DOI: 10.1594/pangaea.52064
title: Radionuclides and silicate measured on water bottle samples during POLARSTERN cruise ANT-X/6

description: In this thesis the distribution of 210Po and 210Pb in the upper 600 m of the Antarctic Circumpolar Current and the Weddell Sea was investigated along north-south transects in austral spring and autumn. 210Po and 210Pb can serve as sensitive tracers for the special hydrographic conditions of the Antarctic Circumpolar Current and the Weddell Sea as well as for biological processes during phytoplankton blooms. The 210Po/210Pb disequilibrium was used as a tracer for particle export. This tracer integrates export on a timescale of 276 days because of the 138 day half-life of 210Po and complements the 234Th/238U disequilibrium as another tracer for plankton production arid export on a shorter timescale of several weeks.The distribution of 210Pb and its granddaughter 210Po is determined by the hydrographic conditions, through decay of 226Ra in the water column and low atmospheric 210Pb input, especially in the study area, and is modified by biological processes in the surface water. Low 226Ra activity north of the Polar Front causes low activities of its daughter nuclides 210Pb and 210Po. South of the Polar Front, upwelling Circumpolar Deep Water transports higher activities of the three nuclides into the surface water.The phenomenon of a minimum in total 2l0Po at the base of the euphotic Zone reaching from the Polar Front to the central Weddell Sea was observed in austral spring and fall. It may be a characteristic feature of Antarctic Surface Waters and the following explanations are favoured:- Upwelling of Circumpolar Deep Water with high 210Po activity at the southern boundary of the Antarctic Circumpolar Current and northward advection with the Antarctic Surface Water can cause higher 210Po activity in the southern surface water. A water layer of low motion with more intense scavenging of 210Po by particles may exist at the interface between the southwardly rising Circumpolar Deep Water and northwardly moving Antarctic Surface Water, because of the prolonged water residence time. It is hypothesized that this effect, together with the preferential adsorption of 210Po (in excess of 210Pb) onto particles, creates the observed 210 minimum layer. - The 210Po minimum covers the layer of the temperature minimum of the Antarctic Surface Water. High 210Po, 210Pb and 234Th activities in sea ice samples support the hypothesis that particle-reactive radionuclides from the water column are adsorbed onto ice crystals during their formation.- The occurrence of large salp swarms in the spring conceivably produced part of the subsurface 210Po minimum. The salps graze on algae and accumulate the algal protein containing the Po.The deficiency of total 210Po relative to 210Pb was observed in the water column down to 600 m. This is due to the preferential adsorption of onto particles. During the development of a phytoplankton bloom in the Polar Frontal Region the increase in diatom abundance was reflected in the change of the particulate 210P0/210Pb ratio with time. The ratio dropped from 4 to 2 indicating that more 210Pb was adsorbed. 210Pb is better scavenged by siliceous plankton and 210Po adsorbs more effectively onto non siliceous particles. For this reason, 210Pb is a good tracer for biogenic silica export and 210Po for POC export.To quantify the particle export from the surface water in the study area, 210Po and export was calculated using the one-dimensional steady-state model of BACON et al. (1976). Because of the 138 day half-life of 210Po the calculated export integrates over a time scale of about nine months. To calculate recent export from changes in the 210Po and 210Pb activity with time during a phytoplankton bloom a non-steady-state solution for 210Po and 210Pb to the vertical scavenging model was developed after the 234Th model by BUESSELER et al. (1992). It transpired that with increasing half-life of the radionuclide the influence of water mass advection increases. Neglecting the advective and diffusive fluxes of 210Po and 210Pb under the hydrographic conditions of the ACC causes the calculated export to be to high or to small. The model gives realistic results only when calculating export from stations with identical temperature and salinity characteristics.The comparison of 210Po and 210Pb export, calculated with the steady-state model, out of the upper 100 m of the Polar Frontal Region, the southern ACC and the eastern Weddell Sea suggested highest export rates in the eastern Weddell Sea. This is in contradiction to the low sedimentation rates obtained from sediment traps and the low 210Pbxs sediment inventory. However, the high particle export signal could also be caused by adsorption of radionuclides onto sea ice, rather than by adsorption onto sinking particles.
description: Supplement to: Friedrich, Jana (1997): Polonium-210 und Blei-210 im Südpolarmeer: Natürliche Tracer für biologische und hydrographische Prozesse im Oberflächenwasser des Antarktischen Zirkumpolarstroms und des Weddellmeeres (Polonium-210 and Lead-210 in the Southern Polar Ocean: Naturally occurring tracers of biological and hydrographical tracers of biological and hydrographical processes in the surface waters of the Antarctic Circumpolar Current and the Weddell Sea). Berichte zur Polarforschung = Reports on Polar Research, 235, 155 pp