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The AAPG/Datapages Combined Publications Database
The ratios among general types of radiolarians are useful tools in the correlation of late Quaternary sediments in piston cores from the continental margin off the west coast of Guatemala. Cores from the outer slope and lower inner slope contain practically no calcareous microfossils because of solution, so that methods of using planktonic forams to locate the Pleistocene-Holocene boundary are useless. Upper-slope cores, however, show a down-core increase in the abundance of planktonic forams. Previous workers have correlated a similar increase in foram abundance offshore Oregon with the Pleistocene-Holocene boundary. The ratio of nassellarian (cone-shaped) radiolarians versus spumellarian (spherical) radiolarians decreases abruptly down core to a distinct minimum in cores from the up er slope as well as in cores from deeper water. This radiolarian minimum occurs just below the increase in planktonic foram abundance and makes a good marker for identifying the Pleistocene-Holocene boundary in deeper water cores which contain no forams.
The radiolarian ratio minimum (which shows a dominance of spumellarian radiolarians in the late Pleistocene) might be a response to a lowering of sea level, to changes in climate or circulation, or to the core sites being closer to shoreline during times of lowered sea level. The last explanation is supported by a study of radiolarian populations from the south Texas outer continental shelf of the Gulf of Mexico, which shows an increase in spumellarian abundance inshore.
This radiolarian ratio is useful not only in locating the Pleistocene-Holocene boundary in noncalcareous sediments, but also might be a tool for qualitatively indicating proximity to shoreline in older samples.
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