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This Article Figures Only Full Text Full Text (PDF) All Versions of this Article: B26453.1v1 121/11-12/1584    most recent Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Suarez, M. B. Articles by Alvarado-Ortega, J.

Isotopic composition of low-latitude paleoprecipitation during the Early Cretaceous

Marina B. Suarez^1,, Luis A. González¹, Gregory A. Ludvigson², Francisco J. Vega³ and Jesús Alvarado-Ortega³

¹ Department of Geology, University of Kansas, 1475 Jayhawk Blvd., Lawrence, Kansas 66045, USA
² Kansas Geological Survey, University of Kansas, 1930 Constant Ave., Lawrence, Kansas 66047, USA
³ Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacan, 04510, México City D.F., Mexico

Correspondence: E-mail: msuarez{at}ku.edu.

The response of the hydrologic cycle in global greenhouse conditions is important to our understanding of future climate change and to the calibration of global climate models. Past greenhouse conditions, such as those of the Cretaceous, can be used to provide empirical data with which to evaluate climate models. Recent empirical studies have utilized pedogenic carbonates to estimate the isotopic composition of meteoric waters and calculate precipitation rates for the Aptian-Albian. These studies were limited to data from mid- (35°N) to high (75°N) paleolati-tudes, and thus future improvements in accuracy will require more estimates of meteoric water compositions from numerous localities around the globe. This study provides data for tropical latitudes (18.5°N paleolatitude) from the Tlayua Formation, Puebla, Mexico. In addition, the study confirms a shallow nearshore depositional environment for the Tlayua Formation. Petrographic observations of fenestral fabrics, gypsum crystal molds, stromatolitic structures, and pedogenic matrix birefringence fabric support the interpretation that the strata represent deposition in a tidal flat environment. Carbonate isotopic data from limestones of the Tlayua Formation provide evidence of early meteoric diagenesis in the form of meteoric calcite lines. These trends in ¹⁸O versus ¹³C were used to calculate the mean ¹⁸O value of meteoric water, which is estimated at –5.46 ± 0.56 (Vienna Standard Mean Ocean Water [VSMOW]). Positive linear covariant trends in oxygen and carbon isotopic values from some horizons were used to estimate evaporative losses of vadose groundwater from tropical exposure surfaces during the Albian, and the resulting values range from 8% to 12%. However, the presence of evaporative mineral molds indicates more extensive evaporation.

The added tropical data improve latitudinal coverage of paleoprecipitation ¹⁸O estimates. The data presented here imply that earlier isotope mass balance models most likely underestimated tropical to subtropical precipitation and evaporation fluxes. The limited latitudinal constraints for earlier isotope mass balance modeling of the Albian hydrologic cycle of the Northern Hemisphere Americas resulted in extrapolated low-latitude precipitation ¹⁸O values that were much heavier (up to 3) than the values observed in this study. The lighter values identified in this study indicate a more pronounced rainout effect for tropical regions and quite possibly a more vigorous evaporation effect. These and additional low-latitude data are required to better constrain changes in the hydrologic cycle during the Cretaceous greenhouse period, and to reduce the uncertainties resulting from limited geographic coverage of proxy data.