Micropalaeontology of the hydrothermal region in the Guaymas Basin, Mexico

Abstract. A micropalaeontological analysis of 40 sedimentary samples from a hydrothermal region in the Guaymas Basin, Gulf of California, Mexico, was carried out to describe microenvironments. These microenvironments were defined through a Q-mode Factor analysis of species abundance data from the benthic foraminiferal biocoenoses and thanatocoenoses. The benthic foraminiferal results were correlated with the biogeographic patterns of diatoms, radiolarians, planktonic foraminifers and ostracods through a ‘graphic-multivariate analysis’. Although the microenvironments have a patchy distribution within the hydrothermal region, it is possible to define the following: (1) the ‘hydrothermal environment’, in which the benthic foraminifera Bulimina mexicana and Globocassidulina sp. cf. C. subglobosa coexist with the mollusc Calyptogena pacifica, even though the almost direct influence of the hydrothermal fluids drastically reduces the presence of the microfauna; (2) the ‘bacterial environment’, in which Trochammina sp. and Recurvoides sp. are associated with bacterial mats; and (3) the ‘cool environment’, further out from the direct hydrothermal influence. In this microenvironment the benthic foraminifera Bulimina spinosa, Bolivina seminuda and Cibicides sp. are common. These three benthic foraminiferal assemblages are different from other assemblages which live in the Guaymas Basin but are not influenced by hydrothermal fluids. In the non-hydrothermal regions, Uvigerina peregrina and Buliminella tenuata are common. The analysis of subsurface samples shows the same patterns as the superficial samples.