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.

以草本植物为主的生态系统覆盖了地球约40%的地表面积，其中热带草本植物贡献了全球净初级生产力的约20%。C3（冷生/温带）与C4（热带及亚热带）草本植物的分布主要受气温调控。本研究利用采自安第斯山脉北部（哥伦比亚与厄瓜多尔）沿海拔与气温梯度布设的植被样地中的植硅体（phytolith）组合，构建了适用于该区域的古温标（paleothermometer）。为实现这一目标，我们基于年平均气温（mean annual temperature, MAT）与植硅体基气候指数（1-Ic）的关联构建了传递函数（transfer function）；其中植硅体基气候指数（1-Ic）为C4草本植硅体（GSSCP）占GSSCP总含量的比例。为评估该指数能否准确反映植被样地内C3-C4草本植物的丰度，我们将其与C4-C3草本植物丰度的半定量植物区系估算结果进行了对比。为进一步验证1-Ic指数作为C3-C4草本植物丰度代用指标（proxy）的有效性，我们将其与对应的δ¹³C值（C3-C4植被的独立代用指标）进行了比对。研究结果表明：1）植硅体组合可准确估算植被样地内C3-C4草本植物的丰度；2）相较于δ¹³C记录，1-Ic指数在估算C3-C4草本植物丰度时表现更优，即便在开放植被类型中亦是如此；3）基于1-Ic指数的模型可精准预测年平均气温。这一新校准后的代用指标将有助于优化安第斯山脉北部的古气温重建工作，其适用的时间尺度至少可追溯至中新世晚期该区域C4草本植物出现并扩散的时期。