Combined Stable Isotope and Trace Element Analyses on Single Planktic Foraminifer Shells: Insights from Live Culture Experiments and Paleoceanographic Applications.

摘要

Planktic foraminifers are abundant in the surface ocean, and the calcite shells of fossil foraminifers form a major component of deep sea sediments. These shells comprise a primary archive of sea surface conditions through the Cenozoic, and measurements of stable isotope and trace element ratios are frequently used in paleoclimate reconstructions. Each individual foraminifer calcifies over the course of days or weeks, and the environmental conditions during its life are locked into the geochemistry of the shell. Typically, measurements pool numerous individual shells in a single, bulk analysis to obtain an estimate of mean environmental conditions. However, observations of living foraminifers indicate that contemporaneous individuals may occupy different depth habitats, and individual foraminifers may migrate vertically during their life cycle. The potential thus exists for a population of foraminifers to capture a range of environmental conditions, and for daily-scale environmental variations to be recorded in the intrashell geochemical heterogeneity of a single individual.;This research explores novel analytical methods of measuring geochemical heterogeneity in trace element ratios and stable oxygen and carbon isotope values in the shell calcite of the extant planktic foraminifer Orbulina universa through live culture experiments (Chapters 1 and 2) and measurements on fossil shells (Chapters 3 and 4). Chapter 1 describes the results of a live culture experiment in which oxygen isotope (delta 18O) and trace element (Ba) labels in synthetic seawater were incorporated into shell calcite during calcification in the laboratory. Chapter 2 describes the results of a similar live culture experiment where O. universa calcified in seawater with modified trace element (Ba, 87Sr) and carbon isotope (delta13C) values. Intrashell geochemical measurements were performed using Secondary Ion Mass Spectrometry (SIMS) for delta 18O and delta13C, and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) for trace element analyses. Intrashell trace element analyses were accurately measured with sub-mum resolution using LA-ICP-MS depth profiling. We demonstrate that the full amplitude of isotopic values (labeled and ambient) are resolvable within the spatial resolution of SIMS measurements (3 mum for delta18O; 6 mum for delta 13C). Together, these experimental results provide a quantitative framework for interpreting intrashell delta18O, delta 13C, and trace element ratio measurements in field samples, and open the possibility for reconstructing daily-scale environmental changes from fossil foraminifer shells.;The combination of multiple different geochemical measurements on fragments of the same shell permits researchers to extract novel information from each individual foraminifer. Additionally, a suite of contemporaneous fossil foraminifer shells preserves a record of a range of environmental conditions from different depths in the water column. These types of measurements provide a new dimension of information about the hydrography or stratification of the surface ocean during transient events, such as glacial meltwater entering the ocean. Chapter 3 describes the results of an experiment conducted on fossil specimens of O. universa where each shell was split into fragments and each fragment was subjected to different reagent cleaning protocols. Intrashell LA-ICP-MS depth profiles on shell fragments illustrate the differences between cleaning techniques, and illustrate a method for computing whole-shell Mg/Ca and Ba/Ca ratios from collected depth profiles. In Chapter 4, delta18O, Mg/Ca, and Ba/Ca measurements were combined on multiple O. universa shells from selected 1 cm intervals from a core in the Orca Basin, Gulf of Mexico, deposited during the last deglacation (∼18 to 11 ka). These combined measurements enable the calculation of delta18O seawater and salinity for each individual and the reconstruction of water column hydrography during meltwater pulses from the Laurentide Ice Sheet. Results yield computed delta18Owater values of Laurentide Ice Sheet meltwater, and provide key insight into the dynamics of the ice sheet during its collapse.