Eustatic Lowering Of Sea Level Research Articles

A steady-state model reconstruction of the patagonian ice sheet during the last glacial maximum

During the Last Glacial Maximum (LGM), the Patagonian Ice Sheet (PIS) was the largest Quaternary ice mass in the Southern Hemisphere outside of Antarctica. Although the margins of the LGM ice sheet are now well established through end-moraine mapping and dating, apart from a few modelling and empirical studies, there remains a lack of constraint on its thickness and three-dimensional configuration. Here, we provide a high-resolution steady-state model reconstruction of the PIS at its maximum - LGM - extent applied using Nye's perfect-plastic ice rheology. The yield-strength parameter for the perfect-plastic flow model was calibrated against independent empirical reconstructions of the Lago Pueyrredón Glacier, where the former vertical extent of this major outlet glacier is well constrained by cosmogenically-dated trimlines and lateral and end-moraine limits. Using this derived yield-strength parameter, the perfect-plastic model is then applied to multiple flowlines demarking each outlet across the entirety of the PIS in a GIS framework. Our results reveal that the area of the PIS was ∼504,500 km2 (±8.5%) with a corresponding modelled ice volume of ∼554,500 km3 (±10%), equivalent to ∼1.38 m (±10%) of eustatic sea-level lowering at the LGM. Maximum surface elevation was at least 3500m asl although the majority of the ice sheet surface was below 2500 m asl. We find that our ice sheet reconstruction is in good general agreement with previous estimates of net PIS volume derived from transient modelling studies. We attribute the slightly lower aspect-ratio of our ice sheet (and its concomitant 5% reduction in volume and sea-level equivalent) to the lower yield strength applied, based on more temperate and dynamic ice sheet conditions.

Quaternary subsidence history of Xisha Islands (northern South China Sea): Evidences from the reef-bank system

Subsidence is a potentially important factor influencing the evolution of carbonate platforms. Yet, few studies have been carried out on the quantitative reconstruction of subsidence history of carbonate platforms in the South China Sea. Using paleo-water depth constraints from core XK-1, drilled in the Xisha Islands, northern South China Sea, the tectonic and total subsidence history of Xisha Islands during the Quaternary is quantitatively reconstructed in this study. Slow tectonic subsidence between 1.945 and 0.988 Ma was related with magmatic intrusion beneath the Xisha Islands, while the relatively high tectonic subsidence rate between 0.988 and 0.781 Ma and the subsequent gradual decline in tectonic subsidence rate between 0.781 and 0.07 Ma were linked to the thermal cooling process. A rapid tectonic uplifting process after 0.07 Ma occurred as a result of compression and volcano eruption. Total subsidence rate showed an increase from the stage of 1.945–0.988 Ma to the stage of 0.988–0.781 Ma, and a gradual decline during the stage of 0.781–0.07 Ma. After 0.07 Ma, there was a rapid uplifting in response to the tectonic movement. The high eustatic sea level and intermediate total subsidence rate from 1.945 to 1.0 Ma caused the domination of bank deposits on the Xisha carbonate platform. From 0.988 to 0.65 Ma, the long-term eustatic sea-level lowering and the relatively high total subsidence rate favored the production of thick reef units. By contrast, the low eustatic sea level and the slow total subsidence between 0.65 and 0.42 Ma forced the accommodation space to be limited, resulting in formation of thin reef-bank deposits and occurrence of exposure horizons. During the stage of slow subsidence, the high eustatic sea level around 0.4 Ma allowed the last presence of thick reef unit in the Quaternary section, after which the lower eustatic sea level reduced the accommodation space again and only thin bioclastic bank deposits and iron-stained exposure horizon occurred. Around 0.07 Ma, the Xisha Islands were continuously exposed due to the uplifting and low eustatic sea level. The strong winter monsoon caused a heterozoan factory nearby the Xisha Islands, and blew the carbonate sediments onshore forming aeolianites. Our results proves that the total subsidence has indeed greatly influenced the evolution of Quaternary carbonate system in the Xisha area. The long-term (at scale of Myr) sea-level lowering could facilitate the flourishment of corals, and a moderate total subsidence in the context of secular sea-level lowering would provide more effective accommodation space for the continuous reef accretion, favoring the formation of thick reef units in the tropical areas.

Deglaciation of the Eurasian ice sheet complex

The Eurasian ice sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum with a span of over 4500 km and responsible for around 20 m of eustatic sea-level lowering. Whilst recent terrestrial and marine empirical insights have improved understanding of the chronology, pattern and rates of retreat of this vast ice sheet, a concerted attempt to model the deglaciation of the EISC honouring these new constraints is conspicuously lacking. Here, we apply a first-order, thermomechanical ice sheet model, validated against a diverse suite of empirical data, to investigate the retreat of the EISC after 23 ka BP, directly extending the work of Patton et al. (2016) who modelled the build-up to its maximum extent. Retreat of the ice sheet complex was highly asynchronous, reflecting contrasting regional sensitivities to climate forcing, oceanic influence, and internal dynamics. Most rapid retreat was experienced across the Barents Sea sector after 17.8 ka BP when this marine-based ice sheet disintegrated at a rate of ∼670 gigatonnes per year (Gt a−1) through enhanced calving and interior dynamic thinning, driven by oceanic/atmospheric warming and exacerbated by eustatic sea-level rise. From 14.9 to 12.9 ka BP the EISC lost on average 750 Gt a−1, peaking at rates >3000 Gt a−1, roughly equally partitioned between surface melt and dynamic losses, and potentially contributing up to 2.5 m to global sea-level rise during Meltwater Pulse 1A. Independent glacio-isostatic modelling constrained by an extensive inventory of relative sea-level change corroborates our ice sheet loading history of the Barents Sea sector. Subglacial conditions were predominately temperate during deglaciation, with over 6000 subglacial lakes predicted along with an extensive subglacial drainage network. Moreover, the maximum EISC and its isostatic footprint had a profound impact on the proglacial hydrological network, forming the Fleuve Manche mega-catchment which had an area of ∼2.5 × 106 km2 and drained the present day Vistula, Elbe, Rhine and Thames rivers through the Seine Estuary. During the Bølling/Allerød oscillation after c. 14.6 ka BP, two major proglacial lakes formed in the Baltic and White seas, buffering meltwater pulses from eastern Fennoscandia through to the Younger Dryas when these massive proglacial freshwater lakes flooded into the North Atlantic Ocean. Deglaciation temporarily abated during the Younger Dryas stadial at 12.9 ka BP, when remnant ice across Svalbard, Franz Josef Land, Novaya Zemlya, Fennoscandia and Scotland experienced a short-lived but dynamic re-advance. The final stage of deglaciation converged on present day ice cover around the Scandes mountains and the Barents Sea by 8.7 ka BP, although the phase-lagged isostatic recovery still continues today.

The palaeogeography of Northwest Europe during the last 20,000 years

Please click here to download the map associated with this article. Since the last glacial period, large vertical changes in the height of sea level relative to the land surface have led to considerable horizontal shifts in the position of coastlines around Northwest Europe. Indeed, for much of the last 20,000 years, extensive areas of the present-day shelf seabed were sub-aerially exposed due (primarily) to the glacial eustatic lowering of sea level. Accurate maps depicting these palaeogeographic changes are of great value to a wide spectrum of researchers including (inter alia) archaeologists, marine geomorphologists, climate scientists, biogeographers and palaeobotanists, although a lack of empirical sea level data has often hindered efforts to produce reliable palaeogeographic reconstructions. However, the processes which bring about change in relative sea level can be successfully simulated by computer models that describe the response of the solid Earth to the loading and unloading of glacial ice (‘glacial rebound models’). In addition to simulating relative sea-levels, the output from these models can be combined with modern day bathymetric and topographic data to produce first-order palaeogeographic reconstructions. For this publication and associated map, numerical outputs from a recently published glacial rebound model are used to produce a series of palaeogeographic maps of Northwest Europe since the Last Glacial Termination. These maps, developed using GIS tools and presented here individually at a scale of 1:20,000,000, emphasize that for much of the period from 20,000 years ago to the present, large areas of the Northwest European shelf, now covered by sea, were dry land. However, they also suggest that whilst Britain maintained a ‘land-bridge’ connection with the continent until well into the Holocene interglacial (which began 11,700 years ago), any connection between Britain and Ireland would have been low-lying, probably ephemeral and unlikely to have existed after circa 15,000 years ago.

Changes in the frequency, scale, and failing areas of latest Quaternary (<29.4 cal. ka B.P.) slope failures along the SW Ulleung Basin, East Sea (Japan Sea), inferred from depositional characters of densely dated turbidite successions

The depositional characters of densely dated turbidite successions originating from the southwestern margin of the Ulleung Basin reveal changes in high-resolution frequency, failing areas, and relative volumes of slope failures over the past 29.4 cal. ka. Between 29.4 and 19.1 cal. ka B.P., various thin- to very thick-bedded turbidites accumulated at an average recurrence interval of ca. 605 years. After 19.1 cal. ka B.P., turbidites were deposited with an average recurrence interval of 3,183 years, and their thickness abruptly decreased upward. These features suggest that various-scale slope failures occurred frequently during the eustatic lowering of sea level, and the frequency and relative volumes of slope failures suddenly decreased after sea level began to rise. When sea level was lowest (20.0–19.1 cal. ka B.P.), successive stacks of very thick turbidites can most likely be ascribed to larger-volume mass failures. An upward change from muddy to sandy turbidites around 21.4 cal. ka B.P. suggests that the failing areas retrograded from the muddy upper-middle slope to the sandy uppermost slope when sea level was nearly at its lowest. Based on these findings together with published evidence, frequent mass failures between 29.4 and 19.1 cal. ka B.P. were plausibly triggered by earthquakes, in combination with reduced hydrostatic pressure that promoted gas-hydrate dissolution during the eustatic lowering of sea level. These data on the frequency, scale, failing areas, and triggering causes of slope failures along the southwestern margin over the past 29.4 cal. ka, not documented in earlier studies, provide invaluable information to better understand the basin-scale characters and occurrences of latest Quaternary slope failures in the Ulleung Basin.

Middle Palaeogene dinoflagellate cysts from Tierra del Fuego, Argentina: biostratigraphy and palaeoenvironments

Abstract. Palynological data from four surface sections in northern Tierra del Fuego, southern Argentina, provide a biostratigraphical and palaeoenvironmental framework for the lower member of the La Despedida Formation and the Cabo Peña Formation in their type areas. Selected dinoflagellate cyst (dinocyst) events indicate that the age of the lower member of the La Despedida Formation is Middle Eocene and that of the Cabo Peña Formation is Late Eocene–earliest Oligocene. The age assigned to the La Despedida Formation agrees with determinations based on calcareous microfossils, but there is a potential discrepancy regarding the Cabo Peña Formation. According to recent stratigraphic studies, the Cabo Domingo Group, which includes the Cabo Peña Formation, is Late Eocene–Miocene in age. The palynomorph assemblages from the lower member of the La Despedida Formation contain the endemic ‘Transantarctic Flora’, which reflects marginal marine conditions. The maximum abundance of Enneadocysta spp. reflects more open-sea conditions and a warming event during the late Middle Eocene. The lower part of the Cabo Peña Formation has a high ratio of dinocysts to sporomorphs and an abundance of Nematosphaeropsis lemniscata, Reticulatosphaera actinocoronata and Impagidinium spp., suggesting an oceanic to outer neritic environment. Abundant Gelatia inflata and protoperidiniacean cysts indicate cool surface waters rich in dissolved nutrients. These cold-water markers may reflect the development of the Antarctic Circumpolar Current, an important event in the transition from a greenhouse to an icehouse climate mode. Toward the top of the sections, the lower ratios of dinocysts to sporomorphs, as well as the composition of the dinocyst assemblages, reflect a neritic rather than an oceanic setting. This palynological change may be due to eustatic sea-level lowering caused by cooling during the latest Eocene–earliest Oligocene. A new species, Spiniferites scalenus, is described and the new combination Lingulodinium echinatum proposed; an emendation for the latter species is also proposed.

Late Pleistocene dark laminated mud layers from the Korea Plateau, western East Sea/Japan Sea, and their paleoceanographic implications

A variety of approaches were applied to a 11.1-m-long sediment core (EBP-4) from the Korea Plateau, one of the bathymetric highs in the western part of the East Sea/Japan Sea, to understand late Pleistocene paleoenvironmental change. The chronostratigraphy of the core was established using a radiocarbon age, dark laminated mud (DLM) layers with known ages, the oxygen isotope ( δ 18O) curve of planktonic foraminifera, biogenic opal and total organic carbon (TOC) contents, downcore changes in the carbon/nitrogen (C/N) ratio as well as their isotopic ratio ( δ 13C and δ 15N) in organic matter. Although interrupted by several pumice lapilli layers, the core bottom represents marine isotope stage (MIS) 8, and spans more than 250 kyr of paleoceanographic history of the western East Sea/Japan Sea. Paleoenvironmental conditions during MIS 2 and MIS 6 are clearly distinguishable, represented by the negative shift of δ 18O values of planktonic foraminifera, resulting from surface-water freshening due to basin isolation caused by eustatic sea-level lowering. The corresponding dark laminated mud (DLM I, II a, II b and III) layers formed during glacial maxima are characterized by strong Mo and total sulfur (TS) enrichment in anoxic conditions, which indicates a stagnant and strongly-stratified water-column structure. Biogenic opal contents can be a useful indicator to confirm the chronostratigraphic assessment, and can also identify periods of high surface-water productivity in the East Sea/Japan Sea particularly during interglacial times. In contrast, glacial paleoproductivity might be low, compared to the interglacial because of insufficient nutrient availability, resulting from limited vertical ventilation. The δ 13C and δ 15N values of sedimentary organic matter also emphasize increased terrestrial input during the glacial maxima, exhibiting highly depleted δ 13C and δ 15N values in all DLM layers. Sea-level induced paleoenvironmental changes associated with glacial-interglacial variations are identified from the Korea Plateau, confirming these events occurred throughout the whole East Sea/Japan Sea.

The mid-Holocene sea-level highstand at Bogenfels Pan on the southwest coast of Namibia

The radiocarbon ages of mollusc shells from the Bogenfels Pan on the hyper arid southern coast of Namibia provide constraints on the Holocene evolution of sea level and, in particular, the mid-Holocene highstand. The Bogenfels Pan was flooded to depths of 3 m above mean sea level (amsl) to form a large subtidal lagoon from 7300 to 6500 calibrated radiocarbon years before present (cal yr BP). The mollusc assemblage of the wave sheltered lagoon includes Nassarius plicatellus, Lutraria lutraria, and the bivalves Solen capensis and Gastrana matadoa, both of which no longer live along the wave-dominated southern Namibian coast. The radiocarbon ages of mollusc shell from a gravely beach deposit exposed in a diamond exploration trench indicate that sea level fell to near or 1 m below its present-day position between 6500 and 4900 cal yr BP. The rapid emergence of the pan between 6500 and 4900 cal yr BP exceeds that predicted by glacio-isostatic models and may indicate a 3-m eustatic lowering of sea level. The beach deposits at Bogenfels indicate that sea level rose to 1 m amsl between 4800 and 4600 cal yr BP and then fell briefly between 4600 and 4200 cal yr BP before returning to 1 m amsl. Since 4200 cal yr BP sea level has remained within one meter of the present-day level and the beach at Bogenfels has prograded seaward from the delayed arrival of sand by longshore drift from the Orange River. A 6200 cal yr BP coastal midden and a 600 cal yr BP midden 1.7 km from the coast indicate sporadic human utilization of the area. The results of this study are consistent with previous studies and help to refine the Holocene sea-level record for southern Africa.

Modeling of the northern hemisphere ice sheets during the last glacial cycle and glaciological sensitivity

We present a new three‐dimensional thermomechanically coupled ice sheet model of the northern hemisphere to reconstruct the Quaternary ice sheets during the last glacial cycle. The model includes basal sliding, internally calculated surface mass balance, glacial isostasy, and a treatment for marine calving. The time dependent forcing consists of temperature and precipitation anomalies from the United Kingdom Meteorological Office (UKMO) General Circulation Model scaled to the Greenland Ice Core Project (GRIP) ice core δ18O record. Model parameters were chosen to best match geomorphological inferences on Last Glacial Maximum extent and global eustatic sea level change. For our standard run we find a maximum ice volume of 57 × 106 km3 at 18.5 ka cal BP. This corresponds to a eustatic sea level lowering of 110 m after correction for hydro‐isostatic displacement and anomalous ice resulting from defects in the specified boundary conditions of the Paleoclimate Model Intercomparison Project (PMIP) for which the UKMO GCM results were generated. Of this 110 m, 82 m was stored in the North American ice sheet and 25 m in the Eurasian ice sheet. We determine the qualitative and quantitative response of the model from a comprehensive sensitivity study in which 11 important parameters were varied over their respective ranges of uncertainty. Model outputs comparable to the observational record were explored in detail as a linear function along the axes of parameter space of the reference model. The method reveals the dominance of climate uncertainty when modeling the Last Glacial Maximum configuration of the northern hemisphere ice sheets, but also highlights the role of ice rheology and basal processes for ice sheet thickness, and glacial isostasy and calving for the timing of maximum ice volume.

New evidence for the Hirnantian (Upper Ordovician) in Belgium? An integrated isotopical, biostratigraphic and sedimentologic approach

The Hirnantian is an important time in Earth history as it includes one of the major episodes of continental glaciation and marine mass extinction (e.g. Brenchley et alii, 1994). Positive C-isotope excursions and an eustatic lowering of sea level extend from a level near the base of the Normalograptus extraordinarius-N. ojsuensis Zone (lower Hirnantian) to a level within the upper Hirnantian N. persculptus Zone. C-isotope values decline to pre-glacial levels throughout the extent of uppermost Hirnantian strata. At least two discrete and distinct positive shifts of δ13Corg have been observed globally; they may coincide with two pulses of peak glaciation (Melchin et alii, 2003; see also Sutcliffe et alii, 2000).

Exposing the Sunda shelf: Tropical responses to eustatic sea level change

During the last glacial period, eustatic lowering of sea level exposed large areas of continental shelves that are presently submerged. By a series of direct numerical simulations we investigate the sensitivity of the atmosphere‐ocean system to exposure of the extensive Sunda shelf in the western tropical Pacific. We demonstrate that this specific forcing factor triggers increased convection and upper level divergence over the shelf itself, an increased Walker circulation over the far western Pacific, and subsidence over the central and eastern Pacific that weakens the surface easterlies. A comparison of atmosphere‐only with coupled atmosphere‐ocean integrations demonstrates that dynamic coupling of the ocean enhances the equatorial response, with cooling and drying of the tropics north and south of the Intertropical Convergence Zone. Exposure of the shelf therefore represents one mechanism by which glacial climates are affected by eustatic sea level changes. These changes constitute a fundamental shift in the El Niño background state.

Late Pleistocene sea level variations derived from the Argentine Shelf

Neritic‐littoral marine sedimentary deposits from the Argentine Shelf provide a record of late Pleistocene sea level variation. Sediments interpreted to reflect the last glacial maximum low‐stand are ∼150 m below present. Using a simple Airy isostatic model for hydroisostatic compensation and correcting for a minor tectonic component yields a local eustatic sea level lowering of ∼105 m at the last glacial maximum. The deglacial sea level curve records two rapid sea level rises consistent with MWP1‐A and 1‐B as documented by the Barbados coral‐based sea level curve. Comparison with relative sea level variations predicted by the ICE4G VM2 viscoelastic model have highlighted a deficiency in the model's predicted sea level history for this region. Detailed data‐model comparisons of late Pleistocene sea level variations are necessary in the face of climate change induced sea level perturbations to determine regional and or systematic biases in the treatment of lithosphere viscosity and accurate predictions of future sea level.

Origins and paleoceanographic significance of laminated muds from the Ulleung Basin, East Sea (Sea of Japan)

A detailed facies analysis of long piston-core sediments from the Ulleung Basin reveals four mud facies: laminated mud, crudely laminated mud, bioturbated mud, and homogenous mud. The laminated and homogeneous muds were most likely deposited from turbidity currents, whereas the bioturbated and crudely laminated muds were formed by hemipelagic/pelagic sedimentation under well-oxygenated and poorly oxygenated bottom-water conditions, respectively. A detailed analysis of vertical distribution of the mud facies together with a chronostratigraphic framework derived from correlative tephra layers with known eruption ages reveals paleoenvironmental changes during the late Quaternary. During the period between 49 and 23 ka, eustatic lowering of sea level facilitated frequent slope failure, resulting in repeated deposition of fine-grained turbidites on the basin floor. Normal hemipelagic sediments were intensively bioturbated under well-oxygenated bottom-water conditions. During the period between 23 and ∼13 to 11 ka, deposition from turbidity currents continued, but hemipelagic sediments were non-bioturbated under poorly oxygenated bottom water conditions. The bottom-water deoxygenation is attributed to further lowering of sea level during the last glacial period and consequent intensified stratification in the water column by freshwater supply to the nearly isolated sea. Primary laminae preserved in the crudely laminated mud generally exhibit low compositional bimodality in backscattered electron images, which suggests low surface primary productivity and presence of superficial micro-bioturbation. After rapid rise of sea level at ∼13 to 11 ka, the slopes were stabilized and pelagic sedimentation has prevailed under well-oxygenated bottom-water conditions.

Snowline depression in the tropics during the Last Glaciation

Five primary methods have been used to reconstruct Pleistocene snowlines or equilibrium-line altitudes (ELAs) in the tropics (23.5°N–23.5°S) during the last glaciation, but each has inherent errors that limit the accuracy of the results. Additional potential errors in determining ELA depression involve estimates of modern snowline altitude, dating resolution, topographic reconstruction of former glaciers, orographic effects, the presence of rockfall debris on glaciers, and calculation of regional ELA gradients. Eustatic sea-level lowering during the last glaciation is an additional factor influencing estimates of ELA depression (ΔELA). In cases where modern snowline lies above a mountain summit, only a minimum value for ΔELA can be obtained. At 12 tropical sites in Africa, the Americas (to 10°S latitude), and Pacific islands, estimates of average ΔELA range from 440 to 1400 m, but most fall in the range of 800–1000 m (mean ±1 σ=900±135 m). In a regional study of ELA depression in the southern tropical Andes (8–22°S), an average ΔELA of ca 920±250 m has been reported. Based on the assumption that glacier mass balance was controlled solely by ablation-season temperature, and assuming a full-glacial temperature lapse rate of −6°C/km, depression of mean annual temperature in glaciated alpine areas was ca 5.4±0.8°C. If adjusted for a sea-level fall of −120 m at the glacial maximum, this value is reduced to 4.7±0.8°C. The figure is based on the (unlikely) assumption that accumulation on alpine glaciers has been invariant; nevertheless, it is similar to values of temperature depresson (5–6.4°C) for the last glaciation obtained from various terrestrial sites, but contrasts with tropical sea-surface temperature estimates that are only 1–3°C cooler than present.

Chronostratigraphy and Hydrocarbon Habitat Associated with the Jurassic Carbonates of Abu Dhabi, United Arab Emirates: ABSTRACT

Deposition of Jurassic epeiric shelf carbonates and evaporates were controlled by epeirogenic movement and sea level fluctuations which formed an excellent combination of source rocks, reservoirs and seats in Abu Dhabi. At the end of the Triassic, a relative drop in sea level, caused by eustatic sea level lowering in conjunction with minor tectonic uplift, resulted in non-deposition or erosion. In the Toarcian, deposition of carbonates and terrigenous, clastics produced the Marrat Formation. In the mid-Aalenian, a drop in sea level eroded much of the Marrat and some of the Triassic in offshore U.A.E. The deposition of the Hamlah Formation followed, under neritic, well-oxygenated conditions. The Middle Jurassic was characterized by widespread, normal marine shelf carbonates which formed the cyclic Izhara and Araej formations (reservoirs). In the Upper Jurassic, the carbonate shelf became differentiated into a broad shelf with a kerogen-rich intrashelf basin, formed in response to a eustatic rise coupled with epeirogenic downwarping and marine flooding. The intrashelf basin fill of muddy carbonate sediments constitutes the Diyab Formation and its onshore equivalent, the Dukhan Formation (source rocks). In the late Upper Jurassic, the climate became more arid and cyclic deposition of carbonates and evaporates prevailed, forming alternating peritidal anhydrite,more » dolomite and limestone in the Arab Formation (reservoir). Arid conditions continued into the Tithonian, fostering the extensive anhydrite of the Hith Formation (seal) in a sabkha/lagoonal setting on the shallow peritidal platform, the final regressive supratidal stage of this major depositional cycle.« less

Paleochannels indicating wet climate and lack of response to lower sea level, southeast Georgia

Large meandering paleochannels on the flood plain of the middle Ogeechee River in southeast Georgia represent wet paleoclimate during late Pleistocene and early Holocene time and show no indication of downcutting in response to eustatically lower sea level. Radiocarbon dates indicate that the paleomeanders were active at ∼31–28 ka and ∼8.5–4.5 ka. Bivariate regression models that correlate modern channel dimensions to the discharge of low-magnitude floods (1–5 yr recurrence interval) indicate that paleodischarge of floods during those times was at least double that of modern floods, thus suggesting a wetter paleoclimate. These data corroborate independent studies of pollen and paleoclimate simulations that indicate wet early to middle Holocene (9–3 ka) conditions that were characterized by intensified monsoonal circulation. Paleoclimate conditions at ∼31–28 ka are less well known. Our analysis of the middle Ogeechee River flood plain indicates the absence of base-level response (downcutting) to eustatic sea-level lowering because the beds of the paleochannels are at approximately the same elevation as the bed of the modern channel. This supports recent arguments that the geomorphic response of coastal-plain streams to sea-level lowering is most apparent in deltaic and shelf environments and may not be recognized very far upstream from the coast.

Interprétation séquentielle de l'Oligo-Miocène (Tunisie centrale et Nord-Orientale). / New sequential interpretation of Oligocene and Miocene (Central and Northeast of Tunisia)

New sequential interpretation of Oligocene and Miocene (Central and Northeast of Tunisia) Sedimentological and sequence stratigraphie study of Oligocene to Langhian deposits represented by Fortuna (Oligocene to Aquitanian), Messiouta (Aquitanian), Grijima (Upper Burdigalian), Oued el Hammam (Lower Langhian) and Aïn Grab (Upper Langhian) formations shows the following results : Three major eustatic lowering of sea level have marked the tunisian platform during the Oligocene to Langhian times. The first one, between Rupelian and Chattian times, is characterized by the installation of a lagoonal domain followed by important detrital discharges. The second, in upper Aquitanian, induces the exundation of the platform and the installation of strongly erosional fluvial channels. The third one, at the top of Burdigalian, is similar to the last one, with a base level fall causing regional gullyings. Three important flooding periods : the first one during Rupelian time which induces rétrogradations of the detrital supply provided by the erosion of the highland located to the West and the South-West. The second during Burdigalian time, overtaken southeastward by the Langhian sea. This observation is in agreement with the HaQ et al. (1988) chart which exhibits a global sea level rise between maximum flooding periods of Burdigalian and Langhian.

Groundwater residence time and palaeohydrology in the Otway Basin, South Australia: 2H, 18O and 14C data

Stable isotopes (δ 2H, δ 18O and δ 13C) and radiocarbon ( 14C) have been used in conjunction with chemical data to evaluate recharge mechanisms, groundwater residence time and palaeohydrology within the confined Dilwyn sand aquifer in the Gambier Embayment of the Otway Basin. This aquifer does not receive recharge down-gradient of the hydraulic hinge-line and data have been interpreted along two discrete flow lines. The mean residence time of groundwater (determined by 14C) in the confined aquifer from the hydraulic hinge position to the sea (a distance of about 50 km) along an inferred flow path is approximately 12 800 years. The corresponding hydraulic travel time calculated from Darcy's law is approximately 49 000 years. The apparent discrepancy may be a result of eustatic sea-level lowering during the last glacial. Because the groundwater system is hydraulically connected to the sea, lower sea-level would result in increased gradients and a decrease in groundwater residence time. Variations in stable isotopic composition along flow lines suggest a number of recharge mechanisms. Stable isotope data indicate progressive depletion of 2H and 18O in the groundwaters over the past 30 000 years. Groundwaters older than about 10 000 years B.P. were recharged either during a cooler climate climatic regime (lower precipitation/evapotranspiration and temperature) and/or the vapour source(s) had travelled over greater continental mass than those recharged over the past 10 000 years. Lower chloride concentrations in some of the older groundwaters indicate lower evapotranspiration rates in the recharge areas before 10 000 years ago.

Pleistocene slope instability of gas hydrate‐laden sediment on the Beaufort sea margin

Abstract The Beaufort Sea continental slope is disrupted by a belt of massive bedding‐plane slides and rotational slumps. This zone coincides with a region of sediment containing gas hydrate, an inclusion compound of gas and water. Quantitative studies suggest that elevated pore‐fluid pressures generated as a by‐product of gas hydrate disassociation during repeated episodes of eustatic sea level lowering during Pleistocene time were a major cause of these slides. Eustatic sea level fall causes reduced pressures acting on seafloor sediment. In oceanic areas underlain by sediment with gas hydrate, the reduction of sea level initiates disassociation along the base of the gas hydrate, which, in turn, causes the release of large volumes of gas into the sediment and creates excess pore‐fluid pressures and reduced slope stability. A quantitative approach was taken to predict excess pressures at the base of the gas hydrate zone, and the stability of the overlying sediment for a variety of sediment types. Our studies suggest that during eustatic sea level fall, fluid diffusion properties dominate the disassociation process in fine‐grained marine sediment. Slope failure appears likely for this sediment type on moderate slopes unless pressures can be adequately vented away from the gas hydrate base. These models give credence to the theory that fluctuations in global climate, manifest in periodic Pleistocene eustatic‐sea level regressions, likely triggered seafloor landslides on the continental slope of the Beaufort Sea and other margins where gas hydrate is present in seafloor sediment.

By-the-wind-sailors from a Late Devonian foreshore environment in western Montana

Three remarkable fossil medusoid hydrozoans (Plectodiscus latinautilus n. sp.) with disc, topsail, and pendant tentacular appendages were recovered from the latest Late Devonian beds of the Sappington Member of the Three Forks Formation in the Gallatin Range, southwest Montana. This is the first velellid reported from a Paleozoic beach paleoenvironment. Top and underside patterns of these chondrophorine velellids display well-preserved casts and mold imprints in fine siltstone. Outlines of the circular disc and the profile of the wide sail and tentacular structures embedded in soft tissue can be recognized.These floating colonial animals were moved along the southeast shores of the Sappington Basin during a time of eustatic sea level lowering and regression in a tropical setting. Colonies were washed up on the upper beach where they were stranded in the bubbling swash left behind by the surf. The foreshore was constructed of tabular, planar cross-bedded, seaward-dipping, foreset accretionary beds which contain parallel laminations and lime-coated grains. Bedding is inclined 17° to 24° seaward.