Seasonal spillover and varve formation in the Santa Barbara Basin, California

Abstract

A temporal record of oxygen and NO 3 − concentrations in the bottom water of the Santa Barbara Basin indicates that outside waters spill over into the basin seasonally. It is proposed that an annual bottom‐water cycle leads ultimately to varve production. As evidence in support of a benthic mechanism for varve formation, we present results of surface sediment and pore‐water analyses, from box‐core collections during three seasons in 1988. The greater degree of bottom‐water oxygen depletion that occurs regularly in late summer and fall was coincident with a smaller pool of pore‐water Fe, penetration of ∑H 2 S up to the sediment‐water interface, maximal numbers of benthic Foraminifera, and bacterial mat growth. The latter was indicated by increases in concentrations of extractable ATP, organic C, and total N in sediments from the depth intervals 0–0.25 and 0.25–0.5 cm. The greatest numbers of siliceous phytoplankton skeletons were found on the sea floor soon after the late spring, surface‐water, Chl a maximum. This event coincides with additions of new and more‐oxygenated seawater to the deep basin.