Miriam A. Bertram

Morphological and compositional evidence for biotic precipitation of marine barite

Barite formation in the surface oceans is generally assumed to be dominated by abiotic precipitation. Acceptance of this pathway is largely the result of the absence of a pelagic marine organism known to precipitate the ovoid to rounded-rectangular barite crystals typically observed in marine waters and sediments. Barite crystals observed in net-tow particles and on substrates retrieved from the seafloor (both in the central North Pacific) were examined by scanning electron microscopy and energy dispersive X-ray spectrometry. Three distinct crystalline forms of bar&e were observed: ovoid and hexagonal crystals several microns in diameter, and aggregates of submicron-sized crystals. Ovoid and hexagonal-type crystals contained between 0 and 26 mole percent SrS04. The microcrystalline barite contained no detectable Sr (CO.05 percent). Hexagonal-type crystals were precipitated by an unusual benthic foraminifera. Comparison of the morphology and composition of the barite crystals observed in this study to crystals precipitated by a variety of biotic and abiotic processes suggests a biotic origin for the ovoid barite crystals, the most common form of barite observed in this region.

Diagenetic stabilization pathways of magnesian calcites

Stabilization of high magnesian calcites (>4 mole% MgCO3) to low magnesian calcite (0–4 mole% MgCO3) and dolomite involves a reduction in the solubility of these phases during diagenetic alteration. The solubility of a magnesian calcite is controlled not only by the Mg concentration, but also other chemical and physical properties of the solid. These other properties include the amount of: 1) trace element diluents other than Mg (e.g., sodium, sulfate, adsorbed or structural water); 2) carbonate ion positional or cation ordering: 3) microstructural and surface defects; and 4) adhered small particles. Crystal size also may affect the solubility of a magnesian calcite. A magnesian calcite may become more stable in the natural environment by a decrease in Mg concentration, by loss of other trace elements and/or changes in its physical properties. Few studies exist of magnesian calcites in sediments and limestones undergoing diagenetic alteration that can be used to document the typical stabilization pathways followed by magnesian calcites. Several stabilization pathways are proposed, based mainly on experimental and theoretical arguments, to encourage further investigation of magnesian calcite diagenesis.

Geomicrobial transformation of manganese in Gorda Ridge event plumes

Abstract Event plumes form as episodic discharges of large volumes of hydrothermal solutions in response to magmatic diking/eruptive events. In consequence, event plumes represent the sudden injection of exploitable reduced chemical substrates, as well as inhibitory constituents, and likely induce successional changes in the microbial community structure and activity within event plume waters. In response to a major seismic event detected beginning 28 February 1996 at the northern Gorda Ridge, a series of three rapid response and follow-up cruises (GREAT 1, 2 and 3) were mounted over a period of three months. This report focuses on time-series measurements of manganese geomicrobial parameters in the two event plumes found in association with this seismic event. Scanning transmission electron microscopy, elemental microanalysis, and radioisotope (54Mn) uptake experiments were employed on samples collected from vertical and tow-yo casts from the three cruises. Numbers of bacteria and ratios of metal precipitating capsuled bacteria to total bacteria were greatest in the youngest (days old) plume, EP96A, found during GREAT 1; however, when normalized to the hydrothermal temperature anomaly, the greatest values were found in a second event plume, EP96B, discovered during GREAT 2 (up to 1 month old). Early capsule bacteria and particulate Mn distributions may have been influenced by entrainment of resuspended sediment, while those of the oldest (2–3 months) plume sample may have been subjected to preferential aggregation and particle settling.

Biomineralization in Agglutinating Foraminifera: An Analytical SEM Investigation of External Wall Composition in Three Small Test Forms

The walls of many deep-sea foraminiferal tests containabiogenic and biogenic, precipitated and agglutinated,components. Both environmental and genetic factorscan contribute to the great diversity in test form andcomposition in benthic foraminifera. Yet, smallspecimen size and the remoteness of the deep-seaenvironment have limited our ability to describe therelative influence of these biological and chemicalfactors. The use of fossilized foraminiferal tests aspaleo-indicators requires that we understand thecontrols on test composition. Test wall morphologyand composition were examined inforaminifera that colonized experimental substratesdeployed on a seamount in the central North Pacific. Three types of agglutinated forms were identified. Atriserial (Eggerella-like) and two-chambered(Hyperammina-like) form contained a Ca-rich(CaCO3) precipitate and the chamber walls of anencrusting two-chambered form was Ba-rich(BaSO4). We discuss the composition of thesebiologically precipitated minerals in the context ofthe environmental conditions during the life of theseforaminifera.

Testate rhizopod growth and mineral deposition on experimental substrates from Cross Seamount

Abstract We present the results of an experiment on Cross Seamount (18°40′N, 158°17′W) in which basalt, ferromanganese-oxide and CaCO 3 substrates were deployed for 19 months. An experimental block design, of identical sets of well-characterized substrates arranged on three distinct panels, was used. Alterations resulting from exposure at 800 m water depth were documented by analytical Scanning Electron Microscopy. Agglutinated rhizopods and irregularly shaped chambers occupied an average of 37% of the basalt surfaces, and 13 and 20% of the ferromanganese-oxide and CaCO 3 substrates, respectively. This coverage is high, relative to that observed on dredged ferromanganese nodules and crusts. High coverage can be attributed to the low abundance of other agglutinated and calcareous foraminifera. Metal-rich deposits composed of BaS, AlSi, Mn and Fe were common on substrate surfaces. Barite (BaS) particles, which originated in the water column, were observed attached to the substrate by agglutinated foraminiferal tests. Fe-enriched AlSi deposits were often adhered to the substrate. These deposits appeared to have been produced by benthic rhizopods, and may persist on the seafloor over time. Fe-coated bacteria-like colonies were numerically scarce, but covered large areas. Mn-oxide precipitates, also bacteria-like, were numerically abundant on ferromanganese-oxide substrates, but never observed on other substrates. The occurrence and morphological features of these Fe- and Mn-oxide coatings reinforce the idea that bacteria play an important role in the accretion of ferromanganese-oxides on the seafloor.