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CHAPTER 4 Tropical Marine Geosphere
4.1 Major Sedimentary Patterns
Because most riverine discharge to the oceans occurs in the tropics and that glacial sedimentation occurs at the poles, there are clear latitudinal patterns in sediment types and composition on the world’s continental shelves (Figure 4.1). The major factors responsible for these global patterns are weathering, the presence or absence of large rivers and coral reefs, glaciation, tectonics, authigenic mineral and detrital formation, and ice‐rafting (Flemming 2011). Recent changes in sedimentary habitats are due to extensive degradation from coastal development, reduced sediment delivery from rivers due to damming, increased coastal erosion, and sea‐level rise. Mud and coral/carbonate deposits of biogenic origin are most abundant in the tropics, whereas sand is globally dominant, decreasing with higher latitudes to be proportionally displaced by glacial rock and gravel (Figure 4.1).
The latitudinal pattern of inner shelf sediment types is somewhat deceptive because a large proportion of mud in the tropics occurs in proximity to the major rivers, particularly the deltaic systems of the Amazon, Orinoco, Mekong, Ganges, and Brahmaputra Rivers. Muds along coastlines near major river plumes are stirred up sufficiently due to cyclones that disrupt wave trains and form mud banks. This phenomenon occurs predominantly in the tropics and is well documented along the coast of SW India (Muraleedharan et al. 2018), northern South America (Proisy et al. 2021), and northern Australia (Caitcheon et al. 2012). On the Kerala coast of SW India, certain inshore areas produce zones of calm water by dampening wave action. Large quantities of riverine‐derived matter in colloidal suspension lead to the dissipation of wave energy. These mud banks, locally known as ‘chekara’, extend over areas of at least 25 km2 and are characterised by silty‐clay sediments, oxygen‐deficient bottom waters, and possibly by generation of gases. These ‘chekara’ are generally 1–2 m thick.
Formation and migration of the Kerala coast mud banks are associated with high‐period waves and their refraction pattern along the sea bottom (Muraleedharan et al. 2018). The size of each bank is determined by the location of converging intertidal currents and offshore flow. The mud bank is supplied continuously with mud from both directions and compensates for losses due to settlement and from export by currents moving offshore. Particulate nutrient concentrations in mud bank deposits are generally high and positively correlated with decreasing grain size.
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,
