A 35Myr North American leaf-wax compound-specific carbon and hydrogen isotope record: Implications for C_4 grasslands and hydrologic cycle dynamics

摘要

Hydrology, source region, and timing of precipitation are important controls on the climate of the Great Plains of North America and the composition of terrestrial ecosystems. Moisture delivered to the Great Plains varies seasonally and predominately derives from the Gulf of Mexico/Atlantic Ocean with minor contributions from the Pacific Ocean and Arctic region. For this work, we evaluate long-term relationships for the past ~35million years between North American hydrology, climate, and floral change, using isotopic records and average carbon chain lengths of higher plant n-alkanes from Gulf of Mexico sediments (DSDP Site 94). We find that carbon isotope values (δ~(13)C) of n-alkanes, corrected for variations in the δ~(13)C value of atmospheric CO_(2,) provide minor evidence for contributions of C_4 plants prior to the Middle Miocene. A sharp spike in C_4 input is identified during the Middle Miocene Climatic Optimum, and the influence of C_4 plants steadily increased during the Late Miocene into the Pleistocene - consistent with other North American records. Chain-length distributions of n-alkanes, indicative of the composition of higher plant communities, remained remarkably constant from 33 to 4Ma. However, a trend toward longer chain lengths occurred during the past 4million years, concurrent with an increase in δ~(13)C values, indicating increased C_4 plant influence and potentially aridity. The hydrogen isotope values (δD) of n-alkanes are relatively invariant between 33 and 9Ma, and then become substantially more negative (75‰) from 9 to 2Ma. Changes in the plant community and temperature of precipitation can solely account for the observed variations in δD from 33 to 5Ma, but cannot account for Plio-Pleistocene δD variations and imply substantial changes in the source region of precipitation and seasonality of moisture delivery. We posit that hydrological changes were linked to tectonic and oceanographic processes including the shoaling and closure of the Panamanian Seaway, amplification of North Atlantic Deep Water Production and an associated increase of meridional winds. The southerly movement of the Intertropical Convergence Zone near 4Ma allowed for the development of a near-modern pressure/storm track system, driving increased aridity and changes in seasonality within the North American interior.