The ikaite to calcite transformation: Implications for palaeoclimate studies
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Marine sedimentary ikaite is the parent mineral to glendonite, stellate pseudomorphs found throughout the geological record which are most usually composed of calcite. Ikaite is known to be metastable at earth surface temperatures and pressures, readily breaking down to more stable carbonate polymorphs when exposed to warm (ambient) conditions. Yet the process of transformation of ikaite to calcite is not well understood, and there is an ongoing debate as to the palaeoclimatic significance of glendonites in the geological record. This study uses a combination of techniques to examine the breakdown of ikaite to calcite, outside of the ikaite growth medium, and to assess the palaeoclimatic and palaeoenvironmental significance of stable and clumped isotope compositions of ikaite-derived calcite. Powder X-ray diffraction shows that ikaite undergoes a quasi- solid-state transformation to calcite during heating of samples in air, yet when ikaite transforms under a high temperature differential, minor dissolution-recrystallisation may also occur with the ikaite structural waters. No significant isotopic equilibration to transformation temperature is observed in the resulting calcite. Therefore, in cases of transformation of ikaite in air, clumped and stable isotope thermometry can be used to reconstruct ikaite growth temperatures. In the case of ancient glendonites, where transformation of the ikaite occurred in contact with the interstitial waters of the host sediments over unknown timescales, it is uncertain whether the reconstructed clumped isotope temperatures reflect ikaite crystallisation or its transformation temperatures. Yet clumped and stable isotope thermometry may still be used conservatively to estimate an upper limit for bottom water temperatures. Furthermore, stable isotope along with element/Ca ratios shed light on the chemical environment of ikaite growth. Our data indicate that a range of (bio)geochemical processes may act to promote ikaite formation at different marine sedimentary sites, including bacterial sulphate reduction and anaerobic oxidation of methane. The colours of the ikaites, from light brown to dark brown, indicate a high organic matter content, favouring high rates of bacterial sulphate reduction as the main driver of ikaite precipitation. Highest Mg/Ca ratios are found in the most unstable ikaites, indicating that Mg acts to destabilise ikaite structure.
| Originalsprog | Engelsk |
|---|---|
| Tidsskrift | Geochimica et Cosmochimica Acta |
| Vol/bind | 334 |
| Sider (fra-til) | 201-216 |
| Antal sider | 16 |
| ISSN | 0016-7037 |
| DOI | |
| Status | Udgivet - 1 okt. 2022 |
Bibliografisk note
Funding Information:
Funding for this study was gratefully received from the European Commission, Horizon 2020 (ICECAP; grant no. 101024218), and the Danish Council for Independent Research -Natural Sciences grant DFF-7014-00142 to C. Korte. This project has also received funding from the Research Council of Norway through the Centres of Excellence funding scheme, project number 223272. We thank EPSRC CNIE research facility service (EPSRC Award EP/S03305X/1) for DSC/TGA data collection. We are very grateful to Sabine Kasten for aiding collection, storage and transportation of three of the ikaites used in these experiments. We thank Phil Holdship, University of Oxford, for running the ICP-MS analysis, and Michael Bedington, Plymouth Marine Lab, for modelling the bottom water temperatures at the ikaite collection sites. We extend thanks to Erwin Suess for giving advice and helping to provide some of the ikaites for the experiments. MLV designed the study, co-ran the PXRD, DSC and TGA, co-interpreted the data, and wrote the manuscript, additionally securing funding for the study. MV ran the PXRD analysis, undertook the Rietveld analysis and made the interpretations thereof, and co-wrote the manuscript. REMR: Co-conceived the study, provided expert knowledge and helped with the interpretation of the data; provided access to the Congo Fan and Bransfield Strait ikaites, and oversaw the running and interpretation of the ICP-MS analysis. HW ran the TGA and DSC analysis and aided the interpretation thereof. SMB ran and processed the clumped isotope data and co-interpreted the data. CVU ran the ICP-OES analysis and provided the interpretation of the data. GB Provided expert knowledge and aided interpretation of the data, and enabled access to the Nankai Trough ikaite. RFS provided expert knowledge and aided interpretation of the data. HK led the expedition to Russia to collect the Laptev Sea ikaites and preserved the specimens after collection. BPS and CA undertook SEM surface imaging of ikaite breakdown. NT helped with the interpretation of the data. CK secured funding for this study.
Funding Information:
Funding for this study was gratefully received from the European Commission, Horizon 2020 (ICECAP; grant no. 101024218), and the Danish Council for Independent Research -Natural Sciences grant DFF-7014-00142 to C. Korte. This project has also received funding from the Research Council of Norway through the Centres of Excellence funding scheme, project number 223272. We thank EPSRC CNIE research facility service (EPSRC Award EP/S03305X/1) for DSC/TGA data collection. We are very grateful to Sabine Kasten for aiding collection, storage and transportation of three of the ikaites used in these experiments. We thank Phil Holdship, University of Oxford, for running the ICP-MS analysis, and Michael Bedington, Plymouth Marine Lab, for modelling the bottom water temperatures at the ikaite collection sites. We extend thanks to Erwin Suess for giving advice and helping to provide some of the ikaites for the experiments.
Publisher Copyright:
© 2022 The Authors
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