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

Abstract

Depth profiles of total 234Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous 234Th studies. Significant zones of particle export ( 234Th deficiency) and particle remineralization ( 234Th excess) were measured both temporally and with depth. 234Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/ 234Th and PN/ 234Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. 234Th 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 December 1999, when traps undercollected for 234Th by a factor of 2 to 4. 234Th export ( ThE) ratios based on 234Th derived PC flux/ 14C 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.