Calcite Crusts Research Articles

Planktonic Foraminifera as Indices of Paleoceanography: ABSTRACT

Each major oceanic water mass today is characterized by its own peculiar planktonic foraminiferal indices. These indices may be totally different groups of species, differently coiled populations of the same species, differences in abundance, differences in form, or some combination of these criteria. Furthermore, selective elimination of planktonic species occurs toward inshore areas, especially toward deltas. It is also known that forms with a calcite crust covering the normal wall of the test develop this crust in bathyal depths, providing in this way a useful index to depth of deposition. Using some of the above criteria it is now known that cold-water planktonic faunas (Globigerina pachyderma sinistral populations), identified with summer surface water temperatures of less than about 5°C, expanded into the temperate areas during colder periods of the Pleistocene, during the middle Pliocene, and during a part of the late Miocene. In contrast, the warmer intervals of the early Pliocene, the late Pliocene, the Pleistocene, and the present are identified by the warmer water types of planktonic faunas (G. pachyderma dextral populations) indicating summer surface water temperatures of about 15° to 18°C, or possibly slightly higher. Eustatic changes in sea level, associated with these cold and warm paleoclimatic alternations in the Pleistocene, brought marked egressive-transgressive cycles in shallow-marine deposits; similar cycles are indicated for the Pliocene and the later Miocene. Thus, in the absence of complications due to variable rates of sedimentation and tectonism, shallow-water depositional sites may reflect a late Miocene nonmarine phase or an unconformity, an early Pliocene marine phase, a middle Pliocene nonmarine phase or an unconformity, a late Pliocene marine phase, and a major regression at the base of the Pleistocene. Other criteria can be used to show former oceanic connections with marine basins by abundance patterns of planktonic Foraminifera. These patterns also may suggest current patterns of the past. For example, the Miocene planktonic foraminiferal facies of the San Joaquin Valley indicate that the major marine connection End_Page 559------------------------------ was across the San Andreas fault zone at the southwestern edge of the valley and that the primary marine current was counterclockwise around the southern end of the basin. End_of_Article - Last_Page 560------------

The Influence of Depth on Shell Growth in Globigerinoides sacculifer (Brady)

Most living planktonic foraminifera thicken their tests in increasingly deeper habitats. Sphaeroidinella dehiscens (Parker and Jones), widely distributed in lower latitudes of the Atlantic and Pacific oceans, is shown to be an aberrant, terminal form of Globigerinoides sacculifer (Brady). The 9dehiscens9 stage is achieved in a small percentage of forms by secretion of a thick calcite crust upon the test of G. sacculifer. Thickening reaches a maximum at mesopelagic and bathypelagic depths (300-2,000 m). G. sacculifer first occurs in early lower Miocene sediments, and 9S. dehiscens9 in late middle Miocene. Possible evolutionary development of the cortex, and classification problems of polymorphic species, are discussed.

Shell Growth and Structure of Planktonic Foraminifera

Planktonic foraminiferal tests show bilamellar additions to each chamber and develop a "secondary thickening" in the form of a calcite crust in late ontogeny. The evidence presented indicates that additional calcium carbonate is deposited upon the original bilamellar test and that maximum calcite crust thickness is attained at depths greater than 500 meters.

Electron Microscopic Studies of Planktonic Foraminifera: ABSTRACT

The wall structures and surface features of several modern planktonic foraminiferal species were examined with an electron microscope for the purpose of evaluating the taxonomic and ecologic significance of such microstructural details. Species belonging to spinose Globigerinoides, non-spinose Globoquadrina, and non-spinose Globorotalia were selected because they are three representative taxa among the planktonic Foraminifera. Surface replicas were made using the Triafol method, which has been found superior to direct shadowing and Polystyrene techniques. Photographs have been taken stepwise from low magnifications with the light microscope to higher magnification electron micrographs in stereo pairs. In the juvenile stages when the individuals live near the ocean surface, the test wall is thin and transparent and is composed of small calcite crystals with their c-and major growth axes normal to the shell surface. In the later stages when the organisms descend to lower water depths there is additional crystal growth in the form of a calcite crust producing columnar prisms whose free ends are rhombic pyramids. As some of the prisms increase in size, other prisms are squeezed out by differential growth; the rhombic pyramids are especially well developed along the keel and apertural side of Globorotalia menardii and G. truncatulinoides. Transitional steps in the test thickening of G. sacculifer have been observed from specimens having large, open pores and distinct spine bases to specimens having constricted pores, obscure spine bases, and surficial encrustment of calcite crystals. Distinguishing between variations in primary test features (e.g., keel, pores, apertural lip) and those due to developmental phases as observed with the electron microscope is complicated by variations due to environmental influences. End_of_Article - Last_Page 516------------