Aragonitic Fossils Research Articles

Facies Specificity of Megaporosity in Mesozoic Shelf-Edge Carbonates, Baltimore Canyon Basin: ABSTRACT

Cores from three wells drilled by Shell Offshore Inc. provide insight into the evolution and distribution of porosity within the Mesozoic shelf-edge carbonates of the Baltimore Canyon basin. Significant primary and secondary megaporosity was found only in the late Kimmeridgian-Berriasian aggradational part of the paleoshelf-edge deposits. Megaporosity is patchy but shows an overall seaward decrease in abundance, averaging from about 13% in back-reed rocks to about 4% in reef-front deposits. This porosity trend parallels a basinward change from mainly freshwater to predominantly submarine diagenesis. In grain-supported reef-flat rocks, the porosity consists of some primary pores but dominantly is of secondary moldic pores. The secondary pores are fabric selective after former aragonitic fossils. Both primary and secondary pores are partly or completely filled with freshwater cements. These consist of clear, equant calcite crystals distributed as discontinuous or continuous circumgranular crusts and epitaxial rims typical of stabilized grainstones. Cementation was contemporaneous with or postdated leaching, but predated breakage of empty micritic envelopes. In reef-front deposits, megaporosity occurs as delicately dissolved fossils and porosity haloes in cemented packstones and grainstones within constructional reef cavities. These secondary pores are surrounded by at least two generations of cement. The first generation is a synsedimentary marinemore » phase. It consists of both clear-bladed and inclusion-rich radiaxial crystals distributed chiefly as pore-lining crusts. The second generation is nonmarine, similar to the freshwater cements found in the reef-flat facies.« less

Oxygen and carbon isotope fractionation in biogenic aragonite: Temperature effects

To better interpret the isotopic composition of ancient aragonitic fossils, stable-isotopic analyses have been performed on live and modern specimens of aragonitic foraminifera, gastropods and scaphopods. Samples were collected from the continental margins off southern California and Texas, U.S.A., and Mexico, and provide a range in ambient temperature of 2.6–22.0°C. We observed a strong covariance between the δ 18O of the aragonitic foraminifera Hoeglundina elegans and that of coeval aragonitic mollusks. On the average, Hoeglundina was 0.2 ± 0.2‰ depleted in 18O relative to the mollusks, and 0.6 ± 0.3‰ enriched relative to the calcitic foraminifera Uvigerina. This enrichment in 18O of aragonite relative to calcite is similar to that observed in previous experimental and theoretical studies. The temperature dependences of mollusk and Hoeglundina δ 18O-values were not notably different from that previously determined for inorganically precipitated calcite, and no significant temperature dependence in Hoeglundina-Uvigerina 18O fractionation was observed. Of note is the temperature dependence of the δ 13C of the biogenic aragonite. Relative to the dissolved inorganic carbon (DIC), the δ 13C of Hoeglundina and the mollusks decreased by 0.11 and 0.13‰, respectively, per °C increase in temperature. The temperature dependence in Hoeglundina-DIC 13C enrichment, and the lack of it in Uvigerina-DIC enrichment, accounts for the temperature dependence in Hoeglundina-Uvigerina (calcitic) fractionation noted by us and previous workers. Isotopic differences between coeval specimens of these genera provide a rough measure of paleotemperature without requiring a knowledge of the isotopic composition of the paleo-ocean.

Recent meteoric diagenesis of miocene Mg-Calcite (Hydrobia Beds, Mainz Basin, Germany)

Miocene Mg-calcite with a maximum of 16 mole% MgCO3 was reported in Hydrobia Beds from the Mainz Basin in 1971. Samples were from a road cut near Mainz Weisenau just opended for highway construction. This locality was resampled in 1973 and 1983. 53 samples from a four meter thick vertical section of subhorizontal dark marls and marly clays revealed Mg-calcites with a MgCO3-content of up to 12.4 mole%. Mg-calcites occur (1) as single grains (“wheat grains”) of 4–10 μm length and 1–3 μm diameter, (2) as “clusters” composed of single crystals of about 0.2 μm size, and (3) as oriented coatings on aragonitic fossils. MgCO3-content is highest within homogeneous marls. It decreases gradually, however, approaching the intercalated oncoidsand-layers, each a few centimeters to some decimeters thick, which allow meteoric water to percolate. The diagenetic influence of meteoric water could likewise be demonstrated by analyzing a horizontal section within a single clay layer. MgCO3-content within Mg-calcites decreases at a distance of 25 cm from the present vertical surface from 15 mole% to less than 10 mole%. Mg-calcites within the section studied can, if adequately exposed to meteoric water, be transformed quantitatively into stable calcite within 10–20 years. In this paper we studied the section in the Mainz Basin in further detail in order to reveal conditions for the stability of the Miocene Mg-calcites reported earlier.

Oxygen isotopic composition of Neogene molluscan fossils from the Eel River Basin of California

Temporal changes in the oxygen isotopic composition of specimens of fossils (mainly the bivalves Macoma and Pecten ) from four sections of Neogene rocks exposed in the Eel River Basin of northern California resulted from changes in temperature and global glacier volume. The temperature was higher (probably 10–15 °C) during the early history of the basin than later. The temperature was probably 5–10 °C during much of the Plio-Pleistocene and showed no well-defined trends, even though the depth of deposition decreased considerably during that time interval. This uniformity was due to strong upwelling of deep water that kept the shallow-water temperatures low. Samples from shallow-water and relatively inland settings at Holmes and Garberville have lower 18 O/ 16 O ratios than do samples from the other two sections. This is because of either higher temperature of these shallow waters or dilution of the sea water by isotopically light fresh water. The difference in fractionation of oxygen isotopes between calcite ( Pecten ) and aragonite ( Macoma ) fossils potentially provides a means of determining oxygen isotope–based paleotemperatures that is independent of the isotopic composition of the water in which the animals lived. The carbon isotopic composition shows a vital effect and is more variable in the nearshore samples.

Diagenesis of a Stromatoporoid Patch Reef

ABSTRACT At Mustoe, Virginia, four biofacies of the Lower Devonian Helderberg Group, including a 13-m patch reef, are well exposed. A detailed petrographic analysis indicates that these lime sediments were lithified and altered in two subsurface environments. After growth ceased, the reef was blanketed by nearshore and shoreline facies; deposition at this time was regressive, terminating in regional subaerial exposure. Fresh water then invaded the near-surface sediments, and diagenesis in the meteoric-phreatic zone included rapid neomorphism of some aragonitic fossils, leaching of others, and precipitation of calcite cement with three crystal forms. Overlying limestone facies reflect subsequent subsidence, and the reef was buried progressively deeper. Diagenesis continued in the deep subsurfac and included pressure solution, probable smectite-illite conversion, precipitation of ferroan saddle dolomite and coarse calcite cements, and replacement of argillaceous matrix by dolomite. In both subsurface environments, several interrelated diagenetic processes were operating, and the final crystal textures, fabrics, and mineralogies were a consequence of this interplay.

Carbonate Petrology of Arun Limestone, Arun Field, Sumatra, Indonesia: ABSTRACT

End_Page 524------------------------------The Arun gas and gas distillate field (estimated 13.7 tcf hydrocarbon gas in place) is a large Miocene coral-algal reef complex located on an intrabasin high in the North Sumatra basin. It was discovered in 1971 by Pertamina/Mobil Oil Indonesia following definition of a reflection seismic anomaly. The field is a large, asymmetric stratigraphic trap 18.5 km (11 mi) long and 5 km (3 mi) wide. The Arun Limestone which forms the reservoir is overlain, underlain, and possibly surrounded by shale. The limestone ranges in thickness from zero west of the field to about 1,200 ft (365 m) in well A-10. Closure on the Arun Limestone is at least 1,200 ft (365 m). The reservoir rock is made up of several carbonate rock types, including coral-algal boundstones, foraminiferal packstones and wackestones, mixed-skeletal wackestones and packstones, and dolomite. Interstitial fill of the reef consists of lime and reef detritus (i.e., skeletal wackestones and packstones); grainstone fabrics are notably absent in the Arun reef and related facies. Diagenesis has had a strong effect on the original sediments; (1) the lower part of the reservoir is completely dolomitized; (2) patches of limestone throughout the Arun are recrystalized to sparry calcite; and (3) much, if not most, of the reservoir is strongly micritized. The most pronounced effect of diagenesis has been the formation of secondary moldic and vuggy porosity formed by the dissolution of aragonite fossils (mainly branching corals, mollusks, and foraminifera). Had there been no diagenesis of the original limestone, it is doubtful that a hydrocarbon reservoir would be present on the Arun structure. End_of_Article - Last_Page 525------------

Facies Control of Cementation and Porosity, Pennsylvanian Fan-Delta Sandstones, Texas Panhandle: ABSTRACT

Interbedded fan-delta sandstones and limestones were deposited on a shallow carbonate shelf in the southern Anadarko basin during Missourian time. Hydrocarbon production from the fan-delta sandstones at Mobeetie field, Wheeler County, is controlled both by structure and facies-determined porosity distribution. Distal margins of some fan-delta lobes were reworked by marine processes, and carbonate fossil fragments and oolites were mixed with terrigenous clastics. Diagenetic history of the distal, marine-reworked sandstones was strikingly different from that of the more proximal, non-reworked sandstones. The first cement to precipitate in the reworked sandstones was a thin, isopachous rim of Mg-calcite cement that probably precipitated in the submarine environment soon after deposition. Next, establishment of a freshwater, phreatic environment in the sediments resulted in extensive calcite cementation in the calcareous sandstones. Dissolution of aragonitic oolites and fossils provided the source of the calcite that occluded primary porosity. In contrast, the non-reworked sandstones were not cemented because they lacked a calcite source, and so they retained high porosity. Rims of authigenic chlorite, which reduced porosity by only a few percent, were the earliest cements to precipitate in the non-reworked facies. With increasing burial, porosity in both the reworked and non-reworked fan-delta sandstones was reduced by precipitation of authigenic quartz, feldspar, kaolinite, Fe-calcite, and ankerite. These cements are generally minor in volume and do not influence porosity distribution. Generation of secondary porosity by dissolution of feldspars and rock fragments occurred in all sandstones but was more extensive in non-reworked facies. However, the main control of present porosity distribution is the presence or absence of early, freshwater calcite cement. End_of_Article - Last_Page 565------------

Quaternary Mixing-Zone Dolomite, Eastern Yucatan Peninsula: ABSTRACT

Dolomite occurs in Pleistocene limestones a few meters below the water table on the eastern margin of the Yucatan Peninsula. It is present in five of seven cores (surface to depth of 11.5 m) which penetrate three stratigraphic units, the youngest of which is dated at 122,000 years B.P. Each unit consists of platform-margin reefs and back-reef facies, and there are no indications of evaporites or restricted conditions during deposition. The most extensive dolomitization (up to 48 wt. %) is in the only reef-facies core from the middle unit. This core is calcite and dolomite in contrast to three cores in the back-reef facies which are calcite and aragonite. Dolomite occurs in a variety of forms including: (1) coarsely and finely crystalline pore-lining cement, (2) finely crystalline replacement of matrix and bioclasts, and (3) internal sediment in dissolution cavities. Most of the dolomite cement is precipitated in molds of aragonitic fossils. There are three types of dolospar cement: (1) limpid euhedral to subhedral crystals (Ca57 Mg43 CO3100), (2) zoned crystals of dolomite and calcian dolomite (Ca63 Mg37 CO3100), and (3) corrugat d layers of alternating calcian dolomite and calcite. The calcian-dolomite layers in the zoned crystals and in the corrugated layers are partly dissolved. Typically, adjacent pores within the same sample contain different types of dolomite and dolomite-calcite intergrowths. The complex mineralogy of the middle unit is evidence that this limestone was subjected to several changes in phreatic-water geochemistry. Concurrent work by Hanshaw and Back demonstrated the existence of a geochemically active and fluctuating phreatic environment in the zone of freshwater and seawater mixing immediately inland from the Yucatan coastline today. Similar mixing zones must have passed through the Pleistocene limestones during past sea-level changes. The geologic setting, textures, and mineralogy of these young limestones suggest that their complex variety and occurrence of dolomite is best explained by mixing-zone diagenesis. End_of_Article - Last_Page 640------------

Structure of Upper Cretaceous chalk benthic communities, southwestern Arkansas

Paleoecological investigation of chalk facies in the Saratoga and Annona Formations includes paleoenvironmental analysis as well as examination of benthic macro-invertebrate faunas. The Saratoga sandy chalk facies was deposited in a middle shelf environment (25–50 m depth), while the Annona chalk-marly chalk facies was deposited in a outer shelf environment (75–125 m depth). Each facies is characterized by a distinct community of benthic macro-invertebrate fossils dominated in occurence by epifaunal suspension-feeding bivalves and deposit-feeding echinoids. However, sandy chalk also contains moderately common infaunal suspension-feeding bivalves and gastropods, and rare infaunal deposit-feeding bivalves, while chalk-markly contains only rare infaunal bivalves and no reported gastropods. Differences in composition within and between the two communities apparently reflect original distribution of shelled taxa. Trace fossil preservation indicates that substrates in both facies were thixotropic. Functional analysis demonstrates that many taxa show adaptations for living on soft substrates. Differences in diversity within each facies may be owing to non-uniformity of substrate softness, because many suspension-feeders were unable to cope with the softest substrates and are therefore sparse in rocks apparently representing these substrates. Variations in substrate softness may have been caused by differing rates of bioturbation. Presence of infaunal suspension-feeding bivalves in the Saratoga, but relative lack in the Annona, may be primarily a manifestation of softer Annona substrates caused by generally finer grain size of Annona sediments. Several studies of chalk faunas have concluded that sparseness of originally aragonitic fossils was caused by secondary sea-floor dissolution or diagenetic processes. Results of this study demonstrate that primary organism-sediment interactions could have also played a significant role in causing the general lack of originally aragonitic fossils in Upper Cretaceous chalks.

EARLY SUBMARINE LITHIFICATION IN THE RED CHALK AND LOWER CHALK OF EASTERN ENGLAND: A BACTERIAL CONTROL MODEL AND ITS IMPLICATIONS

SUMMARY Early submarine lithifications are common in the Albian and Cenomanian Red Chalk and Lower Chalk of eastern England. They have developed generally in shallow water sediments deposited relatively slowly in the aphotic zone of the Chalk sea. Five varieties of early lithification are described and interpreted in detail. The lithification has occurred during the intrinsic diagenesis of the sediment ranging from very close to the water/sediment interface to some metres below. It has been caused by the precipitation of a ferroan calcite cement (2940–8900 ppm Fe 2+ ) under anaerobic alkaline conditions. The preservation of aragonitic fossils indicates that sources from within the sediment and the external reservoir of sea water supplied the necessary Ca 2+ and CO 3 2− . Indirect evidence suggests that (1) bacterial ammonification and sulphate-reduction have effected the precipitation of the ferroan calcite cement, and (2) porewater temperature, depositional rates, and the content of aragonite and organic matter in the sediment were important factors in controlling the development of early lithification. The importance or the bacterial mechanism as a general lithification process in the aphotic marine environment is discussed.

Sandstone pseudomorphs of aragonite fossils in an Ordovician vadose zone

ABSTRACTMoulds after aragonite fossils from two Upper Ordovician limestones in the Oslo Region are filled with well sorted clastic fine sand. The fossil moulds are thought to have been formed by selective dissolution of aragonite shell material by fresh water in the vadose zone. Internal sedimentation post‐dates precipitation of a thin veneer of iron poor drusy calcite cement, but predates precipitation of ferroan blocky calcite cement. These age relationships and the texture of the fine sand suggests sedimentation in semiconsolidated sediment in the vadose zone of an island during early emergence.

Subaerial Diagenesis of Axial Corallite of Acropora Cervicornis: ABSTRACT

In the subaerial environment the axial corallite of Acropora cervicornis is micritized. The micritization starts in the biologically secreted aragonite; later, the pore-infilling cement (recrystallized or not) is micritized. If the micritized fibers are leached out, the nonmicritized fibers are left without support and the corallite becomes crumbly (chalky). As more fibers are micritized, the corallite may be leached out entirely. If the pores of the corallite are infilled with minerals resistant to leaching, a reticular three-dimensional frame will be left in the space occupied by the corallite. If the new voids are infilled by drusy calcite a cast of the corallite will be created. The aragonitic fossil corals located in the splash zone may be replaced by calcite crystal , with much of the texture of the original cement, septa, septothecae, and costae reappearing and being preserved as ghosts in the calcitic crystals. The spherulitic texture of septa, septothecae, and costae produced by a submarine recrystallization process is not preserved. End_of_Article - Last_Page 515------------

Biofacies and Development and Differential Occlusion of Porosity in A Lower Cretaceous (Edwards) Reef

ABSTRACT An Edwards reef, exposed in a large quarry in Scurry County, Texas, exhibits the following biofacies, outward from the reef core: 1) caprinid - Chondrodonta zone (reef core), 2) Eoradiolites - Chondrodonta zone (upper forereef), 3) Toucasia - Eoradiolites - Chondrodonta zone (middle forereef), 4) Cladophyllia zone (lower forereef). Beyond the forereef and below the reef are tabular biostromes in which Eoradiolites, Toucasia, Chondrodonta and Cladophyllia are dominant. The Edwards reef was subaerially exposed soon after deposition, and stabilization of carbonate mineralogy, lithification, solution, of aragonite fossils and differential cementation occurred in a fresh water diagenetic environment. Microcavernous porosity developed in the upper Edwards from solution of aragonitic caprinid and corm shells of the reef core. Adjacent to large caprinid molds former body cavities of Eoradiolites are filled by coarse calcispar cement which initiated as overgrowths on large crystals of inverted (from aragonite) calcite of the inner shell layer. Primary porosity (body cavities of calcitic pelecypods) became occluded by calcite cement, while secondary porosity (molds of aragonitic fossils) was preserved. A sharp boundary, representing a paleo-groundwater table, separates the upper paleovadose zone of microcavernous porosity from a lower paleophreatic zone wherein all former voids, including caprinid molds, became occluded by calcispar cement. A heavy concentration of vadose internal sediment terminates at the palco-groundwater table. Coarse aggrading recrystallization of micrite matrix to pseudospar is well developed in the paleophreatic zone, but is lacking in the paleovadose zone. Differential porosity occlusion was largely controlled by vadose-phreatic groundwater dynamics.

Helium-uranium dating of corals

Fanale and Schaeffer (1965) determined He U ages of corals and other aragonitic fossils and found that the results generally agreed with independent age estimates. As a more extensive and rigorous test of the reliability of He U ages of fossil corals, I have determined He U ages on forty-five independently dated Cenozoic corals. Uranium and thorium isotope compositions and Rn 222 retentivities were also determined as consistency checks. The results indicate that reliable ages are obtained when the following consistency tests are met and the following corrections are made: 1. (1) corals are unrecrystallized aragonite. 2. (2) Th 232 is absent, indicating that the samples are free of contaminating detritus. 3. (3) U 234 U 238 and Th 230 U 234 ratios are concordant with the He/U age. 4. (4) Rn 223 leakage is less than 5 per cent. 5. (5) a correction is made for inherited helium. 6. (6) a correction is made for alpha particles ‘shot out’ from the coral walls during U and U daughter decay.

General considerations on isotopic paleotemperature determinations and analyses on Jurassic ammonites

Paleotemperature analyses have been carried out on the aragonitic outer shells and septa of three Jurassic ammonites. Postmortal alterations can be recongnized easily by the degree of conversion of original aragonite to secondary calcite. The amount of secondary calcite is used as an indication of an isotopic exchange. It often proves difficult, however, to distinguish primary from secondary calcite in calcitic fossils. Therefore, calcitic fossils should only be used for paleotemperature determinations, if they are associated with aragonitic fossils, for instance ammonites, showing a good preservation of the more unstable aragonite.

The Correlation of the Lower Chalk of South-East England

The Lower Chalk of south-eastern England is a richly fossiliferous sequence 54-104 m. thick, spanning most of the Cenomanian Stage. It rests with a slight break or without interruption on Upper Albian dispar-perinflatum Sub-zone sediments of Gault or Upper Greensand facies. The upper limit as here adopted is marked by a slight break at the base of the Cenomanian-Turonian plenus Marls ( Metoicoceras gourdoni Zone), the sub- plenus erosion surface of Jefferies (1962, 1963). Of the many groups of fossils present, the ammonites provide the most useful means of subdivision and correlation. The distribution of these, and of other originally aragonitic fossils, is largely controlled by preservation, and there are levels at which there is evidence of seafloor dissolution of aragonite. Nevertheless, three ammonite zones can be recognised: 3. Zone of Calycoceras naviculare 2. Zone of Acanthoceras rhotomagense 1. Zone of Mantelliceras mantelli Within the M. mantelli Zone three assemblages are identified, characterised by: (a) Hypoturrilites carcitanensis, (b) Mantelliceras saxbii, (c) Mantelliceras gr. dixoni. There are also three assemblages in the Zone of A. rhotomagense, characterised by: (a) Turrilites costatus, (b) Turrilites acutus, (c) Acanthoceras jukes-brownei. The C. naviculare Zone is not subdivided. These zones are comparable to those proposed by Hancock (1959) for the type Cenomanian, and are equivalent to the Lower, Middle and Upper parts of the Stage. Bivalves, corals and brachiopods also have a use in correlation, and even fragments of the commonest fossils— Inoceramus and Holaster—are of local use.

Aragonite in fossils

The distribution of aragonite in the skeletal parts of living organisms is reviewed, and its distribution in fossils is described on the basis of several hundred new determinations. Aragonitic fossils are extensively preserved in Tertiary sediments, and are common in Mesozoic rocks, particularly where the enclosing lithology is argillaceous. No aragonite was found by the authors in fossils from Palaeozoic rocks. The most important requirement for the preservation of aragonite is the presence of reducing conditions. It is suggested that the preservation of aragonite is due to the protective effect of organic skeletal matrix, which in turn requires reducing conditions for its preservation. The protective action of the organic matrix is attributed to the formation of a hydrophobic monomolecular layer on the crystal surface, composed of aminoacids derived from the breakdown of skeletal matrix proteins.

Processes of Conversion of Aragonite to Calcite with Examples from the Cretaceous of Texas

ABSTRACT Many invertebrate skeletons and skeletal fragments in modern carbonate sediments are aragonitic, but, when found in carbonate rocks, they are nearly always calcitic. Two processes of conversion of this aragonite to calcite have been suggested: 1. a process of in situ conversion, and 2. a solution-deposition process in which the aragonite is dissolved and replaced by drusy calcite. This basic process can be further subdivided into four types depending on the relative timing of this solution of aragonitic skeletons and skeletal fragments, lithification of carbonate mud matrix, and deposition of the drusy calcite. If solution of the aragonitic allochems occurs before lithification of the carbonate mud matrix, the resulting void will collapse. Solution-deposition may occur after li hification of the carbonate mud matrix but before any drusy calcite has formed in the originally existing voids within fossils, resulting in fossil replacement without preservation of the outlines of internal cavities. If drusy calcite has started to form in original voids within fossils before the solution conversion process begins, replacements retaining outlines of internal cavities will result. If solution of the aragonite occurs before lithification of the carbonate mud and after drusy calcite infilling of original voids, only internal molds are formed. Originally aragonitic fossils from well cores from the lower Cretaceous of Texas give evidence of all but one of these processes. Important variables in determining which conversion process occurs are original shell structure, fluid hemistry, grain size, and time. The process of conversion has an important bearing on the development of carbonate petroleum reservoirs.

Comparison of the Composition of Fossil and Recent Mollusk Shells

Unrecrystallized shells of five species of mollusks were collected from Miocene, Pliocene and Pleistocene deposits and from a variety of Recent environments. The manganese, iron, magnesium, strontium and barium contents, and the mineralogy of these shells, were determined. The average magnesium and manganese contents of the aragonitic fossil shells are lower, and the strontium, barium, and iron contents higher than in Recent shells of the same species. The mean strontium and magnesium contents are lower in the fossil shells of the single calcitic species studied, whereas the remainder of the elements in the shells of this species do not differ statistically. The differences in composition of fossil and Recent shells are attributed to the effects of weathering and diagenesis. Since the trace-element content of both aragonite and calcite can change without recrystallization, geochemical and paleoecological studies based on assumptions of unchanging composition of carbonate skeletons should be used with caution.

Trace Elements in Recent and Fossil Mollusk Shells: ABSTRACT

Analyses were performed for iron, magnesium, manganese, strontium, barium, and mineralogy in the shells of seven species of mollusks collected over much of their present-day environmental range. Correlations between the shell composition and water temperature and salinity were determined in order to evaluate the feasibility of a paleoecological tool using these elements. Significant relationships were observed, but they are generally too weak to be used for paleoecological determinations and are not consistent between species. Differences in salinity cause greater changes in shell composition than differences in temperature, but salinities above 25 ppt. do not greatly affect the shell composition. Unrecrystallized Miocene and Pleistocene shells of five of the same species were analyzed for comparison with the Recent shells. The average magnesium and manganese contents of the aragonitic fossil shells are lower, whereas the strontium, barium, and iron contents are higher than in Recent shells of the same species. The mean strontium and magnesium contents are lower in the fossil shells of the single calcitic species studied, and the remainder of the elements do not differ significantly. The difference in composition of fossil and Recent shells are attributed to post-depositional effects. Paleoecological studies based on the composition of carbonate skeletons, radioactive disequilibrium age dating, and the oxygen isotope paleotemperature method should be used with the realization that changes in the trace element content of both aragonite and calcite can occur without recrystallization. End_of_Article - Last_Page 366------------