Database of global glendonite and ikaite records throughout the Phanerozoic

. This database of Phanerozoic occurrences and isotopic characteristics of metastable cold-water calcium carbonate hexahydrate (ikaite; CaCO 3 ·6H 2 O) and their associated carbonate pseudomorphs (glendonites) has been compiled from academic publications, explanatory notes and reports. Our database including more than 700 occurrences reveals that glendonites characterize cold-water environments, although their distribution is highly irregular in space and time. A significant body of evidence suggests that glendonite occurrences are restricted mainly to cold-water settings, however they 15 do not occur during every glaciation or cooling event of the Phanerozoic. While Quaternary glendonites and ikaites have been described from all major ocean basins, older occurrences have a patchy distribution, which may suggest poor preservation potential of both carbonate concretions and older sediments. The data file described in this paper is available on Zenodo at https://doi.org/10.5281/zenodo.4386335 (Rogov et al., 2020).

Occurrences of ikaite pseudomorphs (glendonites) in the geological record are characterized by a slightly less diverse range of palaeo-environments. Ancient glendonites have been mainly described from marine clastic rocks, although they have been 35 discovered in caves and lacustrine deposits of Neogene age.
The first attempts at a comprehensive review of glendonite occurrences through geological time were undertaken by M.E. Kaplan (1978Kaplan ( , 1980, who summarized the available information about glendonite findings throughout the world and publicized their importance for palaeoclimatic studies. During the last 40 years, numerous papers on glendonite and ikaite occurrences have been published, along with new geochemical data. These geochemical data mainly include stable carbon 40 and oxygen isotope values from ikaite and glendonite, along with a limited number of clumped isotope and Sr isotope data (Nenning, 2017;Rogov et al., 2018). Information on glendonite occurrences is scattered throughout hundreds of papers on regional geology, explanatory notes to geological maps, and other reports, but until now, no single reference database of global glendonite distribution throughout the Phanerozoic has been compiled. This paper presents a new single reference database of ikaite and glendonite records from the Early Cambrian until 45 the present. It is based on the analysis of numerous papers (including unpublished reports) and museum collections, as well as on data collected during the last 15 years by the authors. All records were averaged to substages (if possible) without further subdivision, and nearby occurrences were considered as a single locality if the distance between separate data points was less than 1−2 km. As glendonite occurrences were frequently mentioned but not imaged or fully described in many publications, consideration of such records as true glendonites was mainly based on additional lines of evidence supporting 50 glendonite occurrence within the same region or stratigraphic interval.
2 General information about ikaite and glendonite composition 2.1 Mineralogy Single crystal X-ray structure determinations (e.g., Hesse and Kuppers, 1983;Swainson and Hammond, 2001;Lennie et al., 2004) have shown that ikaite crystallizes in the spacegroup C2/c, and its structure consists of CaCO 3 ·6H 2 O 55 units with each Ca bound to six water molecules and a bidentate carbonate group to yield 8-fold coordination. Hydrogen bonding links CaCO 3 ·6H 2 O moieties to form the crystal structure (Lennie et al., 2004).
Outside of the aqueous environment, ikaite rapidly disintegrates into a mush of water and anhydrous calcium carbonate (e.g., Pauly, 1963;Bischoff et al., 1993), which can be in the form of vaterite (e.g., Shaikh, 1990;Ito, 1996;Ito et al., 1999), aragonite (e.g., Stein and Smith, 1986;Council and Bennett, 1993), or calcite (e.g., Pauly, 1963;Bischoff et al., 1993). Ito (1998) suggested that the rate of ikaite transformation is dependent on the availability of water (which increases 75 the rate of transformation) and the presence of magnesium ion (which inhibits the transformation to both calcite and vaterite). Purgstaller et al. (2017) showed that the formation of anhydrous calcium carbonates is controlled mainly by the prevailing physicochemical conditions, such as the Mg/Ca ratio of the aqueous medium and water availability.
Ikaite-glendonite transformation begins with ikaite destabilization, resulting in a cloudy (Suess et al, 1982) or whitish (Kodina, 2003) interior of the ikaite crystals. The replacement of ikaite by anhydrous calcium carbonate 80 (CaCO 3 ·6H 2 O → CaCO 3 + 6H 2 O) leads to a primary volume loss of 70−80% (Suess et al, 1982;Fairchild et al., 2016) and disintegration, as the density of ikaite is much less than of anhydrous calcium carbonates. However, the primary morphology of ikaite can be preserved due to fast seawater cementation (Huggett et al., 2005;Selleck et al., 2007). Seawater diagenesis of ikaite takes place in the sulphate-reduction zone and glendonite can be subsequently altered during burial diagenesis, where it can be replaced by younger carbonate generations or non-carbonate minerals such as silica (Wang et al., 2017), 85 dolomite (Loog, 1980) or gypsum (Mikhailova et al., 2019).

Morphology
Unlike calcite and aragonite, ikaite is typically characterized by pyramidal, spear-like (e.g., Dieckmann et al., 2008;Tang et al., 2009;Rysgaard et al., 2014) or stellate crystals (e.g., Selleck et al., 2007) with square-prismatic cross-sections (e.g., Swainson and Hammond, 2001) ( Fig. 1). In some cases, the shape of the original ikaite crystal is preserved as a 90 pseudomorph (e.g., Kaplan, 1978;Shearman and Smith, 1985;Kemper, 1987). The size of natural modern ikaite clusters and solitary crystals varies from ~ 5 μm to ~10 cm. Glendonites display a number of internal structures, including (1) visible core and rim with no apparent zonation; (2) core and alternating mm-scale rims, forming distinct zonation and (3)  bodies from the centre to the edges. Glendonites are characterized by a size range which mainly lies between 0.5 and 15−20 cm, with some of the largest specimens ranging up to 1 m in diameter or in length. Microglendonites are rarely reported; 95 however, their size is similar to that of ikaite microcrystals, ranging from 5 to 10 μm (Oehlerich et al., 2013). Glendonite colour varies from light yellow and whitish if weathered to dark brown.
Due to water removal during ikaite to anhydrous calcium carbonate transformation, glendonites are frequently porous and contain small pieces of adjacent sediment (such as sand grains, microfossils, clay particles etc) and several successive carbonate generations. 100
Ikaite-derived calcite does not form the frame of the glendonite and is always supported by cements of different morphology. The first ikaite-derived calcite generation can be overgrown by needle-like or spherulitic calcite cement (e.g., Boggs, 1972;Kaplan, 1979;Huggett et al., 2005). Crystals are a yellowish or amber colour under the optical microscope with a bright cathodoluminescence (Frank et al., 2008;Teichert, Luppold, 2013, among others). This carbonates are typically 115 Mg or/and Fe-rich. These carbonates can contain inclusions of pyrite (either as idiomorphic crystals or with framboidal habits; Greinert and Derkachev, 2004). Blocky, sparry or radiaxial fibrous calcite can occupy the residual pore space (Teichert, Luppold, 2013) or replace carbonate rims and ikaite-derived calcite (Vasileva et al., 2019), typically displaying an orange to dark red cathodoluminescence.

Database contents
The database is provided as an Excel 2003 file (.xls). The main sheet of the database includes the following information: locality, age (substage/formation/Ma), modern coordinates, palaeolatitudes (calculated using paleolatitude.org; see van Hinsbergen et al., 2015), references, depositional settings, and host rock. Some datapoints lack information about palaeolatitude (as they were derived from terrains with an uncertain geographic position) or host rock (in the case of some 150 museum specimens). We aim to provide locality and coordinate information as precisely as possible. However, in some cases the available locality information was rather imprecise (for example, 'basin of river 'X'', or 'mountain ridge 'Y''), although the effects of such imprecise location details on our analysis of (palaeo)latitude distribution are considered insignificant. Palaeolatitudes of Paleogene and Neogene glendonites from the northern margin of the Pacific cannot be determined using paleolatitude.org. We chose a similar palaeolatitude to the present-day for the Paleogene and Neogene 155 based on Bazhenov et al. (1992), Harbert et al. (2000) and Kovalenko and Chernov (2003). Palaeomagnetic data from Sakhalin (Weaver et al., 2003;Zharov, 2005) are more complicated and suggest a northward drift during the Cenozoic. The complex tectonic structure of the island precludes an accurate calculation of palaeolatitude for each time slice and locality from Sakhalin.
Additional sheets include stable carbon and oxygen isotope data and references. 160 5 Data overview 5.1 Glendonite distribution in space and time A significant irregularity in Phanerozoic glendonite distribution is apparent in Fig. 4, which illustrates glendonite locations plotted on present-day geography. However, when the palaeolatitudes of glendonite samples are taken into account, it becomes clear that they mainly formed in high latitudes throughout the Phanerozoic (Fig. 5). Interestingly, most glendonite 165 occurrences have been reported from the northern Hemisphere, which is challenging to explain. Only Quaternary records of ikaite/glendonite occurrences are fairly evenly distributed across the cold-water environments of both the northern and southern Hemispheres and within all ocean basins, including deep-water sites at low and near-equatorial latitudes.
Glendonite occurrences within the Arctic Ocean (Kara Sea, Laptev Sea, Chukchi Sea) and the Sea of Okhotsk are more abundant compared to other regions, which can be partially explained by sampling bias, as these regions have been 170 particularly actively studied during the past few decades. In contrast to other time slices, Neogene glendonites are restricted to the northern margin of the Pacific, except for a single reported occurrence in Arctic Canada. Glendonite occurrences are particularly common at numerous localities along the western coast of the Pacific Ocean, including from Japan, Sakhalin and Kamchatka, where their presence is used as a diagnostic feature for numerous formations and members. Furthermore, one of these formations (Gennoishi Formation of Sakhalin Island) is named after the Japanese name for glendonites (gennoishi, 玄 175 能石). Coeval glendonite records also occur along the eastern Pacific coast, where they were first recognized by Dana (1849). Paleogene glendonites typically occur in the same region as Neogene occurrences but are also abundant in the North Atlantic area. North Atlantic occurrences include giant glendonites from the Mors and Fur islands (Denmark), as well as abundant Spitsbergen glendonites. It should be noted that the giant glendonites from Denmark are mainly embedded in post-PETM rocks, but clumped isotope data from the glendonites are indicative of near-freezing temperatures (Nenning, 2017). 180 The Late Cretaceous was characterized by a global greenhouse climate and lacks any glendonite occurrences.
Lower Cretaceous glendonites are known from all stages and occurred in high latitudes of both the northern and southern Hemispheres. However, no occurrences are known from the most prominent warming event (early Aptian) and only a single occurrence has been reported from the Barremian. The paucity of Barremian glendonites may be related to the HALIPinduced regression in the Arctic and scarcity of marine deposits of this age in the high northern latitudes. Glendonites are 185 particularly abundant within the Valanginian, Aptian and Albian sediments of Svalbard (Kemper, Schmitz, 1981;Vickers et al., 2018) and Arctic Canada (Herrle et al., 2015), along with the Aptian of Australia (de Lurio, Frakes, 1999). Berriasian glendonites, which mark the initiation of the Early Cretaceous cold interval, are rare and mainly described from Siberia (Rogov et al., 2017).
Peak glendonite abundance, comparable to that of the Quaternary, occurs during the Middle Jurassic, but nearly all 190 occurrences of this age have been described from northern Eurasia, with no occurrences in the southern Hemisphere. The numerous Middle Jurassic glendonite occurrences coincide with long-term cooling of the Arctic Ocean, primarily instigated by changes in oceanic circulation following closure of the Viking corridor (Korte et al., 2015). The Late Jurassic is characterized by a significant decrease in glendonite abundance towards the end of this time interval. Lower Jurassic glendonite occurrences are restricted to the upper Pliensbachian and a few in the upper Toarcian. 195 No glendonites have been confidently identified from the Triassic Period. Only two suspicious reports of pseudomorphs resembling glendonites from low-latitude lagoonal environments were mentioned by Kaplan (1979), based on reports by van Houten (1965) and Kostecka (1972).
Permian glendonite occurrences display a similar distribution to Lower Cretaceous occurrences. They are also known from both the northern and southern Hemisphere, but are especially abundant in Tasmania and Australia (Selleck et 200 al., 2007), while in the northern Hemisphere they are mainly found in north-east Asia (Biakov et al., 2013), accompanied by rare findings in the rest of Siberia and Novaya Zemlya. Possible glendonites from Turkey and Saudi Arabia are poorly described.
Carboniferous glendonites are uncommon and mainly known from the Upper Carboniferous of Gondwana. In addition, glendonites were also reported from low-latitude carbonate deposits of Alberta, Canada (Brandley, Krause, 1994). 205 There is a notable absence of glendonite occurrences for a prolonged period of time below the Carboniferous, spanning the Devonian, Silurian and Middle−Upper Ordovician. Lower Ordovician glendonites have only recently been discovered and described (Popov et al., 2019;Mikhailova et al., 2019), as until recently, they were known as a kind of 'anthraconite' and were not considered as glendonites. Their findings are restricted to Baltoscandia. Cambrian glendonites are less frequent but are mainly known from the same region and same facies (black shales) as the Lower Ordovician 210 glendonites. A few Cambrian glendonites were also found in sandstones, and a single occurrence is known from outside the Baltic region in South Korea (Chon, 2018).
However, experiments carried out during the last few years have revealed at least short-term ikaite stability at much higher temperatures, up to 30-35 o C (Clarkson et al., 1992;Rodríguez-Ruiz et al., 2014;Purgstaller et al., 2017;Tollefsen et al., 2020). Ikaite stability at high temperatures had only previously been demonstrated under high pressures ( van Valkenberg 225 et al., 1971). It is important to note that high-temperature experiments were only conducted on ikaite microcrystal precipitates, and there is currently no evidence for stability of 'typical' centimetre to decimetre-sized ikaite crystals and ikaite clusters at similarly high temperatures.
Our analysis of glendonite distribution through space and time has revealed a complex relationship between climate change and glendonite occurrence. On the one hand, nearly all glendonite findings known to date (except for a few doubtful 230 ones) are associated with cold-water environments and are usually found in sediments with high-latitude low diversity faunas and dropstones (e.g., Price, 1999). On the other hand, no glendonites have been discovered from two prominent glacial intervals in Earth's history (the Late Ordovician and Late Devonian), and their pre-Quaternary occurrences are very irregular, changing to the southern Hemisphere in early Paleozoic to mostly northern Hemisphere in Mesozoic and Cenozoic.
However, the roles of additional factors influencing ikaite precipitation remain unclear. 235 6 Data availability The .xls file containing this database is available on Zenodo https://doi.org/10.5281/zenodo.3991964 (Rogov et al., 2020). Updates will be available under the same web address as well as through direct hyperlink (http://jurassic.ru/_pdf/glendonites_database.xls) or can be requested directly from the first author of this paper.

Conclusions 240
Our new reference glendonite and ikaite database can be used to show that glendonites typically occur in cold-water environments throughout the Phanerozoic, but their distribution is highly irregular in space and time. Although there is no evidence for glendonite formation in warm-water settings, not all of Earth's major glaciations and other cooling events are associated with glendonite occurrences. Significant irregularity also occurs in spatial glendonite distribution throughout the Phanerozoic. Quaternary glendonites and ikaites have been described from all ocean basins of the world, while pre-245 Quaternary glendonites have not been found in many regions which would appear to be suitable for ikaite precipitation. Author contributions MR designed the study and collected data on stratigraphic and geographic distribution of glendonite, VE did the overall editing and supervision, OV collected data on morphology and mineralogical composition, KV and KM were responsible for data on isotopic composition, petrography and cathodoluminescence, AK provided data on ikaite distribution 250 and diagenesis. All authors discussed the results and participated in preparation of the paper.

Competing interests
The authors declare that they have no conflict of interest.