On stable isotopic variation and earliest Paleocene planktonic foraminifera

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doi: 10.1029/93PA00952
Author(s): D'Hondt, Steven; Zachos, James C.
Author Affiliation(s): Primary:
University of Rhode Island, Graduate School of Oceanography, Narragansett, RI, United States
University of California, Santa Cruz, United States
Volume Title: Paleoceanography
Source: Paleoceanography, 8(4), p.527-547. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0883-8305 CODEN: POCGEP
Note: In English. 119 refs.; illus. incl. 1 table
Summary: Extant planktonic foraminifera display positive covariance between δ13C signals and test size. As documented by other studies, primary causes of increased δ13C values with increased test size may include increased reliance on ambient CO2 for calcification at larger test sizes, decreased kinetic fractionation during calcification at larger test sizes, and increased photosymbiotic activity in larger symbiont-bearing planktonic foraminifera. Planktonic foraminiferal δ18O values also often covary with test size, although the direction of this covariance is taxon dependent. Possible explanations for relationships between δ18O signals and test size include changing habitat depth over ontogeny, correlations between adult test size and environmental conditions, and changing isotopic disequilibrium with size, ontogenetic stage, or photosymbiont density. In order to assess the magnitude and implications of similar size dependence in earliest Paleocene planktonic foraminifera, we measured the stable isotopic signals of multiple size fractions of 10 earliest Paleocene species. All of these taxa exhibit a strong positive correlation between δ13C and test size. The slope and magnitude of this trend varies between species, with Woodringina claytonensis displaying the largest shift (1.1 per mil over a 130 µm range in mean sieve size) and Guembelitria cretacea displaying the smallest (0.2 per mil over a 38 µm range). By analogy with modern planktonic foraminifera, this general relationship between δ13C and size probably resulted from increased reliance on ambient CO2 for calcification at larger test sizes. The high magnitude of this shift in some taxa may reflect either photosymbiotic enhancement of the general trend or relatively greater changes in the proportions of metabolic and ambient CO2 used for calcification at different test sizes. Failure to account for relationships between test size and δ13C signals can lead to underestimation of early Paleocene surface ocean δ13C values by 1 per mil or more. These size-related δ13C effects provide an alternative explanation for decreases in whole-rock δ13C values and some decreases in planktonic-tobenthic foraminiferal δ13C gradients documented at marine K/T boundary sequences. At all size fractions, the 10 Paleocene taxa display a very limited interspecies range of δ18O derived paleotemperatures. Despite this limited range, paleobiogeographic patterns and δ18O signals appear to provide realistic estimates of relative paleodepth and seasonal affinities of earliest Paleocene planktonic foraminiferal species. Earliest Paleocene δ18O and biogeographic data are consistent with a general trend of surface-to-deep diversification of microperforate planktonic foraminifera following the K/T boundary. Such a trend may simply result from exploitation of a near-surface open-ocean habitat by the epicontinental K/T survivor G. cretacea. Copyright 1993 by the American Geophysical Union.
Year of Publication: 1993
Research Program: DSDP Deep Sea Drilling Project
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; C-13/C-12; Carbon; Carbon dioxide; Cenozoic; Cretaceous; Foraminifera; Invertebrata; Isotope ratios; Isotopes; K-T boundary; Lower Paleocene; Mesozoic; Microfossils; O-18/O-16; Oxygen; Paleocene; Paleogene; Planktonic taxa; Protista; Stable isotopes; Stratigraphic boundary; Tertiary; Upper Cretaceous
Record ID: 1994018821
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.

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