RESPONSE OF RADIOLARIAN BIOMINERALIZATION TO THE INCREASING ROLE OF DIATOMS IN THE CENOZOIC SILICA CYCLE

Polycystine Radiolaria are major biological agents in the silica cycle. Morphological changes in radiolarian skeletons are therefore an important lens through which the history of the silica cycle can be viewed. A well-documented pattern in the radiolarian fossil record is that skeletons decrease in mass through the Cenozoic. Two reasonable hypotheses to explain this are decreased body size, or decreased silicification, perhaps due to increased competition for silica by diatoms. We measured morphometric data (length, width, shell thickness, relative pore area) in 23 samples from low-latitude ODP and DSDP holes to construct a Cenozoic time series. These data were analyzed using idealized geometric models of radiolarian morphology to obtain estimates of silica use (volume of SiO₂ per cell) and degree of silicification (volume of SiO₂ per unit cell volume). The Cenozoic lightening trend is reproduced in our time series of silica use. Body size, one component of silica use, lacks a clear trend, but shows a marked peak in the Late Eocene. Interestingly, unlike many other microfossil groups, the radiolarian body size record does not appear to be closely correlated to the general Cenozoic climatic cooling trend. Our metric for the degree of silicification, however, shows a clear, consistent decrease toward forms with thinner shells and greater pore area through time. This trend is complementary and antithetical to diatom species richness, an indicator of diatom participation in the silica cycle. These data support the hypothesis that the Cenozoic trend toward lighter radiolarian skeletons reflects more economical use of silica, probably in response to decreasing dissolved silica levels resulting from the rise of the diatoms.