Red Sea Holocene carbonates: Windward platform margin and lagoon near Al-Wajh, northern Saudi Arabia

ABSTRACT Analysis of Holocene sedimentary seascape is focused on the Red Sea windward Al-Wajh platform margin, its central lagoon, and nearby isolated platforms based on data that include mapped ecological facies (habitats), water depths, grain sizes, and allochem types and abundances determined from thin sections. On this basis, a depositional model applicable to Red Sea Plio-Pleistocene and other ancient icehouse carbonate platforms is presented. The model highlights favorable reservoirs in analogous ancient systems to include coral crests and columnar framework habitats with primary porosity developed in boundstone lithologies and windward platform margins to contain considerable open pore space, including cavernous openings, of which not all should be anticipated to be occluded with marine cements and sediments. Meteoric diagenesis is expected to be minor as limited freshwater is available due to extreme aridity, but may play a role during glaciation. Most habitats have potential for secondary (enhanced) porosity resulting from dissolution of aragonite skeletons, particularly mollusk shells and calcareous coral (Scleractinia) endoskeletons. Central-lagoon habitats are expected to have the least favorable reservoir potential of environments considered because they are dominated by peloids. Central-lagoon sediment differs from other published localities, having higher peloid abundances, greater peloid distribution, and little or no association with Halimeda and quartz grains. Under the likely scenario that platform-interior sediments are completely bioturbated and comprise peloid-rich, grain-dominated fabrics, with many smaller peloids (most of them likely fecal pellets) at or near 4 μm in size (i.e., mud fraction), it is possible that grain size will control pore size once the considered deposits are lithified. If so, platform-interior sediments will lithify as mudstones, wackestones, or very fine-grained grainstones, an outcome which might otherwise be unexpected given the abundance of coarse peloid grains. The Al-Wajh platform is compared with 15 Holocene analogs and found to be unique with respect to rift-margin type, restricted-marine circulation, in having a lagoon with high peloid content, and lack of karst. In further comparison with ancient reservoir analogs, two greenhouse and four icehouse, it compares favorably to icehouse platforms deposited in rift basins with respect to mineralogy of deposition, meter-scale cycle thicknesses, and general peloid content and distribution. It provides a snapshot as to how an icehouse platform might have nucleated and attached along an active rift margin; it is a broadly applicable carbonate analog for the Red Sea Plio-Pleistocene and similar icehouse, rift basins.

Changes in particle sizes and geochemistry of Siyah Keshim lagoon sediment of Gilan province to determine origin and tectonic position of sediment

Useful information was obtained about the environmental condition of this region such as energy, sediment location, origin, sediment carrying path, pond evolutionary process, and tectonic conditions and origins of sediments by studying physical and geochemical sediment changes processes in place and time, distribution of sediments and elements in lagoon bed, and also identification the effective factors on sediment distribution model. In this regard, 59 sediment samples were taken from Siyah Keshim lagoon and were analyzed chemically and aggregation by XRF and ICP-MS technic. Adaptation of data by Folk diagrams showed that this region is placed in a range of sand, muddy sand, sand with a little gravel, muddy sand with a little gravel, and silty sand. Weak to medium sorting and negative tilting shows sediment in a coastal area. Geochemical evidence shows that SiO2/Al2O3 ratio in these sediments is 2.6-3.7. In addition, Na2O value shows relatively low sediment investigation for sediments of this lagoon. Moreover, determination of the weathering effect in origin place by the chemical index of alteration (CIA) and chemical weathering index (CWI) shows intensive chemical alternation on sediments. Index of combined variety was averagely 1.54 in the studied sediments and show that sediments resulted from the first cycle sediment. Using tectonic separating diagrams based on the primary and secondary oxidants percentage show the sediment in the active continent margin (ACM), continent-island arc (CIA), and oceanic island arc (OIA) and shows that the studied sediments are related to subduction margin.

Response of a coastal lagoon sediment budget to extreme events and climate change implications: The case of the Bay of Cadiz

<p>Climate change has resulted in increased storminess and sea-level change that affects the morphodynamics of bays worldwide, impacting both the ecosystem and local infrastructure. This study explores the impact of differing storm winds on the sediment budget of the Bay of Cadiz. The Bay of Cadiz is a highly altered coastal lagoon located in Southwest Spain surrounded by ports, navigation channels, and urban developments and is of high socioeconomic and environmental importance. The human interactions with the bay have already caused morphological impacts, which could be exacerbated by increased storminess. Potential impacts on the sediment budget of the Bay of Cadiz will be modeled using a Delft3D model previously calibrated and tested using field data from December 2011 to January 2012. The model will consider a variety of storm wind scenarios and observe their impacts on sediment transport within the bay, identifying sources and sinks. This will help to estimate the potential impacts of climate change and increased storminess on the bay and the surrounding areas.</p>

Mercury Budget and Scenario Analysis for the Marano-Grado Lagoon, Using Modelling and Observations

The Marano-Grado Lagoon (MGL) is extensively contaminated by mercury (Hg) from local sources and long-term (500 years) tidally delivered inputs from the Idrija Hg mine (Slovenia) through Isonzo River suspended loads. Mercury-polluted coastal sites, often become sites of increased mercury methylation and act, in the long term, as secondary mercury sources for the Mediterranean Basin. Methylmercury (MeHg) produced upon Hg methylation bioaccumulates and biomagnifies in the trophic webs, and it is eventually transferred to humans via fish intake. We implemented a dynamic model released by US-EPA (WASP-Merc7) to the MGL (North Adriatic Sea, Italy) in order to assess the concentration of mercury species in water, sediment and particulate, and to quantify the mercury fluxes and budget within the lagoon itself and between the lagoon, the atmosphere and the Adriatic Sea. Furthermore, the model was used to simulate the mercury long-term dynamics to estimate the recovery time for Hg in lagoon sediment (about 600 years) and to explore future scenarios of climate change and rivers capping. Still several gaps exist in the knowledge on mercury species concentration and kinetics, the model results take also into account the major sources of uncertainty.