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Micropaleontological and taphonomic characteristics of mass transport deposits in the northern Gulf of Eilat/Aqaba, Red Sea
Abstract
Submarine mass transport deposits (MTDs) are a well-known phenomenon in tectonically active regions. Evidence for such deposits is commonly found in the continental slope sedimentary records, as distinct units with coarser grain size compared to the usual and continuous pelagic sedimentation. The Gulf of Eilat/Aqaba is located between the southernmost end of the Dead Sea transform and the spreading center of the Red Sea, and is considered as an active tectonic region.
... The introduction of the comment by Reijmer (2022) certainly describes the major aspects that were addressed in our paper and we thank him for his critical reading of our work. In our response, we refer to both the paper published in Sedimentology (Ash- Mor et al., 2022) and to Ash-Mor et al. (2017), published in Marine Geology, because Reijmer's comments also refer to it. ...
... The gulf continental shelf is very narrow, and averages 585 m in width at the western subbasin (Tibor et al., 2010) where the cores were collected from, meaning the biogenic sediment sources are very restricted. Noticeably, only LBF are sensitive enough to define depth differences along the shelf (Reiss & Hottinger, 1984;Perelis-Grossowicz et al., 2008; fig. 1 in Ash-Mor et al., 2017). Additional analysis on cores from the gulf is presented in an upcoming paper (Ash-Mor et al., in review), which includes the identification of: (i) a 'Holocene assemblage' dominated by the stenohaline symbiont-bearing larger species Operculina ammonoides, Amphistegina papillosa and Amphistegina bicirculata; and (ii) a 'Glacial assemblage' dominated by the hypersaline tolerant species Elphidium cf. ...
... The coarse LBF-bearing units correlate with catastrophic pre-historical events described from submerged fossilized coral reefs (Shaked et al., 2004(Shaked et al., , 2011. In the historical period, which is documented both in seismic catalogues and geological records (Ambraseys et al., 1994;Amit et al., 2002;Lefevre et al., 2018) the upper units correlate with two known earthquakes dated to 1068 CE and 1458 CE (Kanari, 2016;Ash-Mor et al., 2017). ...
The introduction of the comment by Reijmer certainly describes the major aspects that were addressed in our paper and we thank him for his critical reading of our work. In our response, we refer to both the paper published in Sedimentology (Ash‐Mor et al., 2022) and to Ash‐Mor et al. (2017), published in Marine Geology, because Reijmer's comments also refer to it.
... The study by Ash-Mor et al. (2022) discusses three main aspects: (i) the use of carbonate biota as sediment transport indicators linking the shallow-water environment with the adjacent slopes and basins; (ii) sediment sorting processes related to the size and shape of individual skeletal grains; and (iii) the impact of earthquakes on marine sedimentation patterns. This comment aims to evaluate part of the research presented in Ash- Mor et al. (2022), but also to discuss the results from the precursor study by Ash-Mor et al. (2017) discussing deposits from the same sedimentary setting. ...
... Ash-Mor et al. (2022) present a detailed study of a series of experiments in which the sediment transport capacity of continental shelf sediments and especially of large benthic foraminifera (LBF) is evaluated. The study builds on the results discussed in an earlier publication by Ash-Mor et al. (2017). The experimental results were compared with variations in the occurrence of different LBF species present within definite layers in a set of cores obtained from slopes and canyons of the Gulf of Aqaba/Eilat (Ash-Mor et al., 2017). ...
... The study builds on the results discussed in an earlier publication by Ash-Mor et al. (2017). The experimental results were compared with variations in the occurrence of different LBF species present within definite layers in a set of cores obtained from slopes and canyons of the Gulf of Aqaba/Eilat (Ash-Mor et al., 2017). The use of the Gulf of Aqaba/Eilat terminology sequence is preferred instead of Gulf of Eilat/Aqaba as used in the Ash-Mor et al. (2017,2022) publications as in the literature the Gulf of Aqaba name already has been in use for many years (e.g. ...
... Drinia and Dermitzakis (2010), Duros et al. (2017) and Mojtahid et al. (2020), analyzed turbidites, by identifying and quantifying BF and their known environmental habitat. Ash-Mor et al. (2017, 2021, used large BF as an indicator for MTDs in the Gulf of Aqaba-Eilat, by analyzing their abundance, size and preservation state. The current study uses benthic foraminiferal assemblages and their shell taphonomy to document both the source (implying for the transport distance) and the transport mechanism of sediments. ...
... Ammonia spp., Cribroelphidium spp., Elphidium spp., Porosononion spp.), although most of them are poorly preserved with a yellowish color, associated with diagenetic processes, such as pyrite reoxidation (e.g. Ash-Mor et al., 2017). A high percentage of broken shells is also observed (Figs. 3, 7). ...
... within these intervals also supports their shallow water allochthonous origin (Fig. 8). These findings confirm the basic assumption regarding allochthonous shells, which are poorly preserved (Ash-Mor et al., 2017;Duros et al., 2012Duros et al., , 2017Hayward et al., 2019;Tsujimoto et al., 2020). A visual comparison of the allochthonous to the autochthonous species (above 150 μm) did not reveal any shell-size differences, unlike Hayward et al. (2019). ...
Nile derived siliciclastic sediments are the main source of sedimentation along the Levant continental margins of the Mediterranean Sea. These sediments are transported along the southeastern Mediterranean coast via well-documented longshore currents, mainly operating along the shelf. However, the cross shelf component of sediment transport, responsible for conveying sediments toward the upper slope, is less known. To better understand the cross-shelf vs. the longshore component of sediment transport we studied a ~ 6 m long piston core (DOR280) sampled on the upper continental slope (280 m water depth) and analyzed benthic foraminiferal assemblages and their shell taphonomy alongside the particle size distribution and mineralogy of bulk sediments, to document both the source and the transport mechanism of those upper continental-slope sediments. The radiocarbon dating at the core-base is ~650 Cal. Yrs BP, indicating an exceptionally high average sedimentation rate of ~800 cm/kyr.
DOR280 consists of two alternating distinct sedimentary facies: (1) laminated (L) intervals up to ~40 cm thick each and showing a high ratio of allochthonous vs. autochthonous (allo/auto) benthic foraminiferal species and a high percentage of broken shells, indicating contribution of transported sediments originating from mid-shelf habitats; (2) non-laminated (NL) intervals up to ~200 cm thick each and showing a low allo/auto ratio and low percentages of broken shells, indicating mostly in-situ hemipelagic deposition. The L intervals are interpreted here as turbidites. The sedimentation rates calculated only for the NL intervals are still exceptionally high, thus excluding hemipelagic sedimentation as the sole mechanism for the NL deposition. Therefore, a contour bottom current transported component is suggested here as a source of the NL sedimentation.
We conclude that a mixed contourite-turbidite system actively prevails along the upper continental slope of the Levant coast, offshore Israel. The mid-shelf to upper slope sediment transport is nearly continuous and presents occasional turbidite events of average decadal reoccurrence time.
... Displaced benthic organisms, and particularly foraminifera, are one of the characteristics used for identification of turbidity current, and other mass transport deposits in the geological record, along with coarser grain size, different mineralogy and higher organic carbon content (Gao & Collins, 1994;Zabel & Schulz, 2001;de Haas et al., 2002;Masson et al., 2006;Tesi et al., 2010;Ducassou et al., 2013;Ash-Mor et al., 2017). The composition of turbidites differs from the deepsea pelagic sediments and may also vary with distance along the transport pathway (Mulder & Alexander, 2001;Stevenson et al., 2015;Mulder et al., 2019). ...
... Composition of displaced foraminifera assemblages in turbidites serve as useful indicators for the primary living environment and deposition depth, sediment source area and transport distance, at a variety of marine environments related to gravitational transport (Maslin et al., 2005;Ducassou et al., 2013;Ash-Mor et al., 2017), shallow long-shore transport (Benavente et al., 2005) and deep open ocean currents (McCave, 1995). Changes in the assemblage preservation along the transport pathway can also serve as a powerful tool to study the dynamic properties during transportation. ...
... For instance, Japan is frequently affected by earthquakes and tropical storms (Yordanova & Hohenegger, 2002;Sugawara et al., 2009), while in Italy turbidites are essentially a combination of high sedimentation rate, due to river discharge, and tectonic activity occurrence (Polonia et al., 2015). Turbidites were previously described by Ash-Mor et al. (2017) in sediment cores collected from the continental slope of the tectonically active Gulf of Eilat/Aqaba (GEA). These deposits were identified as turbidites due to their graded bedding character, typical for this type of deposit. ...
Transport of continental shelf sediments to the deep ocean can be studied from displaced symbiont‐bearing larger benthic foraminifera found in turbidity current deposits. The larger benthic foraminifera habitat depth, physical characteristics and preservation serve as indicators for understanding sediment transport dynamics near the seabed and in the water column. Here, an experiment was designed to explore sediment transport in a closed flume system using simulated high current velocities. Shelf sediments from the Gulf of Eilat/Aqaba, dominated by Amphistegina papillosa and Operculina ammonoides, were subjected to 60 and 80 cm/sec current velocities while collected in a 10 cm vertical sediment trap. Larger benthic foraminifera abundance, shell physical properties and preservation were analyzed and compared with the original bulk sediments. The experiment results showed that at 80 cm/sec velocity, larger benthic foraminifera shells of all sizes and preservations are efficiently resuspended and transported in large quantities throughout the water column, as opposed to their transport as bedload by the lower velocity current. Larger benthic foraminifera shape also has a role in the transport distances and accumulation depths. Operculina ammonoides shells were found more portable, compared to Amphistegina papillosa, due to their flatter discoid shape. The results suggest that a threshold velocity of ca 80 cm/sec was needed to generate the thick coarse deposits found in the Gulf of Eilat/Aqaba slope sedimentary record, which were previously suggested to be triggered by large magnitude seismic events. Lower velocities probably winnowed minor amounts of larger benthic foraminifera shells (with little or no coarser sediments) that were deposited as thin sand layer may point to lower magnitude seismic triggers. In conclusion, larger benthic foraminifera shells are transported and deposited in accordance to their hydrodynamic properties, resulting in assemblage differentiation along the transport pathway. This study shows that the fossil biogenic composition in slope sediments include valuable information on current velocities, transport dynamics and possible triggers in the geological record.
... These hyperpycnal flows can deposit several cm of sediment on the shelf. Given the small size of the bay, material reworking during transport events can bring shallowwater material into the deepest waters, at times with significant abrasion (Ash-Mor et al., 2017). ...
... Furthermore, based on the lithology alone we cannot rule out that wave action and not only the cascading hyperpycnal plume would have erode and transported material from the shelf. Sediment transport from the shelf to the slope was also described in seismic-induced turbidite records along the slope of the GoA, although mass flow units exhibit much coarser sediments and unconformities when correlated with known historical mass flow events (Ash-Mor et al., 2017;Ash-Mor et al., 2022). ...
Storms have long been recognized as a force that agitates sediments, even in the deep-water. In shallow water depths (<40 m) sediments are affected dominantly by wave actions, while in deeper-water (>100 m) the impact is primarily due to suspended sediment flow. While such transport events have been studied sedimentologically extensively, their impact on porewater chemistry is yet poorly understood. The March 2020 tropical storm which impacted the Gulf of Aqaba (GoA) offered a unique opportunity to investigate this particular topic. Detailed sedimentological, mineralogical and porewater analyses were carried out on two sets of short cores collected along a transect from 270 m to 700 m water depths before and after the storm on the western margins of the GoA, which represents a steep slope with a potential for high energy transport events. The cores exhibited a decrease in grain size in the mid-slope water depth and an increase at the base of the slope after the storm, with the top of the core being enriched with terrestrial material. Both Fe 2+ and NO x profiles exhibited a marked shift in the aftermath of the storm. NO x concentrations increased in the top 5 cm of all the cores and the ferruginous zone migrated upwards in both the top and bottom of the slope. We postulate that the storm event remixed the top of the sediment column and infused it with fine material. This filled burrows and decreased the diffusive coefficient across the sediment-water interface, limiting the exchange of ions. As a result, the porewater trapped in the sediment has developed along the terminal electron acceptor chain.
... The slopes are spotted with fresh scars and collapsed materials (Tibor et al., 2010), implying submarine landslides, which may be tsunamigenic. Based on underwater drill cores, Kanari et al. (2014) and Ash-Mor et al. (2017) suggested correlating such landslides with the strong 1068 AD and 1458 AD earthquakes along the DST. ...
... Indirect evidence of past tsunamis might be the finding of submarine mass-transport deposits in two different marine boreholes at the HGEA (Kanari et al., 2014;Ash-Mor et al., 2017). The two studies suggest a correlation between the submarine landslides and paleo and historic (1068 AD and 1458 AD) earthquakes. ...
Unique geological and seismotectonic settings may trigger a multicascading hazard and should be identified beforehand. Such is the head of the Gulf of Elat-Aqaba (HGEA) at the northeastern end of the Red Sea where its geology, tectonics, bathymetry, and earthquake and tsunami history exhibit clear potential for earthquake and submarine-landslide tsunami generation. We thus investigated the possible tsunamigenic sources in the gulf and evaluated the resulting hazard at the HGEA. First, we assembled a bathymetric grid and adopted GeoClaw software to simulate most of the earthquake-tsunami scenarios. Next, we resolved the scheme of the largest possible tsunamigenic earthquakes along the deep basins of the Gulf of Elat (GEA) and the associated Dead Sea rift valley, as well as the potential tsunamigenic submarine landslides in the HGEA. The use of GeoClaw was verified against the 1995 tsunami generated by the Nuweiba Mw 7.2 earthquake, and then operated to simulate a suite of earthquake scenarios. Results showed that the marginal faults of Elat Basin pose the highest tsunami hazard to the Israeli part of the HGEA. To better assess that hazard, we screened the geology and seismotectonics of the HGEA and found that the Elat normal fault presents the worst-case scenario for Elat city. It is capable of generating a multicascading threat of earthquake and submarine-landslide tsunami, local subsidence that can increase inundation, and above all, destructive ground motion. Scenarios of a tsunami caused by the worst-case earthquake on the Elat fault simulated by GeoClaw and Ward's (Tsunami, The encyclopedia of solid earth geophysics. 2011, 1473-1493) approach, and submarine landslide in the HGEA simulated by Wang et al.'s (Geophys. J. Int., 2015, 201, 1534-1544) 'Tsunami Squares' approach, demonstrated waves as high as 4 m along these coasts. Accordingly, we constructed a map of the evacuation zone. We also show that strong ground-shaking and retreat of the sea at the HGEA should be considered a tsunami warning, although false alarms are inevitable. Furthermore, tsunami hazard exists all along the gulf and further assessments are needed to quantify this hazard and increase awareness among the area's population.
... Contrasting displaced foraminifera with modern autochthonous fauna allows the identification of the sedimentary source (original depth of deposition) and the inference of transport processes (Ash-Mor et al., 2017;Ash-Mor et al., 2021;Usami et al., 2017). The inclusion of gravitationally displaced individuals (i.e., allochthonous fauna) in the autochthonous foraminiferal assemblage from deeper part of the basins will alter the diversity and dominance parameters of the local fauna (Hayward et al., 2019;Schröder-Adams et al., 2008). ...
The calcite tests of foraminifera are an important biogenic component of marine sediments. The abundance of foraminiferal tests in marine sediments broadly varies with bathymetry, thus has been used to reconstruct paleobathymetry. It is also promising as a tracer for downslope transport triggered by earthquakes and typhoons, especially if the displaced material from shallow locality contrasts strongly with the background autochthonous sediments in terms of foraminiferal abundance, such as the ratio of benthic and planktic foraminifera termed %P. However, its applicability in sediments off Taiwan has not been assessed. Taiwan is located in the path of typhoons and at tectonic plate margins, where typhoons and earthquakes may trigger submarine geohazards. This, combined with the fact that its seafloor spans a large bathymetric range, render this region an ideal natural laboratory to evaluate the applicability of %P as a proxy for tracing submarine geohazards and bathymetry. Here we report foraminiferal abundance, %P, grain size and elemental data from 148 surface sediment samples off 6 sectors off Taiwan, namely Southern Okinawa Trough, Hoping-Nanao-Hateruma Basins, Taitung-Hualien, Hengchun Ridge, Gaoping, and Changyun Sand Ridge. Of all the hydrographic and sedimentological parameters assessed, seafloor bathymetry is the major driver of foraminiferal abundance and %P in these regions. Notably, several data points deviate from the regional %P-water depth relationship. Based on sedimentological parameters and previous studies, we posit that these outliers may have to do with local sedimentation setting. These processes include earthquake-induced sediment transport via submarine canyon in the Southern Okinawa Trough, typhoon-triggered sediment flushing in Gaoping Canyon, cross-shelf and northward advection of planktic foraminifera on the Gaoping shelf, and carbonate dissolution in the deep Hateruma Basin. Off Taiwan, the %P value in sediments increases exponentially with bathymetry (R2 = 0.72, n = 81), and agrees well with the global calibration obtained by combining data from several regions of the global ocean (R2 = 0.86, n = 1004). The regional %P-water depth relationship may be useful for reconstructing paleobathymetry here, albeit with an uncertainty in the range of 14–1600 m. The uncertainty increases with water depth. Our results also highlight the potential of the %P index as a tracer for downslope transport and lateral advection in the water column. In conclusion, the downcore application of %P has the potential to reconstruct past geohazard events while also identifying autochthonous sediment sequences that are suitable for paleoceanographic reconstruction.
... Hence, dating samples below and above the slide deposit, provide the maximum and minimum ages of the slide event, respectively. Sediments from within the slide deposit can also be dated, although their age may represent the sediment deposition before being transported by the actual failure event (Lee, 2009;Ash-mor et al., 2017). Usually, the minimum age represents a better estimation for the event age, since the dated sediments have been deposited immediately after sliding (Owen et al., 2007). ...
Unravelling the failure history of submarine slides and delineating its environmental controls are crucial for practical hazard assessments, but are commonly challenging and costly. Here we address this challenge in investigating the Goliath slide complex, one of the largest documented slide scars on the ~200 km long continental slope offshore Israel. Geophysical evidence suggest that the Goliath complex is a relatively young feature. However, despite its proximity to submarine infrastructure and to one of the most populated coastal areas in the Mediterranean, its recent temporal development was never determined.
Based on a detailed analysis of sub-meter resolution multi-channel seismic profiles, four sediment cores were retrieved from the northern head scarp and toe domains of the Goliath slide complex. Selective sedimentological and chronological analyses were carried out following a detailed computed tomography (CT) analysis of the cores. The results reveal two generations of deposits on the northern head scarp of Goliath, separated by a detachment surface. Two ¹⁴C ages of 7.46±0.13 cal ka BP (core PHS-1) and 7.38±0.16 cal ka BP (core PHS-5) were obtained for the sediments immediately overlying this surface, implying that the Goliath northern head scarp was formed, at least in part, during a substantial ~7.4 cal ka BP event. A ~3.7 kyrs hiatus within the late Holocene sedimentary sequence of core PHS-5 may suggest an additional, more recent and presumably smaller, slide event that occurred at ~2.4 ka BP. The sedimentary succession from Goliath’s toe domain (core PTL-3), located ~32 km downslope from the head scarp, show an upper 1.2 m long continuous undisturbed sequence dated to the last ~14 ka BP and including sapropel S1 deposits. This sequence overlies three disturbed units, interpreted as mass transport deposits (MTDs) containing various deformation features, large-scale burrows, sharp contacts and fold structures. A ¹⁴C age dates the base of the hemipelagic sequence to 13.91±0.23 cal ka BP, representing the minimum age of the slide event that deposited this MTD. Ages within the MTD units are substantially older (18.21±0.27 to 28.66±0.33 cal ka BP) than in the overlying sequence and an age-inversion within the MTDs supports their interpretation as transported sediments.
The timing of the prominent slide events in the Goliath complex corresponds with the late stages of meltwater pulses 1A (~14.6-13.7 ka BP) and 1C (~8.2-7.5 ka BP), periods of accelerated sea level rise and probably increased sediments supply. Our results suggest that climate-driven factors likely contributed significantly to slope instability in this area, while earthquakes on the Dead Sea Transform may have been the triggering factor for the Goliath Slide events.
The focus of this study was to determine the long-term fate of oil-residues from the 2010 Deepwater Horizon (DwH) oil spill due to remobilization, transport, and re-distribution of oil residue contaminated sediments to down-slope depocenters following initial deposition on the seafloor. We characterized hydrocarbon residues, bulk sediment organic matter, ease of resuspension, sedimentology, and accumulation rates to define distribution patterns in a 14,300 km2 area southeast of the DwH wellhead (1,500 to 2,600 m water depth). Oil-residues from the DwH were detected at low concentrations in 62% of the studied sites at specific sediment layers, denoting episodic deposition of oil-residues during 2010–2014 and 2015–2018 periods. DwH oil residues exhibited a spatial distribution pattern that did not correspond with the distribution of the surface oil slick, subsurface plume or original seafloor spatial expression. Three different regions were apparent in the overall study area and distinguished by the episodic nature of sediment accumulation, the ease of sediment resuspension, the timing of oil-residue deposition, carbon content and isotopic composition and foram fracturing extent. These data indicate that resuspension and down-slope redistribution of oil-residues occurred in the years following the DwH event and must be considered in determining the fate of the spilled oil deposited on the seafloor.
It is hypothesized that submarine transport of sediments down a continental slope induces physical disintegration of pristine (non-broken) foraminiferal shells, and thus mass transport deposits should include a significant percentage of fragmented shells. To validate this hypothesis, we studied two gravity-cores from the eastern Mediterranean continental slope, offshore Israel: AM113 sampled within a landslide lobe at 848 m water depth, and AM015 located away from a landslide at 1080 m. At least one interval, c. 0.5 m thick, of heterogeneous sediments (i.e. debrite) was identified within each core. The timing of these debrites, based on biostratigraphy, oxygen isotopes and total organic carbon data, predates sapropel S1 in both cores and is contemporaneous (AM113) or slightly predates (AM015) the most recent deglaciation.
We found a noticeable increase in the fragmentation of benthic and planktic foraminiferal shells through the last deglaciation and up to the base of S1. This strongly fragmented sequence is located in the debrite of AM113 but overlays the debrite of AM015. Accordingly, we suggest two possible mechanisms for the increased fragmentation of foraminiferal shells in both cores: sediment transport and turbulence related to submarine mass-transport events, or geochemical changes in the lower water column properties at the transition from MIS-2 to the Holocene.
The study focuses on description, analysis and mapping of the thick succession of calciturbidites facies for the first time on the Early Cretaceous Arabian Platform Passive Margin from Western Zagros Fold–Thrust belt, Northeastern Iraq (Kurdistan Region). Along the margin, the study revealed active source of shedding voluminous calciturbidites sediments to the periplatform and deep basin of Neo-Tethys sea during the Early Cretaceous. Turbidity currents reworked pure coarse and fine detrital carbonates, from the platform, and transported long distance to the basin passing through the slope of the basin. Along this distance, the sediments crossed the marginal slope scoring submarine channels and depositing megabreccias in addition to coarse detrital carbonates (rudstone) before reaching the basin plain. On the plain mostly the fine grain facies of calciturbidites lay down and mixed with pelagic sediments such as radiolarias and calcispheres. The calciturbidite evidence can be tracked for more than 50 km in the studied area from platform to the basin across the slope. These latter facies are occasionally graded, laminated and cross-laminated with deep basin fossil such as planktonic foraminifera, and radiolarians. Stratigraphically, Qamchuqa, as carbonate factory, and Balambo Formations represent the platform and the basin facies, respectively. The periplatform (slope) was northeastward-facing and its toe was the depositional locus of shared lithology between the two aforementioned units. These formations are two Early Cretaceous rocks successions that crop out in Zagros Fold–Thrust Belt and occur in most famous oil fields in Middle East. These calciturbidites are keys for picturing boundary condition of the Passive Arabian Continental (platform) Margin, which is proved to be dynamic, stormy and with contrasted bathymetry (high topography). The study totally changes all characteristics of Balambo Formation from bathyal fine grain deposits to mixture of different carbonate grain of calciturbidite and pelagic origins.
The Gulf of Aqaba is an oligotrophic marine system with an oxygen-rich water column. Aeolian dust from the Arabian, Sinai, and Sahara deserts is an important source of sedimentary material to the gulf, especially at 700 m water depth. The head of the gulf is affected by sediment transport from the Arava desert during winter flash floods. In this work, we have studied the speciation of iron in the dust, dry creek sediments and sediments of the Gulf of Aqaba at various water depths in order to understand sources and transformations of iron. Two sources of iron, dust and flash floods transported material, were found to possess distinct geochemical signatures: dust was found to be enriched in total and highly reactive iron relative to the sediments in creek beds. Combination of these two sources leads to an increase of highly reactive iron in sediments with water depth. This increase, in turn, results in formation of a lateral redox gradient with sulfidic pore-waters near the shore, and ferruginous-manganous pore-waters and cryptic sulfur cycling at the deeper water sites. Another result of dry aeolian deposition of desert dust to the sediments of the Gulf of Aqaba, overlaid by deep well‑oxygenated waters, is anomalously high ratios of highly reactive to total iron, which have been proposed to be diagnostic for anoxic, iron-rich water columns, when applied as a paleoproxy.
The distribution of living (Rose Bengal-stained), dead and fossil benthic foraminifera was investigated in six short cores
(multicores, 30–32 cm total length) recovered from the central Red Sea. The ecological preferences as well as the relationship
between the live and dead/fossil assemblages (preserved down-core) were examined. The sites, located along a W-E profile and
between the depth of 366 and 1782 m, extend from the center of the oxygen minimum zone (OMZ, ~200–650 m), through its margin
at ~600 m, and down to the well-aerated deep-water environment. Live (Rose-Bengal stained) and coexisting dead foraminifera
were studied in the upper 5 cm of each of the sites, and the fossil record was studied down to ~32 cm. Q-mode Principal Component
Analysis was used and four distinct foraminiferal fossil assemblages were determined. These assemblages follow different water
mass properties. In the center of the OMZ, where the organic carbon content is highest and the oxygen concentration is lowest
(≤0.5 ml O2/1), the Bolivina persiensis-Bulimina marginata-Discorbinella rhodiensis assemblage dominates. The slightly more aerated and lower organic-carbon-content seafloor, at the margin of the OMZ, is characterized
by the Neouvigerina porrecta-Gyroidinoides cf. G. soldanii assemblage. The transitional environment, between 900–1200 m, with its well-aerated and oligotrophic seafloor, is dominated
by the Neouvigerina ampullacea-Cibicides mabahethi assemblage. The deeper water (>1500 m), characterized by the most oxygenated
and oligotrophic seafloor conditions, is associated with the Astrononion sp. A-Hanzawaia sp. A assemblage.
Throughout the Red Sea extremely high values of temperature and salinity are constant below ~200 m depth, but the flux of
organic matter to the sea floor varies considerably with bathymetry and appears to be the main controlling factor governing
the distribution pattern of the benthic foraminifera. Comparison between live and the dead/fossil assemblages reveals a large
difference between the two. Processes that may control this difference include species-specific high turnover rates, and preferential
predation and loss of fragile taxa (either by chemical or microbial processes). Significant variations in the degree of loss
of the organic-cemented agglutinants were observed down core. This group is preserved down to 5–10 cm at the shallow OMZ sites
and down to greater depths at well-aerated and oligotrophic sites. The lower rate of disintegration of these forms, in the
deeper locations of the Red Sea, may be related to low microbial activity. This results in the preservation of increasing
numbers of organic-cemented shells down-core.
Submarine gravity flows are a key process for transporting large volumes of sediment from the continents to the deep sea. The location, volume, and character of the sediment bypassed by these flows dictates the areal extent and thickness of the associated deposits. Despite its importance, sediment bypass is poorly understood in terms of flow processes and the associated stratigraphic expression. We first examine the relationships between the physical parameters that govern bypass in flows, before assessing the variable stratigraphic expression of bypass from modern seafloor, outcrop, and subsurface datasets. Theoretical and numerical approaches distinguish grain size, slope, flow size, and sediment concentration as parameters that exert major controls on flow bypass. From field data, a suite of criteria are established to recognize bypass in the geological record. We identify four bypass-dominated zones, each of which is associated with a set of diagnostic criteria: slope-channel bypass, slope-bypass from mass wasting events, base-of-slope bypass, and basin-floor bypass. As the expression of bypass varies spatially and is dependent on the scale of observation, a range of scale-dependent criteria are required for robust interpretation of these zones in the field or subsurface. This synthesis of deep-water sediment bypass highlights the challenge in quantitatively linking process with product. The establishment of criteria to recognize sediment bypass, qualitatively linked with flow processes, is an important step towards improving our understanding of submarine flow dynamics and resultant stratigraphic architecture.
Examination of fossil reefs in 3-D reveals a series of tectonic, climatic, and biological processes in the northwestern corner of the Gulf of Elat-Aqaba (GEA). We excavated a reef buried beneath clastic beach sediments, examined its morphology, and collected coral samples in situ. The reef is preserved in pristine condition due to the sedimentary cover and its position below the sea level. Coastal geomorphology and sediments are combined with evidence from the buried reef to describe the late Holocene evolution of this stretch of the coast that is shaped by activity of the Dead Sea Transform system. Radiocarbon and uranium series ages of coral samples from the reef set the temporal framework for events. At least two down-faulting events are inferred from the buried reef, at ~4.7 ka and ~2.4 ka. Following the second event the reef was completely bur-ied and fossilized, and the shoreline propagated 100 m eastwards into the sea. Hence the present shoreline was shaped by recurrent tectonic displacements followed by redistribution of sediments along the coast.
The Gulf of Aqaba (Gulf of Eilat) is a terminal elongated basin that
exchanges water with the northern Red Sea via the Straits of Tiran. The
gulf's hydrography exhibits strong seasonal variability, with deep
mixing during February-March and stable stratification afterward. We use
an oceanic model to investigate the annual cycle of the general
circulation and hydrographic conditions in the gulf. We demonstrate that
on a subannual time scale, the general circulation deviates from the
standard depiction of inverse estuarine circulation. During the
restratification season (April-August), the exchange flux with the
northern Red Sea is maximal and is driven by density differences between
the basins, while atmospheric fluxes actually counteract this exchange
flow. The observed warming of the surface layer is mainly due to
advection of warm water from the northern Red Sea, with a smaller
contribution from surface heating. During the mixing season
(September-March), the exchange flux and the advection of heat are
minimal and atmospheric fluxes drive convection rather than the exchange
flow. We estimate the seasonality of the exchange flow through the
Straits of Tiran. The seasonal variability in the exchange flow is large
and ranges from 0.04 Sv during early spring to 0.005 Sv during early
winter.
The Dead Sea Transform Fault constitutes the northwestern boundary of the Arabian plate, accommodating the plate’s lateral movement relative to the African plate. A complete and homogeneous catalogue of historical earthquakes has been compiled and used in the subdivision of the fault area into the following segments: 1) Araba segment, which extends along Wadi Araba and the southernmost part of the Dead Sea (29.5°-31.3°N) and trends SSW-NNE with scarce historical and instrumental seismicity; 2) Jordan-valley segment, which extends along the central and northern parts of the Dead Sea and the Jordan valley to the Huleh depression (31.3°-33.1° N) and trends S-N with moderate historical seismicity; 3) Beqa’a segment, which extends along the western margin of the Beqa’a valley in Lebanon (33.1°-34.5°N) and trends SSW-NNE with strong historical seismicity; 4) El-Ghab segment, which extends along the eastern flank of the coastal mountain range of Syria (34.5°-35.8°N) and trends S-N with moderate historical seismicity; 5) Karasu segment, which extends along the Karasu valley in SE Turkey (35.8°-37.3°N) and trends SSW-NNE, exhibiting the strongest historical seismicity of the area. Probabilities for the generation of strong (M > 6.0) earthquakes in these segments during the next decade are given, by the application of the regional time and magnitude predictable model.
It can be argued that Japan's giant 11 March 2011 Tohoku earthquake and its tsunami should not have come as geophysical surprises. GPS had shown that the entire subduction zone along the Japan Trench behaves as one enormous unit rather than segmented sections that rupture with different frequencies and strengths [Suwa et al., 2006]. Historical documents and coastal sand deposits had shown a history of Sendai area tsunamis larger than those of recent centuries [Sawai et al., 2008]. Such hindsight brings to mind the Cascadia subduction zone, with its history of great earthquakes that, despite being literally in the backyards of many Pacific Northwest geologists and geophysicists, went unrecognized until late in the twentieth century. In 1969, at the dawn of the development of plate tectonic theory, scientists lacked a record of significant historical earthquakes along the U.S. northwestern coast and had no reason to believe that there were any seismic hazards in the region. What was known was that deep-sea cores taken offshore in 1965-1967 contained extensive turbidity current deposits--- graded beds of sand and silt--- interbedded with pelagic clays, sequences that by 1969 I had just finished studying for my doctoral dissertation. At the time, those turbidites (the sediments deposited by turbidity currents or submarine mudflows) posed a great enigma. What process or disturbance could have generated these recurring turbidity currents?
Symbiont-bearing benthic foraminifera are restricted to the euphotic zone of tropical and warm-temperate seas. Species distribution is correlated with depth, and the continuous alteration of community structures represents a coenocline. Since depth is a composite environmental gradient, the coenocline of larger foraminifera is not stable but alters with changes in primary limiting factors: temperature, light, water movement, substrate, and nutrients. Temperature determines geographic distribution and affects the depth distribution of larger foraminifera by the development of a shallow thermocline that truncates the distribution of shallower species and excludes species adapted to the deepest euphotic zone. Within these constraints, light is the most important primary factor because larger foraminifera are at least partly dependent upon photosynthesis by their algal endosymbionts for growth and calcification. The microalgae show distinct intervals along the light gradient and the foraminiferal host develops various strategies for regulating light intensity. First, well-structured environments in shallow waters allow shelter against irradiation by protecting in shadow areas. Second, wall and test structures enable regulation of light penetration. A range of mechanisms allows species to resist the highest energies in the breaker zone of the reef edge and crest, where foraminifera attach to inorganic or organic hard substrates. Concentrations of dissolved and particulate organic matter in the water column, as well as sediments or other inorganic particles, influence depth distributions by changing water transparency and, therefore, photosynthesis. Permanent or episodic elevations of concentrations therefore compress the coenocline upward. Species adapted to hard substrates must compete for the reduced space, while species living in the deepest euphotic zone are at a disadvantage because they are insufficiently motile to surmount large depth differences. Changing light transparencies due to nutrient input and different hydrodynamic conditions alter relations between the light coenocline and water depth. Thus, paleodepth interpretations based on larger foraminiferal assemblages should be based not only on foraminiferal taxa and ecology, but also on environmental evidence for climate, terrigeneous influence, water transparency, and hydrodynamic conditions based on sedimentology, geochemistry, and associated fossil biota.
In the shallow-water area (0–70 m) of the Northern Bay of Safaga, 73 surface samples were studied with respect to total foraminiferal fauna. The samples cover a great variety of shallow-water environments and yielded 239 foraminiferal taxa. Based on q-mode cluster analysis, and tested by canonical discriminant analysis, the samples are grouped into 13 foraminiferal associations, each characterized by several species:(1) Quinqueloculina mosharrafai-Borelis schlumbergeri-Brizalina simpsoni Ass.,(2) Heterostegina depressa-Amphlstegina lessonii/bicirculata Ass.,(3) Cibicidids-Rosalina-Amphistegina lobifera-Pseudoschlumbergerina ovata Ass.,(4) Peneroplis planatus Ass.,(5) Peneroplis planatus-Varidentella neostriatula Ass.,(6) Peneroplis planatus-Coscinospira hemprichii-Varidentella neostriatula Ass.,(7) Quinqueloculina spp. Ass.,(8) Hauerina diversa-Sorites orbiculus Ass.,(9) Verneuilina sp.-n Articulina pacifica-Reussella simplex Ass.,(10) Textularia agglutinans/rugulosa-Bolivina variabilis Ass.,(11) Textularia agglutinans-Challengerella bradyi-Elphidium jenseni/simplex Ass.,(12) Operculina ammonoides-Adelosina laevigata-Brizalina striatula/subspathulata Ass.,(13) Bolivina variabilis—Miliolinella-Nonion fabum-Elphidium simplex Ass.The characteristic forms of each association are linked to specific environmental parameters and modes of life. As long as taxonomic uniformitarianism can be applied, similar associations with comparable ecological demands may also be detected in the fossil record. For comparisons with taxonomically different material the 13 associations are summarized into 4 categories each reflecting a specific type of substrate. In the fossil record these categories may be recognized by general morphological characteristics of the foraminiferal tests and by sedimentological data. The 4 substrate types are: hardground, sand (with or without seagrass and/or corals), firmground, and soft bottom.
A total of 256 recent benthic foraminiferal species belonging to 111 genera is identified from 73 carbonate-siliciclastic sediment samples collected along the coast of southern Somalia (Burgao channel) and eastern Kenya (Lamu Archipelago). This represents the first survey of recent foraminifers in this region of East Africa. The sediment samples are representative of different environments (e.g., mangrove flats, tidal channels, restricted shelf, and open shelf), water depths (0-60 m), and salinities (strongly brackish to normal marine conditions). Q-mode Hierarchical Cluster Analysis reveals that nine assemblages can be distinguished; for each assemblage, faunal composition and distribution, diversity indices and dominance
are assessed. As predicted by ecological models, there is an overall trend of increasing species diversity from the backwater zone within channels to open marine conditions, and from intertidal to subtidal settings. Salinity, suspended sediment, nutrient levels, and tidal exposure are the most influential factors in determining benthic foraminiferal distribution patterns. An interesting feature is the nearshore contraction of the depth gradient determining the shallower distribution of several larger foraminiferal species, as evidenced by the depth ranges of five species of Amphistegina.
Summary The depth distributions of larger foraminifera (27 species) were investigated along two transects in the fore reef areas of
a NW Pacific fringing reef. One transect is distinguished by a strong flattening below the steep reef slope (−30 m), whereas
further steepening characterizes the equivalent part in the other transect. According to the different taphonomic processes
affecting foraminiferal tests before final sedimentation, empty tests were classified into the three categories ‘optimally’,
‘well’ and ‘poorly’ preserved. The depth distribution of each preservation state was compared with living individuals. While
distributions of optimally preserved tests almost coincide with living individuals, well-preserved tests are characterized
by significant depth shifts that are stronger at the upper-most slope compared with the deeper parts. Since the time-averaged
traction forces are similar in both investigated transects, differences between the distributions of living individuals and
well-preserved tests are more intensive on steep versus flat slopes. Poorly preserved tests signalize allochthonous origin
or reworking of relict sediments.
Using specimens of Amphistegina lessonii, A. lobifera and A. papillosa from the Gulf of Elat, changes in shell shape with depth observed in previous studies can be quantitatively accounted for by a corresponding trend in thickness of secondary laminae. -Authors
Three multicores were studied in the Central Red Sea off Port Sudan at a W—E transect in order to reconstruct palaeoceanographic conditions of the past ~6000 years. Downcore fluctuations in the relative abundance of the epipelagic planktic foraminifera Globigerinoides ruber versus that of the deeper dweller G. sacculifer, together with the presence/absence pattern of the mesopelagic pteropod Limacina bulimoides, reflect variations in the mixed layer and intermediate water column properties and enabled a division of the past ~6000 years into five zones. These palaeoceanographic changes reflect late-Holocene climate fluctuations. Two distinct climate systems regulate the hydrography of the central Red Sea: the Mediterranean climate system to the north controls the formations of the deeper waters, at times of drier and harsher winters, with periods of increased abundance of G. sacculifer reflecting a vigorous deep-water formation in the northern Red Sea. The second is the monsoonal climate system that regulates the nutrient input from the south and the shifting northward position of the Intertropical Convergence Zone (ITCZ) at times of monsoon intensification. Increasing abundances in G. ruber indicate greater influence of the monsoonal climate system. The most arid interval (4200—3400 yr BP) is associated with high abundance of G. sacculifer and maximum abundance of Limacina bulimoides , linked to the Mediterranean climate system. This period is of a wide regional scale and also coincides with a pulse of weakening in the Indian Ocean monsoonal system.
1] A comprehensive multisite paleoseismic archive of the late Holocene Dead Sea basin (past 2500 years) is established by constructing two age‐depth chronological models of two sedimentary sections exposed at the retreating shores of the modern Dead Sea. Two new paleoseismic study sites studied are the Ein Feshkha Nature Reserve outcrop located at the northern part of the basin and close to an active underwater transverse fault and the east Ze'elim Gully outcrop at the southern part of the basin. Age‐depth regression models are calculated for these sections based on atmospheric radiocarbon ages of short‐lived organic debris calibrated with a Bayesian model. The uncertainties on individual model ages are smaller than 100 years. The new chronological records are compared to a laminae‐counting study of the Ein Gedi core (Migowski et al., 2004) located at the central Dead Sea basin. The Ein Feshkha outcrop yielded the largest number of seismites in the studied time interval (n = 52), while lower numbers of seismites are recovered from the Ze'elim outcrop and Ein Gedi core (n = 15 and 36, respectively). The seismites show no strong dependence on the limnological‐sedimentological conditions in the particular sampling sites (they coappear in both shallow and deep water environments and in different sedimentary facies). During time intervals when the chronologies are comparable it appears that the number of seismites is significantly larger in the northern part of the basin (Ein Gedi and Ein Feshkha). Seismic quiescence intervals are apparent at all three sites from 2nd–4th century A.D. and at 500–150 B.C. at Ze'elim and Ein Gedi. Several synchronous seismites appear in all sections (termed here the intrabasin seismites (IBS)). Among them: 1927, 1293, 1202/1212, 749, 551, 419, and 33 A.D. and 31 and mid‐2nd century B.C. The recurrence time of the IBS from the 2nd century B.C. to the 14th century A.D. is ∼200 years, compared with ∼100 years for all earthquakes. On a diagram of epicentral distance versus magnitude, historic earthquakes that are correlated with IBS plot in a field of high local intensity. The farther and stronger IBS earthquakes require lower local intensities to be recorded. This study demonstrates that a painstaking effort is still needed for unraveling the seismic history of the Dead Sea basin. The results also indicate that such a study will likely be highly rewarding.
A high-resolution marine geophysical study was conducted during October-November 2006 in the northern Gulf of Aqaba/Eilat,
providing the first multibeam imaging of the seafloor across the entire gulf head spanning both Israeli and Jordanian territorial
waters. Analyses of the seafloor morphology show that the gulf head can be subdivided into the Eilat and Aqaba subbasins separated
by the north-south-trending Ayla high. The Aqaba submarine basin appears starved of sediment supply, apparently causing erosion
and a landward retreat of the shelf edge. Along the eastern border of this subbasin, the shelf is largely absent and its margin
is influenced by the Aqaba Fault zone that forms a steep slope partially covered by sedimentary fan deltas from the adjacent
ephemeral drainages. The Eilat subbasin, west of the Ayla high, receives a large amount of sediment derived from the extensive
drainage basins of the Arava Valley (Wadi ’Arabah) and Yutim River to the north–northeast. These sediments and those entering
from canyons on the south-western border of this subbasin are transported to the deep basin by turbidity currents and gravity
slides, forming the Arava submarine fan. Large detached blocks and collapsed walls of submarine canyons and the western gulf
margin indicate that mass wasting may be triggered by seismic activity. Seafloor lineaments defined by slope gradient analyses
suggest that the Eilat Canyon and the boundaries of the Ayla high align along north- to northwest-striking fault systems—the
Evrona Fault zone to the west and the Ayla Fault zone to the east. The shelf–slope break that lies along the 100m isobath
in the Eilat subbasin, and shallower (70–80m isobaths) in the Aqaba subbasin, is offset by approx. 150m along the eastern
edge of the Ayla high. This offset might be the result of horizontal and vertical movements along what we call the Ayla Fault
on the east side of the structure. Remnants of two marine terraces at 100m and approx. 150m water depths line the southwest
margin of the gulf. These terraces are truncated by faulting along their northern end. Fossil coral reefs, which have a similar
morphological appearance to the present-day, basin margin reefs, crop out along these deeper submarine terraces and along
the shelf–slope break. One fossil reef is exposed on the shelf across the Ayla high at about 60–63m water depth but is either
covered or eroded in the adjacent subbasins. The offshore extension of the Evrona Fault offsets a fossil reef along the shelf
and extends south of the canyon to linear fractures on the deep basin floor.
The general Pleistocene architecture of the Amazon Fan has been reconstructed using sediment recovered by Ocean Drilling Program Leg 155. Huge regional mass-transport deposits (MTDs) make up a significant component of the Amazon Fan. These deposits each cover an area over 15,000 km2 (approximately the size of Jamaica), reach a maximum thickness of 200 m, and consist of ∼5000 Gt of sediment. Benthic foraminiferal fauna analysis and sedimentology indicate that the MTDs originated on the continental slope, which is at least 200 km laterally and 1500 m above their present position. Each mass-failure event was formed by the catastrophic failure of the continental slope and has been dated and correlated with climate-induced changes in sea level. Studies of the benthic foraminiferal assemblages in the Amazon Fan has been essential to our reconstruction of the origin and cause of these failures. The MTDs contain rare shelf (Quinqueloculina cf. stalkeri, Brizalina aenariensis, Q. lamarckiana, and Pseudononion atlanticum) and dominant upper-middle bathyal species (cassidulinids and buliminids). We conclude that the MTD originated between 200 and 600 m water depth, approximately the same zone in which gas hydrates occur. We suggest that the glacial MTDs referred to as Deep Eastern MTD (35–37 ka) and Unit R MTD (41–45 ka) correlate with rapid drops in sea level which destabilized continental slope gas-hydrate reservoirs causing catastrophic slope failure. An alternative explanation is required for the deglacial MTDs referred to as Western and Eastern Debris Flows (13–14 ka) which occurred as sea level rose rapidly during the Bølling-Allerød period. We suggest that the deglaciation of the Andes and the consequent enhanced sediment supply coupled with a shift of the depo-centre to the continental shelf, caused over-burdening and thus slope failure. Evidence for a 2‰ negative δ13C shift in both planktonic foraminifera and organic matter coeval with these failures suggest that whatever the cause, there was a large release of methane hydrate associated with each failure.
Huge landslides, mobilizing hundreds to thousands of km3 of sediment and rock are ubiquitous in submarine settings ranging from the steepest volcanic island slopes to the gentlest muddy slopes of submarine deltas. Here, we summarize current knowledge of such landslides and the problems of assessing their hazard potential. The major hazards related to submarine landslides include destruction of seabed infrastructure, collapse of coastal areas into the sea and landslide-generated tsunamis. Most submarine slopes are inherently stable. Elevated pore pressures (leading to decreased frictional resistance to sliding) and specific weak layers within stratified sequences appear to be the key factors influencing landslide occurrence. Elevated pore pressures can result from normal depositional processes or from transient processes such as earthquake shaking; historical evidence suggests that the majority of large submarine landslides are triggered by earthquakes. Because of their tsunamigenic potential, ocean-island flank collapses and rockslides in fjords have been identified as the most dangerous of all landslide related hazards. Published models of ocean-island landslides mainly examine ‘worst-case scenarios’ that have a low probability of occurrence. Areas prone to submarine landsliding are relatively easy to identify, but we are still some way from being able to forecast individual events with precision. Monitoring of critical areas where landslides might be imminent and modelling landslide consequences so that appropriate mitigation strategies can be developed would appear to be areas where advances on current practice are possible.
In this 2006 volume John Murray investigates the ecological processes that control the distribution, abundance and species diversity of benthic foraminifera in environments ranging from marsh to the deepest ocean. To interpret the fossil record it is necessary to have an understanding of the ecology of modern foraminifera and the processes operating after death leading to burial and fossilisation. This book presents the ecological background required to explain how fossil forms are used in dating rocks and reconstructing past environmental features including changes of sea level. It demonstrates how living foraminifera can be used to monitor modern-day environmental change. Ecology and Applications of Benthic Foraminifera presents a comprehensive and global coverage of the subject using all the available literature. It is supported by a website hosting a large database of additional ecological information (www.cambridge.org/0521828392) and will form an important reference for academic researchers and graduate students in Earth and Environmental Sciences. © Cambridge University Press, 2009 and John Murray 2006. All rights reserved.
Seagrasses are one of the most valuable marine ecosystems on earth, yet they are declining worldwideat alarming rates. With most of seagrass monitoring based on long term responses to environmentalpressures, there is growing interest in developing alternative diagnostic tools that more effectively iden-tify changes in seagrass ecological status at an early stage. Besides morphological indicators, functionaland biochemical descriptors may provide a good understanding of plant’s responses to environmentalchanges. Moreover, the epiphytic microbial communities of seagrasses may also shift in response tochanges in environmental conditions, although these have been seldom used as a descriptor of environ-mental change. In this study three Halophila stipulacea (Forsk.) Aschers meadows, found in the Gulf ofAqaba (northern Red Sea), were characterized using an integrated approach to highlight possible differ-ences in the meadows ecological status. Plant descriptors, including leaves morphometrics (leaf size, leafnumber/plant, leaves with lost apex), photosynthetic pigments (Chlorophylls, Carotenoids) and total phe-nols contents, were investigated and coupled with the plants’ epiphytic microbial community structureand composition, studied using pyrosequencing. The entire suite of descriptors highlighted differencesamong the meadows ecological status based on changes in plants’ morphology and biochemistry, andtheir associated microbial communities, in response to the different environmental conditions (watercolumn turbidity, seawater and sediment nutrients) and the geomorphological features (bottom slope,granulometry) of the stations. Leaf morphology and photosynthetic pigment content were modulated inH. stipulacea in response to light availability and hydrodynamics in the Gulf of Aqaba. The highest leafsurface area and photosynthetic pigment contents were observed at the lowest irradiance and hydro-dynamics/granulometry among stations. Total phenol content showed differences among stations withincreasing concentrations from north to south. The microbial communities showed differences amongstations and plant compartments, with high incidence of Gammaproteobacteria and Bacteroidetes in lightlimiting conditions, while Cyanobacteria and Rhodobacteraceae thrived in conditions of high light avail-ability and hydrodynamics. The mutual response of the seagrass plants and the microbial communitiesprovided evidence of their functional relationship, which undoubtedly needs further investigation. Tothe best of our knowledge, this is the first time that such descriptors have been used in an integratedapproach. We provide evidence of their effectiveness in discriminating seagrass ecological status, even atsmall spatial scales. This work constitutes a new approach to the assessment of seagrasses and a steppingstone in the application of microbial communities as a putative marker in a changing environment.
Seagrasses are one of the most valuable marine ecosystems on earth, yet they are declining worldwide at alarming rates. With most of seagrass monitoring based on long term responses to environmental pressures, there is growing interest in developing alternative diagnostic tools that more effectively identify changes in seagrass ecological status at an early stage. Besides morphological indicators, functional and biochemical descriptors may provide a good understanding of plant's responses to environmental changes. Moreover, the epiphytic microbial communities of seagrasses may also shift in response to changes in environmental conditions, although these have been seldom used as a descriptor of environmental change. In this study three Halophila stipulacea (Forsk.) Aschers meadows, found in the Gulf of Aqaba (northern Red Sea), were characterized using an integrated approach to highlight possible differences in the meadows ecological status. Plant descriptors, including leaves morphometrics (leaf size, leaf number/plant, leaves with lost apex), photosynthetic pigments (Chlorophylls, Carotenoids) and total phenols contents, were investigated and coupled with the plants’ epiphytic microbial community structure and composition, studied using pyrosequencing. The entire suite of descriptors highlighted differences among the meadows ecological status based on changes in plants’ morphology and biochemistry, and their associated microbial communities, in response to the different environmental conditions (water column turbidity, seawater and sediment nutrients) and the geomorphological features (bottom slope, granulometry) of the stations. Leaf morphology and photosynthetic pigment content were modulated in H. stipulacea in response to light availability and hydrodynamics in the Gulf of Aqaba. The highest leaf surface area and photosynthetic pigment contents were observed at the lowest irradiance and hydrodynamics/granulometry among stations. Total phenol content showed differences among stations with increasing concentrations from north to south. The microbial communities showed differences among stations and plant compartments, with high incidence of Gammaproteobacteria and Bacteroidetes in light limiting conditions, while Cyanobacteria and Rhodobacteraceae thrived in conditions of high light availability and hydrodynamics. The mutual response of the seagrass plants and the microbial communities provided evidence of their functional relationship, which undoubtedly needs further investigation. To the best of our knowledge, this is the first time that such descriptors have been used in an integrated approach. We provide evidence of their effectiveness in discriminating seagrass ecological status, even at small spatial scales. This work constitutes a new approach to the assessment of seagrasses and a stepping stone in the application of microbial communities as a putative marker in a changing environment.
Rose Bengal stained foraminifera from box cores, collected in the southern Adriatic Sea along a transect ranging from 146 to 1,200 m water depth, have been studied. Total numbers of supposedly deposit-feeding foraminifera decrease in a fairly regular manner with increasing water depth and distance from land, probably as a consequence of a decreasing flux of organic matter. Suspension-feeding astrorhizid taxa have an irregular distribution, apparently not related to water depth. Highest numbers of stained foraminifera are invariably found at the sediment surface, whereas numbers decrease exponentially deeper in the sediment. Although most species have maximum densities near the sediment surface, a few of the rarer species are concentrated at deeper levels in the sediment. The vertical distribution patterns of a number of common species are variable with depth along the transect, apparently determined by several different environmental parameters.
An approach to grain size trend analysis is developed on the basis of a semi-quantitative filtering technique. Using this technique, grain size trends identified from a grid of surficial sediment samples are transformed into a "residual pattern' representing net sediment transport paths. The proposed method is applied to the analysis of grain size trends over the Christchurch Bay area, southern England. Mean grain size, sorting and skewness are used to form eight possible grain size trends; two of these are used to derive a residual pattern. The pattern obtained shows general agreement with transport patterns derived from other sediment dynamics investigations undertaken for the region. -from Authors
The recurrence of mass-flow units within sapropel S1, an organic carbon-rich lower Holocene marker bed in the Eastern Mediterranean Sea, was used to study the interplay between earthquakes and sedimentation along the seismically active Calabrian Arc (Ionian Sea).
Nine turbidite beds interrupt anoxic conditions during the deposition of sapropel S1. Each of these turbidites is associated with sharp grain size and geochemical elemental anomalies (high Al and Si, low Ca and coarse-grained basal part marked by Zr peaks), and with displaced foraminiferal species from different bathymetric ranges. We used these proxies to identify turbidite beds also above and below the sapropel, where turbidite signature is less clear due to the absence of major color changes. Turbidite structure and composition, as well as comparison with historical seismoturbidites, suggest a seismic triggering for such mass flow events. The peculiar color, well-known composition, geochemistry and age of sapropel S1, make this unit a key bed within which turbidites may be considered a sort of sedimentary “bar code” recording high-energy events within the background pelagic sedimentation; deciphering this code will reconstruct paleo-seismicity in this well-defined stratigraphic interval.
The pelagic units bracketing turbidite beds were radiometrically dated, and the age of the sapropel S1, deduced through age modeling, is between 6.0 and 10.2 kyr cal BP. The emplacement age of each turbidite was estimated considering the average time-interval between successive turbidite beds (from pelagic sediment thickness and sedimentation rate). Subsequently these ages were further refined through age modeling. In this way, we compiled a catalogue of mass flow events during sapropel S1 deposition, a time span long enough to include several earthquake cycles and allow reliable seismic and tsunami hazard assessment in this area.
The recurrence of mass-flow units within sapropel S1, an organic carbon-rich lower Holocene marker bed in the Eastern Mediterranean Sea, was used to study the interplay between earthquakes and sedimentation along the seismically active Calabrian Arc (Ionian Sea).
A. Introduction.- A.1 Background and History of Research.- A.2 List of Participants and Their Contributions.- A. 3 Purpose of the Book.- B. Synopsis.- C. The Gulf of Aqaba - a Rift-Shaped Depression.- D. A Desert-Enclosed Sea.- D.1 Climate.- D.2 Hydrography.- D.3 Circulation Pattern.- D.4 Seasonality.- D.5 Nutrients.- D.6 Primary Production.- D.7 Composition of Plankton.- D.8 Light.- D.9 Characteristics of Water Masses.- E. Shell Producers in the Water Column.- E.1 Calcareous Plankton.- E.2 Coccolithophorida.- E.3 Foraminiferida.- E.4 Pteropoda.- E.5 Aqaba Calcareous Plankton: Significance and Problems.- F. The Sea Bottom - a Mosaic of Substrates.- F.1 Methods of Investigation.- F.2 Selected Areas.- F.2.1 The "Shelf" in Front of the H. Steinitz Marine Biology Laboratory.- F.2.2 Geziret Fara'oun ("Coral Island").- F.2.3 Ras Burka.- F.2.4 Dahab.- F.2.5 Mangroves.- F.2.6 Marset el At.- F.2.7 Ras Muhammed.- F.2.8 Grafton Passage, Tiran.- F.3 Significance of the Depth Gradient.- F.4 Significance of Substrates.- F.5 Seasonality.- G. Benthic Foraminifera: Response to Environment.- G.1 Larger Foraminiferans.- G.1.1 Soritines.- G.1.2 Alveolinids.- G.1.3 Amphisteginids.- G.1.4 Nummulitids.- G.2 "Smaller" Benthic Foraminifera.- G.3 Significance of Shell Morphogenesis.- G.4 Significance of Symbiosis.- G.5 Stable Isotopes and Related Problems.- G.6 Thanatocoenoses.- H. 150,000 Years Gulf of Aqaba.- H.1 Deep Sea Cores.- H.2 Microfossil Assemblages from Cores.- H.3 Stratigraphy.- H.4 Paleoenvironments.- H.5 Paleoceanographic History.- References.- Taxonomic Index.
On 22 November 1995 the largest earthquake instrumentally recorded in the area, with magnitude Mw 7.3, occurred in the Gulf of Aqaba. The main rupture corresponding to the strike-slip mechanism is located within the gulf of Aqaba, which forms the marine extension of the Levantine fault, also known as the Dead Sea fault. The Levantine fault accommodates the strike-slip movement between the African plate and the Arabian plate. The Gulf of Aqaba itself is usually described as the succession of three deep pull-apart basins, elongated in the N-S direction. Concern-ing historical seismicity, only two large events have been reported for the last 2000 years, but they are still poorly constrained. The seismicity recorded since installation of regional networks in the early 1980s had been characterized by a low background level punctuated by brief swarmlike activity a few months in duration. Three swarms have already been documented in the Gulf of Aqaba in 1983, 1990, and 1993, with magnitudes reaching at most 6.1 (Mw). We suggest that the geometry of the rupture for the 1995 event is related to the spatial distribution of these previous swarms. Body-wave modeling of broadband seismograms from the global network, along with the analysis of the aftershock distribution, allow us to propose a well-constrained model for the rupture process. Northward propagation of the rupture has been found. We have demonstrated that three successive subevents are necessary to obtain a good fit between observed and synthetic wave forms. The total seismic moment released was 7.42 × 1019 N-m. The location of the subevents shows that the three stages of the rupture involve three different segments within the gulf. Substantial surface breakage showing only normal motion (up to 20 cm) affecting beachrock was ob-served along the Egyptian coast. We show that these ruptures are only a secondary feature and are in no case primary ruptures. The stress tensor derived from striations collected in quaternary sediments shows radial extension. This result supports land-sliding of the beach terraces under the action of the earthquake shaking.
Abstract Down-faulting at the north-west margins of the Gulf of Aqaba is inferred to have triggered a catastrophic sedimentary event at 2.3 ka that killed the Elat fringing coral reef. Whereas segments of the Holocene reef were perfectly fossilized and preserved beneath a veneer of siliciclastic sediments, other segments were abraded, settled by nomads, and later re-submerged under 4 m of water. Repeated damage triggered by down-throwing earthquakes degenerate the fringing reefs of the north-west end of the gulf. Conversely, on the north-eastern and southern parts of the gulf, where earthquakes uplift the margins, modern reefs are thriving, attached to uplifted fossil reef terraces. Therefore, coastal subsidence moderates the development of fringing coral reefs during the late Holocene sea-level stand still. Terra Nova, 00, 1–6, 2004
Recent surface and subsurface geological investigations in Israel and Jordan provide new data for the re-examination of Dubertret's (1932) hypothesis of the left-hand shear along the Dead Sea rift. It is found that while none of the pre-Tertiary sedimentary or igneous rock units extend right across the rift, all of them resume a reasonable palaeographical configuration once the east side of the rift is placed 105 km south of its present position. It is therefore concluded that the 105 km post-Cretaceous, left-hand shear along the Dead Sea rift is well established. The 40 to 45 km offset of Miocene rocks and smaller offsets of younger features indicate an average shear movement rate of 0.4 to 0.6 cm a-1 during the last 7 to 10 Ma. Unfortunately, the 60 km pre-Miocene movement cannot be dated yet. Along the Arava and Gulf of Aqaba and in Lebanon the shear is divided over a wide fault zone within and outside the rift.
Black and brown speckled foraminiferal sand occurs in the western part of the Capricorn Reef Complex and on adjacent parts of the Inner Shallow Shelf in the zone of mixing of terrigenous and carbonate sediments. The colored materials, which are found preferentially on foraminifera tests, are iron and manganese sulphides, hydroxides and/or oxides. The iron and manganese appear to be derived from associated terrigenous sediments and are being reprecipitated under reducing conditions and in the presence of sulphate-reducing bacteria around centers of organic material such as the foraminifera tests in the sediment. Removal of sediment by erosion and the activity of burrowing organisms brings reduced sediment back into the oxidizing zone where sulphides (black) are oxidized to ferric hydro ides and/or oxides (brown).
A study of major earthquake occurrence along the Dead Sea transform (35.5°–36.5° E; 27.2°–37.5° N) during the past four millenia has been attempted. Geological, archaeological, biblical, historical, and seismological evidence were integrated in an effort to quantify the space-time distribution of seismicity in the said province. The overall earthquake activity in the conterminous Near East indicates a stable pattern and appeared to have been stationary over the examined time window. About 110 earthquakes in the magnitude range 6.7 ≤ ML ≤ 8.3 affected the area during the past 2500 years. Of these, 42 originated along the Dead Sea fault system itself, while 68 were imported from the Helenic-Cyprian arcs and the Anatolian-Elburz-Zagros fault systems. These events were responsible for the repeated destruction of many cultural centers. In the Dead Sea region proper, the major seismic activity since 2100 B.C.E. (Before Christian Era), has been confined to the vicinity of its eastern shore with extremal seismicity at its southern tip near the prehistorical site of Bab-a-Dara'a (31° 15'N, 35° 32'E). This may constitute the first solid evidence that the Biblical “cities of the Plain” (Sodom, Gommorah, etc.) were located there. Recent studies of earthquake deformations in the Lisan deposits near Bab-a-Dara'a, agree with our findings. At the present time, a magnitude 6¾ earthquake is pending at the northern edge of the Levant rift, with its average recurrence interval (83 years) exceeded by one standard deviation (32 years).
Recent geophysical and geological studies of the Gulf of Elat including bathymetry, bottom photographs, continuous seismic profiles, seismic refraction, gravity, magnetics, heat flow, and coring, provide new information about the shallow and deep crustal structure of the important segment of the world rift system. Overall, it seems that within the S Gulf of Elat a transition occurs between crustal spreading that takes place in the Red Sea to a rifting without spreading that takes place along the Dead Sea transform. Spreading activity propagates from S to N. The most active place is the central basin which is being propagated N into the shallow N basin.-from Authors
Landslides are common on inclined areas of the seafloor, particularly in
environments where weak geologic materials such as rapidly deposited,
fine-grained sediment or fractured rock are subjected to strong
environmental stresses such as earthquakes, large storm waves, and high
internal pore pressures. Submarine landslides can involve huge amounts
of material and can move great distances: slide volumes as large as
20,000 km³ and runout distances in excess of 140 km have been
reported. They occur at locations where the downslope component of
stress exceeds the resisting stress, causing movement along one or
several concave to planar rupture surfaces. Some recent slides that
originated nearshore and retrogressed back across the shoreline were
conspicuous by their direct impact on human life and activities. Most
known slides, however, occurred far from land in prehistoric time and
were discovered by noting distinct to subtle characteristics, such as
headwall scarps and displaced sediment or rock masses, on
acoustic-reflection profiles and side-scan sonar images. Submarine
landslides can be analyzed using the same mechanics principles as are
used for occurrences on land. However, some loading mechanisms are
unique, for example, storm waves, and some, such as earthquakes, can
have greater impact. The potential for limited-deformation landslides to
transform into sediment flows that can travel exceedingly long distances
is related to the density of the slope-forming material and the amount
of shear strength that is lost when the slope fails.
The convective/advective balance at the northern end of the Gulf of Elat was investigated by comparing observed data to a numerical model's predictions. The data, monthly temperature and salinity profiles collected from July 1988 to August 1989, indicate a continuously developing annual cycle, with the water column reaching vertical homogeneity in February and the new thermocline beginning to develop in March. In the summer, an upper 200 m thermally stratified layer (surface temperatures reaching 26°C) overlies a thermally homogeneous layer of 21°C. Salinity is close to 40.5% and varies by less than 0.5% throughout the year, although a salinity minimum develops in the upper layer in late spring and erodes and deepens in the fall and winter as the water column becomes mixed.The strictly one-dimensional convective model successfully reproduces the thermal structure, with a slight lag in summer thermocline development, but is unable to predict the observed salinity minimum. The addition of some advected Red Sea water (40.3%) to the model enables the reproduction of the salinity minimum. This inflow of warmer water also may account for the relatively early thermocline development.
Many early Tertiary nummulitic limestones contain broken Nummulites tests (commonly referred to as nummulithoclastic debris) that display breakage ranging from damage to the terminal chamber, to disintegration into sand- and silt-sized (and probably finer) fragments. Little consideration previously has been given to the processes responsible for this damage, or whether test abrasion can be used as an indicator of the degree of transportation or wave reworking. Studies of modern larger benthic foraminifera suggest that transport-induced abrasion is a likely candidate for the test damage seen in many fossil Nummulites. However, experimental reconstruction of the transportation of Nummulites within a traction carpet of skeletal material, using the structurally similar and related extant form Palaeonummulites venosus, failed to reproduce the degree of test damage seen in fossil forms, despite simulating transport up to approximately 71 km. Evidence from experimental and field observations suggests that the additional damage noted in Eocene Nummulites possibly is the result of transportation within turbidity currents and/or predation by relatively large bioeroders, such as fish and echinoids. Processes such as dissolution and microboring are considered to have played only a minor, if any, role in the comminution of Nummulites. These observations have been used to define a scale of taphonomic features observed in fossil Nummulites, to aid identification of autochthonous and allochthonous Nummulites populations in thin-section studies of nummulitic limestones, and to facilitate comparison between different facies and carbonate-platform environments.
Texturally immature glacial sediment on the continental shelf and slope of the E Weddell Sea remains unsorted, except by processes of sediment gravity flow. Piston core coverage demonstrates that downslope transitions between slumps, debris flows, and turbidity currents are active on the unstable intercanyon slope areas and on the glacially scoured continental shelf. Sediment textural analyses indicate that size sorting and mineralogic maturity are achieved within sediment gravity flow transitions and that lithic (more than 40% lithics) glacial sediment is transformed into sorted, arkosic sand (less than 3% lithics) over distances of less than 10km on the upper continental slope and shelf. Arkosic or quartz sand thus generated is often interbedded with diamictons, which are either glacial or nonglacial (slumps and debris flows) in origin. Therefore, such associations in ancient sequences do not rule out a glacial origin for diamictites (or mixtites), nor do they mandate deep-water sedimentation.-from Author
The 1995 November 22, Mw= 7.2 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform (DST) in the Gulf of Elat (Aqaba). It was the largest earthquake along the DST in at least 160 yr. The main shock was preceded by earthquake swarms north and south of its NE-striking rupture since the early 1980s, and was followed by about 6 months of intense aftershock activity, concentrated mainly northwest and southeast of the main rupture. In this study we re-analyse ERS-1 and ERS-2 InSAR data for the period spanning the main shock and 5 post-seismic years. Because the entire rupture was under the Gulf water, surface observations related to the earthquake are limited to distances greater than 5 km away from the rupture zone. Coseismic interferograms were produced for the earthquake +1 week, +4 months and +6 months. Non-linear inversions were carried out for fault geometry and linear inversions were made for slip distribution using an ascending–descending 2-frame data set. The moment calculated from our best-fitting model is in agreement with the seismological moment, but trade-offs exist among several fault parameters. The present model upgrades previous InSAR models of the Nuweiba earthquake, and differs from recent teleseismic waveform inversion results mainly in terms of slip magnitude and distribution.
The moment released by post-seismic deformation in the period of 6 months to 2 yr after the Nuweiba earthquake is about 15 per cent of the coseismic moment release. Our models suggest that this deformation can be represented by slip along the lower part of the coseismic rupture. Localised deformation along the Gulf shores NW of the main rupture in the first 6 months after the earthquake is correlated with surface displacements along active Gulf-parallel normal faults and possibly with shallow M > 3.9, D < 6 km aftershocks. The geodetic moment calculated by modelling this deformation is more than an order of magnitude larger than expected for a single M∼ 4 aftershock, but could be a result of a sequence of aftershocks and/or aseismic slip. The major aftershocks and the slip along Gulf-parallel normal faulting NW of the main rupture are associated with positive Coulomb stress changes induced by the main event.
The Elat fault (a segment of the Dead Sea Transform) runs along the southern Arava valley (part of the Dead Sea Rift, Israel) forming a complex fault zone that displays a time-dependent seismic behaviour. Paleoseismic evidence shows that this fault zone has generated at least 15 earthquakes of magnitude larger than M 6 during the late Pleistocene and the Holocene. However, at present the Elat fault is one of the quietest segments of the Dead Sea Transform, lacking even microsesimicity. The last event detected in the southern Arava valley occurred in the Avrona playa and was strong enough to have deformed the playa and to change it from a closed basin with internal drainage into an open basin draining to the south.Paleoseismological, geophysical and archaeological evidences indicate that this event was the historical devastating earthquake, which occurred in 1068 AD in the eastern Mediterranean region. According to the present study this event was strong enough to rupture the surface, reactivate at least two fault branches of the Elat fault and vertically displace the surface and an early Islamic irrigation system by at least 1 m. In addition, the playa area was uplifted between 2.5 and 3 m along the eastern part of the Elat fault shear zone. Such values are compatible with an earthquake magnitude ranging between M 6.6 and 7. Since the average recurrence interval of strong earthquakes during the Holocene along the Elat fault is about 1.2 ± 0.3 ky and the last earthquake occurred more about 1000 years ago, the possibility of a very strong earthquake in this area in the future should be seriously considered in assessing seismic hazards.
Current global warming necessitates a detailed understanding of the relationships between climate and global ice volume. Highly resolved and continuous sea-level records are essential for quantifying ice-volume changes. However, an unbiased study of the timing of past ice-volume changes, relative to polar climate change, has so far been impossible because available sea-level records either were dated by using orbital tuning or ice-core timescales, or were discontinuous in time. Here we present an independent dating of a continuous, high-resolution sea-level record in millennial-scale detail throughout the past 150,000 years. We find that the timing of ice-volume fluctuations agrees well with that of variations in Antarctic climate and especially Greenland climate. Amplitudes of ice-volume fluctuations more closely match Antarctic (rather than Greenland) climate changes. Polar climate and ice-volume changes, and their rates of change, are found to covary within centennial response times. Finally, rates of sea-level rise reached at least 1.2 m per century during all major episodes of ice-volume reduction.
The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post-depositional consolidation and soft-sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain-support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non-cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle-support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain-to-grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle-support mechanism depends upon flow conditions, particle concentration, grain-size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse-grain tail (or dense-grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large-scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensuBagnold, 1962), in which fluid turbulence is the main particle-support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge-like flows and quasi-steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge-like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi-steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening-up units capped by fining-up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near-steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co-exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.
Settling and traction velocities were measured on optimally preserved tests of larger foraminifera using a settling tube and flume tank. Within larger foraminifera with porcelaneous tests, the peneroplids, Peneroplis antillarum, P. planatus, P. pertusus and Dendritina cf. D. zhengae, are distinguished by low test densities (ca 1·2) that do not change with growth. Buoyancy is high because of low Reynolds numbers and increases in large individuals because of the allometric change of test shape. The fusiform Alveolinella quoyi, with test densities ca 1·6, is characterized by high Reynolds numbers, inducing the weakest buoyancy within porcelaneous larger foraminifera. The highest buoyancy was recorded for the three soritids, Parasorites orbitolitoides, Sorites orbiculus and Amphisorus hemprichii, because of their low test densities (ca 1·25) and the extremely flat, biconcave, plate-like shape. Flat tests, however, reduce traction and entrainment from smooth surfaces. Within hyaline larger foraminiferat, the amphisteginids show thick-lenticular (Amphistegina lobifera, A. radiata) to thin-lenticular tests (A. bicirculata, A. papillosa), influencing buoyancy. Here, high test densities (ca 1·8) decrease with growth in A. lobifera, A. lessonii and A. bicirculata, and remain constant in A. radiata and A. papillosa. Minimum velocities required for entrainment are lower for thick-lenticular tests and higher for thin-lenticular tests. Test densities remain constant with growth in the calcarinid Baculogypsina sphaerulata (ρ ∼ 1·78) and decrease slightly in Calcarina gaudichaudii and Neorotalia calcar (starting at ρ ∼ 1·85), all living under extreme hydrodynamic conditions. Density decreases the most in Baculogypsinoides spinosus (starting at ρ ∼ 1·8), resulting in higher buoyancy through low Reynolds numbers. Traction is promoted in spherical tests of Baculogypsina and Baculogypsinoides. Within nummulitids, the thick-lenticular Palaeonummulites venosus (test density decreasing with size; starting at 1·78) is less buoyant, expressed in high Reynolds numbers, but easily entrained. Thick-lenticular juveniles and extremely flat adults distinguish Operculinella cumingii, Heterostegina depressa and the giant Cycloclypeus carpenteri. Test densities increase during growth, starting from ca 1·6 and attaining a maximum of 1·8. Buoyancy is low in small tests and high in large tests, while entrainment velocities are reduced as the tests flatten. High buoyancy is also a characteristic of the entirely flat tests in Operculina ammonoides (from deeper regions) and Planostegina operculinoides, which is expressed in the lowest Reynolds numbers within larger foraminifera.
Larger foraminifera living in the upper 50 m in front of the fringing coral reef northwest off Sesoko Island, Japan show strong habitat differences. This study closely examines the distributions of larger foraminifers and relates these to a number of key environmental factors using rigorous statistical methods. Since all larger foraminifera house symbiotic algae, light attenuation by the water column is the most important limiting factor that must be dealt with wall structures. Water energy is also countered by test structure. The local topography is responsible for different intensities of hydrodynamic forces, which are expressed in various substrates, mostly coral rubble and coarse-grained sand. The genus Peneroplis, very common on the reef flat, clearly prefers hardgrounds of the shallowest slope parts down to 30 m, while Dendritina is restricted to sandy bottoms and avoids the uppermost meters of the slope. It can be found down to 50 m at least. Alveolinella shows a similar depth distribution to Dendritina, but is common on hard bottom. The distribution of Parasorites, which is restricted to sandy substrates, starts at 20 m and extends to 80 m. Sorites, on the other hand, was found only on firm substrates between the reef edge and 50 m. The same depth distribution was recorded for Amphisorus, but this genus is not correlated with specific substrates. Most of the Amphistegina species prefer hardgrounds, while Amphistegina radiata is also common on sand. The calcarinids, capable of withstanding high water energy, are abundant on firm substrates close to the reef edge. Only Baculogypsinoides inhabits deeper parts of the slope on sandy bottom and avoids the shallowest parts. Sections with hard substrates are settled by Heterostegina, even down to 80 m, although this genus was occasionally found on sandy bottoms. Nummulites, in contrast, is restricted to sands between 20 and 70 m. Operculina, starting at 20 m, also prefers sandy substrates, but rare individuals were detected on coral rubble and macroids.
New multichannel seismic and bathymetric data are presented, which clarify the Plio-Quaternary evolution of the northern Gulf of Aqaba (Elat) and the Dead Sea Transform (DST). The seismic data reveal two main seismic sequences, a lower (pretectonic) and an upper (syntectonic) unit, separated by a prominent unconformity. These units are each linked to a distinct tectonic phase in the history of the DST. Parallel horizons and an undisturbed internal structure point to a tectonic quite time or pure strike-slip without extension or compression during the first (pretectonic) phase. The second (syntectonic) phase, which begins in the early Pliocene, is characterized by a major change in the activity of the DST. The pretectonic sedimentary unit subsided and, consequently, dips southward with a supplementary inclination to the east. The coeval sedimentation of the syntectonic unit is recorded by the divergent reflection pattern and onlap terminations on the unconformity. The apparent fault system seems to be rearranged in the second phase. The stepover of the main strand of the DST from the eastern side of the Elat Deep to the western side of the northern Gulf of Aqaba was mapped in detail for the first time. The very smooth shape of the stepover and the apparent lack of extensional tectonics do not fit with the classical pull-apart basin model for the Elat Deep and point to a decoupling of the crystalline basement from the sedimentary overburden. Comparisons of the new geophysical findings with analog models support this assertion.
The preservation of Corg in various shelf seas from different geological, hydrological and climatological settings is compared. Most of these shelves have been studied extensively over the last two to three decades, often within the framework of large (inter)national research projects. Various factors (physical oceanography, chemical conditions, sedimentation rates) that could influence the preservation of Corg are discussed. Not only recent conditions, but also longer time scale fluctuations (glacials versus inter-glacials, sea level variations) are dealt with. Most (>95%) of the Corg introduced onto shelves by primary production and imported from the oceans and continents is mineralized in the water column and sediments. It appears that the role of shelves as sinks for Corg is sometimes overestimated. Large areas of the continental shelves do not show any accumulation of organic matter under present day conditions. Only locally, where hydrological and sedimentological conditions are favourable for organic matter accumulation considerable amounts of Corg are buried. On longer time scales the role of shelves as sinks for Corg is limited, even more than under recent conditions. Continental slopes, canyons and deep-sea fans are thought to be the main sinks for Corg.
A large turbidity current was detected in the Zaire submarine valley at 4000 m water depth. Current meters, turbidimeter and sediment trap deployed on a mooring located in the channel axis, although they were damaged, recorded the signature of a very high energy event. An average velocity of more than 121 cm s−1 was measured 150 m above the channel floor. Coarse sand and plant debris were collected at 40 m height. The turbidity current clearly overflowed the edges of the valley as demonstrated by the large quantity of turbiditic material (464 mg organic carbon m−2 d−1) found in the sediment trap moored 13 km south from the channel axis.
The quantitative distribution of planktonic foraminifera, pteropods, and coccolithophorids, as well as oxygen-isotope variations were analyzed in four deep-sea cores from the Gulf of Aqaba (Elat) and the northernmost Red Sea. The core record covers about 150,000 yr. Detailed stratigraphic subdivision is facilitated by combining all calcareous plankton groups. Time-stratigraphic correlation and dating beyond the radiocarbon range are possible by comparison of the oxygen-isotope curves. During the glacial maximum salinity rose to more than 50‰, while winter temperature of the upper waters fell by at least 4°C compared to the present. The rise in salinity can be accounted for by sea-strait dynamics and lowering of sea level. The Gulf of Aqaba and the Red Sea were continuously connected through the Straits of Tiran, and there is no indication of desiccation during the glacial maximum.
Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at many locations, which has been attributed to anoxic sulfide oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, about 10 m thick, consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate (ikaite: CaCO3·6H2O). Ikaite δ13C values are indicative of a methane carbon contribution to the CO2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate–methane transition zone within the turbiditic sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resulting from this slide emplacement, and the development towards a new steady state are fully sufficient to explain all features related to the pore-water profiles, particularly, SO42− and dissolved inorganic carbon (DIC). The model shows that the downslope transport took place about 300 yr ago.
Two sediment wave fields have been identified on the flanks of the western Canary Islands of La Palma and El Hierro, using a high-quality 2-D and 3-D dataset that includes GEOSEA and TOBI imagery, 3.5-kHz profiles, and short sediment cores. The La Palma sediment wave field covers some 20,000 km2 of the continental slope and rise, and consists of sediment waves with wave heights of up to 70 m and wavelengths of up to 2.4 km. The wave crestlines have a complex morphology, with common bifurcation and a clear sinuosity. Waves have migrated upslope through time. Cores recovered from the wave field contain volcaniclastic turbidites interbedded with pelagic/hemipelagic layers. The wave field is interpreted as having formed beneath unconfined turbidity currents. A simple, previously published, two-layer model is applied to the waves, revealing that they formed beneath turbidity currents flowing at 10–100 cm/s−1, with a flow thickness of 60–400 m and a sediment concentration of 26–427 mg/l. The El Julan sediment wave field lies within a turbidity current channel on the southwest flank of El Hierro. The sediment waves display wave heights of about 6 m and wavelengths of up to 1.2 km. The waves are migrating upslope, and migration is most rapid in the centre of the channel where the flow velocity is highest. This wave field has been formed by channelised turbidity currents originating on the flanks of El Hierro.