Stable carbon isotopic variation of long chain n-alkanoic acids in the equatorial Pacific sediments over the last 40 Ma: Implications for expansion of C4 grassland in South America

The stable carbon isotope ratio (delta C-13) values of sedimentary long chain (>C-20) n-alkanoic acids from the equatorial Pacific (IODP Sites U1331-1338) were measured for historical reconstruction of C-4 plant contribution from South America over the last 40 Ma. The values for C-30 and C-32 acids ranged between -36.9 parts per thousand and -25.4 parts per thousand, suggesting that they likely originated from higher plant wax associated with C-3-dominated vegetation. The molecular distribution of alkanoic acids in the sediments showed a marked change across the Intertropical Convergence Zone (ITCZ), indicating a potential contribution from the Northern Hemisphere to the sediments located at >10 degrees N. In contrast, the abundances of n-alkanoic acids significantly decreased in the sediments at <10 degrees N. The results suggest that the ITCZ may have acted as an effective barrier for the atmospheric transport of terrigenous biomarkers from the Northern Hemisphere, and the records presumably reflect the plant wax signal from the northern part of South America. The latitudinal variation in the delta C-13 values suggest that the differences in the core locations possibly caused north-and southward shifts of ca. -1.0 parts per thousand/degrees N and ca. +0.5 parts per thousand/degrees S from the equator, respectively. The corrected delta C-13 values of C-30 and C-32 acids did not cross the C-3 end member threshold until 1.7 Ma, despite the onset of C-4 plants in central South America at ca. 7 Ma. This implies that vegetation in the low latitudes of South America was less affected by hydrological changes induced by the uplift of the Andes during the late Miocene. (C) 2014 Elsevier Ltd. All rights reserved.