Abstract The TEX86 paleotemperature proxy is widely used to reconstruct ocean surface temperatures over the past 100 million years. However, archaeal culture experiments show that nutrient stress elevates TEX86 values by increasing glycerol dialkyl glycerol tetraether (GDGT) cyclization. Here, we demonstrate that this “nutrient effect” is also recorded in sedimentary GDGTs. Using an expanded core‐top database, we find a significant negative correlation between TEX86 and nitrate concentrations (ρ = −0.31; P < 0.001) once the thermal effect is removed. There are stronger correlations (ρ −0.73 to −0.91; P < 0.001) in regions with steep nitrate gradients. Comparisons between TEX86 and U₃₇K′‐based reconstructions from the Arabian Sea and Tasman Sea suggest that nutrient stress influenced GDGT distributions during glacial‐interglacial cycles. Our findings underscore the need to account for nutrient effects when applying TEX86 paleothermometry.

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