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ABSTRACT: Isotopic and Chemical Signatures of Eustasy: The Record at Enewetak Atoll

T. M. Quinn, K. C. Lohmann, A. N. Halliday,

The post-Miocene eustatic record of Enewetak Atoll has been examined using lithology, mineralogy, stable isotope, strontium isotope, geochemical, and petrographic data. These data are evidence for at least 14 subaerial unconformities and numerous thin (< 3 m) paleophreatic lenses. The vast majority of these subaerial exposure surfaces are characterized by development of caliche, oxidized and laminated crusts, depletion in Sr and Mg, and ^dgrC13 values depleted at the exposure surface and become progressively enriched with depth. Strontium isotope stratigraphy of the upper 200 m of core KAR-1, developed from coral, bivalve, and whole-rock samples, indicates the presence of four major subaerial unconformities, which have been confirmed by a newly developed str ntium isotope stratigraphy on age and facies equivalent sediments of core OOR-17.

Oxygen, carbon, and strontium isotope and concentration data of microsamples (0.5 to 1.0 mg) of sparry calcite cements and adjacent rock matrix document the record of meteoric phreatic diagenesis attendant with these sea level lowstands. Strontium concentration positively covaries with Sr87/Sr86 and ^dgrC13. In contrast, Sr87/Sr86 negatively covaries with ^dgrC13 variations. Sr87/Sr86 values indicate the source of carbonate cementation because strontium is derived from dissolution of overlying aragonitic allochems. When the dissolving aragonite is of similar age, and hence of similar strontium isotopic composition, lowstand phreatic calcites preserve the depositional strontium isotope age. However, when the aragonite source has a significantly different strontium isotope composition (e.g., younger age sediments), lowstand phreatic calcites record a diagenetic strontium isotope age. Documentation of the spatial and depth distribution of diagenetically induced strontium age anomalies provide constraints on the timing and magnitude of sea level lowstands.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990