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<p>A series of papers published shortly after the Integrated Ocean Drilling Program Arctic Coring Expedition (ACEX, 2004) on Lomonosov Ridge indicated remarkably high early Eocene sea surface temperatures (SSTs; ca. 23 to 27&thinsp;<span class="inline-formula">^∘</span>C) and land air temperatures (ca. 17 to 25&thinsp;<span class="inline-formula">^∘</span>C) based on the distribution of isoprenoid and branched glycerol dialkyl glycerol tetraether (isoGDGT and brGDGT) lipids, respectively. Here, we revisit these results using recent analytical developments – which have led to improved temperature calibrations and the discovery of new temperature-sensitive glycerol monoalkyl glycerol tetraethers (GMGTs) – and currently available proxy constraints.</p> <p>The isoGDGT assemblages support temperature as the dominant variable controlling TEX<span class="inline-formula">₈₆</span> values for most samples. However, contributions of isoGDGTs from land, which we characterize in detail, complicate TEX<span class="inline-formula">₈₆</span> paleothermometry in the late Paleocene and part of the interval between the Paleocene–Eocene Thermal Maximum (PETM; <span class="inline-formula">∼</span>&thinsp;56&thinsp;Ma) and the Eocene Thermal Maximum 2 (ETM2; <span class="inline-formula">∼</span>&thinsp;54&thinsp;Ma). Background early Eocene SSTs generally exceeded 20&thinsp;<span class="inline-formula">^∘</span>C, with peak warmth during the PETM (<span class="inline-formula">∼</span>&thinsp;26&thinsp;<span class="inline-formula">^∘</span>C) and ETM2 (<span class="inline-formula">∼</span>&thinsp;27&thinsp;<span class="inline-formula">^∘</span>C). We find abundant branched GMGTs, likely dominantly marine in origin, and their distribution responds to environmental change. Further modern work is required to test to what extent temperature and other environmental factors determine their distribution.</p> <p>Published Arctic vegetation reconstructions indicate coldest-month mean continental air temperatures of 6–13&thinsp;<span class="inline-formula">^∘</span>C, which reinforces the question of whether TEX<span class="inline-formula">₈₆</span>-derived SSTs in the Paleogene Arctic are skewed towards the summer season. The exact meaning of TEX<span class="inline-formula">₈₆</span> in the Paleogene Arctic thus remains a fundamental issue, and it is one that limits our assessment of the performance of fully coupled climate models under greenhouse conditions.</p>