Tropical Coasts Lectures

Coral reefs; characteristics and occurrence ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● Shallow, warm, clear tropical and subtropical marine settings High rates of in situ calcium carbonate framework and sediment production Coral reef sedimentary environment; active coral growth, reef accumulation and the associated carbonate sedimentary environments eg lagoons or seagrass beds Primary building blocks of reefs are the coral polyps themselves (the individual skeletons are corallites) Skeleton composed of bundles of aragonite needle cements Morphological groups and corallite structure defines taxonomy Calcification dependent upon amount of calcium and carbonate ions in seawater and interaction between respiration in corals and photosynthesis in zooxanthellae Aragonite saturation tropical average approx 4 but optimum for reef building >4 SST global average 20-18C From nearshore to open ocean; fringing reef, patch reef, bank barrier reef, barrier reef, atoll reef Fringing and bank barrier reefs; adjacent to coast, small, restricted narrow lagoons Patch reefs; small (meter scale) often within lagoons Barrier reef; large linear complexes, develop at shelf edges, separated from land by a wide lagoon Atolls; ringed reefs rising from deep ocean, deep central lagoon, no adjacent land, form as a succession driven by long-term subsidence of ocean floor Marked variations in reef geomorphology can also occur at intra-reef scales so we also describe reef habitats in terms of geomorphic zones (eg depth) and ecological zones Geomorphic zonation; different wave energies, depths, light regimes (back reef, reef crest, reef flat etc) Ecological zonation; spatial distribution of organisms across a reef, under optimum conditions a stratified community evident reflecting environmental controls Biological competition coral vs coral and coral vs algae (eg overgrowth by fast growing algae) Predation by organisms feeding directly on polyps such as molluscs or invasive species eg Crown of Thorns starfish Coral reefs: Holocene evolution and responses to sea-level change ● Long-term controls on reef development ○ Style and extent of reef development and rates of reef accretion are influenced by longer-term controls - rates and timing of past sea-level change, underlying substrate ○ Sea level influences the style of reef development (vertical or lateral accretion), the rate of reef accretion, the contemporary evolutionary state of reefs ○ Rate and timing of post glacial sea-level rise varied in different parts of the tropics ○ ○ ● ● Modern SL attained at different times in different regions In some regions SL was above present and has since fallen → magnitude of the highstand and the nature of the subsequent fall remain poorly understood → more sophisticated recent approaches have used multiple proxies → impact on geomorphology eg reef crests or reef flats which control reef ecology (species favoured) ○ Core records suggest different reefs have responded in different ways even within individual regions ○ In some regions there has been clear evidence for rapid jumps in SL linked to meltwater pulse events → some evidence for this leading to localised drowning of reefs or back stepping (relocation of reef growth further inshore or upshelf) ○ Corals require hard substrates to settle on thus following submergence there is an assumed time lag before reef establishment as lowstand soils reworked and removed → influences the timing of reef initiation ○ Topographic highs often inferred as a focal point for reef growth ○ Along the GBR the inner shelf has fluvial erosion and incision, the middle shelf has bedrock islands/high stands → the GBR was colonised from outer shelf inwards as sea level rose Reef growth models and rates of reef growth ○ Evidence that growth models for each (fringing, barrier, atolls) very varied Reef morphogenetic states ○ As reefs grow they pass through different stages of development (morphogenetic or evolutionary stages) → implications for contemporary zonation and habitat diversity ○ All reefs, under prevailing sea level states, have a finite life span → at some point they will reach sea level, infill their accommodation space and cease to accrete Reefs as sediment systems ● What processes and species produce reef sediment ○ Derived almost entirely from skeletal remains (thus mostly calcium carbonate and often aragonitic) ○ Distinct from temperate and cooler marine carbonate sediments ○ Broad variation in production evident between ocean basins reflecting marine conditions (carbonate sediment production responds to salinity, nutrients and turbidity) ○ Habitat scale differences in sediment reflect reef ecology ○ Physical breakdown produces coral rubble (gravel-grade clasts) ○ Biological erosion (eg parrotfish) excrete milled coral or coralline algae (fine sands to mud) ○ Derived from echinoids that breakdown into fragments, spines and teeth ○ Derived from green algae species that secrete carbonate in their tissues ○ Derived from epiphytic species living on seagrass blades that produce lightly calcified coralline encrusted and small foraminifera ○ ● ● In some settings externally derived sediments are also important eg where reefs develop close to sources of fluvial sediment discharge How much sediment is produced and how does this vary between reef environments ○ Spatial variations in sediment composition reflect both types of producers and rates of production ○ Important because it determines how much material is available to supply shorelines/islands and to feed into reef building ○ Calculations depends on species abundance which is not easy to measure and assumptions about turnover rates Sediment role in reef-development and reef landform construction ○ Reef islands are low lying <3m above mean SL accumulations of biogenically derived sediments that form above reefs and reef platforms ○ Sand cays island; sand dominated margin and interior but grain sizes can vary, dominant in areas of low cyclone frequency ○ Motus islands; cobble/gravel seaward margins and sandy lagoon ward shore, occur in areas of high cyclone frequency or along high energy margins of atolls ○ Two main settings for reef islands - atolls and barrier reefs ○ In atoll settings reef islands form both along margins and interior (but dependent on underlying geomorphology) ○ In barrier reef settings reef islands form along the barrier ○ Key controls on formation; physical foundation (accommodation space) and sediment supply, process regime eg the seasonality, frequency and intensity of storms Reef islands; form and development ● ● ● What are reef islands and where do they form When and how have reef islands formed Key controls on reef island formation and change ○ Physical foundation (accommodation space) and sediment supply ■ Past sea level history (various SL rise/fall scenarios under which islands can form) ■ Reef platform growth rates and/or rates of lagoon infilling ■ Presence of islands in Caribbean (with no SL highstand) suggest higher SL not a prerequisite ■ Reef size also is a control on rates of reef lagoon infilling ○ Process regime; seasonality, frequency and intensity of storms ■ To build islands waves/currents need to move sediment to points of accumulation ■ Reefs filter and focus wave energy governing nodal locations favourable for island accumulation ■ Sediment supply rates must remain high ■ Reef shape influences wave refraction and accumulation point position ■ ● Seasonal fluctuation in shoreline position as changes in wave direction differentially move sediment across reefs/around the islands ■ Impact of extreme events for accretion, wave overtopping and sediment movement during extreme events drive island vertical building How will island geomorphology change in the future ○ Two approaches; to look at how islands form/change under past and recent SL rise scenarios from field evidence, or by modelling ○ Remote sensing analysis reveals most exhibit little change <5% increase/decrease in area, only 30% islands changed more than 5% ○ Many recent analyses show islands maintaining size under SLR but changing position on the reef platforms