Rate Of Relative Sea-level Rise Research Articles

Different fates of the Yangtze and Mississippi deltaic wetlands under similar riverine sediment decline and sea-level rise

The Yangtze River has experienced a sharp sediment decrease as a result of extensive dam constructions since the 1980s, similar to the sediment decline in the Mississippi River since the 1950s. Historical land losses in the Mississippi Delta have been reported extensively, but this issue in the Yangtze Delta is still under debate. Here we presented the morphological responses of deltaic wetlands to the sediment decline with the comparison of hydrodynamic forces in the Yangtze and Mississippi deltas. We found that the wetlands of the Yangtze Delta continued their expansion even after the sediment decline since 2003 when the Three Gorges Dam began operation, in contrast to the historical scenario in the Mississippi. Based on observations at water depth around 10 m in both of the open subaqueous deltas, the bed shear stresses caused by waves and currents (τcw) during neap and spring tides were 0.18–0.30 N/m2 in the Yangtze Delta and 0.09–0.10 N/m2 in the Mississippi Delta under fair weather conditions. The periods of τcw > τcr (the critical bed shear stress for erosion) occupied on average 80% of a tidal cycle in the Yangtze Delta but only 47% of a cycle in the Mississippi Delta. The rates of relative sea-level rise and riverine sediment decrease in these two deltas are similar, but the sediment-supply capacity to the deltaic wetlands from the Yangtze's subaqueous delta is much larger than that of the Mississippi Delta. Thus, we infer that in contrast to the historical land loss after sediment decline in the Mississippi Delta, the Yangtze deltaic wetlands stand still with a good opportunity to survive for years to decades with the aid of abundant sediment from the subaqueous delta due to higher capacity of seabed erodibility by stronger coastal hydrodynamics.

Decreasing land growth and unique seasonal area fluctuations of two newborn Mississippi subdeltas

A total of 491 Landsat satellite images from 1973 to 2017 were used to study the evolution of two newborn subdeltas of the Mississippi-Atchafalaya system, the Wax Lake Delta (WLD) and the Atchafalaya Delta (AD). The exposed delta areas derived from the satellite images using an unsupervised classification method were applied to evaluate the long-term land growth and seasonal area variations in the WLD and AD, as well as their main influencing factors. The results show that the land growth rate of the WLD decreased relative to the mean sea level from 1.67 km2/year to 0.35 km2/year from 1983–1999 to 1999–2017, and the growth rate of the AD decreased from 1.51 km2/year to 0.80 km2/year from 1973–1999 to 1999–2017. The land building of the two subdeltas during the 1980s to 1999 was more efficient than that from 1999 to 2017. The significant decreases in the land growth rate and land building efficiency are attributed both to the increased impact from relative sea level rise and to the decrease in river sediment discharge, and the extension of the distributary channels, as well. The satellite-derived exposed areas of the WLD and AD show unique seasonal loops affected by the growth of vegetation during the warmer weather from April to June and the decay of vegetation mainly due to the cold fronts in December, which is different from the monotonic changes in the areas of other deltas, mainly driven by the rise and fall of water levels. A new method based on the correction between the exposed areas and water levels from December to April was used to estimate the vertical growth rates of the WLD and AD. After deducting the relative sea level rise rate, the net sediment deposition rates of the WLD and AD were estimated to be 20.8 and 24.2 mm/year, respectively. Approximately 72% and 62% of the sediments deposited in the WLD and AD were used to offset the effects of relative sea level rise when building deltaic lands from 1999 to 2017. This study not only improves our understanding of the growth of newborn subdeltas but also aids in assessing whether river course shifts and sediment diversions can restore deltas or deltaic wetlands that are at risk of drowning.

Subsurface geomorphology of wave-dominated nearshore deposits: Contrasting styles of reservoir heterogeneity in response to shoreline trajectory

Wave-dominated nearshore deposits are commonly regarded as laterally-continuous, homogeneous, sheet-like reservoirs that can be resolved at the parasequence scale. Many reservoirs, however, have an internal complexity that becomes clearer during production, indicating unexpected internal compartmentalization. Modern wave-dominated nearshore environments are extremely variable due to the effects of allogenic and autogenic processes. Relative sea-level change and shoreline trajectory has been indicated as key controlling factors on the distribution of sand in Quaternary coastal systems worldwide. In this paper we use 3D seismic data to describe the geomorphology of buried, Campanian and Eocene wave-dominated coastal deposits in two stratigraphic intervals within shelf-margin clinoforms in the Santos Basin, SE offshore Brazil. By applying seismic attributes and comparison with modern analogues we interpret the paleoenvironments and correlate them with well-log motifs and styles of shelf-margin trajectories in order to discuss the balance of allogenic and autogenic controls on nearshore reservoir heterogeneity. The sand-rich nearshore deposits appear in the seismic data as distinct topset amplitude anomalies recording shoreline transit across the shelf. The Campanian interval comprises reflectors formed in a predominantly normal-regressive context, while the Eocene interval depicts partially-eroded topset reflectors associated with forced regressions and retrograding reflectors associated with transgressions. Nearshore seismic-geomorphic elements were grouped in associations of strandplains, barrier islands/spits and lagoons, and partially-eroded nearshore. Strandplains are related sub-horizontal to ascending regressive clinoform trajectories where wider strandplains are associated with sub-horizontal trajectories. Nearshore erosion and fluvial incision are associated with descending trajectories within forced regressions, while barrier islands/spits and lagoons are associated with transgressive trends. Transgressive trajectories generated coastal systems with high mud content and discontinuous sand bodies along strike and dip, while forced regressions resulted in sand-rich systems that are complex and highly compartmentalized by subaerial erosion. The most attractive reservoirs are associated with normal-regressive intervals under low rates of relative-sea level rise, forming relatively homogeneous sand-rich strandplains.

Nonlinear absolute sea-level patterns in the long-term-trend tide gauges of the West Coast of North America

AbstractThe research issue of which are the present relative and absolute rates of rise and accelerations for North America is here addressed. The data of the 20 long-term-trend (LTT) tide stations of the West Coast of North America with more than 80 years of recorded data are shown. The absolute rates of rise are computed by considering the absolute vertical velocity of Global Navigation Satellite System (GNSS) antennas near the tide gauges, and the relative rate of sea-level rise from the tide gauge signals. The 20 LTT stations along the West Coast of North America show an average relative rate of rise of -0.38 mm/yr., an average acceleration of +0.0012 mm/yr2, and an average absolute rate of rise of +0.73 mm/yr. This is the first paper publishing a comprehensive survey of the absolute sea-level rates of rise along the West Coast of North America using the reliable information of relative sea-level rates of rise from LTT tide gauges plus the absolute subsidence rates from different GNSS antennas close to the tide gauge installations.

Field‐Based Estimate of the Sediment Deficit in Coastal Louisiana

AbstractCoastal and deltaic sediment balances are crucial for a region's sustainability. However, such balances remain difficult to quantify accurately, particularly for large regions. We calculate organic and mineral sediment mass and volume balances using field measurements from 273 Coastwide Reference Monitoring System sites across the Louisiana Coast between 2006 and 2015. The rapid relative sea level rise rate (average 13.4 mm/year) is offset by the small dry bulk densities observed (average 0.3 g/cm3) to produce a 16.2 ± 41.1% mass deficit and 24.1 ± 14.0% volume deficit, significantly smaller than recent predictions for 2000–2100 (73–79% mass deficit). Geostatisical estimates show that this deficit is primarily located in areas not directly nourished by major rivers, yet these regions still accumulate ~24 MT/year of mineral sediment. A fluvial sediment discharge of 113.8 MT/year suggests a coast‐wide trapping efficiency of 31.5 ± 15.8% of the riverine sediment, excluding subaqueous deposition. Organic accumulation accounts for 25% of all mass accumulation during our study period, and total organic mass accumulation per unit area is relatively constant in both directly and indirectly nourished regions. Sediment characteristics in the modern coastal wetlands differ from the Holocene deposit, suggesting secular changes within the system that will likely continue to influence coastal dynamics over the coming decades. Our results suggest that the gap between accommodation and accumulation (mass or volume) during this decade was not as large as the previously predicted century average.

Modeling the spatial dynamics of marsh ponds in New England salt marshes

Ponds are common features on salt marshes, yet it is unclear how they affect large-scale marsh evolution. We developed a spatially explicit model that combines cellular automata for pond formation, expansion, and drainage, and partial differential equations for elevation dynamics. We use the mesotidal Barnstable marsh (MA, USA) as a case study, for which we measured pond expansion rate by remote sensing analysis over a 41-year time span. We estimated pond formation rate by comparing observed and modeled pond size distribution, and predicted pond deepening by comparing modeled and measured pond depth. The Barnstable marsh is currently in the pond recovery regime, i.e.,every pond revegetates and recovers the necessary elevation to support plant growth after re-connecting to the channel network. This pond dynamic creates an equivalent (i.e.,spatially and temporally averaged over the whole marsh) 0.5–2 mm/yr elevation loss that needs to be supplemented by excess vertical accretion. We explore how the pond regime would change with decreased sediment supply and increased relative sea-level rise (RSLR) rate, focusing on the case in which the vegetated marsh keeps pace with RSLR. When the RSLR rate remains below the minimum unvegetated deposition rate, the pond dynamics is nearly unaltered and ponds always occupy ~10% of the marsh area. However, when RSLR rate exceeds this threshold, the ponds in the marsh interior – which receive the least amount of suspended sediment – do not recover after drainage. These ponds transition to mudflats and permanently occupy up to 30% of the marsh area depending on RSLR rate. For marshes with a small tidal range, such as the microtidal Sage Lot Pond marsh on the opposite side of the peninsula from Barnstable marsh, high RSLR rates could bring every portion of the marsh into the pond runaway regime, with the whole marsh eventually converting into mudflats. In this regime, the existing marsh would disappear within centuries to millennia depending on the RSLR rate. Because of the spatial and temporal components of marsh evolution, a single RSLR threshold value applied across the entire marsh landscape provides a limited description of the marsh vulnerability to RSLR.

Drowning of microbial mounds on the slopes of the Latemar platform (middle Triassic)

Two microbial mound-shaped carbonate bodies buried below the slope deposits of the middle Triassic Latemar platform (Dolomites, Italy) have been studied. The two sedimentary bodies, after having reached different stages of evolution, drowned and were covered by the slopes of the Latemar carbonate platform. The estimation of the depth and time at which they were located when they were buried made it possible to infer the average rates of relative sea-level rise to which they were subjected, revealing that these latter exceeded the growth rate of the main Latemar buildup. Given the estimated rates of sea-level rise, the two satellite bodies reached a critical depth at which microbial carbonate production stops, or it is significantly reduced, before being buried. As extensional tectonics was active in the area of the Dolomites during the Anisian, subsidence is the more likely cause of sea level rise. It is therefore hypothesized that the cause of the ultimate drowning of the “mounds” were subsidence rates exceeding their aggradation potential and inducing a progressive deepening that brought them below the lower depth threshold of microbial carbonate production.

Surface elevation and sedimentation dynamics in the Ganges-Brahmaputra tidal delta plain, Bangladesh: Evidence for mangrove adaptation to human-induced tidal amplification

In the Ganges-Brahmaputra (G-B) delta, periodic flooding of the land surface during the tidal cycle coupled with enormous sediment delivery during the monsoon promotes sediment accretion and surface elevation gain through time. However, over the past several decades, widespread embankment (“polder”) construction in the G-B tidal delta plain has led to numerous environmental disturbances, including channel siltation and tide range amplification. While previous research indicates that rates of sediment accretion are relatively high in the G-B tidal delta plain, it remains unclear if and how surface elevation is maintaining pace with relative sea-level rise (RSLR) in this region. In this study, we utilize an array of surface elevation tables, sediment traps, and groundwater piezometers to provide longitudinal trends of sedimentation and elevation dynamics with respect to local platform elevation and associated hydroperiod. Two hydro-geomorphic settings of the Sundarbans mangrove forest are compared: higher elevation stream-bank and lower elevation interior. Seasonal measurements over a time span of 5 years reveal that elevation gain is occurring in all settings, with the highest rates observed at elevated stream-bank zones. Elevation gain occurs primarily in response to sediment accretion, with possible minor contributions from pore-water storage and swelling of clay minerals during the monsoon season (i.e., belowground biomass and organic contribution is minimal). As a result, elevation loss and shallow subsidence in the G-B delta is unlikely to be caused by compaction of organic-rich soils, but rather appears to be controlled by seasonal lowering of the groundwater table and compaction of clay minerals. Rates of surface elevation gain in the Sundarbans greatly exceed rates of RSLR and more closely follow rates of RSLR augmented from tide range amplification, indicating that this landscape is adapting to human-induced environmental change. The proceedings of this study underscore the adaptability of the natural G-B tidal delta plain to local environmental disturbances, with the caveat that these defenses may be lost to future upstream reductions in sediment supply.

Sea Level Rise Explains Changing Carbon Accumulation Rates in a Salt Marsh Over the Past Two Millennia

AbstractHigh rates of carbon burial observed in wetland sediments have garnered attention as a potential “natural fix” to reduce the concentration of carbon dioxide (CO2) in Earth's atmosphere. A carbon accumulation rate (CAR) can be determined through various methods that integrate a carbon stock over different time periods, ranging from decades to millennia. Our goal was to assess how CAR changed over the lifespan of a salt marsh. We applied a geochronology to a series of salt marsh cores using both 14C and 210Pb markers to calculate CARs that were integrated between 35 and 2,460 years before present. CAR was 39 g C·m−2·year−1 when integrated over millennia but was upward of 148 g C·m−2·year−1 for the past century. We present additional evidence to account for this variability by linking it to changes in relative sea level rise (RSLR), where higher rates of RSLR were associated with higher CARs. Thus, the CAR calculated for a wetland should integrate timescales that capture the influence of contemporary RSLR. Therefore, caution should be exercised not to utilize a CAR calculated over inappropriately short or long timescales as a current assessment or forecasting tool for the climate change mitigation potential of a wetland.

Late Quaternary evolution and stratigraphic framework influence on coastal systems along the north-central Gulf of Mexico, USA

Coastal systems in the Gulf of Mexico are threatened by reduced sediment supply, storm impacts and relative sea-level rise (RSLR). The geologic record provides insight into geomorphic evolution thresholds to these forcing mechanisms to help predict future barrier evolution in response to climate change. This study synthesizes ∼2100 km of geophysical data, 700 + sediment cores, and 62 radiocarbon dates to regionally map two lowstand sequence boundaries, multiple ravinement surfaces and fourteen depositional facies demonstrating stratigraphic and antecedent topographic influences on coastal evolution. The Mississippi-Alabama (MSAL) barriers are anchored by a marine isotope stage (MIS) 5e section of Dauphin Island coupled with an MIS 2 surface gradient change. Sand for the modern MSAL barriers were largely sourced through Holocene transgressive ravinement of relict valley fill deposits, providing up to 300 × 106 m3 of sand. Mud-filled MIS 2 tributaries correspond to areas of repeated storm breaching or tidal inlets.A Holocene geomorphic evolutionary model was created for Petit Bois and Dauphin Islands, highlighting RSLR rates, changes in sediment supply and the antecedent geologic framework. As the MIS 2 surface was flooded, tidal/wave scour supplied sand to migrating marine shoals. These transgressing shoals converted drowned paleovalleys to estuaries ∼9ka BP. Islands formed in their modern positions ∼6ka BP, when sediment supply was high and RSLR rates were 2 mm/yr. Between ∼4ka-1750 CE, islands prograded from reduced RSLR rates of 1-0.4 mm/yr and sufficient sand supply from alongshore/inner shelf sources. Currently, the islands experience 3.74 mm/yr of RSLR and reduced sediment supply, resulting in barrier degradation.

Holocene sea-level variability from Chesapeake Bay Tidal Marshes, USA

We reconstructed the last 10,000 years of Holocene relative sea-level rise (RSLR) from sediment core records near Chesapeake Bay, eastern United States, including new marsh records from the Potomac and Rappahannock Rivers, Virginia. Results show mean RSLR rates of 2.6 mm yr−1 from 10 to 8 kilo-annum (ka) due to combined final ice-sheet melting during deglaciation and glacio-isostatic adjustment (GIA subsidence). Mean RSLR rates from ~6 ka to present were 1.4 mm yr−1 due mainly to GIA, consistent with other East Coast marsh records and geophysical models. However, a progressively slower mean rate (<1.0 mm yr−1) characterized the last 1000 years when a multi-century-long period of tidal marsh development occurred during the ‘Medieval Climate Anomaly’ (MCA) and ‘Little Ice Age’ (LIA) in the Chesapeake Bay region and other East Coast marshes. This decrease was most likely due to climatic and glaciological processes and, correcting for GIA, represents a fall in global mean sea level (GMSL) near the end of Holocene Neoglacial cooling. These pre-historical climate- and GIA-driven Chesapeake Bay sea-level changes contrast sharply with those based on Chesapeake Bay tide-gauge rates (3.1–4.5 mm yr−1) (back to 1903). After subtracting the GIA subsidence component, these rates can be attributed to long-term (millennial) global factors of accelerated ocean thermal expansion (~1.0 mm yr−1) and mass loss from alpine glaciers and Greenland and Antarctic Ice Sheets (1.5–2.0 mm yr−1).

Rapid inundation of southern Florida coastline despite low relative sea-level rise rates during the late-Holocene

Sediment cores from Florida Bay, Everglades National Park were examined to determine ecosystem response to relative sea-level rise (RSLR) over the Holocene. High-resolution multiproxy analysis from four sites show freshwater wetlands transitioned to mangrove environments 4–3.6 ka, followed by estuarine environments 3.4–2.8 ka, during a period of enhanced climate variability. We calculate a RSLR rate of 0.67 ± 0.1 mm yr−1 between ~4.2–2.8 ka, 4–6 times lower than current rates. Despite low RSLR rates, the rapid mangrove to estuarine transgression was facilitated by a period of prolonged droughts and frequent storms. These findings suggest that with higher and accelerating RSLR today, enhanced climate variability could further hasten the loss of mangrove-lined coastlines, compounded by the reductions in natural flow to the coast caused by water management. Climate variability is nonlinear, and when superimposed on increases in RSLR, can complicate estimated trajectories of coastal inundation for resource management and urban planning.

Outer- to inner-shelf response to stepped sea-level rise: Insights from incised valleys and submerged shorelines

Shorelines respond to rising sea-level through processes such as erosion, landward migration and in-situ drowning (i.e. overstepping). Submerged and preserved shorelines on the continental shelf play a key role in examining coastal response to rising sea levels as they provide important information on how modern shorelines may evolve in time and space within the context of changing climate and post-glacial sea-level rise. This study identifies and assesses the response of a continental shelf to stepped rises in sea level with particular focus on the stepwise evolution of incised valleys and shorelines from the shelf-edge to the inner shelf. Multibeam bathymetry from the mid-outer allows for the analysis of seafloor morphology, including the Protea Banks Reef (a palaeo-shoreline complex), and the adjacent incised, sediment starved continental shelf. Six seismic units and intervening surfaces are identified using interpretations from sub-bottom profiles, these include the incised acoustic basement, variable incised valley fill successions, aeolianite ridges and post-transgressive shoreface and associated sediments that withstood wave ravinement processes. The incised valleys of the outer-shelf are manifested as distinctive seafloor depressions, filled at their bases by fluvial deposits overlain, in the unfilled valley, by deposits derived from cascading subaqueous dunes which comprise the upper-most post-transgressive sediments. A core intersecting the dune material yields a maximum age of deposition of 1191–1263 cal yr BP (68% range), synchronous with a period of higher than present sea-levels in the region suggesting reworking and redistribution of coastal sediment as shelf sediment post-transgression. During the stepped rises in sea level, the shoreface has disconnected from the contemporary shoreline and is preserved by means of topographic barriers formed by antecedent topography as relict shoreface deposits. We provide a new perspective of shoreline response to stepped rises in sea level by integrating the seismic architecture of incised valley fills and shorelines across the continental shelf thus allowing for the assessment of variation in rates of relative sea-level rise since the last glacial maximum.

Modeled Sediment Availability, Deposition, and Decadal Land Change in Coastal Louisiana Marshes under Future Relative Sea Level Rise Scenarios

The ability, or lack thereof, for wetlands in coastal Louisiana to maintain elevation capital has been well documented in the literature to be a function of local and regional factors as well as environmental conditions. The Integrated Compartment Model (ICM) framework developed for the state of Louisiana’s Coastal Master Plan models hydrologic, vegetation, and wetland elevation dynamics and captures regional and local dynamics of wetland elevation, inundation and sedimentation processes. It provides insights into the relative sensitivities of wetland evolution to environmental drivers under uncertain future environmental conditions. A systematic, and computationally efficient modeling exercise was conducted to test coastal marsh survival across a wide range of possible future relative sea level rise rate scenarios. Model results indicate a diverse response with respect to sediment deposition and marsh survival driven by regional subsidence rates and proximity to suspended sediment sources. Sediment poor regions of coastal Louisiana are particularly sensitive to relative sea level rise under all but the most optimistic of future sea level rise rates simulated. Coastal marshes with high sediment availability fare much better under most scenarios tested, despite high rates of relative sea level rise.

THE VERTICAL LAND MOTION OF TIDE GAUGE AND ABSOLUTE SEA LEVEL RISE IN BOHAI SEA

Abstract. The ground vertical movement of the tide gauges around the Bohai sea was firstly analyzed by using the observation data from 2009 to 2017 of the nine co-located GNSS stations. It was found that the change rate of ground vertical motion of four stations was in the same order of magnitude as the sea level change. In particular, the land subsidence rate of BTGU station reaches 11.47 mm/yr, which should be paid special attention to in the analysis of sea level change. Then combined with long-term tide gauges and the satellite altimetry results, the sea level changes in the Bohai sea and adjacent waters from 1993 to 2012 were analyzed. The relative and absolute sea level rise rates of the sea area are 3.81 mm/yr and 3.61 mm/yr, respectively, both are higher than the global average rate of change. At the same time, it is found that the vertical land motion of tide gauge stations is the main factor causing regional differences in relative sea level changes.

Holocene sea-level database for the Rhine-Meuse Delta, The Netherlands: Implications for the pre-8.2 ka sea-level jump

This paper provides the most-accurate sea-level index points (SLIPs) for the Rhine-Meuse Delta (RMD), The Netherlands, identified amidst a wealth of data accumulated in sixty years of research. Following documented protocols, 106 selected radiocarbon dates from peat beds from transgressed valley floor, upper estuarine and back-barrier lagoonal settings were first listed individually, then screened in ensemble on quantified age-depth position and inshore palaeotidal setting. The database contains 50 SLIPs and 56 upper limiting data points spanning a depth range of −34 to −1.5 m. The dates in the database originate from dedicated sea-level research, which kept surface elevation and sampling depth uncertainty small, and sampling of basal-peat beds was favoured, keeping compaction uncertainties small too.The SLIPs cover an age-range of 8.8–3.0 ka BP and are available from Greater Rotterdam and its near offshore. For upper limiting data points, the coverage extends offshore, tracing the Rhine palaeovalley, and reaches back to 11.5 ka BP. The age-depth data density and spatial concentration of subsets of dates allow to perform Bayesian radiocarbon calibration and to reduce temporal uncertainty by 25% for the average sample (1σ age uncertainty ≈ 75 cal. yr), when depth position and rates of relative sea-level rise (RSLR) are used to assign sequence order. Between 8.0–4.5 ka BP, rates of RSLR gradually decreased from 9 to 1 mm/yr. In the millennium before, rates of RSLR were much higher, on average 10 mm/yr, and marked by a sea-level jump between 8.45–8.2 ka BP. A two-phased nature of this jump starts to be resolved, with local magnitudes of 1.7 ± 0.6 m (1σ, first phase) and 0.2 ± 0.2 m (second phase), corresponding to globally-averaged jumps of 2.5 m ± 0.9 m for the first and 0.3 ± 0.3 m for the second phase. For the pre-9 ka BP period, only offshore upper limiting data points are currently available. Until new offshore data are gathered, trends of RSLR before 9 ka BP can only be estimated, fed with global insights in post-glacial eustasy, regional estimates regarding rates of glacio-isostatic adjustments and tectonic subsidence, and sedimentary geological interpretations of scattered basal-peat presence.

Assessment of Sea Level Rise at West Coast of Portugal Mainland and Its Projection for the 21st Century

Based on the updated relative sea level rise rates, 21st-century projections are made for the west coast of Portugal Mainland. The mean sea level from Cascais tide gauge and North Atlantic satellite altimetry data have been analyzed. Through bootstrapping linear regression and polynomial adjustments, mean sea level time series were used to calculate different empirical projections for sea level rise, by estimating the initial velocity and its corresponding acceleration. The results are consistent with an accelerated sea level rise, showing evidence of a faster rise than previous century estimates. Based on different numerical methods of second order polynomial fitting, it is possible to build a set of projection models of relative sea level rise. Applying the same methods to regional sea level anomaly from satellite altimetry, additional projections are also built with good consistency. Both data sets, tide gauge and satellite altimetry data, enabled the development of an ensemble of projection models. The relative sea level rise projections are crucial for national coastal planning and management since extreme sea level scenarios can potentially cause erosion and flooding. Based on absolute vertical velocities obtained by integrating global sea level models, neo-tectonic studies, and permanent Global Positioning System (GPS) station time series, it is possible to transform relative into absolute sea level rise scenarios, and vice-versa, allowing the generation of absolute sea level rise projection curves and its comparison with already established global projections. The sea level rise observed at the Cascais tide gauge has always shown a significant correlation with global sea level rise observations, evidencing relatively low rates of vertical land velocity and residual synoptic regional dynamic effects. An ensemble of sea level projection models for the 21st century is proposed with its corresponding probability density function, both for relative and absolute sea level rise for the west coast of Portugal Mainland. A mean sea level rise of 1.14 m was obtained for the epoch of 2100, with a likely range of 95% of probability between 0.39 m and 1.89 m.

Are coal balls rare? A cyclostratigraphic analysis of coal-ball occurrence in North America

From the perspective of Phanerozoic time, coal balls are rare, apparently limited to a 24 m.y. interval (323–299 Ma) in the Pennsylvanian and earliest Permian. Yet within this interval, coal balls occur in many coals. Approximately 82 transgressive-regressive sedimentary cycles have been described for the Midcontinent, Illinois and Appalachian basins of North America during the mid-to-late Pennsylvanian. One third (27/82) have coal balls, including 57% of major cycles, 36% of intermediate cycles and 16% of minor cycles. Coal-ball occurrence in the Donets Basin is similar: over an interval of about 4 m.y. (~315–311 Ma, latest Bashkirian to mid-Moscovian), 39% (11/28) of transgressive-regressive cycles have coal balls. As North American paleoclimate became drier, tree ferns replaced lycopsids as the dominant plant in peat swamps at the Desmoinesian/Missourian boundary, and coal-ball occurrence declined. Overall, 47% of cycles (19/40) with lycopsid or cordaitean dominance have coal balls, whereas 19% (8/42) of cycles with tree fern dominance have coal balls (p < .004). While 32% (6/19) of minor cycles with lycopsid or cordaitean dominance have coal balls, no minor cycle with tree fern dominance has coal balls.This pattern may reflect the relative abundance of coal in the Late Pennsylvanian, with drier paleotropical climates in the Missourian-Virgilian leading to less paleotropical coal, fewer mines and a lower probability of discovering Missourian and Virgilian coal balls. However, average cycle duration decreased from 150 kyr in the Desmoinesian to ~100 kyr in the Missourian and Virgilian, and faster rates of relative sea-level rise could have affected coal-ball abundance, particularly if coal balls formed in marine swamps, with more rapid transgression leading to thinner coals, or to freshwater coals directly overlain by marine sediments. Rygel et al. (2008) suggested that Missourian-Virgilian cycles recorded more erosional relief than Desmoinesian cycles, consistent with increased amplitude and rates of sea-level rise during each glacial-eustatic cycle. Changes in Missourian-Virgilian cycles may have been augmented by stratigraphic attenuation as sediment filled the antecedent topography.

Hydrodynamic controls on sediment retention in an emerging diversion-fed delta

The morphodynamics of river-dominated deltas are largely controlled by the supply and retention of sediment within deltaic wetlands and the rate of relative sea-level rise. Yet, sediment budgets for deltas are often poorly constrained. In the Mississippi River Delta, a system rapidly losing land due to natural and anthropogenic causes, restoration efforts seek to build new land through the use of river diversions. At the Davis Pond Freshwater Diversion, a new crevasse splay has emerged since construction was completed in 2002. Here, we use beryllium-7 activity in sediment cores and USGS measurements of discharge and turbidity to calculate seasonal sediment input, deposition, and retention within the vegetated Davis Pond receiving basin. In winter/spring 2015, which included an experimental period of high discharge through the diversion, Davis Pond received 106,800 metric tons of sediment, 44% of which was retained within the basin. During this time, mean flow velocity was 0.21 m s−1 and mean turbidity was 56 formazin nephelometric units (FNU). In summer/fall 2015, the Davis Pond basin received 35,900 metric tons of sediment, 81% of which was retained. Mean flow velocity in summer/fall was 0.10 m s−1 and mean turbidity was 55 FNU. The increase in sediment retention from winter/spring 2015 to summer/fall 2015 may be due in part to the corresponding drop in water flow velocity, which allowed more sediment to settle out of suspension. Although high water discharge increases sediment input and deposition, increased turbulence associated with higher current velocity appears to increase sediment throughput and thereby decrease the sediment trapping efficiency. Sediment retention in Davis Pond is on the high end of the range seen in deltaic wetlands, perhaps due to the enclosed geometry of the receiving basin. Future diversion design and operation should target moderate water discharge and flow velocities in order to jointly maximize sediment deposition and retention and provide optimal conditions for delta growth.

Measuring rates of present-day relative sea-level rise in low-elevation coastal zones: a critical evaluation

Abstract. Although tide gauges are the primary source of data used to calculate multi-decadal- to century-scale rates of relative sea-level change, we question the usefulness of tide-gauge data in rapidly subsiding low-elevation coastal zones (LECZs). Tide gauges measure relative sea-level rise (RSLR) with respect to the base of associated benchmarks. Focusing on coastal Louisiana, the largest LECZ in the United States, we find that these benchmarks (n=35) are anchored an average of 21.5 m below the land surface. Because at least 60 % of subsidence occurs in the top 5 m of the sediment column in this area, tide gauges in coastal Louisiana do not capture the primary contributor to RSLR. Similarly, global navigation satellite system (GNSS) stations (n=10) are anchored an average of &gt; 14.3 m below the land surface and therefore also do not capture shallow subsidence. As a result, tide gauges and GNSS stations in coastal Louisiana, and likely in LECZs worldwide, systematically underestimate rates of RSLR as experienced at the land surface. We present an alternative approach that explicitly measures RSLR in LECZs with respect to the land surface and eliminates the need for tide-gauge data in this context. Shallow subsidence is measured by rod surface-elevation table–marker horizons (RSET-MHs) and added to measurements of deep subsidence from GNSS data, plus sea-level rise from satellite altimetry. We show that for an LECZ the size of coastal Louisiana (25 000–30 000 km2), about 40 RSET-MH instruments suffice to collect useful data. Rates of RSLR obtained from this approach are substantially higher than rates as inferred from tide-gauge data. We therefore conclude that LECZs may be at higher risk of flooding within a shorter time horizon than previously assumed.