Jamaica is situated in the northwest region of the Caribbean Sea, one of the four of the Greater Antilles. Fringing reef systems extend over much of the north coast, very important natural ecosystems of Jamaica (Goreau, 1992), (Moses, 2008). To gain a greater a greater understanding these reef environments of the geology, hydrogeology and anthropogenic land use changes governing their formation and health must be evaluated. Through this report I will look to dissect these processes from past and present occurring within these coastal environments.
2. Major Geological features of Jamaica
The island itself is split into two main sections. The east is dominated by the Blue Mountains, rising to a height of 2,256 km. This is separated from the west by a plateau of cretaceous crystalline and limestone outcrops, the Central Inlier (Donovan, 2002). The west is much flatter, rising to a maximum height of roughly 900m and is mostly composed of limestone, often referred to as the Cockpit Country (Moses, 2008).
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Figure 1. Simplified geoligcal map of Jamaica, with vertical cross section. Shows the dominance of the white limetone formations and the crystalline outcrops in the east (Brown et al, 1973).
The whole island was formed through four phases. Firstly the cretaceous volcanic island arc phase, followed by rifting of the arc system in the Palaeocene to early Eocene. Next carbonates were vastly deposited in the middle Eocene to Miocene. Finally deformation in the Cenozoic led to major uplift creating the Blue Mountain region in the west (Mitchell, 2002).
The Central Inlier is divided into four distinct sections. The Arthurs Seat Formation is the oldest, composed of volcanic sediment and poorly sorted conglomerates with successions of intruding dykes and small plutons. This is overlain by the Crift Synthem composed of limestone. The Slippery Rock Formation is composed of pebble conglomerates, this is part of the younger sedimentary sequence of the Kellites Synthem and tertiary white and yellow limestone groups. The final formations are the Guinea Corn Formation which is succeeded by the Summerfield Group. These consist of shallowing upwards marine and volcaniclastic sedimentary rock (Mitchell, 2002), (Mitchell 2005).
Early rifting continued throughout the Early Palaeogene, forming extensive block and trough topographies. Yellow and white limestone groups were deposited during the deep water and shallow water successions within these blocks (Mitchell, 2002), (Mitchell, 2005).
Carbonates were deposited vastly over the edge of the Clarendon Basin, within protected lagoonal environments (Mitchell, 2005). The major growth of coral environments initially started into the late Oligocene, shelf edge environments providing the home for extensive reefs. The early Pleistocene was characterised by high clastic input in to the shallow coastal regions creating unfavourable conditions. Regeneration occurred in the late Pliocene and early Pleistocene away from regions of high turbidity and sediment input (Donovan, 2002).
3. Hydrology of Jamaica
3.1 Karstic landscape
The hydrology of Jamaica is controlled by two main factors; the geology of the region, mostly karstic formations, secondly the level of precipitation throughout the year (Nkemdirim 1979).
Karst regions have very characteristic hydrological features, due to the ability of water to dissolve the calcium carbonate of the limestone. This dissolution leads to the formation of extensive underground networks, allowing surface rivers and streams to disappear into an underground labyrinth of caves and dissolution channels (Fleurant, 2008). These karstic formations dominate in the Cockpit Country of Jamaica, composed Eocene White Limestone formations (Fleurant, 2008). Cave, sinkhole and gorge formations occur throughout the region, another product of vast dissolution of these limestone's (Donovan, 2002). A large proportion of this water doesn't re-surface until these drainage networks reach the coast. Submarine vents and surface fissures allow the fresh groundwater to resurface, often within embayment regions such as Discovery Bay (Greenway, 2006), (Brown et al, 1973).
Rainfall is also incredibly important, imperative for the recharge of these karstic networks. Precipitation controls the recharge of the groundwater, governing how much freshwater is discharged at the coast. This freshwater provides the main nutrient supply to the coastal embayment's, therefore is highly influential on the biological productivity of the reefs (Greenway, 2006).
There are two distinct wet and dry periods throughout the year in Jamaica. The first wet period occurs in May, with 167mm of precipitation on average falling over the month, the second in October, with 166mm of rainfall falling. The dry seasons fall between the months of February to April, and the second from June to August, with between 57-91mm over each month respectively (Creary, 2012). The Blue Mountain regions tend to receive significantly more rainfall than the rest of Jamaica, around 7.5mm of precipitation falls over the course of the year (Berglund & Johanson, 2004).
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It's important to point out that the rainfall over the year is incredibly variable, rainfall patterns may not necessarily conform to these wet and dry seasons (Greenway, 2006).
Figure 3. Relationship between rainfall and runoff within Jamaica, shows the high importance of rainfall (Nkedirim, 1979
Figure 2. Mean monthly maximum and minimum air temperature and rainfall within Discovery Bay, over (?) (Creary, 2012)
4.Bauxite mining and deforestation
Aluminium is one of the world's most important metals. In the form of its ore, bauxite, it is found in significant quantities within Jamaica. The bauxite industry is incredibly important for the Jamaican economy, making up around 20% of the country's total income (Moses, 2008). As of 2004 only Australia and Guinea produced more bauxite than Jamaica (Berglund & Johanson, 2004).
Throughout the island there's thought to be over 1.8 billion tonnes of bauxite, of which 1 billion tonnes is able to be accessed with relative ease (Berglund & Johanson, 2004). Most of these deposits reside in the White Limestone Groups in the central plateau region (Grant et al, 2005), where sharp basal contacts occur (Donovan, 2002).
There are three main theories to why these bauxite deposits have formed within Jamaica. The 'Residual Theory', secondly the 'Alluvial Theory, but it's the third theory, 'the Volcanic Ash Theory' that's now generally accepted. Here bauxite is thought to have formed through the weathering of igneous rocks formed during the eruptions in the Cenozoic which was deposited on the top of the karstic limestone (Donovan, 2002).
Although economically important, the bauxite mining has some serious implications on the Jamaican environment. The mining processes involve the extensive removal of soil to create open cast mines so the bauxite can be easily accessed. The soil that's removed is replaced after the completion of the mining. However the properties of this soil are often highly affected, with local vegetation suffering as water retention is poor (Berglund & Johanson, 2004).
Deforestation is also a major problem. It's necessary to remove significant quantities of local forest to make way for new access roads to the mining area. This doesn't just lead to the destruction of local habitats, but the large extent of this deforestation may be altering the hydrosphere of the region. Consequently it's proposed an increase in abnormal rainfall patterns and more severe draughts could be occurring (Berglund & Johanson, 2004).
Figure 4. Major land use changes between 1989 to 1998, showining major forest change, and large increase in bauxite mining (Berglund & Johanson, 2004)
5. Hydrography of the northern Jamaican Waters
Jamaica is dominated by North East Trade Winds, mainly due to both the relief of the island and its latitude. In general these winds will remain calm throughout the night (Gayle & Woodley, 1998). At around 09.30am the winds begin to pick up as the solar heating increases. This steady sea breeze remains constant throughout the day, subsiding at around 6p.m. as the temperatures cool (Moses, 2008).
Figure 5. Change in wind speed throughout the day within Discovery Bay Jamaica, shows clear trend of increasing wind speeds throughout daylight hours, starting around 9pm and diminishing around 6pm (Genovese & Witman, 2004)
5.2.1 Caribbean currents
There are several strong currents and ocean gyre systems surrounding Jamaica and the Caribbean.
There are three main currents, these include; the Yucatan Current, the Loop Current and the Florida Current. The main gyre is found in the Columbian Basin, which is cyclonic in nature. This cyclonic gyre creates 'drifter' currents, which move North West over the Jamaican Ridge. This passage of water results in the formation of two strong jests which leads to the formation of the Yucatan Current (Centurioni & Niiler, 2003).
In Northern Jamaica, wave action is mostly low due to the protection of the fringing reefs, with currents flowing in a westerly direction (Wade, 1991), (Brucks, 1971).
5.2.2 Discovery Bay currents
Embayment's seem to have their own current circulation system, primarily controlled by wind patterns, therefore are prominent during the daytime. This becomes apparent as flow rates in the back reef during the day are larger than those during the night. Two key forces act upon the currents, the waters move from east to west within the bay through the force of the Caribbean westerly currents, whilst the trade winds create waves which travel into the bay from the northeast. These two forcing create circular currents that rotate clockwise within the bay itself (Genovese & Witman, 2004).
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Figure 6. Major ocean currents surrounding the Jamaican coastline, shows the dominance of westerly surface currents (Goreau, 1992)
Sediment transport within Northern Jamaican coastal regions is primarily controlled by the currents plus the inflow within the coastal embayments. The trade winds and westerly currents creating the clockwise currents within the bay can often re-suspend the sediment within the bay when they are strong (Gayle & Woodley, 1998). However Discovery Bay has no fluvial input, therefore no terrigenous sediment supply other than from wind-blown sources from off of land and the bauxite loading plant in the west of the bay. This leads to iron, manganese and aluminium rich material in the west corner of the bay, particularly through wind-blown sediment. Hotspots of bauxite accumulate in the North East and North West of the loading terminal (Perry et al, 2006).
Jamaica dominated by White Limestone formations formed in the Tertiary, making up the large karstic formations of the cockpit country in the West, whilst higher, older crystalline rocks dominate the east with the Blue Mountains (Donovan, 2002).
Hydrology of Jamaica shows characteristic karst groundwater flow (Fleurant, 2008), with over 80% (ECLA) of human supply coming from these sources. Level of groundwater outflow is controlled by the level of precipitation, which tends to re-submerge at the coastal embayment networks (Greenway, 2006).
Bauxite mining is the main industrial processes within Jamaica, contributing to over 20% of the country's total income (Moses, 2008). This has led to deforestation within the country, overall there has been over a 50% reduction in forested areas (Berglund & Johanson, 2004).
The Trade winds dominate throughout the island, winds pick up along the coast in the late morning and last until 6pm (Moses, 2008).
Main currents dominating Caribbean waters are the Yucatan Current, the Loop Current and the Florida Current, including a cyclonic gyre found in the Columbian Basin (Centurioni & Niiler, 2003). The north coast is dominated by westerly currents, where the fringing reefs protect the coast from battering waves (Genovese & Witman, 2004).
Sediment transport follows the main current flows, with industrial bauxite activity influencing the sediment loading within Discovery Bay itself (Perry et al, 2006).