Alternation of sparry and micritic laminae in modern nonmarine travertine stromatolites from creeks in the Arbuckle Mountains of Oklahoma and from Eocene hot water deposits in the Palm Park Formation of New Mexico represent seasonal accumulations. The sparry laminae, with upright biogenic structures, represent spring-summer accumulations, whereas the micritic laminae, with flat-lying microbial remains, represent fall-winter accumulations. The delta 18 O values of the modern creek waters average -5.04 (spring-summer) and -5.25 (fall-winter) (SMOW), and calcite precipitated in equilibrium with these waters should have delta 18 O values of approximately -6.4 and -5.2 (PDB) for spring-summer and fall-winter precipitates, respectively. delta 18 O values for calcite which precipitated during three-month periods on test objects in the creek waters average -6.5 for spring-summer and -4.9 for fall-winter. The sparry and micritic laminae comprising the naturally occurring modern stromatolites from these creeks average -6.4 for sparry (spring-summer) and -5.2 for micritic (fall-winter) laminae. Thus, the agreement between delta 18 O values for precipitates which formed in isotopic equilibrium with the creek water composition, precipitates on test objects, and the sparry and micritic laminae comprising naturally occurring modern stromatolites confirm the seasonal origin of the laminae. The delta (super 13C values for the creek waters, the test objects, and the naturally occurring laminae of the stromatolites are higher in the sparry (spring-summer) than the micritic (fall-winter) deposits. The delta 18 O values obtained as a result of lamina-by-lamina analyses of the Eocene hot water travertine stromatolites of New Mexico do not display a seasonal trend, but the delta 13 C values are consistently higher in the sparry laminae than in the micritic laminae. In addition, increased concentrations of trace elements occur in the sparry relative to the micritic laminae of the hot water stromatolites. The lamina-by-lamina stable isotopic and trace elemental analyses provide confirmation of the seasonal origin of the sparry (spring-summer) and micritic (fall-winter) laminae comprising the ancient stromatolites.