LITMIR.BIZ

FIGURE 4.1 Latitudinal distribution of sediment types and composition on the world’s inner continental shelves.

      Source: Flemming (2011), figure 9, p. 15. © Elsevier.

      The coast of the Guianas (French Guiana, Surinam, British Guiana) of South America is also characterised by migrating mud banks. The coast is bordered by the sources of mud in the region, in the east by the mouth of the Amazon, and in the west by the Orinoco River (Jolivet et al. 2019). Mud banks attached to the shore are gigantic (about 200 km²) and are composed of fluid mud, forming a temporary storage for silt and clay, and colonised and recolonised by mangroves after alternating episodes of accretion and erosion in relation to river discharge (Proisy et al. 2021). As off the SW coast of India, the subtidal nearshore seabed is shallow, gently sloped, and fronted by mangrove forests at the water’s edge, leading to similar wave and tidal current patterns which foster accretion and migration of fluid mud. These linear mud shoals change rapidly in space and time and are transported westward by wave‐induced currents on the inner shelf and by the Guiana Current on the outer shelf (Jolivet et al. 2019).

      Land‐use change is an important factor in changing mud coastlines. For example, along the Mahin mud section of the Nigerian coast, Gulf of Guinea, 58% of the coastline experienced serious erosion, with a rapid rate of coastline retreat triggering a land loss of 10.6 km² to the Atlantic over the last three decades (Dada et al. 2019). Although the changing wave climate had a strong influence on the observed patterns of erosion and accretion along the Mahin mud coast, both marine and anthropogenic processes act in unison to influence coastal retrogradation along the coast; mangrove deforestation has contributed greatly to the increased rate of retreat.

Tropical coastlines characterised by migrating mud banks are the exception rather than the rule. Excluding these areas and other regions such as off tropical river deltas, the Peruvian upwelling system, and the Bight of Biafra where highly reducing, sulphidic blue mud persists, the largest area of tropical shelves is sand dominated. Several shelves are dominated by carbonates and in many instances, bordered landward by extensive tidal flats, mangroves, and seagrasses or fringed seawards by coral reefs (Martini 2014).

      Modern carbonate shelves in the subtropics and tropics fall into two categories: (i) protected shelf lagoons (the Bahamas, Florida, Belize, Cuba, and the Great Barrier Reef) and (ii) open shelves (Yucatan, western Florida, the Persian Gulf, and northern Australia). Shelf lagoons are characterised by the presence of fringing barrier reefs, islands, and shoals, and commonly have across‐shelf gradients of mixed terrestrial‐carbonate deposits. On the Belize shelf, the Grand Bahamas Bank, and the Great Barrier Reef, the lagoons consist of gradients from inshore terrigenous quartz sand and mud, grading to mixed terrigenous/carbonate deposits and then to carbonate sand and mud out to the edge of the shelf (Vieira et al. 2019). Carbonate shelves are not limited to the low latitudes, but protected shelves are latitudinally restricted because only in the low latitudes has the production of carbonate at the shelf margin sufficient to keep pace with Holocene sea‐level rise. On many tropical shelves, soft sandy‐mud sediments derived from continental drainage dominate inshore areas with varying mixtures of quartz sand and carbonate sand deposits dominating the middle and outer shelf areas, the extent of which vary with shelf width (Vieira et al. 2019).

      Many coastal lagoons which formed behind barrier islands are either hypersaline in arid regions due to excessive evaporation (e.g. the Persian Gulf) or form gigantic interconnecting waterways in the humid tropics, as along the Gulf of Guinea off Africa. Sediment composition varies greatly among lagoons depending on their openness to the sea and the presence or absence of rivers and coastal vegetation. Lagoons in highly arid regions are often trapped with aeolian dunes that have been cemented by the precipitation of calcium carbonate, within which biogenic material is rapidly produced. Microbial stromatolites develop particularly in arid Indo‐Pacific areas (e.g. Shark Bay, Western Australia) where other biota is excluded by accretion of precipitated inorganic carbonates. The formation of cyanobacterial mats constitutes the final stages in the formation of coastal gypsum lakes.

      On sandy beaches, ecosystem structure and function are highly dependent on the beach type. Beaches are defined by the interactions among wave energy, tides, and the nature of the available sand (McLachlan and Defeo 2018). The beach slope is the simplest index of beach state and is the product of all three variables; beach face slopes flatten as wave energy or tidal range increases or particle size decreases, assuming other factors are kept constant. The flattest beach thus occurs in macro‐tidal regions of high wave energy and fine sand while the steepest beaches occur in micro‐tidal regions with low wave energy and coarse sand. A range of beach morphological types can be distinguished between these extremes.

In a micro‐tidal regime, beaches are wave dominated, and three beach types can be recognised: reflective, intermediate, and dissipative (Figure 4.2). A reflective beach is characterised by a steep face and absence of a surf zone with gentle wave and coarse sand. Dissipative beaches are characterised by a flat beach face and wide surf zone; waves break far out from the beach face and dissipate their energy while traversing the surf zone before expiring as swash on the beach face. Dissipative beaches are thus the product of large waves moving over fine sand. Between these two extremes are intermediate beaches distinguished by the presence of surf zones that are smaller than for dissipative beaches and are generally 20–100 m wide. The surf zone of an intermediate beach has well‐developed sand banks and channels with rip currents.