Early marine diagenesis of biogenic aragonite and Mg-calcite: new constraints from high-resolution STEM and AEM analyses of modern platform carbonates

Prior studies of porewater compositions from modern shallow marine carbonates have established that dissolution and recrystallization of metastable aragonite and Mg-calcites occur during the earliest stages of sedimentation. Yet, significant microstructural and mineralogical changes in sediment allochems have not been observed. In part, this is due to the high resolution required to identify diagenetic changes in these finely crystalline materials. Herein, we present ultrastructural and microgeochemical evidence for early marine diagenesis using scanning electron microscopy (SEM) and high resolution scanning transmission/analytical electron microscopy (STEM/AEM) methods to characterize original biogenic materials and sediment boxcores from modern deposits on the Floridan–Bahamas platform. We selected the two most common sediment producers, a Mg-calcite foraminifer, Peneropolid, and an aragonitic green algae, Halimeda, for study. Both are composed of submicron-sized crystallites, whose excess free energy could provide a thermodynamic driving force for recrystallization. STEM observations of grain mounts containing dispersed Mg-calcite and aragonite crystallites from gently disaggregated individual Peneropolids and Halimeda segments indicate that living specimens contain euhedral prismatic crystals with blunt terminations and smooth faces. Specimens from the sediment column show abundant evidence for alteration. The most dramatic changes involve crystallite dissolution and precipitation of thin overgrowths, causing the overall aspect ratio of crystallites to shorten and widen. Direct measurements of several hundred individual crystallites confirm that crystallite length and width distributions differ significantly between original and altered materials. These changes are consistent with crystal-growth processes that tend to minimize surface free energy by increasing overall crystal size and by producing more equant shape. The precipitated material has a composition and structure which is grossly similar to the host crystallite, as determined by AEM and electron diffraction analyses. AEM analyses of hundreds of Mg-calcite crystallites from living specimens of Peneropolid foraminifera indicate a composition of ∼13–14 mol% MgCO3, whereas crystallites from altered tests are ∼1–3 mole% lower. Taken together, these data suggest that early diagenesis in shallow marine carbonates is indeed manifested in the solids as well as the sediment pore waters, producing significant changes in biogenic Mg-calcite and aragonite crystallite size and shape. This alteration would not produce changes in the mineral assemblage because dissolution is accompanied by precipitation of compositionally similar overgrowths, under major element conditions not drastically dissimilar from overlying seawater. However, crystallite recrystallization likely has implications for incorporation of minor elements which undergo cycling via organic matter decomposition or redox changes (e.g., uranium, cadmium, manganese, rare earth elements).