Trull et al., JGR-Oceans, accepted for publication.

This is a deep sediment trap paper which includes 230Th-based assessments

of collection efficiency , which I include because I think that extension

of our considerations to deep traps, and to 230Th and 231Pa may be useful.

In that regard, the recent paper by Yu et al., on the use of 231Pa/230Th

ratios in the assessment of deep trap efficiency may be of interest (E.-F.

Yu et al., 2001. Trapping efficiency of bottom-tethered sediment traps

estimated from the intercepted fluxes of 230Th and 231Pa. Deep-Sea

Research, 48(3): 865-889).

Moored Sediment Trap Measurements of Carbon Export

in the Sub-Antarctic and Polar Frontal Zones

of the Southern Ocean, south of Australia.

T.W. Trull1, S.G. Bray1, S.J. Manganini2, S. Honjo2, and R. François2

1. Antarctic Cooperative Research Centre, University of Tasmania, Hobart,

Australia

2. Woods Hole Oceanographic Institution, Woods Hole, MA, U.S.A.

Revised Draft March 28, 2001

Abstract

The SAZ project organized by the Antarctic CRC has a continuing program of

moored sinking particle trap studies in the Sub-Antarctic and Polar Frontal

Zones southwest of Tasmania along 140°E longitude. The first deployment

obtained weekly or higher resolution samples through the austral summer

from September 1997 through February 1998 at three locations: the central

Sub-Antarctic Zone (~47°S, traps at 1060, 2050, and 3850m depth), the

Sub-Antarctic Front (~51°S, 1 trap at 3080m) and above the Southeast Indian

Ridge in the Polar Frontal Zone (~54°S, 2 traps at 830 and 1580m).

Particle fluxes were high at all the sites (18 to 32 g m-2 yr-1 total mass

flux and 0.5 to 1.4 g organic carbon m-2 yr-1 at ~1000m depth, assuming

minimal flux outside the sampled summer period). These values are similar

to other recent Southern Ocean deep sediment trap results, and to the

median estimated for the global ocean by Lampitt and Antia (1997).

For the 153 day collection period, particulate organic carbon export was

somewhat higher in the SAZ (range 0.57 to 0.76 gC m-2 yr-1) than in the PFZ

(range 0.31 to 0.53). The fraction of surface organic carbon export

(estimated from seasonal surface water nutrient depletion) reaching 1000m

was indistinguishable in the two zones, despite considerably different

phytoplankton communities, in particular the more frequent occurrence and

greater abundance of large diatoms in the PFZ.

Pronounced seasonality was observed at all the sites. The SAZ site

exhibited similar late-spring and late-summer periods of elevated flux and

was dominated by carbonate export (>50% of total mass), while the PFZ site

exhibited higher flux in summer than spring and was dominated by biogenic

silica export (>50% of total mass) throughout the year. The spring

increase in export and late summer decrease were similar to the timing

observed by the AESOPS program near 170°W, but the presence of a low flux

period in all the SAZ project traps in December was a strong contrast.

Comparison with sea-surface height records suggests movement in frontal

positions was not a major factor in the observed SAZ and PFZ seasonal

variations, although it did affect the SAF results. The deployment year

experienced unusually cool sea surface temperatures throughout summer, and

it is possible this influenced export, although surface nutrient depletion

for the deployment year was similar to a previous year with a more typical

amplitude of surface warming. With the caveat of possibly strong

interannual variability, consideration of remote sensing and hydrographic

results suggests the SAZ project particle fluxes can be taken as

representative of the SAZ and PFZ for the East Antarctic sector of the

Southern Ocean from 90 to 145°E.