VERTICAL ACCRETION IN MARSHES WITH VARYING RATES OF SEA LEVEL RISE

Persistence of tidal wetlands under conditions of sea level rise depends on vertical accretion of organic and inorganic matter, which vary in their relative abundance across estuarine gradients. We examined the relative contribution of organic and inorganic matter to vertical soil accretion using lead-210 (210Pb) dating of soil cores collected in tidal wetlands spanning a tidal freshwater to brackish gradient across a Chesapeake Bay subestuary. Only 8 out of the 15 subsites had accretion rates higher than relative sea level rise for the area, with the lowest rates of accretion found in oligohaline marshes in the middle of the subestuary. The mass accumulation of organic and inorganic matter was similar and related (R2 = 0.37). However, owing to its lower density, organic matter contributed 1.5–3 times more toward vertical accretion than inorganic matter. Furthermore, water/porespace associated with organic matter accounted for 82%–94% of the total vertical accretion. These findings demonstrate the key role of organic matter in the persistence of coastal wetlands with low mineral sediment supply, particularly mid-estuary oligohaline marshes.

Download Full-text

Thresholds of mangrove survival under rapid sea level rise

Science ◽

10.1126/science.aba2656 ◽

2020 ◽

Vol 368 (6495) ◽

pp. 1118-1121 ◽

Cited By ~ 14

Author(s):

N. Saintilan ◽

N. S. Khan ◽

E. Ashe ◽

J. J. Kelleway ◽

K. Rogers ◽

...

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Carbon Sink ◽

Vertical Accretion ◽

Mangrove Forests ◽

Relative Sea Level ◽

Relative Sea Level Rise ◽

Emissions Scenarios ◽

Organic Sediments ◽

Timing Of Initiation

The response of mangroves to high rates of relative sea level rise (RSLR) is poorly understood. We explore the limits of mangrove vertical accretion to sustained periods of RSLR in the final stages of deglaciation. The timing of initiation and rate of mangrove vertical accretion were compared with independently modeled rates of RSLR for 78 locations. Mangrove forests expanded between 9800 and 7500 years ago, vertically accreting thick sequences of organic sediments at a rate principally driven by the rate of RSLR, representing an important carbon sink. We found it very likely (>90% probability) that mangroves were unable to initiate sustained accretion when RSLR rates exceeded 6.1 millimeters per year. This threshold is likely to be surpassed on tropical coastlines within 30 years under high-emissions scenarios.

Download Full-text

Processes Influencing Autocompaction Modulate Coastal Wetland Surface Elevation Adjustment With Sea-Level Rise

Frontiers in Marine Science ◽

10.3389/fmars.2021.694039 ◽

2021 ◽

Vol 8 ◽

Author(s):

Kerrylee Rogers ◽

Neil Saintilan

Keyword(s):

Organic Matter ◽

Sea Level Rise ◽

Sea Level ◽

Carbon Storage ◽

Surface Elevation ◽

Vertical Accretion ◽

Elevation Change ◽

Mean Sea Level ◽

Accommodation Space ◽

Surface Elevation Change

The fate of coastal wetlands and their ecosystem services is dependent upon maintaining substrate elevations within a tidal frame that is influenced by sea-level rise. Development and application of morphodynamic models has been limited as few empirical studies have measured the contribution of key processes to surface elevation change, including mineral and organic matter addition, autocompaction of accumulating sediments and deep subsidence. Accordingly, many models presume that substrates are in equilibrium with relative sea-level rise (RSLR) and the composition of substrates are relatively homogenous. A 20-year record of surface elevation change and vertical accretion from a large tidal embayment in Australia coupled with analyses of inundation frequency and the character of sediments that have accumulated above mean sea level was analyzed to investigate processes influencing surface elevation adjustment. This study confirms the varying contribution of addition, decomposition and compression of organic material, and mineral sediment consolidation. Autocompaction of substrates was proportional to the overburden of accumulating sediments. These processes operate concurrently and are influenced by sediment supply and deposition. Vertical accretion was linearly related to accommodation space. Surface elevation change was related to vertical accretion and substrate organic matter, indicated by carbon storage above mean sea level. Surface elevation change also conformed to models that initially increase and then decrease as accommodation space diminishes. Rates of surface elevation change were largely found to be in equilibrium with sea-level rise measured at the nearest tide gauge, which was estimated at 3.5 mm y–1 over the period of measurements. As creation of new accommodation space with sea-level rise is contrary to the longer-term history of relative sea-level stability in Australia since the mid-Holocene, striking stratigraphic variation arises with deeper sediments dominated by mineral sands and surficial sediments increasingly fine grained and having higher carbon storage. As the sediment character of substrates was found to influence rates of surface elevation gain, we caution against the unqualified use of models derived from the northern hemisphere where substrates have continuously adjusted to sea-level rise and sediment character is likely to be more homogenous.

Download Full-text

Vertical Accretion and Relative Sea Level Rise in the Ebro Delta Wetlands (Catalonia, Spain)

Wetlands ◽

10.1007/s13157-010-0092-0 ◽

2010 ◽

Vol 30 (5) ◽

pp. 979-988 ◽

Cited By ~ 43

Author(s):

Carles Ibáñez ◽

Peter James Sharpe ◽

John W. Day ◽

Jason N. Day ◽

Narcís Prat

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Ebro Delta ◽

Vertical Accretion ◽

Relative Sea Level ◽

Relative Sea Level Rise

Download Full-text

Caribbean Reef Drowning During Slow Mid-Holocene Sea-Level Rise

10.31219/osf.io/tfz8g ◽

2017 ◽

Author(s):

Paul Blanchon ◽

José Estrada ◽

Simon Richards ◽

Juan-Pablo Bernal ◽

Sergio Cerdeira-Estrada ◽

...

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Surf Zone ◽

Sediment Flux ◽

Vertical Accretion ◽

Acropora Palmata ◽

Coral Recruitment ◽

Drowned Reefs ◽

Dune Ridge ◽

High Sediment

Predicting if reefs can keep pace with future sea-level (SL) rise is problematic because accretion occurs over geological timescales. For example, although meltwater pulses drowned reefs during postglacial SL rise, drowning has also been reported during the mid Holocene, when SL rise was slower and meltwater pulses unlikely. Here we report the discovery of a drowned incipient reef-crest on the widest part of northeast Yucatan shelf. Our data show the reef is an array of closely-spaced patches that crest at -14 m. These patches consist of 3 m-thick stands of Acropora palmata that grew over a Pleistocene dune ridge ~8 ka ago, but subsequently failed to keep pace with rising SL. We hypothesize that this failure is due to the suppression of coral recruitment by high sediment flux on wide shelves. Although recruitment does occur as the surf zone crosses topographic residuals during the transgression, suppression in sandy reef-fronts and back-reefs limits accretion of these incipient crests, causing their slow drowning during SL rise. We conclude that although reefs can slowly develop breakwaters in such settings, they may be incapable of vertical accretion and thus vulnerable to drowning. Identifying reefs with this vulnerability will be key to managing future SL rise.

Download Full-text

Projected Responses of Tidal Dynamics in the North Sea to Sea-Level Rise and Morphological Changes in the Wadden Sea

Frontiers in Marine Science ◽

10.3389/fmars.2021.685758 ◽

2021 ◽

Vol 8 ◽

Author(s):

Christian Jordan ◽

Jan Visscher ◽

Torsten Schlurmann

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

North Sea ◽

Wadden Sea ◽

Morphological Changes ◽

Vertical Accretion ◽

Tidal Dynamics ◽

Intertidal Flats ◽

The North ◽

The North Sea

This study explores the projected responses of tidal dynamics in the North Sea induced by the interplay between plausible projections of sea-level rise (SLR) and morphological changes in the Wadden Sea. This is done in order to gain insight into the casual relationships between physical drivers and hydro-morphodynamic processes. To achieve this goal, a hydronumerical model of the northwest European shelf seas (NWES) was set-up and validated. By implementing a plausible set of projections for global SLR (SLRRCP8.5 of 0.8 m and SLRhigh−end of 2.0 m) by the end of this century and beyond, the model was run to assess the responses of the regional tidal dynamics. In addition, for each considered SLR, various projections for cumulative rates of vertical accretion were applied to the intertidal flats in the Wadden Sea (ranging from 0 to 100% of projected SLR). Independent of the rate of vertical accretion, the spatial pattern of M2 amplitude changes remains relatively stable throughout most of the model domain for a SLR of 0.8 m. However, the model shows a substantial sensitivity toward the different rates of vertical accretion along the coasts of the Wadden Sea, but also in remote regions like the Skagerrak. If no vertical accretion is assumed in the intertidal flats of the Wadden Sea, the German Bight and the Danish west coast are subject to decreases in M2 amplitudes. In contrast, those regions experience increases in M2 amplitudes if the local intertidal flats are able to keep up with the projected SLR of 0.8 m. Between the different scenarios, the North Frisian Wadden Sea shows the largest differences in M2 amplitudes, locally varying by up to 14 cm. For a SLR of 2.0 m, the M2 amplitude changes are even more amplified. Again, the differences between the various rates of vertical accretion are largest in the North Frisian Wadden Sea (> 20 cm). The local distortion of the tidal wave is also significantly different between the scenarios. In the case of no vertical accretion, tidal asymmetry in the German estuaries increases, leading to a potentially enhanced sediment import. The presented results have strong implications for local coastal protection strategies and navigation in adjacent estuaries.

Download Full-text

Impacts of poldering: elevation change, sediment dynamics, and subsidence in the natural and human-altered Ganges Brahmaputra tidal deltaplain

10.5194/egusphere-egu21-13928 ◽

2021 ◽

Author(s):

Carol Wilson ◽

Sharmin Akter ◽

Masud Rana ◽

Michael Steckler ◽

Bar Oryan

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Well Being ◽

Dry Season ◽

Surface Elevation ◽

Direct Result ◽

Vertical Accretion ◽

Elevation Change ◽

Cyclonic Storm ◽

The Past

<p>In the Ganges-Brahmaputra Delta (GBD) and other tide-dominated low-lying regions, periodic tidal and cyclonic storm surge flooding of the land surface promotes sediment accretion and surface elevation gain which offsets elevation losses from eustatic sea level rise and subsidence. However, over the past several decades, anthropogenic modification of the GBD tidal deltaplain through embankment construction has precluded sediment delivery to densely populated embanked islands, locally-termed polders, resulting in landscapes 1-1.5 m lower than adjacent natural mangrove platforms. Recent discussion on GBD sustainability includes whether land surfaces (natural or anthropogenic) are keeping pace with local sea-level rise rates, and the quantification of continued elevation change, vertical accretion, and land subsidence. To provide local-scale, longitudinal trends of landscape dynamics, an array of Rod Surface Elevation Tables (RSETs) and sediment marker horizons was deployed in natural and embanked settings near Polder #32 and monitored seasonally over the past 6 years (expanded throughout the SW delta in 2019). These data are compared to existing and new co-located continuous GPS measurements (also expanded 2019). Near Polder #32, elevation gain is taking place in both natural and embanked regions (1-3 cm/yr), though it appears to be slightly greater (30%) within the poldered areas. This may be due to increased accommodation space and/or embankment sloughing. There also is a distinct seasonal pattern in both regions, with greater elevation change documented after the wet monsoon season (May-Sept), and either less elevation gain, or even elevation loss after the winter dry season (Jan-May). Elevation gain is a direct result of exceptionally large sediment vertical accretion (2-3 cm/yr), as measured from marker horizons and sediment tiles, and rates appear to be keeping pace with local effective sea-level rise documented by Pethick and Orford (2013). Seasonal shallow subsidence (0.8-1.1 cm/yr) is also observed, exacerbated in poldered regions during the dry season. These measurements of shallow subsidence are 30-50% greater than deeper subsidence measured with GPS (0.3-0.7 cm/yr) but consistent with resurveys of geodetic monuments (see Steckler et al. abstract). Preliminary results delta-wide show shallow subsidence can be as much as 3 cm over the course of one year. These data provide critical information to local stakeholders about the natural versus human-altered delta dynamics, and have cross-disciplinary implications for ecological productivity, social well-being, and flood risk mitigation.</p>

Download Full-text

Glacial Contributions to 21st Century Sea Level Rise

Eos ◽

10.1029/2020eo143700 ◽

2020 ◽

Vol 101 ◽

Author(s):

Kate Wheeling

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

21St Century ◽

Mass Change ◽

Sources Of Uncertainty

Researchers identify the main sources of uncertainty in projections of global glacier mass change, which is expected to add about 8–16 centimeters to sea level, through this century.

Download Full-text