THE NEOGENE EXPANSION OF C4 DOMINATED ECOSYSTEMS: AN AUSTRALIAN PERSPECTIVE

Widespread expansion of C₄ dominated ecosystems occurred by the late Miocene in many regions, as indicated by carbon isotope shifts in terrestrial plant derived carbon records. Today, much of Australia is dominated by C₄ taxa, including grasses, forbs and chenopod shrubs, but little is known about the origin and development of these widespread ecosystems and whether they also expanded during the late Miocene. Here we present on our organic geochemical study to constrain the timing of proliferation of C₄ dominated ecosystems on the Australian continent. We utilise compound specific isotope analysis (CSIA) of terrestrial plant derived long chain n-alkanes (n-C27 to n-C33) extracted from deep marine sediments off of the eastern and western coasts of Australia. Carbon isotope signatures of these homologs are used to quantify shifts in photosynthetic pathway dominance through time. Measures of n-alkane homolog abundance and distribution assist in the interpretation of records, particularly in the context of organic matter sources. Presented are time series records of n-alkane homolog carbon isotope signatures, concentration, average chain length (ACL) and carbon preference index (CPI) from two late Neogene marine sedimentary successions. To date, we have observed a shift towards less negative n-C31 isotope ratios across the study interval indicative of the expansion of C₄ vegetation. In addition, each of the measures of n-alkane homolog abundance and distribution show an increase across the same interval. We interpret this as an increased input of terrestrial plant derived organic matter to the marine sedimentary record during this period, with an increasing proportion of that coming from C₄ plants. These observations and interpretation are consistent with and complement previously observed increases in the abundance of plant phytoliths as well as Poaceae, Asteraceae, and chenopod-type pollen within the same sedimentary successions.