210Pb in the ocean: A pilot tracer for modeling particle reactive elements     

Ernst Maier-Reimer  Gideon Henderson 《Journal of Earth System Science》1998,107(4):351-357

We discuss the basic requirements for a successful modeling of²¹⁰Pb in the ocean as a test tracer for at least other lead isotopes, but also of other elements that behave similarly to lead. With the aid of realistic models of the oceanic circulation and the major biogeochemical cycles in it, the result is a dynamically consistent model of lead cycling that reproduces observed profiles within 10%.  相似文献   

Radiocarbon Ages Constraints on the Origin and Shedding of Bank-Top Sediment in the Bahamas during the Holocene     

Niall C. Slowey  Gideon M. Henderson 《Aquatic Geochemistry》2011,17(4-5):419-429

Great quantities of fine-sized aragonite needles are produced in the shallow waters that cover the tops of the Bahama Banks and then exported to the bank margins where they accumulate with shells of pelagic organisms. To better understand these processes, we investigated Holocene-aged sediments in a core from the southwestern margin of Little Bahama Bank. The aragonite content of the sediments, ??¹⁸O of planktonic foraminifera shells, and radiocarbon ages of aragonite-rich <63 ??m sediments and coexisting planktonic foraminifera shells were determined. Sediment deposition was rapid overall, and a significant increase in deposition rate occurred 3,500?C4,000 years ago, shortly after rising sea level flooded the bank top with seawater and caused a dramatic increase in the shallow water area where aragonite production occurred. During the latest Holocene when high deposition rates minimize effects of bioturbation, aragonite-rich <63 ??m sediments are 400?C600 years older than coexisting foraminifera. This difference indicates the net age of aragonite when it was exported from the bank top. It is consistent with expectations of the ??hip-hop??n?? model (Morse et al. in Geochimica et Cosmochimica Acta 67: 2819?C2826, 2003) whereby aragonite needles on the bank top, formed initially by biologic or other processes, continue to grow for hundreds of years via precipitation of epitaxial carbonate cement from seawater. Earlier in the Holocene, when sea level was lower and the top of Little Bahama Bank was subaerially exposed, the deposition rate and aragonite content of the sediments were less, and the aragonite-rich <63 ??m sediments are about 1,000 years younger than coexisting foraminifera. This age difference can be explained by downward mixing of latest-Holocene <63 ??m material into older early-Holocene sediments.  相似文献   

Ba, Ra, Th, and U in marine mollusc shells and the potential of Ra/Ba dating of Holocene marine carbonate shells     

Michael Staubwasser  Gideon M. Henderson  Brenda L. Hall 《Geochimica et cosmochimica acta》2004,68(1):89-100

The geochemistry of Ba, Ra, Th, and U and the potential of using ²²⁶Ra/Ba ratios as an alternative dating method are explored in modern and Holocene marine mollusc shells. Five modern shells of the Antarctic scallop Adamussium colbecki collected from the present day beach and six radiocarbon dated specimens from Holocene beach terraces of the Ross Sea region (Antarctic) between 700 and 6100 calibrated yr BP old have been analysed by mass spectrometry. In clean shells ²²⁶Ra concentrations and ²²⁶Ra/Ba ratios show a clear decrease with increasing age, suggesting the possibility of ²²⁶Ra dating. Limiting factors for such dating are Ba and ²²⁶Ra present in surface contaminants, and ingrowth of ²²⁶Ra from U present within the shell. Surface contamination is difficult to clean off entirely, but moderate levels of residual contamination can be corrected using ²³²Th. Sub-samples from the same shell with different proportions of contamination form a mixing line in a ²²⁶Ra/Ba-²³²Th/Ba graph, and the ²²⁶Ra/Ba of the pure shell can be derived from the intercept on the ²²⁶Ra/Ba axis. Contaminant corrected ²²⁶Ra/Ba ratios of late-Holocene ¹⁴C-dated samples fall close to that expected from simple ²²⁶Ra excess decay from seawater ²²⁶Ra/Ba values. ²²⁶Ra ingrowth from U incorporated into the shell during the lifetime of the mollusc can be corrected for. However, the unknown timing of post mortem U uptake into the shell makes a correction for ²²⁶Ra ingrowth from secondary U difficult to achieve. In the A. colbecki shells, ²²⁶Ra ingrowth from such secondary U becomes significant only when ages exceed ∼2500 yr. In younger shells, ²²⁶Ra/Ba ratios corrected for surface contamination provide chronological information. If evidence for a constant oceanic relationship between ²²⁶Ra and Ba in the ocean can be confirmed for that time scale, the ²²⁶Ra/Ba chronometer may enable the reconstruction of variability in sea surface ¹⁴C reservoir ages from mollusc shells and allow its use as a paleoceanographic tracer.  相似文献   

The evolution of a Late Cretaceous–Cenozoic intraplate basin (Duaringa Basin), eastern Australia: evidence for the negative inversion of a pre-existing fold–thrust belt     

Babaahmadi  Abbas  Sliwa  Renate  Esterle  Joan  Rosenbaum  Gideon 《International Journal of Earth Sciences》2018,107(5):1895-1910

International Journal of Earth Sciences - The Duaringa Basin in eastern Australia is a Late Cretaceous?–early Cenozoic sedimentary basin that developed simultaneously with the opening of the...  相似文献   

The application of an expert knowledge‐driven approach for assessing gully erosion susceptibility in the subtropical Nigerian savannah     

Didams Gideon  Firuza Begham Mustafa  Idakwo Victor 《Singapore journal of tropical geography》2021,42(1):107-131

This study assessed gully erosion susceptibility in Southern Gombe State, Nigeria. The objectives of the study were to: (1) prepare gully inventory of Southern Gombe State, (2) apply the Analytical Hierarchy Process to assess the contribution of gully erosion predisposing factors, and (3) produce a gully erosion susceptibility map of Southern Gombe State. Based on geomorphological study involving interpretation of Google Earth images and field surveys, 127 gullies were identified and 13 gully erosion predisposing factors assumed to influence gully erosion susceptibility were selected. Identified gullies were randomly split into training (89 or 70 per cent) and validation (38 or 30 per cent) datasets. The contribution of each gully erosion predisposing factor was obtained using the Analytical Hierarchy Process. The results indicated that slope (0.130), stream density (0.121), and distance from stream (0.121) play crucial roles in gully erosion susceptibility. By overlaying the gully erosion susceptibility factor maps, a gully erosion susceptibility map was created. A natural break method was then used to classify gully erosion areas into relatively safe (6.04 km²), less susceptible (3332.46 km²), moderately susceptible (1811.49 km²), highly susceptible (1146.35 km²), and extremely susceptible (1726.77 km²) categories. Field verification confirmed that the map accurately classified 92.11 per cent of the validation datasets, signifying the Analytical Hierarchy Process as a reliable method for gully erosion susceptibility assessment. The created gully erosion susceptibility map can assist land planners to identify critical gully erosion areas where prevention and mitigation actions should be implemented.  相似文献   

A Robust Procedure for High‐Precision Determination of Rare Earth Element Concentrations in Seawater          下载免费PDF全文

Xin‐Yuan Zheng  Jingjing Yang  Gideon M. Henderson 《Geostandards and Geoanalytical Research》2015,39(3):277-292

This study reports a robust procedure that permits precise measurement of all fourteen naturally occurring rare earth element (REE) concentrations, present at ng kg^?1 to sub ng kg^?1 levels, in ~ 100 ml seawater. This procedure is simple and can be routinely applied to measure seawater REEs with relatively high sample throughput. The procedure involves addition of a ¹⁴²Ce‐¹⁴⁵Nd‐¹⁷¹Yb‐enriched spike mixture, iron co‐precipitation, REE purification with chromatographic separation and the use of a magnetic‐sector‐field ICP‐MS (Element 2) coupled with a desolvating sample introduction system (Aridus 1). Critical steps of the procedure, including co‐precipitation pH and matrix removal, have been optimised through a set of experiments described here. The accuracy of the new procedure was assessed against a gravimetric mixture of REEs, and the precision was demonstrated by repeated measurement of two well‐mixed natural seawaters. Repeated analyses of these seawater reference materials (RMs), using ~ 100 ml seawater for each aliquot, indicate precision of 3% (1s) for the REEs. Measured REE concentrations of two uncertified seawater RMs (CASS‐4 and NASS‐5) are consistent with published values, and REE concentrations of the GEOTRACES intercalibration samples show good agreement with those reported by other participant laboratories. REE concentrations for other intercalibration samples (SAFe and Arctic PS70) are also reported.  相似文献   

Analysis of submarine slumping in the continental slope off the southern coast of Israel     

Gideon Almagor  Gdalyah Wiseman 《Marine Georesources & Geotechnology》2013,31(1-4):349-388

Abstract

The continental slope off the coast of Israel is riddled with numerous large slump scars at depths greater than 400 m. Recent scar slumps are situated in the steepest central portions of the continental slope (400–450 m depth, α=6°), frequently disfiguring older slump scars in its lower portions. The slumping materials were probably largely transported downslope in the form of density currents, and occasionally by sliding of large sediment chunks. Upslope retrogressive slumping phases progressively disfigure the shape of the slump scars until they totally disappear, causing net reduction of the thickness of the sedimentary column. To provide a basis for the quantitative analysis of slumping, laboratory vane tests, triaxial consolidated, undrained compression tests with pore‐pressure measurements, drained direct shear tests, and consolidation tests were performed oh undisturbed samples. Because the sediments consist of normally consolidated silty clays, the geotechnical properties measured on the core samples can be readily extrapolated for greater depths, assuming the sediments are homogeneous. Angles of internal friction measured by direct shearing under drained conditions are ?_d =24°‐25°, designating the maximum possible angle of a stable infinite slope. These angles are appreciably higher than the steepest slopes in the investigated area, and a drained slumping mechanism is therefore considered unlikely. The slopes of the slump scar walls are about 20°; therefore, in the absence of active erosional, sedimentological, or tectonic agents, these walls have long‐term stability (drained shear). Undrained shear failure resulting in slope instability may be attributable to rapid changes in slope geometry (undercutting or oversteepening of the slope), fluctuations in pore pressure, or accelerations associated with earthquakes. Undrained shear‐strength parameters were determined by both laboratory consolidated‐un‐drained triaxial tests and by miniature vane shear tests. The angles of internal friction that were measured are ?_cu =15°‐17°, and the c_u/p_o values range between 0.22 and 0.75. An analysis of the force equilibrium within the sediments leads to the conclusion that horizontal earthquake‐induced accelerations, as little as 5–6% of gravity, are sufficient to cause slope failure in the steepest slope zone (400–450 m depth, α = 6°, c_u /p_o =0.25). Collapse resulting from liquefaction is unlikely, as the sediments are normally consolidated silty clays with intermediate sensitivity, S_t =2–4.

The existence of slump scars in the lower portion of the continental slope, characterized by gentle slopes (α=1°‐3°) and sediments with high shear strength (c _u /p _o=0.30–0.50) is attributed to large horizontal accelerations(k=12–16% of gravity). Owing to the wide range of geotechnical properties of the sediments (c_u /p_o = 0.20–0.75) and the inclination of the continental slope (α=1°‐6°), the same earthquake may generate a wide range of horizontal accelerations in different portions of the continental slope, and slumping may occur wherever the stability equilibrium is disrupted.  相似文献   

Simple shear testing for the study of the earthquake response of clay from the Israeli continental slope     

S. Frydman  M. Talesnick  G. Almagor  G. Wiseman 《Marine Georesources & Geotechnology》2013,31(3):143-171

Abstract

A study was carried out to investigate the simple shear behaviour of a clay from the Israeli continental slope, and to consider the relevance of this behaviour to the stability of the slopes during earthquake conditions. Norweigian simple shear apparatus was modified in order to enable both static and cyclic shearing of undisturbed samples taken from the slope.

Static tests performed on virgin samples and on samples which had undergone extensive cycling indicated similar static strengths, despite development of large cyclic shear strains during cycling. This suggests that the stability of the slopes following completion of an earthquake would not be significantly less than it was before the onset of the earthquake; the critical period for instability would be during the earthquake.

The results of the cyclic tests indicated that shear strain development accelerated once the cyclic strain reached the order of 3%. A failure criterion for the clay, relevant to cyclic loading conditions, was developed, based on this strain value, and was incorporated into a cumulative damage type of analysis for earthquake loading, using Miner's rule. This analysis was used to develop a computer program for the stability analysis of the continental slope during any given earthquake.  相似文献   

Marine Sand Resources Offshore Israel     

Gideon Almagor  Dan Gill  Ithamar Perath 《Marine Georesources & Geotechnology》2013,31(1):1-42

Abstract

The continental margin of northern Sinai and Israel, up to Haifa Bay, is the northeastern limb of the submarine Nile Delta Cone. It is made up predominantly of clastics from the Nile and its predecessors. The continental shelf and coastal plain of Israel are built of a series of shore-parallel ridges composed of carbonate-cemented quartz sandstone (locally named kurkar), a lithification product of windblown sands that were piled up into dunes during the Pleistocene. The drop in global sea level and regression during the last glacial period exposed the continental shelf to subaerial erosion and created a widespread regional erosional unconformity which is expressed as a prominent seismic reflector at the top of the kurkar layers. The subsequent Holocene transgression abraded much of the westernmost kurkar ridges, drowned their cores, and covered the previous lowstand deposits with marine sands, which were in turn covered by a sequence of sub-Recent clayey silts.

The Mediterranean coasts of Sinai and Israel are part of the Nile littoral cell. Since the building of the Aswan dams the sand supplied to Israel's coastal system is derived mainly from erosion of the Nile Delta and from sands offshore Egypt that are stirred up by storm waves. The sands are transported by longshore and offshore currents along the coasts of northern Sinai and Israel. Their volume gradually declines northward with distance from their Nile source. The longshore transport terminates in Haifa Bay where some sand is trapped, and the test escapes to deeper water by bottom currents and through submarine canyons, thus denying Nile-derived sand supply to the 40-km-long Akko-Rosh Haniqra shelf.

The sand balance along Israel's coastal zone is a product of natural processes and human intervention. Losses due to the outgoing longshore transport, seaward escape, and landward wind transport exceed the natural gains from the incoming longshore transport and the abrasion of the coastal cliffs. The deficit is aggravated by the construction of (1) seaward-projecting structures that trap sands on the upstream side and (2) offshore detached breakwaters that trap sands between themselves and the coast. The negative sand balance is manifested by the removal of sand from the seabed and the consequent exposure of archaeological remains that were hitherto protected by it.

The sediments that escape seaward from the longshore transport system form a 2.5- to 4-km-wide sandy apron adjacent to the shore that extends to where the water is 30–40 m deep. The apron's slope (0.5–0.8°) is steeper than the theoretical equilibrium slope for the median grain-size diameter in this zone (0.1–0.3 mm).

The beach sands and the apron's surficial sands are well sorted. Their grain size decreases with distance from shore, from 0.2–0.3 mm nearshore to 0.11–0.16 mm by the drowned ridge. The coarse-grained fraction consists of skeletal debris (commonly 5–12% carbonate matter) and wave-milled kurkar grains (locally named zifzif). In deeper water, the basal sands underlying the fine-grained sediment cover consist of 1- to 30-cm layers whose composition ranges from silty sands to various types of sands (fine, medium, coarse, and gravelly) to zifzif. For the most part, they contain large amounts of skeletal debris (20–60%) and small fragments of kurkar.

Two types of kurkar rock were encountered offshore: a well-sorted, fine- to medium-grained (0.074–0.300 mm) lithified dune sand with variable amounts of carbonate cement, ranging from hard rock of low permeability to loose sand; and a porous sandstone made up predominantly of algal grains and skeletal debris (calcarenite).  相似文献   

Recognition of non-Milankovitch sea-level highstands at 185 and 343 thousand years ago from U–Th dating of Bahamas sediment     

Gideon M. Henderson  Laura F. Robinson  Katharine Cox  Alex L. Thomas 《Quaternary Science Reviews》2006,25(23-24):3346

Thirty-one new bulk-sediment U–Th dates are presented, together with an improved δ¹⁸O stratigraphy, for ODP Site 1008A on the slopes of the Bahamas Banks. These ages supplement and extend those from previous studies and provide constraints on the timing of sea-level highstands associated with marine isotope stages (MIS) 7 and 9. Ages are screened for reliability based on their initial U and Th isotope ratios, and on the aragonite fraction of the sediment. Twelve ‘reliable’ dates for MIS 7 suggest that its start is concordant with that predicted if climate is forced by northern-hemisphere summer insolation following the theory of Milankovitch. But U–Th and δ¹⁸O data indicate the presence of an additional highstand which post-dates the expected end of MIS 7 by up to 10 ka. This event is also seen in coral reconstructions of sea-level. It suggests that sea-level is not responding in any simple way to northern-hemisphere summer insolation, and that tuned chronologies which make such an assumption are in error by ≈10 ka at this time. U–Th dates for MIS 9 also suggest a potential mismatch between the actual timing of sea-level and that predicted by simple mid-latitude northern-hemisphere forcing. Four dates are earlier than that predicted for the start of MIS 9. Although the most extreme of these dates may not be reliable (based on the low-aragonite content of the sediment) the other three appear robust and suggest that full MIS 9 interglacial conditions were established at 343 ka. This is ≈8 ka prior to the date expected if this warm period were driven by northern-hemisphere summer insolation.  相似文献