Sea-level Lowstand Research Articles

A new method for the U–Th dating of a carbonate chimney deposited during the last glaciation in the northern Okinawa Trough, East China Sea

Authigenic carbonates from cold seep systems, such as chimney and crust carbonate deposits, are important archives of biogeochemical processes associated with the migration and seepage of methane-rich fluids and seepages on the seafloor. Here we use a new U–Th dating method to precisely determine the ages of the authigenic carbonates of a chimney in the Okinawa Trough (OT), East China Sea. The method uses a series of heavy liquids (HL, a mixture of tribromomethane and alcohol with a density of 2.71–2.80 g/cm 3 ) to separate subsamples of carbonate contaminated by detrital material. The contents of relevant elements (Al, Rb, Zr, Nb, and rare earth elements) of the subsamples reveal that the separation of the mineral phase was successful. Five samples from along the radius of a well-structured chimney (PES-1) from station D2-5 in the OT were analyzed and yielded U–Th ages of 28.8 ± 0.4 ka, 26.1 ± 0.5 ka, 24.4 ± 0.7 ka, 22.5 ± 0.6 ka and 20.1 ± 0.6 ka, from the exterior to the interior of the chimney section, and AMS 14 C dating of the same samples produced the similar results. The agreement between the 230 Th/U and AMS 14 C ages suggests that the application of heavy liquid methods in U–Th dating can be very successful, and it reduces the influence of detrital contamination. Therefore, the approach has a great potential space for studying the evolutionary history of authigenic carbonates in cold seep systems. Moreover, the new 230 Th/U ages show that the carbonate chimney was formed during the Last Glacial Maximum (LGM), which corresponding to the large sea-level fall in the East China Sea. Together with information on biogeochemical processes, the results will provide valuable geochronological information for the future study of active fluid venting on continental margins. • Carbonates chimney from metals-rich sediments can be precisely dated with U-series. • Heavy liquids were used to separate subsamples of seep impure carbonate. • Multiple-point HL regressions provide the more reliable chronology. • U–Th ages between 28.8 and 20.1 ka point to sea-level low stands during the LGM.

Molluscan aminostratigraphy of the US Mid-Atlantic Quaternary coastal system: Implications for onshore-offshore correlation, paleochannel and barrier island evolution, and local late Quaternary sea-level history

The Quaternary record of the US Mid-Atlantic coastal system includes onshore emergent late Pleistocene shoreline deposits, offshore inner shelf and barrier island units, and paleovalleys formed during multiple glacial stage sea-level lowstands. The geochronology of this coastal system is based on uranium series, radiocarbon, amino acid racemization (AAR), and optically stimulated luminescence (OSL) methods. We report over 600 mollusk AAR results from 93 sites between northeastern North Carolina and the central New Jersey shelf, representing samples from both onshore cores or outcrops, sub-barrier and offshore cores, and transported shells from barrier island beaches. AAR age estimates are constrained by paired 14C analyses on specific shells and associated U-series coral ages from onshore sites. AAR data from offshore cores are interpreted in the context of detailed seismic stratigraphy. The distribution of Pleistocene-age shells on the island beaches is linked to the distribution of inner shelf or sub-barrier source units. Age mixing over a range of time-scales (~1 ka to ~100 ka) is identified by AAR results from onshore, beach, and shelf collections, often contributing insights into the processes forming individual barrier islands. The regional aminostratigraphic framework identifies a widespread late Pleistocene (Marine Isotope Stage 5) aminozone, with isolated records of middle and early Pleistocene deposition. AAR results provide age estimates for the timing of formation of the three major paleochannels that underlie the Delmarva Peninsula: Persimmon Point paleochannel ≥800 ka; Exmore paleochannel ~400–500 ka (MIS 12); and Eastville paleochannel > 125 ka (MIS 6). The results demonstrate the value of synthesizing abundant AAR chronologic data across various coastal environments, integrating multiple distinct geologic studies. The ages and elevations of the Quaternary units are important for current hypotheses about relative sea-level history and crustal dynamics in the region, which was likely influenced by the Laurentide ice sheet, the margin just ~400 km to the north.

Geomorphological and stratigraphic evolution of a fluvial incision in the coastal plain and inner continental shelf in southern Brazil

This paper describes the evolution of a sector of the southern coastal plain of the Pelotas Basin, southern Brazil, between the last Pleistocene sea-level lowstand and the Holocene highstand, based on geomorphology and high-resolution stratigraphy. A fluvial drainage system associated with an incised valley developed during the Last Glacial Maximum (LGM) was identified. Two high-resolution 2D and 2.5D geophysical profiles using Ground-Penetrating Radar (GPR) show a Pleistocene fluvial system's former path, starting close to its present mouth inside Mirim Lagoon crossing the coastal barriers and reaching the inner continental shelf, covering some 60 km across the coastal plain. During the LGM, with the sea-level at −120 to −130 m and the coastline positioned close to the shelf edge (distant 200 km from the present coastline), the fluvial system excavated a ~31.5-m deep channel. Two cores, 36 and 32 m long, show that the paleochannel was filled with sedimentary deposits after 19.4 ka BP, and the facies succession represents the transition from fluvial, to estuarine, to the coastal barrier, and finally to inner shelf depositional environments, generated by processes connected to LGM and the Postglacial Marine Transgression (PMT). The study region behaved as a sediment by-pass zone, connecting the adjacent coastal plain's inner portion with the inner continental shelf. The studied area presents an unusual record of the LGM at a coastal plain domain.

East Asian monsoon and sea-level controls on clay mineral variations in the southern South China Sea since the Last Glacial Maximum

Clay minerals preserved in marine sediments carry important information about sediment transport processes from sources to sinks. To reveal the dynamic transport processes of clay minerals and related controlling factors in the southern South China Sea, spatiotemporal variations of clay mineral assemblages from previous studies (Cores MD05-2893, CG2, SO18287-3, MD01-2393, SO18383-3, and MD97-2150) are integrated to assess their controlling factors over the past 30 ka. Based on the smectite/(illite + chlorite) ratio in six cores, we found that the clay mineral variations on the continental slope off Sunda Shelf (Cores MD05-2893, CG2, and SO18287-3) have been controlled by the sea-level changes during the last glacial sea-level lowstand and by the East Asian monsoon during the Holocene sea-level highstand periods. Meanwhile, the clay mineral variations on the continental slope off the Mekong River (Cores MD01-2393, SO18383-3, and MD97-2150) have been driven by the chemical weathering in the Mekong River basin that associates with the East Asian summer monsoon evolution. Accordingly, we conclude that the glacial-interglacial variation of clay minerals in an integrated picture of the southern South China Sea region since the Last Glacial Maximum has been mainly controlled by sea-level change and East Asian monsoon evolution. However, these controlling factors play independent roles in different parts of the southern South China Sea, which appears more heterogeneous than generally expected.

Anatomy of a gas-bearing submarine channel-lobe system on a topographically complex slope (offshore Nile Delta, Egypt)

The characterization and predictability of submarine channel-lobe systems on topographically complex slopes have proven challenging, due to the complex responses of such systems to interacting flows and seafloor topography, in terms of their temporal evolution and spatial changes in morphology and architecture. Detailed subsurface studies can reveal important depositional and morphological elements in such settings and help develop predictive models for hydrocarbon exploration. Here, we document a gas-bearing submarine channel-lobe system on a topographically complex slope, from the Pliocene succession offshore the west Nile Delta where the Giza gas field has been discovered. Based on an integrated analysis of 3D seismic reflection and borehole data, several distinct deep-water depositional elements, including amalgamated channel-fills, aggradational channel-fills, mud-prone channel-fills, lobes, levees and mass-transport deposits are recognized. They are developed on a tectonically influenced continental slope, forming a channel-lobe system that features four main evolutionary phases over a period of ca. 0.26 Myr: 1) Channel belt incision, 2) infill of channel belt by laterally amalgamated channel deposits, 3) development of leveed aggradational channels, with lobes developed within and outside the channel belt, 4) channel avulsion leading to the abandonment of the channel belt, resulting in the formation of a new ponded lobe adjacent to the original channel belt.Spatially, the channel-lobe system shows marked changes in architecture and planform morphology in the areas affected by folds and faults. As the channels cut across a faulted anticlinal structure, they show straightening with some degree of diversion on the upstream limb, while on the downstream limb, channels exhibit increased lateral migration and sinuosity, widespread high-amplitude reflections (interpreted as sand-rich channel-fills), and channel to lobe transitions. In contrast, the last-stage avulsion channel appears to divert along several normal faults at a high angle to the regional slope, and forms a ponded lobe external to the main channel belt. This study shows how the channels and lobes throughout these evolutionary stages respond differently to the presence of folds and faults. It provides a unique analog for submarine channel-lobe systems developed during a third-order sea-level lowstand to transgressive phase with the influence of structurally induced topography and mass transport. The results of this study provide insights into the spatio-temporal development of various types of slope channels and lobes, and may greatly enhance the prediction of associated reservoirs in deep-water systems in tectonically active areas.

Reply to: Climate did not drive Common Era Maldivian sea-level lowstands

Reply to: Climate did not drive Common Era Maldivian sea-level lowstands

Climate did not drive Common Era Maldivian sea-level lowstands

Climate did not drive Common Era Maldivian sea-level lowstands

Enhanced terrestrial organic matter burial in the marine shales of Yangtze platform during the early Carboniferous interglacial interval

The supply and preservation of terrestrial organic matter (TOM) during early Carboniferous glacial-interglacial intervals in South China were investigated with respect to their roles in paleo-environmental variation and organic matter (OM) accumulation in black shales. The marine redox proxies such as V/Cr, Corg/P ratios, and Mo–U covariation indicate a fully oxic water condition during the earliest Visean, while dominant oxic with occasional suboxic water conditions prevailed during the early to middle Visean. High sedimentary δ15N values (1.9–6.3‰) suggest the existence of enhanced water-column denitrification in the expanded oxygen minimum zones (OMZs) during the earliest Visean, whereas lower δ15N values (avg. 2.6‰) during the early to middle Visean indicate a decreasing water column denitrification in the reduced OMZs. Both the organic matter compositions and the relationship between Corg/N ratios and δ13Corg values indicate a significant contribution of terrestrial OM in the marine sediments, and the low abundance of marine OM is mainly attributed to the low paleoproductivity as evidenced by low P/Ti and Ba/Al ratios. One reason for such a high contribution of TOM is that a large amount of plant debris and wildfire-derived OM was preserved during Tournaisian glacier time and then released and buried rapidly in the marine sediments during the interglacial interval of Visean due to the temperature-driven glacial melting. The rapid transportation of large quantity of TOM from land to ocean may have resulted in remarkable drawdown of atmospheric pCO2 and subsequent cool climate, as evidenced by the decoupled relationship between δ13Corg and δ13Ccarb values. In summary, the sinking of TOM flux during the sea level lowstand and highstand may be the controlling factor for the paleo-environmental variation as well as OM accumulation in the Visean black shales.

Large-scale margin collapses along a partly drowned, isolated carbonate platform (Lansdowne Bank, SW Pacific Ocean)

The Lansdowne Bank is a partly drowned, isolated carbonate platform of around 4000 km2 located 300 km west of New Caledonia, in the SW Pacific Ocean, in water depths of 20 to 100 m. New multibeam bathymetric data, high resolution seismic reflection profiles and sediment gravity cores have been acquired on the bank top and adjacent slopes. This dataset reveals an almost continuous 4 km wide outer reef rim located in ca. 50 m water depth, surrounding a gently deepening inner platform, reaching up to 100 m water depth. The bank is bordered by very steep slopes showing numerous erosional morphologies such as canyons, channels and gullies. Along with these bypass features, spectacular bank margin collapses and slope failures are evidenced by up to 20 km-wide bank edge and intraslope failure scars, respectively, resulting in a typical “scalloped” geometry of the bank margin. These failure scars can lead to a complete collapse of the outer reef rim and impact subsequent reef development. Bank margin collapses are evidenced by hectometer to kilometer-scale blocks and debris shed on the slope, likely emplaced by rock fall/avalanching processes originating from the brittle failure of early cemented bank edge and upper slope sediments. In turn, failures triggered on the un-cemented mud-prone middle to lower slopes likely generate more cohesive, submarine debris flows that could be at the origin of erosive morphologies within the debris fields. Estimated individual failure volumes can reach up to 3 km3. Quaternary sea-level lowstands, that would have led to platform exposure, fracturing and karstification, and the development of an erosional sea cliff, as well as subsequent rising sea-level are believed to play a significant role in mass wasting event emplacement, yet “bottom up” submarine processes such as the upslope propagation of bypass morphologies by retrogressive headward erosion cannot be ruled out. In terms of geomorphic and stratigraphic constraints, the documented bank margin collapses affect a terrace located in 70 m water depth around the bank, which, depending on its age and origin, could provide a minimum age for collapse events. Finally, considering the shallow water depth of failure headscarps, the volumes of material involved in the slides as well as their vicinity to the nearby main island of New Caledonia, numerical simulations of the tsunamigenic potential of submarine slides have been performed. They showed that these slides would have been able to produce a meter-scale wave that would reach the northern coast of the island in less than an hour.

Climate changes in the pre-Hirnantian Late Ordovician based on δ18Ophos studies from Estonia

Results of conodont-based δ18Ophos studies from two core sections in Estonia (Mehikoorma-421 and Valga-10) indicate that the pre-Hirnantian Late Ordovician was characterized by a general cooling trend: a rapid climate cooling in the early Sandbian and a less intense cooling in the late Sandbian through Katian. During this time interval, sea-surface temperatures probably decreased by up to 9o C. This fits with earlier ideas of a colder Late Ordovician climate before the Hirnantian. On the background of general cooling trend in the pre-Hirnantian Late Ordovician, tendencies in the temperature changes were quite variable. Several episodes of SST decrease and increase of different magnitudes and durations are evident from the δ18Ophos curves in the studied sections. In the interval sampled in detail, 7 cooling events are recognized and tentatively called (in stratigraphical order): Early Kukruse, Haljala, Keila, Early Nabala, Vormsi, Early Pirgu and Middle Pirgu CE-s. Duration of these events and amplitudes of changes in seawater temperatures increased from Sandbian to late Katian. All cooling events may have triggered the waxing of polar and subpolar Gondwana icecaps and are reflected in the sedimentary succession by major sea-level lowstands recognized also outside the study region. Results of our research do not support a simple, straightforward correlation between temperature trends inferred from δ18Ophos measurements and changes in the δ13C record. Comparison of δ18Ophos and δ13C curves reveal that these two variables behave quite independently: in some intervals their trends fit quite well, in others they might show completely the opposite. For this comparison we provide also a new, detailed δ13C curve from the Valga-10 core section.

The geographic, environmental and phylogenetic evolution of the Alveolinoidea from the Cretaceous to the present day.

The superfamily Alveolinoidea is a member of the Order Miliolida, and comprises three main families, the Alveolinidae, the Fabulariidae and the Rhapydioninidae. They are examples of Larger benthic foraminifera (LBF), which are single-celled organisms with specific characteristic endoskeletons. Alveolinoids are found globally from the Cretaceous to the present day, and are important biostratigraphic index fossils in shallow-marine carbonates. They are often associated with hydrocarbon reservoirs, and exhibit provincialism with characteristic genera often confined to one of the American, Tethyan or Indo-Pacific provinces. Previously, the systematic study of the global interrelationship between the various alveolinoid lineages has not been possible because of the absence of biostratigraphic correlation between the geographically scattered assemblages, and the scarcity of described material from the Indo-Pacific province. Here we use the literature and new material from the Americas, the French Alps, Iran, Tibet, India and South East Asia, coupled with the use of the planktonic foraminiferal zonal (PZ) correlation scheme to propose a comprehensive, global, systematic analysis of the biostratigraphic, phylogenetic and paleogeographic evolution of the alveolinoids. The alveolinoids originated in the Cretaceous in the Tethyan province. During a global sea-level low stand, a westward migration of some alveolinoids species to the Americas occurred, a behaviour previously reported in contemporaneous orbitolinid LBF. After the Cretaceous/Palaeogene (K-P) event, which saw the extinction of all Cretaceous alveolinoids, rare new forms of alveolinoids evolved again, first in the Americas and later independently in Tethys. As was found in previous studies of rotalid LBF, sea-level low stands in the Paleocene also allowed some alveolinoid forms to migrate, but this time in an eastward direction from the Americas to Tethys, and from Tethys on to the Indo-Pacific province. Alveolinoids still exist today (Borelis and Alveolinella), the former of which is cosmopolitan, while the latter is restricted to the Indo-Pacific province. Throughout their phylogenetic history, alveolinoids characteristically exhibit convergent evolution, with the repeated re-occurrence of certain morphological features. Understanding this propensity to homoplasy is essential in understanding and constructing the phylogenetic relationships within the alveolinoid superfamily.

Deep-sea submarine erosion by the Kuroshio Current in the Manila accretionary prism, offshore Southern Taiwan

The Kenting Plateau is characterized by unusual low relief surfaces that straddle the topographic crest of the northern Manila accretionary prism off southern Taiwan at 400–700 m water depth. Multibeam bathymetric data, reflection seismic data, Acoustic Doppler Current Profiler (ADCP) data, surface grab samples, and sediment cores were collected in and around the Plateau to identify evidence of erosion in the Kenting Plateau and understand how the morphological evolution has been influenced by submarine erosion over geological time scales. The most distinctive feature on the Kenting Plateau is a 3 km × 7 km bean-shaped flat elevated platform (Kuroshio Knoll) revealed by multibeam bathymetry. Seismic data show almost no reflections beneath the seafloor and erosional truncations at the seafloor, especially in the Plateau's eastern half, evidencing widespread erosion. The P-wave velocity of the gravels recovered from the top of the Plateau ranges from 2.2 to 4 km/s. After comparing the velocity with the borehole data from nearby basin the burial depth of the parent rocks was found to be around 2 to 4 km below the seafloor, indicating that the parent rocks have been uplifted and gravels were formed due to erosion of the Plateau. The truncation of the seafloor shown on seismic sections suggests significant erosion on the Plateau. Sand content of the sediment cores decreases away from the Plateau, suggesting that sediment transport is effective in this area with high energy deposition, thereby accumulating coarse sediments on the Plateau and removing fine particles away from it. The presence of a dune field migrating northward of the Plateau, parallel to the Kuroshio Current also evidences active sediment transport in the area. Flow velocity of the Kuroshio Current observed from the ADCP data is very high, reaching up to 1.8 m/s on top of the Kuroshio Knoll (SE domain). We thus interpret that the observed intense erosion is caused by the Kuroshio Current, while the uplift of the Kenting Plateau is partially due to isostatic rebound caused by sediment removal through erosion and compression of the accretionary wedge. The higher sedimentation rate and coarser in grain size during sea level lowstand (20,000–12,000 yrs. BP) suggests that the erosion was more intense during the glacial period compared to that of deglacial period (< 12,000 yrs. BP) as seen from the MD97–2145 core. Submarine erosion is predominant throughout the Plateau, and it controls the geomorphology of the Plateau, especially the Kuroshio Knoll.

Formation of undulating topography and gravel beds at the bases of incised valleys: Last Glacial Maximum examples beneath the lowlands facing Tokyo Bay

Recent studies using well density distributed borehole logs have revealed undulating topography at the bases of incised valleys formed during the Last Glacial Maximum (LGM). In this study, from analysis of 4702 borehole logs, undulating topography forming a series of pits 1–2 km long, < 1 km wide, 5–10 m deep, and spaced at 1–2-km intervals was discovered at the bases of LGM incised valleys beneath the Tama River Lowland on the west coast of Tokyo Bay. This undulating topography can be attributed to scouring at braided river channel confluences. In the study area, single borehole logs are available within each 187 m × 187 m grid cell, and the logs sample both the bottom and marginal portions of the scouring, which suggests that this undulating topography is not an artifact of erroneous values arising from mathematical interpolation. The morphologies and incision depths of two incised valleys in the study area show a cover effect of the gravel bed at the base of the post-LGM incised-valley fills. The basal age of this basal gravel bed (BG) is confirmed at < 30 ka because the LGM incised valleys dissect the MIS 3 Tachikawa buried terrace overlain by the AT tephra dated 30.0 ka. This means that the BG, which represents braided-river sediments, is interpreted as resulting from the LGM sea-level lowstand after 30 ka.

Paleo-facies distribution and sequence stratigraphic architecture of the Oligo-Miocene Asmari carbonate platform (southeast Dezful Embayment, Zagros Basin, SW Iran)

A facies and sequence stratigraphic analysis was made of the Oligo-Miocene Asmari carbonate platform situated in the east and southeast of the Dezful Embayment (Zagros Basin, SW Iran). This study provides new paleofacies and depositional sequence distribution models for the eastern part of the Dezful Embayment and discusses the impact of the factors controlling sediment deposition being eustatic sea-level changes and tectonic processes. The petrographic study reveals that the variations in facies cover three depositional settings, e.g. open marine, lagoon and tidal flats. In total, six depositional sequences (DS) were identified within the sedimentary sequence of the Asmari Formation. DS1 and DS2 were deposited during the Rupelian (Oligocene) within deeper open marine platform settings. Sediments forming DS3 and DS4 were deposited during the Chattian in a carbonate platform environment within an open lagoon to protected lagoon setting. The Miocene DS5 and 6 sequences, Aquitanian and Burdigalian in age, were deposited in shallower environments compared to other DS's, within protected lagoon to tidal flat settings. A comparison with the sedimentary development in other parts of the Dezful Embayment showed that the deposition of the Asmari strata started earlier (Early Rupelian) in the eastern, south-eastern and north-eastern part of the basin. In the western and central parts of the Dezful Embayment it began in the Late Rupelian or even Rupelian/Chattian. The eastern and south-eastern parts of the Dezful Embayment displayed a platform margin setting during the Rupelian while the north-eastern and central areas showed a basin setting. During the Early Chattian, clastic sediments originating from the Arabian Plate were deposited in the west, southwest, northwest and centre of the basin in response to a sea-level lowstand. The two very thin sandstone units occurring in the Late Aquitanian succession across the west of the Bibi Hakimeh oilfield are the only sandstone layers related to a sea-level lowstand throughout E/SE of the Dezful Embayment. During the Chattian the sediments in the eastern parts of the Dezful Embayment were deposited in a proximal inner ramp setting, and corals and coralline red algae (mainly Lithophyllum sp. and Lithothamnion sp.) dominated the carbonate platform in the east, while imperforate benthic foraminifera (e.g. Peneroplis sp., Archaias sp.) and rare perforate benthic foraminifera (e.g. Neorotalia sp., and Amphistegina sp.) were the most frequent sediment contributors in the southeast of the basin (Bibi Hakimeh area). At the same time, sedimentation occurred on a distal inner to mid-ramp settings through the central areas of the Dezful Embayment. A eustatic fall in sea-level and tectonic processes resulted in the isolation of the basin. During the Aquitanian two exposure horizons formed in the western and central Dezful Embayment basin while the eastern part only displays one. During the Burdigalian, sedimentation in the entire Dezful Embayment occurred within an innermost, very shallow ramp setting. The current study shows that sea-level fluctuations and sometimes combined with regional tectonic processes controlled the variability in platform geometry and paleofacies distribution of the Oligo-Miocene sediments across the basin.

Quaternary evolution of the southeastern Korean continental shelf, East Sea: Paleo-incised valley and channel systems

About 1000 km of high-resolution seismic reflection profiles were used to study the morphology, distribution, nature, and sequence stratigraphy of incised valleys and valley fills within the Quaternary (<ca. 2.0 Ma) sedimentary section of the southeastern Korean continental shelf, East Sea. Seven sequences, S1 to S7 from bottom to top, were analyzed on the modern continental shelf. These sequences comprise a ~300 m thick sedimentary section containing, at minimum, one cycle of sea-level lowstand, transgression, and high stand deposits. The lowstand sections are composed mainly of laterally and vertically accreting fluvial channel deposits (i.e., point bars, channel lags) formed on an alluvial plain during a lowstand in sea level. The transgressive sections consist of confined, low-to moderate-amplitude continuous reflections interpreted as estuarine facies. The high stand sections, in contrast, are comprised of laterally extensive low amplitude reflections interpreted as open marine facies. Each of these sequences rests unconformably on its’ predecessor, particularly at the mid-southeastern edge of the shelf where angular, toplap, and onlapping relationships can be discerned.The sequence boundaries (SB1 to SB7) bounding these sequences are produced by fluvial incision (i.e., valleys and channels) during sea-level fall. SB4, SB5, and SB7 form the most prominent unconformities on the shelf and are incised by extensive paleo-incised valleys and channels. These valleys and distributary channels display simple U- or V-shaped cross sections or complex morphologies; they are ~1–8 km wide and ~10–60 m deep. Incised-valley fills reach a maximum thickness of over 80 m. The largest incised valley fills (~8 km wide, ~60 m thick) are found near the shelf break in the SE, indicating relative sea-level fall on this location (ca. 150 m lower than the present sea level). In the SE, the incised valleys are comprised dominantly of stacked lowstand successions (point bars and channel lags) and a lesser degree of transgressive successions (estuarine mud and levees) that are capped by subsequent high stand deposits (marine mud). The youngest channel above the valley was inferred to have been formed during the Last Glacial Maximum (LGM) on the shelf edge trajectory about 140 m lower than the present. These large incised valleys are thought to have developed through a complex history of repeated avulsion and reoccupation during the lowstand sea level. The most important factors regulating the valley morphology are the duration of sea-level fall, lithologic variations, climate (i.e., fluvial discharge), and the river gradient relative to the shelf gradient. The ancient drainage patterns are more-or-less northeast-southwest orientated, consistent with the trend of the present-day submerged fluvial channel. Superimposed upon this general pattern are the effects of tectonic activity (uplift, folds, and faults), which appear to have modified the courses of the rivers. Our results, therefore, provide a good example of how external factors (i.e., climate, tectonics, sediment supply, and paleogeomorphology) can determine the location and geometry of the valleys and the architecture of the valley-filling deposits of continental shelves.

A ~130 ka terrestrial-marine interaction sedimentary history of the northern Jiangsu coastal plain in China

Coastal deposits are ideal archives to decipher the history of terrestrial-marine interactions in response to sea-level changes. Over the last decades, numerous investigations have been implemented to uncover the stratigraphic framework and the paleoenvironment evolution of the Jiangsu coastal plain (JCP), China. However, there have been limited studies on the sedimentary history over the glacial-interglacial timescales because of the lack of reliable chronostratigraphy. Here, we selected a 30-m-long sediment core in the northern JCP for luminescence dating, and carried out comprehensive sedimentological and paleontological analyses to ascertain the sedimentary history. Multiple luminescence dating techniques, including quartz optically-stimulated luminescence (OSL) and feldspar (polymineral) post-infrared (IR) IR stimulated luminescence (pIRIR) dating, were applied to determine the chronology of the sediment core. The obtained chronostratigraphy of core JCP01 spanning ~130 ka reveals that terrestrial deposition occurred in Marine Isotope Stage (MIS) 6, MIS 4–2 and the late stage of MIS 1. In contrast, marine influenced deposition is identified in MIS 5e and early MIS 1. The terrestrial interval of core JCP01 mainly consists of stiff clay and floodplain facies occurring at depths of 30–24 m, 12–5 m and 2–1 m, formed in relation to sea-level lowstands; whereas marine-influenced sediments at depths of 24–12 m and 5–2 m correspond to sea-level highstands, respectively. Our results highlight two sets of marine influenced sedimentary stratigraphy (T1, T2) dated to ~7 ka and around 130–120 ka, but no obvious MIS 3 transgression recorded in this core. In general, the conclusion is consistent with previous studies in East Asia and beyond, but not with previous work in this region that relied on radiocarbon dating. This suggests that re-dating with luminescence techniques is required for deposits correlated with the MIS 3 transgression in the northern JCP. Our reconstructed terrestrial-marine alternation sedimentary history of the northern JCP also shows a strong linkage with changes in the relative sea-level over the glacial-interglacial cycles.

The Breede River estuary (Cape Province, South Africa): A historical perspective on hydrology, geomorphology, and sedimentology

A hitherto unpublished historical dataset of the Breede River estuary, Western Cape, South Africa, that was collected in the summer season of 1983 is presented. Bathymetric, physiographic, and sedimentological data were collected at 1–1.5-km intervals between the mouth and the Malgas ferry crossing at km 35. The remaining estuary up to km 52 was not surveyed. Sedimentologically, the estuary could be divided into a lower marine sand reach (mouth to km 5), a mixed sand/mud reach (km 5–18.5), and a fluvial sand reach upstream of km 18.5. Hydrological data were collected at three midstream anchor stations (mouth, km 14.5, and km 24) which were occupied for complete tidal cycles. Five parameters were recorded: tidal elevation, current velocity, salinity, temperature, and light transmittance. Pronounced velocity asymmetries of the tides were revealed by phase delays between the times of low (high) water and corresponding slack water (turn of the tide), as well as considerable up-estuary delays in the occurrence of high and low tides. The mode of tidal wave propagation was synchronic (constant height) up to a distance of ~23 km at spring high tide and ~13 km at neap high tide, from where it proceeded in hyposynchronic mode (progressive decrease in height). Peak surface velocities reached 1.5 m/s at Station 1 (mouth), 0.6 m/s at Station 2 (km 14), and 0.45 m/s at Station 3 (km 24). The marine sand reach and parts of the mixed sand/mud and fluvial sand reaches were distinctly flood dominated as revealed by the orientation of bedforms. Salt water intrusion reached up to km 25, where river background levels were reached. Suspended sediment concentrations (turbidity) varied from 55–85 mg/l at the mouth, 65–200 mg/l in the mixed sand/mud reach, and 55–85 mg/l in the fluvial sand reach. At the time of observation, the Breede River estuary was in a well-mixed hydrological state. The fluvial sand reach displayed numerous, up to 18-m-deep scour pools. Grain-size distributions revealed distinct differences between the individual estuary sections, the sand/mud, and marine sand reaches, in particular, being characterized by up to four, mostly mixed hydraulic populations. Offshore seismic profiles suggest that the Duiwenhoks River, located 14 km to the east of the Breede mouth, was a tributary of the latter during Pleistocene sea-level lowstands. From a global perspective, the Breede River estuary conforms physically to typical small estuaries that are only marginally impacted by human interferences.

Geologic controls on paleodrainage incision and morphology during sea level lowstands on the Cascadia shelf in Oregon, USA

The lowstand extensions of two fluvial systems from the Cascadia subduction margin are investigated using high-resolution seismic reflection data to determine the relative controls on paleodrainage morphology. We document distinct differences between the two systems, located ~40 km apart, attributed to underlying structure and lithology, shelf gradient, and drainage basin characteristics. A cross-shelf valley exists for only one of the drainage systems, the Umpqua River. The outer shelf part of the valley is a syncline, which formed a natural structural low for lowstand drainage, while the inner shelf part of the valley exhibits incision and is associated with a mid-shelf convex change is slope. The other drainage system, the Siuslaw River, did not develop an across-shelf valley and is only denoted in the seismic data by an erosional subaerial unconformity. Structural trends are perpendicular to cross-shelf drainage, supporting a more anastomosing stream morphology for the paleo-Siuslaw. The shelf edge for this system was not exposed for a long period of time during the last lowstand, preventing substantial outer shelf incision. Differences in fluvial elevation, discharge, and hinterland geology, also played a role in the cross-shelf morphologies of the paleo-Umpqua and paleo-Siuslaw. These results highlight how controls on lowstand paleodrainage morphology can change over short distances on an active margin.

Offshore Freshened Groundwater in Continental Margins

AbstractFirst reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 106 km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography‐driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three‐dimensional hydrological models. We show that such advances, combined with site‐specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub‐seafloor geomicrobiology.

The role of sea-level and climate changes in the assembly of Sri Lankan biodiversity: A perspective from the Miocene Jaffna Limestone

As part of the Western Ghats-Sri Lanka biodiversity hotspot, Sri Lanka harbors a rich and diverse fauna with a considerable number of endemic species, especially in the wet southwestern part of the island. Understanding the origin of this biodiversity requires consideration of the land bridge history between the island and southern Peninsular India. To examine the influence of fluctuating sea levels and climate on island connectivity and isolation during the Miocene, we describe the lithological succession of the Jaffna Limestone in surface outcrops of north(west)ern Sri Lanka and interpret it with respect to palaeoenvironments and sequence stratigraphy. A stack of three marine intervals was identified, which were deposited in seagrass-, coral reef-, and nearshore siliciclastic environments of a land-attached carbonate platform and are separated by caliche and palaeokarst unconformities. They represent high sea-level peaks during the long-term sea-level maximum related to the Middle Miocene Climate Optimum (MMCO). In support of divergence time from Sri Lankan and south Indian species, it is argued that the Palk Isthmus connected Sri Lanka to mainland India prior the MMCO (~17–15 Ma) and after the Middle Miocene Climate Transition (~15–13 Ma), whereas a marine barrier continued to persist during low sea-level stands associated with the Mi2, Mi3a and Mi3 glaciations within this period of time. The early and middle Miocene landbridge connections opened up dispersal opportunities for terrestrial animals. Species migrations via the Palk Isthmus were, however, impacted by a climatic filter, which promoted a high level of ancient endemism on Sri Lanka. Our results indicate that the Sri Lankan sub-center of biodiversity started to evolve in response to increased sea-level and climate variability associated with Antarctic ice dynamics and the initiation of the South Asian monsoon system during the early stage of the current ice house period.