Supplementary material for: A paleothermometer for the Northern Andes based on C3/C4 grass phytoliths

Grass-dominated ecosystems cover ~40% of Earth’s surface with tropical grasses accounting for ~20% of global net primary productivity. C3 (cool/temperate) and C4 (tropical and subtropical) grass distribution is driven primarily by temperature. In this work, we used phytolith assemblages collected from vegetation plots along an elevation and temperature gradient in the Northern Andes (Colombia and Ecuador) to develop a paleothermometer for the region. To accomplish this, we created a transfer function based on the relationship between mean annual temperature (MAT) and the phytolith-based Climatic Index (1-Ic), which is the proportion of C4 grass phytoliths (GSSCP) over the sum of GSSCP. To evaluate how accurately the index reflects C4-C3 grass abundance in vegetation plots, we compared it with semi-quantitative floristic estimates of C4-C3 grass abundance. To further evaluate the 1-Ic index as a proxy for C4-C3 grass abundance, we compared it with corresponding ∂13C values (an independent proxy for C4-C3 vegetation). Results indicate that 1) GSSCP assemblages correctly estimate C4-C3 grass abundance in vegetation plots; 2) the Ic index outperforms the ∂13C record in estimating C4-C3 grass abundance, even in open vegetation types; and 3) our Ic index–based model accurately predicts MAT. This new calibrated proxy will help improve paleotemperature reconstructions in the northern Andes since at least the emergence and spread of C4 grasses in the region during the late Miocene.