Modern Radiolarian (Polycystina) from Carmen Basin, Gulf of California, México: an ecological approach of the last 200 years

Different taxa of radiolarians are adapted to different ocean conditions such as temperature and productivity, and specific assemblages characterize water masses and upwelling environments. Radiolarians are very common fossils in open marine sediments and in the Gulf of California are well preserved. In deep water deposits in slope sediments of the southern Gulf, up to 50 percent of the coarse fraction may consist of radiolarians, except in areas where dilution by terrigenous material is high. In the central part, diatoms are quantitative much more important in the sediments. However, south of latitude 26 degrees N, biogenous silica is almost exclusively radiolarians. Thus, in order to reconstruct the environmental conditions (i.g. water masses and ocean circulation patterns) a laminated sequence (core DIPAL V C-33, with 40 cm long) was collected in the eastern Carmen Basin in the gulf, at 600 m depth were the oxygen minimum impinges on the slope. Two hundred continuous samples along the core were prepared for radiolarian research. Rich and relatively well-preserved fauna is found and 160 taxa have been identified. A preliminary analysis shows that the orders Nasellaria and Spumellaria represent around 70% and 30%, respectively, indicating a more nearly oceanic conditions. Preliminary model age based on 210Pb dates indicates a sedimentation rate of 1.9 mm/yr, thus the core covers the past c. 200 years. Ecological diversity indices and statistical analysis are considered for the interpretation of radiolarian records and environmental reconstruction.