Search

The objects of the current study are glendonite pseudomorphs forming the central part of cannon-ball carbonate concretions found within Miocene terrigeneous sediments of Sakhalin island (easternmost part of Russia). Twelve samples of glendonites and host carbonate concretions were examined using optical and cathodoluminescence microscopy, EDX analysis, powder X-ray diffraction and isotopic analysis. The aim of the study is to determine the origin of the concretions and the relationships between the concretion and glendonite occurrence.Glendonites and host cannon-ball concretions were found within terrigeneous sediments of Bora (Lower Miocene) and Vengeri (Upper Miocene) formations. These formations are composed of laminated sandstones, siltstones, argillites and siliceous rocks. Dropstones are often found within these sediments as well as cannon-ball carbonate concretions, some of them with glendonites in central part. 60-90% of the cannon-ball concretion is occupied by sandy limestone (with high-magnesium calcite) and occasionally contains dolomite and pyrite. Central part of the cannon-ball concretion is occupied by glendonite (single crystal-like or star-like cluster of crystals). Glendonites are composed of several calcite generations. Rosette-like calcite crystals (“ikaite-derived calcite”) are composed of low-magnesium calcite, they are non-luminescent. Needle-like calcite cement is composed of high-magnesium calcite or dolomite and show bright-yellow cathodoluminescence. The rest of the glendonite is occupied with low-magnesium radiaxial fibrous or sparry calcite with dark-red cathodoluminescence.Isotopic ratios of glendonites are close to those of host concretions. For host concretions δ13С varies from -20.3 to -14.9 ‰PDB, δ18О varies from +1.7 to +2.7 ‰PDB; for glendonites δ13С varies from -18.1 to -1.9 ‰PDB, while δ18О varies from +0.7 to +3.4 ‰PDB.Close mineralogical and isotopic composition of the studied glendonites and host cannon-ball concretions suggest they were formed in similar geochemical environment. Association of glendonite occurrence along with dropstones is an indicator of cold conditions, which is well-corresponding with view on glendonites as a proxy for cooling events. Cementation of surrounding sediment (formation of the cannon-ball concretions) and glendonite formation was simultaneous and occurred during early diagenesis in the sulfate-reduction zone. The source of calcium and magnesium ions was seawater (δ18О values are characteristic for seawater). Ikaite was replaced with low-magnesium calcite; the replacement was favored by organic matter decay (δ13C values are characteristic for organic matter). Cementation of the cannon-ball concretion with high-magnesium calcite occurred together with needle-like high-magnesium calcite growth in the glendonite with increasing concentration of magnesium due to calcite extraction from the pore water. The remaining pore space was subsequently filled with radiaxial fibrous or blocky sparry calcite during burial diagenesis.The study is supported by RFBR, project number 20-35-70012.

Glendonites often used as paleoclimate indicator of cold near-bottom temperature, as these are calcite pseudomorphs of ikaite, a metastable calcium carbonate hexahydrate, precipitates mostly under low temperature (mainly from 0-4oC) and may be stabilized by high phosphate concentrations that occurs due to anaerobic oxidation of methane and/or organic matter; dissolved organic carbon, sulfates and amino acid may contribute ikaite formation as well. Therefore, glendonites-bearing host rocks frequently include glacial deposits that make them useful as a paleoclimate indicator of near-freezing temperature.Our study is based on material collected from five wells drilled in eastern Barents Sea: Severo-Murmanskaya, Ledovaya – 1,2; Ludlovskaya – 1,2. The studied glendonites, mainly represented by relatively small rhombohedral pseudomorphs (0,5-2 cm) and rarely by stellate aggregates, collected from Middle Jurassic to Lower Cretaceous shallow marine clastic deposits. They scattered distributed throughout succession. Totally 18 samples of glendonites were studied. The age of host-bearing rocks were defined by fossils: bivalves or ammonites, microfossils or dinoflagellate. Bajocian-Bathonian glendonites were collected from Ledovaya – 1 and Ludlovskaya – 1 and 2 wells; in addition to these occurrences Middle Jurassic glendonites are known also in boreholes drilled at Shtockmanovskoe field. Numerous ‘jarrowite-like’ glendonites of the Middle Volgian (~ latest early Tithonian) age were sampled from Severo-Murmanskaya well. Unique Late Barremian glendonites were found in Ledovaya – 2 well.δ18O values of Middle Jurassic glendonite concretions range from – 5.4 to –1.7 ‰ Vienna Pee Dee Belemnite (VPDB); for Upper Jurassic – Lower Cretaceous δ18O values range from – 4.3 to –1.6 ‰ VPDB; for Lower Cretaceous - δ18O values range from – 4.5 to –3.4 ‰ VPDB. Carbon isotope composition for Middle Jurassic glendonite concretions δ13C values range from – 33.3 to –22.6 ‰ VPDB; for Upper Jurassic – Lower Cretaceous δ13C values range from – 25.1 to –18.4 ‰ VPDB; for Lower Cretaceous - δ13C values range from – 30.1 to –25.6 ‰ VPDB.Based on δ18O data we supposed that seawater had a strong influence on ikaite-derived calcite precipitation. Received data coincide with δ18O values reported from other Mesozoic glendonites and Quaternary glendonites formed in cold environments. Values of δ13C of glendonites are close to bacterial sulfate reduction and/or anaerobic oxidation of methane or organic matter. Glendonites consist of carbonates forming a number of phases which different in phosphorus and magnesium content. Mg-bearing calcium carbonate and dolomite both include framboidal pyrite, which can indicate (1) lack of strong rock transformations activity and (2) presence of sulfate-reduction bacteria in sediments.To conclude, Mesozoic climate was generally warm and studied concretions indicate cold climate excursion in Middle Jurassic, Upper Jurassic-Early Cretaceous and Early Cretaceous. The study was supported by RFBR, project number 20-35-70012.