On the intermediate particle maxima associated with oxygen-poor water off western South America

Abstract

Abstract The distribution of suspended particulate matter was measured during 21 May and 18 June 1977 between 4 and 23°S from the coast of South America to about 500 nautical miles offshore. A well-defined maximum was observed over the continental margins at depths of about 200 m between ∼9 and 23°S, At 4°S, the main particle maximum was at approximately 400 m, but in the nearshore zone the maximum extended upwards to ∼200 m. A comparison of the particle and chemical data shows that the particle maxima are usually at approximately the core depth of the oxygen minimum layer. A nitrite maximum and a nitrate minimum were also observed at or near the particle maximum core depth south of ∼9°S. Near 4°S, a weak nitrite maximum was observed within the oxygen minimum layer at some stations. The protein distribution near 15°S suggests that the material in the particle maximum contains significant amounts of organic matter. The distribution of the particle maximum layer between 9 and 23°S and its relations to the density field and the cross-shelf flow suggest that most of the particles could originate in the bottom waters over the outer continental shelf and be transported offshore in a quasi-horizontal path. Offshore particle transport near the equator is probably supported by a westward current off northern Peru between and under the eastward extension of the Equatorial Undercurrent and the Subsurface South Equatorial Countercurrent. However, the source of the particles in this ∼400-m maximum has not been determined.