RECONSTRUCTING ANCIENT TERRESTRIAL ECOSYSTEMS USING ISOTOPIC PROXIES FROM PALEOSOL MINERALS, ORGANICS, AND FOSSILS

A variety of geological and paleontological material can be analyzed to interpret aspects of ancient climate and environments. By combining traditional isotopic and clumped isotopic analysis of paleosol organic matter and carbonate as well as coeval vertebrate fossils, we can more accurately interpret a wide range of climatic and environmental aspects such as mean annual precipitation (MAP); warm-season and mean annual temperatures; precipitation δ¹⁸O; and orographic effects. We provide an ongoing case-study from the Early Cretaceous of Gansu Province, China. The Early Cretaceous Hekou Group preserves many transitional and primitive dinosaurs common throughout the rest of the Cretaceous. To investigate paleoclimate and paleoenvironment of the upper Hekou, we have incorporated stable and clumped isotope analysis of fossils and surrounding paleosols. Teeth of Lanzhousaurus magnidens were analyzed for carbonate C and O and phosphate O to determine drinking water isotopic composition, seasonality, diet isotopic composition, and MAP. Isotopic analysis of calcite nodules were used to estimate temperature and meteoric water δ¹⁸O. Turtle, crocodile and fish remains were analyzed to determine meteoric water δ¹⁸O and lake water temperature. Average Lanzhousaurus isotopic composition include: δ¹³C_CO3 = -6.24 ± 0.80‰ VPDB, δ¹⁸O_CO3 = 24.46 ± 1.0‰ VSMOW and δ¹⁸O_p = 20.9 ± 1.6‰ VSMOW. Pedogenic carbonate isotopes include: δ¹³C = -3.0 ± 0.3‰, δ¹⁸O = 20.7 ± 1.1‰, and D₄₇ = 0.690 values. Turtle and crocodile δ¹⁸Op averaged 13.0 ± 0.4‰ and 12.3 ± 0.2‰ respectively, while fish average 15.7 ± 0.6‰. Data suggest a warm-month mean of 26 to 30°C, while lake water temperatures were approximately 14°C suggesting strong seasonal temperature changes. The herbivorous diet of Lanzhousaurs was made up of typical C₃ vegetation though slightly enriched suggesting arid to semi-arid conditions. Calculated δ¹⁸Ow of turtles and crocodiles were ¹⁸O-depleted suggesting draining from the surrounding mountains. Soil water estimates using clumped isotope values and δ¹⁸O of microdrilled soil carbonates ranged between ~ -5.5 and -7.6‰ VSMOW. The soil carbonate data are in line with seasonal variations estimated with Lanzhousaurs teeth analyses (δ¹⁸O_{ingested water} = -6.7 ± 1.1‰) and suggest enrichment due to soil water evaporation.