Deep Sea Research Part II: Topical Studies in Oceanography
234Th in surface waters: Distribution of particle export flux across the Antarctic Circumpolar Current and in the Weddell Sea during the GEOTRACES expedition ZERO and DRAKE
Introduction
Particle flux is a major factor in biogeochemical cycles. For the interpretation of the distribution of trace elements in the ocean, information on the geographical distribution of particle rain is therefore badly needed. Such information is available in a variety of time and space scales.
The geological record shows us rain rates averaged over hundreds to thousands of years. The sedimentary record in the southern Atlantic shows large latitudinal changes across the Antarctic Circumpolar Current with maximum flux in the Last Glacial Maximum north of the present position of the Antarctic Polar Front (PF) shifting to a present maximum located south of the PF (Frank et al., 2000, Kumar et al., 1995). Most studies agree on very low present particle fluxes in the central Weddell Sea, supported by radionuclide data (Walter et al., 2000). Whereas we know that the sedimentary record is biased by sediment redistribution, a map of 230Th-based present particle rain rates supports the distribution with maximum rain rates in the zone between the PF and the Southern ACC Front and very low rain rates in the central Weddell Sea (Geibert et al., 2005). However: in the Weddell Sea a contradiction was observed between surface data and sedimentary record (Leynaert et al., 1993) which may be related to atypically shallow mineralization in this area (Usbeck et al., 2002).
Averages over years to decades are obtained from compilations of hydrographic data. Schlitzer (2000) used inverse modeling to derive present net nutrient uptake and consequently POC export production from published oxygen, nutrient and carbon distributions. His analysis supports an enhanced production and export in the ACC around 45–55°S at the zero meridian. The Dahlem map of global productivity (Berger, 1989), based on 14C primary production measurements, phosphate utilization and satellite observations, gives a productivity maximum in the 50–60°S latitude band in the SE Atlantic. Distributions in a similar time frame are obtained from measurements of benthic fluxes (Jahnke, 1996) and oxygen penetration depth in the sediment (Sachs et al., 2009).
The seasonal evolution of particle fluxes is shown by sediment trap deployments, from the high fluxes near the Polar Front to the extremely low fluxes in the Weddell Sea (Fischer et al., 1988, Fischer et al., 2000). Satellites give us the pigment distribution in the ocean on a daily basis, which can be converted to primary production rates (Antoine et al., 1996, Behrenfeld and Falkowski, 1997). Satellites frequently show thin filaments of high phytoplankton biomass sometimes associated with the fronts, as one might expect related to mesoscale eddies causing local upwelling of nutrients and shallow mixed layers (Strass et al., 2002). Export events of such filaments can even be observed on the sediment (Sachs et al., 2009). However Sokolov and Rintoul (2007), in their investigation of the relationship between satellite Chl-a and the location of fronts, find that productive regions are separated by fronts rather than associated with them, similar to the findings from the geological record. They also show the major importance of seafloor topography in stimulating production through upwelling (cf. KEOPS, Blain et al., 2007; CROZEX, Pollard et al., 2009).
For the modeling of the distribution of tracers in the deep ocean, where even the most particle reactive elements have residence times of the order of decades, the multi-year average rain rate fields are appropriate. But for the understanding of trace element behavior in the surface ocean where wax and wane of plankton blooms occur in time scales of days or weeks we need flux information at higher time and space resolution. This information cannot be obtained from the literature but must be measured along with the studies of the other trace elements. Fluxes based on 234Th disequilibrium have been measured in several Southern Ocean studies and were summarized by Savoye et al. (2008).
During the expedition ZERO and Drake an extensive GEOTRACES program was executed in the Atlantic sector of the Southern Ocean along the Zero Meridian and across the Drake Passage. A major objective of the GEOTRACES program is the contemporaneous determination of a range of trace elements and isotopes in order to enable an integrated interpretation. We report here the distribution of 234Th and of the export rate of 234Th and of POC during this expedition. We investigate to what extent the 234Th-based fluxes are related to the development of phytoplankton as seen from space and to the removal of trace metals, notably iron and manganese, from the surface water.
Section snippets
Sampling
The cruise track (Fig. 1) of Polarstern Expedition ANTXXIV/3 (February–March 2008, Fahrbach and De Baar, 2010) joined the zero meridian at 52°S and followed this meridian to the ice shelf at 70°S (Fig. 1). At this place the floating shelf ice extends far north, an extension of the Fimbul Ice Shelf called Trolltunga. Whereas the sea ice on the zero meridian section had disappeared before our expedition, the crossing of the Weddell Sea to the tip of the Antarctic Peninsula was influenced by sea
Zero meridian
The Cape Town—zero meridian section (Fig. 1) crossed the Subantarctic Front (SAF) at about 45°S, the Polar Front (PF) at about 49°S and the southern boundary of the ACC (SB ACC) at about 56°S (Middag et al., 2011). The surface mixed layer (SML) was 80–120 m thick in the northern region from 42°S to 55°S and decreased to 25–50 m in the latitude range 60–69°S (Klunder et al., 2011, cf. Fig. 2A). Close to the edge of the Fimbul Ice Shelf (near the protruding floating shelf ice called Trolltunga) a
Discussion
The 234Th depletion is an integral result of export processes in the preceding approximately 2 months. For the interpretation of the 234Th results we must therefore consider the development of phytoplankton in the few months before our sampling. We discuss here data of Chl-a from remote sensing and data from a preceding Polarstern expedition to the zero meridian. For the phytoplankton distribution during our own sampling campaign we use data of beam attenuation and of particulate 234Th in
Conclusions
In this study, we have combined the analysis of precise 234Th profiles at a 1–2° (100–200 km) horizontal resolution with data of surface water at a substantial higher resolution.
The high-resolution distribution of particulate 234Th in surface water helps to characterize the distribution of phytoplankton and complements data of light transmission and Chl-a (shipboard pigment analyses or satellite-derived).
The total 234Th monitoring in surface waters with the automated procedure gives a higher
Acknowledgments
We are grateful to captain Schwarze and the crew of FS Polarstern for their support during the expedition. We thank Eberhard Fahrbach for the way in which he prepared and managed the expedition, Gerd Rohardt and Sven Ober for their help in collecting and providing the hydrographic data and the entire GEOTRACES team under guidance of Hein de Baar for excellent cooperation. We acknowledge thoughtful comments by Brad Moran and an anonymous reviewer. Remote sensing data of chlorophyll
References (71)
- et al.
Carbon and nitrogen export during the JGOFS North Atlantic Bloom Experiment estimated from 234Th:238U disequilibria
Deep-Sea Research
(1992) Upper ocean export of particulate organic carbon and biogenic silica in the Southern Ocean along 170°W
Deep-Sea Research II
(2001)The effect of marginal ice-edge dynamics on production and export in the Southern Ocean along 170°W
Deep-Sea Research II
(2003)An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy
Marine Chemistry
(2006)Particle fluxes associated with mesoscale eddies in the Sargasso Sea
Deep-Sea Research II
(2008)- et al.
An improvement in the small-volume technique for determining thorium-234 in seawater
Marine Chemistry
(2006) - et al.
238U, 234U and 232Th in seawater
Earth and Planetary Science Letters
(1986) - et al.
Low particulate organic carbon export in the frontal zone of the Southern Ocean (Indian sector) revealed by 234Th
. Deep-Sea Research I: Oceanographic Research
(2005) - et al.
Organic carbon fluxes in the Atlantic and the Southern Ocean: relationship to primary production compiled from satellite radiometer data
Deep-Sea Research II
(2000) - et al.
Determining true particulate organic carbon: bottles, pumps and methodologies
Deep-Sea Research II
(2003)
Actinium-227 as a deep-sea tracer: sources, distribution and applications
Earth and Planetary Science Letters
Annual uptake of atmospheric CO2 by the Weddell Sea derived from a surface layer balance, including estimations of entrainment and new production
Journal of Marine Systems
CO2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter
Marine Chemistry
Dissolved Fe in the Southern Ocean
Deep-Sea Research II
Consistent merging of satellite ocean color data sets using a bio-optical model
Remote Sensing of Environment
Dissolved manganese in the Atlantic sector of the Southern Ocean
Deep-Sea Research II
Differences in seawater particulate organic carbon concentration in samples collected using small- and large-volume methods: the importance of DOC adsorption to the filter blank
Marine Chemistry
234Th/238U disequilibrium in the central Arctic Ocean: implications for particulate organic carbon export
Deep-Sea Research II
Particulate matter and nutrient distributions in the ice-edge zone of the Weddell Sea: relationships to hydrography during late summer
Deep-Sea Research
U-salinity relationships in the Mediterranean: implications for 234Th:238U particle flux studies
Marine Chemistry
Two repeat crossings of Drake Passage in austral summer 2006: Short-term variations and evidence for considerable ventilation of intermediate and deep waters
Deep-Sea Res. II
234Th-based carbon export during an ice-edge bloom: sea-ice algae as a likely bias in data interpretation
Earth and Planetary Science Letters
Comparison of carbon and opal export rates between summer and spring bloom periods in the region of the Antarctic Polar Front, SE Atlantic
Deep-Sea Research II
Carbon export during the spring bloom at the southern Polar Front, determined with the natural tracer 234Th
Deep-Sea Research II
234Th sorption and export models in the water column: a review
Marine Chemistry
234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)
Deep-Sea Research II
Mesoscale physics, biogeochemistry and ecology of the Antarctic Polar Front, Atlantic sector: an introduction to and summary of cruise ANT XIII/2 of R.V. Polarstern
Deep-Sea Research II
Scavenging of 231Pa and thorium isotopes based on dissolved and size-fractionated particulate distributions at Drake Passage (ANTXXIV-3)
Deep-Sea Research II
Reduced scavenging of 230Th in the Weddell Sea: implications for paleoceanographic reconstructions in the South Atlantic
Deep-Sea Research I
Oceanic primary production, 2. Estimation at global scale from satellite (coastal zone color scanner) chlorophyll
Global Biogeochemical Cycles
Circulation in the Ona Basin, southern Drake Passage
Journal of Geophysical Research
A consumer's guide to phytoplankton primary productivity models
Limnology and Oceanography
Testing a new small-volume technique for determining thorium-234 in seawater
Journal of Radioanalytical and Nuclear Chemistry
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