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Radiolarian Ratios and the Pleistocene-Holocene Boundary
Kenneth J. McMillen (2)
The ratio of general types of radiolarians is a useful tool 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 due to dissolution, so that methods of using planktonic foraminiferans to locate the Pleistocene-Holocene boundary are unavailable. Upper-slope cores, however, do show a down-core increase in the abundance of planktonic foraminiferans. Previous workers have correlated a similar increase in foraminiferan abundance in samples from offshore Oregon with the Pleistocene-Holocene boundary. The ratio of nassellarian (cone-shaped) radiolarians vs. spumellarian (spherical) radiolarians decreases rapidly down core to a distinct minimum in both upper-slope and deeper-water cores. The radiolarian minimum occurs just below the increase in planktonic foraminiferan abundance and makes a good marker for identifying the Pleistocene-Holocene boundary in deeper-water cores which contain no foraminiferans.
The radiolarian ratio minimum (which shows a dominance of spumellarians in the late Pleistocene) might be a response to changes in climate or circulation; or it might be due to core sites being closer to shoreline during times of lowered sea level. The latter explanation is supported by a study of living 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 it also might be a tool for qualitatively indicating proximity to shoreline in older samples.
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