Ikaite nucleation at 35 °C challenges the use of glendonite as a paleotemperature indicator

Glendonites have been found worldwide in marine sediments from the Neoproterozoic Era to the Quaternary Period. The precursor of glendonite, ikaite (CaCO3 · 6H2O), is metastable and has only been observed in nature at temperatures <7 °C. Therefore, glendonites in the sedimentary record are commonly used as paleotemperature indicators. However, several laboratory experiments have shown that the mineral can nucleate at temperatures>7 °C. Here we investigate the nucleation range for ikaite as a function of temperature and pH. We found that ikaite precipitated at temperatures of at least 35 °C at pH 9.3 −10.3 from a mixture of natural seawater and sodium carbonate rich solution. At pH 9.3, we observed pseudomorphic replacement of ikaite by porous calcite during the duration of the experiment (c. 5 hours). These results imply that ikaite can form at relatively high temperatures but will then be rapidly replaced by a calcite pseudomorph. This finding challenges the use of glendonites as paleotemperature indicators.

The narrow temperature range of ikaite formation observed in nature has motivated using the mineral and its pseudomorphs as an indicator of paleotemperature conditions 29 . Glendonites have also been suggested as an indicator of extreme low temperature metamorphism in Neoproterozoic sediments 30 . However, in addition to cold temperatures, ikaite precipitation requires high alkalinity 31 , high pH 32 and the inhibition of calcite growth which has been argued to occur due to the presence of phosphate 31 or magnesium 33 . Moreover, in laboratory experiments ikaite has been reported to precipitate at a wider range of temperature than seen in natural environments. Clarkson et al. (1992) precipitated ikaite from supersaturated solutions at 15 °C with triphosphate as an inhibitor of calcite nucleation 34 . Stockmann et al. (2018) also precipitated ikaite at 15 °C but without phosphate 35 . In another set of experiments, magnesium from natural seawater inhibited nucleation of calcite in favour of ikaite 33 . The results from these experiments may suggest that ikaite nucleation can also occur at temperature >7 °C in the natural environment. A recent study by Popov et al. (2019) of fossil records in glendonite bearing strata suggested that ikaite nucleation probably occurred at water temperature >40 °C 36 . These findings call into question the use of ikaite as a paleotemperature indicator. In this study, we address this issue with a series of experiments on ikaite nucleation as a function of temperature and pH.

Methods
Experimental setup and material. In total 39 experiments were performed at different temperatures and pH to investigate the precipitation of ikaite. The experimental setup was similar to the methods described by Stockmann et al. (2018) and Tollefsen et al. (2018) with the following modifications 33,35 : The samples were precipitated from a 50-50 mixture of two solutions: Solution 1) was surface seawater collected nearby the island of Andøya, Norway (Table 1). Solution 2) was prepared by mixing 0.1 M Na 2 CO 3 and 0.1 M NaHCO 3 at different proportion, 1:5, 1:3, 1:2, 1:1 and 3:1, which represent solutions with pH 9. 49, 9.70, 9.86, 10.14 and 10.59 at 5 °C respectively.
The seawater was filtered through Munktell Qualitative Filter Paper grade 3 before use and stored in a cooling room at 5 °C. This removes any particulate matter >10 µm in size. We consider it unlikely that finer particulate matter, if present, affected the outcome of our experiments. This is because Tollefsen et al. (2018) used the same approach and obtained comparable results for synthetic seawater and natural seawater from the same location. Solution 2 was prepared using Na 2 CO 3 and NaHCO 3 powders from Merck dissolved in 1 litre of ultrapure deionized water (MilliQ resistivity >18.2 MΩ cm).
The two solutions and their mixtures were kept in water baths during the experiments to maintain the desired temperature. We ran experiments at temperatures from 5 to 35 °C. The mixing rate was 0.67 ± 0.21 ml/min and when ~300 ml of mixture was obtained, the experiment was stopped. The resulting mixture was put in a cooling room (5 °C) overnight. The next day, the mixture was filtered through Munktell Qualitative Filter Paper 00 K and kept overnight to dry in the cooling room. Thereafter, the precipitate was collected, weighed and stored in a freezer (−18 °C). XRD analysis. Precipitated minerals were identified by X-ray powder diffraction (XRD) using an X Pert Pro instrument from PANalytical and the program Data Colllector at the Swedish Museum of Natural History. The sample holder was kept in a freezer (−18 °C) for 10 min before analysis to preserve the ikaite crystals. The measurement program used was Absolute Scan 5-70°2θ with a runtime of 11 min. The program Topas version 6 (Bruker AXS product) was used to identify and quantify the different phases in the samples by Rietveld refinement. In 7 samples, amorphous calcium carbonate (ACC) was present, which was quantified by Rietveld refinement of the XRD data after addition of a quartz standard to the sample in weighed proportions. Distinguishing calcite from Mg-calcite was based on unit cell data obtained from the Rietveld refinement ( Supplementary Fig. S1). ICP and IC20 analyses. We dissolved and diluted 27 of 39 samples for cation analysis with an Inductively coupled plasma -atomic emission spectroscopy (ICP-OES, Thermo ICAP 6500) with autosampler ASX520 from CETAC and using a spray nebulizer. The seawater sample from Andøya was also analysed with the same instrument. The concentrations of anions in the seawater were measured with an IC20 Ion Chromatograph from Dionex.

Results
From the experiments, we obtained 39 samples precipitated from by mixing natural seawater (  Table 2). Ikaite did not precipitate at any temperature in series 1. Instead Mg-calcite precipitated together with aragonite or together with aragonite and monohydrocalcite (MHC) ( Table 2). Ikaite precipitated in most of the series 2 experiments, either as the main phase (at 15 °C, 30 °C and 35 °C) or subsidiary to MHC and/or calcite (Table 2). In a few series 2 experiments (at 5 °C and 25 °C), ikaite precipitates were not observed ( Table 2).
Ikaite precipitated in all experiments in series 3, 4 and 5. In series 3 pure ikaite precipitated at all temperatures up to 20 °C in all but one experiment in which calcite also precipitated, and together with calcite at temperatures of 25 °C and higher (Table 2). In series 4 pure ikaite precipitated at all temperatures up to 30 °C in all but two experiments in which calcite also precipitated. In the experiment run at 35 °C (exp. 25) the precipitate was 80% amorphous calcium carbonate (ACC) and 20% ikaite ( Table 2). In series 5 pure ikaite precipitated at 5 °C, and thereafter together with ACC at 10 °C and together with ACC and nesquehonite from 15 to 35 °C. The proportion of ACC increased with temperature ( Table 2).
In three of the samples from series 2, we did not observe pseudomorphs after ikaite. In the first sample (exp. 9 run at 25 °C), we observed large spherical MHC and spherical Mg-calcite. In the second sample (exp. 10 run at 30 °C), the main phase was ikaite which occurred as euhedral crystals. In the third sample (exp. 7b run at 15 °C), ikaite and Mg-calcite were the main phases observed. Some of the ikaite crystals in this sample had cavities ( Fig. 2A). This texture was also observed in exp. 11 run at 35 °C together with euhedral ikaite crystals and pseudomorphs after ikaite (Fig. 2B,C). (2020) 10:8141 | https://doi.org/10.1038/s41598-020-64751-5 www.nature.com/scientificreports www.nature.com/scientificreports/ In sample 18 from series 3 (run at 35 °C), we observed ikaite but also partly replaced ikaite crystals (Fig. 4). In one crystal (Fig. 4A), replacement occurs in the centre of the ikaite crystal. In a second crystal from the same sample the replacement occurs at the edge of the crystal (Fig. 4B,C). Fig. S3) showed that calcite, aragonite, ikaite, MHC and ACC were supersaturated at all temperatures. Calcite and aragonite had a SI between 2 and 3, and MHC had an SI of ~1.5. Ikaite was nearly at equilibrium at low pH and high temperature (SI = 0.04), but its SI was higher (up to 1.4) at higher pH and lower temperature. Amorphous calcium carbonate was nearly at equilibrium at low pH and low temperature (SI = 0.04), but its SI was higher (0.57) at higher pH and  www.nature.com/scientificreports www.nature.com/scientificreports/ temperature. Nesquehonite was undersaturated except for exp. 29 to 32, which is in agreement with the occurrence of this mineral in the samples from these experiments ( Table 2).

Summary of results.
The experimental results showed that at pH 9.50 and 9.82 (series 3 and 4), ikaite precipitated at temperatures between 5 °C and 35 °C, and at pH 10.31 (series 5), ikaite precipitated together with ACC at temperatures between 10 °C and 35 °C. The amount of ACC increased with temperature and at 35 °C, the sample contained 94% ACC (Table 2). At pH 9.09 (series 1), ikaite did not precipitate. Instead, Mg-calcite precipitated together with aragonite at all temperatures.
Because ikaite precipitated at 35 °C in series 2-5, we infer that the upper temperature limit for ikaite nucleation exceeds 35 °C. However, because ikaite content diminished with increasing temperature in series 4 and 5, we infer that this limit is close to 35 °C for these series. On the other hand, because ikaite was the main phase at 35 °C in series 2 and 3 and because Cryo-SEM images of these samples indicate that minor calcite (23 and 13%, respectively) occurred as pseudomorphs after ikaite (Figs. 2B,C and 4), we infer that the upper temperature limit for ikaite nucleation is higher than 35 °C for these series.

Discussion
We constructed a stability diagram for nucleation of some anhydrous and hydrous carbonate phases as a function of pH and temperature (Fig. 5). We defined three distinct zones based on our experiments: (1) Mg-calcite/ aragonite zone, (2) ikaite zone, (3) ikaite/ACC zone. Between the Mg-calcite/aragonite and ikaite fields, we define a transition zone in which several phases precipitated, and most notably, pseudomorphs after ikaite appeared within the duration of the experiments (~5 hours). In the following discussion, we will use this diagram and our experimental findings to consider the nucleation of ikaite and its pseudomorphic replacement by calcite.
ikaite nucleation. Ikaite has been found to occur naturally in modern settings at temperatures below 7 °C 10,12 . However, in our experiments, ikaite nucleated at much higher temperatures (up to 35 °C) in series 2-5. The upper temperature limit for ikaite nucleation was approached in series 4 (pH 9.82). In the experiment run at 35 °C, the main precipitate was ACC, which occurred as spheroids closely surrounding euhedral ikaite crystals (Fig. 6A).
Based on previous experimental work which showed that ikaite nucleation is favoured by the presence of calcite growth inhibitors such as phosphate and Mg 31,33,35 , we infer that Mg in seawater was a contributory factor which allowed for ikaite nucleation at higher temperatures in our experiments. Also, several studies have shown that the uncharged ion pair species CaCO 3 0 plays an important role in the multistage pathways whereby anhydrous and hydrous carbonates form 10,35,41,42 . Specifically, Gal et al. (1996) showed that CaCO 3 0 is easily hydrated and initially forms ACC and, at low temperatures, ikaite 41 . The importance of the CaCO 3 0 ion pair for ikaite nucleation is confirmed by our experiments: The main precipitates in series 1 (pH 9.09), which had the lowest CaCO 3 0 concentration in the parent solution (<1.37 mmol/kg: Supplementary Fig. S4), were Mg-calcite and aragonite together with only small amounts of MHC. In contrast, the main precipitate in series 2 (pH 9.32) at and above T = 30 °C at which the CaCO 3 0 concentrations approaches the Ca 2+ concentrations (1.76 and 2.22 mmol/kg respectively) in the parent solution ( Supplementary Fig. S3), was ikaite ( Table 2). This is despite the fact that its SI is lower than at 5 °C ( Supplementary Fig. S3). The activity of the CaCO 3 0 ion pair is higher than the activity of the Ca 2+ ion in all experiments (Supplementary Fig. S6). We suggest that this could highlight the importance of the CaCO 3 0 ion pair for ikaite nucleation as Buchardt et al. (2001) and Stockmann et al. (2018) have also previously suggested 10,35 . pseudomorphic replacement of ikaite. In some of our experiments, ikaite which had precipitated at temperatures up to 35 °C, was thereafter pseudomorphically replaced by calcite within the experimental runtime of 5 hours.
Several experimental studies have investigated the breakdown of the ikaite crystal [43][44][45] . These studies demonstrate that the ikaite unit cell expands anisotropically with increasing temperature. The CaCO 3 0 ion pair in ikaite is surrounded by water molecules held together with hydrogen bonding. When ikaite is exposed to higher temperatures, the unit cell expands along the a-axis followed by the b and c-axis. This volume increase occurs just before the breakdown of the hydrogen bonds and the release of the water molecules from the ikaite structure 43,45 . In our experiments, this was observed in series 4 (pH 9.82). Here, unit cell data show a sudden increase along the a-axis and in volume at 35 °C (exp. 25; Supplementary Fig. S5). This was the only series where we found similarities in the crystal lattice data with previous studies of ikaite breakdown. One reason could be that in series 2 and 3 the temperature limit of ikaite nucleation at 35 °C with pH 9.32-9.50 was not reached despite the observation of pseudomorphs after ikaite.
In the transition zone, we observed pseudomorphs after ikaite in most samples (Table 2; Fig. 5). We speculate that the pseudomorphic replacement of ikaite is controlled not only by temperature but also by pH. A possible scenario could be that ikaite precipitation lowers the pH in the parent solution beyond the limit for metastable nucleation of ikaite and thereafter the mineral starts to transform and is replaced by calcite pseudomorphically. Calcite has been reported as the primary replacement mineral in naturally occurring pseudomorphs (glendonite) after ikaite 11,24,36,46 . Our experiments confirm this common observation. They also point to replacement of ikaite by calcite occurring at higher temperatures than previously thought.
Based on the perfect shapes of pseudomorphs, porous nature of the replacement calcite and a sharp reaction front, we propose that ikaite replacement occurs by a coupled dissolution -reprecipitation mechanism at the    www.nature.com/scientificreports www.nature.com/scientificreports/ ikaite-calcite interface. Putnis and Putnis (2007) describe this mechanism as a coupled dissolution and nucleation process within the fluid boundary layer at the parent solid surface 47 . The generation of porosity by the reaction allows fluids and mass transport to and from the reaction interface. In this process, the original morphological structure is generally well preserved 47 . In Fig. 6B a reaction front can be seen in the crystal, with a solid surface, the interface and the generated porosity. This mechanism could explain why calcite grows in an otherwise Mg-rich solution, as follows. It has been shown that the fluid at the sharp interface can be distinct and isolated from the bulk solution 48 . The fluid at the reaction interface in our experiments is probably composed of H 2 O released by the dissolution of ikaite, and therefore calcite growth is not inhibited at the interface. We also postulate that this process took place in the solution before filtering, because otherwise we should have observed pseudomorphic replacement of ikaite in all ikaite samples. This suggests that the replacement of ikaite can occur at an early stage and at high rate if the conditions change slightly (e.g. pH and/or temperature).
We observed two distinct textures of the ikaite crystals in the transition zone. The first type of texture (Figs. 3, 4 and 6B) is the perfect pseudomorphic replacement of ikaite by porous calcite. The second texture was seen in two samples (7b and 11) from the transition zone (Figs. 2 and 7). The ikaite crystals have rounded cavities, which contain undefined material and small crystals with calcite shape (Fig. 7B). Sample 7b contains both intact ikaite crystals and ikaite with cavities ( Figs. 2A and 7B). Sample 11 contains ikaite crystals and ikaite with cavities but also pseudomorphs after ikaite (Figs. 2B,C, 7A). Vickers et al. (2018) proposed 2 models for the ikaite to glendonite transformation, based on observations and data from their study and previous studies on glendonite 49 (Fig. 8). The model in Fig. 8A was suggested for rapid change in conditions and the model in Fig. 8B for no change or small change in conditions 49 . The model in Fig. 8A corresponds to exps. 7b and 11 (Figs. 2 and 7), however there were no abrupt change in the conditions during our experiments that could explain this change. The model in Fig. 8B corresponds to exps. 5, 6b, 7a, 8a, 8b, 11 and 18 (Figs. 3, 4 and 6B) and indeed our results indicate that a slight change in conditions (e.g. pH and/or temperature) induced this transformation. Sanchez-Pastor et al.
(2016) made similar observation when they exposed ikaite crystals to air at 10 °C and at 20 °C. Ikaite crystals exposed at 10 °C were pseudomorphically replaced, whereas crystals exposed at 20 °C recrystallized to calcite and vaterite 50 . It could be that an abrupt change in temperature or other conditions would provoke partial dissolution of ikaite and crystallization of calcite or other phases, whereas a slight change in conditions favours pseudomorphic transformation.  www.nature.com/scientificreports www.nature.com/scientificreports/ controls on Mg-rich Amorphous calcium carbonate (Acc) formation. In our study we focused on ikaite nucleation, however we made some interesting observations on Mg-rich ACC formation that are worth discussing. With only one exception, we did not detect ACC below pH 10.31 in the parent solution (Table 2), even though ACC had a slightly higher SI than ikaite in all series at 35 °C (Supplementary Fig. S3). However, it is possible that the SI for Mg rich ACC differs from pure ACC.
In series 5 (pH 10.31) ikaite precipitated together with Mg rich ACC at temperatures ≥10 °C and the amount of ACC increased with temperature. Blue and Dove (2015) demonstrated from experimental studies on ACC that the incorporation of Mg into ACC was controlled by Mg/Ca ratio and pH 51 . For experiments at pH 9.5 to 10.3 the Mg content in ACC in their study was 31-65% which is in agreement with our result of 35 ± 10%. It has also been found that the incorporation of Mg into ACC stabilises this otherwise unstable phase 52,53 . This would explain why we did not observe any transformation of ACC in our samples. In the experiments at pH 10.31 > 5 °C ikaite and ACC co-precipitate and both phases have similar predicted SI (Supplementary Fig. S3). However, ACC is favoured by the increase in temperature, whereas the amount of ikaite diminished (Table 2).
conclusions Pseudomorphs (glendonites) after ikaite are widely used as paleotemperature indicators because ikaite has only been observed in nature at temperatures below 7 °C. The results from our study show that ikaite can nucleate at temperatures up to 35 °C for pH ranging from 9.3 to 10.3. Based on geochemical modelling, we infer that the concentration of the CaCO 3 0 ion pair in the parent solution is an important control of ikaite precipitation.