Calibration of sediment traps and particulate organic carbon export using 234Th in the Barents Sea
Introduction
Particle fluxes are usually measured by sediment traps to estimate particle abundance and particle sinking speed. Variations of the quantity and presumably of the quality of particulate matter (including particulate organic carbon i.e. POC) recovered in a trap reflect the characteristics of the surface water dynamics and biomass composition. However, hydrodynamic effects and swimmers might seriously bias trapping efficiency Buesseler, 1991, Gardner, 1996. Therefore, the use of traps alone is not sufficient to estimate correctly particle fluxes. Independent measurements of vertical flux from 234Th activities in the water column can be used to calibrate the sediment trap catchment efficiency (Buesseler, 1991). This method is based on the fact that 238U (half-life 4.5×109 years) has a conservative behavior in the ocean, whereas its daughter product, 234Th (half-life t1/2=24.1 days), is highly particle-reactive and hence sticks to all particle surfaces. From the 234Th deficiency (relative to 238U) in surface waters, we can predict the particulate 234Th flux exported from these waters Coale and Bruland, 1985, Coale and Bruland, 1987. By comparing this flux with 234Th measurement in a trap, we can estimate if the trap collects 234Th-bearing particles quantitatively (Gardner, 1996). The particulate organic carbon export can be derived if the ratio POC/234Th is known. However, the relationships between POC and 234Th are not well understood.
The Barents Sea is an arctic shelf sea of the eastern North Atlantic, which supports one of the most productive marine ecosystems in the Arctic region (Wassmann et al., 1999). The annual estimates of primary production range from 18 to 73 g C m−2 year−1 between the northern and the southern part of the Barents Sea (Wassmann and Slagstad, 1993). This region is characterized by large regional differences in hydrographic conditions and ice coverage Loeng, 1991, Loeng et al., 1997. Atlantic water flowing into the Barents Sea from south–west and Arctic water entering from north–east are separated by the Polar Front. The northern, central and eastern Barents Sea is periodically ice covered. The marginal ice zone (MIZ) is a unique frontal system (Wassmann et al., 1999). During late spring and early summer, the ice melt gives rise to a stratified and nutrient-rich euphotic zone. It is followed by a phytoplankton bloom (Slagstad and Wassmann, 1997). Temporal and spatial variability of vertical flux of biogenic matter is important to understand elemental dynamics and food webs in the ocean Legendre, 1990, Smetacek et al., 1984, Wassmann, 1991. The dynamics of this flux and the composition of the exported production were investigated during previous studies along a section in the central Barents Sea during the 1984–1993 period (Wassmann and Slagstad, 1993). Along this south–north transect, the zonal structure of the Barents Sea with its main water masses (Atlantic water, the Polar Front, the ice edge and Arctic water) was investigated (Andreassen and Wassmann, 1998). Physical processes may also influence the dynamics of the vertical flux in the Barents Sea through their impact on ice coverage, ice melt, stabilization, advection and retention of zooplankton Slagstad and Wassmann, 1997, Wassmann and Slagstad, 1993.
We present water column measurements of dissolved and particulate 234Th and 234Th collected in sediment traps at five stations representing different trophic features corresponding to different ice coverage conditions and different water masses (Arctic and Atlantic waters). These results are used to calibrate sediment traps, to quantify POC export in this region and to compare these estimates with direct POC export estimates.
Section snippets
Sample collection
Samples were collected onboard RV “Jan Mayen” from June 28, 1999 to July 12, 1999, during ALV3 cruise (Arctic Light and Heat), along a transect in the central Barents Sea. The sampling area was from 78°15′N–34°09′E to 73°49′N–31°43′E (Fig. 1). Heading for the north, we tried to penetrate as deep as possible into the MIZ. Along this south–north section, POC in small particles, nutrients and plankton were measured during short stations. When the northernmost station was reached (78°N), we
Hydrography
The main water masses encountered along the transect are identified with the temperature section (Fig. 2a). Atlantic water was predominant in the south and in the center of the section. It was divided in two branches which penetrated the Barents Sea from the south–west. The South Barents Atlantic-derived Water (SBAW) was 4–7 °C while the North Barents Atlantic-derived Water (NBAW) was colder at 2–4 °C. On the Sentralbanken, a specific water mass (Sentralbanken Water i.e. SW) of Arctic origin
Origins of 234Th/238U disequilibrium
As 234Th is highly particle-reactive and sticks to all particle surfaces, the loss of 234Th (i.e. 234Th/238U disequilibrium) from surface waters is a direct indication of the removal rate of sinking particulate matter. Coale and Bruland (1985) showed that there is a link between 234Th removal rates and new production in the open ocean. However, other mechanisms can also explain a 234Th/238U disequilibrium. 234Th/238U disequilibrium throughout the water column has been previously observed in the
Conclusion
Measurements of 234Th depth profiles in the Barents Sea indicate that 234Th/238U disequilibrium is present in July in the entire sampling area. This is due to the high concentrations of biogenic matter in the water column. The 234Th method confirms the accuracy of sediment traps in the Barents Sea. This allows to compare POC fluxes estimated with 234Th model and measured POC fluxes in sediment traps. We suggest that high POC fluxes previously reported in the Arctic Ocean during the summer based
Acknowledgements
It is a pleasure to acknowledge the sedimentation group of the Norwegian College of Fishery Science in Tromsø. We are specially grateful to C. Wexels Riser, C. Svensen and M. Reigstad for their cooperation and assistance during the cruise ALV3 and the visit of L. Coppola during 1 month in the lab. We thank F. Lyard for sharing his expertise for the barotropic calculations. We wish also to acknowledge the officers and the crew of the R/V Jan Mayen for their efforts in sampling. J. Dunne, M.
References (42)
- Andersson, P.S.Ö.G., Roos, P., Broman, D., Toneby, A., 2000. Particle mediated surface water export: comparison of...
- et al.
Vertical flux of phytoplankton and particulate biogenic matter in the marginal ice zone of the Barents Sea in May 1993
Mar. Ecol., Prog. Ser.
(1998) The behavior of Al, Mn, Ba, Sr, REE and Th isotopes during in vitro degradation of large marine particles
Mar. Chem.
(2001)- et al.
Removal of thorium-234 by scavenging in the bottom nepheloid layer of the ocean
Earth Planet. Sci. Lett.
(1989) - et al.
Export flux of carbon at the equator during EqPac time-series cruises estimated from 234Th measurements
Deep-Sea Res., Part II
(1996) Do upper-ocean sediment traps provide an accurate record of particle flux?
Nature
(1991)- et al.
Carbon and nitrogen export during the JGOFS North Atlantic Bloom Experiment estimated from 234Th:238U disequilibria
Deep-Sea Res.
(1992) - et al.
A three dimensional time-dependent approach to calibrating sediment trap fluxes
Glob. Biogeochem. Cycles
(1994) - et al.
Regional estimates of the export flux of particulate organic carbon derived from thorium-234 during the JGOFS EQPAC Program
Deep-Sea Res., Part II
(1995) Upper ocean export of particulate organic carbon in the Arabian Sea derived from thorium-234
Deep-Sea Res.
(1998)
Rates of particle scavenging and particulate organic carbon export estimated using 234Th as a tracer in the subtropical and equatorial Atlantic Ocean
Deep-Sea Res., Part II
234Th as a tracer of particulate organic carbon export in the subarctic northeast Pacific Ocean
Deep-Sea Res., Part II
238U–234U and 232Th in seawater
Earth Planet. Sci. Lett.
Labyrinth of doom: advice to minimize the “swimmer” component in sediment trap collections
Limnol. Oceanogr.
234Th:238U disequilibria within the California current
Limnol. Oceanogr.
Oceanic stratified euphotic zone as elucidated by 234Th:238U disequilibria
Limnol. Oceanogr.
Thorium-234/Uranium-238 disequilibrium as an indicator of scavenging rates and particulate organic carbon fluxes in the Northeast Water Polynya Greenland
J. Geophys. Res.
Physical and ecological processes in the marginal ice zone of the northern Barents Sea during the summer melt period
J. Mar. Syst.
Accumulation of Th, Pb, U and Ra in marine phytoplankton and its geochemical significance
Limnol. Oceanogr.
The significance of microalgal blooms for fisheries and for the export of particulate organic carbon in oceans
J. Plankton Res.
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