A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on Th-234 : U-238 disequilibrium

Depth profiles of total Th-234 (dissolved+particulate) were collected at St ation ALOHA (22 degrees 45N, 158 degrees 00W) in the North Pacific Subtropi cal Gyre during 9 cruises from April 1999 to March 2000. Samples were colle cted and processed by a new 2 L technique that enables more detailed depth resolution then previous Th-234 studies. Significant zones of particle expo rt (Th-234 deficiency) and particle remineralization (Th-234 excess) were m easured both temporally and with depth. Th-234 derived particulate carbon ( PC) and nitrogen (PN) fluxes were determined with steady-state and non-stea dy-state models and PC/Th-234 and PN/Th-234 ratios measured with both in si tu pumps and free-drifting particle interceptor traps deployed at 150 m. Th -234 based export estimates of 4.0 +/- 2.3 mmol C m(-2) d(-1) and 0.53 +/- 0.19 mmol N m(-2) d(-1), were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4 +/- 0.2 mmol C m (-2) d(-1) and 0.32 +/- 0.08 mmol N m(-2) d(-1). Most of this difference is attributable to two large export events that occurred during October and D ecember 1999, when traps undercollected for Th-234 by a factor of 2 to 4. T h-234 export (ThE) ratios based on Th-234 derived PC flux/C-14 based primar y production ranged from 4% to 22% (average = 8.8%). Our results confirm th e recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) an d Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C e xport from the oligotrophic ocean must be considered when discussing C sequ estration in global climate change. (C) 2001 Elsevier Science Ltd. All righ ts reserved.