Oxygen Isotope Offsets in Deep-Water Benthic Foraminifera

ABSTRACT Despite the extensive use of the benthic foraminiferal oxygen isotope composition (δ18O) as a proxy for paleoclimatic reconstructions, uncertainties remain regarding the consistency of interspecies offsets and the environmental factors controlling 18O fractionation. We investigated δ18O offsets of some frequently used Uvigerina, Bulimina, and Cibicidoides species in core top samples from different hydrographic and sedimentary regimes in the South China Sea, Makassar Strait, and Timor Strait/Eastern Indian Ocean. The δ18O values of the epifaunal taxa Cibicidoides mundulus and Cibicidoides wuellerstorfi showed no significant offset in all investigated regions, whereas shallow infaunal Cibicidoides species exhibited higher variability and were less reliable. We found no offsets between species of Uvigerina and Bulimina and assume that these genera can be measured together and/or substituted. Our results show that epifaunal taxa are close to equilibrium with ambient seawater and thus provide more reliable records of past ice volume and/or bottom water temperature variations than infaunal taxa. Offsets among equilibrium calcite, epifaunal taxa, and infaunal taxa are not constant “vital effects” but are influenced by changing gradients in bottom to pore water pH and carbonate ion concentrations that depend on deep-water ventilation and export flux of particulate carbonate and organic carbon. Offsets between epifaunal and infaunal taxa varied between 0.58 and 0.73‰, depending on regional bottom and pore water conditions. Our findings highlight the importance of regional and temporal variations in organic carbon flux/degradation and dissolution of calcite that may lead to slight under- or overestimates of the amplitude of δ18O fluctuations, especially during times of rapidly changing calcite-saturation of bottom and pore water.

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Oxygen isotope equilibrium in brachiopod shell fibres in the context of biological control

Mineralogical Magazine ◽

10.1180/minmag.2008.072.1.239 ◽

2008 ◽

Vol 72 (1) ◽

pp. 239-242 ◽

Cited By ~ 1

Author(s):

M. Cusack ◽

A. Pérez-Huerta ◽

P. Chung ◽

D. Parkinson ◽

Y. Dauphin ◽

...

Keyword(s):

Oxygen Isotope ◽

Isotope Composition ◽

Seawater Temperature ◽

Environmental Data ◽

Ambient Seawater ◽

Secondary Layer ◽

Vital Effect ◽

Vital Effects ◽

Stable Oxygen ◽

Isotope Equilibrium

With their long geological history and stable low-Mg calcite shells, Rhynchonelliform brachiopods are attractive sources of environmental data such as past seawater temperature (Buening and Spero, 1996; Auclair et al., 2003; Brand et al., 2003; Parkinson et al., 2005). Concerns about the influence of vital effects on the stable isotope composition of brachiopod shells (Popp et al., 1986), led to isotope analyses of different parts of brachiopod shells in order to identify those parts of the shell that are influenced by any vital effect and those parts that may be suitable recorders of seawater temperature via stable oxygen isotope composition (Carpenter and Lohmann, 1995; Parkinson et al., 2005). Such detailed studies demonstrated that the outer primary layer of acicularcalcite is isotopically light in both δ18O and δ13C while the secondary layer, composed of calcite fibres, is in oxygen-isotope equilibrium with ambient seawater(Fig. 1) (Parkinson et al., 2005).

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Oxygen isotope composition of the Phanerozoic ocean and a possible solution to the dolomite problem

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1719681115 ◽

2018 ◽

Vol 115 (26) ◽

pp. 6602-6607 ◽

Cited By ~ 28

Author(s):

Uri Ryb ◽

John M. Eiler

Keyword(s):

Oxygen Isotope ◽

Isotope Exchange ◽

Isotope Composition ◽

Temporal Variations ◽

Oxygen Isotope Exchange ◽

Diagenetic Alteration ◽

Isotope Studies ◽

Dolomite Problem ◽

Clumped Isotope

The18O/16O of calcite fossils increased by ∼8‰ between the Cambrian and present. It has long been controversial whether this change reflects evolution in the δ18O of seawater, or a decrease in ocean temperatures, or greater extents of diagenesis of older strata. Here, we present measurements of the oxygen and ‟clumped” isotope compositions of Phanerozoic dolomites and compare these data with published oxygen isotope studies of carbonate rocks. We show that the δ18O values of dolomites and calcite fossils of similar age overlap one another, suggesting they are controlled by similar processes. Clumped isotope measurements of Cambrian to Pleistocene dolomites imply crystallization temperatures of 15–158 °C and parent waters having δ18OVSMOWvalues from −2 to +12‰. These data are consistent with dolomitization through sediment/rock reaction with seawater and diagenetically modified seawater, over timescales of 100 My, and suggest that, like dolomite, temporal variations of the calcite fossil δ18O record are largely driven by diagenetic alteration. We find no evidence that Phanerozoic seawater was significantly lower in δ18O than preglacial Cenozoic seawater. Thus, the fluxes of oxygen–isotope exchange associated with weathering and hydrothermal alteration reactions have remained stable throughout the Phanerozoic, despite major tectonic, climatic and biologic perturbations. This stability implies that a long-term feedback exists between the global rates of seafloor spreading and weathering. We note that massive dolomites have crystallized in pre-Cenozoic units at temperatures >40 °C. Since Cenozoic platforms generally have not reached such conditions, their thermal immaturity could explain their paucity of dolomites.

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Oxygen isotope composition in Modiolus modiolus aragonite in the context of biological and crystallographic control

Mineralogical Magazine ◽

10.1180/minmag.2008.072.2.569 ◽

2008 ◽

Vol 72 (2) ◽

pp. 569-577 ◽

Cited By ~ 13

Author(s):

M. Cusack ◽

D. Parkinson ◽

A. Freer ◽

A. Pérez-Huerta ◽

A. E. Fallick ◽

...

Keyword(s):

Biological Control ◽

Oxygen Isotope ◽

Environmental Influence ◽

Isotope Composition ◽

Seawater Temperature ◽

Crystal Habit ◽

Ambient Seawater ◽

Oxygen Isotope Fractionation ◽

Modiolus Modiolus ◽

Horse Mussel

AbstractLiving systems exert exquisite control on all aspects of biomineral production and organic components, including proteins, are essential to this biological control. The protein-rich extrapallial (EP) fluid of bivalve molluscs is a strong candidate for the source of such proteins. Differences in calcium carbonate polymorphs between Modiolus modiolus and Mytilus edulis are concurrent with differences in EP fluid protein profiles. In conjunction with this biological control is the environmental influence which is interpreted using proxies such as δ18O to determine the history of ambient seawater temperature. In the horse mussel, Modiolus modiolus, the difference in oxygen isotope fractionation in the nacreous aragonite and the prismatic aragonite layer results in respective δ18O values of 2.1±0.2% and 2.5±0.2%. These δ18O values result in estimates of ambient seawater of 12.1±0.6°C and 10.2±0.6°C for nacreous and prismatic aragonite, respectively. Electron backscatter diffraction is used here to determine the crystallographic orientation at high spatial resolution, allowing the measurements of stable isotopes to be accurately mapped in terms of shell architecture. These preliminary data suggest that it is essential to account for both polymorph and crystal habit when deciphering ambient seawater temperature using δ18O as a proxy.

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