Victoria C. Hover

GEOCHEMICAL SIGNATURES OF PALEODEPOSITIONAL AND DIAGENETIC ENVIRONMENTS: A STEM/AEM STUDY OF AUTHIGENIC CLAY MINERALS FROM AN ARID RIFT BASIN, OLDUVAI GORGE, TANZANIA

Olduvai Gorge, Tanzania (East African Rift) exposes a 100 m thick Plio-Pleistocene sequence of dominantly volcaniclastic sediments deposited in a 50 km wide closed basin containing a playa lake. A scanning transmission electron and analytical electron microscopy (STEM/AEM) study of authigenic clay minerals in sediments from representative depositional environments in the basin (pyroclastic fan, fluvial plain, wetland, lake margin and lake basin) was undertaken to determine whether clay compositions and textures could provide unique geochemical fingerprints characteristic of source area (Plio-Pleistocene trachytic volcanics vs. Precambrian quartzose-feldspathic basement) or paleoenvironmental conditions.Our study shows that compositional signatures obtained by clay minerals during early pedogenesis are inherited from their parent source rocks. Sediments sourced from volcanics contain highly disordered, dioctahedral smectite. Those sourced from Precambrian basement are similar, but are more Al-rich. Subsequent neoformation in the pedogenic (soil) or diagenetic (lake-margin, lake) environments results in the modification of original clay mineralogy, compositions, and textures, and unique paleoenvironmental fingerprints are acquired. Soils developed on the distal pyroclastic fan contain smectite with more Fe(III) and Mg than smectite from the proximal pyroclastic fan sediments. A trend of decreasing Al and increasing Mg content occurs in smectite compositions in samples from the fluvial to lake-margin and lake environments as a result of partial replacement of original dioctahedral Al-rich smectite by neoformed trioctahedral Mg-rich smectite (stevensite). Neoformed celadonite replaces smectite in the most saline lake sediments.The STEM/AEM data collectively indicate that diagenesis in the saline-alkaline lake results in the replacement of Al-rich dioctahedral smectite by Mg-rich trioctahedral smectite (stevensite) and Mg- and Al-rich celadonite. Thus, determination of clay mineral compositions at a basin-wide scale provides a useful tool for interpreting the spatial distribution of depositional and diagenetic environments.

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).

ARTESIAN BLISTER WETLANDS, A PERENNIAL WATER RESOURCE IN THE SEMI-ARID RIFT VALLEY OF EAST AFRICA

A cluster of artesian springs encircled by mounds of marsh and wet meadows was discovered near the equator in Kenya, East Africa. Each spring is capped by a dense fibrous root mat that covers a mound of clayey peat with a blister of water in the center. Individual mounds are ∼15 m wide, 1–2 m high, and affect an area of ∼50 m2. The central water-blister volume is <1 m3. The arched semi-permeable vegetation cap appears to be buoyed upward by slow artesian flow that leaks through the cap and moves slowly away. Lush plant growth (Poaceae and Cyperaceae, algae, diatoms, and filamentous cyanobacteria) is supported even through the dry season (Dec.–Feb.). The term “artesian blister wetland” is proposed for this unusual marsh, which has not been described previously. Approximately 20 small, circular-to-oval artesian blister wetlands occur within a large spring and wetland complex (∼1.3 km2) that includes several freshwater Typha marshes fed by ground-water seeps. The springs discharge along a rift-related fracture system near the contact between volcanic bedrock and late Quaternary sediments. Cores (1–2 m) through the mounds revealed a dense root mat underlain by water in the center and a clayey peat that is locally pebbly at the base surrounding the blister. LOI in the clayey peat decreases from 75% at surface to ∼ 10% at 0.4–1.2 m. Blister water is cool, fresh, and dysaerobic (T=30–33 °C; pH=6.2–7.2; conductivity ∼600 ìS/cm; and DO=50%; 0.6–3.5 mg/l). The spring/wetland mounds likely form by the blanketing of the land around the spring orifice with vegetation (paludification). Plants and cyanobacteria seem to trap sediment transported by surface run-off and wind. The mound grows with time, but its height is limited by the magnitude of the hydraulic head. These ecological niches are important freshwater resources for animals and humans in semi-arid environments.

Possible Late Holocene equatorial palaeoclimate record based upon soils spanning the Medieval Warm Period and Little Ice Age, Loboi Plain, Kenya

Wetland and floodplain soils in the East African Rift of Kenya provide a record of changing palaeoclimate and palaeohydrology compatible with climate records for the mid-Holocene through the late Holocene Medieval Warm Period (~AD 800–1270) and Little Ice Age (~AD 1270–1850), documented previously in nearby lacustrine sites. Soils forming from volcaniclastic source materials in both Loboi Swamp and laterally adjacent Kesubo Marsh, two wetland systems of latest Holocene age, were investigated using micromorphology, whole-soil geochemical analysis, and stable isotope analysis of soil organic matter (SOM). Wetland formation was abrupt and possibly related to climate shift from drier conditions associated with the mid-Holocene and Medieval Warm Period, to wetter conditions associated with the Little Ice Age. Pre-wetland sediments are floodplain volcanic sandy silts comprising buried Inceptisols (SOM dC= –15x PDB) that fine upward to fine silt and clay, which are overlain by clays and organic-rich sediment (peat) (SOM dC= –26x PDB). Stable isotopes record an abrupt shift from 20 to 40% C3 vegetation (scrubland mixture of warm-season grasses and Acacia) to 100% C3 (wetland dominated by Typha) that occurred about 680F40 years BP (C-14 date from seeds). Soils developed on the periphery of the wetland show evidence for fluctuations in hydrologic budget, including siderite and redoximorphic features formed during wetter phases, and vertic (shrink–swell) and clay illuviation features developed during drier phases. Soils at Kesubo Marsh, located 2–3 km east of Loboi Swamp, consist of two buried mid-Holocene, 4000–4600 years BP (two C-14 dates from bulk SOM) Inceptisols developed from fluvially derived volcanic sand (SOM dC= –15x PDB) and separated from the latest Holocene surface soil (SOM dC= –17.5x PDB) by an unconformity and prominent stone line. Both the Loboi Swamp and Kesubo Marsh surface soils show increases in Zr, Fe, and S relative to buried soils, as well as higher leaching indices. Elevated Zr may reflect zircon

K Uptake by Modern Estuarine Sediments During Early Marine Diagenesis, Mississippi Delta Plain, Louisiana, U.S.A.

ABSTRACT Marine sediment porewaters are commonly depleted in K+ relative to conservative mixing trends, but the mineralogical sink for K+ in the sediment has not been well characterized. Results are presented from a geochemical study of surface waters and porewaters and associated muddy sediments in the Mississippi Delta plain estuary spanning the salinity gradient from 0-12‰. Evidence of K+ depletion in sediment porewaters is integrated with scanning transmission and analytical electron microscopy (STEM/AEM) analyses of clay components to determine possible mineralogical sinks for K+ in the sediments. Conservative mixing between the freshwater influx from the Atchafalaya/Mississippi river systems and Gulf of Mexico seawater controls the surface-water major-element composition. Porewaters from the most saline site, however, are depleted in K+ by up to 33% relative to overlying water, implying uptake of K+ by the sediment. Textural characterization of the sediment at the same location by TEM indicates that it is dominated by aggregates of highly disordered smectite-rich clay material, which is mixed with a small component of more-ordered illite-rich material. AEM compositions obtained on the most smectite-like component in these aggregates indicates that the average K+ contents increase by up to 1.0 wt % K2O relative to similar smectite-rich clay in freshwater sediments. Ca2+ concentrations in smectite-rich clay decrease concomitantly, whereas Na+ concentrations are similar between the sites. X-ray diffraction (XRD) analyses confirm that smectite-rich clay from the freshwater site has a larger interlayer spacing than smectite-rich clay from the brackish-water site, consistent with exchange of smaller hydrated interlayer K+ (± Na+) for larger Ca2+ (± Mg2+). AEM data indicate that the discrete illite-rich component also takes up K+. The integrated porewater and sediment analytical data imply that uptake of K+ by smectite-rich clay is a rapid process (decadal) occurring at the earliest stages of diagenesis in estuarine environments upon exposure to brackish water. Our study shows that K+ uptake in marine sediments may not require the formation of authigenic minerals. Instead, exchange of K+ for other interlayer cations in smectite-rich clay and weathered illite may provide a substantial sink for K+ in the marine environment.

Diversity and distribution of macrophytes in a freshwater wetland, Loboi Swamp (Rift Valley) Kenya

ABSTRACT An inventory of Loboi swamp was undertaken to determine the macrophyte diversity and distribution. A total of 36 vascular plant species in 13 families were recorded, with Cyperaceae forming over 30% of macrophytes. Two vegetation zones were observed, characterised by the presence of Typha and papyrus. The Typha zone, comprising over 70% of the swamp, is dominated by T. domingensis and is species rich with 35 plant species whereas the papyrus zone includes the dominant Cyperus papyrus and only one other macrophyte species. Distribution of macrophytes is correlated with depth and period under water, with the Typha zone seasonally flooded while the papyrus zone is permanently under water at depths over 0.5m. Water chemistry has little influence on the distribution of macrophytes in the swamp, but at the edges there is predominance of Cyperus laevigatus in high alkalinity soils. Current uses of the swamp include dry season grazing, harvesting of papyrus and other plant material for mat making and house thatching, and use of the swamp water for domestic and irrigation agriculture. Further monitoring is needed to evaluate the effect of the resource uses on the swamp.

Significance of early-diagenetic water-rock interactions in a modern marine siliciclastic/evaporite environment: Salina Ometepec, Baja California

Although marine brines are a significant component of pore waters in sedimentary basins, there are few geochemical studies of modern analogues of such systems, especially in siliciclastic settings. For these reasons, we chose the evaporite-associated siliciclastic sediments deposited in the salt flats of the Salina Ometepec, Baja California, for an integrated investigation of sediment, pore-water, and overlying brine geochemistry. Here, the detrital components include quartz, K-feldspar, plagioclase, chlorite, biotite, and smectite, and authigenic minerals are dominated by halite, gypsum, and K-rich magnesium smectite. Thermal and saline stresses on the sediments of the Salina Ometepec keep both organic and inorganic carbon concentrations in the sediments unusually low relative to other coastal marine environments. Sediment pore waters exhibit little microbial sulfate reduction, and dissolved inorganic C contents are also very low. As a result, we did not observe carbonate and sulfide mineral authigenesis in the Salina Ometepec sediments. Instead, pore-water geochemical evolution is largely controlled by evaporative concentration of seawater, evaporite-mineral dissolution and recrystallization, and diagenetic alteration of detrital aluminosilicates. Evaporite-mineral recycling affects the compositional evolution of surficial brines even before they infiltrate the sediment. Specifically, Na+ and Cl− concentrations are increased owing to halite dissolution. We see significant Br− enrichment relative to expected seawater evaporation trends in near-surface pore water, secondary to dissolution of K- and Mg salts. Because bacterial sulfate reduction is inhibited in the Salina Ometepec sediments, sulfate concentrations are more accurate indicators of the degree of evaporation than Br−, a usually conservative element during geochemical reactions. Pore waters exhibit down-core increases in dissolved Mg2+, K+, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(H\_{4}SiO\_{4}^{0}\) \end{document} over the upper 1 m. Authigenic K-rich Mg-smectite formation is promoted by the concurrent processes of brine concentration, selective dissolution of K- and Mg-bearing salts, and dissolution of detrital aluminosilicates. Pore waters at a depth of 1 m have 87Sr/86Sr ratios that require input of Sr that is less radiogenic than that of Gulf of California seawater. This Sr is likely derived from weathering of detrital aluminosilicates from nearby volcaniclastic sources. These results show that significant chemical interactions among marine brines, evaporite minerals, and detrital aluminosilicates can occur relatively soon after sedimentation.