Carbon and nitrogen export during the JGOFS North Atlantic Bloom experiment estimated from 234Th: 238U disequilibria

Abstract

The disequilibrium between the particle-reactive tracer ²³⁴Th ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 24.1}$ days) and its soluble parent, ²³⁸U, was used to examine Th scavenging and export fluxes during the U.S. JGOFS North Atlantic Bloom Experiment (24 April–30 May 1989) at ∼47°N, 20°W. Four profiles of dissolved and particulate ²³⁴Th in the upper 300 m and a non-steady box model were used to quantify dissolved ²³⁴Th uptake and particle export rates. The highest export fluxes occured during the first half of May. From POC/²³⁴Th and PON/²³⁴Th ratios, particulate organic C and N fluxes were calculated. Results were 5–41 mmol C m⁻² day⁻¹ and 0.9–6.5 mmol N m⁻² day⁻¹ from the 0–35 m layer. The ratio of POC export flux to primary production ranged from 0.05 to 0.42, peaking in the first half of May. The estimated fluxes agree with the observed losses of total C and N from the upper ocean during the bloom, but yield significantly higher fluxes than were measured by floating traps at 150 and 300 m.