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open file report > OFR 94-276 > rock analysis
The term caliche as used in Table III is synonymous with soilstone crust, calcrete, and paleosol and typically forms on rock surfaces during subaerial exposure, forming subaerial unconformities. Selected core components, mainly brown and gray infillings and unconformity surfaces (caliche in Table III), generally contained more phosphorous than host limestone. This phosphorous, however, is considered natural. For example, SB-3 15 is a relatively soft brown soil-like carbonate (paleosol) that completely infills a large void. This soil material contained 125 ppm P, Al (455 ppm) and Fe (152 ppm). These values are higher than are generally found in host rock and are considered typical of subaerial-unconformity-related soils because of enrichment of dust. Saharan dust is present in most caliches or soilstones throughout the Caribbean (Muhs et al., 1990). Saharan dust contains clays (aluminum silicates) and iron that oxidizes, lending caliches and carbonate soils their typical rusty brown color. Phosphorous is also a component of Saharan aerosols. SB-3 (20) is a little farther down in the same core and consists of well-cemented rusty-brown lining of voids, which are not completely filled. This sample contained only slightly more Al, Fe, and P over that found in white unaltered coral (see Table III). KL-5 (16) is the caliche on top of the Pleistocene beneath the reef rock in the KL transect. For reasons that are not understood, this sample has elevated P but reduced Al and Fe. Samples with the most elevated P are from OR-5 which, like KL-5, is the core farthest from shore. These samples are from the Holocene reef framework. The high sodium and magnesium are typical of submarine cementation, typically Mg calcite and aragonite, which have high strontium. The elevated P is most likely associated with natural marine cementation. These data, combined with groundwater chemical data discussed below, indicate that dissolution was the most important process at the location of our monitoring wells. A significant amount of phosphate may be taken up by the host limestone in the immediate vicinity (a few tens of meters) of injection wells. This study, however, was not designed to monitor close-in effects of individual injection wells but rather to take a broad view of offshore ground water. Lapointe et. al. (1990) provided evidence of phosphate uptake by limestone in the immediate vicinity of septic tanks. The low levels of PO4 in offshore ground waters versus higher levels in onshore ground waters may indeed be an indication of removal by limestone interactions near the source.
U.S. Department of the Interior, U.S. Geological Survey
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