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

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Abstract

As part of the GEOTRACES Polarstern expedition ANTXXIV/3 (ZERO and DRAKE) we have measured the vertical distribution of 234Th on sections through the Antarctic Circumpolar Current along the zero meridian and in Drake Passage and on an EW section through the Weddell Sea. Steady state export fluxes of 234Th from the upper 100 m, derived from the depletion of 234Th with respect to its parent 238U, ranged from 621±105 to 1773±90 dpm m−2 d−1. This 234Th flux was converted into an export flux of organic carbon ranging from 3.1 to 13.2 mmol C m−2 d−1 (2.1–9.0 mmol C m−2 d−1) using POC/234Th ratio of bulk (respectively >50 μm) suspended particles at the export depth (100 m). Non-steady state fluxes assuming zero flux under ice cover were up to 23% higher. In addition, particulate and dissolved 234Th were measured underway in high resolution in the surface water with a semi-automated procedure. Particulate 234Th in surface waters is inversely correlated with light transmission and pCO2 and positively with fluorescence and optical backscatter and is interpreted as a proxy for algal biomass. High resolution underway mapping of particulate and dissolved 234Th in surface water shows clearly where trace elements are absorbed by plankton and where they are exported to depth. Quantitative determination of the export flux requires the full 234Th profile since surface depletion and export flux become decoupled through changes in wind mixed layer depth and in contribution to export from subsurface layers.

In a zone of very low algal abundance (54–58°S at the zero meridian), confirmed by satellite Chl-a data, the lowest carbon export of the ACC was observed, allowing Fe and Mn to maintain their highest surface concentrations.

An ice-edge bloom that had developed in December/January in the zone 60–65°S as studied during the previous leg had caused a high export flux at 64.5°S when we visited the area 2 months later (February/March). The ice-edge bloom had then shifted south to 65–69°S evident from uptake of CO2 and dissolved Fe, Mn and 234Th, without causing export yet. In this way, the parallel analysis of 234Th can help to explain the scavenging behavior of other trace elements.

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

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