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

Depth profiles of total Th (dissolved+particulate) were collected at Station ALOHA (22145N, 158100W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2L technique that enables more detailed depth resolution then previous Th studies. Significant zones of particle export (Th deficiency) and particle remineralization (Th excess) were measured both temporally and with depth. Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/Th and PN/Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150m. Th based export estimates of 4.072.3mmol Cm 2 d 1 and 0.5370.19mmol Nm 2 d , were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.470.2mmolCm 2 d 1 and 0.3270.08mmolNm 2 d . Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for Th by a factor of 2 to 4. Th export (ThE) ratios based on Th derived PC flux/C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change. r 2001 Elsevier Science Ltd. All rights reserved.