Margin Of Great Bahama Bank Research Articles

Spatial, seasonal and climatic drivers of suspended sediment atop Great Bahama Bank

ABSTRACTSuspension is the key mechanism by which fine‐grained sediment (≤125 μm) is winnowed and transported across shallow‐water carbonate platforms into adjacent deep waters. Unlike sliding and saltation, which deliver sedimentary structures via bedload, the sedimentological signature of suspended sediment is more cryptic. This study focuses on suspended sediment, and its drivers – wind, waves and tides – to better constrain the processes responsible for the transport of fine‐grained sediments. By building forward from remote sensing algorithms developed for deep‐waters, sediment suspension in the shallow water column can be mapped from satellite. By applying machine learning to Moderate Resolution Imaging Spectroradiometer data for Great Bahama Bank, this study demonstrates how the drivers of sediment suspension change over 18 years across this 100 000 km2 carbonate platform. Through time, both seasonal patterns of suspension, as well as those induced by El Niño‐Southern Oscillation and, more subtly, the Atlantic Meridional Overturning Circulation were tracked. El Niño‐Southern Oscillation modulates wind‐induced currents, while Atlantic Meridional Overturning Circulation affects local sea level. Across space, this study shows how the eastern margin of Great Bahama Bank, which is traditionally considered to be wind‐dominated, primarily owes its suspended sediment to tidal currents focused between islands. Sediment suspension across the leeward margin of Great Bahama Bank, meanwhile, can be attributed to wind‐induced currents and waves. The authors consider this work a step towards delivering a quantitative, reproducible, process‐based understanding of sediment suspension atop carbonate platforms using Earth observation data.

Geochemical fingerprints of dolomitization in Bahamian carbonates: Evidence from sulphur, calcium, magnesium and clumped isotopes

AbstractIn an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow‐water carbonate sediments the δ34S, Δ47, δ13C, δ18O, δ44/40Ca and δ26Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ34S values similar to those found in contemporaneous seawater and δ44/40Ca, δ26Mg values, Sr contents and Δ47 temperatures (25 to 30°C) indicating relatively shallow dolomitization in a fluid‐buffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ34S values indicating dolomitization in a more sediment‐buffered diagenetic system where bacterial sulphate reduction enriches the residual in 34S, consistent with high sediment Sr concentrations and low δ44/40Ca and high δ26Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ34S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive δ44/40Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid‐buffered or closed sediment‐buffered diagenetic system have similar δ26Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ47 values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow‐water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.

Co‐existence of skeletal and ooid shoals as a result of antecedent topography—Cat cay shoal complex, Bahamas

AbstractHigh‐resolution seismic data reveal an unexpected Pleistocene topography underneath the Cat Cay shoal complex along the western margin of Great Bahama Bank, illustrating how Pleistocene topography focuses tidal flow to create different types of grainstone shoals. The 1–3 km wide and 35 km long shoal complex is composed of the Cat Cay ooid shoal that is a laterally continuous 8 m thick ooid shoal and a sequence of 300–600 m wide and less than 6 m thick skeletal‐dominated tidal deltas south of Ocean Cay. The skeletal tidal deltas overlie an irregular Pleistocene surface, while the Cat Cay ooid shoal is situated on a flat Pleistocene surface east of a Pleistocene rock ridge. This finding challenges the assumption that an antecedent high is needed for ooid shoal initiation. The base of the Cat Cay ooid shoal is an up to 4 m thick skeletal‐peloidal unit that is similar in composition to the skeletal tidal deltas south of Ocean Cay but their deposition was followed by an up to 4 m thick accumulation of ooids. The Pleistocene ridge west of the Cat Cay ooid shoal allowed accumulation of mud and peloids (the nucleus source), while to the south, muddy sediment was winnowed away and no ooids formed. The evolution of the two shoal types is ultimately the result of the presence and absence of antecedent topography adjacent to the shoal system, resulting in variations of mud accumulations and the formation of the nucleus in the ooid shoal. The coeval occurrence of ooid and skeletal shoals in the same complex implies that in the rock record, a vertical succession from oolitic to skeletal shoals does not indicate an environmental change such as climate or an anoxic event but rather a change in flow conditions created by antecedent topography.

Rolling window regression of δ13C and δ18O values in carbonate sediments: Implications for source and diagenesis

AbstractDiagenetic alteration can produce, modify or erase significant biogeochemical information recorded in carbon and oxygen (δ13C and δ18O) values of marine carbonates throughout geological time. Understanding the type and extent of alteration experienced by a carbonate deposit can improve sedimentological and geochemical interpretations of events in Earth history. In this study, we present a new application of a statistical approach to aid in the identification and interpretation of sedimentological surfaces in a shallow marine carbonate sequence using a rolling window regression (RWR) analysis. RWR analysis evaluates the degree of covariation between two records and how it changes through time. Geological application of this statistical technique permits a new perspective on the fine scale variability in carbon and oxygen isotope records and the processes that generate them and provides a complementary tool for sedimentological interpretations. In this study, we apply RWR to δ13C and δ18O values from the Clino drilled into the western margin of Great Bahama Bank, a core that has been extensively altered by diagenetic processes, within the vadose, phreatic and marine burial zones. This core penetrates ~676 m of shallow marine to deeper slope deposits and contains a variety of both sedimentological and diagenetic events, including facies transitions, subaerial exposure surfaces, marine hardgrounds, firmgrounds and periods of reduced sedimentation rate. Using more than 1,200 analyses of paired δ13C and δ18O values, we have applied a RWR analysis to evaluate how the correlation between these two proxies changes at scales of 10, 30 and 100 m. The results of this study highlight the dynamic evolution of correlations between δ13C and δ18O values within diagenetic zones and in association with diagenetic surfaces and provide context for interpreting covariance in δ13C and δ18O values from the geological record.

Preface: Celebration of the Career of Robert Nathan Ginsburg

Preface: Celebration of the Career of Robert Nathan Ginsburg

A diagenetic origin for isotopic variability of sediments deposited on the margin of Great Bahama Bank, insights from clumped isotopes

Abstract The clumped isotope temperature proxy has been used to investigate the diagenetic history of carbonate sediments in two cores recovered during ODP Leg 166 on the margin of Great Bahama Bank. While periplatform sediments constitute a tempting archive of paleo ocean chemistry as they are unlikely to be subducted, their primary limitation is a well-documented susceptibility to post-depositional diagenetic reworking. The crystallization temperatures reconstructed using the clumped isotope proxy, as well as the mineralogy and δ13C and δ18O values have been used to determine the relative effects of sediment mixing and sediment recrystallization. Results show that as sediments undergo diagenetic alteration at the seafloor, their initially “warm” clumped isotope composition is overprinted at cooler benthic temperatures. This process appears to occur in an environment with sufficient fluid exchange to overprint carbon isotopes; an observation confirmed in a separate study by analyses of calcium, a similarly rock-buffered element. This early reactive exchange between carbonates and fluids is likely driven by the conversion of metastable aragonite to calcite. This partially occurs within the “flushed zone”, where porewater compositions remain compositionally similar to seawater throughout the upper ∼40 meters of the sediment column. Sediments dominated by open system isotopic compositions correspond to a period of minimal sediment accumulation between 2 and 3 Ma. More deeply buried Miocene sediments of the more platform-proximal Site 1003 show evidence of subsequent recrystallization, incorporating the warmer geothermal TΔ47 values, and more as well as modified water δ18O values, likely driven by co-evolving porewater and carbonate oxygen isotopes. Reconstructed water δ18O values of these deeper sediments at Site 1003 are considerably more positive than the measured modern values, suggesting that porewater δ18O values were more positive during the Miocene. Sediments deposited at the platform distal Site 1006 between the early and middle Miocene did not show evidence for this deeper recrystallization. Differences in diagenetic behavior between the two sites cannot be solely accounted for by differences in sediment accumulation rate. To illustrate this, time-integrated models were constructed which simulated the burial of identically reactive material through the depositional history of each site, the sediments deposited at Site 1006 appear to “stabilize” after an initial phase of neomorphism, whereas the more platform proximal Site 1003 continues to recrystallize during deeper burial, apparently in the presence of a parent fluid with a more positive δ18O value than observed today. We conclude that despite consisting of the same end-member sediment sources, and being spatially separated by less than 30 km, the difference in clumped and oxygen isotopic composition between Sites 1003 and 1006 can be predominantly attributed to differences in the rate and duration of recrystallization during burial.

Geochemical and petrographical characterization of fine-grained carbonate particles along proximal to distal transects

The origin of carbonate ooze particles is often poorly understood. This is due to their polygenic origin and potential post-depositional alteration. Here, the outcome of a physical separation study with regard to different component classes of micritic carbonates is shown. The focus is on grain size and morphology, mineralogy and isotope signatures. Two contrasting proximal-to-distal transects were investigated: (1) the Miocene leeward margin of Great Bahama Bank (ODP Leg 166) and (2) the transition between the Maiella platform and the Umbria–Marche basin in central Italy near the Cenomanian–Turonian boundary. In both case settings, carbonate particles of biogenic origin include at least three groups of organisms: (i) planktonic foraminifera, (ii) calcareous nannofossils and (iii) fragments of unspecified neritic skeletal material. Two further particle types lack diagnostic structures, and based on particle size and mineralogy, are here referred to as (iv) macroparticles (5–20μm, mainly xenomorphic) and (v) microparticles (<12μm, mainly automorphic to sub-automorphic). Macro- and microparticles represent 50 to 80% of the carbonate phase in slope and toe-of-slope domains and share characteristic carbon and oxygen isotope signatures. Macro- and microparticles are considered shallow-water precipitation products subsequently exported into the slope and toe-of-slope domains. Macroparticles are probably related to the fragmentation of neritic skeletal components while microparticles point to inorganic and/or bioinduced precipitation in the water column. In some cases, macro- and microparticles may have an early diagenetic origin. The identification of the origin of fine-grained particles allows for a quantitative assessment of exported, in situ and diagenetic carbonate materials in periplatform environments. The data shown here represent an important step towards a more complete characterization of carbonate ooze and micrite.

The stable carbon isotopic composition of organic material in platform derived sediments: implications for reconstructing the global carbon cycle

AbstractIn pelagic carbonate sediments, the degree to which the δ13C values of inorganic and organic fractions co‐vary has been used to interpret rates of production, burial and decomposition of organic carbon. This relationship is relatively consistent through time, permitting estimates of organic carbon production and preservation. However, as the majority of pelagic sediments older than 200 Myr have been subducted, carbonate sediments deposited in epeiric seas and platforms are often substituted for pelagic carbonates in analyses of ancient global carbon cycling. There are well‐known pitfalls to using shallow marine carbonate materials, including diagenesis, semi‐isolation of depositional environments and input of different types of sediments with varying inorganic δ13C (δ13Cinorganic) values, which can obscure any global signatures. One method used to assess whether global changes in δ13C are accurately represented by δ13Cinorganic records is to examine variations in the δ13C of co‐occurring organic material (δ13Corganic). If a δ13Corganic record co‐varies with a co‐occurring δ13Cinorganic record, it is argued that the signals must be related to variations in the global carbon cycle. This assumption has been investigated by analysing the isotopic composition of the organic carbon preserved in the uppermost 150 m of periplatform sediments recovered during ODP Leg 166 from the western margin of Great Bahama Bank. The δ13Corganic values measured in this study were compared to previously published δ13Cinorganic records measured on identical samples, thus allowing a study of the correlation between the two records through time. These analyses showed that the correlation coefficient between δ13Cinorganic and δ13Corganic increased from the proximal location (Site 1005, r2 = 0·1), to the distal site (Site 1006, r2 = 0·63). The importance of platform‐derived carbonate and organic material at the proximal location, Site 1005, is reflected in the absence of a co‐variation between inorganic and organic δ13C records, which exhibit no correlation on the platform itself. In contrast, the co‐variance in δ13C values at the basinal location, Site 1006, is explained by a two‐point mixing model, which demonstrates the importance of both pelagic and platform‐derived carbonate and organic carbon in generating the positive correlation between the organic and inorganic δ13C values; this results in a correlation between δ13Cinorganic and δ13Corganic records at Site 1006 that is unrelated to global carbon cycling. Such data question the applicability of using δ13Corganic values to support the ability of δ13Cinorganic values to record global carbon cycling in carbonates recovered from environments where multiple sources of carbonate and organic carbon contribute to the bulk δ13C signal.

Whiting-related sediment export along the Middle Miocene carbonate ramp of Great Bahama Bank

Modern aragonite needles are present all along the modern leeward margin of Great Bahama Bank (ODP Leg 166), while Middle Miocene sediments contain needles only in more distal areas (Sites 1006 and 1007). In contrast to the rimmed, flat-topped platform topography during the Plio-Pleistocene, the Miocene Great Bahama Bank morphology is a carbonate ramp profile. This might imply a different location and precipitation type for Miocene aragonite needles. In this study, aragonite needles in Miocene sediments were isolated using a granulometric separation method. Furthermore, the isolation of the various carbonate components enables the identification of primary versus diagenetic components. The Miocene aragonite needles are concentrated in the finest granulometric sediment fractions (<12 μm). The fraction-specific geochemical analyses (δ13C, δ18O and Sr elemental abundance) represent useful tools to assess the possible sources of the aragonite mud. The geochemical variation of the fractions, rich in pristine aragonite needles, and the characteristics of the needle morphology point to whiting phenomena as the main sediment source and algal fragmentation as a minor component. Both components indicate shallow-water environments as the main sediment source area. Ramp-top-related fine-grained particles now present at distal sites were likely exported as suspended material similar to present-day transport mechanisms. The scarcity of needles at proximal sites is probably linked to hydrodynamic processes but dissolution and recrystallization processes cannot be excluded. The granulometric separation approach applied here enables a better characterization of the finest carbonate particles representing an important step towards the discrimination between primary and diagenetic fine-grained components.

Nature and origin of carbonate particles along a transect on the western margin of Great Bahama Bank (Middle Miocene): sedimentary processes and depositional model

Abstract During ODP Leg 166, Middle Miocene sediments were collected along the western margin of the Great Bahama Bank (GBB) at four sites, distributed along a proximal-distal transect. Site 1006 is located in the basin, Site 1007 at the toe-of-slope and Sites 1003 and 1005 on the mid and upper slope. The carbonate slope deposits of GBB consist of periplatform oozes, an admixture of platform-derived aragonite and high-magnesium calcite particles, and pelagic low-magnesium calcite. An assessment of carbonate sedimentation is made in order to estimate the proportion of platform-derived versus pelagic components. The originality of this study is based on the application of a separation method giving access to homogeneous granulometric fractions, ranging from 63 to 3 μm in grain diameter. This method, associated with a multidisciplinary approach (micropaleontology, optical and electronic microscopy, mineralogy – X-ray diffractometry, and trace elements geochemistry – strontium and magnesium), allows the quantification and characterization of different kinds of carbonate particles. In Miocene sediments, three types of particles display a biogenic structure: planktonic foraminifera, calcareous nannofossils and fragments of neritic organisms. Two types of particles do not exhibit any structures that allow for a determination of their origin. Based on their size and their mineralogy, we have distinguished macroparticles (calcite and dolomite) and microparticles (calcite and aragonite). The detailed study of the composition of the separated fractions highlights major differences in carbonate ooze composition between the different sites along the transect. The unlithified samples of Sites 1006 and 1007 are dominated by pelagic components (planktonic foraminifera and calcareous nannofossils) and contain aragonite microparticles. In contrast, lithified sediments of Sites 1003 and 1005 (and 1007) are characterized by higher proportions of neritic debris and particles without biologic structure, the pelagic phase is impoverished and aragonite needles are absent. The origin of particles without biological structure has been demonstrated by their spatial distribution and by mineralogical as well as geochemical criteria. The rhombohedral calcitic microparticles mainly occur at slope sites. Their high magnesium contents support a formation on the bank implying an allochthonous origin. This suggests that calcitic microparticles correspond to the bank micrite exported towards slope environments. Rhomboedral calcitic macroparticles, which fill sediment voids, originate from in situ transformations of metastable carbonates due to pore fluids, which preferentially flow in the slope permeable sediments. Two sedimentary sources are proposed: 1) an autochthonous pelagic source with low-magnesium calcitic components and 2) an allochthonous neritic source with both aragonitic and calcitic components. Export and depositional processes can be differentiated for the allochthonous particles. The neritic debris and calcitic microparticles in slope Sites 1003 and 1005 (and less frequently in Site 1007) were probably exported by gravity currents whereas the aragonite microparticles, due to their shape and granulometry, were perhaps transported as suspension and deposited at the distal Sites 1006 and 1007.

Controls on grain-size patterns in periplatform carbonates: Marginal setting versus glacio-eustacy

The importance of grain size as a fundamental property controlling other physically derived properties is well known. Recent grain-size studies on periplatform sediments of the Caribbean and western north Atlantic have shown that grain-size variability is coupled with changes in carbonate platform productivity and export in response to Pleistocene–Holocene glacio-eustacy. These findings showed that periplatform sediments are dominated by fine-grained material during sea-level highstands and coarser material during sea-level lowstands. This paper addresses the question of grain-size patterns in periplatform settings and their relationship to glacio-eustacy. Furthermore, it considers whether these patterns are applicable to different types of margin settings (accretionary-erosional). Here, existing data (Pleistocene–Holocene) from the accretionary western leeward margin of Great Bahama Bank (ODP Leg 166) and new data from the erosional eastern windward margin (ODP Leg 101) are used. The results show that the grain-size patterns of both margins are controlled by late Neogene sea-level fluctuations. The accretionary leeward margin sediments indicate the same grain-size pattern as previously described in the literature. However, the erosional windward margin shows a reverse pattern, with coarser sediments in interglacials and finer deposits in glacials. This proposes that the leeward margin grain-size pattern is controlled by the productivity-export mode of the platform. This is indicated by the platform-top-derived fine muds, transported off-bank, to form a thick sediment wedge on the low-angled slope during interglacials. In contrast, on the windward margin, the sediment flux pattern is controlled by the impact of mass transport processes. This is indicated by the increased occurrence of turbidites during interglacials that might be more frequent at this time because the steep-angled erosional slope is less apt for the deposition of excess fine material. This material is eroded and/or bypasses the slopes and is redeposited in the periplatform basin setting.

Holocene Atlantic climate variations deduced from carbonate periplatform sediments (leeward margin, Great Bahama Bank)

A marine sediment core from the leeward margin of Great Bahama Bank (GBB) was subjected to a multiproxy study. The aragonite dominated core MD992201 comprises the past 7230 years in a decadal time resolution and shows sedimentation rates of up to 13.8 m/kyr. Aragonite mass accumulation rates, age differences between planktonic foraminifera and aragonite sediments, and temperature distribution are used to deduce changes in aragonite production rates and paleocurrent strengths. Aragonite precipitation rates on GBB are controlled by exchange of carbonate ions and CO2 loss due to temperature‐salinity conditions and biological activity, and these are dependent on the current strength. Paleocurrent strengths on GBB show high current velocities during the periods 6000–5100 years BP, 3500–2700 years BP, and 1600–700 years BP; lower current speeds existed during the time intervals 5100–3500 years BP, 2700–1600 years BP, and 700–100 years BP. Bahamian surface currents are directly linked to the North Atlantic atmospheric circulation, and thus periods with high (low) current speeds are proposed to be phases of strong (weak) atmospheric circulation.

Sedimentation cycles and their diagenesis on the slope of a Miocene carbonate ramp (Bahamas, ODP Leg 166)

Numerous small-scale depositional cycles are present in the Miocene sediments of seismic sequence m along the margin of Great Bahama Bank (ODP Leg 166). These cycles consist of decimetre- to metre-scale alternations between light-grey and dark-grey/black wackestones/packstones. The light-grey layers are well cemented and nearly uncompacted. They contain planktonic and benthic foraminifera, and bioclasts. Bioturbation in these layers is moderate. The dark-grey wackestones/packstones are uncemented, strongly compacted and normally strongly bioturbated. The main components are planktonic foraminifera and fine-grained bioclasts. The dark layers are rich in aragonite and organic carbon and contain around 80% carbonate. The light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97.5%. Light δ 13C and slightly negative δ 18O values were observed in the dark uncemented layers while the cemented intervals show heavy δ 13C values and slightly more positive δ 18O values. The carbon isotope signal between the dark and the light layers shows variations of up to 1.45‰. Both δ 13C and δ 18O co-vary with carbonate content. The Formation MicroScanner images reflect the changes in carbonate mineralogy. The Natural Gamma-Ray Tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy. The internal stacking pattern of the cycles is closely tied to sea level. The dark layers are deposited during rising sea level, while the light layers reflect sediment production and export during highstand and falling sea level. These cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat-topped platforms. Most measured parameters suggest that primary sediment composition played an important role in the cementation process. The primary differences in composition were enhanced during shallow burial diagenesis. The initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite-rich layers leading to the formation of the light beds. Dissolution of metastable high-Mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite. Diagenesis was enhanced by the coarse grain size of the primary aragonite-rich layers. In contrast the more pelagic, dark calcite-rich layers were only slightly affected by diagenesis and could preserve their aragonite content. The uncemented layers subsequently were subject to strong mechanical compaction, whereas the cemented beds stayed nearly uncompacted.

Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas

Carbonate diagenetic models have been heavily influenced by numerous studies of exposed Quaternary limestones. As a result, meteoric diagenesis is often assumed to be the principle means of altering aragonite-rich sediments into calcitic limestones. However, these models are limited by the scarcity of examples of aragonite-rich sediments buried in seawater that have never been influenced by meteoric fluids. The Bahamas transect cores recovered originally aragonite-rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms. The Bahamas transect consists of seven cores drilled in the prograding western margin of Great Bahama Bank. The two proximal cores (Clino and Unda) were drilled on the platform top and recovered shallow-water platform to reef facies overlying deeper margin and proximal slope facies. The five distal cores were drilled by ODP Leg 166 in up to 660 m of water and recovered carbonate slope facies. All studied sections are Neogene to Pleistocene in age. Diagenetic environments were identified based on petrographic and scanning electron microscopy (SEM) observations, XRD mineralogy, carbon and oxygen stable isotopic data, and trace elements. The upper 100–150 m of the two proximal cores were altered in meteoric to mixing-zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor, marine-burial, and deep-burial diagenesis. Several of the findings of this study question current carbonate diagenetic paradigms. These include: (1) large-scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the −120 m elevation of the latest Pleistocene lowstand. Rather, phreatic meteoric diagenesis appears restricted to within ≈10 m of the land surface. (2) Mixing-zone diagenesis includes aragonite dissolution and minor LMC cementation but does not show the cavernous porosity or dolomitization predicted by mixing-zone diagenetic models. Current models are based on coastal mixing zones, which do not appear to be applicable to these more inland, and perhaps more typical, locations. (3) Marine-burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity, aragonite neomorphism, blocky calcite spar and calcite microspar. However, oxygen stable isotopic data (average δ 18O=+1‰) indicate alteration in marine pore fluids only. The character of marine-burial diagenesis is partially controlled by the nature of the sediment being altered. We have identified two end-member styles, an open-system style characterized by dissolution of aragonite without significant cementation and a more closed-system style with aragonite dissolution accompanied by calcite cementation. The sediments examined were deposited well above the aragonite compensation depth, so seawater entering the sediment is saturated with respect to aragonite. The under-saturation needed to drive diagenesis is likely the result of bacterial oxidation of organic matter using sulfate. (4) Microspar forms in these sediments as a cement based on petrographic and SEM examination of partly to completely altered samples. This contradicts the common assumption that microspar forms by aggrading neomorphism of micrite. (5) Strontium content of sediments altered in marine pore fluids can show an extreme range of values, formerly thought to indicate different environments. The opportunity to finally examine the diagenesis of aragonite-rich sediments buried in seawater challenges current diagenetic paradigms and emphasizes the importance of integrated studies.

ABSTRACT: Calibration of Seismic Data from the Western Margin of Great Bahama Bank with Exposed Strata in the Maiella Mountains (Italy)

ABSTRACT: Calibration of Seismic Data from the Western Margin of Great Bahama Bank with Exposed Strata in the Maiella Mountains (Italy)

ABSTRACT: Quaternary Slope Development of the Western, Leeward Margin of Great Bahama Bank

ABSTRACT: Quaternary Slope Development of the Western, Leeward Margin of Great Bahama Bank

The Origin of Dolomites in Tertiary Sediments from the Margin of Great Bahama Bank

ABSTRACT Based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on Great Bahama Bank, we have determined three different environments of formation for the dolomites that are common throughout the Pliocene and Miocene parts of these cores. The first environment of dolomitization occurs in association with development of nondepositional surfaces. Dolomite typically forms below each of these surfaces, the concentration and extent of which is governed by the length of the period of nondeposition. These dolomites are recognized by their association with the nondepositional surfaces, characteristic positive 18O values indicative of formation from cold bottom waters, and 18O and Sr profiles with depth that suggest formation in the presence of diffusive temperature and Sr gradients. The second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes. The dolomite forming here is termed background dolomite. This is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space. Dolomitization by this mechanism uses a local source of Mg2+, and consequently the dolomite never constitutes more than between 5 and 10% of the sediment. This type of dolomite is characterized by extremely high Sr concentrations, which reflect concentrations of Sr2+ in the pore fluids from which it formed. The high concentrations of Sr2+ in the pore fluids arise through the continued recrystallization of metastable aragonite and high-Mg calcite to dolomite and low-magnesium calcite driven by oxidation of organic material by sulfate. Not only does sulfate reduction provide an additional thermodynamic drive for recrystallization, but because the absolute concentration of Sr2+ in the pore fluids is governed by the solubility of celestite (SrSO4), removal of sulfate allows the Sr2+/Ca2+ ratio of the interstitial fluid to become much higher than normally encountered. The final type of environment of dolomitization is associated with coarse-grained reefal sediments. The pervasive nature of the dolomitization and the relatively normal Sr concentrations suggest the circulation of normal marine water in a relatively open system.

The Origin of Dolomites in Tertiary Sediments from the Margin of Great Bahama Bank

The Origin of Dolomites in Tertiary Sediments from the Margin of Great Bahama Bank

Facies and early diagenetic influence on the depositional magnetization of carbonates

Research Article| September 01, 1997 Facies and early diagenetic influence on the depositional magnetization of carbonates Donald F. McNeill Donald F. McNeill 1Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information Donald F. McNeill 1Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (9): 799–802. https://doi.org/10.1130/0091-7613(1997)025<0799:FAEDIO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Donald F. McNeill; Facies and early diagenetic influence on the depositional magnetization of carbonates. Geology 1997;; 25 (9): 799–802. doi: https://doi.org/10.1130/0091-7613(1997)025<0799:FAEDIO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Rock-magnetic data from a core (Clino) on the margin of Great Bahama Bank show that both depositional facies and early diagenesis contribute to preservation of depositional remanent magnetization. This primary magnetization, carried by single-domain magnetite, and resultant directional data recorded in carbonates are influenced by diagenetic alteration that is largely controlled by facies and to a lesser degree relative sea level. Although polarity does not change, there is usually inclination shallowing relative to the axial dipole value, and the inclination values are more dispersed as a result of cementation. Four magneto-diagenetic zones were identified in Clino: from top down, these are (I) a low-Mg calcite, a cemented, shallow-water facies with dispersed inclinations, relatively high remanence coercivity, and magnetostatic interaction; (II) an upper-slope facies, moderately cemented, that has original aragonite mineralogy, produced more statistically consistent inclination angles, lower magnetostatic interactions and coercivity, and higher remanence intensity; (III) a cemented, lower-slope facies having characteristics of both the overlying zones, but containing some dolomite and abundant fractures; and (IV) a partially dolomitized, basal lower-slope interval with magnetic characteristics (dispersed inclination, weaker intensity, higher coercivity, higher magnetostatic interaction) similar to the shallow-water facies. The relatively uncomplicated diagenetic history, excellent age control, and depositional facies provide a spectrum of magnetization destructive processes and characterization of polarity data from platform carbonates. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Recurrent neomorphic and cement microtextures from different diagenetic environments, Quaternary to Late Neogene carbonates, Great Bahama bank

The cores Clino and Unda, taken near the western margin of Great Bahama Bank, contain a diverse suite of Quaternary to Late Neogene carbonates that have a variety of neomorphic and cement microtextures. The microtextures of each of the different neomorphosed aragonitic grain types, such as corals, mollusks, and plates of the calcareous green algae Halimeda, are very similar wherever the neomorphosed grains are present in the cores. Limestone samples that contain neomorphosed aragonitic fossils are cemented predominantly with blocky calcite. Despite the similar neomorphic and cement microtextures, light-stable isotope data reveal that these calcites precipitated in different diagenetic environments in different parts of the cores. The neomorphic calcites and associated calcite cements in the upper parts of Clino and Unda precipitated in meteoric-influenced pore waters, as indicated by δ 18O values ranging from −1.9%. to −3.7%., whereas in the lower part of Clino the calcites precipitated in marine pore waters, as indicated by δ 18O values ranging from −0.1%. to +2.4‰. Essentially identical diagenetic microtextures were thus produced in different diagenetic environments, revealing a significant limitation in the use of microtextural data to determine the diagenetic histories of carbonate rocks.