Seismic sequence stratigraphy of Miocene deposits related to eustatic,tectonic and climatic events,Cap Bon Peninsula,northeastern Tunisia

The Cap Bon Peninsula, belonging to northeastern Tunisia, is located in the Maghrebian Alpine foreland and in the North of the Pelagian block. By its paleoposition, during the Cenozoic, in the edge of the southern Tethyan margin, this peninsula constitutes a geological entity that fossilized the eustatic, tectonic and climatic interactions. Surface and subsurface study carried out in the Cap Bon onshore area and surrounding offshore of Hammamet interests the Miocene deposits from the Langhian-to-Messinian interval time. Related to the basin and the platform positions, sequence and seismic stratigraphy studies have been conducted to identify seven third-order seismic sequences in subsurface (SM1-SM7), six depositional sequences on the Zinnia-1 petroleum well (SDM1-SDM6), and five depositional sequences on the El Oudiane section of the Jebel Abderrahmane (SDM1–SDM5). Each sequence shows a succession of high-frequency systems tract and parasequences. These sequences are separated by remarkable sequence boundaries and maximum flooding surfaces (SB and MFS) that have been correlated to the eustatic cycles and supercycles of the Global Sea Level Chart of Haq et al. (1987). The sequences have been also correlated with Sequence Chronostratigraphic Chart of Hardenbol et al. (1998), related to European basins, allows us to arise some major differences in number and in size. The major discontinuities, which limit the sequences resulted from the interplay between tectonic and climatic phenomena. It thus appears very judicious to bring back these chronological surfaces to eustatic and/or local tectonic activity and global eustatic and climatic controls.     

Hydrochemical characterization and geospatial analysis of groundwater quality in Cap Bon region,northeastern Tunisia

The hydrogeochemical characteristics of shallow groundwater in the Grombalia region, northeastern Tunisia, were investigated to evaluate suitability for irrigation and other uses and to determine the main processes that control its chemical composition. A total of 21 groundwater samples were collected from existing wells in January–February 2015 and were analyzed for the major cations and anions concentrations. Conductivity, pH, T°, O₂ and salinity were also measured. Interrelationships between chemical parameters were determined by using the scatter matrix method. The suitability of groundwater for irrigation and other uses was assessed by determining the sodium adsorption ratio, soluble-sodium percentage, total dissolved solids, total hardness, Kelly’s index and permeability index values of water samples. The spatial distribution of key parameters was assessed using a GIS-based spatial gridding technique. This analysis indicated that the chemical composition of groundwater in the study area is of Cl–SO₄–Na–Ca mixed facies with concentrations of many chemical constituents exceeding known guideline values for irrigation. The salinity of groundwater is controlled by most dominant cation and anion (Na–Cl). A correlation analysis shows that Na⁺ is the dominant cation and that reverse ion exchange is a dominant process that controls the hydrogeochemical evolution of groundwater in the area. Geospatial mapping of hydrochemical parameters and indices analyzed with the USSL and Wilcox diagrams show distinctive areas of irrigation suitability. In contrast, 76.2% of samples fall in the highly doubtful to unsuitable category and indicate that the central and north-eastern parts of the study area are unsuitable for irrigation due to a high salinity and alkalinity.     

Use of time-domain electromagnetic (TDEM) method to investigate seawater intrusion in the Lebna coastal aquifer of eastern Cap Bon,Tunisia

A multi-method geophysical survey, using time-domain electromagnetic (TDEM) fields, is conducted on the north-eastern coastal aquifer of Cap Bon, Tunisia. The aim of the study is to investigate the inland extent saltwater intrusion, to estimate the fresh-saltwater transition zone, and to characterize the lithologic spatial variability of the underground layers. Moreover, the TDEM is used to determine both fluid resistivity and bulk resistivity which are required to calculate the porosity by applying the Archie’s equation. The survey of TDEM profiles is carried out along the left and right overbanks of the Lebna river from the coast until the Lebna dam. The interpretation and the analysis of these profiles allow us to better identify the flux exchanges between the river and the underlying aquifer. On the basis of the 1D and 2D longitudinal and transversal resistivity profiles as well as the existing boreholes data and groundwater quality, it seems that the freshwater-saltwater interface is moving forward inland. Furthermore, electromagnetic results combined to the Archie’s law show that the porosity values are at the ranges of 12–36 and 5–31% for the Quaternary outcropping and Pliocene formation, respectively.     

Understanding the origin of salinization of the Plio-quaternary eastern coastal aquifer of Cap Bon (Tunisia) using geochemical and isotope investigations

As in many other semi-arid regions, the Plio-quaternary aquifer of the eastern coast of Cap Bon peninsula (NE Tunisia) shows a parallel increase in overexploitation and mineralization of groundwater resources and so the water quality is deteriorating. Different methods using geochemistry (ions Na⁺, Cl⁻, Ca²⁺, Mg²⁺, Br⁻) and stable isotopes (¹⁸O, ²H) are compared with the hydrodynamic information for identifying the main processes involved in the increase of salinization. Along the coast, intrusion of seawater resulting from groundwater overexploitation is identified, but is not the only cause of qualitative degradation: the development of irrigation that induces soil leaching and transfer of fertilizers to groundwater over the whole aquifer extent is another major reason for the increase in salinization. A total of 48 groundwater wells were sampled to obtain additional information on the hydrochemical characteristics of the groundwater defined in previous studies.     

Late Pliocene and Pleistocene small mammal chronology in the Italian peninsula

The abundant documentation of small mammals in the Italian peninsula, collected over recent years, furnishes a detailed biochronological sequence mainly from the Late Pliocene onwards. An updated stratigraphic framework is here presented, based on the European small mammal biozonation. The Early Villanyian is characterized by Mimomys hassiacus, M. stehlini, and, later, poorly documented M. polonicus. The Late Villanyian localities are well characterized with M. pliocaenicus, M. pitymyoides and M. tigliensis. The older part of the Early Biharian is documented by assemblages containing Microtus (Allophaiomys) ex gr. pliocaenicus, M. pusillus, M. cf. ostramosensis and M. tornensis, while the later part of the Early Biharian, is characterized by advanced Microtus (Allophaiomys) species occurring together with M. pusillus or M. blanci. In the Late Biharian M. savini, Microtus hintoni-gregaloides, Microtus (Iberomys) ex gr. huescarensis-brecciensis and Terricola arvalidens occur. The Early Toringian with A. mosbachensis, Allocricetus bursae, Pliomys episcopalis and small-sized Microtus brecciensis, and the Late Toringian with A. terrestris in diversified associations are quoted. The most important faunal events are calibrated by independent chronological controls, thanks to the record of small mammals from lacustrine deposits. Several figures illustrate the most significant rodent species occurring in the succession of selected Italian localities.     

Sea‐level rise,depth‐dependent carbonate sedimentation and the paradox of drowned platforms

A mathematical model of carbonate platform evolution is presented in which depth‐dependent carbonate growth rates determine platform‐top accumulation patterns in response to rising relative sea‐level. This model predicts that carbonate platform evolution is controlled primarily by the water depth and sediment accumulation rate conditions at the onset of relative sea‐level rise. The long‐standing ‘paradox of a drowned platform’ arose from the observation that maximum growth rate potentials of healthy platforms are faster than those of relative sea‐level rise. The model presented here demonstrates that a carbonate platform could be drowned during a constant relative sea‐level rise whose rate remains less than the maximum carbonate production potential. This scenario does not require environmental changes, such as increases in nutrient supply or siliciclastic sedimentation, to have taken place. A rate of relative sea‐level rise that is higher than the carbonate accumulation rate at the initial water depth is the only necessary condition to cause continuous negative feedbacks to the sediment accumulation rates. Under these conditions, the top of the carbonate platform gradually deepens until it is below the active photic zone and drowns despite the strong maximum growth potential of the carbonate production factory. This result effectively resolves the paradox of a drowned carbonate platform. Test modelling runs conducted with 2·5 m and 15 m initial sea water depths at bracketed rates of relative sea‐level rise have determined how fast the system catches up and maintains the ‘keep‐up’ phase. This is the measure of time necessary for the basin to respond fully to external forcing mechanisms. The duration of the ‘catch‐up’ phase of platform response (termed ‘carbonate response time’) scales with the initial sea water depth and the platform‐top aggradation rate. The catch‐up duration can be significantly elongated with an increase in the rate of relative sea‐level rise. The transition from the catch‐up to the keep‐up phases can also be delayed by a time interval associated with ecological re‐establishment after platform flooding. The carbonate model here employs a logistical equation to model the colonization of carbonate‐producing marine organisms and captures the initial time interval for full ecological re‐establishment. This mechanism prevents the full extent of carbonate production to be achieved at the incipient stage of relative sea‐level rise. The increase in delay time due to the carbonate response time and self‐organized processes associated with biological colonization increase the chances for platform drowning due to deepening of water depth (> ca 10 m). Furthermore this implies a greater likelihood for an autogenic origin for high‐frequency cyclic strata than has been estimated previously.     

Late quaternary sea‐level changes,crustal movements and coastal evolution in Northumberland,UK

Northumberland lies in the transition between Holocene emergence and submergence and is thus a critical zone for testing models of isostatic rebound. We have collected data from this area to reconstruct relative sea‐level changes and lateral coastline movements for the last 14000 y. These are deposits from tidal marsh, back‐barrier wetland and terrestrial environments producing 47 sea‐level index points from 12 sites. There is no unequivocal evidence for Late Devensian sea levels above present and the reliable sea‐level index points are restricted between −6 m and +2.5 m relative to present and 9.0–2.5 kyr cal. BP. Analysis of these quantifies differential responses to glacio‐ and hydroisostatic rebound, with the northern sites recording a mid‐Holocene sea‐level maximum ca. 2.5 m above present, whereas the southern sites show a maximum ca. 0.5 m above present. These observations show a reasonable fit with the predictions from quantitative models of glacio and hydroisostatic rebound, but there is currently no unique solution of Earth and ice model parameters that will explain all the sea‐level observations. Copyright © 2000 John Wiley & Sons, Ltd.     

Combining boron isotopes and carbamazepine to trace sewage in salinized groundwater: A case study in Cap Bon,Tunisia

The Korba aquifer on the east coast of Cape Bon has been overexploited since the 1960s with a resultant reversal of the hydraulic gradient and a degradation of the quality due to seawater intrusion. In 2008 the authorities introduced integrated water resources planning based on a managed aquifer recharge with treated wastewater. Water quality monitoring was implemented in order to determine the different system components and trace the effectiveness of the artificial recharge. Groundwater samples taken from recharge control piezometers and surrounding farm wells were analyzed for their chemical contents, for their B isotopes, a proven tracer of groundwater salinization and domestic sewage, and their carbamazepine content, an anti-epileptic known to pass through wastewater treatment and so recognized as a pertinent tracer of wastewater contamination. The system equilibrium was permanently disturbed by the different temporal dynamics of continuous processes such as cation exchange, and by threshold processes linked to oxidation–reduction conditions. The B isotopic compositions significantly shifted back-and-forth due to mixing with end-members of various origin. Under the variable contribution of meteoric recharge, the Plio-Quaternary groundwater (δ¹¹B of 35–40.6‰, a mean B concentration of 30 μmol/L, no carbamazepine, n = 7) was subject to seawater intrusion that induced a high δ¹¹B level (δ¹¹B of 41.5–48.0‰, a mean B concentration of 36 μmol/L, and n = 8). Fresh groundwater (δ¹¹B of 19.89‰, B concentration of 2.8 μmol/L, no carbamazepine) was detected close to the recharge site and may represent the deep Miocene pole which feeds the upper Plio-Quaternary aquifer. The managed recharge water (δ¹¹B of 10.67–13.8‰, n = 3) was brackish and of poor quality with a carbamazepine content showing a large short term variability with an average daily level of 328 ± 61 ng/L. A few piezometers in the vicinity of the recharge site gradually acquired a B isotopic composition close to the wastewater signature and showed an increasing carbamazepine content (from 20 to 910 ng/L). The combination of B isotopic signatures with B and carbamazepine contents is a useful tool to assess sources and mixing of treated wastewaters in groundwaters. Effluent quality needs to be greatly improved before injection to prevent further degradation of groundwater quality.     

Middle to Late Pleistocene lake‐level fluctuations of Lake El'gygytgyn,far‐east Russian Arctic

Lake El'gygytgyn, located in central Chukotka, Russian Arctic, was the subject of an international drilling project that resulted in the recovery of the longest continuous palaeoclimatic and palaeoenvironmental record for the terrestrial Arctic covering the last 3.6 million years. Here, we present the reconstruction of the lake‐level fluctuations of Lake El'gygytgyn since Marine Isotope Stage (MIS) 7 based on lithological and palynological as well as chronological studies of shallow‐water sediment cores and subaerial lake terraces. Reconstructed lake levels show an abrupt rise during glacial–interglacial terminations (MIS 6/5 and MIS 2/1) and during the MIS 4/3 stadial–interstadial transition. The most prominent lowstands occurred during glacial periods associated with a permanent lake‐ice cover (namely MIS 6, MIS 4 and MIS 2). Major triggering mechanisms of the lake‐level fluctuations at Lake El'gygytgyn are predominantly changes in air temperature and precipitation. Regional summer temperatures control the volume of meltwater supply as well as the duration of the lake‐ice cover (permanent or seasonal). The duration of the lake‐ice cover, in turn, enables or hampers near‐shore sediment transport, thus leading to long‐term lake‐level oscillations on glacial–interglacial time scales by blocking or opening the lake outflow, respectively. During periods of seasonal ice cover the lake level was additionally influenced by changes in precipitation. The discovered mechanism of climatologically driven level fluctuations of Lake El'gygytgyn are probably valid for large hydrologically open lakes in the Arctic in general, thus helping to understand arctic palaeohydrology and providing missing information for climate modelling.     

Tectonic,sea-level,and climatic controls on Late Paleozoic sedimentation in the western basins of Argentina

Tectonic activity, sea-level changes, and the climate controlled sedimentation in Late Paleozoic basins of western Argentina. The role of each factor is investigated from the geologic record of the Río Blanco and Paganzo basins using three hierarchical orders of stratigraphic bounding surfaces. First-order surfaces correspond to regional unconformities, second-order ones to local unconformities with a lesser regional extent, and third-order surfaces represent locally extended sedimentary truncation. Using this methodology, the Carboniferous–Permian record of the Paganzo and Río Blanco basins may be divided into two megasequences, four sequences, and 12 stratigraphic sections. Megasequences are bounded by regional unconformities that result from tectonic events important enough to cause regional paleogeographic changes. Sequences are limited by minor regional extension surfaces related to local tectonic movements or significant sea-level falls. Finally, stratigraphic sections correspond to extended sedimentary truncations produced by transgressive events or major climatic changes. Sequence I is mainly composed of marine deposits divided into basal infill of the basin (Section 1) and Tournaisian–Visean transgressive deposits (Section 2). Sequence II is bounded by a sharp erosional surface and begins with coarse conglomerates (Section 3), followed by fluvial and shallow marine sedimentary rocks (Section 4) that pass upward into shales and diamictites (Section 5). The base of Sequence III is marked by an extended unconformity covered by Early Pennsylvanian glacial sedimentary rocks (Section 6) that represent the most important glacial event along the western margin of Gondwana. Postglacial deposits (Section 7) occur in the two basins and comprise both glaciolacustrine (eastern region) and transgressive marine (central and western regions) deposits. By the Moscovian–Kasimovian, fluvial sandstones and conglomerates were deposited in most of the Paganzo Basin (Section 8), while localized volcanic activity took place in the Río Blanco Basin. Near the end of the Carboniferous, an important transgression is recorded in the major part of the Río Blanco Basin (Section 9), reaching the westernmost portion area of the Paganzo Basin. Finally, Sequence IV shows important differences between the Paganzo and Río Blanco basins; fluvial red beds (Section 10), eolian sandstones (Section 11), and low-energy fluvial deposits (Section 12) prevailed in the Paganzo Basin whereas volcaniclastic sedimentation and volcanism dominated in the Río Blanco Basin. Thus, tectonic events, sea-level changes and climate exerted a strong and complex control on the evolution of the Río Blanco and Paganzo basins. The interaction of these allocyclic controls produced not only characteristic facies association patterns but also different kinds of stratigraphic bounding surfaces.     

Tectonic, sea-level, and climatic controls on Late Paleozoic sedimentation in the western basins of Argentina

Tectonic activity, sea-level changes, and the climate controlled sedimentation in Late Paleozoic basins of western Argentina. The role of each factor is investigated from the geologic record of the Río Blanco and Paganzo basins using three hierarchical orders of stratigraphic bounding surfaces. First-order surfaces correspond to regional unconformities, second-order ones to local unconformities with a lesser regional extent, and third-order surfaces represent locally extended sedimentary truncation. Using this methodology, the Carboniferous–Permian record of the Paganzo and Río Blanco basins may be divided into two megasequences, four sequences, and 12 stratigraphic sections. Megasequences are bounded by regional unconformities that result from tectonic events important enough to cause regional paleogeographic changes. Sequences are limited by minor regional extension surfaces related to local tectonic movements or significant sea-level falls. Finally, stratigraphic sections correspond to extended sedimentary truncations produced by transgressive events or major climatic changes. Sequence I is mainly composed of marine deposits divided into basal infill of the basin (Section 1) and Tournaisian–Visean transgressive deposits (Section 2). Sequence II is bounded by a sharp erosional surface and begins with coarse conglomerates (Section 3), followed by fluvial and shallow marine sedimentary rocks (Section 4) that pass upward into shales and diamictites (Section 5). The base of Sequence III is marked by an extended unconformity covered by Early Pennsylvanian glacial sedimentary rocks (Section 6) that represent the most important glacial event along the western margin of Gondwana. Postglacial deposits (Section 7) occur in the two basins and comprise both glaciolacustrine (eastern region) and transgressive marine (central and western regions) deposits. By the Moscovian–Kasimovian, fluvial sandstones and conglomerates were deposited in most of the Paganzo Basin (Section 8), while localized volcanic activity took place in the Río Blanco Basin. Near the end of the Carboniferous, an important transgression is recorded in the major part of the Río Blanco Basin (Section 9), reaching the westernmost portion area of the Paganzo Basin. Finally, Sequence IV shows important differences between the Paganzo and Río Blanco basins; fluvial red beds (Section 10), eolian sandstones (Section 11), and low-energy fluvial deposits (Section 12) prevailed in the Paganzo Basin whereas volcaniclastic sedimentation and volcanism dominated in the Río Blanco Basin. Thus, tectonic events, sea-level changes and climate exerted a strong and complex control on the evolution of the Río Blanco and Paganzo basins. The interaction of these allocyclic controls produced not only characteristic facies association patterns but also different kinds of stratigraphic bounding surfaces.     

Late Pleistocene and Holocene glaciation of the Fish Lake valley,northeastern Alaska Range,Alaska

We reconstructed a chronology of glaciation spanning from the Late Pleistocene through the late Holocene for Fish Lake valley in the north‐eastern Alaska Range using ¹⁰Be surface exposure dating and lichenometry. After it attained its maximum late Wisconsin extent, the Fish Lake valley glacier began to retreat ca. 16.5 ka, and then experienced a readvance or standstill at 11.6 ± 0.3 ka. Evidence of the earliest Holocene glacial activity in the valley is a moraine immediately in front of Little Ice Age (LIA) moraines and is dated to 3.3–3.0 ka. A subsequent advance culminated at ca. AD 610–900 and several LIA moraine crests date to AD 1290, 1640, 1860 and 1910. Our results indicate that ¹⁰Be dating from high‐elevation sites can be used to help constrain late Holocene glacial histories in Alaska, even when other dating techniques are unavailable. Close agreement between ¹⁰Be and lichenometric ages reveal that ¹⁰Be ages on late Holocene moraines may be as accurate as other dating methods. Copyright © 2009 John Wiley & Sons, Ltd.     

Late Pleistocene loess‐palaeosol sequences in the Vojvodina region,north Serbia

Late Pleistocene loess‐palaeosol sequences are widespread in the Vojvodina region, with thicknesses reaching a maximum of about 20 m. Our investigations include more than 40 of these loess sections. Geochronology of the last glacial loess‐palaeosol sequences, based on luminescence dating and amino acid racemisation, provides correlations between Upper Pleistocene loess‐palaeosol sediments in Vojvodina and comparable deposits at other European localities. Sedimentary logs of magnetic susceptibility, grain‐size measurements and carbonate content, combined with malacological evidence, indicate two main relatively cold and arid phases during the last glacial period, related to intensive accumulation of loess units L1L1 and L1L2, as well as many brief episodes of dry and windy climatic conditions, suggesting a possible relationship with cold events recorded in the North Atlantic region. Generally, late Pleistocene climate in the region was dry and relatively warm, compared with glacial period sites in central Europe, and was characterised by sharp differences between glacial and interglacial modes. New data and interpretations presented in this study emphasise the significance of loess‐palaeosol sequences in Vojvodina for the reconstruction of the temporal and spatial evolution of late Pleistocene palaeoclimate in this part of Europe. Copyright © 2007 John Wiley & Sons, Ltd.     

Optically stimulated luminescence age controls on late Pleistocene and Holocene coastal lithosomes, North Carolina, USA

Luminescence ages from a variety of coastal features on the North Carolina Coastal Plain provide age control for shoreline formation and relative sea-level position during the late Pleistocene. A series of paleoshoreline ridges, dating to Marine Isotope Stage (MIS) 5a and MIS 3 have been defined. The Kitty Hawk beach ridges, on the modern Outer Banks, yield ages of 3 to 2 ka. Oxygen-isotope data are used to place these deposits in the context of global climate and sea-level change. The occurrence of MIS 5a and MIS 3 shorelines suggests that glacio-isostatic adjustment (GIA) of the study area is large (ca. 22 to 26 m), as suggested and modeled by other workers, and/or MIS 3 sea level was briefly higher than suggested by some coral reef studies. Correcting the shoreline elevations for GIA brings their elevation in line with other sea-level indicators. The age of the Kitty Hawk beach ridges places the Holocene shoreline well west of its present location at ca. 3 to 2 ka. The age of shoreline progradation is consistent with the ages of other beach ridge complexes in the southeast USA, suggesting some regionally contemporaneous forcing mechanism.     

Late Pleistocene fans and terraces in the Majes valley, southern Peru, and their relation to climatic variations

This study investigates the connection between sediment aggradation, erosion and climate in a desert environment of the Majes valley, southern Peru. Luminescence dating of terraces and fans shows that sediment aggradation correlates with wet time intervals on the Altiplano, suggesting a climatic influence on the aggradation–degradation cycles. Major periods of aggradation occurred between ~110–100, ~60–50 and 12–8 ka. More precipitation in the Majes catchment resulted in increased erosion and transportation of sediment from the hillslopes into the trunk river. As a result, the sediment loads exceeded the transport capacity of the Majes River and aggradation started in the lower reaches where the river gradient is less. Depletion of the hillslope sediment reservoirs caused a relative increase in the capacity of the trunk river to entrain and transport sediment, resulting in erosion of the previously deposited sediment. Consequently, although climate change may initiate a phase of sediment accumulation, degradation can be triggered by an autocyclic negative feedback and does not have to be driven by climatic change.     

Late Pleistocene and Holocene environmental changes in the coastal zone of northwestern Portugal

This paper examines the Quaternary changes in the coastal zone of northwestern Portugal through the relationship between sediments and landforms. By interpolating the available data from geomorphology, lithostratigraphy, sedimentology and geochronology, it was possible to reconstruct existing palaeoenvironments, the oldest of which date back to the last interglacial. The main landforms are a high and a low platform bounded by scarps. River sands and silty sediments formed in a lacustrine environment (Antas Formation, MIS 5) are associated with the high platform. Two deposits are associated with the low platform: the Cepães Formation (MIS 3) consisting of a lower set of river sand beds and an upper gravel beach bed, and the Aguçadoura Formation (MIS 1) consisting of Holocene lagoonal beds. Copyright © 2006 John Wiley & Sons, Ltd.     

Late Pleistocene and Early Holocene lake‐level fluctuations in the Lahontan Basin,Nevada: Implications for the distribution of archaeological sites

The Great Basin of the western U.S. contains a rich record of Late Pleistocene and Holocene lake‐level fluctuations as well as an extensive record of human occupation during the same time frame. We compare spatial‐temporal relationships between these records in the Lahontan basin to consider whether lake‐level fluctuations across the Pleistocene‐Holocene transition controlled distribution of archaeological sites. We use the reasonably well‐dated archaeological record from caves and rockshelters as well as results from new pedestrian surveys to investigate this problem. Although lake levels probably reached maximum elevations of about 1230–1235 m in the different subbasins of Lahontan during the Younger Dryas (YD) period, the duration that the lakes occupied the highest levels was brief. Paleoindian and Early Archaic archaeological sites are concentrated on somewhat lower and slightly younger shorelines (_1220–1225 m) that also date from the Younger Dryas period. This study suggests that Paleoindians often concentrated their activities adjacent to large lakes and wetland resources soon after they first entered the Great Basin. © 2008 Wiley Periodicals, Inc.     

Relationship between Late Pleistocene sea‐level variations,carbonate platform morphology and aragonite production (Maldives,Indian Ocean)

A piston core from the Maldives carbonate platform was investigated for carbonate mineralogy, grain‐size distributions, calcium carbonate content and organic carbon. The sedimentary record was linked to Late Pleistocene sea‐level variations, using an age model based on oxygen isotopes obtained from planktonic foramanifera, nannofossil biostratigraphy and ¹⁴C age determinations. The correlation between the sedimentary record and Late Pleistocene sea‐level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the Maldives carbonate platform. Platform flooding events were characterized by strongly increased deposition of aragonite and mud within the Inner Sea of the Maldives. Exposure events, in contrast, can be recognized by rapid decreases in the values of both proxy records. The results show that sediments on the Maldives carbonate platform contain a continuous record of Pleistocene sea‐level variations. These sediments may, therefore, contribute to a better understanding of regional and even global sea‐level changes, and yield new insights into the interplay between ocean currents and carbonate platform morphology.     

Stratigraphy and chronology of Provo shoreline deposits and lake‐level implications,Late Pleistocene Lake Bonneville,eastern Great Basin,USA

The Provo shoreline of Lake Bonneville formed following the Bonneville flood, and, based on previous dating, was formed during a period of overflow from about 17.5 to 15.0 cal. ka. In many places the Provo shoreline consists of a pair of distinct shorelines, one ～3 m higher than the other. We present data from two cuts through double beaches to show that the upper beach is younger and represents sedimentation after a lake‐level rise. In addition, the lower beach deposits are internally stratified by beds that suggest three more lake‐level rises during its development. The Provo beach complex thus appears to have been built during rising lake levels, which can be explained by rises in the overflow threshold by sequential landslide deposition. Evaluation of beach altitudes demonstrates that the two beach crests throughout the Bonneville basin experienced equivalent rebound from removal of the lake load, and therefore they formed after the rebound associated with the Bonneville flood occurred in early Provo time. However, radiocarbon ages on gastropods collected within the beach deposits suggest both that the sequence of five beach deposits formed from c.18.1 to c. 17.0 cal. ka, and that the Bonneville flood occurred before 18 cal. ka. These ages are discordant with previous dates on shells within offshore sands, and raise questions about the validity of radiocarbon ages for shells in Lake Bonneville as well as about the age of the Bonneville flood and Provo shoreline. The timing for maximum Provo lake depths and its association with climate stages during deglaciation remain unresolved.