Mangrove Sediments and Geomorphology

Over the last 25 years considerable information on the geomorphological evolution of the world's largest coral reef system, the Great Barrier Reef, has become available. This book reviews the history of geomorphological studies of the Great Barrier Reef and assesses the influences of sea-level change and oceanographic processes on the development of reefs over the last 10,000 years. It presents analyses of recently attained data from the Great Barrier Reef and reconstructions of the sequence of events which have led to its current geomorphology. The authors emphasise the importance of the geomorphological time span and its applications for present management applications. This is a valuable reference for academic researchers in geomorphology and oceanography, and will also appeal to graduate students in related fields.

The Geomorphology of the Great Barrier Reef: Development, Diversity and Change

Hurricanes occur in belts 7° to 25° north and south of the equator. Reefs growing in these belts suffer periodic damage from hurricane-generated waves and storm surge. Corals down to 20m depth may be broken and removed, branching colonies being much more susceptible to breakage than upright massive forms. Sand cays may be washed away and former storm ridges may migrate to leeward across reef flats to link with islands. Reef crest and reef front coral debris accumulate as talus at the foot of the fore-reef slope, on submarine terraces and grooves, on the intertidal reef flat as storm ridges of shingle or boulders and isolated blocks of reef framework, as accreting beach ridges of leeward migrating shingle, as lobes and wedges of debris in back-reef lagoons, as drapes of carbonate sand and mud in deep off-reef locations in the fore-reef and lagoonal areas. In addition to the coarse debris deposited, other features may aid the recognition of former hurricane events, including the assemblage of reef biota, its species composition and the structure of the skeletons; graded internal sediments in framework cavities; characteristic sequences of encrusting organisms; characteristic shapes of reef flat microatoll corals; and submarine cement crusts over truncated reef surfaces. The abundance of reef flat storm deposits whose ages cluster around 3000–4000 y BP in certain parts of the world most likely relate to a slight fall in relative sea level rather than an increase in storminess during that period. A higher frequency of storms need not result in more reef flat storm deposits. The violence of the storm relative to normal fair-weather conditions influences the extent of damage; the length of time since the previous major storm influences the amount of coral debris created; the length of time after the hurricane, and before a subsequent storm influences the degree of stabilization of reef-top storm deposits and hence their chances of preservation.

The geological effects of hurricanes on coral reefs and the interpretation of storm deposits

There are extensive mangrove forests along the tropical shorelines of the world; they are particularly well developed in, though not exclusively restricted to, areas that are sheltered and muddy. Stratigraphic studies of the sediments underlying both the mangrove forests themselves and associated wetlands indicate that there have been considerable changes in the extent of these wetlands as a result of past sea-level fluctuations. Further changes in the area covered by mangroves are to expected if the sea rises at the rates anticipated as a result of the greenhouse effect. The exact response of a mangrove coastline will depend upon shoreline topography, sources of sediment, rates of sediment supply, and the rate of sealevel rise. The stratigraphy and chronology of Holocene deposits on mangrove shorelines allow reconstruction of the way that mangrove forests have responded

The impact of sea-level rise on mangrove shorelines:

This paper describes recent geomorphic change on reef islands of Ontong Java atoll, Solomon Islands and discusses the nature of the 'steady state' equilibrium that has been proposed for cays and motus. The effects of a hurricane in 1967 are analysed through detailed field mapping in 1971, 1972 and 1986, and with reference to evidence from earlier charts and air photographs. It is concluded that the hurricane represents a high-magnitude low-frequency event necessary for the long-term replenishment of sediment on shorelines, but that in the short term such storms will seem to have mainly destructive effects. Since 1967 rubble ramparts thrown up by the hurricane have been largely reworked into stable landforms, mainly through the agency of more frequent, lower-magnitude storms. In parts of the atoll where coarse sediment was not deposited in 1967 there is a widespread retreat of seaward beaches. Such coastal erosion does not necessarily imply that islands have been destabilized by vegetation removal, climatic change or a rising sea level. It is only by considering the long-term sediment budget and the relaxation time of landforms in relation to major hurricanes that decisions can be reached about whether particular reef islands are in equilibrium with process.

The role of hurricanes in the development of reef islands, Ontong Java atoll, Solomon Islands

This paper brings together the evidence from shallow coring, surface geomorphology, lithology of exposed rocks, superficial sediment accumulations, vegetation patterns, and the historical record derived from radiometric dating to suggest a sequence of reef and island development on the northern Great Barrier Reef in Holocene time. Reefs initially grew vertically as the sea rose rapidly from glacial low levels. This continued until vertical growth was limited by the air/sea interface as the rate of sea level rise slowed. Vertical growth was then replaced by reef flat formation at low intertidal levels, and by the lateral extension of reefs, especially to leeward. Superficial sediment accumulations on the reef flat define a series of changing habitats for further organic growth, and also record the sequence of Holocene events. Controls of the transition from vertical to horizontal reef growth will be discussed and some comments offered on latitudinal variation in reef form along the Great Barrier Reef.

Evolution of Reefs and Islands, Northern Great Barrier Reef: Synthesis and Interpretation

https://journals.uair.arizona.edu/index.php/radiocarbon/article/download/712/717

B. G. Thom

Papers

Evolution of Reefs and Islands, Northern Great Barrier Reef: Synthesis and Interpretation

Australian National University radiocarbon date list VIII.