Vertical fluxes of diatoms and silicoffagellates in the eastern equatorial Atlantic, and their contribution to the sedimentary record

Sediment trap data from water depths of 853 m and 3921 m retrieved from the eastern equatorial Atlantic (01 degrees 47.5'N, 11 degrees 07.6'W; sampling time March 1, 1989, to March 16, 1990) reveal seasonal changes in the vertical flux of opal. Biogenic opal constitutes 3-20% of the total flux at 853 m and 10-19% at 3921 m. Important contributors to the opal fraction are diatoms and, to a lesser extent, silicoflagellates. At 853 In, the fluxes of both siliceous groups show a distinct seasonal pattern related to upwelling and the location of the ITCZ. The flux values are highest between March and April, and in August/September. The seasonal pattern with the spring and summer peaks can be recognized in the samples from 3921 m in a modified form: The flux curve is smoother and the time lag varies between 0 and 95 days. Higher flux values for diatoms, silicoflagellates and total particles in the lower trap suggests an additional source of material. Lateral advection of particles from the coastal regime seems to be an important factor at 3921 m, adding material at depth and distorting the original record. Also, freshwater diatoms were relatively important in the spring and winter samples at 853 m, and throughout the year at 3921 m, suggesting continental influence. A total of 202 diatom taxa were identified at both depths; only a few were responsible for > 50% of the total diatom assemblage. Nitzschia bicapitata dominated the assemblage. Differences in the relative abundances of strongly and weakly silicified diatom taxa between trap levels indicate dissolution in the water column. However, comparison between trap and sediment surface assemblages point to extreme dissolution at the sediment/water interface; approximately 98% of the siliceous shells are lost to the sedimentary record. Despite this loss, the changes between the surface sediment assemblages reflect the overlying hydrographic conditions of the surface waters.