Foraminifers In Sediments Research Articles

Surface Bottom Sediments of the North Atlantic on the Transect along 59.5° N

An analysis of the lithological, isotope-geochemical, and micropaleontological parameters of the surface layer of bottom sediments in the North Atlantic was made at 26 stations on the profile along 59.5° N. The distribution of humidity and granulometric composition of sediments reflects vertical/circumcontinental zonality, since fine-grained and water-saturated material accumulates in the deepest parts of the Iceland Basin and the Labrador and Irminger seas, which are farthest from land. Larger masses of calcium carbonate are also formed there, but organic carbon is unevenly distributed. Both vertical/circumcontinental and climatic (circulation of the warm North Atlantic water) zonalities affect the mineral composition of the sediment fraction 0.063 mm. The accumulation of diatoms, radiolarians, and planktic foraminifers in sediments reflects both vertical/circumcontinental and, to a greater extent, climatic zonality. Variations in the isotopic oxygen and carbon ratios in the shells of benthic and planktic foraminifers require a complex interpretation involving data on the parameters of various surface and bottom water masses. The distribution of planktic microfossil assemblages according to cluster analysis shows areas of distribution of different water masses with a clear boundary along the eastern margin of the Subpolar Gyre. The distribution of “cluster” assemblages of benthic foraminifers does not correspond much to that of planktic microfossils, reflecting the division of the water area into abyssal and shallow water areas.

Distribution of Benthic Foraminifera in Intertidal Sabkha of Al-Kharrar Lagoon, Saudi Arabia: Tools to Study Past Sea-Level Changes

Contemporary foraminiferal sediment samples were collected from the intertidal sabkha of Al-Kharrar Lagoon, Saudi Arabia, to study the vertical distribution of foraminifers and, based on a modern training set, their potential to develop a predictor of former sea-level changes in the area. Based on hierarchical cluster analysis, the intertidal sabkha is divided into three vertical zones (A, B, and C) represented by three foraminiferal assemblages, where agglutinated species occupied Zone A and calcareous species occupied the other two zones. In Zone A (high intertidal), Agglutinella compressa, Clavulina angularis and C. multicamerata are dominant species with a minor presence of Peneroplis planatus, Coscinospira hemprichii, Sorites orbiculus, Quinqueloculina lamarckiana, Q. seminula, Ammonia convexa and A. tepida. In contrast, in Zone B (middle intertidal) the most abundant species are P. planatus, C. hemprichii, S. orbiculus, Q. lamarckiana, Q. seminula and Q. laevigata, while Zone C (low intertidal) is characterized by C. hemprichii, Q. costata, S. orbiculus, P. planatus, A. convexa, A. tepida, Spiroloculina communis, and S. costigera. A transfer function for sea-level reconstruction was developed using a modern dataset of 75 contemporary sediment samples and 99 species collected from several transects across the sabkha. The model provided an error of 0.12 m, suggesting that intertidal foraminifers can be used to assess past sea-level changes with high precision in Al-Kharrar Lagoon, and adjacent regions.

Research on the Construction of a Petrophysical Model of a Heterogeneous Reservoir in the Hydrate Test Area in the Shenhu Area of the South China Sea (SCS)

The occurrence characteristics of hydrates in the Shenhu area reflect a typical inhomogeneity in terms of spatial distribution. It is difficult to accurately describe the petrophysical properties of a reservoir using a petrophysical model considering a single cementation factor parameter. According to the analysis of a mathematical model and the estimation results of V p and V s , the unique structure of foraminiferal sediment particles provides opportunities for forming a diversified hydrate occurrence in the foraminiferal area. In areas where hydrates are thin and interbedded, hydrate reservoirs are generally three-phase media, with obvious thermoelastic properties. Therefore, the parameters of the three characteristic models of the pore-filling model, particle cementation model, and thermodynamic elastic model are all included in the correction model. The weights of the influence factors are then changed to realize an accurate description of the petrophysical characteristics of the correction model in different drilling areas and at different formation depths, reducing the limitations of using a single petrophysical model to describe the petrophysical characteristics of heterogeneous regions under the influence of multiple factors.

Effect of Euxinic Conditions on Planktic Foraminifers: Culture Experiments and Implications for Past and Future Environments

AbstractThe oceanic redox state is an essential determinant for life on Earth, and “anoxic events,” periods when large parts of the oceans were anoxic or euxinic (free hydrogen sulfide (H2S) present), may have substantially affected the oceanic biosphere. We present the first culture data showing the effects of euxinic conditions on planktic foraminifers. We cultured six species of planktic foraminifers (n = 31) in three treatments: ~2 mg H2S/L, ~9 mg H2S/L, and a control, without H2S. Under the H2S treatments, planktic foraminifers did not survive for more than 48 hr, and gametogenesis percentages were very low (8% and 17%), while the time to gametogenesis was short (less than 1 day). However, some species (e.g., Neogloboquadrina dutertrei) survived episodic or temporary exposure to H2S for a relatively short time (i.e., <24 hr, such as a tidal cycle), indicating species‐specific tolerance levels for H2S. The biological response of foraminifers to euxinia thus is fundamentally different from that of dysoxia (i.e., low dissolved oxygen; ~0.7 mg O2/L or ~22 μmol O2/L), when they survive, add chambers, feed, and undergo gametogenesis. The presence/absence of planktic foraminifers in sediments thus may help to elucidate the redox state of the euphotic zone in the oceans of the past, with planktic foraminifers absent in sediments deposited when euphotic waters became euxinic.

Planktonic foraminiferal biostratigraphy and assemblages in the Bering Sea during the Pliocene and Pleistocene: IODP sites U1340 and U1343

IODP Site U1340 and Site U1343 in the Bering Sea have been investigated with regard to planktonic foraminifers and fragmentation. The base of Site U1340 dates back to the Early Pliocene and the base of Site U1343 to the Early Pleistocene. Site U1340 is situated at Bowers Ridge, the southern Bering Sea. Site U1343 is situated near the gateway to the Arctic Ocean in the northern Bering Sea. At both sites there are none or very few planktonic foraminifers during the Pliocene and early Pleistocene. After 1.3–1.4Ma the planktonic foraminifers are continuously present for most of the samples examined. Three stratigraphic events have been identified in this study. The first occurrence (FO) of Neogloboquadrina inglei is observed at 1.4–1.5Ma, although this event may be affected by poor preservation of foraminifers in older sediments. The observed age of the change in the coiling ratio of Neogloboquadrina pachyderma from right to left at 1.2Ma agrees with the dating of the same event at the Californian margin. The age of the last occurrence (LO) of N. inglei also seems to match the same event from the Californian margin at 0.7Ma. This implies that these events are robust regional events for the entire northern Pacific. Multivariate analyses of the quantitative planktonic foraminifer data show three main faunal assemblages. The oldest assemblage from 1.3–1.4Ma to 1.2Ma is dominated by N pachyderma s.l. (dex) together with Globigerina bulloides. Other species in this fauna are N. inglei, N. pachyderma s.l. (sin), Globigerina umbilicata and Turborotalita quinqueloba. After 1.2Ma the faunal assemblage is dominated by N. pachyderma s.l. (sin), but the remaining species are the same as before. At 0.7Ma N. inglei disappears, whilst the remaining fauna assemblage stays the same, with N. pachyderma s.l. (sin) still dominating, reflecting subpolar–polar conditions. Prior to 1.4–1.3Ma there are very few or no planktonic foraminifers. Low shell fragmentation and lower TOC suggest that the lack of planktonic foraminifers in these sediments cannot be explained either by a shoaling of the carbonate compensation depth or by enhanced supralysoclinal dissolution. Instead, we argue that the appearance of abundant foraminifers in the sediment reflects the same evolutionary adaption of N. pachyderma (sin) to cold conditions to have occurred after 1.1–1.0Ma as in the North Atlantic.

Mount Woolnough: the submerged volcano, SE of Sydney, NSW

Part of a larger investigation of the sea bed off Sydney was a study of the extinct submarine volcano Mount Woolnough. It is located approximately 41 km east of Kurnell, NSW, and protrudes 175 m above the sediment cover at depths of approximately −550 to −375 m. Volcanic rock, approximately 2.2 km in diameter, is exposed above the sediment sea floor and is much smaller than its magnetic expression (approximately 13 km in diameter). Samples dredged from Mount Woolnough were conglomerates with phosphatic nodules and volcanic fragments set in a fine foraminiferal sediment matrix. Zircons within the mafic fragments yielded a minimum age of 261 Ma.

Quaternary paleoceanology of Benguela upwelling based on microfossils (borehole 362 DSDP)

The study of the upper part of the section of borehole 362 DSDP (southeastern Atlantic) allowed the recognition of five zonal units in the Quaternary based on nannofossils and refinement of the UpperLower Pleistocene boundary based on benthic foraminifers. The study of planktonic and benthic foraminifers in sediments display the history of the Benguela upwelling. The temperature of the surface water layer reconstructed based on Upper Pleistocene planktonic foraminifers reaches 17°–19°C and decreases downward in the section to 2°–3°C, suggesting that, in the Early Pleistocene, the upwelling was more extensive in area. The increasing proportion of benthic foraminifers and large nannofossils downward in the section is evidence of more active dissolution of carbonates in the Early Pleistocene and the high content of terrigenous matter suggests that the Benguela Current occupied a westerly position. The existence of the Benguela upwelling throughout the Late Pliocene-Pleistocene was accompanied by a decrease in its activity.

SHE Analysis for Biozone Identification among foraminiferal sediment assemblages on reefs and in associated sediment around St. Kitts, Eastern Caribbean Sea, and its environmental significance

Abstract Sixteen nearshore (≤ 1 m water depth) sediment samples, termed Sample Set 1, were collected from the eastern and south-western coasts of the South East Peninsula (SEP) of St. Kitts. Twelve samples termed Sample Set 2 were taken from nearby fringing (6–17 m) and offshore (~ 18–25 m) reefs. The sample sets were analysed separately using SHE Analysis for Biozone Identification (SHEBI), α and β diversities and complementarity. SHEBI is a statistical technique that determines abundance biozones (ABs) using the entire vector of species abundances in samples, species richness S , the information function H , and the equitability index E . Alpha diversity expresses the diversity within an AB, while β diversity quantifies diversity changes between adjacent ABs. Complementarity assesses the permeability of AB boundaries. SHEBI divided Sample Set 1 into five ABs but did not distinguish separate bays. Beta diversities reflected the highest and lowest α diversities at Mosquito Bay and South Friar's Beach\Frigate Bay respectively. Complementarity was highest between two ABs in South Friar's Bay, indicating a close relationship between shoreline sediment and material marginally farther offshore. It was lowest between Whitehouse and South Friar's Bays. The assemblages along this transect were rich in Discorbis rosea . For Sample Set 2, samples were accumulated for SHEBI by depth. SHEBI distinguished four ABs (three on the fringing reefs) characterised by differing percentages of Asterigerina carinata , Archaias angulatus and Amphistegina gibbosa . Comparison between Sample Sets 1 and 2 indicates that during storms sediment moves shoreward from the fringing reefs. Foraminifera show that there is less movement of sediment > 63 μm from the shoreline (Sample Set 1) to the fringing and deeper reefs.

Archaeological Investigation of the Landscape History of an Oceanic Atoll: Majuro, Marshall Islands

Historical ecology has provided the field of geoarchaeology in Oceania with the concept of an island landscape as a historical product, invented from the dynamic interactions between natural processes and human agency. Since Davidson's work in Nukuoro (1971) and Dye's introduction to the prehistory of Majuro in the Marshall Islands (1987), systematic excavations of atoll islets have also been based on this tenet. Following this concept, this study presents a geoarchaeological examination of the long-term history of the pitagricultural landscape in Laura Islet of Majuro Atoll, which now consists of 195 pits showing remarkable undulation and anthropogenic vegetation on their spoil banks. Our excavations, conducted since 2003, have revealed that human habitation on Laura began as early as 2,000 years ago, soon after the emergence of the core islet, which probably followed a relative drop in sea level in the late Holocene. Some centuries later, the inhabitants started excavating agricultural pits for the cultivation of wet taro, probably Cyrtosperma spp. The subsequent sea-level decline would have enlarged the foraminiferal sediment; the islet then extended its landform both oceanward and lagoonward as well as along the longitudinal axis stretching north to south. The land accretion caused its inhabitants to increasingly extend their activity space and readjust areas for habitation. It would also have enlarged the volume of the freshwater lens, prompting additional construction of agricultural pits even in the area just behind the lagoonside beach ridge. Most of the current landscape was formed by around 1,000 years ago at the latest. Geoarchaeological synthesis of Pacific atolls will enable the precise elucidation of local chronological relationships between land accretion and expansion of human activities.

Depths of the Sea of Okhotsk sedimentary basin in the Cenozoic

The biofacies analysis of benthic foraminifers in sediments of the Sea of Okhotsk paleobasin revealed the presence of typical abyssal species in their Oligocene-Miocene assemblages that are missing in the Pliocene and Pleistocene section. The development of the abyssal fauna in the Sea of Okhotsk was determined by its relatively large depths (>2000 m) and intense water exchange with the Pacific Ocean. The Sakhalin folding phase at the Neogene-Quaternary transition resulted in the uplifting of the Japan-Kurile cordillera and separation of deep basins of the Japan and Okhotsk seas from the ocean, which was responsible for the formation of unfavorable conditions for migration and existence of the Pacific abyssal fauna. The taxonomic similarity and general tendencies in the development of the Neogene benthic foraminifers common for the Japan and Okhotsk seas imply the lack of narrow and shallow thresholds between these basins similar to the present-day Nevel’skoi, La Perouse, and Sangar straits. Such bottom topography stimulated the intense northward water flow, which determined the similarity between the benthic foraminiferal assemblages of the Japanese and Okhotsk paleobasins.

Surficial sediments of the wave-dominated Orange River Delta and the adjacent continental margin off south-western Africa

The textural and compositional characteristics of the surficial shelf sediments north and south of the Orange River Delta are reviewed and compared. Sediments are fractionated and dispersed both north- and southwards of the Orange River mouth by wave action, longshore drift and subsurface currents. The mean grain size decreases both offshore and southwards in response to decreased wave influence at the seabed and the competence of the weak poleward undercurrent respectively. The increasing dominance of marine biogenic components in sediments south of the prodelta indicates a greater marine influence, modifying previous inferences that the Namaqualand mudbelt is primarily derived from the southward transport of Orange River sediments. A sharp distinction can be drawn between sediments of the Orange Shelf to the south and the Walvis Shelf to the north. Foraminiferarich deposits that dominate the Orange middle shelf and slope indicate that upwelling is an inner-shelf phenomenon. On the Walvis Shelf, foraminiferal sediments are confined to the slope and outer shelf. Fish debris is more common in Walvis Shelf sediments. Although phosphorite and glauconite sands often occur together in the same deposits on the Orange Shelf, the two minerals are concentrated in separate deposits on the Walvis Shelf.

Do inverted depositional sequences and allochthonous foraminifers in sediments along the Coast of Kachchh, NW India, indicate palaeostorm and/or tsunami effects?

The study of 134 subsurface sediment samples from three cores collected along the Coast of Kachchh, off Gujarat Province, NW India, shows the presence of 151 foraminifer species. Q-mode cluster analysis on foraminifer data for each core and radiocarbon dating at ten different downcore depths reveal an inverted sequence, with fine-grained sediments and small foraminifers sandwiched between normal detrital sediments at water depths of 10–20 m. It is postulated that ∼8,000 years B.P., sediments ranging in age from ∼10,000 to ∼12,000 years B.P. were eroded from deeper offshore deposits by storm/tsunami(s), and were subsequently transported and redeposited in shallow regions, resulting in an inverted sequence, followed by a normal detrital sequence deposited between ∼8,000 and ∼7,000 years B.P. A second, similar event resulted in the deposition of ∼10,000 year old sediments over a normally deposited, ∼7,000 year old sequence. The second inverted sequence was subsequently overlain by a normal sequence. It is hypothesised that the erosion and transportation of fine-grained sediments from deeper water, and their deposition in shallower water against the rule of gravity, are the result of storm/tsunami influence.

Biostratigraphy and paleoenvironment of foraminiferal assemblages from neogene and quarternary deep-sea sediments collected during the nautimate cruise (Pacific Ocean, off Mexico, 18°N-21°N). Geodynamic implications

Deep-sea dives with the submersible Nautile allow to reconstruct the lithostratigraphic series, and to collect samples of plutonic rocks and sediments along the inner wall of the Acapulco trench, off Mexico (Manzanillo area, 18°N-19°N, Pacific Ocean). Plutonic rocks are unconformably overlain by brown and grey sandstones and siltstones, constituting a continuous sequence of sediments, several hundreds meters thick. Assemblages of planktonic foraminifera give a late Miocene-early Pliocene age for the lowest part of the sedimentary sequence. Benthic foraminifera indicate that the minimum depth of deposition was located into the middle bathyal zone. Characteristic species are the following: Bolivina foraminata, Bulimina mexicana, Galliherina avigerinaformis, Gyroidina healdi, Hansenisca rotundimargo, Nodogenerina lepidula, Parafrondicularia miocenica, Fontbotia wuellerstorfi, Plectofrondicularia californica, Protoglobobulimina pseudotorta, Pyrgo murrhina, Siphonodosaria advena. These assemblages are dominated by endofaunic species (bolivinids, buliminids, uvigerinids and cassidulinids) and oxygen-minimum zone indicators (Buliminella subfusiformis, Epistominella smithi, Uvigerina subperegrina). These late Miocene-early Pliocene sediments were probably deposited into the oxygen-minimum zone, at the boundary between the lower and upper parts of the middle bathyal zone, corresponding approximately to a depth of 1000 m. Their present position (between 2500 and 4000 m) reveals a strong and rapid subsidence of this area (between 1500 and 3000 m) since the early Pliocene. These stratigraphic and paleoenvironmental results indicate the importance of these stepwise downward movements of this zone since Eocene. An additional study of late Miocene-Quarternary foraminiferal sediments collected during deep-sea dives conducted to the North (South of Tres Marias Islands, 20°N-21°N) gives complementary data on the stratigraphy and paleoenvironment of these deposits. It completes the understanding of the complex geodynamic history affecting the western boundary of the North America plate.

Experimental Analysis of Abrasion and Dissolution Resistance of Modern Reef-Dwelling Foraminifera: Implications for the Preservation of Biogenic Carbonate

Fringing coral reefs at Discovery Bay, Jamaica, exhibit a pronounced depth-related gradient in water turbulence and associated physicochemical taphonomic factors (abrasion, dissolution), and thus provide ideal settings for investigating the influence of taphonomic processes on the formation of fossil assemblages. Foraminifera are prominent constituents of bioclastic sediments at Discovery Bay, and exhibit a high diversity of test sizes, shapes, wall compositions, architectures, and microstructures which may potentially affect their post-mortem behavior. We have developed a taphofacies model for Jamaican north coast fringing reefs and associated environments that has allowed us to generate hypotheses about the formation of foraminiferal sediment assemblages

Relation of counting methods to taphonomic gradients and biofacies zonation of foraminiferal sediment assemblages

The routine procedure for most distributional studies of foraminifera has been to count approximately 300 specimens of all sizes greater than some specified minimum (usually between 63 and 125 μm), and determine percent abundance of each species using total counts. This method fails to take into account that foraminiferal species may be found predominantly within certain size fractions, either because of species-specific size ranges or taphonomic processes (e.g., sorting, transport, abrasion, dissolution). Use of an alternative counting procedure (“sieve method”) takes into account foraminiferal size distributions. The sieve method utilizes counts of up to 300 specimens in each sand size fraction (0.125–0.25, 0.25–0.5, 0.5–1.0, 1–2 mm) of each sample. Counts are then totaled for each sample (up to 1200 specimens per site) and used in determination of species abundances for each site. The sieve method has been of considerable utility in recognition of foraminiferal bathymetric zonation preserved in sediment assemblages of fringing reef environments at Discovery Bay, Jamaica. Well-documented reef zones (originally based on corals and physiography) are relatively well-defined in Q-mode cluster analysis (UPGMA) of species abundances determined using the sieve method. In contrast, reef zones are not recognized in cluster analysis of foraminiferal species abundances based on the standard method, nor by cluster analysis of species abundances within individual size fractions. The sieve counting procedure compensates for operator bias in specimen counts introduced by large and unusually abundant species (e.g., Amphistegina gibbosa) and by small but colorful forms (e.g., Discorbis rosea), which mask the zonation using the standard procedure. The sieve method does not alter overall depth-related trends in species abundance, distribution, or diversity as determined by the standard method. Thus, foraminiferal sediment assemblages contain more paleoenvironmental information than had previously been thought.

Foraminiferal Biofacies on a North Coast Fringing Reef (1-75 m), Discovery Bay, Jamaica

Recent foraminifera inhabiting Jamaican north coast fringing reefs exhibit variation in distribution patterns that are related to bathymetry and reef morphology. Parallel traverses were run from the back reef (depth 1-4 m) across the reef crest, fore-reef terrace (5-14 m), escarpment (24 m), slope (30-55 m), and upper (75 m) deep fore-reef zones at Discovery Bay, Jamaica. Approximately 150 species belonging to 80 genera were identified. Diversity indices (S, H', J') are lowest on the fore-reef terrace, increase toward the back-reef andfore-reef escarpment and slope, and tend to be highest on the mid-lower slope and deep fore reef. Major operational taxonomic groups of foraminifera (suborders Textulariina, Miliolina, benthic and planktonic Rotaliina) exhibit distinct bathymetric trends, with the Miliolina occurring most abundantly in the back reef, and Textulariina and planktonic Rotaliina being most abundant below 30 m. Benthic rotaliines are prominent across the entire reef environment and may compose >60% of foraminiferal sediment assemblages. Among the largest and most common species, Amphistegina gibbosa (benthic Rotaliina) dominates fore-reef assemblages, whereas Archaias angulatus (Miliolina) is most abundant in back-reef environments. Individual fore-reef zones are poorly delineated in Q-mode cluster analysis of foraminiferal species abundances (2 5% of foraminiferal assemblages at any site) based on standard counting procedures (300 foraminifera from all size-fractions >0.125 mm). Reef zones are, however, clearly defined when the standard countingprocedures are applied to individual size-fractions (1-2, 0.5-1.0, 0.25-0.5, and 0.125-0.25 mm) and species abundances determined from total counts of all four size-fractions combined ('sieve method). Based on the sieve method, Q-mode cluster analysis of common (3% of foraminiferal assemblages at any site) species delineates back reef, fore-reef terrace, and fore-reef slope and upper (75 m) deep fore-reef zones. The alternative counting procedure appears to compensate for operator bias in specimen counts introduced by large and unusually abundant species (e.g., Amphistegina gibbosa) and by small but colorfulforms (e.g., Discorbis rosea), which appear to mask the zonation when using standard counting procedures.

Phosphorus in foraminiferal sediments from North Atlantic Ridge cores and in pure limestones.

Samples of foraminiferal ooze from two North Atlantic cores were cleaned and progressively dissolved. Most of the P, Fe, and Mn was released during the first of two reductive cleaning steps. Most of the remainder of these elements and most of the Ca were released during the final acid dissolution step. The P in these samples is present largely in Fe- and Mn-rich coatings, not as a constituent of the foraminiferal shells themselves. Our results are consistent with those of earlier studies. The concentration of P in carbonate oozes in which it is clearly associated with coatings is similar to that of modern calcareous sediments in general, and with that of reasonably pure limestones of all ages. Phosphorus is apparently associated with (Fe, Mn)-oxide coatings in many carbonate sediments. The rate of removal of P from the oceans as a constituent of such sediments depends on the rate of formation of (Fe, Mn) coatings, not on the rate of incorporation of P into calcium carbonate.

Sedimentation of planktonic foraminifera

Fossil assemblages of planktonic Foraminifera contain many valuable clues to paleoclimate and paleo-oceanography. Unfortunately, our understanding of production, dissolution, redeposition, and other processes of foraminiferal sedimentation is but rudimentary. Lacking direct observations, information largely rests on comparisons between abundance and composition patterns of life-, death-, and sediment-assemblages. Standing stock and production of life assemblages vary greatly, depending on the fertility of the water. Fertility is greatest near continents and along the equator. The shell supply in these fertile regions is several times greater than in the sterile subtropical areas. The distribution of living and empty shells in the water column suggests that virtually all production takes place within the mixed layer, possibly near its base. Size distributions suggest that most formainiferal production becomes food for predators and is not available for sedimentation. Morphology patterns of living and empty shells confirm this suggestion. Thus, very rapid turnover of populations is required to produce the sedimentation rates observed. A residence time of about one week for living Foraminifera larger than 150 μ is proposed for fertile regions, implying life spans of no more than about 2 weeks. Most of the foraminiferal shells reaching the ocean floor are redissolved. Dissolution is complete below the calcite compensation depth (CCD), but much solution takes place above this level also. This fact has long escaped notice, because percentages of calcite are poor indicators of solution patterns. Any reasonable solution profile, even a linear one, would produce a “sudden” decrease of calcite content at the level where the amount of calcite becomes comparable to the amount of non-calcareous dilutants. This is evident from discussion of the formula: L=100(1− Ro R ) where L is the loss of sediment necessary to increase the insoluble fraction R 0 to R percent. (For example, set R 0 = 5%; R = 10%, yielding L = 50%; i.e., 50% of the sediment, or more than half of the carbonate, was lost while the carbonate content went from 95 to 90%!). The same formula yields minimum dissolution in formainiferal assemblages, if resistant planktonic species (or alternatively, benthonic forms) are assumed insoluble. Resulting solution estimates improve understanding of the sedimentary lysocline, a depth level that separates well-preserved from poorly preserved assemblages. Calcite saturation levels inferred from thermodynamic calculations are conceptually and actually different from both lysocline and CCD. While the depth (i.e., pressure) control of solution patterns is obvious, circulation and fertility patterns also influence preservation of foraminiferal sediments. In general, fertile areas tend to have poorly preserved calcareous assemblages, even at shallow depths, because much CO 2 is developed in the sediments. The associated CCD may be quite deep, however, because of high sedimentation rates of calcium carbonate. The ultimate reason for dissolution of shells is that organisms supply more CaCO 3 to the ocean floor than is being introduced from external sources into the oceanic system. This excess supply leads to undersaturation. Excess supply and dissolution rates must balance for geochemical steady state to prevail. Undersaturation, therefore, proceeds to a value where dissolution rates are just right to provide this balance.

Planktonic foraminifera; sediment production in an oceanic front

Faunal composition and phenotypic variation of four assemblages, productivity of planktonic foraminiferal sediments controlled by water movement, food supply, reproduction, predation, Pacific Ocean off Baja California, Mexico

Stages of Glauconite Formation in Modern Foraminiferal Sediments

ABSTRACT Examination of 325 bottom samples taken at 68 stations off the southeast coast of the United States by the United States Fish and Wildlife Service revealed the common occurrence of glauconite in foraminiferal sands. The glauconite is variable in color and in morphology, ranging from dark greenish-black to greenish-white. The dark green glauconite generally occurs in the smooth, bulbous pellet form, whereas the lighter green glauconite commonly occurs as rough, porous pellets in the form of internal molds of microorganisms. However, in some internal mold pellets, dark green and light green glauconite occur together. Magnetic fractions were taken from all samples. The samples composed primarily of foraminiferal tests yielded a glauconite-rich sparate. In the magnetic separates occurred many intact shells of foraminifers which, upon solution in acid, generally yielded internal molds of the light-colored glauconite. Analysis by x-ray diffraction of single pellets and of composite samples of hand-picked grains revealed the glauconite to be essentially a mica-type clay varying in degree of hydration and disorder. Examination of oriented slides of the composite samples following vapor glycol treatment showed that the dark-green glauconite was unaffected, but that the pale green glauconite was expandable. Debye-Scherrer films of single pellets of the light green glauconite showed the presence of kaolinite which was not found in the darker green glauconite. A decrease in sharpness and a shift of the basal mica diffraction line to higher d values as well as a decrease in the potassium content and an increase in the water content were directly related to the color change of the glauconite from dark green to pale green. The glauconite in these foraminiferal sands is interpreted as representing various stages of glauconitization of clay material which was originally enclosed within the tests of microorganisms.