Chapter 7. Calcium Carbonate Production and Contribution to Coastal Sediments
Contributors: Colin D. Woodroffe, Frank R. Hall, John W. Farrell an Peter T. Harris (Lead member)
1. Calcium carbonate production in coastal environments
Biological production of calcium carbonate in the oceans is an important process Although carbonate is produced in the open ocean (pelagic, see Chapter 5), thi chapter concentrates on production in coastal waters (neritic) because thi contributes sediment to the coast through skeletal breakdown producing sand an gravel deposits on beaches, across continental shelves, and within reefs. Marin organisms with hard body parts precipitate calcium carbonate as the minerals calcit or aragonite. Corals, molluscs, foraminifera, bryozoans, red algae (for example th algal rims that characterize reef crests on Indo-Pacific reefs) are particularl productive, as well as some species of green algae (especially Halimeda). Upo death, these calcareous organisms break down by physical, chemical, and biologica erosion processes through a series of discrete sediment sizes (Perry et al., 2011) Neritic carbonate production has been estimated to be approximately 2.5 Gt year (Milliman and Droxler, 1995; Heap et al., 2009). The greatest contributors are cora reefs that form complex structures covering a total area of more than 250,000 km (Spalding and Grenfell, 1997; Vecsei, 2004), but other organisms, such as oysters may also form smaller reef structures.
Global climate change will affect carbonate production and breakdown in the ocean which will have implications for coastal sediment budgets. Rising sea level wil displace many beaches landwards (Nicholls et al., 2007). Low-lying reef islands calle sand cays, formed over the past few millennia on the rim of atolls, are particularl vulnerable, together with the communities that live on them. Rising sea level ca also result in further reef growth and sediment production where there are health coral reefs (Buddemeier and Hopley, 1988). In areas where corals have already bee killed or damaged by human activities, however, reefs may not be able to keep pac with the rising sea level in which case wave energy will be able to propagate mor freely across the reef crest thereby exposing shorelines to higher levels of wav energy (Storlazzi et al., 2011; see also Chapter 43).
Reefs have experienced episodes of coral bleaching and mortality in recent year caused by unusually warm waters. Increased carbon dioxide concentrations are als causing ocean waters to become more acidic, which may affect the biologica production and supply of carbonate sand. Bleaching and acidification can reduc coral growth and limit the ability of reef-building corals and other organisms t produce calcium carbonate (Kroeker et al., 2010). In some cases, ocean acidificatio may lead to a reduced supply of carbonate sand to beaches, increasing the potentia for erosion (Hamylton, 2014).
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1.1 Global distribution of carbonate beaches
Beaches are accumulations of sediment on the shoreline. Carbonate organisms particularly shells that lived in the sand, together with dead shells reworked fro shallow marine or adjacent rocky shores, can contribute to beach sediments Dissolution and re-precipitation of carbonate can cement sediments formin beachrock, or shelly deposits called coquina. On many arid coasts and islands lackin river input of sediment to the coast, biological production of carbonate is th dominant source of sand and gravel. Over geological time (thousands of years) thi biological source of carbonate sediment may have formed beaches that ar composed entirely, or nearly entirely, of calcium carbonate. Where large river discharge sediment to the coast, or along coasts covered in deposits of glacial til deposited during the last ice age, beaches are dominated by sediment derived fro terrigenous (derived from continental rocks) sources. Carbonate sediment comprise a smaller proportion of these beach sediments (Pilkey et al., 2011).
Sand blown inland from carbonate beaches forms dunes and these may be extensiv and can become lithified into substantial deposits of carbonate eolianite (wind blown) deposits. Significant deposits of eolianite are found in the Mediterranean Africa, Australia, and some parts of the Caribbean (for example most of the islands o the Bahamas). The occurrence of carbonate eolianites is therefore a useful proxy fo mapping the occurrence of carbonate beaches (Brooke, 2001).
Carbonate beaches may be composed of shells produced by tropical to sub-pola species, so their occurrence is not limited by latitude, although carbonate productio on polar shelves has received little attention (Frank et al., 2014). For example Ritchie and Mather (1984) reported that over 50 beaches in Scotland are compose almost entirely of shelly carbonate sand. There is an increase in carbonate conten towards the south along the east coast of Florida (Houston and Dean, 2014) Carbonate beaches, comprising 60-80 per cent carbonate on average, extend fo over 6000 km along the temperate southern coast of Australia, derived fro organisms that lived in adjacent shallow-marine environments (James et al., 1999 Short, 2006). Calcareous biota have also contributed along much of the wester coast of Australia; carbonate contents average 50-70 per cent, backed by substantia eolianite cliffs composed of similar sediments along this arid coast (Short, 2010) Similar non-tropical carbonate production occurs off the northern coast of Ne Zealand (Nelson, 1988) and eastern Brazil (Carannante et al., 1988), as well a around the Mediterranean Sea, Gulf of California, North-West Europe, Canada, Japa and around the northern South China Sea (James and Bone, 2011).
On large carbonate banks, biogenic carbonate is supplemented by precipitation o inorganic carbonate, including pellets and grapestone deposits (Scoffin, 1987). Bal (1967) identified marine sand belts, tidal bars, eolian ridges, and platform interio sand blankets comprising carbonate sand bodies present in Florida and the Bahamas This is also one of the locations where ooids (oolites) form through the concentri precipitation of carbonate on spherical grains. Inorganic precipitation in the Persia Gulf, including the shallow waters of the Trucial Coast, reflects higher wate temperature and salinity (Purser, 1973; Brewer and Dyrssen, 1985).
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1.2 Global distribution of atolls
The most significant social and economic impact of a possible reduction in carbonat sand production is the potential decrease in supply of sand to currently inhabited low-lying sand islands on remote reefs, particularly atolls. Atolls occur in the war waters of the tropics and subtropics. These low-lying and vulnerable landforms ow their origin to reef-building corals (see Chapter 43 which discusses warm-wate corals in contrast to cold-water corals dealt with in Chapter 42). The origin of atoll was explained by Charles Darwin as the result of subsidence (sinking) of a volcani island. Following an initial eruptive phase, volcanic islands are eroded by waves an by slumping, and gradually subside, as the underlying lithosphere cools an contracts. In tropical waters, fringing coral reefs grow around the volcanic peak. A the volcano subsides the reef grows vertically upwards until eventually the summi of the volcano becomes submerged and only the ring of coral reef (i.e., an atoll) i left behind. The gradual subsidence can be understood in the context of plat tectonics and mantle “hot spots”. Many oceanic volcanoes occur in linear chain (such as the Hawaiian Islands and Society Islands) with successive islands being olde along the chain and moving into deeper water as the plate cools and contract (Ramalho et al., 2013).
Most atolls are in the Pacific Ocean (in archipelagoes in the Tuamotu Islands Caroline Islands, Marshall Islands, and the island groups of Kiribati, Tuvalu an Tokelau) and Indian Ocean (the Maldives, the Laccadive Islands, the Chago Archipelago and the Outer Islands of the Seychelles). The Atlantic Ocean has fewe atolls than either the Pacific or Indian Oceans, with several in the Caribbean (Vecsei 2003; 2004). The northernmost atoll in the world is Kure Atoll at 28°24' N, alon with other atolls of the northwestern Hawaiian Islands in the North Pacific Ocean The southernmost are the atoll-like Elizabeth (29°58' S) and Middleton (29°29' S Reefs in the Tasman Sea, South Pacific Ocean (Woodroffe et al., 2004). Th occurrence of seamounts (submarine volcanoes) is two times higher in the Pacifi than in the Atlantic or Indian Oceans, explaining the greater frequency of atolls.
Corals, which produce aragonite, are the principal reef-builders that shape an vertically raise the reef deposit, and there are secondary contributions from othe aragonitic organisms, particularly molluscs, as well as coralline algae, bryozoans an foraminifera which are predominantly made of calcite. Carbonate sand and gravel i derived from the breakdown of the reef. Bioerosion is an important process in reefs with bioeroders, such as algae, sponges, polychaete worms, crustaceans, se urchins, and boring molluscs (e.g., Lithophaga) reducing the strength of th framework and producing sediment that infiltrates and accumulates in the porou reef limestone (Perry et al., 2012). Erosion rates by sea urchins have been reporte to exceed 20 kg CaCO; m™” year™ in some reefs, and parrotfish may produce 9 k CaCO; m” year (Glynn, 1996). Over time, cementation lithifies the reef. Wherea the reef itself is the main feature produced by these calcifying reef organisms, loos carbonate sediment is also transported from its site of production. Transporte sediment can be deposited, building sand cays. Broken coral or larger boulder eroded from the reef by storms form coarser islands (termed motu in the Pacific) Sand and gravel can be carried across the reef and deposited together with fine mud filling in the lagoon (Purdy and Gischler, 2005).
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Carbonate production on reefs has been measured by at least three differen approaches; hydrochemical analysis of changes in alkalinity of water moving across  section of reef, radiometric dating of accretion rates in reef cores, and census-base approaches that quantify relative contributions made by different biota (includin destruction by bioeroders). These approaches indicate relatively consistent rates o ~10 kg CaCO; m” year™ on flourishing reef fronts, ~4 kg CaCO3 m” year“ on ree crests, and <1 kg CaCO3 m” year” in lagoonal areas (Hopley et al., 2007; Montaggion and Braithwaite, 2009; Perry et al., 2012; Leon and Woodroffe, 2013). These rate have been described in greater detail in specific studies (Harney and Fletcher, 2003 Hart and Kench, 2007), and have been used to produce regional extrapolations o net production (Vecsei, 2001, 2004).
2. Changes known and foreseen —sea-level rise and ocean acidification.
Several climate change and oceanographic drivers threaten the integrity of fragil carbonate coastal ecosystems. Anticipated sea-level rise will have an impact on th majority of coasts around the world. In addition, carbonate production is likely to b affected by changes in other climate drivers, including warming and acidification Tropical and subtropical reefs would appear to be some of the worst affecte systems. However, it is also apparent that already many degraded systems can b attributed to impacts from social and economic drivers of change; pollution overfishing and coastal development have deteriorated reef systems and man severely eroded beaches can be attributed to poor coastal management practices.
2.1 Potential impacts of sea-level rise on beaches
Sea-level rise poses threats to many coasts. Between 1950 and 2010, global sea leve has risen at an average rate of 1.8 + 0.3 mm year’; approximately 10 cm o anthropogenic global sea-level rise is therefore inferred since 1950. Over the nex century, the mean projected sea-level rise for 2081-2100 is in the range 0.26-0.54  relative to 1986-2005, for the low-emission scenario (RCP 2.6). The rate of rise i anticipated to increase from ~3.1 mm year™ indicated by satellite altimetry to 7-1 mm year by the end of the century (Church et al., 2013). The rate experienced o any particular coast is likely to differ from the global mean trend as a result of loca and regional factors, such as rates of vertical land movement or subsidence. Beac systems can be expected to respond to this gradual change in sea level, and the low lying reef islands on atolls appear to be some of the most vulnerable coastal system (Nicholls et al., 2007).
Based on predictions from the Bruun Rule, a simple heuristic that uses slope of th foreshore and conservation of mass, sea-level rise will cause erosion and ne recession landwards for many beaches (Bruun, 1962). Although this approach ha been widely applied, it has been criticized as unrealistic for many reasons, includin that it does not adequately incorporate consideration of site-specific sediment
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budgets (Cooper and Pilkey, 2004). Few analyses consider the contribution o biogenic carbonate and none foreshadow the consequences of any reduction i supply of carbonate sand. This is partly because of time lags between production o carbonate and its incorporation into beach deposits, which is poorly constrained i process studies and which is subject to great variability between different coasta settings, ranging from years to centuries (Anderson et al., 2015). In view o uncertainties in rates of sediment supply and transport, probabilistic modeling o shoreline behavior may be a more effective way of simulating possible responses including potential accretion where sediment supply is sufficient (Cowell et al. 2006).
2.2 Potential impacts of sea-level rise on reef islands
Small reef islands on the rim of atolls appear to be some of the most vulnerable o coastal environments; they are threatened by exacerbated coastal erosion inundation of low-lying island interiors, and saline intrusion into freshwater lense upon which production of crops, such as taro, depends (Mimura, 1999). Sand cays on atolls as well as on other reefs, have accumulated incrementally over recen millennia because reefs attenuate wave energy sufficiently to create physicall favourable conditions for deposition of sand islands (Woodroffe et al., 2007), as wel as enabling growth of sediment-stabilizing seagrasses and mangrove ecosystem (Birkeland, 1996). Sand cays are particularly low-lying, rarely rising more than a fe metres above sea level; for example, <8 per cent of the land area of Tuvalu an Kiribati is above 3 m above mean sea level, and in the Maldives only around 1 pe cent, reaches this elevation (Woodroffe, 2008). This has led to dramatic warnings i popular media and inferences in the scientific literature that anthropogenic climat change may lead to reef islands on atolls submerging beneath the rising ocean, wit catastrophic social and economic implications for populations of these atoll nation (Barnett and Adger, 2003; Farbotko and Lazrus, 2012).
However, reef islands may be more resilient than implied in these dire warning (Webb and Kench, 2010). Unlike the majority of temperate beaches that have a finit volume of sediment available, biogenic production of carbonate sediments mean that there may be an ongoing supply of sediment to these islands. Although coral is  major contributor, it is not necessarily the principal constituent of beaches; larg benthic foraminifera (particularly Calcarina, Amphistegina and Baculogypsina contribute more than 50 per cent of sediment volume on many islands on Pacifi atolls (Woodroffe and Morrison, 2001; Fujita et al., 2009). One survey of Pacific cora islands (Webb and Kench, 2010) reported that 86 per cent of islands had remaine stable or increased in area over recent decades, and only 14 per cent of island exhibited a net reduction in area; however, the greatest increases in area resulte from artificial reclamation (Biribo and Woodroffe, 2013). Further studies of shorelin changes on atoll reef islands using multi-temporal aerial photography and satellit imagery indicate accretion on some shorelines and erosion on others, but with th most pronounced changes associated with human occupation and impacts (Rankey 2011; Ford, 2012; Ford 2013; Hamylton and East, 2012; Yates et al., 2013).
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The impacts of future sea-level rise on individual atolls remain unclear (Donner 2012). Healthy reef systems may be capable of keeping pace with rates of sea-leve rise. There is evidence that reefs have coped with much more rapid rates of ris during postglacial melt of major ice sheets than are occurring now or anticipated i this century. Reefs have responded by keeping up, catching up, or in cases of ver rapid rise giving up, often to backstep and occupy more landward location (Neumann and Macintyre, 1985; Woodroffe and Webster, 2014). Geologica evidence suggests that healthy coral reefs have exhibited accretion rates in th Holocene of 3 to 9 mm year” (e.g., Perry and Smithers, 2011), comparable t projected rates of sea-level rise for the 21* century. However, reef growth is likely t lag behind sea-level rise in many cases resulting in larger waves occurring over th reef flat and affecting the shoreline (Storlazzi et al., 2011; Grady et al., 2013). It i unclear whether these larger waves, and the increased wave run-up that is likely, wil erode reef-island beaches, overtopping some and inundating island interiors, o whether they will more effectively move sediments shoreward and build ridge crest higher (Gourlay and Hacker, 1991; Smithers et al., 2007). Dickinson (2009) inferre that reef islands on atolls will ultimately be unable to survive because once sea leve rises above their solid reef-limestone foundations, which formed during the mid Holocene sea-level highstand 4,000 to 2,000 years ago, formerly stable reef island will be subject to erosion by waves.
2.3 Impact of climate change and ocean acidification on production
The impact of climate change on the rate of biogenic production of carbonat sediment is also little understood, but it seems likely to have negative consequences Although increased temperatures may lead to greater productivity in some cases, fo example by extending the latitudinal limit to coral-reef formation, ocean warmin has already been recognised to have caused widespread bleaching and death o corals (Hoegh-Guldberg, 1999; Hoegh-Guldberg, 2004; Hoegh-Guldberg et al., 2007) Ocean acidification will have further impacts, and may inhibit some organisms fro secreting carbonate shells; for example reduction in production of the Pacific oyste has been linked to acidification (Barton et al., 2012). Decreased seawater p increases the sensitivity of reef calcifiers to thermal stress and bleaching (Anthony e al., 2008). Based on the density of coral skeleton in >300 long-lived Porites coral from across the Great Barrier Reef, De’ath et al. (2009) inferred that a decline i calcification of ~14 per cent had occurred since 1990 manifested as a reduction i the extension rate at which coral grows, which they attributed to temperature stres and declining saturation state of seawater aragonite (which is related to a decreas in pH). However, this extent of the apparent decline has been questioned because o inclusion of many young corals (Ridd et al., 2013); it is not observed in coral collected more recently from inshore (D’Olivo et al., 2013).
There has been some debate about the role of carbonate sediments acting as  chemical buffer against ocean acidification; in this scenario, dissolution o metastable carbonate mineral phases produces sufficient alkalinity to buffer pH an carbonate saturation state of shallow-water environments. However, it is apparen that dissolution rates are slow compared with shelf water-mass mixing processes such that carbonate dissolution has no discernable impact on pH in shallow waters
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that are connected to deep-water, oceanic environments (Andersson and Mckenzie 2012). The seawater chemistry within a reef system can be significantly differen from that in the open ocean, perhaps partially offsetting the more extreme effect (Andersson et al., 2013; Andersson and Gledhill, 2013). Corals have the ability t modulate pH at the site of calcification (Trotter et al. 2011; Venn et al. 2011; Falte et al., 2013). Internal pH in both tropical and temperate coral is generally 0.4 to 1. units higher than in the ambient seawater, whereas foraminifera exhibit no elevatio in internal pH (McCulloch et al., 2012).
Changes in the severity of storms will affect coral reefs; storms erode some islan shorelines, but also provide inputs of broken coral to extend other islands (Marago et al., 1973; Woodroffe, 2008). Alterations in ultra-violet radiation may also have a impact, as UV has been linked to coral bleaching. Furthermore, if reefs are not in  healthy condition due to thermal stress (bleaching) coupled with acidification an other anthropogenic stresses (pollution, overfishing, etc.), then reef growth an carbonate production may not keep pace with sea-level rise. This could, in the long term, reduce carbonate sand supply to reef islands causing further erosion, althoug ongoing erosion of cemented reef substrate is also a source of sediment on reefs indicating that supply of carbonate sand to beaches is dependent upon severa interrelated environmental processes. Disruption of any one (or combination) of th controlling processes (carbonate production, reef growth, biological stabilization bioerosion, physical erosion and transport) may result in reduction of carbonat sand supply to beaches.
3. Economic and social implications of carbonate sand production.
More than 90 per cent of the population of atolls in the Maldives, Marshall Islands and Tuvalu, as well many in the Cayman Islands and Turks and Caicos (which all hav populations of less than 100,000), live at an elevation <10 m above sea level an appear vulnerable to rising sea level, coastal erosion and inundation (McGranahan e al., 2007). The social disruption caused by relocating displaced people to differen islands or even to other countries is a problem of major concern to many countrie (Farbotko and Lazrus, 2012, see also Chapter 26). Beach aggregate mining is a small scale industry on many Pacific and Caribbean islands employing local people (McKenzie et al., 2006), but mining causes environmental damage when practised o an industrial scale (Charlier, 2002; Pilkey et al., 2011, see also Chapter 23). In th Caribbean, illegal beach mining is widespread but there is little information on wha proportion is carbonate (Cambers, 2009). Beach erosion reduces the potentia opportunities associated with tourism (see Chapter 27), and decreases habitat fo shorebirds and turtles (Fish et al., 2005; Mazaris et al., 2009).
Without coral reefs producing sand and gravel for beach nourishment and protectin the shoreline from currents, waves, and storms, erosion and loss of land are mor likely (see also Chapter 39). In Indonesia, Cesar (1996) estimated that the loss due t decreased coastal protection was between 820 United States dollars (for remot areas) and 1,000,000 dollars per kilometre of coastline (in areas of major touris infrastructure) as a consequence of coral destruction (based on lateral erosion rates
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of 0.2 m year™, and a 10 per cent discount rate [similar to an interest rate] over  25-year period). In the Maldives, mining of coral for construction has had sever impacts (Brown and Dunne, 1988), resulting in the need for an artificial substitut breakwater around Malé at a construction cost of around 12,000,000 dollar (Moberg and Folke, 1999).
4. Conclusions, Synthesis and Knowledge Gaps
There has been relatively little study of rates of carbonate production, and furthe research is needed on the supply of biogenic sand and gravel to coastal ecosystems Most beaches have some calcareous biogenic material within them; carbonate is a important component of the shoreline behind coral-reef systems, with reef island on atolls entirely composed of skeletal carbonate.
The sediment budgets of these systems need to be better understood; direc observations and monitoring of key variables, such as rates of calcification, would b very useful. Not only is little known about the variability in carbonate production i shallow-marine systems, but their response to changing climate and oceanographi drivers is also poorly understood. In the case of reef systems, bleaching as a result o elevated sea temperatures and reduced calcification as a consequence of ocea acidification seem likely to reduce coral cover and production of skeletal material Longer-term implications for the sustainability of reefs and supply of sediment t reef islands would appear to decrease resilience of these shorelines, althoug alternative interpretations suggest an increased supply of sediment, either becaus reef flats that are currently exposed at low tide and therefore devoid of coral, ma be re-colonized by coral under higher sea level, or because the disintegration of dea stands of coral may augment the supply of sediment.
Determining the trend in shoreline change, on beaches in temperate settings and o reef islands on atolls or other reef systems, requires monitoring of beach volumes a representative sites. This has rarely been undertaken over long enough time periods or with sufficient attention to other relevant environmental factors, to discern  pattern or assign causes to inferred trends. Although climate and oceanographi drivers threaten such systems, the most drastic erosion appears to be the result o more direct anthropogenic stressors, such as beach mining, or the construction o infrastructure or coastal protection works that interrupt sediment pathways an disrupt natural patterns of erosion and deposition.
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