Relative Sea Level Research Articles

Late Holocene Climate Warming Events and Their Linkage to Hydraulic Engineering on the Coast of Hangzhou Bay, East China

The coastal lowlands in East China are very sensitive to climate change and marine disasters, and much large-scale hydraulic engineering was recorded in the historical documents of the Late Holocene. In this study, AMS 14C and OSL were used to date three sedimentary profiles from the north and south coasts of inner Hangzhou Bay, and grain size and geochemical analyses including organic carbon, macro-elements, and alkaline earth metals were performed, while hydraulic engineering records in historical documents were compiled, in an attempt to reveal the sedimentary records of extreme climatic and hydrological events over the past 3000 years and to probe into the correlation between them and hydraulic engineering. The results show that the intensified chemical weathering during ca. 200 BCE to 900 CE in East China corresponded to the warm and humid climate during the Qin-Han and Sui-Tang dynasties. Salinity intrusion with rising local water levels occurred in the lowland plains along the south coast of Hangzhou Bay from 120 to 895 CE. Low-salinity water intrusion from 32 to 488 CE was also recorded in the stratigraphy of lowland plains along the north coast of Hangzhou Bay. The sedimentary records of the East Tiaoxi River basin show river floods about 2000 years ago. The above sedimentary records indicate that the relative sea level rose in the Hangzhou Bay area during the Qin-Han and Sui-Tang Warm Periods, resulting in frequent salinity intrusion and river floods, which coincided with the historical records of hydraulic engineering such as the construction of seawalls, river levees, and the enclosure of lakes for restoration of river floods during the Han and Tang dynasties. Such coincidence reflects that climate change profoundly affected the hydrological environment of the coastal areas in East China as well as the response of the human societies.

Geomorphic Response of the Georgia Bight Coastal Zone to Accelerating Sea Level Rise, Southeastern USA

Synthesis of geologic and chronologic data generated from Holocene sedimentary sequences recovered along the inner continental shelf, shoreface, and modern coastal zone of the Georgia Bight reveal a synchronous sequence of paleoenvironmental events that occurred in response to rate of sea level rise tipping points. During the early Holocene (11.7–8.2 cal kyr BP), the paleoshoreline was overstepped and submerged by rapidly rising seas that averaged ~5 mm yr−1. Rates of rise during the middle Holocene (8.2–4.2 cal kyr BP) averaged ~2 mm yr−1 and this deceleration resulted in the formation of coastal environments and sedimentary sequences that were subsequently reworked as the shoreface continued its landward and upward migration. The modern coastal zone emerged commensurate with the late Holocene (4.2–0 cal kyr BP), when the rate of sea level rise averaged <1 mm yr−1. Analysis of water level data collected at six NOAA tide gauge stations located along the Georgia Bight coast indicates the rate of relative sea level rise has increased from a historical average of 3.6 ± 0.2 mm yr−1 (<1972 to 2022) to 6.6 ± 0.8 (1993 to 2022) and during the 21st century it has averaged 9.8 ± 0.3 mm yr−1 (2003 to 2022). The current rate of sea level rise is nearly double the early Holocene rate of rise. Based upon a novel application of the principle of uniformitarianism (i.e., the past is the key to the future), the likely geomorphic trajectory of the Georgia Bight coastal zone under conditions of 21st century accelerating sea level rise will be one of increasing instability (e.g., coastal erosion) and flooding (e.g., overwash, breaching). Evidence of an emerging instability within the coastal zone has been previously reported throughout the region and supports the trajectory of geomorphic change proposed herein. This will ultimately result in the submergence of existing landscapes and replacement by estuarine and marine environments, which may hasten in pace and scale given the current rate of sea level rise is expected to continue accelerating throughout this century. These findings have not been previously reported and should be considered by coastal practitioners responsible for conceptualizing risk, as well as the formulation and implementation of adaptation action plans designed to mitigate threats to the built and natural environment induced by climate change.

Sea level rise and the drivers of daily water levels in the Sacramento-San Joaquin Delta

Water levels in deltas and estuaries vary on multiple timescales due to coastal, hydrologic, meteorologic, geologic, and anthropogenic factors. These diverse factors increase the uncertainty of, and may bias, relative sea level rise (RSLR) estimates. Here, we evaluate RSLR in San Francisco Bay and the Sacramento-San Joaquin Delta, USA by applying a physics-based, nonlinear regression to 50 tide gauges that determines the spatially varying controls on daily mean water level for water years 2004–2022. Results show that elevated river flow and pumping (99th percentile) raise water level up to 6 m and lower it up to 0.35 m, respectively, and coastal water level variations are attenuated by 30-60% within the Delta. Strong westerly winds raise water level up to 0.17 m, and tidal-fluvial interaction during spring tides and low discharge raises water level up to 0.15 m. Removal of these interfering factors greatly improves RSLR estimates, narrowing 95% confidence intervals by 89–99% and removing bias due to recent drought. Results show that RSLR is spatially heterogeneous, with rates ranging from − 2.8 to 12.9 mm y-1 (95% uncertainties < 1 mm y-1). RSLR also exceeds coastal SLR of 3.3 mm y-1 in San Francisco at 85% of stations. Thus, RSLR in the Delta is strongly influenced by local vertical land motion and will likely produce significantly different, location-dependent future flood risk trajectories.

Causes of accelerated High-Tide Flooding in the U.S. since 1950

The U.S. coastlines have experienced rapid increases in occurrences of High Tide Flooding (HTF) during recent decades. While it is generally accepted that relative mean sea level (RMSL) rise is the dominant cause for this, an attribution to individual components is still lacking. Here, we use local sea-level budgets to attribute past changes in HTF days to RMSL and its individual contributions. We find that while RMSL rise generally explains > 84% of long-term increases in HTF days locally, spatial patterns in HTF changes also depend on differences in flooding thresholds and water level characteristics. Vertical land motion dominates long-term increases in HTF, particularly in the northeast, while sterodynamic sea level (SDSL) is most important elsewhere and on shorter temporal scales. We also show that the recent SDSL acceleration in the Gulf of Mexico has led to an increase of 220% in the frequency of HTF events over the last decade.

Growth of the sandy isthmus of tyre and ensuing relocation of its harbors

The renowned Phoenician city of Tyre was settled on a small coastal island from which it resisted invasions and sieges for centuries. In 332 BCE, the city was taken when Alexander-the-Great built a 750 m-long causeway to seize the island. The causeway interrupted longshore sediment transport, forcing sand to accumulate along the causeway, creating a isthmus that today still connects the island to the mainland. This isthmus is no less than 530 m wide and 13 m high. We studied the impact of isthmus growth on city development during Antiquity, by combining archeological data, core stratigraphy, and multibeam bathymetry to track the paleogeographic evolution of the land-facing coast of Tyre, from the Holocene marine transgression up to today. In Phoenician time (900-300 BCE), sea-level markers indicate that relative sea level lied more than 2.5 m below current sea level, defining a 1500 × 600 m rocky island, 450 m longer in the south than the modern rocky headland. Between the island and the mainland, diffracted/refracted waves built a submerged sand bank that rests on eroded transgressive lagoonal clays. Along the mainland-facing coast of Tyre Island, Phoenician walls were built on the emerged end of the sand bank. Middle infrared spectroscopy (MIRS) shows that the sand of the bank and its successor isthmus is made of carbonate clasts (90–100%). 36Sr/37Sr ratios further indicate that these are modern bioclastic sands. Coeval, heterogeneous clay-rich sediments were deposited between the rocky island, and the sand bank. We interpret these as harbor sediments, deposited behind buried and/or submerged Phoenician breakwaters. The sand bank inflated dramatically after the building of Alexander's causeway, rising 5 m during the Hellenistic and Roman periods. It rose another 4 m–8 m in successive stages during Byzantine and Medieval times, up to the 19th century. The rapid growth of the sandy isthmus after 332 BCE led to the demise and burial of the harbors that had been developed along the mainland-facing coast. The Phoenician southern harbor of Tyre appears to have been buried in Roman times during the repurposing of the SE corner of the island for monumental baths. We interpret this repurposing as a response to the growth of the sandy isthmus, and suggest that the northern Phoenician harbor was then relocated to its present position. Subsequent relative sea level rise led to the erosion of the landfill overlying the southern harbor, exposing the formerly buried port on the seafloor.

Climate and sea-level fluctuations on the western coast of Bohai Bay during the 4.2 ka event: Multi-proxy evidence from the Nandagang area of Cangzhou, China

The 4.2 ka event was an important climatic event during the mid–late Holocene, with significant effects on the ecological environment and climatic evolution of different regions. Coastal wetlands, as a transitional ecosystem between land and sea, are sensitive to changes in climate and sea level; hence, research on coastal wetlands is important for reconstructing past climate and sea-level changes and for predicting future changes. However, little research has been conducted on the characteristics of the 4.2 ka event on the western coast of Bohai Bay in China. We analyzed the pollen assemblages (214 samples), grain size, and geochemical element contents of a mid–late Holocene sedimentary sequence from the Nandagang (NDG) wetland on Bohai Bay, with the chronology based on 13 AMS 14C dates. The results revealed the history of climate and sea-level changes on the western coast of Bohai Bay during the interval of 5000–3600 cal yr BP. High sedimentary arboreal pollen concentrations indicated that the NDG area had a relatively dense cover and humid climate during 5000–4450 cal yr BP. During the 380-year interval of 4450–4070 cal yr BP, corresponding to the 4.2 ka event, the decreased representation of arboreal pollen indicated a decrease in the regional forest cover and an arid climate. The internal structure of the 4.2 ka event recorded in core NDG showed a dry-wet-dry temporal structure during the intervals of 4450–4290 cal yr BP, 4290–4170 cal yr BP, and 4170–4070 cal yr BP, respectively, together with a relatively low sea level. Comparison of the relative sea level (RSL) record of the NDG wetland with similar records from other coastal regions in East Asia during the 4.2 ka event revealed contrasting trends. During 4070–3600 cal yr BP, the tree pollen concentration increased again, indicating that the forest vegetation coverage increased and the climate became wetter. Overall, our results suggest that the 4.2 ka event, as recorded in Bohai Bay, was related to the reduced influence of the western Pacific Ocean Warm Pool and the more frequent occurrence of the El Niño–Southern Oscillation.

Imprint of relative sea level histories on Last Interglacial coral preservation

SUMMARY Fossil corals are commonly used to reconstruct Last Interglacial (∼125 ka, LIG) sea level. Sea level reconstructions assume the water depth at which the coral lived, called the ‘relative water depth’. However, relative water depth varies in time and space due to coral reef growth in response to relative sea level (RSL) changes. RSL changes can also erode coral reefs, exposing older reef surfaces with different relative water depths. We use a simplified numerical model of coral evolution to investigate how sea level history systematically influences the preservation of corals in the Bahamas and western Australia, regions which house &amp;gt;100 LIG coral fossils. We construct global ice histories spanning the uncertainty of LIG global mean sea level (GMSL) and predict RSL with a glacial isostatic adjustment model. We then simulate coral evolution since 132 ka. We show that preserved elevations and relative water depths of modelled LIG corals are sensitive to the magnitude, timing and number of GMSL highstand(s). In our simulations, the influence of coral growth and erosion (i.e. the ‘growth effect’) can have an impact on RSL reconstructions that is comparable to glacial isostatic adjustment. Thus, without explicitly accounting for the growth effect, additional uncertainty is introduced into sea level reconstructions. Our results suggest the growth effect is most pronounced in western Australia due to Holocene erosion, but also plays a role in the Bahamas, where LIG RSL rose rapidly due to the collapsing peripheral bulge associated with Laurentide Ice Sheet retreat. Despite the coral model's simplicity, our study highlights the utility of process-based RSL reconstructions.

The ancient slipways and shipsheds of the Aegean: Accurate indicators of relative sea level change?

The ancient slipways and shipsheds of the Aegean: Accurate indicators of relative sea level change?

Global distribution and dynamics of muddy coasts

Muddy coasts provide ecological habitats, supply food and form a natural coastal defence. Relative sea level rise, changing wave energy and human interventions will increase the pressure on muddy coastal zones. For sustainable coastal management it is key to obtain information on the geomorphology of and historical changes along muddy areas. So far, little is known about the distribution and behaviour of muddy coasts at a global scale. In this study we present a global scale assessment of the occurrence of muddy coasts and rates of coastline change therein. We combine publicly available satellite imagery and coastal geospatial datasets, to train an automated classification method to identify muddy coasts. We find that 14% of the world’s ice-free coastline is muddy, of which 60% is located in the tropics. Furthermore, the majority of the world’s muddy coasts are eroding at rates exceeding 1 m/yr over the last three decades.

Reconstruction of the sedimentary environment of Nayband Bay during the last 1600 years; implications for relative sea level and climate change in Northern Persian Gulf

This paper reconstructs the relative sea level (RSL) and palaeoclimate of the northern coasts of the Persian Gulf (PG) during the last 16 centuries using physiochemical and ecological proxies from a radiocarbon-dated sediment core from the Nayband Bay (NB) estuary. The results show that between 1600 and 1150 cal yr BP, the NB was a marine lagoon with high river discharge, indicated by low organic matter content sandy silt sediments, high foraminiferal abundance (7900 tests per 10 cm3), and balanced agglutinated and hyaline taxa. During the Medieval Warm Period, the NB became a shallow lagoon, and consequently, a playa with high eolian input and low river discharge formed in the studied site that extended until 500 cal. yr BP. This condition is indicated by sediment finning trend, elevated magnetic susceptibility in sandy mud sediments, and low foraminiferal frequency dominated by hyaline taxa (mainly opportunist Ammonia beccarii). During the stillstand condition corresponding to 500–250 cal yr BP, the evolution of tidal channels in the area provided a platform for developing the upcoming mangrove habitat. In the last 250 years, RSL in the north of NB dropped enough to create a low-energy intertidal flat sheltered by a sandbar, which allowed mangrove growth. We conclude that the ecological and sedimentological evolution of the shallow marginal NB is mainly driven by eustatic RSL decline superimposed by local uplift. We suggest that the current orbitally-induced increasing insolation trend, enhanced by human-induced global warming, could increase aridity in the PG and the southern Zagros.

Organic carbon stocks of Great British saltmarshes

Coastal wetlands, such as saltmarshes, are globally widespread and highly effective at capturing and storing ‘blue carbon’ and have the potential to regulate climate over varying timescales. Yet only Australia and the United States of America have national inventories of organic carbon held within saltmarsh habitats, hindering the development of policies and management strategies to protect and preserve these organic carbon stores. Here we couple a new observational dataset with 4,797 samples from 26 saltmarshes across Great Britain to spatially model organic carbon stored in the soil and the above and belowground biomass of Great British saltmarshes. Using average values derived from the 26 marshes, we deliver first-order estimates of organic carbon stocks across Great Britain’s 448 saltmarshes (451.66 km2). The saltmarshes of Great Britain contain 5.20 ± 0.65 Mt of organic carbon, 93% of which is in the soil. On average, the saltmarshes store 11.55 ± 1.56 kg C m-2 with values ranging between 2.24 kg C m-2 and 40.51 kg C m-2 depending on interlinked factors such as geomorphology, organic carbon source, sediment type (mud vs sand), sediment supply, and relative sea level history. These findings affirm that saltmarshes represent the largest intertidal blue carbon store in Great Britain, yet remain an unaccounted for component of the United Kingdom’s natural carbon stores.

Offsets in tide-gauge reference levels detected by satellite altimetry: ten case studies

Comparing measurements of absolute sea level by satellite altimetry and relative sea level by a tide gauge can reveal errors in either measurement system. Combining the measurements can determine vertical land motion (VLM) at the tide gauge. We here discuss ten case studies in which a tide gauge has likely experienced a small (≤10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\le 10$$\\end{document} cm), discontinuous offset in the vertical, suggesting inadvertent loss of reference-level stability. Proper interpretation of offsets is helped if independent VLM measurements from nearby geodetic stations are available. In two cases, earthquake-induced VLM cannot be ruled out, although it appears unlikely. Offsets as small as 1–2 cm can be detected when both altimeter and tide gauge successfully observe the same ocean signal. This is most likely to occur for tide gauges located on small, open-ocean islands. Tide gauges near large land masses are typically more challenging owing to inadequacies of satellite altimetry near land and to differences between coastal and open-ocean sea levels. The case studies highlight the utility of satellite altimetry for tide-gauge quality control.

Middle and Late Holocene relative sea level changes and coastal development at Rugård, Denmark

Denmark has been subject to complex interactions of isostatic uplift and eustatic sea level changes since the last deglaciation. Prominent coastal beach ridges as well as lagoonal and lake deposits from this period have been investigated at a number of sites in the region to constrain the relative sea level (RSL) changes. However, despite the common occurrence of former coastal lagoons and lakes in proximity to raised beach ridges, they have rarely been studied in combination. In this study, we use a multiproxy approach including geospatial data, lake sediment coring, ground penetrating radar and optically stimulated luminescence dating to investigate the Holocene coastal evolution and RSL history at Rugård in Mols Bjerge National Park, on the east coast of the Jutland Peninsula. Our results show that the coastal area at Rugård was transgressed between c. 7.6 and 7.0 cal. ka BP and that RSL was ~4.5 m higher than present between c. 6.6 and 5.9 ka ago, when the highest section of the beach ridge plain was deposited. The elevation and timing of this relative highstand are in good agreement with previous estimates of the Littorina transgression and contribute to our combined knowledge about RSL history and coastal evolution in the southern Kattegat. Subsequently, isostatic adjustment has caused uplift and erosion of the beach ridge plain, but renewed progradation and deposition of a lower beach plain have taken place since c. 1740 CE. Our results demonstrate the value of using a multiproxy approach to study RSL changes and coastal evolution.

Depositional environment and source rock distribution of the Late Cretaceous Rakopi formation in the deepwater Taranaki Basin, New Zealand

The interpretation of depositional environments and evaluation of potential source rocks in deepwater frontier basins are challenging because of the limited data availability. The Deepwater Taranaki Basin, the northwest extensional area of the Taranaki Basin in New Zealand, hosts the Late Cretaceous Rakopi Formation, which is one of the most important source rocks and potential reservoirs in the Taranaki Basin. This study utilized seismic data and well ties from the Romney-1 well to interpret three seismic units in the study area, representing different depositional environments and evolutions. These units were correlated with the regional Megasequences B2–B4 in the Deepwater Taranaki Basin. Sediment transport and progradation mainly occurred from the northeast to the southwest direction. Fluctuations in relative sea levels influenced the shifting of the shelf and marginal environments, potentially creating coal-bearing strata in the study area. The key source rock strata were identified within units 2 and 3 characterized by kerogen type III, coaly source rocks in a marginal environment. A minor source rock interval was found in unit 2, characterized by kerogen type II, marine organic matter in a bathyal environment in the depocenter of the study area. Coal seams were identified using seismic and well log data. However, thin coal seams were challenging to distinguish clearly, sometimes showing a similar response to sandstone beds, because of the limited seismic resolution. Thus, within limited data areas, source rock predictions can be made in the context of depositional interpretation.

Constraining the end of the Last Interglacial (MIS 5e) relative sea-level highstand in central Mediterranean: New data from Grotta delle Capre, central Italy

The current rapid change of the Earth's climate has resulted in an increasing interest for the past warm periods as potential long-term scenarios of the effects of the present global warming. The last such a period occurred 129–116 ka, known as the Last Interglacial (LIG), when the continental ice volume was significantly smaller than present, leading to a global sea-level (GSL) higher than present one. Detailed morpho-stratigraphic data, supported by a robust U/Th chronology, from Grotta delle Capre, central Italy, provided new chronological insights on the relative sea-level (RSL) dynamic during the LIG in the Mediterranean region. Our results indicate that, on Tyrrhenian Sea coasts of the central Italy, after having stationed at ∼9 m a.s.l., the LIG RSL fell at an elevation <3 m a.s.l. as early as before 123 ka, and then no longer rose above this elevation either during the later stages of the LIG or afterwards. The results match previous studies based on U/Th dating of terrestrial limiting points from Grotta Infreschi, ∼200 km SE from Grotta delle Capre along the same Tyrrhenian Sea coasts, and are in agreement with the Red Sea RLS and GSL records and the probabilistic LIG sea level assessments based on globally distributed records. On the other hand, our reconstruction is not supported by implications of U/Th dating of corals and phreatic overgrowth on speleothems from the Balearic Island of Mallorca. Such an inconsistency in the overall knowledge around the LIG RSL reconstruction results in a high uncertainty in modelling the ice and sea-level dynamic during this warm period, which needs to be reduced through more and more high-resolution, stratigraphic and chronological investigations of the morphological and sedimentary sea-level records.

The search for “Jenyon’s Channel”: The missing link between the Permian basins in the North Sea

The Mid North Sea High (MNSH) Seaway, otherwise known as Jenyon's Channel, was the only major marine connection between the Northern and Southern Zechstein Basins during the latest Permian. Current understanding favours a model where marine replenishment began with a northern connection to the Panthalassic Ocean (along a network of basins that were precursors to the North Atlantic Rift System), after which marine water passed into the Northern Zechstein Basin, travelled through the MNSH Seaway before finally reaching the Southern Zechstein Basin. This study delineates evaporite formations in an extensive petrophysical dataset to analyse marine connections and whether they influenced Zechstein facies distribution. The first four Zechstein Cycles (Z1-Z4) are identified on the MNSH platform which shows that this structure was always covered by a thin water column during sea-level highstand; however, during Z2 sea-level lowstand, the MNSH Seaway provided the main connection between the basins. As relative sea-level fell, the MNSH Seaway became increasingly constricted resulting in hypersalinity due to limited volumes of marine water reaching the Southern Zechstein Basin. This system precipitated vast volumes of Z2 (late Wuchiapingian) halite in the Southern Zechstein Basin, whereas this facies remains reduced in the Northern Zechstein Basin. In Z3 and Z4 (Changhsingian) times relative average sea-level was higher resulting in sustained communication between the basins, even in times of sea-level lowstand. This is evidenced through a regionally traceable blanket of Z3 halite, along with occasional examples of Z2 carbonate platforms becoming entombed in Z3 halite. This work provides revised palaeoenvironmental understanding for the latest Permian. The workflow could also aid in the study of other oceanic gateways with further implications for hydrocarbon exploration efforts and emerging energy transition technologies such as subsurface storage on the UK Continental Shelf.

Placing constraints on the nature of short‐term eustatic curves

AbstractThe isolation of the eustatic signal from the sedimentary record is a challenging task, and accordingly, there is no consensus on the magnitude and pace (rate) of eustatic events in the geological record. Here we critically assess various published short‐term Cretaceous eustatic curves using insights from forward stratigraphic modelling. We generate a range of simulations with varying eustatic rates and sediment supply against a background of constant subsidence. From these, we generate statistics on the accommodation change associated with the various systems tracts for different sediment supply. We quantify the minimum rate needed to generate transgressive systems tracts (TST). Using this threshold and average subsidence rates for passive margins and intracratonic basins, we document some key challenges with a range of Cretaceous eustatic curves. While it is possible to complexify, this approach through the inclusion of other parameters, our results provide a framework for evaluating eustatic (or relative sea level) curves in terms of the implied rate of change of accommodation. Given these caveats, we also show that many estimates of the magnitude of short‐term transgressions are of insufficient rate to generate observable TST. Further, our work places an upper limit on the time frame over which aquifer and thermo‐eustasy can have observable impacts on the rock record, providing support for the action of glacio‐eustasy during the Cretaceous.

Geochemical and sedimentary features of the unusual association between hybrid carbonates and beachrocks in the Southeastern Brazilian Coast

Rocks formed in beach environments during the Holocene constitute strong evidences of relative sea level variations (RSL) and supply important data for paleoenvironmental reconstruction. At the southeastern Brazilian coast, an unusual association of lithofacies composed of hybrid carbonates and beachrocks was identified on the Parati beach in the Espírito Santo state. In this study, radiocarbon dating and sedimentary and geochemical characterization of five lithofacies were conducted, with the aim of reconstructing the paleoenvironmental conditions of deposition and cementation of this facies association. The lithofacies were classified as quartzarenites, bioclastic quartzarenites, hybrid carbonate and hybrid boundstones, with ages ranging from 2600 to 700 cal yr BP. Sedimentary and geochemical changes observed in these five lithofacies of the shallow shelf off southeastern Brazil reinforce evidence relative sea level variations during the Holocene.

Mapping the vulnerability of groundwater to saltwater intrusion from estuarine rivers under sea level rise

Surface water bodies connected to the ocean such as estuarine rivers can act as pathways for saltwater intrusion (SI, i.e., the displacement of fresh groundwater by saltwater in an aquifer) far inland of the coast, presenting one of the earliest risks associated with relative sea level rise (SLR). However, SI vulnerability mapping approaches have largely focused on SI from the coast and do not consider estuarine surface water bodies, except for GALDIT–SUSI (Kazakis et al., 2019); a weighted indexing approach designed for regional-scale mapping applications using a Geographic Information Systems (GIS) framework. However, GALDIT-SUSI is subjective in ranking the importance of factors that can contribute to SI and combining these into a vulnerability index. A less subjective approach to assessing SI vulnerability than weighted indexing methods involves the use of physically-based analytic solutions such as Strack (1976), but these have not been applied in a GIS framework or along estuaries and rivers previously. Here, these analytic solutions, surface water salinity, and surface water-groundwater freshwater head gradients are used in the development of a new SI vulnerability tier system. This new approach was applied in the low-lying coastal city of Ōtautahi Christchurch, Aotearoa New Zealand, along 70 km of coastal, estuary, and river margins under current sea level and SLR using GIS, then compared to GALDIT–SUSI. The main advantage of the SI vulnerability tiers method is that it considers surface water behaviour, e.g., increased saltwater encroachment up rivers under SLR, which exposed new areas of the aquifer to saltwater and increased SI vulnerability upstream. In contrast, GALDIT-SUSI does not consider surface water conditions and accounts for topography and groundwater level as the main SI vulnerability drivers in this application. The SI vulnerability tiers proposed here are more theoretically robust than GALDIT-SUSI and provide a physically-based, large-scale, relatively low-budget and rapid screening tool to highlight areas most vulnerable to SI under current and future conditions for further monitoring and management; a gap of national and international relevance.

Marsh migration and beyond: A scalable framework to assess tidal wetland resilience and support strategic management

Tidal wetlands are critical but highly threatened ecosystems that provide vital services. Efficient stewardship of tidal wetlands requires robust comparative assessments of different marshes to understand their resilience to stressors, particularly in the face of relative sea level rise. Existing assessment frameworks aim to address tidal marsh resilience, but many are either too localized or too general, and few directly translate resilience evaluations to recommendations for management strategies. In response to the deficiencies in existing frameworks, we identified a set of metrics that influence overall marsh resilience that can be assessed at any spatial scale. We then developed a new comprehensive assessment framework to rank relative marsh resilience using these metrics, which are nested within three categories. We represent resilience as the sum of results across the three metric categories: current condition, adaptive capacity, and vulnerability. Users of this framework can add scores from each category to generate a total resilience score to compare across marshes or take the score from each category and refer to recommended management actions we developed based on expert elicitation for each combination of category results. We then applied the framework across the contiguous United States using publicly available data, and summarized results at multiple spatial scales, from regions to coastal states to National Estuarine Research Reserves to finer scale marsh units, to demonstrate the framework’s value across these scales. Our national analysis allowed for comparison of tidal marsh resilience across geographies, which is valuable for determining where to prioritize management actions for desired future marsh conditions. In combination, the assessment framework and recommended management actions function as a broadly applicable decision-support tool that will enable resource managers to evaluate tidal marshes and select appropriate strategies for conservation, restoration, and other stewardship goals.