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Influence of solution chemistry on the boron content in inorganic calcite grown in artificial seawater
- Source :
- Geochimica et Cosmochimica Acta. 218:291-307
- Publication Year :
- 2017
- Publisher :
- Elsevier BV, 2017.
-
Abstract
- The ratio of boron to calcium (B/Ca) in marine biogenic carbonates has been proposed as a proxy for properties of seawater carbonate chemistry. Applying this proxy to planktic foraminifera residing in the surface seawater largely in equilibrium with the atmosphere may provide a valuable handle on past atmospheric CO 2 concentrations. However, precise controls on B/Ca in planktic foraminifera remain enigmatic because it has been shown to depend on multiple physicochemical seawater properties. To help establish a firm inorganic basis for interpreting the B/Ca records, we examined the effect of a suite of chemical parameters ([Ca 2+ ], pH, [DIC], salinity and [PO 4 3− ]) on B/Ca in inorganic calcite precipitated in artificial seawater. These parameters were primarily varied individually while keeping all others constant, but we also tested the influence of pH and [DIC] at a constant calcite precipitation rate ( R ) by concurrent [Ca 2+ ] adjustments. In the simple [Ca 2+ ], pH and [DIC] experiments, both R and B/Ca increased with these parameters. In the pH–[Ca 2+ ] and [DIC]–[Ca 2+ ] experiments at constant R , on the other hand, B/Ca was invariant at different pH and decreased with [DIC], respectively. These patterns agree with the behavior of solution [B Total /DIC] ratio such that, at a fixed [B Total ], it is independent of pH but decreases with [DIC]. Based on these results, R and [B Total /DIC] ratio appear to be the primary controls on B/Ca in inorganic calcite, suggesting that both B(OH) 4 − and B(OH) 3 are possibly involved in B incorporation. Moreover, B/Ca modestly increased with salinity and [PO 4 3− ]. Inorganic calcite precipitated at higher R and in the presence of oxyanions such as SO 4 2− and PO 4 3− in growth solutions often undergoes surface roughening due to formation of crystallographic defects, vacancies and, occasionally, amorphous/hydrous CaCO 3 . These non-lattice sites may provide additional space for B, particularly B(OH) 3 . Consequently, besides the macroscopic influence of R and bulk solution chemistry, molecular-scale processes associated with calcite nucleation can be an important consideration for B incorporation, especially in complex ionic solutions. Lastly, the covariance of B/Ca with [DIC] and salinity observed here qualitatively agrees with those in planktic foraminifers. It follows that their impact on foraminiferal B/Ca is partly inorganically driven, which may explain why the effect is evident across different species.
- Subjects :
- Calcite
010504 meteorology & atmospheric sciences
Analytical chemistry
chemistry.chemical_element
Mineralogy
Artificial seawater
Isotopes of boron
Calcium
010502 geochemistry & geophysics
01 natural sciences
Salinity
chemistry.chemical_compound
chemistry
Geochemistry and Petrology
Carbonate
Seawater
Boron
Geology
0105 earth and related environmental sciences
Subjects
Details
- ISSN :
- 00167037
- Volume :
- 218
- Database :
- OpenAIRE
- Journal :
- Geochimica et Cosmochimica Acta
- Accession number :
- edsair.doi...........c6b563a0a0fa5e11102a753f98c2963b