Spatial Developments in the Aruban Landscape Error! Reference source not found. January 2015 Landscape series: 3 A review of Geology, Climate and Hydrology in Aruba Photo by: Stan Norcom - Limestone Terraces along the Northeast shoreline From: www.lago-colony.com The Aruban landscape has undergone many changes in history. This paper is part of the landscape series: “Spatial Developments in the Aruban Landscape: A multidisciplinary GIS-based approach derived from geologic, historic, economic and housing information” 2016 1 Contents A review of Geology, Climate and Hydrology in Aruba ............................................................................................................ 1 Contents ..................................................................................................................................................................................... 2 Geological history of Aruban landforms ‘Great (Caribbean Islands) Arc’ 3 Basaltic Oceanic Plateau 3 Magmatic intrusions 3 Collision with South American Plate 3 Uplifting 3 Geomorphology of different rocks 4 Cretaceous Aruba Lava Formation (ALF) 4 Cretaceous Aruba Batholith 5 Dykes and Veins 5 Paleogene (Eocene) Lime stone deposits 6 Neogene Seroe Domi Formation 6 Quaternary Pleistocene Eolianite sanddunes 6 Quaternary Late Pleistocene Limestone Terraces 6 Paleoclimate records 6 Wet and dry periods 7 Current Climate Conditions 9 Temperature 10 Precipitation 10 Number of rain days 10 Humidity 10 Hydrogeological structure 10 Watershed and Salt Spray Park 10 Differences in the drainage pattern of dry-river beds 11 Tanki’s and Aquifers in the Batholith 12 Groundwater 13 Chemical and Physical weathering 13 Weathering in the Limestone Terraces 13 Pressure release, expansion cracks and weathering in the Batholith rocks 13 Physical weathering 14 Sheeting of rock 14 Stream sediments 14 Today’s influences on soil and groundwater 14 Soil enrichment and pollution in (sub) urban areas 15 In conclusion 15 Works Cited 16 Author: Ruud R. W. M. Derix, PhD Head department Spatial and Environmental Statistics Central Bureau of Statistics Aruba 2016 We would like to thank the following persons for their support and critical remarks: Geologist Armando Curet, Drs. – Director Compania Arubano di Petroleo N.V Palynologist Kees van Nooren, PhD student – Mexican Institute Cultural Anthropologist Luc Alofs, PhD – President Fundacion Herencia Cultural Inmaterial Aruba II This paper is part of a series on the developments that Basaltic Oceanic Plateau ~ 91Ma relate to the Aruban landscape. To bring perspective to A large basaltic flooding on the Farallon Plate, the current environmental threats and developments we Caribbean Large Igneous Province (CLIP), occurred below review in this paper the geological and (paleo)-climate sea level, at approximately 91-88 Ma ago, at a location off history of Aruba. Good knowledge of present but also of the coast of present-day Colombia in todays’ Pacific past processes is vital to understanding the effects that (White, Tarney, Klaver, & Ruiz, 1996). The Aruba-Curacao urbanization and economic progress pose on land and lava basement is a detached remnant part of this marine ecosystems. intrusion and the origin of both the Aruba Lava Formation (ALF) as well as the Curacao Lava Formation (CLF). In its Geological history of Aruban landforms relative movement eastwards it collided against the The tectonic history of the Caribbean is not yet fully Proto-Caribbean Ocean crust and against the Great understood. Different models attempt to explain the Caribbean Islands Arc. different processes involved, in particular with respect to the position and origin of the Caribbean plate. The most Magmatic intrusions ~ 88Ma popular theory explains the formation of the Caribbean As the Farallon Plate moved farther in between the North Plateau by the entrapment and relative movement (in and South American Plates, there was a reversal of the east-west direction relative to a fixed North American subduction zone. The Proto-Caribbean Ocean crust went continent) in between the separating and westward on now descending under the Farallon Plate and causing migrating North- and South American Plates1. Ahead of new magmatic activity, but now to the west of the the Plateau developed the Caribbean Great Arc by a series subduction zone. Consequently, about 3 Mio years (~88 of volcanic eruptions, relatively pushed eastwards in Ma) after the proto-Aruba Lava Formation had stopped between the separating North and South American intruding the Farallon plate and had been cooling, a new continents, forming todays’ Islands on the edge of the magmatic intrusion occurred on one section of the Caribbean Basin. An overview of this so-called Pacific- Oceanic Plateau, named the Aruba Batholith (see page 5). origin model is given by Pindell (2011) and Boschman The Batholith is typical for Aruba and is not present in (2014), and outlined below in order to gain better Curacao (or in Bonaire) (Lelij, et al., 2010). understanding of the history of Aruban geological features. Following an alternative, so-called in-situ model, The forces of the collision and magmatic intrusions caused originally proposed by James (2005), the Caribbean Great metamorphic rock formations in the older Aruba Lava Arc never existed and the origin of the Caribbean Basin Formation rocks (see page 4). In time, the Aruba-Curacao and the Islands in the region developed in-situ, i.e. they basement moved on the edge of the new Caribbean Plate origin from a Proto-Caribbean Ocean crust at about the towards the southern margins and underwent a complex place where these situate now and not in the distant series of deformations against the South American Pacific. However, evidence from Pacific type fauna in the continent. The Great Caribbean Island Arc moved further Caribbean and from the complex distribution and pattern on the edge of the deformation zone north and southeast. in geochemistry of old rocks against the South American The part that was to become the Greater Antilles moved continent favor the Pacific-origin model (references can all the way towards its current position, at the northeast be found in Lely et al., 2010). We describe the more border of the Caribbean Plate. popular models below. Collision with South American Plate ~ 75Ma ~ 93Ma ‘Great (Caribbean Islands) Arc’ At about 75-73 Ma at the margins of the forming In the Late Cretaceous, some 100-93 Ma (Mio yrs. ago; Caribbean Plate, accretion, convergence and thickening of see endnote), a volcanic islands arc system formed, the the oceanic crust took place. As the north-eastwards ‘Great Caribbean Islands Arc’ (the later Caribbean Islands), migrating Caribbean Plate collided with the South east of a subduction zone where the Pacific Ocean crust American Plate, the submarine basement of Aruba and (Farallon Plate) submerged under the Proto-Caribbean Bonaire (defined as discrete areas within a single Block) Ocean crust. As the North and South American Plates was deposited successively against the South American were separating and moving northwestwards this gap in Plate (first Aruba at about 70-60 Ma ago and later Bonaire between, the Proto-Caribbean Ocean crust, was so to say at ~50 Ma ago). The basements of Bonaire and Aruba/ in collision with the Pacific Ocean crust. The volcanic Curacao had a different origin, but were positioned next basement of Bonaire (Bonaire Washikemba Formation- to each other. Their distinct history from the collision, BWF) cropped out at the southern end of the Great Arc. strike slip displacements and accretion against the South Accordingly, the basement of Bonaire has in its earliest American Plate and renewed heating of the rocks (Lelij, et origin a different tectonic evolution then the Aruba - al., 2010) shaped a somewhat different landscape. Curacao basement (Lelij, et al., 2010) that is argued to have its origin more to the West as part of a basaltic Uplifting ~ 70Ma intrusion pushing upwards inside the Farallon Plate. At the margin of the Caribbean Plate, Aruba was positioned in a very complex zone with tectonic interactions. The relative motion of the Caribbean Plate 1 The movement of the Earth's continents relative to each other against the South American Plate caused a deformation zone with several plate fragments and blocks to strike and is called continental drift; after Wegener, 1912. The Caribbean slip against the South American Plate. The Aruba-Curacao- Plate was engulfed by westward migrating North and South Americas. Bonaire Block is such a block and estimated to be about 3 1,000 km long and 300 km wide. Along the thrust, strike Geomorphology of different rocks and slip faults, moments of subsistence and land uplift Most of the Aruban basement consists of solidified occurred. Figure 1 gives an impression of the many faults molten rock from magma that has its origin deep below that are still recognizable at the surface. The uplifting of the surface of the earth crust during different episodes. the leeward Antilles occurred 70-60 Ma ago, about 500 These rocks are generally called igneous rocks with km westwards from Aruba’s current relative location. volcanic origin (when the magma erupted and quickly During the uplifting, parts of the original Basalt formation, cooled as was the case with the Aruba Lava Formation) or came to the surface in all three islands. Evidence from plutonic origin (when later magmatic bursts remained paleo magnetic studies suggest that over subsequent within the earth's crust and cooled slowly, such as with displacements, the islands rotated clockwise at least 90° the Aruban Batholith). Plutonic rocks are more coarse- relative to a more stable South America continent grained than volcanic rocks and have larger crystals (references in Boschman et al. (2014)). because with the slow cooling, the minerals had more time to move and crystalize. Grain size and chemical Evidence is found today in the strike and slip fault composition of the rocks is an important determinant for patterns that run more or less parallel to the diffuse the resistance against later erosion. boundary between the two plates and A nice overview of geological studies in the Leeward evidence is also found in the direction Antilles, up to 1977, is compiled in the “Guide to the field of Quartz and calcite veins within excursions on Curacao, Bonaire and Aruba” in: GUA paper the rocks (Beardsley & Avé of Geology, Series 1 No 10. Lallemant, 2005). Today, in Aruba, the strike and slip Cretaceous Aruba Lava Formation (ALF) ~ 91Ma fault activity can still be So, the oldest rocks in Aruba have volcanic origin and are felt from shocks and from the time when the island arc was formed below sea temblors on a level off the coast of present-day Colombia during the quite regular Late Cretaceous, some 90-93 million years ago. Today, in basis2. the area of Parke Arikok we can still find some of the rocks (Pillow-lava) that formed below sea level (Arikoknieuws, 1999). Original basalt-dolerite rocks in the ALF are very fine-grained, hard and very resistant against weathering and erosion. This is why the old remains of the formation can still be seen in the landscape and also created an attractive hilly landscape with different types of rock at the surface (Figure 1 and 2). Hardness and a fine grain are typical features for rocks from lava eruptions with fast cooling and insufficient time for the complete crystallization of the minerals. Also found in the ALF are typical volcanic rock such as Tuff. These are ashes Figure and other erupted 1 The materials that are map shows compacted and the Aruban contour topography cemented into a (based on 1 m height rock. Another isoclines) and the location of type of rock dry-river beds (blue lines), 20th that can be century water holes (blue drops), and found are rainwater plains (green-blue colored areas). Red lines indicate the location of geological faults as is described by Beets (1996). The reconstruction of tectonic processes and movements of the Aruba-Curacao-Bonaire Block is difficult because of the complexity of interactions. Most of the block is submerged beneath the sea and most of the uplifted land is under thick layers of sediments. Also there is still Conglomerates that are formed from loose particles and continental growth of South America as the South other clast sediments cemented together by the heat of American Plate interacts with the Caribbean Plate (Curet, pressure underneath the earth crust. 1992). 2 http://es.earthquaketrack.com/p/aruba/recent 4 Most of these rocks however are transformed under the The big magmatic body, the Batholith, composes influence of pressure and heat3 and become predominantly of tonalite and quartz-diorite rocks5. metamorphic4 like schist rocks. Earlier smaller intrusions with Gabbro (near Bushiribana and Matividiri) and later with Hooibergite (hornblende- ~ 88Ma Cretaceous Aruba Batholith rich diorite) are also part of the Aruba Batholith. Most of The magma that cracked from under the partially these rocks have about the same origin but differ in solidified lava and intruded the older volcanic rocks is chemical composition and silicate (quartz) content. called the Aruba Batholith. Studies show that the The Hooibergite intrusion was one of the later thermal batholith intrusion occurred in a sequence pulses during the development of the Batholith. of several bursts not too long after Thus, the Hooiberg is not an old volcano even if it looks the Aruba Lava Fm. stopped like one, but the remains of a magmatic intrusion of the erupting (in less than 3 Ma) Aruba Lava Formation that contained relatively hard rock (Lelij, et al., 2010). Earlier material and survived deformation, uplifting, erosion and magma had not been weathering (van den Oever, 2000). cooled completely yet The large Diorite boulders in the Aruban landscape only (White, Tarney, exist in Aruba and clearly show the processes of physical Klaver, & Ruiz, and chemical weathering that we will discuss later. 1996). Dykes and Veins The contraction or expansion in rocks upon solidification, folding of land masses, earthquake shocks and line displacements cause fissures. Such fractures and cracks were later filled with magmatic intrusions and with sediments and minerals that solved in water and seeped into the cracks. Large longitudinal fissures are sometimes recognizable as narrow often straight-walled dykes that still exist because their harder material survived erosion better that than the original rock did. / Seroe Domi Formation Figure 2 GIS layer representation of the map by Beets (Beets, Metten, & Hoogendoorn, 1996). The batholith and the ALF are cross-cut by numerous such dykes but also by veins. Veins are similar in origin but distinct because they have irregular, shorter and discontinuous shapes. The diorite embedded quartz veins in Aruba are known as they sometimes contain gold ores 3 A nice introduction to Petrology, the study of the origin, (van den Oever, 2000). occurrence, structure and history of rocks, is found online: http://www.brocku.ca/earthsciences/people/gfinn/petrology/def n.htm 4 Rock metamorphism occurs when the original rock has been 5 The difference is based on quartz or SiO2 (Silica) content: subjected to high pressures and temperatures and has been quartz-diorite contains >5% quartz and tonalite contains >20% transformed into another form. quartz. 5 ~35- 24Ma Paleogene (Eocene) Lime stone deposits Quaternary Late Pleistocene Limestone Terraces ~ 0.6- 0.1Ma From the time of Late Cretaceous to Middle Miocene only The Pleistocene is commonly known for the alternating an incomplete record of coral fore-reef debris and periods of advance and retreat of the Arctic and Antarctic sediments from the South American mainland remained. ice cap. As a consequence of the advance and retreat of The oldest remains of the erosion sedimentation are from the ice sheets the sea levels slowly changed worldwide. Early Oligocene/Eocene (approx. 35 Ma), observable on The climate change came with dryer and colder climate the surface in Butucu. Slabs from a borehole in Oranjestad during Glacial and warmer and more humid climate (Helmers & Beets, 1977) reveal Eocene Limestone conditions during Interglacial. At the more regional level sediments from Early Miocene (approx. 24 Ma). however, climatic conditions may have varied more abruptly followed by prolonged periods of change in ~ 15- 0.5Ma Neogene Seroe Domi Formation regional temperature, precipitation and humidity. More articulate are the uplifted layered thick carbonate sediment depositions with underneath different A difference of approx. 120 m exists between the lowest limestone deposits that date from Early/Middle Miocene seawater highstand only some 18,000 yrs. ago at the end until probably the Middle to Late Pleistocene (approx. 15 of last Pleistocene glacial period and the highstand in Ma-0.5 Ma). This is the so-called Seroe Domi Formation current Holocene Interglacial (Lambeck, Yokoyama, & that typically consists of large flat multi-layers of Purcell, 2002). Today, global sea levels are still rising, but limestone coral debris with eroded earlier Reef and Fore less strong as in early Pleistocene and with only a few Reef from a Miocene high sea level stand. The Seroe meters since the middle Holocene (over the last 5,000 Domi Fm. exists in Aruba, Bonaire as well as in Curacao yrs.) (Hodell, et al., 1991). The cause of this sea level rise where it is more visible in the landscape. is not to be confused with the very recent sea level rise caused by global warming and the buildup of greenhouse In Aruba, the areas northeast of Pos Chiquito, Savaneta gasses in the atmosphere. and San Nicolas and an area east of Bubali Plas and at Seroe Cristal near the Northeast coastline (Figure 2) show The subsequent sea level cycles in the Caribbean during the remains of Seroe Domi Formation. Like in Curacao, the Pleistocene (following the Glacial and Interglacial periods) stratification of these layers appears at some places with a continuous uplift of the land, have created a clearly tilted. Today, it is the generally accepted view that staircase of coral reefs banks by the interplay of land the long-term processes of deformation and land uplift of uplift and reef growth and erosion. The terraces the Aruba-Curacao-Bonaire block against the South surrounding the Aruba Batholith and the Aruba Lava American continental margin has been accompanied by a Formation are the fossilized deposits and remains from folding and tilting of the Seroe Domi Formation complex. these processes, cemented together into sedimentary There is, however, still some debate about whether these limestone rock. Each of the raised shorelines is found to undulating limestone layers reflect the folding of a top correspond to a specific period of highstand of the sea earth crust (compression during early Pleistocene) or level (Eisenhauer & Blanchon, 2001). simply is the consequence of sedimentation along a dip in In Aruba, only a few terraces have remained at different the original sea bedding. In his PhD Thesis (1979), heights above sea level. Initial studies by Westermann Herweijer studied the Seroe Domi Formation and the (1932) and De Busonjé (1974) broadly discern a ‘Higher’, likely occurrence of compression after deposition of the ‘Middle’ and ‘Lower’ Terrace surrounding the Aruba thick carbonate sediment layers on top of the heavily Batholith\ ALF complex. These limestone terraces are the eroded Cretaceous basement (the remains of the remaining evidence of coral reef deposits during the final ALF/Batholith complex). phases of different Pleistocene high sea level stands. The continuous uplifting of the ALF/Batholith has brought Quaternary Pleistocene Eolianite sanddunes these Terraces above current sea level. The oldest terrace ~1.1- 0.1Ma In the National Parke Arikok and the area of Jaburibari we is situated on the highest grounds, but terrace building is find fossilized cliffs of Pleistocene eolianite6 rocks, i.e. a process that is still active today. solidified grains of former wind-blown sand dunes. These lime-sands hardened and fossilized into the eolianite Based on a more complete record of Terraces in Barbados limestone rocks. They are a reminder of the rich shallow- (Muhs, 2001), we know that in Aruba some terraces have marine life and coralgal communities with carbonate not survived and must have been eroded completely. The content that after depositing and surfacing have been thick Seroe Domi Fm. eroded and washed away for a large blown by the winds into undulating sand dunes part. Along the southwest coast, where waters had been (Herweijer, 1979). calmer, the Limestone deposits and erosion fields (and the remaining parts of the eroded Seroe Domi Fm.) however still cover most of the areas today. It is on the lowest and youngest of these terraces that most of the Aruban Aloe cultivation took place in early 20th century (see Figure 5). 6 Eolianite refers not to a specific time period but to the type of Paleoclimate records process that formed the rock, that is, Eolianite rocks found their Little information is available about the more recent origin in compaction of sediments that have been accumulated paleoclimate events that shaped the Aruban landscape. by wind into for instance coastal dunes (formed into coastal limestone or sand dunes). 6 Sediment core records show that in the past the winter seasons. The process behind these fluctuations landscape must have been quite different. was argued to be intense rainfall during the summer A pollen record from a bore hole in Oranjestad (Helmers seasons. & Beets, 1977) shows spores of ferns and palms (A. Curet, pers. comm., 2015), possibly from early Miocene origin Today, in contrast to the situation in Mid-Holocene, (23-15 Ma). Proxy7 studies, like palynological research marine hydrological conditions in the Southern Caribbean (pollen and spore research), oxygen isotope8 and more Sea are best characterized by the elevated evaporation in recently x-ray fluorescence measurements of sediment winter and the strong (wind-driven) oceanic surface components have shed some light on the paleoclimate currents that carry large freshwater concentrations from history in the Caribbean. We describe some major far away, from the seasonal discharge of the Orinoco and findings, next. the Amazon rivers. Wet and dry periods The dynamic relationship between the marine hydrology In a detailed study of sediment cores in Lake Miragoane, and the regional climate and the dominating effect Haiti, published in Nature (Hodell, et al., 1991), changes in thereof on local circumstances reflect a southwards shift the lake water levels have been reconstructed on the of the so-called Inter Tropical Convergence Zone12 (ITCZ) basis of oxygen isotope9 analyses. Information about past throughout the Holocene. wet and dry conditions was compared with a corresponding analysis of vegetation communities, based Aside from these general trends that we mentioned on pollen zonation research. The overall pattern in the above, Hodell (1991) described in-between climate ~ 5400 BP Haitian Lake samples shows that after the end of the last alterations that may have maintained over many years Glacial10, from early Holocene at about 10,500 BP11 up to and that seem to have been stronger than might be about 5,400 BP, precipitation levels and climate expected on the basis of the annual shifts in the received temperatures had increased. Hence, the circumstances in solar radiation13 alone. The study suggests that while the Caribbean have been (over thousands of years) much long-term fluctuations in received solar energy dictate wetter than today. From about 5,400 BP onwards, from climate and sea level changes (Glacial and Interglacial the mid to late Holocene up to today, climate turned to Periods) and consequently propel regional changes dry conditions. towards dry or wetter conditions, additional forces such as a drastic shift in the salinity of sea currents may have Nearer to Aruba, in Bonaire, another study, by Giri (2013), created temporal alterations in local climate (Metcalfe, shows similarly that during mid-Holocene local climate Barron, & Davies, 2015). was dominated by high and intense levels of precipitation, particularly during the summer. These findings were One such abrupt variation in climate conditions (thus a based on oxygen isotope analysis from (marine) coral large climatic change in a relatively short time span) ~ 8200 BP samples, off the coast in Bonaire. occurred at about 8,200 yrs. BP, when conditions Major deviations in the annual isotope ratios revealed suddenly turned more humid due to an increase in fluctuations in marine salinity between summer and precipitation (Hodell, Curtis, & Brenner, 1995). A rather ~ 3200 BP abrupt onset of dry conditions occurred in Haiti at about 3,200 BP and again at about 2,400 BP. Based on the 7 Climate proxy studies are based on preserved physical findings from the lake in Haiti, the ratio of ~ 2400 BP characteristics of the past that enable scientists today to precipitation/evaporation only switched back to the levels reconstruct the past climatic conditions. Definition: wikipedia.org from before 2,400 BP at about 1,500 BP (500 AD), 8 Oxygen Isotope analyses is based on the difference in weight indicating the end of period of temporary drought that between the light oxygen atom 16O (8 protons and 8 neutrons) lasted 900 years (Hodell, et al., 1991). and the heavy oxygen 18O (more neutrons). Since water molecules with oxygen16 are lighter these molecules will Beside information about local climate variations, there is evaporate more readily. The ratio between the two thus tells evidence that suggests a coincidence of climatic something about the conditions of evaporation or precipitation in fluctuations across the Caribbean. Recent studies, for the environment at the time of allocation in the sediments. 9 instance, confirmed that the fall of the Mayan Culture in Due to a difference in mass, differences in relative Oxygen isotope concentrations (O18/O16) express differences in Mexico was indeed consequential to repeated periods of ~ 1240 BP 18 a Caribbean-wide drought between about 1,240 BP (760 evaporation relative to precipitation (H O is a fraction heavier 2 16 AD) and 1,090 BP (910 AD) (Peterson & Haug, 2005). and precipitates easier that the lighter H O ). 2 10 During this time span there have been at least three Ice ages (Glacials) typically occur in intervals of about 40-100 Ma. In between there are shorter interglacial periods such as the current one (Holocene) that are characterized by a retreat of the 12 The ITCZ is the area encircling the earth near the equator ice sheets and a warmer favorable climate. As the ice sheets where the northeast and southeast trade winds come together. retreat, sea levels rise first rapidly but then gradually up to the The location of the ITCZ varies over time influenced by the sun's level of today. There is little change (3-5m) over the last 7,000 position and the differentially warming hemisphere (Schneider, yrs. (Lambeck, Yokoyama, & Purcell, 2002) The general believe is Bischoff, & Haug, 2014) (Haug, Hughen, Sigman, & Röhl, 2001). however that human action contributes significantly to recent 13 There is an annual shift in the received solar radiation that global warming and sea level rise reflects the slow shift of the annual orbit of the earth around the 11 BP indicates the timescale Before Present (dd. 1950) and is not sun. This movement dictates long-term but slowly varying levels to be confused with BC (Before Christ). 8,200 BP equals approx. in received solar radiation and these are largely responsible for 6,200 BC. the Glacial and Interglacial periods. 7 multiple-year periods of drought in the Yucatan Peninsula erosion) was observed together with charcoal14 remains, with an accumulation of the known social implications whereas such type of sediment layer was absent from the that led to the end of the Mayan Culture. The authors periods before. The presence of charcoal and clastic supported their conclusions on the basis of x-ray sediments coincided with a peak in the occurrence of Pal’i fluorescence measurements of trace elements and sia Cora pollen (Bursera simaruba) in these layers. The foraminifera data from cores in the Cariaco Basin, off the high occurrence of ‘Pal’i sia Cora’ pollen is indicative that coast of Venezuela (at quite some distance from the there was a dry tropical forest in the surroundings of the Yucatan). Their data coincided with the earlier findings by lagoon up to that time. The timeframe clearly Hodell in a lake in the Yucatan Peninsula, Mexico (Hodell, corresponds to the period of early colonization when Curtis, & Brenner, 1995), and provided very detailed deforestation occurred from the intense wood harvesting information about the shifts in precipitation and in marine and the free-roaming of herbivore grazers. Consequential salinity levels. increased erosion seems in line with these events. Such (preliminary) data also show that the inland bay in ~ 1200 BP The change towards generally dry conditions in the Spanish Lagoon was much larger in the past than it is Caribbean in Late Holocene (approx. ~1,200 BP) is also today. Mangrove forests (Rhizophora mangle) were supported by a study by Gregory et al. (2015) in two present already since approximately 7,000 BP. Pollen coastal lagoons in Cuba. Differential presence of specific analyses of the old mangrove peat indicate relatively wet foraminiferal assemblages reveals a shift in relative conditions (pers. comm. Nooren, 2015) with the presence lagoon water salinity while a shift in the composition of of many fern spores, which were absent in the younger trace elements in sediment core samples reveals a change deposits. in rainwater runoff into the lagoons corresponding to less Today, both bays are filled with sediments. The bay at precipitation. Boca Prins is almost completely filled with erosion material from the hills in the hinterland. In the Boca Prins A nice overview of the spatiotemporal pattern of climatic bay inlet there is only one thin organic layer found, at 3.3 fluctuations across Central America and the Caribbean m depth, that reveals a temporary presence of Mangrove during the Holocene (including an overview of underlying forests (Rhizophora mangle), whereas the remainder of studies), is given by Metcalfe (2015). The spatial, multi- the sediment core is almost exclusively from mineralogical annual, and even seasonal mapping of the climate in the content. Slabs suggest that the Mangrove vegetation was Caribbean is complex, however. Their summary of studies displaced by a more terrestrial tree species, Buttonwood suggests that wetter and drier conditions occur (Conocarpus erectus). Interesting is the fact that the core alternatively under the influence of factors, other than records from before the loss of Mangroves do also show the decline in seasonal insulation and the displacement of spores of fern that were absent in all the younger records. the ITCZ alone. The authors reason, that the successive Such abrupt destruction of mangrove assemblages and pulses of glacial ice water entering the region may have the alteration of pollen spectra reveal a subsequent an impact on the climate as well. The occurrence of domination of more dry and open hinterland pollen types. intense hurricane seasons, for instance, may also Findings by Engel et al. (2009) in Bonaire suggest the challenge a proper understanding of the paleoclimate occurrence of extreme wave events at about the same conditions as these may mask periods with relative time. Whether there is any concurrence of events may be drought (Frappier et al., 2014; referenced in Metcalfe et interesting to investigate in more detail. Such studies, al., 2015). however, suggest intense paleoclimate events that may have had a lasting impact on vegetation and landscape. Unfortunately, paleoclimate information from Aruba is scarce. It would be interesting to reconstruct more Data collected relatively near to Aruba, in the Cariaco precisely the climatic conditions that have dictated the Basin off the coast of Venezuela, show that dry conditions vegetation growth and fauna abundance in Aruba for with precipitation minima occurred from 3,800 to 2,800 instance during recent Holocene and be able to BP (Haug et al., 2001). The climate and vegetation cover ~ 3800 BP understand the current landscape more precisely. then may accordingly have been different from today15. Artifacts and large shell midden show that pre-ceramic A pilot study in Aruba (Nooren, 2008), based on pollen people may already have visited Aruba incidentally in analyses from sediment cores at the Boca Prins bay inlet Mid-Holocene, i.e. since ~4,000 BP16 (Versteeg & Ruiz, (Northeast coast) and at a site in the Spanish Lagoon 1995). Preceramic Indian inhabitants in Aruba had a (Southwest coast), suggests a change in vegetation type hunting/fishing and gathering lifestyle and mainly and cover in line with described climatic fluctuations. The occupied the coastal areas, including the Spanish Lagoon. characterization of the sediment cores from the pilot study even suggests evidence that relates to the dramatic impact by man on the landscape from deforestation, 14 Micro-charcoal remains in sediments associate clearly with grazing and heavy erosion in recent colonial times, as has fires. The distinction between natural and anthropogenic fire been described in literature already (Hartog, 1953). In the regimes is however difficult to establish and are cause for debate. youngest of sediment records from the Spanish Lagoon 15 Based on received solar radiation today we typically live in a site, for instance, clastic material (seen as evidence for relatively dry time zone 16 The Preceramic period in Aruba is estimated to start at about 4,000 BP and last until 1,000 BP. The Ceramic period is described to last from 1,000 BP to about 500 BP when Spanish colonization began. 8 The oldest dated shell deposits associated with human Current Climate Conditions occupation on the island were found in Rooi Bringamosa Regular monitoring of climatic conditions occurs since the near the Spanish Lagoon. A charcoal sample from this early 20th century (Meteorological-Yearbook, 1933-1972). shell layer in the pilot study of Nooren revealed an AMS17 We present in figure 3 and figure 4 climate measurements ~ 3500 BP 14C date of 3,260 +/-35 BP (calibrated age of 3,500 +/-50 from 2000-2011 at Reina Beatrix Airport in Aruba (Year yr. BP at 68.2% probability) (pers. comm. Nooren, 2015, reports Statistics of the Meteorological Observations in unpublished data). Nooren also confirms that fossil pollen The Netherlands Antilles: 1955 - 1972). Earlier data can be and spores from that time span identify plant species that found at www.meteo.aw. are now hardly found (pers. comm., 2015). Figure 3 a-d (top) show climatological records collected at Reina Interesting is the fact that Ceramic (Dabajuran) Indians Beatrix Airport during the period 2000-2011. The four graphs arrived at about 1,200 BP (800 AD) in Northern Venezuela show the average monthly records for temperature, total and at about 1,100 BP (900 AD) in Aruba (NAM, 1999). It is precipitation, number of rain days and relative humidity. possible that climate change-induced migration (Gupta, Note: A rain day is a day with at least 1 mm rain. Anderson, Pandey, & Sanghvi, 2006) (Laczko & Aghazarm, Source: Statistical Yearbooks 2000-2011. CBS, Aruba Figure 4 a-d (below) show climatological records collected at 2009) played a role here as well, and, that the onset of Reina Beatrix Airport during the period 2000-2011. The four dryer climate conditions, such as occurred at about 3,800 graphs show the average annual records for temperature, total BP and again at about 1,200 BP, was coincidental to the precipitation, number of rain days and relative humidity. timing of settlement of the new arrivers. Average monthly temperature (°C) Average monthly precipitation (mm) 2000 - 2011 2000 - 2011 30 200 28 100 Figure 3a Figure 3c 26 0 Jan. Feb.Mar.Apr. May Jun. Jul. Aug.Sep. Oct.Nov.Dec. Jan. Feb.Mar.Apr. May Jun. Jul. Aug.Sep. Oct.Nov.Dec. Average monthly number of rain days Average monthly relative humidity (%) (N) 2000 - 2011 15 80 2000 - 2011 10 75 5 0 Figure 3b 70 Figure 3d Jan. Feb.Mar.Apr. May Jun. Jul. Aug.Sep. Oct.Nov.Dec. Jan. Feb.Mar.Apr. May Jun. Jul. Aug.Sep. Oct.Nov.Dec. Figure 4a Figure 4c Figure 4b Figure 4d 17 Accelerator mass spectrometry (AMS) is a type of carbon dating technique. 9 Temperature Over the period 2000 to 2011 the average monthly In Aruba, we observe little variation in daily temperature humidity was highest in the month November (77.7%) and (Figure 3a). Values fluctuate between an average low of lowest in August (74.2%), but the difference remains small about 27 °C during the coldest months, January and (Figure 3d). February, up to 30°C during the hot summer months, Sea surface temperature is an important determinant for August and September. Year averages remain quite precipitation (Karmalkar, 2013). Measurements of sea uniform as well in the range of 28°C and 29°C (Figure 4a). surface temperatures in the Southern Caribbean show that the highest annual sea surface temperature in the Precipitation Southern Caribbean is in September18 (no CBS data In contrast to the slight fluctuation in temperature the available), which is consistent with the timing of the onset fluctuation in annual precipitation is more extreme of the wet season. (Figure 4b). Over the 10 year period, extremes occur in The descriptions of rainfall above provide no information 2001 (137mm) and in 2004 (906mm). Considerable about the intensity of rainfall. Commonly, rains pour fluctuation exists between consecutive years. The latest down in heavy short showers. Under such conditions the decennium appears to have been quite wet at Queen runoff is strong and the effect of erosion on top soils Beatrix Airport, as we have to go back to the mid-50s to severe. In particular when logging and land clearance has observe similar levels of rainfall. Rainfall in 1955 and 1956 just taken place, heavy rains amass into large brownish (respectively 816 and 679mm average annual rainfall) is runoff streams that carry topsoil and sediments towards comparably high to the level of rainfall in 2004 (906 mm), the sea. 2010 and 2011 (respectively 906 and 826mm). The average annual rainfall over the period 2000-2011 is 587.9 Hydrogeological structure mm and that is well above the long-term average of 410 Next, we will review and describe some hydro- mm over the long period 1953-1972. morphological processes in Aruba to better understand the effects of erosion, weathering and soil formation. Number of rain days Earlier, we described how the Aruban geological Precipitation is strongly influenced by the presence of formation has undergone tectonic displacement, uplifting, tropical storms and/or hurricanes in the region. sea level rises, and deformation activities. Subterranean Therefore, it is important to realize that successive years temperature and pressure regimes during its formation with heavy rainfall as in the recent decade can be caused distinct differences between rocks in morphology, deceiving and do not necessarily represent a change in mineral composition and physico-chemical characteristics. local climate conditions. These differences in resistance against the influences Rainfall generally peaks in November-December (rainy from sea, sun, wind and rain have shaped the relief season), but in the rest of the year significant number of patterns in the Aruban landscape as we know it today. We rainy days exist as well (Figure 3b and 3c). describe some of these relief patterns in the For instance, August 2011 had 211.6 mm of rain but this is ALF/Batholith complex and Pleistocene terrace landscape exceptional. The month of April is generally with the least and explain how processes of erosion, weathering and rain, though it is not necessarily the driest month. sedimentation created new opportunities for soil development and vegetation (Finkel & Finkel, 1975). Interesting to note is that in contrast to the north-western Caribbean there are no distinct two rainy seasons. The Watershed and Salt Spray Park precipitation pattern in Aruba is a unimodal late annual Interestingly, because of the elevated topography at the rainy season as described for south-eastern Caribbean windward side, the central watershed line19 (Figure 6) is at islands. a close distance of only 0.6 km from the Northeast coastline and up to approximately 4 km from the August and September are considered the hot and dry Southeast coastline. With the exception of the more summer months (March is considered dry as well) and central area, the watershed line roughly defines the November and December are the coolest and wettest border of an intended Salt Spray Park that is envisioned to months. protect and cover the relatively still untouched natural environment along the Northeastern coastline (DIP, Finkel and Finkel (1975) analyzed total rainfall per location 2009). In the North and in the South, the watershed areas and estimated rainwater drainage area. They suggested a east of the central watershed line have little or no history descending gradient in rainfall from Southeast to in agricultural activity20 and are still sparsely inhabited Northwest in Aruba, in concurrence with the orientation today. Current housing projects, however, develop of the Northeast Passat winds and the descending beyond former agricultural terrain and advance towards gradient in height topography of the island. the Northeastern coastal zone. Humidity Average annual humidity ranges between approx. 73% 18 www.meteo.aw and 78% (compare Figure 4b, 4c and 4d) and roughly co- 19 The watershed is an imaginary line that separates one drainage varies with annual number of rain days (ranges between basin from another one. A drainage basin or catchment area 47 and 97 days) and annual precipitation (ranges between covers the total land area that is drained by a single (dry-) river approx. 350 mm and 950 mm). system. 20 For information on the original agricultural extent in 1911, we refer to another paper in this landscape series (Derix R., 2016d). 10