William D. Bischoff

Stabilities of synthetic magnesian calcites in aqueous solution: Comparison with biogenic materials

Abstract Free-drift dissolution data and inverse time plots were used to evaluate the stabilities of synthetic and biogenic magnesian calcites in aqueous solutions at 25°C and 1 atm total pressure. Synthetic phases with MgCO 3 concentrations below 6 mole percent have stoichiometric ion activity products that are less than the value for calcite, whereas the values for phases with higher concentrations are greater than that of calcite. For synthetic phases, stability is a smooth function of composition and all phases (up to 15 mole percent MgCO 3 ) have values of ion activity products less than that for aragonite. These results agree with those of Mucci and Morse (1984) derived from precipitation of magnesian calcites in aqueous solutions. “Average” seawater at 25° and 1 atm total pressure is supersaturated with respect to all synthetic phases in the compositional range studied. Biogenic samples are less stable than synthetic phases of similar Mg concentrations and stability is not a smooth function of composition. Biogenic materials with compositions greater than 11–13 mole percent MgCO 3 have ion activity products greater than that for aragonite. The difference in stability between biogenic materials and synthetic phases is due to greater variation in chemical and physical heterogeneities found for the biogenic samples. If it is assumed that the results of the dissolution experiments reflect only differences in Gibbs free energies of formation between synthetic phases and biogenic materials of similar Mg concentration, the biogenic materials are 200–850 j/mol less stable than the synthetic phases. Only the results of synthetic dissolution experiments should be used to model the thermodynamic behavior of the magnesian calcite solid solution. The results for the synthetic phases, however, may not be appropriate to use for interpreting diagenetic reaction pathways for magnesian calcites in modern sediments, except as a basis of comparison with the behavior of natural materials.

Holocene shallow-subtidal dolomitization by near-normal seawater, northern Belize

Calcic dolomite cements compose an average of 5% of the upper 4.3 m of subtidal deposits 18 O (+2‰) compositions of the high-Sr dolomites (mean October 1900 ppm), together with near-normal salinity and inherently normal Mg/Ca ratio of pore fluids, suggest precipitation from near-normal seawater. Tidal and wind-driven circulation of seawater through the sediments supplies most of the Mg for dolomitization, which appears to be promoted by elevated pore-water alkalinity resulting from bacterially mediated oxidation of organic matter and, locally, early stages of methanogenesis. Rapid dolomitization here supports the idea that significant quantities of dolomite can form syndepositionally, from normal seawater, in shallow subtidal deposits.

Dolomitization of Holocene Shallow-Marine Deposits Mediated by Sulfate Reduction and Methanogenesis in Normal-Salinity Seawater, Northern Belize

ABSTRACT Dolomite constitutes an average of 12% of the Holocene organic-rich sediments over a 15 km2 area of the Cangrejo Shoals mudbank in northern Belize. Although it defines a laterally persistent stratiform body that averages 3 m thick, it is present throughout the 7.6-m-thick sediment section. These transgressive sediments are less than 6400 years old and were deposited in shallow-marine environments of normal salinity. The dolomite is dominantly cement, and average crystal size is 7 m. There are no significant correlations among amount of dolomite vs. sediment texture, mineralogy, porosity, or mole % MgCO3 in associated particulate high-Mg calcite, depth, or location on the shoals. The dolomites are poorly ordered and calcic (39.5-44.5 mole % MgCO3), with low mean Mn (210 ppm) and relatively high mean Sr (1034 ppm) concentrations. There is no evidence of recrystallization or geochemical alteration of the dolomite. 18O values of the dolomites range from 0.5 to 2.8o/ooPDB, and the mean value (2.1o/oo) suggests that the dolomite precipitated from normal-salinity pore water. Dolomite 13C values range from -5.2o/oo to +11.6o/ooPDB (mean seawater 13C = 0.5o/oo), which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water. Dolomitization attending these organodiagenetic reactions apparently was reversible over time, and episodic rather than continual precipitation is indicated. Requisite Mg and Ca were provided by seawater and by some dissolution of host sediments. The most rapid period of dolomitization may have been during early transgression, when relatively high sedimentation rates sustained high levels of organodiagenesis and pore-water alkalinities.

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.

Dissolution Enthalpies of Magnesian Calcites

A series of synthetic and biogenicmagnesian calcites was dissolved in weak acetic acidsolutions to measure the enthalpies of dissolution at25°C. For the synthetic phases, heat releasedwas 33.5 kJ/mol for calcite, decreasing to 33 kJ/molfor a phase of 2 mol % MgCO3, and increasing to35 kJ/mol for a phase of 15 mol %. Values of excessenthalpies, ΔHxs, calculated using calciteand magnesite end-members, average about -1 kJ/mol forthe synthetic phases. Total entropies of solidsolution formation, ΔSSS, also werecalculated using available data on Gibbs free energiesof formation and these excess enthalpies. Values ofΔSSS range from -2 J/(mol-K) at 2 mol % to-5 J/(mol-K) at 15 mol % MgCO3. These negativevalues of ΔHxs and ΔSSSsuggest that some form of ordering (cation?) isobtained in the synthetic phases, and that vibrationalentropies of the solid solution are diminished incomparison to the end-members.In contrast, biogenic samples generally have positivevalues of ΔHxs, increasing from +1 kJ/molat 5 mol % to +3 kJ/mol at 20 mol % MgCO3. Mostvalues of ΔSSS are equal (within errors)to values expected from configurational enthalpyalone. Thus, in most biogenic materials cationordering probably is not obtained, and most phases aremore typical of equivalent-site solid solutions.

Coral diversity and mode of growth of lateral expansion patch reefs at Mexico rocks, northern Belize shelf, central America

Mexico Rocks is a large patch reef complex on the outer shelf of northern Belize, to the lee of the barrier reef. The complex consists of approximately 100 patch reefs, clustered on a topographic ridge of Pleistocene limestone, and is composed predominantly (83%) of head corals (Montastrea annularis). Biotic zonation is not apparent on any of the patch reefs. Concomitant with increasing area of individual reefs is an increase in surficial areas of dead coral, areas degraded by physical and bio-erosion, and cavities. Such areas enhance habitat complexity and provide zooid-free substrates that are colonized rapidly by more cryptic coral species. The result of such processes is an increase in spatial heterogeneity and species richness on larger reefs relative to smaller patch reefs in the complex.Initial colonization of the rocky substratum (Pleistocene limestone bedrock) by single head corals began about 3.5 KaBP, and the patch reefs mostly accreted vertically as growth kept pace with slowly rising sealevel. Continued growth involved lateral expansion and coalescence of adjoining patch reefs to form larger composite reef masses as a consequence of the rapid growth rates ofMontastrea annularis and lack of verticla accommodation space. Such a lateral-expansion style of reef growth, together with dominance of head corals, may mimic the biotic composition and geometry of give-up (drowned) reefs common in early transgressive systems tracts in modern and ancient shelf settings.

Environmental setting of Holocene sabellariid worm reefs, northern Belize

Communities of sabellariid worms (Polychaeta) occur as areally discontinuous, unlithified reefs on an irregular depositional topography of Holocene and older sediments at the mouth of the Northern River Lagoon, Belize, Central America. They are found in nearshore marine, moderate energy, tidally influenced environments of normal to low salinity. These colonies, as much as 30 cm thick, are composed of dense thickets of agglutinated worm tubes (1.0 mm diameter, 3.0 cm length) that trap and bind sand- to silt-sized bioclastic debris, microorganisms, and micrite

Meteoric Calcitization and Incipient Lithification of Recent High-Magnesium Calcite Muds, Belize

High-magnesium calcite-dominated muddy sediments on subaerially exposed rays in northern Belize are being pervasively altered to and lithified by low-magnesium calcite in the meteoric environment. Such diagenesis, occurring in sediments less than 1000-1500 years old, is indicated by the rapid loss of Mg from muds and foraminifera, progressively lighter mean isotopic compositions in LMC-cemented muds relative to those in unaltered marine muds, and concurrent reduction in Sr and increase in Mn concentrations. The fabrics of these LMC deposits are micrites and microspars to pseudospars with pore-filling LMC cements of similar crystal size. These fabrics are similar to calcitized aragonite-dominated precursor micrites except that aragonite relics in microspar or pseudospar are relatively rare in these samples.

Coral mortality, recovery and reef degradation at Mexico Rocks Patch Reef Complex, Northern Belize, Central America: 1995–1997

The 1995 coral bleaching event in the western Caribbean was the first reported episode that significantly affected the Belize barrier and lagoonal patch reefs. Bleaching was attributed to a 2 mo period of warm water temperatures above 30°C. Near Ambergris Caye, barrier and patch reefs experienced up to 50% bleaching. At Mexico Rocks patch reef complex, the bleaching resulted in changes in reef health, community, and physical structure. Prior to the hyperthermal episode, patch reef surface area consisted of 47% healthy framework coral coverage, 12% secondarily colonized biotic coverage, 35% dead coral surfaces that were degraded by biological activity and physical erosion, and 6%cavities. six months after bleaching, most corals had regained their color, but, owing to coral mortality, areas of surface degradation had increased to an average 49% (p=0.029 based on Kruskal–Wallis analyses). Eighteen months after bleaching, degraded surface areas expanded to 53% (p=0.0366). Although re-coloring indicates rapid recovery for surviving corals, the persistence in dead coral surfaces suggests that reef skeletal structure recovery lags behind that of individual corals. Initial results of framework measurements indicate that bleaching events may result in an ‘imbalance’ in the carbonate production rate of coral reefs and produce mass wasting of the skeletal structure. Remapping of reef skeletal structure should establish quantitative measures for the long-term effects of bleaching on patch reef frameworks.

Chemical Differences Among the Shells of Two Euryhaline Species of Fossil Ostracoda (Crustacea): A Preliminary Study

Microfossils were extracted from a Florida Bay sediment core, and brackish and marine environments were interpreted on the basis of fossil ostracode and foraminiferid assemblages. A total of 48 hand-picked specimens of two species of euryhaline ostracodes (Cyprideis salebrosa and Peratocytheridea setipunctata) were chemically analyzed for Ca, Mg, Sr, and Fe concentrations to determine the effect of salinity on bulk skeletal chemistry. Results indicate that adult specimens of the two species have similar Mg and Sr concentrations, but Fe is more concentrated in the shells of C. salebrosa. There are no differences in trace element concentrations in adult specimens from brackish or marine sections of the core. Nodose and non-nodose instars of P. setipunctata contain similar concentrations of Sr, but greater concentrations of Mg and Fe than conspecific adults or adults of C. salebrosa. The enrichment of Mg and Fe in instars may be the result of rapid shell growth rate. Rapid carapace calcification may represent an adaptive strategy for survival that is maintained throughout the ontogeny of an individual. Ostracodes (Cambrian-Recent) are microscopic bivalved crustaceans ubiquitous to most water bodies. After hatching from an egg, an ostracode grows by molting (ecdysis). As a result of this life cycle, a series of carapaces graded in size and shape are shed in the sediment. Each ostracode can potentially molt 7 to 8 times prior to development of the genitalia that characterize the adult stage. When fossilized, these shells provide a record of ontogenic development and abundant specimens for paleontological analysis primarily in the fields of paleoecology and biostratigraphy. The use of suites of fossil ostracodes for interpretation of paleoecology stems from their well documented sensitivity to diverse environments in the Recent and the extensive geologic ranges of some ostracode taxa. Ostracodes abundantly occupy fresh, brackish, and marine waters, and their fossilized carapaces can provide evidence to support paleoenvironmental