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Isotopic composition of low-latitude paleoprecipitation during the Early Cretaceous

Marina B. Suarez, Luis A. Gonzalez, Gregory A. Ludvigson, Francisco J. Vega and Jesus Alvarado-Ortega
Isotopic composition of low-latitude paleoprecipitation during the Early Cretaceous
Geological Society of America Bulletin (November 2009) 121 (11-12): 1584-1595

Abstract

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 degrees N) to high (75 degrees N) paleolatitudes, 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 degrees 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 delta (super 18) O versus delta (super 13) C were used to calculate the mean delta (super 18) O value of meteoric water, which is estimated at -5.46+ or -0.56 per mil (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 delta (super 18) O 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 delta (super 18) O values that were much heavier (up to 3 per mil) 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.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 121
Serial Issue: 11-12
Title: Isotopic composition of low-latitude paleoprecipitation during the Early Cretaceous
Affiliation: University of Kansas, Department of Geology, Lawrence, KS, United States
Pages: 1584-1595
Published: 200911
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 46
Accession Number: 2009-100324
Categories: StratigraphyIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 3 tables, geol. sketch map
N18°34'60" - N18°34'60", W97°54'15" - W97°54'15"
Secondary Affiliation: Universidad Nacional Autonoma de Mexico, MEX, Mexico
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200953
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