The Properties of the Surface, Boundary Layer and Interfacial Water Controls Mineral Dissolution and Kinetics

Since the mid-1990s, I am of the opinion that the minute pore spaces within the sediments may have a different chemistry than the bulk media due to the surface charge of the sediment grains, their sharp curvature, and therefore enhanced electric field strength under such an environment.

In continuation of those thoughts a recent work (Ma et al., 2018) using a combination of cryo-microscopy and single-particle 3D reconstruction suggested the existence of silica cages of around 10 nm size that they christened as "silicages" with dodecahedral structure. These microenvironments are different from bulk media properties and have the potential for the separation of isotopes as these "silicages" are effective substrates for column separation. Their extended uses are in the areas of wastewater treatment and separation of organic molecules.

Such a change in the properties of bulk water within the microenvironment can be ascribed to the low dielectric constant of ~2 for the water (Fumagalli et al., 2018). The consequences of such a very low dielectric constant of water within nanoscale pore spaces has ramifications for geochemistry. Some of them include dissolution and precipitation of minerals, weathering and remobilization of major and trace elements in the hyporheic zone of the riverine environment, the hydrocarbon potential of oil-bearing formations (water shall have increased miscibility with oil in the nano-pore spaces), solubility of calcium carbonate in the karst regions, pore water chemistry and isotopic fractionation during early diagenesis, and adsorption.

I believe that the anomalous characteristics of water within the confined space is also applicable to the boundary layers of the mineral grains. For example, the leaching of trace elements from the aerosols (for a review, please read Mahowald et al., 2018) is a highly complex process. Now consider an aerosol particle in the upper atmosphere with mineral dust as a nucleus and condensed water around it. Also, assume that the aerosol has natural organic molecules such as formate and acetate that can be excited by irradiation of light (or, UV).

1) The aerosols with the fine mineral particle as nuclei can have a boundary layer around it where the properties of water shall be different from its bulk chemistry; 2) The fine mineral particles shall have the surface charges that may impact the chemical reactions occurring within the boundary layer. 3) The natural organics shall interact with dissolved trace elements in the aerosol, such as Fe and Cu, to generate reactive oxygen species by photochemical reactions. Thus, the aerosols present an exciting phase for the study of biogeochemical cycling of trace elements - especially that of Fe, Mn, Cu.)

The same principles are applicable for the trace elements speciation in the boundary layer surrounding the phytoplankton cells. The redox chemistry of the trace elements within the boundary layer is highly complex and difficult to study. However, the scattering and polarisation of light within the boundary layer may serve as a probe for the environmental conditions in the boundary layer. From those results, it could be possible to understand the speciation of trace elements. For example, the Circular Dichroism Spectroscopy (CDS) can probe the secondary structure of macromolecules like the protein. Moreover, the secondary structures of proteins are prone to environmental conditions such as pH and temperature. Similarly, the nature of the boundary layers surrounding algae cells can be probed using Vibrational Optical Activity (Vibrational Circular Dichroism, and Vibrational Raman Optical Activity). Since biological molecules are chiral, and the algal exudates meant to manipulate the speciation of trace elements are expected to exhibit chirality, studying their optical properties using the techniques mentioned above may provide more insight into the conditions prevailing within the boundary layer.

Thus, I consider that the surface change, electric field strength, the dielectric constant of water, the surface energy of "adsorption sites" or, "sites of molecular interaction" can have a tremendous impact on the properties of the interface. Understanding the chemistry of the interface has ramifications for many fields in geochemistry.

A recent investigation by Schaefer et al. (2018) of silica mineral dissolution kinetics at the "boundary layer" using surface-specific spectroscopy (V-SFG), revealed that the dissolution products reached saturation within a very short time (tens of hours) compared to silica dissolution kinetics observed using bulk solutions, indicating that the mineral-water interface has distinct properties. I am of the opinion that such an attainment of fast equilibrium conditions during silica dissolution arises from the distinctive properties of the interface (or, boundary layer) described above.

I extend that "boundary layer theory"  of  interface chemistry to the application of mineral dissolution. And, I state that the surface "sites of molecular interaction" of a mineral have their surface energy distribution described by the Boltzmann function, akin to that of a porous media in that, the energetic heterogeneity of the "sites of molecular interaction" can be derived from the theory of surface adsorption (Anandaraj et al., 2017).  Then, I propose that the surface roughness of the mineral introduces an "energetic heterogeneity" onto the "sites of molecular interaction" that affects the Boltzmann energy distribution function of that surface. Then I suppose that the surface charge of the mineral, the electric field strength of curved surfaces that arises due to surface roughness (that introduces the geometric heterogeneity), the dielectric constant of water, the surface energy of "adsorption sites" or, "sites of molecular interaction" are the forces that play a major role in the dissolution kinetics of a mineral. The same principles are applicable to describe trace elements speciation and biological uptake within the boundary layers.

Note: This article is based on my earlier posts in the LinkedIn with most of the portions being reproduced here in a coherent way.

a) https://www.linkedin.com/feed/update/urn:li:activity:6439327245055156224
b) https://www.linkedin.com/feed/update/urn:li:activity:6417815034390474753
c) https://www.linkedin.com/feed/update/urn:li:activity:6424147843132555264
d) https://www.linkedin.com/feed/update/urn:li:activity:6422264668093546496

References:

Anandaraj, B., Eswaramoorthi, S., Rajesh, T.P., Aravind, J., Suresh Babu, P., 2018. Chromium(VI) adsorption by Codium tomentosum: evidence for adsorption by porous media from sigmoidal dose–response curve. Int. J. Environ. Sci. Technol. 15, 2595–2606. https://doi.org/10.1007/s13762-017-1488-7

Fumagalli, L., Esfandiar, A., Fabregas, R., Hu, S., Ares, P., Janardanan, A., Yang, Q., Radha, B., Taniguchi, T., Watanabe, K., Gomila, G., Novoselov, K.S., Geim, A.K., 2018. Anomalously low dielectric constant of confined water. Science 360, 1339–1342. https://doi.org/10.1126/science.aat4191

Ma, K., Gong, Y., Aubert, T., Turker, M.Z., Kao, T., Doerschuk, P.C., Wiesner, U., 2018. Self-assembly of highly symmetrical, ultrasmall inorganic cages directed by surfactant micelles. Nature 558, 577–580. https://doi.org/10.1038/s41586-018-0221-0

Mahowald, N.M., Hamilton, D.S., Mackey, K.R.M., Moore, J.K., Baker, A.R., Scanza, R.A., Zhang, Y., 2018. Aerosol trace metal leaching and impacts on marine microorganisms. Nat Commun 9, 2614. https://doi.org/10.1038/s41467-018-04970-7

Schaefer, J., Backus, E.H.G., Bonn, M., 2018. Evidence for auto-catalytic mineral dissolution from surface-specific vibrational spectroscopy. Nat Commun 9, 3316. https://doi.org/10.1038/s41467-018-05762-9


 

MagPySV With Unlimited Applications in the Time Series Analysis of Environmental Data in the Earth Sciences.

The MagPySV (Cox et al., 2018) processes the geomagnetic field data served by the Edinburg World Data Centre and presents the hourly Secular Variation (SV). This package applies the correction to the baseline and optionally removes outliers in the geomagnetic field caused by solar disturbances. Further to that this package is capable of noise reduction using Principal Component Analysis (PCA) of the covariance matrix of residuals between observed and predicted geomagnetic field.

We understand that the earthquakes influence ionospheric and geomagnetic fields disturbances. Moreover, changes in the ionosphere (Imtiaz and Marchand, 2012) and geomagnetic fields (More details at https://geomag.usgs.gov/research/geomagnetism-earthquakes.php) can predict the earthquakes. Since MagPySV presents a robust and statistically validated geomagnetic secular variation data, it shall improve our ability to predict the earthquake.

MagPySV is an Open Source package. Modified source code shall find application in denoising and baseline correction of any time series data in the Earth Sciences. Some of the applications are in hydrology (river flow modelling and derivation of base flow), suspended sediment transport, river water quality, instrumental analysis (where baseline correction and outlier detection are very much necessary), and wave dynamics (on the seashore).

References:

Cox, G.A., Brown, W.J., Billingham, L., Holme, R., 2018. MagPySV: A Python Package for Processing and Denoising Geomagnetic Observatory Data. Geochem. Geophys. Geosyst. 19, 3347–3363. https://doi.org/10.1029/2018GC007714

Imtiaz, N., Marchand, R., 2012. Modeling of ionospheric magnetic field perturbations induced by earthquakes: MAGNETIC FIELD PERTURBATIONS. J. Geophys. Res. 117, n/a-n/a. https://doi.org/10.1029/2011JA017475

Modelling the depth profile of εNd in the ocean for the reconstruction of weathering patterns and siliciclastic input from the river basins.

Xu et al. (2018) explored the Nd isotope variability at two different depths viz., 500 m and 2800 m, in the western Philippine Sea. While the Nd isotopic signature did not exhibit any seasonal variability at 500 m depth, at 2800 m depth, it showed such a variation. Such an observation pointed out:

"rapid modification of εNd values during the settling of planktonic foraminifera by the precipitation of Mn coatings derived from water masses at deposition depths"

The εNd values are essential in deciphering the siliciclastic input, and a suitably designed work can lead to a better understanding of the variability of weathering in the river basins.

I suggest that the depth profile of εNd can be modelled akin to that of Total CO₂ by Goyet and Davis (1997). The results obtained from such a study might be very much useful to explore the water column properties of the paleo ocean and to reconstruct the siliciclastic input using the Nd isotopes extracted from well-preserved foraminifers.

For example, such research has potential applicability in the exploration of temporal variability in the weathering pattern and the exploration of siliciclastic input to the South China Sea from the island of Borneo. The Borneo has faced rapid upliftment, and intensive weathering is underway. Such a place is ideally suited for quantifying the variability of siliciclastic input to the adjacent South China Sea. Some other areas where this type of work can be carried out include the Southern Alps in New Zealand and the Himalaya mountains.

References:

Xu, Z., Li, T., Colin, C., Clift, P.D., Sun, R., Yu, Z., Wan, S., Lim, D., n.d. Seasonal variations in the siliciclastic fluxes to the western Philippine Sea and their impacts on seawater εNd values inferred from one year of in situ observations above Benham Rise. Journal of Geophysical Research: Oceans. https://doi.org/10.1029/2018JC014274

Goyet, C., Davis, D., 1997. Estimation of total C02 concentration throughout the water column. Deep Sea Research Part I: Oceanographic Research Papers 44, 859–877. https://doi.org/10.1016/S0967-0637(96)00111-2
 

Photochemical oxidation of marine DOC can modulate δ13C signature of the carbonates.

A recent work by  Mitnick et al. (2018) revealed that authigenic carbonate precipitation influences the δ¹³C record of the ocean. In the backdrop of this work, the photochemical oxidation of marine dissolved organic carbon in the surface seawater, and how it could affect the δ¹³C signature of authigenic and biogenic carbonates are discussed. The δ¹³C record of the ocean is widely being used for the reconstruction of paleoclimate. Thus, consideration of the effect of photochemical oxidation on the δ¹³C signature of the carbonates is of paramount importance.

In brief, DOC is the primary carbon reservoir of the Earth. The photochemical reactions degrade the labile as well as recalcitrant DOC (which is around 95%) in the ocean. The mineralisation of DOC generates dissolved inorganic carbon (DIC) with a distinct carbon isotopic fractionation (δ¹³C). The DIC, when used by the biotic and abiotic processes, carry over this δ¹³C signature. Since the intensity of solar radiation that depends on the orbital parameters (Milankovitch cycle) affects the extent of photochemical oxidation of DOC, the interpretation of δ¹³C signature from paleoclimatic records need a further assessment.

Discussion:

Dissolved organic carbon (DOC) in the ocean is the primary carbon reservoir compared to the atmospheric CO₂ (Kim et al., 2015). Many biogeochemical processes are utilising the DOC in the ocean. Among all such processes, the photochemical oxidation accounts for the mineralisation of ~2.5% of DOC in the California Current Upwelling System (CCUS; Day and Faloona, 2009) and an exact estimate for the world oceans is lacking.

In the sunlit surface layers of the seawater, photochemical reactions involving dissolved organic carbon and trace elements results in the generation of reactive oxygen species namely superoxide radical and hydroxyl radical (Zika, 1987; Sikorski and Zika, 1993; Zika et al., 1993). These photochemical reactions are quite active in the upper ~200 m of the ocean, where sunlight is available. There are diurnal and seasonal changes effected by the solar input at a given time (Zika et al., 1993). During such a photochemical reaction, the mineralisation of dissolved organic carbon results in the generation of CO and DIC (Miller and Zepp, 1995) with a significant carbon isotopic fractionation. The DIC released during the photochemical oxidation of marine dissolved organic carbon is a part of the dissolved inorganic carbon pool of the ocean. Precipitation of authigenic calcium carbonate and ingestion of DIC by the foraminifers, corals and other biological organisms might consume a significant portion of DIC. Thus, the biogenic and authigenic carbonates carry this δ¹³C signature.

Now consider the Milankovitch cycle that modulates the solar radiation received on the surface of the Earth (for details, please read http://www.leif.org/EOS/197-203-Milankovich.pdf). The time-integrated photochemical oxidation of DOC over a Milankovitch cycle can modulate δ¹³C of DIC. Thus, photochemical processes in the upper ocean influence the δ¹³C of authigenic carbonates. Thus, the inference of the level of atmospheric oxygen using δ¹³C records is incomplete without due consideration for the photochemical reactions. The δ¹³C signature of DIC is affected by other processes in the ocean.

Moreover, it is difficult to quantify the extent of carbon isotopic fractionation arising solely from photochemical reactions in the authigenic and biogenic carbonates. However, the study of photochemical isotopic fractionation of biologically active trace elements iron, manganese and copper, can be helpful. The phytoplankton takes up these biologically active trace elements and could record their isotopic signature in their tissues. From those isotopic signatures, it shall be possible to reconstruct the extent of photochemical reactivity of the oceanic water column and rebuild the paleoclimatic record of CO₂, paleoproductivity, and carbonate production in the ocean.

Trace elements speciation under supercritical conditions: A diagnostic tool for understanding deep Earth carbon cycle

Trace elements are diagnostic tools of redox conditions of their environment. Their mobility and isotopic fractionation are of great interest in the study of the evolution of the crust and mantle. A recent revelation that "arc volcanoes" have different trace elements fingerprint compared to the "hotspot volcanoes" (Edmonds et al., 2018) opens up new frontiers of research needed to understand the deep Earth carbon cycle.

The arc magmatism is the result of subduction, whereas hotspot magmatism is due to the generation of magma in the mantle. The subduction fluids are mostly hydrous. Since the dielectric constant of water is different under these extreme conditions, the ionic interactions are more complicated. Such ionic interactions change the "activity coefficient" of the ions. As the subduction slab moves further and further down, the water content becomes less and less, and the dielectric constant also undergoes significant changes. The application of  Debye–Hückel theory to such fluids poses operation problems as "surface charges" and the "electric field strength" of the ions may add additional components in the description of ionic interactions. Further, the dissolution of carbonates and prevailing P-T conditions can make supercritical subduction fluids rich in CO₂. Laboratory experiments designed to unravel trace elements speciation and their isotopic fractionation under these extreme conditions can provide vital information necessary to understand the deep Earth carbon cycle - because redox conditions in the mantle can affect it.

A carbonate source for carbonatites: Whether mantle is progressively enriched in δ13C over geological time scale?

Carbonatites are of economic importance as they contain high concentrations of Rare Earth Elements (REE), niobium, tantalum, zirconium and platinum group elements (PGE). Moreover, carbonatites contain >50% carbonates and its exposure to the Earth's surface through volcanic activity and further weathering has the potential to impact climate change. Hence, understanding their origin is essential.

Carmody (2012) has found that the isotopic analysis of fumarole gases at the world's only active carbonatite volcano, Oldoinyo Lengai, Tanzania, yielded δ¹³C values of -7.37 to -4.46‰ and concluded that the carbonatites are of mantle origin. Shavers et al. (2016) also expressed the view that the carbon isotopic composition of carbonatites indicates their mantle origin. However, Hulett et al. (2016) found that the temporal evolution of boron isotopic composition of carbonatites, found worldwide with an emplacement time between ~40 and ~2,600 Ma, indicated evidence for ingestion of subducted crustal components by carbonatite melts. Thus, the sedimentary carbonates may become a source of the carbon (or, carbonate) for the carbonatites - though exact mechanisms are not discernible.

The primary barrier in invoking sedimentary carbonates as a source of carbon for carbonatites through a subduction process arises from the low δ¹³C values of primary mantle-derived carbonatites (between -4‰ and -8‰; Xu et al., 2018) compared to crustal sedimentary carbonates (~0 ± 1.5‰; Veizer et al., 1992). For a discussion on δ¹³C values of various carbon reservoirs, please refer, Cartigny et al., 1998). A possible mechanism should be proposed to explain the observed discrepancy in the δ¹³C signatures of carbonatites and the sedimentary carbonates. Such a proposition mandates that 1) the subduction results in the dissolution of sedimentary carbonates; 2) the dissolved carbonates become the source of carbon (or, carbonate) for the carbonatites by specific reactions, 3) during such a process, carbon isotopic fractionation occurs that explain the observed low δ¹³C of the carbonatites compared to sedimentary carbonates.

Now consider a subduction zone, defined as the convergent boundary of tectonic plates where one plate moves under another so that the sinking plate finally reaches the mantle. Usually, the denser oceanic plate moves under, the lighter continental plate. During such subduction, the sediments deposited on the oceanic plate, especially authigenic and biogenic carbonates, are transferred to the mantle. The subducted carbonates, along with the sediments, are either decarbonated or dissolved. The decarbonation is a process by which the carbonates react with the silicates to release CO₂ to the atmosphere through volcanic eruptions. Instead, if carbonates in the subduction slab dissolve, they are transferred to the mantle. The evolution of CO₂, after carbonate dissolution and transfer to the mantle, can only occur if the subduction fluid is subjected to appropriate P-T conditions at a later stage.

In scientific literature two schools of thoughts prevail - one favouring decarbonation of the subducted carbonates and the other dissolution. The dissolution of Mg and Ca carbonates in the subduction zone has been widely studied using carbonatite and related samples from the field. Hoernle et al. (2001) found that calcio-carbonatites had mantle-like stable isotopic compositions. Moreover, they have suggested that these calcio-carbonatites result from the melting of recycled carbonated oceanic crust with a recycling age of 1.6 Ga. Such a long recycling age can be inferred from the subduction-related long-term evolution of boron isotopic composition of the carbonatites over Earth's history (Hulett et al., 2016). 

There is substantial evidence suggesting that the subducted carbonates are getting dissolved at shallower depths, and that serve as a source of carbon to the mantle (Frezzotti et al., 2011). Such a dissolution of carbonates under the conditions prevailing in the upper mantle is also inferred for MgCO₃ from ab initio molecular dynamics by Pan et al. (2013). The dissolved carbonates in the subduction zone either return to the atmosphere as volcanic emanations of CO₂ or, accumulated and transferred to the deep mantle. A recent study proposed that almost all such subducted carbon return to the Earth's crust and atmosphere (Kelemen and Manning, 2015).

Apart from carbonate dissolution, evidence for subducting carbonates as a source of volcanic gaseous CO₂ has emerged. From the study of Eocene Cycladic subduction complex on the Syros and Tinos islands, Greece, Ague and Nicolescu (2016) argued that fluid-mediated carbonate mineral dissolution results in silicate precipitation and the release of CO₂. If carbonate dissolution occurs along the subducting slab, then metasomatic reactions are expected to occur along the subducting slab resulting in elemental mobilisation. The evidence was found by Li et al. (2018) by an investigation of the mélange rocks of the ultramafic blocks in the Franciscan Complex of California for Mg-isotopic signature that recorded fluid metasomatism along the slab-mantle interface in the subduction zone. They have concluded that multi-stage fluid-rock interaction is driving Mg-isotopic variation during fluid metasomatism, and the Mg in the subducted carbonate becomes mobile within the subduction zone. The overall picture seems to emerge from the work of Boudoire et al. (2018) using CO₂-He-Ar systematics that revealed extensive degassing in the upper mantle with multiple steps of magmatic differentiation beneath the Piton de la Fournaise oceanic basaltic volcano of the La Réunion Island. During such a magmatic differentiation, there is an isotopic fractionation that results in lower (<-6 per mil) δ¹³C values of volcanic CO₂ compared to primary phase. Whether such δ¹³C depleted CO₂ are the emanations from crystallising carbonatite magma or, derived by the decarbonation reactions is an unresolved question. Hopefully, high temperatures and oxidising conditions favour decarbonation. The dissolution of carbonates may proceed under oxygen-deficient conditions.

If sedimentary carbonates are the source of the carbon for the carbonatites, then the next fundamental question is when it all started? The study of the eclogite xenolith in Paleoproterozoic carbonatite in North China (Xu et al., 2018) argues for cold subduction as early as 1.8 Ga. The Paleoproterozoic carbonatite of North China has a carbonate carbon signature and Sr-Nd composition indicative of the ocean crustal source. From this study, we understand that the deep carbon cycle has been in operation over an extended geological period influencing the mantle oxidation state and its compositional heterogeneity. Combining the results of Xu et al. (2018) with the evolution of boron isotopic composition (Hulett et al., 2016), we can conclude that the subduction process and its influence on the composition of the mantle can be as old as 2.6 Ga.

From the preceding discussion, it is clear that the sedimentary carbon may constitute a part of carbonatite carbon. The following processes may be thought to prevail:

1) Subduction of the slab and dissolution of sedimentary carbonates possibly aided by lowering of the dielectric constant of the water (Cline et al., 2018).

2) Transfer of the dissolved fluid along with the subducting slab deep into the mantle (Frezzotti et al., 2011).

3) The reaction of carbonate fluid with iron and silicate that results in the replacement of oxygen by carbon in the silicate structure (Sen et al., 2013) and the formation of Fe-Si-carbides (Horitaa and Veniamin, 2015).

4) Upwelling of these materials towards the upper mantle by convection, the oxidation of carbon-rich phases to carbonatite (Horitaa and Veniamin, 2015) due to the high oxygen fugacity of the upper mantle (Cline et al., 2018) that generates CO₂ degassing through the volcanoes (Boudoire et al., 2018) and results in ¹³C isotopic fractionation (Horitaa and Veniamin, 2015).

5) The carbon isotopic fraction during the formation of carbonatite melt results in the low δ¹³C values typical of the mantle within the carbonatite melt. As the carbonatite magma ascents, it loses fluids and volatiles to crystallise carbonatites with low δ¹³C values.

If these assumptions are correct, then the δ¹³C of the mantle should move towards more positive values (i.e., ¹³C/¹²C of the mantle should increase) over geological periods. Because the mantle is magmatically differentiated to form the carbonatite melt enriched in ¹²C due to isotopic fractionation, the mantle becomes comparatively rich in ¹³C. The Paleoproterozoic carbonatites in North China Craton (Xu et al., 2018) have higher positive δ¹³C values (-5.7‰ to -1.6‰) compared to the typical mantle-derived primary carbonatites (-8‰ to -4‰). If the Paleoproterozoic mantle was rich in ¹²C than it is today, then the carbonatites of that time may have higher δ¹³C values. In that case, assimilation of subducted sediments may have contributed little to the observed deviation in the carbon isotopic composition of these carbonatites. After emplacement, the carbonatite weathering and its return to the mantle as sedimentary carbonates (i.e., carbonatite recycling) shall further enhance the isotopic fractionation over geological periods. There might also be a Ca isotope fractionation, and the samples collected by Hulett et al. (2016) can provide essential clues for testing the δ¹³C and Ca isotopes evolution of carbonatites over geological periods.

From the temporal distribution of carbonatites, there is an exponential increase in the frequency distribution of carbonatites emplacement at the least since 1.8 Ga, which was alternatively interpreted as due to the probability of carbonatite preservation against weathering (Veizer et al., 1992). If the frequency of carbonatite emplacement is on the rise, then we shall observe a systematic fall in the δ¹³C of carbonatites with geological time. Thus, age-old carbonatites are expected to be comparatively rich in δ¹³C than the recent carbonatites. Such assumptions are applicable only if the ¹³C/¹²C variations are affected by the carbonatite formation. However, in nature, other biogeochemical processes may have an impact.