Late Pleistocene glacial–interglacial shell-size–isotope variability in planktonic foraminifera as a function of local hydrography

[摘要] So-called "vital effects" are a collective term for a suite of physiologicallyand metabolically induced variability in oxygen (δ¹⁸O) and carbon(δ¹³C) isotope ratios of planktonic foraminifer shells that hamperprecise quantitative reconstruction of past ocean parameters. Correction forpotential isotopic offsets from equilibrium or the expected value isparamount, as too is the ability to define a comparable life stage for eachspecies that allows for direct comparison. Past research has focused uponfinding a specific size range for individual species in lieu of otheridentifiable features, thus allowing ocean parameters from a particular constant(i.e. a specific depth or season) to be reconstructed. Single-shell isotopeanalysis of fossil shells from a mid-latitude North Atlantic Oceanpiston core covering Termination III (200 to 250 ka) highlight theadvantage of using a dynamic size range, i.e. utilising measurements frommultiple narrow sieve size fractions spanning a large range of total bodysizes, in studies of palaeoclimate. Using this methodology, we show thatisotopic offsets between specimens in successive size fractions ofGloborotalia inflata and Globorotalia truncatulinoides arenot constant over time, contrary to previous findings. For δ¹⁸O insmaller-sized globorotalids (212–250 μm) it is suggested that theoffset from other size fractions may reflect a shallower habitat in an earlyontogenetic stage. A reduction in the difference between small and largespecimens of G. inflata between insolation minima and maxima isinterpreted to relate to a prolonged period of reduced water columnstratification. For the shallow-dwelling species Globigerinabulloides, no size–isotope difference between size fractions is observed, andthe variability in the oxygen isotopic values is shown to correlate wellwith the seasonal insolation patterns. As such, patterns in oxygen isotopevariability of fossil populations may be used to reconstruct past seasonalitychanges.

[学科分类] 地球化学与岩石