Abstract
Repeated measurements of depth profiles of 234Th (dissolved, 1-70 and > 70 μm particulate) at three stations (Orca, Minke, Sei) in the Ross Sea have been used to estimate the export of Th and particulate organic carbon (POC) from the euphotic zone. Sampling was carried out on three JGOFS cruises covering the period from October 1996 (austral early spring) to April 1997 (austral fall). Deficiencies of 234Th relative to its parent 238U in the upper 100 m are small during the early spring cruise, increase to maximum values during the summer, and decrease over the course of the fall. Application of a non-steady-state model to the 234Th data shows that the flux of Th from the euphotic zone occurs principally during the summer cruise and in the interval between summer and fall. Station Minke in the southwestern Ross Sea appears to sustain significant 234Th removal for a longer period than is evident at Orca or Sei. Particulate 234Th activities and POC are greater in the 1-70 gm size fraction, except late in the summer cruise, when the > 70 μm POC fraction exceeds that of the 1-70 μm fraction. The POC/234Th ratio in the > 70 μm fraction exceeds that in the 1-70 μm fraction, likely due in part to the greater availability of surface sites for Th adsorption in the latter, Particulate 234Th fluxes are converted to POC fluxes by multiplying by the POC/234Th ratio of the > 70 μm fraction (assumed to be representative of sinking particles). POC fluxes calculated from a steady-state Th scavenging model range from 7 to 91 mmolCm-2 d-1 during late January-early February, with the greatest flux observed at station Minke late in the cruise. Fluxes estimated with a non-steady-state Th model are 85mmolCm-2d-1 at Minke (1/13-2/1/97) and 50mmol Gm-2 d-1 at Orca (1/19-2/1/97). The decline in POC inventories (0-100m) is most rapid in the southern Ross Sea during the austral summer cruise (Smith et al., 2000. The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica. Deep-Sea Research 47, 3119-3140; Gardner et al., 2000. Seasonal patterns of water column particulate organic carbon and fluxes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3423-3449), and the 234Th-derived POC fluxes indicate that the sinking flux of POC is ∼ 30-50% of the POC decrease, depending on whether steady-state or non-steady-state Th fluxes are used. Rate constants for particle POC aggregation and disaggregation rates are calculated at station Orca by coupling particulate 234Th data with 228Th data on the same samples. Late in the early spring cruise, as well as during the summer cruise, POC aggregation rates are highest in near-surface waters and decrease with depth. POC disaggregation rates during the same time generally increase to a maximum and are low at depth (> 200 m). Subsurface aggregation rates increase to high values late in the summer, while disaggregation rates decrease. This trend helps explain higher values of POC in the > 70 m fraction relative to the 1-70 m fraction late in the summer cruise. Increases in disaggregation rate below 100 m transfer POC from the large to small size fraction and may attenuate the flux of POC sinking out of the euphotic zone.
Original language | English |
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Pages (from-to) | 3451-3490 |
Number of pages | 40 |
Journal | Deep-Sea Research Part II: Topical Studies in Oceanography |
Volume | 47 |
Issue number | 15-16 |
DOIs | |
Publication status | Published - 2000 |
Externally published | Yes |
Bibliographical note
Funding Information:We thank the officers and crew of the R/V Nathaniel B. Palmer and support personnel from Antarctic Support Associates for their assistance. Without their help, this study would not have been possible. We are grateful to Ellen Roos and Eileen Goldsmith for their assistance with the field work and logistics. This research has benefited from numerous discussions with our colleagues, including J. Dymond, W. Gardner, W. Smith and I. Walsh. Reviews by R. Anderson, D. DeMaster, J. Dunne and M. Rutgers van der Loeff improved the manuscript. Portions of this work constitute the MS thesis of Hong-Wei Wang, SUNY-Stony Brook. This research was supported by the National Science Foundation as part of the Joint Global Ocean Flux Study (grants OPP-9530861 to KOB and MPB and OPP-9612761 to JKC); we gratefully acknowledge this support. This is Contribution Number 1200 from the Marine Sciences Research Center, 10252 from the Woods Hole Oceanographic Institution and JGOFS Number 564.
ASJC Scopus Subject Areas
- Oceanography