David A. Budd

Poorly cemented coral reefs of the eastern tropical Pacific: Possible insights into reef development in a high-CO2 world

Ocean acidification describes the progressive, global reduction in seawater pH that is currently underway because of the accelerating oceanic uptake of atmospheric CO2. Acidification is expected to reduce coral reef calcification and increase reef dissolution. Inorganic cementation in reefs describes the precipitation of CaCO3 that acts to bind framework components and occlude porosity. Little is known about the effects of ocean acidification on reef cementation and whether changes in cementation rates will affect reef resistance to erosion. Coral reefs of the eastern tropical Pacific (ETP) are poorly developed and subject to rapid bioerosion. Upwelling processes mix cool, subthermocline waters with elevated pCO2 (the partial pressure of CO2) and nutrients into the surface layers throughout the ETP. Concerns about ocean acidification have led to the suggestion that this region of naturally low pH waters may serve as a model of coral reef development in a high-CO2 world. We analyzed seawater chemistry and reef framework samples from multiple reef sites in the ETP and found that a low carbonate saturation state (Ω) and trace abundances of cement are characteristic of these reefs. These low cement abundances may be a factor in the high bioerosion rates previously reported for ETP reefs, although elevated nutrients in upwelled waters may also be limiting cementation and/or stimulating bioerosion. ETP reefs represent a real-world example of coral reef growth in low-Ω waters that provide insights into how the biological–geological interface of coral reef ecosystems will change in a high-CO2 world.

Cenozoic dolomites of carbonate islands: their attributes and origin

Abstract Dolomites found on and below carbonate islands, atolls and oceanic platforms provide useful insights into the origin of dolomite; insights that may not be attainable from the study of more ancient cratonic dolomites. For this reason they have been the subject of study for decades. A critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data. In most cases the association with an island is causal and not genetic, nevertheless, these dolomite occurrences are referred to herein as island dolomites. One type of dolomite on carbonate islands is penecontemporaneous dolomite, a phase that forms while the host sediment is in its original depositional setting. Island examples are Holocene in age, occur in Holocene sediments and originate as a direct precipitate from either normal or evaporated seawater. These are microcrystalline, poorly ordered, Ca-rich 18O-enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed. Post-depositional dolomite is the other type of dolomite associated with carbonate islands. This type of dolomite replaces older precursors and forms as cement. Distinctive characteristics include a dominance of fabric-preserving texture, pore-lining cement rims that may exhibit micron-scale banding with low-Mg calcite, and formation in association with precursor dissolution. Sr-isotopic dating indicates that all examples are Neogene or Quaternary in age. Those formed during the Middle Miocene through Pliocene are massive, laterally continuous, and often multigenerational. In contrast, younger examples tend to be localized partial replacements of a single generation. Sr-isotope ages also suggest global synchroneity in many dolomitization events, which suggests a connection between dolomitization, global eustacy and/or global climatic factors. Geochemical attributes of post-depositional island dolomites are Ca enrichment, positive δ18O and δ13C, low Sr contents (150–300 ppm) and low Fe ( Global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin. Inferred origins, however, depend primarily on how δ18O and Sr data are interpreted. Values for Δ18O and DSr must be assumed due to uncertainties in oxygen isotope fractionation and Sr partitioning. There is no consensus or uniformity in those assumptions, thus interpretations can vary and be biased to a desired result. Covariant trends in δ13C and δ18O, some negative δ13C values, and high Sr all favor a mixing-zone origin, but examples with these attributes are few. Lack of covariance in the isotopes, mean δ18O of +2.0% to +3.5% and low Sr ( Future studies should standardize the type of data collected and the analytical techniques employed. Multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible. Also needed are a better understanding of the kinetic processes that form these dolomites, more careful assessment of their recrystallization status and an improved understanding of DSr and Δ18O at low temperature.

Bathymetric Zonation and Paleoecological Significance of Microborings in Puerto Rican Shelf and Slope Sediments

Sediments collected from 63 sites on the Puerto Rican shelf and slope at depths ranging from intertidal to 530 m were analyzed for associated microborings. Scanning electron microscope examination of three dimensional plastic casts of microboring networks form the primary basis for this study. In all, 27 microboring forms are distinguished on the basis of size and shape of boring casts, mode of branching, overall boring pattern and associated accessory structures. Although most of the microborings exhibit wide bathymetric ranges, many of the forms tend to occur more abundantly within restricted depth zones, suggesting a three-fold subdivision. The upper photic zone assemblage (intertidal to 20 m) is characterized by the endolithic blue-green algae Hyella gigas, Hyella caespitosa, Hyella tenuoir, Hormathonema sp., and an unidentified endolith likely of algal origin (form D). The lower photic zone assemblage (20 m - 85 m) is characterized by the green algae Ostreobium quekettii and Codiolum polyrhizum , the Conchocelis -stage of an endolithic red alga, and a problematic algal form (A). The aphotic zone is characterized by the absence of algal borings and the presence of fungal form Il and a tubular form of uncertain affinity. Microborings in the upper photic zone are typically oriented perpendicular to substrate surfaces; those of the lower photic and aphotic zones parallel substrate surfaces and penetrate only slightly into substrate interiors. Microborings as trace fossils appear to have high paleoecologic potential inasmuch as they are believed to be biologically specific and often are morphologically distinctive. Microborings have been reported in sediments ranging in age form Cambrian to Holocene with many ancient endoliths resembling extant forms. Differences in direction of penetration between photic and aphotic endolithic organisms may be sufficient criteria for differentiating paleophotic zones in ancient carbonate sequences. A Pleistocene example is presented from the Fort Thompson Formation of south Florida which indicates that impregnation techniques utilized in endolithic analysis of modern sediments may find application in studies of ancient microboring assemblages.

Aragonite-to-calcite transformation during fresh-water diagenesis of carbonates: Insights from pore-water chemistry

Dissolved strontium and calcium concentrations in fresh-water lenses (FWL) and associated mixing zones (MZ) on two small, Holocene ooid-sand islands in the Schooner Cays, Bahamas, were monitored during a 1-yr period to quantitatively analyze the transformation of aragonite to calcite. The observed characteristics of this mass transfer are functions of climate and hydrology. Aragonite-to-calcite transformation in all hydrologic zones is primarily associated with meteoric recharge. The transformation occurs throughout the FWL and in the MZ to relative salinities of 19% and 36% sea water on the two islands. Rates of transformation are rapid in all zones and are greatest in the FWL. A limestone composed of 100% calcite should form from an aragonite precursor within 4,700 to 15,600 yr in the FWL, and within 8,700 to 60,000 yr in the upper MZ. Efficiencies of transformation can vary between hydrologic zones due to PCO2 effects; yet, the efficiency of the entire system (FWL + MZ) is high (87%). This indicates that most CaCO3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh-water system or upper mixing zone. CO2 effects, fresh-water-sea-water mixing, and the differing solubilities between aragonite and calcite all drive the mass transfer. The latter is the most significant, accounting for up to nine times more mass transfer than CO2 effects and at least ten times more mass transfer than fresh-water-sea-water mixing. Differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle, but it apparently becomes ineffective after the rainy season, possibly due to the inhibition of calcite precipitation.

Unconformities and Porosity in Carbonate Strata

Any single geologic setting may include a variety of geochemical environments, each capable of producing a different type of carbonate solution porosity. Also, many types of porosity can form in more than one geologic setting. Thus, the interpretation of solution porosity is best approached by first delineating the geochemical processes necessary to form the observed pattern of porosity, and then using these insights to assess the broader geologic context. Throughout most of any carbonate formation the solution process is highly selective, and only those openings of maximum groundwater flow are enlarged, while surrounding openings undergo little or no enlargement. Pervasive macroscopic porosity, in which nearly all initial openings are enlarged by solution, is formed by: (1) meteoric water with high discharge and/or low flow distance, (2) mixing of waters of disparate chemistry, (3) oxidation of hydrogen sulfide, or (4) production of acids by redox reactions involving carbon compounds in reducing environments. Areally extensive solution porosity within a narrow stratigraphic range usually indicates solution or reduction of sulfates. Cavernous solution porosity is negligible where aggressive infiltration is lacking, in deep zones where groundwater chemistry is uniform, and in low-flow areas of diagenetically mature carbonate rocks far from sources of groundwater recharge.

Geochemical Imprint of Meteoric Diagenesis in Holocene Ooid Sands, Schooner Cays, Bahamas: Correlation of Calcite Cement Geochemistry with Extant Groundwaters

ABSTRACT The geochemistry of meteoric calcite cements and their parent waters on three small Holocene ooid-sand islands in the Schooner Cays, Bahamas, is explicated in terms of observed hydrologic and hydrochemical processes. Dissolved Mg is derived soley from admixed seawater or aerosols. The Mg contents of the cements (0.4 to 3.0 mole %) are thus a salinity indicator and suggest cementation in waters composed of 16 mmoles/liter per yr). Variations in Mg and Sr contents across cement mosaics reflect long-term temporal fluctuations in the amou t of aragonite-to-calcite alteration and incursions of brackish waters. The pore-water oxygen reservoir is water-buffered and derived from meteoric rainfall and seawater mixing, but is seasonally enriched in 18O by evaporation. The 18O values of the cements (-3.4 to -5.0 PDB) reflect the relative proportions of cements formed during and immediately after the rainy season versus those formed during the dry season. The pore-water carbon is derived from organic respiration, atmospheric carbon, and dissolved aragonite. Organic respiration dominates the 18C of the vadose cements (-7.8 to + 1.1 PDB) and the 13C of phreatic groundwaters (-2.2 to -14.0% PDB). In contrast, 13C-enriched phreatic cements (-1.2 to +3.7% PDB) reflect carbon from dissolved aragonite, and are not in carbon isotopic equilibrium with the present groundwaters. This case example reveals the complex geochemical signal that can result from even a very simple diagenetic history, such as a single phase of meteoric diagenesis acting on a purely aragonitic sediment.

The destruction of paleoclimatic isotopic signals in Pleistocene carbonate soil nodules of Western Australia

Stable carbon and oxygen isotopic analyses were conducted on small (<10 mm), spherical Pleistocene carbonate soil nodules from 53 different levels in a 9.15-m soil auger collected in northern Western Australia. The δ13C and δ18O values range from −4 to 0‰ and −9 to −6‰ (Pee Dee Belemnite standard), respectively. Comparison of these values to the δ13C values of coexisting soil organic matter (SOM) reveals marked differences between the two records. The SOM values record an enrichment of ∼16‰ through the section, whereas the carbonate values record only a 4‰ variability. Application of a diffusion–production model to the data indicates isotopic disequilibrium between the two carbon records. One possible interpretation of the apparent isotopic disequilibrium is that the nodules formed at depths of ∼1 cm, but this interpretation is not supported by field observations. An alternative is that the soil organic matter currently in the profile was emplaced after nodule formation. Such a scenario would have to have occurred numerous times during the Late Pleistocene as the nodules grew below different soil zones through time. A third explanation is that diagenetic alteration of the carbonates in the presence of dissolved inorganic groundwater carbon has occurred. This interpretation is consistent with the widely fluctuating regional groundwater table. It is also supported by petrographic fabrics and heterogeneous cathodoluminescence patterns indicative of initial pedogenic development of the nodules and then at least one dissolution event, multiple precipitation events, prolonged nodule growth by displacive spars and microspars, cementation, and recrystallization of some micrite. The results illustrate potential problems for (1) the use of some pedogenic carbonate isotopic records as proxies for environmental change, and (2) the use of ‘altered’ δ18O values and ‘unaltered’ δ13C values as screening tools for the presence or absence of diagenesis. The results highlight the necessity of understanding regional environmental conditions and diagenetic histories prior to using pedogenic carbonates in paleoreconstructions.

Micro-rhombic calcite and microporosity in limestones: a geochemical study of the lower cretaceous thamama group, U.A.E.

Abstract Euhedral micro-rhombic calcite forms a microporosity network in limestones of the Lower Cretaceous Thamama Group, U.A.E. Isotopic and trace-element data indicate that micro-rhomb formation was a two-step process associated with diagenesis in meteoric waters. The micro-rhombic calcite, and the micropores are both products of the second event. Sr, Fe, Mn, and Mg contents of the micro-rhombs are all relatively low, ranging from 108 to 310 ppm, 34 to 308 ppm, 3 to 32 ppm, and 0.42 to 0.68 mole %, respectively. The low Sr and Mg contents indicate extensive flushing of these constituents from the rock. The low Fe and Mn contents are probably inherited from the original sediments and indicate the lack of any Fe or Mn in the pore fluids during micro-rhomb formation. The ϵ13C of the micro-rhombs (+ 3.1 to + 4.5% PDM) are also primary, unaltered by diagenesis. The ϵ13O of the micro-rhombs (−4.5 to −6.0% PDB) reflect alteration by mateoric waters. A large number of pore volumes and an open, water-buffered system would be necessary to flush Sr and Mg, yet decreasing ϵ18O with increasing porosity indicates that ϵ18O values in non-porous intervals are partially rock-buffered. Two diagenetic events are thus implicated, the first flushing Sr and Mg and lowering ϵ18O in all rocks, and the second event actually forming the micro-rhombs and further altering ϵ18O as a function of porosity. The first diagenetic event was most likely mineralogical stabilization in an unconfined meteoric aquifer immediately after Shuaiba deposition. If mineralogical stabilization was completed in the first event, then calcite-to-calcite recrystallization is implicated for the second event. The second event may have occurred in the unconfined meteoric aquifer, or it may have occurred in a confined meteoric aquifer during the early Late Cretaceous.

Permeability loss with depth in the Cenozoic carbonate platform of west-central Florida

Relationships between matrix permeability and depth in shallow-buried (< 500 m) Cenozoic limestones of Florida were investigated using more than 12,000 minipermeameter measurements on 1210 m of slim-hole core. These data are considered representative of the matrix permeability reduction that accompanies early burial of shallow-water, subtidal to peritidal, nonreefal carbonates of sta ble cratonic margins, platforms, shelves, and ramps. Measured matrix permeabilities range from less than 4.0 to 6450 md for both limestones and dolostones, although the median permeability of limestones (88 md) exceeds that for dolostones (15 md). Sucrosic dolostones, which have high intercrystalline po rosities, have median matrix permeabilities of 1000 md. Non sucrosic dolostones, which are much less porous, have median matrix permeabilities of only 13 md. Neither population of dolostones exhibits a permeability-with-depth trend. Low-permeability limestones consist of mudstones, wackestones, (tidal) laminites, pedogenetically altered limestone, lime mud-rich breccia, and well-cemented grain-supported rocks. These low-permeability limestones all exhibit median matrix permeabilities less than 35 md, and none exhibit permeability-with-depth trends. In contrast, high-mud packstones, low-mud packstones, very low mud packstones, and grainstones exhibit larger median matrix permeabilities (69, 129, 181, and 418 md, respectively) and a systematic reduction with depth in their mean and maximum permeabilities. Least-squares exponential regressions indicate that burial depth explains 22-31% of the permeability variance in these grain-supported rocks; the balance of the variability within each textural class is attributed to variation in facies (grain types, grain sizes, sorting, etc.) and the amount of early, preburial cementation. In the absence of permeability enhancement by deep subsurface diagenesis or fracturing, extrapolation of the permeability-with-depth trends suggests that the limestones that have the best reservoir potential in the deeper subsurface are grain-supported limestones that have higher than (Begin page 1254) average matrix permeabilities in the near-surface realm. Petrographic analyses and consideration of diagenetic histories suggest that matrix permeability of the dolostones and low-permeability limestones was determined by early near-surface dolomitization and cementation, respectively, and has yet to be affected by burial processes. Petrographic observations also suggest that the depth signal in grain-supported limestones results from mechanical compaction and the onset of grain-to-grain pressure solution. Extrapolation of the matrix permeability data indicates that permeability loss with burial occurs faster than the documented burial-related porosity loss for the same group of rocks. This indicates that limestone permeability is more sensitive to burial compaction than limestone porosity. Thus, preservation of reservoir-quality rock during burial is more dependent on permeability preservation than on porosity preservation.

Unconformities and Porosity Development in Carbonate Strata: Ideas from a Hedberg Conference

The purpose of this paper is to highlight ideas from a Hedberg Conference concerning detection of unconformities and associated porosity in carbonate strata. Many types of information, including cores and outcrops, seismic data, eustatic sea level curves, wireline logs, biostratigraphic data, stable isotope trends, cycle stacking patterns, and tectonic and basin evolution models, can be used to predict and/or detect subaerial unconformities. All methods of detecting subaerial exposure with these types of information have limits and pitfalls. Detailed analysis of the stratigraphy, sedimentology, and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure, though an integrated approach using all a ailable information is recommended. The relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted. Subsurface porosity is the product of many factors operating during deposition, subaerial exposure, and burial. Several concepts should be recognized when evaluating porosity associated with subaerial unconformities. (1) Carbonate diagenesis during subaerial exposure rearranges pore networks, but usually does not increase the total porosity. Near-surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone. In many cases, total porosity in carbonates under unconformities is actually reduced during subaerial exposure. (2) Permeability is likely to change more than porosity during subaerial exposure, increasing in some c ses but decreasing in others. (3) Pore systems evolve with time during subaerial exposure. Short periods of subaerial exposure (10,000-400,000 yr) are often associated with greater porosity than long intervals of subaerial exposure (1-20 million yr). Prolonged subaerial exposure may change permeability less than porosity because high-permeability, karst-related conduits can form quickly and persist for millions of years. (4) Many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction, cementation, and stratal collapse reduce porosity during burial. (5) Unconformity-related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial. (6) Vugs, caverns, and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure. (7) Lithologic changes at unconformities, like shales overlying carbonates, can influence the flow of subsurface fluids during End_Page 857------------------------------ deeper burial, resulting in deep-burial dissolution localized along unconformities. If subaerial exposure and exposure surfaces are integrated into a sequence stratigraphic, paleogeographic, and climatic framework, systematic and hopefully predictable patterns will emerge. However, we do not fully understand the complex interplay between the factors critical to creating and preserving porosity in carbonates. Additional research is needed to quantify critical processes and products so we can reliably predict porosity associated with subaerial exposure during exploration in frontier basins, and on a smaller scale, during reservoir analysis.