Red Sea sediments: formation processes, engineering properties, and implications

Polycyclic aromatic hydrocarbons (PAHs) in Egyptian red sea sediments: Seasonal distribution, source Identification, and toxicological risk assessment

This study investigated the concentrations of 16 polycyclic aromatic hydrocarbons (PAHs) in sediments from the shoreline of the Red Sea. 16 PAH concentrations ranged from 1.43 to 4990.69 µg/g dry weight to 1.64 to 6397.69 µg/g dry weight during spring and autumn, respectively. Most sites with the highest PAH concentrations were (4–6 rings) compounds. The sources of PAHs were analyzed using principal component analysis (PCA) and diagnostic ratios between combinations of PAHs. The results indicated that the pyrogenic source was the primary source of PAHs. The results showed that surface sediment from all sample sites presents low to sporadic toxicity concerns to marine organisms. Total carcinogenicity and mutagenicity of eight priority PAHs ranged from 0.03 to 863.96 and 0.09 to 1197.85 µg/g dry weight, respectively (TEQBaP and MEQBaP). BaP accounted for most of the influence among the eight carcinogenic PAHs and could be employed as a particular PAH toxicity indicator. Using the toxic equivalent quotient (TEQcarc) and incremental lifetime cancer risk (ILCR) to estimate human health risk, it has been determined that sediment contaminated with PAHs in some study area stations may possess the capacity to cause cancer, particularly in children. The ecological risk posed by PAHs was evaluated using a risk quotient (RQ). The RQPAHs estimated for the samples revealed that the study region was at a high-risk level. Anthropogenic hydrocarbon inputs that were more pronounced in locations connected to maritime activity are described in this study. Further monitoring studiesencompassing different environmental matrices about PAHs pollution and their toxicological relevance should be conducted.

Ecological assessment of physico-chemical properties in mangrove environments along the Arabian Gulf and the Red Sea coasts of Saudi Arabia

Urban expansion along the coastal sites is one of the major causes of the deterioration of mangrove habitats. This study aimed to report the current status of different mangrove environments in Saudi Arabia. Mangrove soil and sediment–water samples were collected from the coasts of the Arabian Gulf and the Red Sea for their physical, chemical, and heavy metal analysis. The mean values (n = 42) of heavy metals in the soil showed that Fe (2200 ppm) > Mn (91.9 ppm) > Zn (66.11 ppm) > Cr (62.40 ppm) > Ni (33.64 ppm) > Cu (31.06 ppm) > Pb (30.69 ppm) > Co (27.06 ppm). While the mean values (n = 15) of heavy metals in the sediment–water were as follows; Cr (0.1978 ppm) > Fe (0.1016 ppm) > Pb (0.0792 ppm) > Co (0.0706 ppm) > Mn (0.0356 ppm) > Cu (0.0317 ppm) > Ni (0.0215 ppm). The average values of Zn, Ni, Cu, Pb, and Co were greater than those reported previously in the Red Sea and Arabian Gulf coastal sediments. At the same time, Mo and Cd were not detected in any studied soil sample. The correlation analysis revealed that the EC of mangrove soil was positively correlated with SOM, Ca, Mg, Na, and K, while the EC of sediment–water was positively correlated with Na, K, and Cu. Landfilling, sewage pollution, mismanagement of solid waste, and contamination due to other anthropogenic activities may lead to heavy metal hazards and the loss of large areas of mangrove plants in this region. Therefore, the findings of this study should be considered to understand and design the management strategy of mangrove forests in the area of study.

The exceptionally efficient quorum quenching enzyme LrsL suppresses Pseudomonas aeruginosa biofilm production.

Quorum quenching (QQ) is the enzymatic degradation of molecules used by bacteria for synchronizing their behavior within communities. QQ has attracted wide attention due to its potential to inhibit biofilm formation and suppress the production of virulence factors. Through its capacity to limit biofouling and infections, QQ has applications in water treatment, aquaculture, and healthcare. Several different QQ enzymes have been described; however, they often lack the high stability and catalytic efficiency required for industrial applications. Previously, we identified genes from genome sequences of Red Sea sediment bacteria encoding potential QQ enzymes. In this study, we report that one of them, named LrsL, is a metallo-β-lactamase superfamily QQ enzyme with outstanding catalytic features. X-ray crystallography shows that LrsL is a zinc-binding dimer. LrsL has an unusually hydrophobic substrate binding pocket that can accommodate a broad range of acyl-homoserine lactones (AHLs) with exceptionally high affinity. In vitro, LrsL achieves the highest catalytic efficiency reported thus far for any QQ enzyme with a Kcat/KM of 3 × 107. LrsL effectively inhibited Pseudomonas aeruginosa biofilm formation without affecting bacterial growth. Furthermore, LrsL suppressed the production of exopolysaccharides required for biofilm production. These features, and its capacity to regain its function after prolonged heat denaturation, identify LrsL as a robust and unusually efficient QQ enzyme for clinical and industrial applications.

Hydrothermal metalliferous sediments in Red Sea deeps: Formation, characterization and properties

Sediment accumulations within the Red Sea central deeps have unique genesis and properties. We piece together available information to understand their geological setting and formation history, and conduct an extensive sediment characterization study to assess their geotechnical properties in order to anticipate engineering/mining implications. The various sediment columns reflect slow-rate background sedimentation (biogenic and detrital particles – Valdivia deep) and hydrothermal metalliferous sediments that nucleate and grow within the overlying brine pools (primarily in the Atlantis II, as well as in the Wando deep, and to a lesser extent in Discovery deep). All sediments are fine-grained silt and clay-size particles; smaller particles tend to have higher specific gravity and define the metalliferous content. Hydrothermal sediments exhibit extreme properties when compared to sediments worldwide: they have uncharacteristically large maximum void ratio and compressibility, and their self-compaction is very different from background Red Sea sediments. Their unique self-compaction trends have a strong effect on remote acoustic characterization and sampling, and must be carefully accounted for during field studies and resource assessment. Three distinct properties of hydrothermal metalliferous sediments are relevant for separation and enrichment: high specific surface area, high specific gravity, and ferromagnetic signature. Small grains and low-density flocs have low terminal Stokes' velocities and their residency times may be extended in convective stratified brine pools; this observation affects the environmental analysis of mining operations and tailings disposal.

Asporychalasin, a bioactive cytochalasan with an unprecedented 6/6/11 skeleton from the Red Sea sediment Aspergillus oryzae

The cytochalasan asporychalasin (1) was obtained from the marine fungus Aspergillus oryzae, isolated from the Red Sea sediments collected off Jeddah, Saudi Arabia. The chemical structure of 1 was elucidated by extensive spectroscopic analysis and quantum-mechanical calculations of 13C NMR resonances and ECD to possess an unprecedented 6/6/11-fused tricyclic skeleton, including an isoquinolindione ring in place of the typical isoindolone. Asporychalasin exhibited moderate antiproliferative activity against three human cancer cell lines, lung carcinoma (A549), liver carcinoma (HepG2), and breast carcinoma (MCF7), and no toxicity on zebrafish embryos.

A Review of Heavy Metals in Coastal Surface Sediments from the Red Sea: Health-Ecological Risk Assessments.

The heavy metal (HM) pollution in sediment is of serious concern, particularly in the Red Sea environment. This study aimed to review and compile data on the concentrations of four HMs (Cd, Cu, Pb, and Zn) in the coastal surface sediments from the Red Sea, mainly from Saudi Arabia, Egypt, and Yemen, published in the literature from 1992 to 2021. The coastal sediments included those from mangrove, estuaries, and intertidal ecosystems. It was found that the mean values of Cd, Cu, Pb, and Zn in coastal Red Sea sediments were elevated and localized in high human activity sites in comparison to the earth upper continental crust and to reference values for marine sediments. From the potential ecological risk index (PERI) aspect, 32 reports (47.1%) were categorized as ‘considerable ecological risk’ and 23 reports (33.8%) as ‘very high ecological risk’. From the human health risk assessment (HHRA) aspect, the non-carcinogenic risk (NCR) values (HI values < 1.0) of Cd, Cu, Pb, and Zn represented no NCR for the ingestion and the dermal contact routes for sediments from the Red Sea countries. The reassessment of the HM data cited in the literature allowed integrative and accurate comparisons of the PERI and HHRA data, which would be useful in the management and sustainable development of the Red Sea area, besides being a helpful database for future use. This warrants extensive and continuous monitoring studies to understand the current and the projected HM pollution situation and to propose possible protective and conservative measures in the future for the resource-rich Red Sea ecosystem.

Genome sequence analysis of Zooshikella ganghwensis strain VG4 and its potential for the synthesis of antimicrobial metabolites

With antimicrobial resistance on the rise, the discovery of new compounds with novel structural scaffolds exhibiting antimicrobial properties has become an important area of research. Such compounds can serve as starting points for the development of new antimicrobials. In this report, we present the draft genome sequence of the Zooshikella ganghwensis strain VG4, isolated from Red Sea sediments, that produces metabolites with antimicrobial properties. A genomic analysis reveals that it carries at least five gene clusters that have the potential to direct biosynthesis of bioactive secondary metabolites such as polyketides and nonribosomal peptides. By using in-silico approaches, we predict the structure of these metabolites.

Quorum-Quenching Bacteria Isolated From Red Sea Sediments Reduce Biofilm Formation by Pseudomonas aeruginosa.

Quorum sensing (QS) is the process by which bacteria communicate with each other through small signaling molecules such as N-acylhomoserine lactones (AHLs). Certain bacteria can degrade AHL molecules by a process called quorum quenching (QQ); therefore, QQ can be used to control bacterial infections and biofilm formation. In this study, we aimed to identify new species of bacteria with QQ activity. Red Sea sediments were collected either from the close vicinity of seagrass or from areas with no vegetation. We isolated 72 bacterial strains, which were tested for their ability to degrade/inactivate AHL molecules. Chromobacterium violaceum CV026-based bioassay was used for the initial screening of isolates with QQ activity. QQ activity was further quantified using high-performance liquid chromatography-tandem mass spectrometry. We found that these isolates could degrade AHL molecules of different acyl chain lengths as well as modifications. 16S-rRNA sequencing of positive QQ isolates showed that they belonged to three different genera. Specifically, two isolates belonged to the genus Erythrobacter; four, Labrenzia; and one, Bacterioplanes. The genome of one representative isolate from each genus was sequenced, and potential QQ enzymes, namely, lactonases and acylases, were identified. The ability of these isolates to degrade the 3OXOC12-AHLs produced by Pseudomonas aeruginosa PAO1 and hence inhibit biofilm formation was investigated. Our results showed that the isolate VG12 (genus Labrenzia) is better than other isolates at controlling biofilm formation by PAO1 and degradation of different AHL molecules. Time-course experiments to study AHL degradation showed that VG1 (genus Erythrobacter) could degrade AHLs faster than other isolates. Thus, QQ bacteria or enzymes can be used in combination with an antibacterial to overcome antibiotic resistance.

Degradation of petroleum hydrocarbons and treatment of refinery wastewater under saline condition by a halophilic bacterial consortium enriched from marine environment (Red Sea), Jeddah, Saudi Arabia.

A halophilic bacterial consortium was enriched from Red Sea saline water and sediment samples collected from Abhor, Jeddah, Saudi Arabia. The consortium potentially degraded different low (above 90% for phenanthrene and fluorene) and high (69 ± 1.4 and 56 ± 1.8% at 50 and 100mg/L of pyrene) molecular weight polycyclic aromatic hydrocarbons (PAHs) at different concentrations under saline condition (40g/L NaCl concentration). The cell hydrophobicity (91° ± 1°) and biosurfactant production (30mN/m) confirmed potential bacterial cell interaction with PAHs to facilitate biodegradation process. Co-metabolic study with phenanthrene as co-substrate during pyrene degradation recorded 90% degradation in 12days. The consortium in continuous stirred tank reactor with petroleum refinery wastewater showed complete and 90% degradation of low and high molecular weight PAHs, respectively. The reactor study also revealed 94 ± 1.8% chemical oxygen demand removal by the halophilic consortium under saline condition (40g/L NaCl concentration). The halophilic bacterial strains present in the consortium were identified as Ochrobactrum halosaudis strain CEES1 (KX377976), Stenotrophomonas maltophilia strain CEES2 (KX377977), Achromobacter xylosoxidans strain CEES3 (KX377978) and Mesorhizobium halosaudis strain CEES4 (KX377979). Thus, the promising halophilic consortium was highly recommended to be employed in petroleum saline wastewater treatment process.

Spatial and temporal distribution of heavy metals in coastal core sediments from the Red Sea, Saudi Arabia

Summary Jeddah is the most industrialized city on the west coast of Saudi Arabia and is under increasing influence of human activities. Heavy metals data were obtained from four near-coast Red Sea sediment cores in close proximity to Jeddah. Chromium, manganese, iron, copper, zinc, and lead were analyzed from depth-resolved sections of each core via heavy acid digestion and inductively coupled plasma-mass spectrometry (ICP-MS). The average concentrations of all four sites were 245.96 mg kg −1 , 478.45 mg kg −1 , 8506.13 mg kg −1 , 251.82 mg kg −1 , 623.09 mg kg −1 , and 362.75 mg kg −1 , respectively. The depth-resolved results showed that highest concentrations of Mn, Cu, and Pb were found in the top 15 cm of the core profile distributions compared to other depth sub-samples. Heavy metal concentrations in core sediments are increased near central Jeddah and have become higher in recent years. The results of enrichment factor calculations indicate little anthropogenic supply of Mn and Cr while Pb, Zn, and Cu show strong anthropogenic input. The Pollution Load Index was higher in the two sites closer to central Jeddah where power and desalination plants and wastewater release are known. This indicates that the area has suffered from heavy metal pollution compared to other non-industrialized sites in the Red Sea. Heavy metal contaminations due to anthropogenic activity should be taken into account to protect the Red Sea during future growth. The results of this work should be considered as a baseline for heavy metals monitoring in the sediments of the Red Sea coast near Jeddah, Saudi Arabia.

Microbial-meiofaunal interrelationships in coastal sediments of the Red Sea.

Population density and biomass of bacteria and meiofauna were investigated seasonally in the sediments of the north-western bank of Red Sea. Samples of sediments were collected seasonally from three different stations to determine microphytobenthic biomass (chlorophyll a), protein, lipid, carbohydrate, and total organic matter concentrations. These investigations revealed that microbial components tended to increase their dominancy, whereas sensitive meiofauna were extremely reduced during the entire study period. Thus a very low density of the total meiofauna (with an annual average of 109 ± 26 ind./10 cm2) was recorded whilst the benthic microbial population densities exhibited higher values (ranging from 0.31 ± 0.02 × 108 to 43.67 ± 18.62 × 108/g dry sediment). These changes in the relative importance analysis of benthic microbial components versus meiofaunal ones seem to be based on the impact of organic matter accumulation on the function and structure of these benthic communities. Proteins, lipids and carbohydrates showed very low concentration values, and the organic matter mostly consisted of carbohydrates, reflecting lower nutritional values for benthic fauna in general and meiofauna in particular. The distribution of microbial and meiofaunal communities seems to be dependent on the quality of the organic matter rather than on its quantity. Total organic matter concentrations varied between 5.8 and 7.6 mg/g, with organic carbon accounting for only 32% of the total organic matter. Chlorophyll a attained very low values, fluctuating between 0.11 and 0.56 μg/g, indicating the oligotrophy of the studied area. The very low concentration of chlorophyll a in the Red Sea sediment suggests that the sedimentary organic matter, heterotrophic bacteria and/or protozoa constitute an alternative resource that is consumed by meiofauna when algae are less abundant. Protozoa, therefore, represent the “missing link in bacteria–meiofauna interaction in the Red Sea marine sediment ecosystem.

Red Sea isolation history suggested by Plio-Pleistocene seismic reflection sequences

High evaporation rates in the desert climate of the Red Sea ensure that, during glacial sea level lowstands when water exchange with the Indian Ocean was more restricted, water salinity and δ18O became unusually extreme. Modeling of the effect on Red Sea sedimentary δ18O has been used previously to reconstruct relative sea level to 500 ka and now poses the question of whether that sea-level model could be extended if continuous core material of older sediment became available. We attempt to address this question here by examining seismic reflection data. The upper Pleistocene hemipelagic sediments in the Red Sea contain intervals of inorganic aragonite precipitated during supersaturated conditions of sea-level lowstands. Seismic impedance changes associated with boundaries to those aragonite-rich layers appear to explain seismic reflection sequences. A segment of Chirp sediment profiler data from the central Red Sea reveals prominent reflections at ∼1, ∼5, ∼23, ∼26 and ∼36 ms two-way travel time (TWT) from the seabed. Based on depths to the glacial marine isotope stages (MIS) in cores, we relate the upper three reflections to the tops of aragonite-rich layers and hence the sea level rises immediately following MIS 2, 6 and 12. The reflection at 26 ms is related to an unusually rapid fall into MIS 12 predicted by one sea level reconstruction, which may have created an abrupt lower boundary to the MIS 12 aragonite-rich layer. With the aid of seismogram modeling, we tentatively associate the ∼36 ms reflection with the top of an aragonite-rich layer formed during MIS 16. Furthermore, some segments of lower frequency (airgun and sparker) seismic data from the central and southern Red Sea show a lower (earlier) Plio-Pleistocene (PP) interval that is less reflective than the upper (late) PP interval. This implies less variability in sediment impedance and that extreme variability in water salinity did not develop; water exchange with the Indian Ocean likely continued throughout this interval. These results suggest that viable relative sea level reconstructions may be recovered from Red Sea sediment δ18O data to at least MIS 16 and perhaps even as far back as the early Pliocene.

Phaeocystidibacter marisrubri sp. nov., a member of the family Cryomorphaceae isolated from Red Sea sediment.

Strain G18(T), a Gram-stain-negative, aerobic, rod-shaped, motile, non-fermentative, yellow-pigmented bacterium, was isolated from Red Sea sediment. Phylogenetic analyses based on 16S rRNA gene sequences revealed that strain G18(T) was most closely related to Phaeocystidibacter luteus PG2S01(T) with 95.3% similarity. Growth of strain G18(T) occurred at 10-42 °C (optimum 28-37 °C), pH 5.0-9.0 (optimum pH 6.0-8.0) and in the presence of 0.5-10% NaCl (optimum 2-5%). The major fatty acids of strain G18(T) were iso-C15 : 0, summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c), iso-C15 : 1 G and iso-C17 : 0 3-OH. The major polar lipids were phosphatidylethanolamine, unidentified aminolipids, phospholipids and other lipids. The predominant quinone was menaquinone 6 (MK-6). The G+C content of the genomic DNA from strain G18(T) was 39.0 mol%. Based on the phylogenetic and phenotypic properties, strain G18(T) represents a novel species of the genus Phaeocystidibacter, for which the name Phaeocystidibacter marisrubri sp. nov. is proposed. The type strain is G18(T) ( = CGMCC 1.14954(T)= JCM 30614(T)).

Distribution patterns and risks posed of polycyclic aromatic hydrocarbons contaminated in the surface sediment of the Red Sea coast (Egypt)

Coastal marine sediment samples collected from 17 locations along the Egyptian Red Sea coast in June 2009 were analyzed for aliphatic, polycyclic aromatic hydrocarbons (PAHs), and total organic carbon (TOC). Total concentrations of 16 Polycyclic Aromatic Hydrocarbons-Environmental Protection Agency (EPA-PAHs) in the studied sediment samples varied from 0.132 to 5.182 μg g−1 with an average value of 1.628 μg g−1 (dry weight). The total aliphatic concentrations fluctuated between 4.232 and 71.874 μg g−1 with an average value of 23.022 μg g−1 (dry weight). The carcinogenic PAHs (PAHCARC) concentrations were ranged between 0.063 and 3.522 μg g−1 (dry weight). The highest PAHs contents found in the samples from Ras Suder (∑5.182 μg g−1), El-Tour (4.052 μg g−1), and Sharm (3.367 μg g−1) locations. Good correlations were observed between the individual PAHs concentrations. The average total organic carbon percent (TOC%) varied between not-detected and 0.35%. The concentration of total pyrolytic hydrocarbons (∑COMB) was higher than the total fossil hydrocarbons (∑PHE), which indicated that the atmospheric fall-out was the significant source of PAHs to the Red Sea marine sediments. Principal component analysis (PCA) was used to determine the sources of hydrocarbon pollutants in the Red Sea sediments. Additionally, individual PAHs compound ratios suggested the pyrogenic origins. Human hazard risk assessment of PAHs was also studied to evaluate the possible risk of contaminated sediments to the public health concerning PAHs intakes.

Radioactivity levels in some sediment samples from Red Sea and Baltic Sea

Levels of ²²⁶,²²⁸Ra, ²³²Th, ²¹⁰Pb, ²¹⁰Po and ⁴⁰K in sediments from four monitoring areas, El Hamraween and Ras El Behar (Red Sea, Egypt) and LL3A and JML (Baltic Sea, Finland), have been investigated using alpha and gamma spectrometry. The average activity concentrations were 238±4 Bq kg⁻¹ (²²⁶Ra), 215±11 Bq kg⁻¹ (²¹⁰Pb) and 311±18 Bq kg⁻¹ (²¹⁰Po) for El Hamraween area. In Ras El Behar area, the corresponding values were 16±0.4, 18±1 and 20±5 Bq kg⁻¹, respectively. The activity concentrations for ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po (uranium series) in El Hamraween bottom sediment are much high compared with those in Ras El Behar area, which indicates the enhanced levels due to the activities of phosphate mining and shipment operations in El Hamraween area. Excluding the influence of phosphate mining activities, it can be concluded that the levels of radioactivity in Baltic Sea sediments are higher than those in Red Sea sediments.

Fractionation of trace metals (Mn, Zn, Cu, Pb) in Red Sea sediments, Gulf of Aden, Yemen

A sequential extraction technique was applied to estimate the chemical association of Mn, Zn, Cu and Pb in five chemical phases (exchangeable, carbonate, Fe–Mn oxides, organic matter and residual) in sediments of the Gulf of Aden, Yemen. The results indicated that a higher level of Mn was associated with the residual fraction (natural sources) than the non-residual fraction (anthropogenic sources). Zn fractionations revealed that it was associated more with Fe–Mn oxides and organic fractions than exchangeable and carbonate fractions. Most of the Cu was present in the residual form (60–72%) except for in the main port area (zone III), where it was associated with the organic phase (77% of the total Cu content, the organic matter content was 5%). Similarly, most Pb was bound in the residual fraction (56–71%) except the main port area where ∼ 62% of the total Pb was bound in non-residual fractions. It was also found that the Pb concentration in the exchangeable fraction was very high compared with other metals. The risk assessment code for the metals showed a low risk for Zn and Cu, but low to medium risk for Mn. Fractionation of Pb showed medium risk at most of the regions except at the eastern area, which revealed a high risk for the aquatic environment.

Sensitivity of Red Sea circulation to monsoonal variability during the Holocene: An integrated data and modeling study

[1] We used an oceanic general circulation model to evaluate the sensitivity of the hydrography and circulation of the Red Sea in response to reduced sea level and modified atmospheric conditions during the Holocene. With Holocene sea level close to the modern level, the Red Sea was sensitive to changes in atmospheric conditions, and it only shows a relatively mild response to sea level change. Changes in the monsoon system influence the exchange flow through the Strait of Bab el Mandab, the meridional overturning circulation of the Red Sea, and its hydrography. Forced by humid conditions the (modeled) Red Sea temperature increased by ∼1.5°C, while when arid conditions were imposed, the temperature decreased by ∼2.5°C. Similar heating and cooling events during the early and late Holocene are seen in a sea surface temperature record from the northern Red Sea (derived from the temperature sensitive TEX86 molecular biomarker), which suggests that humid conditions prevailed during the early Holocene and more arid conditions prevailed during the late Holocene. The gradual decline in Red Sea temperature between these two time periods suggests a gradual decline in the summer monsoon strength. This monsoon trend and the resulting changes in the Red Sea circulation are supported by the distribution of crenarchaea fossil lipids in Red Sea sediments from this period. Monsoon-driven changes in the exchange flow through the Strait of Bab el Mandab affected the crenarchaea population structure, and therefore, their molecular fossil distribution in the sediments of the Red Sea potentially provides an index for the summer monsoon strength during the Holocene.