A LATE MIOCENE N-ALKANE δD AND δ13C RECORD FROM THE GULF OF MEXICO

δ¹³C and δD records of n-alkanes from higher-plant leaf waxes have been developed from late Miocene Gulf of Mexico sediments in order to assess the relationship between climate and C₄ grass expansion. Prior to 5.5 Ma, δ¹³C are relatively constant and average ~-29.5‰. However, at 5.5 Ma δ¹³C values shift -1.5‰, while δD values becomes enriched by ~30‰ from a background level of ~-130‰. A subsequent +3‰ change in δ¹³C occurs over the following 1 myr with no change in δD values. At ~3.5 Ma both δD and δ¹³C records return to their baseline values. A plant's carbon and hydrogen isotope ratios are integrally linked through water availability and the plant's capacity to cope with water stress. Under drier conditions, plant δ¹³C values become more ¹³C-enriched as does plant δD. However, in our record δ¹³C values trend more negative as δD becomes increasingly D-enriched. Increased evaporation/evapotranspiration enriches a plant's source water δD, as does changes in moisture source (i.e. distance the moisture traveled). Since these sediments derive from the Gulf of Mexico, the plant's moisture source likely came from the Gulf of Mexico itself, thereby minimizing the likelihood for vast changes in moisture source. Accordingly, our record suggests that evaporation increased over preciptiation between 4.5 and 5.5 Ma on the North American continent and specifically within the Gulf of Mexico watershed. Given concurrent North American terrestrial isotope records show similar trends (Passey et al., 2002), it appears that major changes in the hydrological cycle took place at this time. Obviously, n-alkane δ¹³C values do not solely reflect water stress. The rapid 1.5‰ depletion could indicate a change from a mixed C₃/C₄ community to a C₃-dominated system. However, this is unlikely since C₄ plants are better adapted to more water-stressed environments. If the ecosystem was increasingly influenced by more dry-adapted C₃ plants or if a negative trend in atmospheric carbon dioxide δ¹³C occurred, then a decrease in plant δ¹³C values would be anticipated. However, there is no evidence for either of these scenarios. Nonetheless, following the -1.5‰ shift, the carbon record behaves as expected under dryer conditions with values enriched in ¹³C. Further work will be performed to gain better resolution of this interval.