Ikaite: A palaeoenvironmental proxy

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences

Abstract

In order to predict the consequences of global warming, it is essential to understand the climate response in terms of temperature and ice volume (hence sea level) to atmospheric carbon dioxide. The geological record of past climatic fluctuations has the power to reveal the natural interplay between these three Earth system parameters. One of the great challenges in paleoceanography since its inception, has been to reconstruct an independent record of oceanic temperature, or sea level throughout the Pleistocene ice ages in order to obtain an idea of the sensitivity and phasing of two crucial climate components: ice volume and temperature, during the glacial cycles. Foraminiferal d18O, the traditional proxy, is influenced by a combination of both ice volume and temperature so lack of constraint on seawater d18O prevents complete and quantitative interpretation of any marine carbonate d18O record. We have hypothesised that ikaite (CaCO3.6H2O), a metastable hydrated form of calcium carbonate which precipitates in organic rich sediments, captures the isotopic composition of ambient waters as hydration waters within the crystal (Rickaby et al., 20061), and offers a promising method of probing directly the d18Osw for periods other than the Last Glacial Maximum (LGM). We propose a multi-disciplinary approach to integrate mineralogical and biogeochemical laboratory experiments with field observations to characterise and understand the presence and climatic significance of ikaite and its isotopes within the natural environment. Our primary aims are to lay the foundations for, and apply this novel ikaite proxy to the reconstruction of d18Osw, hence ice volume, and extend application of the proxy to different timeslices and water masses.
 
Description Glendonites are pseudomorphs of the mineral ikaite (CaCO3·6H2O) after loss of hydration water and occur in distinctive euhedral crystalline forms, sometimes clustered as rosettes of up to tens of centimeters in diameter. While it is generally accepted that organic-rich environments, methane seeps, and high phosphate levels are important for ikaite formation, glendonite occurrences in ancient sedimentary sequences are widely considered to reflect near-freezing temperatures, even at high latitudes during periods of greenhouse climates. To fully understand the paleoenvironmental significance of glendonites, a comprehensive examination of the modern ikaite setting is necessary. Temperature is the most important parameter that has been quantitatively constrained for the presence of ikaite. Low bottom-water temperature, while a required condition for formation of the mineral, is not adequate for its growth; other controls are necessary to explain the absence of ikaite in many cold environments. In this study, we discuss the control of carbonate chemistry on ikaite formation. Our compilation of geochemical data from sediment cores with well-preserved ikaite provide further evidence for the importance of phosphate. A phosphate concentration above ~400 µM in shallow and cold porewater may be the requisite parameter for extensive ikaite precipitation. Thus, abundant glendonites in ancient successions mark past periods and regions of elevated porewater phosphorus concentrations, which may also be related to high surface productivity and/or iron fertilization.
Exploitation Route Implications for inference of environment of deposition from glendonite
Sectors Environment
 
Description Press release on ikaite in Antarctica 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Generated our paper to be one of the top 10 most downloaded EPSL papers for that year

NA
Year(s) Of Engagement Activity 2012