Limnol. Oceanogr.. 32(l), 1987, 189-200 0 1987, by the.pdfVIP

Limnol. Oceanogr.. 32(l), 1987, 189-200 0 1987, by the American Society of Limnology and Oceanography, Inc. Oceanic stratified euphotic zone as elucidated by 234Th:238Udisequilibrial Kenneth H. Coale and Kenneth W. Bruland Institute of Marine Sciences, University of California, Santa Cruz 95064 Abstract Profiles of dissolved and particulate 234Th were determined at the VERTEX 2 and 3 stations off Manzanillo, Mexico, and at the VERTEX 4 station about 900 km north of Hawaii. By modeling the disequilibria between 234Th and 238U in the dissolved and particulate form, estimates of scav- enging rates for Th from the dissolved to particulate phases, particle residence times, and the flux of Th via particle removal can be obtained. 234Th: 238U activity ratio profiles indicate that the euphotic zone can be separated into two layers: An upper oligotrophic layer characterized by low new production values, low net scavenging, and long dissolved 234Th residence times; and a sub- surface eutrophic layer with higher new production values, more intense scavenging, and shorter dissolved 234Th residence times. New production, rather than total primary production may determine net scavenging rates of reactive elements from oceanic surface waters. These results contribute to the emerging descriptions of the layered structure of oligotrophic euphotic zones and support the notion that this may be a general and ubiquitous feature of global stratified oligotrophic regimes. These layered systems can be structured not only in biolo