LEAF WAX CARBON AND HYDROGEN ISOTOPIC EVIDENCE OF HYDROLOGIC CONDITIONS IN MIDCONTINENT NORTH AMERICA SINCE THE LAST INTERGLACIAL

Future climate change is expected to have a large impact on precipitation, although the sign and magnitude of this impact is unclear in most regions, especially in the midcontinent North America (NA). Studying past changes in hydrology from midcontinent NA, one of the most drought-sensitive regions in NA, during period of major warming similar to those expected in the future can help us better constrain future hydrologic conditions. The Last Interglacial (LIG; 130-115 kyr) is the most recent warm time period with higher average global temperature (~2°C), higher sea level (~6m), and lower global ice volume than the present. Studying the LIG may offer an understanding of the future hydrologic changes. However, there are insufficient hydrologic records in midcontinent NA during the LIG, and modeled and proxy data are contradicting. To address this gap, we generated a record of hydrologic and vegetation conditions in midcontinent NA for the last ~150 kyr. To generate these records, we measured hydrogen (δD_wax) and carbon (δ¹³C_wax) stable isotopes of plant leaf waxes (n-alkanes and n-alkanoic acids) as proxies for hydrology and vegetation, respectively, that are preserved in marine sediments (ODP 625B) from northeastern Gulf of Mexico. Long-chain n-alkanes showed a strong preference for odd-over-even carbon numbers, indicating leaf waxes mainly derived from higher plants. δ¹³C_wax values were relatively higher during interglacials, and lower in glacial periods, suggesting expansion of C₄ plants during interglacial periods in midcontinent NA. The LIG had highest contribution of C₄ plants during the last ~150 ka based on n-alkanes distributions. δD_wax values were higher during the Holocene and LIG, indicating higher moisture contributions from the Gulf of Mexico and Atlantic Ocean during interglacials. Differences in moisture source and vegetation between and within glacials and interglacials will be discussed. A future study on identifying source and transport mechanisms of leaf waxes in the Mississippi River Basin is suggested to validate the interpretation of this study. This work will help understand regional water cycling and vegetation during the past warm (and cold) periods, and such information will be critical in reducing uncertainties of future hydrologic condition predictions.