Rutgers van der Loeff, Ingrid Vöge, Walter Geibert, Alfred-Wegener-Institut
Work at sea
Particle-reactive trace elements are removed from the water column by adsorption on sinking particles. The efficiency of this scavenging process depends on the particle rain rate and particle composition. The cruise track offered the opportunity to study the scavenging over a wide range of scavenging conditions.
Scavenging of 234Th
234Th is produced by decay of 238U in seawater and decays with a half life of 24.1 days. Scavenging causes a depletion of 234Th with respect to 238U in surface waters.
We expected to find a pattern of depletion that mirrors the combined effect of varying particle flux and composition along the cruise track. We have measured 234Th with two techniques: The classical technique requires the filtration of 20-l samples taken from the ship’s seawater supply to determine the particulate 234Th content and the subsequent co-precipitation of 234Th on MnO2 in the filtrate. This technique was used in coordination with the trace metal sampling (Table 1.1).
In addition we have tested a new automated technique. Five-liter samples of the ship’s seawater supply are automatically filtered and filled into a container where reagents are added to form a MnO2 precipitate. After a period of at least one hour, the precipitate is filtered over a second filter. The filter is washed with distilled water, dried and beta counted on board ship. Quartz fiber filters are used throughout because they have the lowest beta background. The new automated procedure uses 5 cm diameter filter units that are counted in a 10-position beta counter. As the background count rate of this counter on board ship was high (around 1 cpm) and rather variable, we decided to abandon the first filtration and measure total 234Th exclusively.
Preliminary results
The results (Fig. 4.1) show the general validity of the automated procedure.
Negligible scavenging is found around 30°N and around 5°N. Some removal is found in the intermediate zone, 10 - 20 °N and in the Biscay area. The strongest removal is found south of the equator in the South Atlantic Gyre which was apparently more productive than we had expected.
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Fig. 4.1: Distribution of total 234Th/238U in surface waters as measured with the 20-l technique (triangles) and with the automated 5-l procedure (dots)
In parallel to the 234Th measurements we also took samples to determine the distribution of 210Pb and 210Po in surface waters.
Enrichment of inorganic colloids by means of a large volume centrifuge
The colloidal size fraction of seawater, filling the gap between particulate (e.g. algae) and truly dissolved substances (e.g. salt), has been a target of marine geochemistry for many years. A focus of previous investigations had been placed on organic components of the colloidal fraction. Inorganic colloidal substances were not considered to represent an important component of sea water. In order to screen seawater for potential inorganic colloids, we have enriched the smallest possible size fractions from large volumes of sea water in order to obtain a sufficient amount of material for analysis. This was accomplished by a continuous flow centrifuge that had been connected to the seawater supply. Particles and colloids from about 1 m3 of seawater at 18 locations were collected at a flow rate of ~350 l/hour at about 16000 x g. The sample collection is suitable for trace element analysis, as the inlet system and the collecting surface is made from titanium or teflon® only.
The given acceleration should be sufficient to efficiently collect particles much smaller than 2 µm in diameter, dependent on their density. This size fraction is expected to contain clay minerals, which would be overlooked in conventional analytic techniques. In order to identify these clay minerals (if present), analysis will include electron microscopy and the measurement of specific elements (Ti, 232Th, Si).
Sampling problems were arising from the surface of the teflon® sheets, that led in some cases to a potential loss of water drops from the upper part of the centrifuge rotor, where the smalles particles must be expected.
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Th recovery in ageing samples of deep-sea water
with and without presence of the colloidal fraction
0
0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0
time [h]
Th recovery in ageing samples of surface water
with and without presence of the colloidal fraction
0
0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0
time [h]
clay mineral standard (SW-y2 Montmorillonite from clay minerals repository, @ 1 mg/l) were added to these water types to test the hypothesis whether the natural colloidal composition of seawater is affecting the sorption kinetics of Th. For comparison, the same water types were also monitored without particles added. Both experiments were lasting five days, and almost all 300 subsamples (particulate and dissolved separately) could be measured on-board with the seagoing Risoe beta counter GM25-5. Preliminary results for the Bay of Biscay water indicate that the absence of colloids is favouring adsorption of Th to the walls of the vessel in both deep and surface water, as seen in poor recoveries for ultrafiltered water (Fig. 4.2 a, b).
Fig. 4.2: Temporal evolution of Th recovery during in-vitro experiments with water from the Bay of Biscay in deep sea (upper panel, Fig. 4.2 a) and surface water (lower panel, Fig.4.2 b). Errors are
estimated to be 3 % (1 σ).
4.2 b 4.2 a
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0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0
time [h]
Th particulate fraction in samples of deep-sea water
with and without presence of the colloidal fraction
0
0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0
time [h]
particulate fraction of Th [%] [%]
0.2 µm clay1 0.2 µm clay2 0.2 µm, pure SW ultrafiltered, clay ultrafiltered, pure SW
Fig. 4.3: Temporal evolution of the particulate fraction of 234Th during in-vitro experiments with water from the Bay of Biscay in deep sea (upper panel, Fig.4.3 a) and surface water
(lower panel, Fig. 4.3.b). Errors are estimated to be 3 % (1 σ).
Particulate fractions of Th are not zero, not in filtered and not even in ultrafiltered seawater (Fig. 4.3). This must be due to the new formation of particles from the colloidal or dissolved pool of substances. Particle formation is not even decreased in ultrafiltered water. However, a trend can be seen that later adsorption of Th onto particles is favoured by previous ultrafiltration. Together with the observations from the recoveries, it can be preliminarily concluded that colloids play a role in keeping Th apparently dissolved.
4.3 b
56 concentrations.
Cooperation with particle optics
During the expedition, cooperations with groups investigating optical properties of seawater were arising. Studies of angular scattering in seawater due to colloids and small particles accompanying the Th sorption experiments were conducted by D.
Stramski on a daily basis during the sorption experiment with water from the dust station. The combination of chemical and optical monitoring of seawater may turn out to be a powerful tool for colloid and small particle characterization.
Studies on light absorption in seawater due to humic substances were performed by R. Röttgers. Comparisons between filtered, ultrafiltered and organics-depleted seawater (by PPL extraction) were providing some insights into possible effects of the different fractions on light absorption in the sample.
4.3 Intercomparison of techniques for the analysis of TH-230, rare