ABSTRACT

Specimens of the planktonic foraminifer Globorotalia menardii (Parker, Jones, and Brady) were examined in bottom samples from three profiles across the continental shelf and slope of the South China Sea. This species is absent in inner shelf assemblages; middle-shelf populations of G. menardii are dominated by forms with tests characterized by smooth, thin, translucent walls and unthickened keels (G.cultrata” of authors). Specimens with coarsely crystalline crust covering much of the test wall and keel become increasingly common seaward, although the distribution of crust-bearing forms on the outer shelf is irregular because of current transport. The percentage of incrusted tests in bottom sediments reaches values of 80–100 just beyond the shelf edge (180–275 m), and these values are maintained across mosi of the continental slope. A narrow zone of high percentages of noncrusted forms occurs in all three profiles on the upper continental slope at depths of 275–500 ni; these deposits represent concentralions of small tests swept off the shelf.

Young specimens of G. menardii live at relatively shallow depths. By the early adult stage of development, the individual organisms descend in the water column where they continue to grow. At increased depth, development of a secondary crystalline crust begins, first with thickening of the keel and then with incrustation on both dorsal and ventral walls. Secondary calcification on the exterior walls of the last chamber, including the apertural face, is represented by thickening and increased opacity of the wall without the development of a typical crust. Because chambers are added after the onset of crust formation, the lack of secondary calcification on septa and on the ventral walls of chambers enclosed within the last whorl of the test indicates that previously secreted crystalline crust has been resorbed.

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