Vertical water mass circulation and the distribution of radiolaria in surface sediments of the Gulf of California

Abstract

Quantitative analysis of microfossils from sediment cores taken in the Gulf of California shows that the distribution of radiolaria is controlled by both surface ocean conditions and vertical circulation patterns in this marginal sea. Fifty-four surface sediment samples from intermediate water depths were used in this study. Samples were taken from surface scrapings of box cores or large diameter gravity cores to insure that the samples represented recent deposition. Q-mode factor analysis was used to divide the radiolarian population found in the Gulf of California into three assemblages. The first is made up of species typical of the Gulf. These species are, in general, not a major part of the open ocean radiolarian fauna. The general geographic distribution of this assemblage shows a maximum abundance in the north-central part of the Gulf where warmer high salinity water is found. This assemblage is also important in areas of upwelling. The second fauna is dominated by radiolaria typical of the subtropical Pacific Ocean. This assemblage dominates in the southern part of the Gulf. The last assemblage is dominated by species commonly found in the eastern boundary of the North Pacific and is found in areas of upwelling within the Gulf of California. Analysis shows that the first two assemblages are strongly influenced by the vertical structure of the southern part of the Gulf of California. The Gulf assemblage shows a maximum in samples taken between 600 and 800 m water depth. The Subtropical assemblage shows high values in shallow sites and in samples taken in water depths greater than 800 m, and a distinct minimum at intermediate depths. The fauna typical of the North Pacific eastern boundary current shows no depth preference. The maximum in the Gulf assemblage coincides with the depth of strong southerly flow of warmer higher salinity water out of the Gulf of California. This suggests that the abundance of the Gulf assemblage in the southern end of the Gulf of California is in part controlled by advection and thus vertical circulation has an important part in controlling the distribution of radiolaria in the sediments of the Gulf of California.