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

白垩纪中期（Albian）被认为是一个温室世界，大气中二氧化碳浓度、全球年平均气温和全球降水率都比现在高得多。 地质代用数据以及大气环流模型已被用来试图量化气候变量（如温度和降水率）。 我们已经利用了氧同位素组成（18-O）的菱铁矿在球形结核（sphaerosiderites在湿地古土壤中发现）和同位素质量平衡模型来量化降水率的变化沿着白垩纪西部内陆盆地（KWIB）的北美（堪萨斯阿拉斯加）的沿海平原。 我们的计算表明，中高纬度地区的降水率大大超过现代降水率（中纬度地区高出290-390%，高纬度地区高出450%），亚热带干旱带地区的降水量更少（即更干燥），热带-赤道地区的降水率与现代降水率没有显着差异。 虽然我们在中纬度和亚热带干旱带的降水率估计的降水赤字比GCM的估计要大得多，我们的热带赤道地区的降水率比GCM的估计要低得多。 然而，从热带和亚热带的经验数据，需要进一步约束我们的模型是缺乏的。 该项目将扩大北方半球白垩纪中期（阿尔比-赛诺曼期）的古纬度18-O代用指标覆盖范围，以覆盖北方半球白垩纪中期的美洲。 扩大的纬度覆盖范围将允许1）改进同位素质量平衡模型，并限制亚热带到赤道地区的降水蒸发通量; 2）最大限度地减少北方半球水文循环建模的不确定性; 3）更好地量化阿尔比期期间KWIB的蒸汽来源和水分再循环。 古降水18-O组成的干带地区（15至30 N古纬度）将通过采样和分析石灰岩在硅质和碳酸盐沿海平原沉积，石灰岩从孤立的碳酸盐台地在得克萨斯州和墨西哥，和淡水湖在墨西哥中部和南部的碳酸盐。 赤道（~10 S至10 N古纬度）古降水18-O值将通过采样和分析哥伦比亚非海洋和沿海平原沉积的球菱铁矿来确定。

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.