Partitioning of rare earth elements on calcite at saturated and undersaturated stages, mimicking settlement through water columns
Abstract
The partitioning of rare earth elements (REEs) in seawater depends on experimental methods. It is somewhat obscure which values are suitable to apply to naturally occurring processes in water columns. In a real oceanic column, calcite tests are produced by coccolithophores in the photic layer. They then settle down to deep water, where REEs in seawater can adsorb or partition onto their surface (Stage 1). The calcite particles are then subjected to dissolution at a layer deeper than the carbonate saturation depth (Stage 2). Here we have developed a new method to determine partitioning of REEs by mimicking the two stages of natural water columns. The partitioning obtained here should be the most applicable to the discussion of REEs in oceanic water columns.
[Method] Calcite particles with different sizes are used as seeds. The particles are stirred mechanically with 300ppm CO2/N2gas being purged in REE-containing seawater for a certain length of time (Stage 1). The solution is subjected to elutriation to separate particles in sizes. The REE/Ca ratio was determined for both particles and seawater and are plotted against sizes. The asymptotic values were adopted as the true values of Stage 1. Half of the solution is further stirred purging with 5000ppm CO2/N2gas to mimic a carbonate-unsersaturated condition (Stage 2). Similarly to Stage 1, the solution is treated with elutriation, size separation and determination of Ca and REE to obtain the partitioning values of Stage 2.
[Results & Discussion] The pattern of partitioning values at Stage 1 was almost identical to that obtained by Zhong and Mucci (1995), but absolute values are smaller by a factor of 1/10 (Fig. 1). At Stage 2, the partitioning values became significantly smaller. The difference between the two can be explained by difference in free REE concentration (Fig. 1). This shows that redistribution rather than incongruent dissolution should be operative at Stage 2.
Our new method enables us to see the effect of other ions in REE partitioning. We have studied the effect of Mn and Fe. When Fe is present, heavy REEs showed higher partitioning values, whereas when Mn is present, no noticeable change was seen in the partitioning values.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMOS23E1673A
- Keywords:
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- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4805 Biogeochemical cycles;
- processes;
- and modeling;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICALDE: 4870 Stable isotopes;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICALDE: 4875 Trace elements;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL