nom_organisme nom_organisme nom_organisme nom_organisme
Quantifying proxy heterogeneity in marine sediment cores: 14C and δ18O analysis on single foraminifera
Brett Metcalfe
Jeudi 18/05/2017, 14:00-15:00
Bât. 701, P. 17C, LSCE Orme des Merisiers

Marine sediment cores used in palaeoceanography form the basis of our current understanding of past global climate and ocean chemistry. Conventional stable isotope analysis (δ18O; δ13C) of planktonic foraminifera, in which each analysis consists of a group of individuals, reduces the variance associated with each individual shell enabling reconstruction of primary climatic signals free from ‘noise’ (e.g. global ice volume change, sea-level change). However, this variance can be attributable to secondary signals such as seasonality and or migration in the depth that each individual foraminifer experiences throughout their ontogeny, which are themselves important pieces of palaeoceanographic information. Analysis of single foraminiferal shells however, can be used to reconstruct these secondary signals such as seasonal temperature changes of individual specimens.

Likewise, precision and accuracy of geochronological control in these sediment cores are crucial in unravelling the timing of rapid shifts in palaeoclimate and, ultimately, the interdependency of global climate mechanisms and their causality. Aware of the problems associated with bioturbation (the mixing of ocean sediments by benthic organisms) palaeoceanographers generally aim to retrieve sediment cores from locations with high sediment accumulation rates, thus minimising the influence of bioturbation as much as possible. However, the practice of concentrating only on areas of the ocean floor with high sedimentation accumulation rates has the potential to introduce a geographical bias into our understanding of global palaeoclimate. For example, global time averaged sediment accumulation rates for the ocean floor (excluding continental margins) indicate that vast areas of the ocean floor have sediment accumulation rates less than the recommended minimum advised sediment accumulation rates of 10 cm/ka or greater. Whilst many studies have focused on quantifying the impact of bioturbation on our understanding of the past, few have attempted to overcome the problems associated with bioturbation.

Recent pioneering developments in 14C AMS at the Laboratory of Ion Beam Physics at ETH Zürich have led to the development of the Mini Carbon Dating System (MICADAS). This compact 14C AMS system can be coupled to a carbonate handling system, thus enabling the direct AMS measurement of gaseous samples, i.e. without graphitisation, allowing for the analysis of carbonate samples of <100 μg. Likewise, while earlier isotope ratio mass spectrometry (IRMS) technology required a minimum of 100 μg of carbonate to produce a successful δ18O measurement, more recent advances in IRMS technology have made routine measurements of as little as 5 μg possible. Combining both analytical techniques enables improvements on palaeoclimate reconstructions, that can be independent of depth.

Contact : Masa Kageyama
Retour en haut