Relative Sea-level Rise Research Articles

Mid- to late Holocene sea-level rise recorded in Hells Bells 234U/238U ratio and geochemical composition

Hells Bells are underwater secondary carbonates discovered in sinkholes (cenotes) southeast of Cancun on the north-eastern Yucatán peninsula, Mexico. These authigenic calcite precipitates, reaching a length of up to 4 m, most likely grow in the pelagic redoxcline. Here we report on detailed 230Th/U-dating and in-depth geochemical and stable isotope analyses of specimens from cenotes El Zapote, Maravilla and Tortugas. Hells Bells developed since at least ~ 8000 years ago, with active growth until present day. Initial (234U/238U) activity ratios (δ234U0) in Hells Bells calcite decreas from 55 to 15‰ as sea level converges toward its present state. The temporal evolution of the geochemistry and isotope composition of Hells Bells calcites thus appears to be closely linked to sea-level rise and reflects changing hydrological conditions (desalinization) of the aquifer. We suggest that decelerated leaching of excess 234U from the previously unsaturated bedrock traces Holocene relative sea-level rise. Considering this proxy, the resulting mean sea-level reconstruction contains half as much scatter, i.e. improves by a factor of two, when compared to previously published work for the period between 8 and 4 ky BP.

Assessing land elevation in the Ayeyarwady Delta (Myanmar) and its relevance for studying sea level rise and delta flooding

Abstract. With their low lying, flat topography, river deltas and coastal plains are extremely prone to relative sea level rise and other water-related hazards. This calls for accurate elevation data for flood risk assessments, especially in the densely populated Southeast Asian deltas. However, in data-poor countries such as Myanmar, where high accuracy elevation data are not accessible, often only global satellite-based digital elevation models (DEMs), suffering from low vertical accuracy and remote sensing artefacts, can be used by the public and scientific community. As the lack of accurate elevation data hampers the assessment of flood risk, studying available information on land elevation and its reliability is essential, particularly in the context of sea level rise impact. Here, we assess the performance of 10 global DEMs in the Ayeyarwady Delta (Myanmar) against the new, local, so-called AD-DEM, which was generated based on topographical map elevation data. To enable comparison, all DEMs were converted to a common vertical datum tied to local sea level. While both CoastalDEM v2.1 (Kulp and Strauss, 2021) and FABDEM (Hawker et al., 2022) perform comparably well, showing the highest correspondence in comparison with AD-DEM and low-elevation spot heights, FABDEM outperforms CoastalDEM v2.1 by the absence of remote sensing artefacts. The AD-DEM provides a high-accuracy, open and freely available, and independent elevation dataset suitable for evaluating land elevation data in the Ayeyarwady Delta and studying topography and flood risk at large scale, while small-scale investigations may benefit from a FABDEM locally improved with data from the AD-DEM. Based on the latest Intergovernmental Panel on Climate Change (IPCC) projections of sea level rise, the consequences of DEM selection for assessing the impact of sea level rise in the Ayeyarwady Delta are shown. We highlight the need for addressing particularly low-lying populated areas within the most seaward districts with risk mitigation and adaptation strategies while also the more inland delta population should be made aware of facing a higher risk of flooding due to relative sea level rise in the next ∼ 100 years.

Long‐Term Monitoring of Coupled Vegetation and Elevation Changes in Response to Sea Level Rise in a Microtidal Salt Marsh

AbstractTight interplays between physical and biotic processes in tidal salt marshes lead to self‐organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long‐term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation‐vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea‐level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7–4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site‐specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species‐specifically to changes in environmental forcings by modifying species‐preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation‐species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution.

The Miocene record of the Betic Cordillera uplift in Los Guájares valley (Granada province, S Spain)

The Los Guájares valley is located about 35 km south of Granada in the Internal Zones of the Betic Cordillera. The Miocene deposits in this area consist of six sedimentary units separated by unconformities, which are exposed in small and laterally discontinuous outcrops overlying metamorphic rocks of the Alpujárride Complex. The lowest unit comprises upper Serravallian marls with planktonic foraminifera filling neptunian dykes and covering Triassic dolomitic marbles of the Alpujárride Complex. These marls reflect deep marine deposition on the Betic basement under a local extensional regime. Uplift of the region led to emergence and deposition of continental red alluvial-fan conglomerates and foothill breccias. The overlying unit, a shoreline conglomerate with small oyster banks, indicates relative sea-level rise. Shallow-marine conditions continued during the deposition of the following unit, early Tortonian in age, which consists of calcareous sandstones with hermatypic corals. Small patch reefs developed in the overlying unit composed of sandy limestones with corals (Porites, Tarbellastraea, Thegioastraea) and oysters, and sandy limestones with coralline algae in the most distal areas. The coral reefs in Los Guájares and some coral heads in Albuñuelas, a near locality, are the only record of early Tortonian coral buildups in the Mediterranean. The last marine unit comprises lower Tortonian conglomerates, sandstones and siltstones accumulated in the front of a small delta. Oyster banks and concentrations of Turritelines suggest high nutrient levels in the delta-front paleoenvironments. The Los Guájares valley area emerged afterwards, following the onset of a compressional geodynamic regime in the central Betic Cordillera and since the early Tortonian only small bodies of foothill and alluvial-fan deposits formed in a predominantly erosional context.

Holocene relative sea-level histories of far-field islands in the mid-Pacific

Holocene relative sea-level (RSL) data from far-field islands in the mid-Pacific have been used to validate the ice-melting histories of glacial isostatic adjustment (GIA) models. However, a lack of quality control in the reconstruction of RSL hinders the understanding of regional variability that can constrain ice-equivalent sea-level changes. Here, we present a standardised database of Holocene RSL data from five regions in the mid-Pacific (Cook Islands, Tuamotu Islands, Christmas (Kiritimati) Island, Gilbert Islands and Fiji). We categorised the data as high or low quality based on the susceptibility of samples to age and/or elevation errors. Of the 614 data points that were reviewed, 25% were rejected and 100 sea-level index points (SLIPs) were reinterpreted as limiting data. The new database consists of 141 SLIPs and 262 marine and 56 terrestrial limiting data points reconstructed from a variety of sea-level indicators (e.g., coral microatolls, mangrove peat, beachrock, and beach ridges), of which 71% provide high-quality constraints on RSL. The early to mid Holocene RSL evolution in the Cook Islands, Gilbert Islands and Fiji are poorly constrained due to a lack of high-quality SLIPs and limiting data during this period. The Tuamotu Islands provided the only record of early to mid Holocene evolution of RSL, indicating rapid RSL rise from between −22.9 m and −15.2 m at ∼9.0 ka to between −0.2 m and 0.5 m by ∼6.5 ka, at rates as high as 9.8 ± 5.1 mm/a, with a slowdown in the rate of RSL rise sometime between ∼8.2 ka and ∼6 ka. The Christmas (Kiritimati) Island record indicates stable RSL within ∼1.5 m of present-day levels over the past ∼6.6 ka. In the late Holocene, the Cook Islands record suggests a gradual fall in RSL over the past ∼2.9 ka at rates below 0.1 ± 4.3 mm/a. SLIPs at Fiji also indicate a slight fall in RSL at rates of less than 0.5 ± ∼4.4 mm/a at ∼4 ka, following which RSL fell from above 0.9 m at ∼3 ka to between −0.3 m and 0.6 m by ∼2.5– ∼2.1 ka. We highlight the importance of standardisation and quality control to critically evaluate the processes controlling RSL and validate GIA models. Indeed, the new standardised database has implications for the timing of the mid- Holocene highstand, which has been used to support the ICE-4G and ICE-7G_NA models. Due to the poor constraints of data in the mid-Pacific islands, particularly in the early Holocene, there remains no unique solution for a global ice-melting history.

Coastal Depositional Responses to Relative Sea-Level Rise: Insights from a Superimposed Sandstone–Shale–Coal Reservoir in the Linxing Gas Field, China

The Ximing Sandstone-to-No. 9 Coal succession of the Taiyuan Formation in the Linxing gas field records a complex internal architecture of a transgressive succession developed in the western coast of the late Pennsylvanian North China epeiric sea. Facies and sequence stratigraphic analyses reveal its depositional evolution from fluvial channels through fluvial-dominated and tide-influenced inner estuaries to tide-dominated estuaries and finally to wave-dominated barrier lagoons. The evolution from fluvial- to tide-dominated deposition has been ascribed to the funnel-shaped valley coupled with an increased tidal prism induced by the upstepping and backstepping shoreline. The evolution from tide- to wave-dominated deposition has been ascribed to the wide North China epeiric seaway lacking local coastline irregularities after the incised-valley fill that provided sufficient fetch for the occurrence of large storm waves. Grain-size analysis reveals the relative importance of traction, saltation, dispersed suspension, and flocculated suspension in the development of the transgressive estuarine to lagoonal deposits. This study not only contributes to a proper understanding of coastal depositional response to the relative sea-level rise but also provides a context within which to interpret the symbiotic relationship of the superimposed sandstone–shale–coal reservoirs and predict the distribution of favorable unconventional gas production formation.

Different hydrological controls causing variable rates of Holocene peat growth in a lowland valley system, north-eastern Netherlands; implications for valley peatland restoration

The Drentsche Aa valley system in the glacial sandy landscape of the north-eastern Netherlands, is one of the many regional drainage systems in the north-western to central European Lowlands. Following deep incision in the Weichselian, ca 7 m of eutrophic peat has accumulated in the lower to middle reaches of the valley system in the Holocene, completely filling the Weichselian incision. We reconstructed the rate of water-level rise controlling peat growth at three locations in the valley system (representing the upper, middle and lower reaches), using 14C peat dates from compaction-free sampling sites on the sloping valley margin where the peat directly overlies the sandy Pleistocene subsurface. The 14C dates enabled the construction of curves showing the water-level evolution at each location. Our results show variable rates of Holocene peat growth in the valley system in response to different drivers of hydrological change: a strong increase in annual precipitation (onset Holocene), a decrease in evapotranspiration (10,000–8000 cal. years BP) and relative sea-level rise (after ~4500 cal. years BP). Because the identified hydrological controls on peat growth are regional, they probably ruled Holocene peat growth in many peat-filled valley systems in the north-western to central European Lowlands. Currently, there is growing attention for these valley systems in the context of European nature restoration efforts and climate change adaptation and mitigation measures. The present study underscores that large-scale hydrological measures outside the valley system are needed for restoration of degraded valley peatlands.

Forecasting Sea Level Rise-driven Inundation in Diked and Tidally Restricted Coastal Lowlands

Diked and drained coastal lowlands rely on hydraulic and protective infrastructure that may not function as designed in areas with relative sea-level rise. The slow and incremental loss of the hydraulic conditions required for a well-drained system make it difficult to identify if and when the flow structures no longer discharge enough water, especially in tidal settings where two-way flows occur through the dike. We developed and applied a hydraulic mass-balance model to quantify how water levels in the diked and tidally restricted coastal wetlands and water bodies dynamically respond to sea-level rise, specifically applied to the Herring River Estuary in MA, USA, from 2020 to 2100. Sensitivity testing of the model parameters indicated that primary outcomes were not sensitive to many of the chosen input values, though the terrestrial water input rate to the estuary and the flow coefficient for the hydraulic infrastructure were important. The relative importance of parameters, however, is expected to be site specific. We introduced a drainability metric that quantifies the net water volume drained over every tidal cycle to monitor and forecast how rising water levels on either side of the dike affected the net draining or impounding conditions of the system. Ensembles of model results across parameter and sea-level scenario uncertainties indicated that substantial impoundment of the Herring River Estuary was expected within ~ 20 years with the existing flow structures, a sluice and two flap gates. Simulations with up to three additional gates did not dampen this trend toward impoundment, suggesting that rising impounded water levels are likely even with major construction upgrades. Increasingly impounded diked coastal waterbodies present a hydrologic challenge with socioecological implications due to projected flooding and ecosystem impacts. Solutions to this challenge may be to allow coastal wetland restoration pathways or require substantial and recurring infrastructure improvement projects.

Fossil Java Sea corals record Laurentide ice sheet disappearance

Abstract The Laurentide ice sheet was the largest late Pleistocene ice mass and the largest contributor to Holocene pre-industrial sea-level rise. While glaciological dates suggest final ice sheet melting between 8 and 6 ka, inversion of sea-level data indicates deglaciation at ca. 7 ka. Here, we present new chronostratigraphic constraints on Laurentide ice sheet disappearance based on Holocene relative sea-level observations from the tectonically stable north coast of Java, Indonesia. Age-elevation data from the flat upper surfaces of 13 fossil intertidal corals (i.e., microatolls) indicate that the Java Sea experienced a relative sea level of 1.3 ± 0.7 m above present between 6.9 and 5.3 ka. To determine uncaptured relative sea-level trends within the observational uncertainties of this apparently constant highstand, we analyzed the internal structure of three sliced microatolls from the same site to produce a high-resolution data set. These data were used to statistically model relative sea-level rates and trends. Employing the data with the model provided evidence for a short-lived rise of relative sea level from 1.0 ± 0.3 m above present at 6.7 ± 0.1 ka to 1.9 ± 0.3 m above present at 6.4 ± 0.1 ka. The end of this rise likely represents the last input of meltwater from the vast Laurentide ice sheet, which, consequently, collapsed at least 400 yr later than assumed by some widely used models of glacial isostatic adjustment. Incorporating these new results into such predictive models will help to better understand the geographical variability of future sea-level rise as a result of global warming.

Holocene relative sea-level changes in northwest Ireland: An empirical test for glacial isostatic adjustment models

The late-Quaternary relative sea-level (RSL) history of Ireland is complex, positioned at the margins of the former British-Irish Ice Sheet, and subject to the influence of ice unloading and forebulge collapse. Geophysical models of post-glacial isostatic adjustment (GIA) provide estimates of the pattern of RSL change since deglaciation which may be tested and validated with empirical data from proxy records. For the region of northwest Ireland, there is a paucity of high-quality RSL data and, therefore, equivocal evidence to support the GIA models that predict a mid to Late-Holocene RSL highstand of between +0.5 and +2 m above present. This study aims to investigate this model-data discrepancy by reconstructing RSL change from a near continuous salt-marsh sequence at Bracky Bridge, Donegal, spanning the last ca. 2500 years. We develop a transfer function model to reconstruct the vertical position of sea level using a regional diatom training set to quantify the indicative meaning and predict the palaeomarsh elevation of the core samples. A chronology is provided by a combination of 14C and 210Pb data, with sample specific ages derived from an age-depth model using a Bayesian framework. Our reconstruction shows ca. 2 m of relative sea-level rise in the past 2500 years. This is not compatible with some previously published sea-level index points from the region, which we re-interpret as freshwater/terrestrial limiting data. These results do not provide any evidence to support a Mid-Holocene RSL highstand above present sea level. Whilst none of the available GIA models replicate the timing and magnitude of the Late-Holocene RSL rise in our reconstruction, those which incorporate a thick and extensive British-Irish Sea Ice Sheet provide the best fit.

Sea-Level Rise Effects on Changing Hazard Exposure to Far-Field Tsunamis in a Volcanic Pacific Island

Coastal flooding exacerbated by climate change is recognised as a major global threat which is expected to impact more than a quarter of all people currently residing in Pacific Island countries. While most research in the last decade has focused on understanding the dynamics and impacts of future coastal flooding from extreme sea levels, the effects of relative sea level rise (RSLR) on exacerbating tsunami hazards are not well understood. Far-field or distant sourced tsunamis tend to have relatively lower impacts in Pacific Island states compared with locally sourced events, but there is limited understanding of how the impact of far-field tsunamis changes over time due to RSLR. Using the hydrodynamics software BG-Flood, we modelled the Tōhoku-oki tsunami from propagation to inundation in Samoa under incremental SLR to examine the effects that RSLR has on changing the exposure of the built environment (e.g., buildings) to a far-field tsunami. Outputs of maximum tsunami inundation and flow depth intensities which incorporate incremental SLR were then combined with digital representations of buildings and depth-damage functions in the RiskScape multi-hazard risk modelling software to assess the changes in building exposure over time. Results suggest that the impacts of Tōhoku-oki-type far-field tsunamis become significant once RSLR reaches 1 m above present levels. Present-day building exposure will increase by approx. 500% with 1 m RSLR by 2080–2130, and approx. 2350% with 2 m RSLR by as early as 2130–2140. These findings provide useful insights for application to tsunami hazard risk assessments under changing sea level conditions in analogous island environments.

Sediment delivery to sustain the Ganges-Brahmaputra delta under climate change and anthropogenic impacts

The principal nature-based solution for offsetting relative sea-level rise in the Ganges-Brahmaputra delta is the unabated delivery, dispersal, and deposition of the rivers’ ~1 billion-tonne annual sediment load. Recent hydrological transport modeling suggests that strengthening monsoon precipitation in the 21st century could increase this sediment delivery 34-60%; yet other studies demonstrate that sediment could decline 15-80% if planned dams and river diversions are fully implemented. We validate these modeled ranges by developing a comprehensive field-based sediment budget that quantifies the supply of Ganges-Brahmaputra river sediment under varying Holocene climate conditions. Our data reveal natural responses in sediment supply comparable to previously modeled results and suggest that increased sediment delivery may be capable of offsetting accelerated sea-level rise. This prospect for a naturally sustained Ganges-Brahmaputra delta presents possibilities beyond the dystopian future often posed for this system, but the implementation of currently proposed dams and diversions would preclude such opportunities.

Assessment the Impacts of Sea-Level Changes on Mangroves of Ceará-Mirim Estuary, Northeastern Brazil, during the Holocene and Anthropocene.

Predictions of the effects of modern Relative Sea-Level (RSL) rise on mangroves should be based on decadal-millennial mangrove dynamics and the particularities of each depositional environment under past RSL changes. This work identified inland and seaward mangrove migrations along the Ceará-Mirim estuary (Rio Grande do Norte, northeastern Brazil) during the mid-late Holocene and Anthropocene based on sedimentary features, palynological, and geochemical (δ13C, δ15N, C/N) data integrated with spatial-temporal analysis based on satellite images. The data indicated three phases for the mangrove development: (1°) mangrove expansion on tidal flats with estuarine organic matter between >4420 and ~2870 cal yrs BP, under the influence of the mid-Holocene sea-level highstand; (2°) mangrove contraction with an increased contribution of C3 terrestrial plants between ~2870 and ~84 cal yrs BP due to an RSL fall, and (3°) mangrove expansion onto the highest tidal flats since ~84 cal yr BP due to a relative sea-level rise. However, significant mangrove areas were converted to fish farming before 1984 CE. Spatial-temporal analysis also indicated a mangrove expansion since 1984 CE due to mangrove recolonization of shrimp farming areas previously deforested for pisciculture. This work mainly evidenced a trend of mangrove expansion due to RSL rise preceding the effects of anthropogenic emissions of CO2 in the atmosphere and the resilience of these forests in the face of anthropogenic interventions.

Riverbank erosion and char stability along the fluvial-to-tidal transition zone in the Lower Meghna River and Tentulia Channel in the Ganges-Brahmaputra-Meghna Delta, Bangladesh

The Ganges-Brahmaputra-Meghna (GBM) delta has been named one of the world's most vulnerable sinking deltas. It is also one of the most dynamic deltas, with the world's highest sediment discharge and third highest water discharge, with semi-diurnal mesoscale (2–4 m) tides. The high fluvial discharge and significant tidal velocities (1–2 m/s) create an expansive fluvial-to-tidal transition zone (FTTZ) along the entire Lower Meghna River (LMR). Within this FTTZ, tidally elongated channel bars, locally called ‘chars’, rapidly evolve due to the high sediment and water discharges. Chars along the LMR have a total population exceeding 5 million people; thus, evaluating the interactions of fluvial and tidal processes along the FTTZ is crucial to understanding vulnerability among char communities with respect to flooding and sea-level rise. Here, we utilize a multi-faceted approach to assess the geomorphology, sedimentology, and hydrology of three chars longitudinally spaced within the LMR and one of its distributaries – the Tentulia Channel. Decadal land change analyses revealed that the FTTZ along the upper LMR and Tentulia Channel has been gaining land at a rate of 4.7 km2/yr, with erosion being most prevalent upstream and most accretion occurring within 50 km of the coast. Almost all the stratigraphic profiles show fine and/or medium sands at depth (0.5–2 m), implying that the three chars are similarly susceptible to erosion by tidal currents and high river discharge. Thus, we attribute the increased stability of chars in the downstream direction to increased tidal influence rather than sedimentology. Land elevation and water level surveys reveal that elevation relative to sea level decreases as the distance to the mouth of the LMR decreases: from 4.05 m (130 km from the mouth), to 2.56 m (100 km from the mouth), and 1.85 m (70 km from the mouth). By incorporating local water level data with the elevation surveys, we conclude that fluvial, namely monsoonal, processes control the geomorphology of the chars near the confluence of the LMR and the head of Tentulia Channel. The shift between fluvial-dominated and tidal-dominated regions of the FTTZ occurs just downstream of Char 3 (∼70 km from the LMR mouth). Therefore, char communities near the LMR confluence are most vulnerable to riverbank erosion and flooding associated with the monsoon season, and chars proximal to the coast are most vulnerable to erosion and flooding related to storm surges and relative sea-level rise.

Distribution and morphometry of pingos, western Canadian Arctic, Northwest Territories, Canada

Pingos of the western Canadian Arctic (WCA) coastal plain are among the most abundant and well-studied in the northern hemisphere. However, the abundance and morphometric variation of these pingos has not been quantitatively examined with respect to surficial and physiographic settings, likely because they have never been systematically catalogued with precision. We assess the distribution and morphology of pingos in the WCA using the High Resolution Digital Elevation Model (HRDEM) and demonstrate how this population compares to others in the Arctic, and how it varies in relation to surficial and physiographic settings. Of the 2363 pingos identified in the WCA (900 more than previous estimates), relief ranges from a few 10s of centimetres to 46.7 m for Ibyuk Pingo, which is the archetype of closed-system (hydrostatic) pingos in the region. Nearly 18 % (272) are taller than 10 m, although about 35 % of the pingos (830) are <2 m tall, which is the minimum height limit of most other studies. Using this minimum cut-off to compare populations, the mean height of WCA pingos >2 m (n = 1533) is 6.5 m, which is greater than other lowland pingos in the Arctic. WCA pingos are about 10 % larger in radius than pingos in North Alaska but have comparable mean slopes. Nevertheless, about 72 % of WCA pingos >2 m tall are “high-slope” pingos with a mean slope of 5° or greater. Within the WCA, 95 % of all pingos occur within the Tuktoyaktuk Lowlands in relation to the distribution of underlying Pleistocene sands, specifically the extent of braidplain deposits from the paleo-Peel and Anderson rivers known as the Kidluit Formation (Marine Isotope Stage 3). However, the spatial density of pingos within the Tuktoyaktuk Lowlands varies with physiography, and the highest densities relate to ice-rich, topographically-complex terrains that have a history of glaciation and thermokarst. In contrast, pingos within the modern Mackenzie Delta, where the low-lying, Holocene-age terrain is not amenable to lake drainage that promotes closed-system pingo formation, have a low density (5 % of total population) and are smaller. Nearly 30 % of WCA pingos reside at <5 m asl. Consequently, ongoing relative sea level rise and the effects on coastal change are likely to erode some pingos within the current population but may also trigger lake drainage and formation of new pingos.

Influence of Relative Sea-Level Rise, Meteoric Water Infiltration and Rock Weathering on Giant Volcanic Landslides

Recent studies have shown that giant landslides correlate with climatic variations. However, the precise processes involved in this phenomenon need to be better defined. This study investigates the causes of giant landslides using a modeling approach. Here, I show that the effect of meteoric water infiltration could be distinguished from that of the sea level rise in triggering paleo-landslides. It is possible to identify the cause of coastal paleo-landslides based on the age of occurrence and comparison with climatic signals when glacial maxima are wetter than during interglacial periods, as in Polynesia and East Equatorial Africa, but not in other cases (Caribbean, Indonesia). The role of pore-pressure variations and sea water loading variations is discussed. The interaction between the relative sea level rise, pre-existing relief and deep weak structure due to the presence of highly weathered lavas may trigger the conditions for a large landslide. Highly weathered lavas have very low friction angles in volcanic islands. When volcanoes are still active, pressure fluctuations in the magma chamber caused by sea level lowering are expected to play a significant role in the destabilization of the relief. Competing processes in real cases make it difficult to distinguish between these processes.

Holocene environmental evolution and relative sea-level change in the Oka estuary (Urdaibai Biosphere Reserve, northern Spain)

The Holocene environmental evolution in coastal areas, including deltas and estuaries, is vital to understanding coastal dynamics and how they may change in the future. Here, we studied the Holocene environmental evolution of the Oka estuary (Urdaibai Biosphere Reserve, northern Spain) from sedimentary cores that were 11–49 m long. We applied a multi-proxy approach of benthic foraminifera and grain size, and a temporal framework of radiocarbon dates to reconstruct paleoenvironments. The benthic foraminifera assemblages varied from near-marine (allochthonous and autochthonous hyaline species such as Lobatula lobatula, Rosalina irregularis, Ammonia tepida and Haynesina germanica) to brackish intertidal (autochthonous hyaline species such as A. tepida and H. germanica) to salt-marsh (autochthonous agglutinated species Trochammina inflata and Entzia macrescens) environments. Grain size analysis supported the foraminiferal assemblages and the sedimentary sequences were sand-dominated in near-marine intertidal environments and mud-dominated in the brackish intertidal and salt-marsh environments. The chronology was constrained by thirty four radiocarbon dates from plant macrofossils, marine shells and wood fragments. Our paleoenvironmental reconstructions were used to produce an updated local Holocene relative sea-level (RSL) record based on new data, revised reconstructions, and age recalibration of previous research. Marine limiting dates showed that RSL changed from above −20.4 ± 0.2 m at ∼9100 cal. yrs. BP to above −4.2 ± 0.2 m at ∼6700 cal. yrs. BP. In the late Holocene, sea-level index points (SLIPs) reconstructed the RSL position from −2.7 ± 1.2 m at ∼4900 cal. yrs. BP to −1.4 ± 1.2 m at ∼1500 cal. yrs. BP. Estimates of rates of RSL change from the Errors-in-Variables Integrated Gaussian Process (EIV-IGP) model show RSL gradually rose at 0.3 ± 2.1 mm yr−1 in the late Holocene. There was a progressive shallowing of the estuary as rates of RSL rise reduced during the Holocene. Between ∼9000 and 7000-6000 cal. yrs. BP pre-Holocene fluvial gravels (Lowstand Systems Tract) were replaced by both sandy marine and brackish intertidal sediments (Transgressive Systems Tract). After ∼7000-6000 cal. yrs. BP salt-marsh sediments deposited (Highstand Systems Tract). The sequence was overlain by anthropogenic deposits derived from agricultural, dredging and dumping activities since the 18th century (Anthropogenic Systems Tract). The Holocene environmental evolution and relative sea-level change of Oka is broadly consistent with previous studies in northern Spain, although it places the onset of the Highstand Systems Tract ∼2000 years earlier.

Climate change mitigation potential of Louisiana's coastal area: Current estimates and future projections.

Coastal habitats can play an important role in climate change mitigation. As Louisiana implements its climate action plan and the restoration and risk reduction projects outlined in its 2017 Louisiana Coastal Master Plan, it is critical to consider potential greenhouse gas (GHG) fluxes in coastal habitats. This study estimated the potential climate mitigation role of existing, converted, and restored coastal habitats for years 2005, 2020, 2025, 2030, and 2050, which align with the Governor of Louisiana's GHG reduction targets. An analytical framework was developed that considered 1) available scientific data on net ecosystem carbon balance fluxes per habitat and 2) habitat areas projected from modeling efforts used for the 2017 Louisiana Coastal Master Plan to estimate the net GHG flux of coastal area. The coastal area was estimated as net GHG sinks of -38.4 ± 10.6 and - 43.2 ± 12.0 Tg CO2 equivalents (CO2 e) in 2005 and 2020, respectively. The coastal area was projected to remain a net GHG sink in 2025 and 2030, both with and without implementation of Coastal Master Plan projects (means ranged from -25.3 to -34.2 Tg CO2 e). By 2050, with model-projected wetland loss and conversion of coastal habitats to open water due to coastal erosion and relative sea level rise, Louisiana's coastal area was projected to become a net source of greenhouse gas emissions both with and without the Coastal Master Plan projects. However, at year 2050, Louisiana Coastal Master Plan project implementation was projected to avoid the release of +8.8 ± 1.3 Tg CO2 e compared to an alternative with no action. Reduction in current and future stressors to coastal habitats, including impacts from sea level rise, as well as the implementation of restoration projects could help to ensure coastal areas remain a natural climate solution.

Late Holocene Cliff Retreat in Del Mar, CA, Revealed From Shore Platform 10Be Concentrations and Numerical Modeling

AbstractRocky coast cliff retreat presents a hazard to coastal communities and infrastructure that is potentially amplified under rising sea level conditions, among other factors. Unfortunately, constraints on retreat rates are typically limited to those derived from imagery and maps spanning the last ∼100 years. Here, we use a newly developed coupled model of shore platform profile evolution and cosmogenic radionuclide production that considers the influence of relative sea level (RSL) rise, weathering, material resistance, and wave height decay to model cliff retreat over millennial timescales and its potential drivers in Del Mar, California. We demonstrate the ability to use topographic and bathymetric measurements from a narrow shore platform along with a limited data set of nine cosmogenic 10Be concentrations extending ∼125 m from the cliff base to obtain modeled cliff retreat rates that steadily range from 5.0 to 12.5 cm yr−1 over the last two millennia until 100 years before present. These rates are consistent with modern retreat rates of about 2–19 cm yr−1 here. RSL rise in Southern California remained relatively constant during the late Holocene, potentially explaining the relatively stable modeled cliff retreat rate over this time. We also explore the relative influence of weathering, material resistance, and wave erosion efficacy and find that both weathering and wave‐driven erosion are necessary to replicate the measured data at this location, with the latter exerting a stronger control on model acceptance, suggesting that waves may provide a possible mechanism by which RSL rise may influence coastal cliff erosion in southern California.

Water-level changes and subsidence rates along the Saigon-Dong Nai River Estuary and the East Sea coastline of the Mekong Delta

Southeast Asian deltas are highly threatened areas for flooding as a response to the combined effects of natural compaction and subsidence exacerbated by human impacts, e.g. oil, gas and water extraction, retention of sediment discharge due to river damming and sand mining, land use changes, sea-level rise and storm-induced water-level setup. Tide-induced water-level fluctuations on different time- and spatial scales, seasonal variations of freshwater runoff and sea-level setup can amplify the impact of sea-level rise and of storm surges on the coastal environment and its inhabitants. Moreover, increasing populations accompanied by growing societal demands can lead to further pressure on delta areas. For the Vietnamese Mekong Delta (VMD) and the Saigon-Dong Nai River (SGDNR), rates of subsidence of several cm/yr have already been determined, but for the coastal area, which is most vulnerable to sea-level rise and storm surges, a lack of detailed spatial and temporal information of this parameter exists. To assess the influence of tide- and water-level fluctuations, records from 11 stations, from the tectonically stable position Vung Tau north of the SGDNR entry to Ganh Hao in the southern VMD, were analyzed. The results reveal a relative sea-level rise at Vung Tau reaching 2.2 ± 0.3 mm/yr for the period from 1987-2015, while along the VMD those rates show a spatially variation from 5.6 ± 0.3 mm/yr at Ben Trai to 13.5 ± 0.7 mm/yr at My Thanh. Considering Vung Tau as tectonically stable, these numbers indicate subsidence rates of the coastal sections along the VMD ranging from 3.4 ± 0.3 mm/yr to 11.3 ± 0.7 mm/yr. It is likely that the rates of subsidence along the coast of VMD have accelerated since 2005. They are not only up to 4 times higher than the mean sea-level rise but are variable along the whole SGDNR estuary and the VMD East Sea coastline. Additionally, the importance of daily, fortnightly and seasonal water level fluctuations due to tides and atmospheric influence, which are acting on top of the long-term relative sea-level rise, is shown. Especially the identified regional variations make these data valuable for setting regional priorities for protection strategies to mitigate riverine and marine flooding, especially when both coincide.