Oceanic stratified euphotic zone as elucidated by 234Th: 238U disequilibria1

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 scavenging 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 subsurface 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 biological and nutrient parameters, but also in terms of the rates of chemical scavenging and elemental transport.