This study examines the relationship between depositional environment and sedimentary organic geochemistry in Lake Malawi, East Africa, and evaluates the relative significance of the various processes that control sedimentary organic matter (OM) in lacustrine systems. Total organic carbon (TOC) concentrations in recent sediments from Lake Malawi range from 0.01 to 8.80 wt% and average 2.83 wt% for surface sediments and 2.35 wt% for shallow core sediments. Hydrogen index (HI) values as determined by Rock-Eval pyrolysis range from 0 to 756 mg HC g −1 TOC and average 205 mg HC g −1 TOC for surface sediments and 228 mg HC g −1 TOC for shallow core samples. On average, variations in primary productivity throughout the lake may account for ~33% of the TOC content in Lake Malawi sediments (as much as 1 wt% TOC), and have little or no impact on sedimentary HI values. Similarly, ~33% to 66% of the variation in TOC content in Lake Malawi sediments appears to be controlled by anoxic preservation of OM (~1–2 wt% TOC), although some component of the water depth–TOC relationship may be due to physical sediment transport processes. Furthermore, anoxic preservation has a minimal effect on HI values in Lake Malawi sediments. Dilution of OM by inorganic sediment may account for ~16% of variability in TOC content in Lake Malawi sediments (~0.5 wt% TOC). The effect of inputs of terrestrial sediment on the organic character of surface sediments in these lakes is highly variable, and appears to be more closely related to the local depositional environment than the regional flux of terrestrial OM. Total nitrogen and TOC content in surface sediments collected throughout the lake are found to be highly correlated (r 2 = 0.95), indicating a well-homogenized source of OM to the lake bottom. The recurring suspension and deposition of terrestrial sediment may account for significant amounts of OM deposited in offshore regions of the lake. This process effectively separates denser inorganic sediment from less dense OM and allows terrestrial OM to preferentially be transported farther offshore. The conclusion is that for the organic carbon content in these regions to be elevated a mixed terrestrial-lacustrine origin is required. The hydrodynamic separation of mineral and organic constituents is most pronounced in regions with shallow bathymetric gradients, consistent with previous findings from Lake Tanganyika.

A geochemical investigation (major cations and anions, stable isotopes of oxygen and hydrogen, pH, and salinity) was conducted to identify the sources of groundwater recharge to the surficial aquifer system in Everglades National Park. The weighted mean values of δ 18 O and δD of rainfall were −2.83‰ and −10.59‰, respectively. A mean deuterium excess value of 12 suggests that evaporation of Everglades surface water contributes between 7% and 12% to the local precipitation. Most shallow groundwater in the surficial aquifer system (<28 m) is recharged throughout the year by Everglades surface water and or canal water exposed to evaporation. Recharge rates between 2 cm/yr and 12 cm/yr were obtained, with the higher rates in areas of little to no standing water. Deep groundwater in the surficial aquifer system (>28 m) is recharged directly from rainfall far upgradient of the northern boundary of Everglades National Park. Groundwater from the underlying Hawthorn Group is geochemically distinct from the surficial aquifer system and recharges the surficial aquifer system from below. There is no geochemical evidence of surface water or shallow groundwater flow between the two major waterways (Shark Slough and Taylor Slough) in Everglades National Park. In this investigation, a combination of stable isotopes (δ 18 O and δD) and major-ion data was necessary to identify different sources of groundwater recharge to the karst aquifer. The stable isotopes (δ 18 O and δD) were most useful in deciphering between rainfall and surface-water recharge to the shallow aquifer, whereas the major-ion data were used to identify recharge from deeper aquifers and seawater intrusion.

Abstract: Independent analyses to date prograding carbonate platform-margin sediments have enabled development of an integrated chronostratigraphy of the west margin of the Great Bahama Bank (GBB). The chronostratigraphy permits interpretation of periods of deposition, nondeposition, and erosion on the platform and margin. The integrated chronostratigraphy is based on planktic foraminiferal biostratigraphy, calcareous nannofossil biostratigraphy, strontium-isotopestratigraphy, and magnetostratigraphy. Critical to interpreting each type of stratigraphy is a clear understanding of slope dynamics and the depositional system. The biostratigraphic data, in particular, are affected by the extreme dilution of key microfossils by platform-derived sediments during margin progradation. Because of this factor, the highest abundance of microfossils is restricted to thin units of pelagic sediment, deposited during temporary intervals when platform sediment supply was greatly reduced. However, these selective microfossil concentrations are also more likely to yield premature last-appearance datums and delayed first-appearance datums than the diluted intervals, because they represent short periods of time. Despite these problems, the microfossils provide critical age control. The detailed chronostratigraphy allows interpretation of a well-constrained platform-margin evolution. Core Unda, the more landward location, penetrated the oldest sediments (earliest late Miocene). The more seaward core Clino, although deeper, bottomed in younger sediments (latest Miocene). Three major progradational episodes were delineated using seismic stratigraphy, lithostratigraphy, and information on depositional age. Progradation occurred during the late Miocene, late early Pliocene, and latest Pliocene. In the Pliocene shelf and ramp setting margin progradation began during a highstand, but also occurred in a forced-regression-type situation during a fall in sea level. Rapid reef progradation occurred near the end of the Pliocene and early Pleistocene, when the platform had aggraded to a nearly horizontal surface. The transformation from a ramp-type platform topography to a flat-topped platform culminated in the middle Pleistocene. Age constraints across the west margin of GBB indicate that the seismic reflectors that constitute sequence boundaries are synchronous, within our age resolution. As expected in a slope setting, the sequence boundaries represent condensed time periods of both erosion and nondeposition. Downslope plankton-rich onlapping units are correlated to upslope marine hardgrounds and are thought to represent periods of falling sea level. Subsequent high-resolution dating of additional basinal and shallow-platform borings will provide a rich archive of paleoceanographic changes and will serve as a direct link between the deep-ocean and shallow-platform accumulations.

Abstract: Two cores drilled on the western margin of Great Bahama Banks penetrated a total of 1131 m of Quaternary to Late Neogene platform and periplatform carbonate sediments. These carbonates show a wide range of diagenetic textures ranging from largely unaltered primary carbonates to texturally mature limestones and dolostones. Although some of these carbonates have been diagenetically altered under freshwater conditions and show classic geochemical and petrographic indicators of such, other rocks exhibit similar petrographic features yet have never experienced meteoric influences. These features include micrite envelopes, moldic porosity, blocky spar cementation, and aragonite neomorphism The core Unda (453.8 m) includes three successions of shallow-water platform to reef sediments that alternate with deeper shelf deposits of silt to coarse sand. Clino, the more distal core, penetrated 677.3 m. It contains an upper reef to platform (21.6-140 m) overlying a thick package of deep forereef to upper and lower slope sediments (140-677.3 m). Three diagenetic zones were identified on the basis of similar diagenetic fabrics. Diagenetic zone 1 (Unda 0-108.08 m; Clino 0-152.71 m) is characterized by minor early marine diagenesis overprinted by numerous episodes of subaerial exposure with caliches, large-scale dissolution, and blocky spar cementation. Diagenetic zone II includes the lower reef and platform in Unda (292.82360.28 m) and has the most complex alteration in these cores. Minor early marine diagenesis was followed by several episodes of meteoric diagenesis, Most of the early fabrics, however, were destroyed during pervasive dolomitization in the burial environment. Diagenetic zone III includes deeper shelf to slope facies in both cores (Unda 108.08-292.82 m; 360.28-452.94 m; Clino 152.71-677.27 m) and shows only marine to marine-burial diagenesis. The exact fabrics within these intervals varies by lithology. Peloidal packstones to grainstones show minimal alteration (primarily compaction, minor dolomitization, and some recrystallization). Skeletal grainstones show two different styles of modification. Most commonly the grainstones are characterized by nearly complete dissolution of aragonitic components, minor cementation, and 10% to 20% early burial dolomite. Grainstones interbedded with the tight peloidal sediments, on the other hand, show nearly complete blocky spar cementation along with neomorphism of aragonitic skeletal grains (peloids were dissolved). Several marine hardgrounds with penecontemporaneous dolomite and/or phosphate and blackening also occur within the deeper-water facies. Each diagenetic zone in Clino and Unda is characterized by fabrics developed in one or more diagenetic environment. The larger-scale controls, therefore, are those that govern the diagenetic environment. For these young sediments, the most important are the depositional system (i.e., reef and platform versus deeper margin to slope) and the sea-level history. As a result, the diagenetic zones largely coincide with the depositional successions and seismic sequences identified in these cores. This study shows that the end product of marine to marine-burial diagenesis appears very similar to the end product of alteration in the phreatic meteoric environment i.e., a limestone composed of LMC and minor dolomite with micrite envelopes, moldic porosity, blocky spar cementation, and aragonite neornorphism. On this basis, a reevaluation is needed of the criteria for recognizing meteoric diagenesis in ancient carbonate sequences. Without clear physical evidence of subaerial exposure (e.g., caliche horizons or vadosecements) or chemical evidence of meteoric fluids (e.g., negative δ 18 O) great care is needed to identify meteoric diagenesis, In addition, the large-scale (> 100 m) sea-level lowstands of the latest Pleistocene did not result in significant phreatic meteoric diagenesis, perhaps because the fresh-water lens is too far below the recharge zone.

Abstract: Geochemical measurements (Ca 2+ , Sr 2+ , Mg 2+ , Cl − , SO 4 2- , and 87 Sr/ 86 Sr) have been made on fluids obtained from two deep boreholes (Unda and Clino) drilled on the Great Bahama Bank. As a result of the fact that these holes were drilled through cemented Limestones, it was not possible to squeeze sediments to obtain interstitial pore waters, and instead the fluids were retrieved using a combination of pumping from the surface and passive water samplers deployed during logging. Geochemical analyses show that it is possible using these techniques, combined with tritium, to define the extent of contamination of these pore fluids by surface waters. Consequently, with appropriate corrections for surface contamination, estimates of the interstitial concentrations of Ca 2+ , Sr 2+ , Mg 2+ , Cl − , and SO 4 2- and 87 Sr/ 86 Sr ratios can be obtained. In both holes, dissolution and precipitation of carbonate minerals such as aragonjte, calcite, and dolomite are taking place in slightly modified sea water, leading to significant Increases in the concentration of Ca 2+ and Sr 2+ and decreases in Mg 2+ . The absence of significant geochemical gradients in the upper zone of Unda suggests that there is substantial influx of modern seawater. Deeper in Unda and throughout Clino there are large increases in Sr 2+ and Ca 2+ suggesting a more closed diagenetic system. In both Clino and Unda the 87 Sr/ 86 Sr ratio of the pore fluids, when corrected for surface contamination, was either similar to or more radiogenic than the co-occurring sediment. This not only suggests that there is not a significant upward vertical flow through the platform, but also supports the notion that the 87 Sr/ 86 Sr ratios of carbonates in such settings can be used to give the oldest possible age of deposition and the youngest possible age of diagenesis.

Abstract: The Sr-isotope composition of pore fluids and co-occurring sediments (carbonates, phosphorites, and dolomites) have been measured from two cores retrieved from western margin of the Great Bahama Bank. The Sr-isotope compositions of the pore fluids, corrected for possible contamination by surface waters using tritium, indicate that the formation fluids have 87 Sr/ 86 Sr ratios less radiogenic than modern sea water, but in all instances considerably more radiogenic than the co-occurring sediments and rocks. Assuming that similar conditions of fluid movement have been prevalent since deposition, the Sr-isotopic composition of the sediments has been used to constrain the age of deposition and diagenesis. The 87 Sr/ 86 Sr ratios of the solid components can be treated in two ways. First, if it is assumed that the chronostratigraphy determined from biostratigraphic and magnetostratigraphic methods is correct, then the Sr-isotopic composition can be used to place constraints on the timing of diagenesis. Second, if the diagenetic alteration occurred relatively soon after deposition, then the Sr-isotope composition can constrain the age of deposition. The data from the sediments show that the 87 Sr/ 86 Sr ratios are in most instances close to the 87 Sr/ 86 Sr ratio calculated from the chronostratigraphic age. The only substantial disagreement between the Sr-isotope age and the biostratigraphic/magnetostratigraphic age occurs between 300 and 500 mbsf in Clino. This could indicate either recrystalli-zation after deposition by more radiogenic solutions or slight uncertainties in the chronostratigraphy in this interval.

Abstract The concentrations and distribution of uranium and boron have been measured in dolomites and limestones from a core taken on the island of San Salvador in the Bahamas. The analyses reveal a wide range of concentrations both within and between the two predominant types of dolomite. The crystalline dolomites show unexpectedly high concentrations of U in skeletal components (2 to 7 ppm), but low values in void-filling cements (0.5 to 1 ppm). In contrast, the fabric-destructive microsucrosic dolomites are uniformly low in U (0.5 to 1 ppm) with occasional red algal fragments exhibiting concentrations as high as 1.5 ppm. Data presented here suggest that the U concentrations of the dolomites are inherited from original sedimentary and diagenetically altered components. It is suggested that the rocks that have higher concentrations of U, and in which the original fabrics are largely preserved, were dolomitized directly from the aragonite and high-Mg calcite (HMC) precursors. The U concentration is retained during dolomitization because in carbonite-rich fluids the uranyl ion (UO 2 2+ ) is complexed principally with the carbonate ion (CO 3 2- ). As the activity of CO 3 2- is usually limiting in producing solutions supersaturated with respect to dolomite, CO 3 2 produced from the dissolution of metastable precursors is reincorporated into dolomite. In contrast, dolomites with lower U concentration formed from a low-Mg calcite (LMC) precursor which previously lost U during stabilization by meteoric waters. Concentrations of B in the dolomites were similar (1 to 3 ppm) to values determined for modern LMC organisms (this study) and therefore suggest dolomitization from predominantly marine fluids. Comparisons with ranges reported in the literature show B concentrations in this investigation to be much lower. This is attributable to the ability of the nuclear-track technique to recognize contamination within the sample and consequently to allow it to be eliminated from the analysis.