Quantifying Changes in Hydrologic Cycle Fluxes over the Americas During the Mid-Cretaceous (Albian) Greenhouse World

白垩纪中期被认为是一个温室世界，在这个世界中，大气中的二氧化碳浓度、全球年平均气温和全球降雨率都比现在高得多。地质学替代数据以及大气环流模型(GCM‘s)已被用来试图量化气候变量(如温度和降雨率)。我们利用球形结核中菱铁矿的氧同位素组成(18-O)和同位素质量平衡模型来量化北美(堪萨斯州至阿拉斯加)白垩纪西部内陆盆地(KWIB)沿海平原的降水速率变化。我们的计算表明，在中高纬度地区，降水率大大超过现代降水率(中纬度地区高290-390%，高纬度地区高450%)，副热带干旱带地区的降水赤字更大(即更干燥)，热带赤道地区的降水率与现代降水率没有显著差异。虽然我们估计的中纬度和副热带干旱带的降雨率比GCM估计的要大得多，但我们的热带赤道地区的降雨率比GCM估计的要低得多。然而，来自热带和亚热带地区的经验数据缺乏，这是进一步限制我们的模型所需的。该项目将扩大北半球白垩纪中期(阿尔比亚-塞诺马期)的北半球古纬度18-O替代覆盖范围，以覆盖北半球的白垩纪中期美洲。扩大的纬度覆盖范围将使1)改进同位素质量平衡模型并限制副热带到赤道地区的降水-蒸发通量；2)最大限度地减少模拟北半球水文循环的不确定性；以及3)更好地量化阿尔比亚期间KWIB的水汽来源和水分循环。干燥带地区(古纬度15~30N)的古沉淀18-O组成将通过采样和分析硅质碎屑和碳酸盐海岸平原沉积中的钙石、德克萨斯州和墨西哥孤立碳酸盐台地的石灰岩以及墨西哥中部和南部的淡水湖相碳酸盐来确定。赤道(~10 S~10 N古纬度)古沉淀18-O值将通过采样和分析哥伦比亚陆相和沿海平原沉积物中的球铁矿来确定。

The mid Cretaceous (Albian) is recognized as a greenhouse world under which atmospheric carbon dioxide concentrations, mean annual global temperature, and global precipitation rates were all much higher than present day. Geologic proxy data as well as general circulation models (GCM's) have been utilized in attempts to quantify climate variables (e.g. temperature and precipitation rates). We have utilized the oxygen isotopic composition (18-O) of siderite in spherical nodules (sphaerosiderites found in wetland paleosols) and an isotope mass balance model to quantify changes in precipitation rates along the coastal plains of the Cretaceous Western Interior Basin (KWIB) of North America (Kansas to Alaska). Our calculations suggest that at mid-high latitude precipitation rates greatly exceeded modern rates (290-390% greater in mid latitudes and 450% greater at high latitudes), that the subtropical dry-belt region had a greater precipitation deficit (i.e. was drier), and that the precipitation rates in the tropical-equatorial regions did not differ significantly from the modern. While our precipitation rates at mid latitudes and the sub-tropical dry belts precipitation deficit estimates are much greater than those estimated by GCM's, our tropical-equatorial region precipitation rates are much lower than GCM's estimates. However, empirical data from the tropics and sub-tropics that are needed to further constrain our model are lacking. This project will expand the northern hemisphere paleolatitudinal 18-O proxy coverage for the mid-Cretaceous (Albian-Cenomanian) to cover the mid-Cretaceous Americas of the northern hemisphere. The expanded latitudinal coverage will allow 1) refinements of isotope mass-balance models and constrain precipitation-evaporation fluxes for the sub-tropical to equatorial regions; 2) minimize uncertainties on modeling the hydrologic cycle in the northern hemisphere; and 3) better quantification of vapor sources and moisture recycling for the KWIB during the Albian. Paleoprecipitation 18-O compositions in the dry-belt region (15 to 30 N paleolatitude) will be determined by sampling and analyzing calcretes in siliciclastic and carbonate coastal plain deposits, and limestones from isolated carbonate platforms in Texas and Mexico, and freshwater lacustrine carbonates in central and southern Mexico. Equatorial (~10 S to 10 N paleolatitude) paleoprecipitation 18-O values will be determined by sampling and analyzing sphaerosiderites from nonmarine and coastal plain deposits in Colombia.