The n-alkane C(31)/(C(29) C(31)) ratios from surface sediments in the eastern equatorial Pacific (EEP) exhibit higher values to the north and lower values to the south across the southern edge (2-4 degrees N) of the Intertropical Convergence Zone (ITCZ). Since plants tend to synthesize longer chain length n-alkanes in response to elevated temperature and/or aridity, the higher C(31)/(C(29) C(31)) ratios at northern sites suggest a higher contribution of vegetation under hot and/or dry conditions. This is consistent with the observation that northern sites receive higher levels of plant waxes transported by northeasterly trade winds from northern South America, where hot and dry conditions prevail. Furthermore, from a sediment core covering the past 750 ka (core HY04; 4 degrees N, 95 degrees W) we found that C(31)/(C(29) C(31)) ratios exhibit a long-term decrease from MIS (marine oxygen isotope stage) 17 to 13. During this period, the zonal SST (sea-surface temperature) gradient in the equatorial Pacific increased, suggesting an increase in Walker circulation. Such intensified Walker circulation may have enhanced moisture advection from the equatorial Atlantic warm pool to the adjacent northern South America, causing arid regions in northern South America to contract, which may explain long-term decrease in n-alkane chain lengths.

n-alkane; Vegetation change; MIS 13; Eastern equatorial Pacific; Mid-Pleistocene Transition

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