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View of Bahamas Geology 5 1 ABSTRACT GEOCHEMISTRY OF THE PLEISTOCENE AQUIFER, NORTHEASTERN ANDROS ISLAND, BAHAMAS By Derek Wayne Dice In this study, the geochemistry and mineralogy of several rock cores from the upper 12 meters from Andros Island were evaluated along with water samples collected at specific depths from within the aquifer. Mineralogy was determined by x-ray diffraction, and the elemental composition (major and minor elements) was examined in selected rock samples by DCP spectrometry. The elemental composition of the groundwater was determined by DCP spectrometry, high-performance liquid chromotography, and in-situ pH. By comparing the Ca/Sr ratios of both the groundwater and the limestone along with saturation states of the groundwater, we concluded that dissolution was the dominant process operating in the upper, freshwater lens. By linking petrology, rock geochemistry, water geochemistry, and the location of sea-level, we were able to gain a better understanding of the progress and stages of diagenesis that are presently ongoing in this Pleistocene aquifer. GEOCHEMISTRY OF THE PLEISTOCENE AQUIFER, NORTHEASTERN ANDROS ISLAND, BAHAMAS A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science Department of Geology by Derek Wayne Dice Miami University Oxford, Ohio 2003 Advisor________________________ (Mark Boardman) Reader__________________________ (Cindy Carney) TABLE OF CONTENTS Section Page I. Introduction 1 A. Carbonate Diagenesis 1 B. Meteoric Diagenesis 2 C. Scope of this Project 3 II. Study Area 5 A. Overview of Bahamas Geology 5 1. Geographical Background of the Bahamas 5 2. Geologic History of the Bahamas 5 3. Quaternary History of the Bahamas 6 B. Field Area Lithology 7 C. Field Area Hydrology 9 1. Island Hydrology 9 2. Controls on the Freshwater Lens of Andros 10 III. Methods of Investigation 11 A. The Study Area 11 B. Construction and Installation of the Multi-level Samplers 11 C. Sampling of the Wells 12 D. Core Samples 13 E. Laboratory Analysis 13 IV. Results 14 A. Chemistry of the Groundwater 14 B. Alkalinity 15 C. pH 15 D. Saturation States 15 E. Chemistry of the Rainwater 15 F. Rock Specimens 15 G. X-ray Diffraction of the Bulk Rock 16 ii V. Discussion 17 A. Water Chemistries 17 B. Proof of Reactivity 18 C. Reaction-derived Values 19 1. Non-reactive with the Aquifer 19 2. Reactive with the Aquifer (Reduction) 20 3. Reactive with the Aquifer (Dissolution within the Aquifer) 20 D. Evidence of Precipitation and Dissolution within the Aquifer 21 E. Quantifying the Dissolution and Precipitation 22 F. Rate of Mineralogical Transformation 24 G. Geologic Implications of this Rate of Transformation 26 VI. Conclusions 28 VII. References 29 iii LIST OF TABLES Number Page 1. Groundwater Classification Based on Total Dissolved Solids 34 2. Measured Constituents in the Groundwater 35 3. Measured and Calculated Constituents in the Groundwater 37 4. Calculated IAP/K’s in the Groundwater 38 5. Average Rainwater Composition 39 6. Measured Constituents in the Rock 40 7. Calculated Values for Precipitation and Dissolution 42 8. Stabilization Rates 43 9. Stabilization Rates of Related Carbonate Environments 44 10. Half-lives of Aragonite Stabilization 45 iv LIST OF FIGURES Number Page 1. Meteoric Environment 46 2. Map of the Bahamas 47 3. Pleistocene Sea-level Curve 48 4. Sea-level History of Northeastern Andros 49 5. Ghyben-Herzberg Principle for a Coastal Aquifer 50 6. Map of North Andros Island Field Area 51 7. Map of North Andros Island Study Area 52 8. Design of the Multi-level Sampler 53 9. Packing Sequence of the Multi-level Sampler 54 10. Well 69 Multi-level Sampler 55 11. Well C2N Multi-level Sampler 56 12. Well 9N Multi-level Sampler 57 13. Well 4NN Multi-level Sampler 58 14. Well 4SA Multi-level Sampler 59 15. Graph of Chloride in the Groundwater with Depth 60 16. Graph of Sulphate in the Groundwater with Depth 61 17. Graph of Calcium in the Groundwater with Depth 62 18. Graph of Strontium in the Groundwater with Depth 63 19. Graph of Sodium in the Groundwater with Depth 64 20. Graph of Magnesium in the Groundwater with Depth 65 21. Graph of Potassium in the Groundwater with Depth 66 22. Graph of Alkalinity in the Groundwater with Depth 67 23. Graph of pH in the Groundwater with Depth 68 24. Graph of log IAP/KT for Aragonite in the Groundwater with Depth 69 25. Graph of log IAP/KT for Calcite in the Groundwater with Depth 70 26. Graph of Calcium in the Rocks with Depth 71 27. Graph of Strontium in the Rocks with Depth 72 28. Graph of Magnesium in the Rocks with Depth 73 v 29. Graph of Potassium versus Chloride in the Groundwater 74 30. Graph of Sodium versus Chloride in the Groundwater 75 31. Graph of Magnesium versus Chloride in the Groundwater 76 32. Graph of Calcium versus Chloride in the Groundwater 77 33. Graph of Strontium versus Chloride in the Groundwater 78 34. Graph of Sulphate versus Chloride in the Groundwater 79 35. Graph of Reaction-derived Magnesium in the Groundwater with Depth 80 36. Graph of Reaction-derived Potassium in the Groundwater with Depth 81 37. Graph of Reaction-derived Sodium in the Groundwater with Depth 82 38. Graph of Reaction-derived Sulphate in the Groundwater with Depth 83 39. Graph of Reaction-derived Calcium in the Groundwater with Depth 84 40. Graph of Reaction-derived Strontium in the Groundwater with Depth 85 41. Graph of Strontium versus Calcium in the Groundwater 86 42. Graph of Calcium and Strontium Ratios in Carbonate Rocks 87 43. Graph of the Remaining Aragonite versus the Duration of Diagenesis 88 vi INTRODUCTION Carbonate Diagenesis Diagenesis includes the natural changes which occur in sediments and sedimentary rocks from the time of initial deposition to the time when the changes caused by elevated temperatures and pressures are considered metamorphic. The ultimate end-point of the diagenetic process affecting carbonate sediments (comprised of aragonite, high-magnesium calcite, and low-magnesium calcite) is chemical and mineralogical stabilization of an initially heterogeneous mixture of carbonate particles. With increasing time, unstable carbonate particles dissolve and stable carbonate particles precipitate. Thus, structural order (calcite) and chemical purity is attained (Morse and Mackenzie, 1990). The major factors affecting the overall diagenetic process are the mineralogy of the original grains, the chemistry of the interstitial fluids flushing through the system, the duration of these processes, and the physical and chemical constants involved (Scoffin, 1987). The purpose of this paper is to report on the early diagenesis of a Pleistocene aquifer. By definition, carbonate rocks contain > 50% carbonate minerals. The most abundant of these minerals are aragonite, calcite, and dolomite. In these minerals, the = CO3 anions (carbonate) are an equilateral triangle, with oxygen atoms on the corners and a carbon atom in the center. These rock-forming minerals are either rhombohedral (calcite) or orthorhombic (aragonite) in crystal habit, depending on the incorporated cation. In pure calcite, the carbonate ions are in layers that alternate with layers of = calcium cations. Each calcium ion has six CO3 anions in octahedral (6-fold) coordination, building rhombohedral crystals. Divalent cations smaller than calcium (1.0 Å), such as sodium (.97 Å), manganese (.74 Å), iron (.72 Å), or magnesium (.63 Å), may easily be substituted into the cation layers of calcite (Ford and Williams, 1989). In aragonite, the calcium and carbonate ions form unit cells of cubic coordination (9-fold coordination) forming orthorhombic crystals (Scoffin, 1987). This structure will not accept cations smaller than calcium (1.0 Å), and could incorporate barium ( 1.32 Å), 1 lead ( 1.35 Å), or strontium ( 1.06 Å) (Prothero and Schwab, 1996). In aragonite precipitated from seawater, strontium is the most common substitute cation. Both ancient and modern shallow water carbonates are composed predominantly of mixtures of aragonite, high-Mg calcite (>4 mole%Mg+2), and low-Mg calcite (<4 mole %Mg+2). This mineralogy depends upon the composition of the calcium carbonate-secreting organisms and the mineralogy of the abiotic precipitates at that time. The most common tropical, shallow water carbonate grains are composed of aragonite and high-Mg calcite. However, both of these minerals are unstable relative to low- magnesium calcite. At some point, mineralogically unstable carbonates will be converted to the most stable form of CaCO3, low-Mg calcite. Unless dolomitized, the final product of carbonate diagenesis is low-Mg calcite, as seen in most of the ancient carbonate rocks (Tucker and Bathurst, 1990). Meteoric Diagenesis The meteoric environment consists of rainfall-derived water in contact with sediment or rock. Within the meteoric environment, there are two major zones of interest. These are the vadose and phreatic zones, separated by the water-table (Figure 1). Water drains through the vadose zone due to gravity. The pores within this zone are characteristically filled with air, but are subjected to wetting and drying depending upon climatic conditions (Tucker and Wright, 1990). Below the vadose zone and the water-table is the phreatic zone. This zone is characterized by fluid filling all of the pore spaces, and typically includes a horizontal groundwater flow direction. In an island or coastal setting, this meteoric zone is underlain by a denser water (highly saline or marine water); i.e. the less dense freshwater lens rests on top of the more saline water. A more specific account of this salinity difference along with more characteristic island hydrology will be discussed later. Freshwater entering the meteoric zone is undersaturated with respect to all of the carbonate minerals and will immediately begin to dissolve the surrounding limestone (Tucker and Bathurst, 1990). Rainfall-derived water (with its initial CO2 concentration) picks up additional CO2 derived from the oxidation of organic matter in the soil.
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