LAUSR

Abstract In this paper, we use molecular paleothermometry, based on the TEX86 proxy derived from fossilized archaeal lipids, to reconstruct absolute sea surface temperatures (SSTs) for the northwest Tethys Shelf during the late Pliensbachian to early Toarcian (Early Jurassic, ~183 Ma) stages. Our composite record from outcrops in Spain and Italy reveals that tropical SSTs varied between 22 and 32 °C over a ~ 3 Myr time period, including transient temperature excursions of 5–10 °C magnitude with lapse rates of ~0.1 °C/kyr. Changes in reconstructed SSTs covaried with sea level fluctuations and δ18O isotope signatures of marine biocalcifiers, recording recurrent shifts between icehouse and greenhouse states. Parallel trends of reconstructed SST and δ13C isotopes of atmospheric CO2 may reflect storage of isotopically light carbon in cryosphere reservoirs during icehouse phases and release during greenhouse phases. The existence of a labile cryosphere is considered a prerequisite to explain the inferred rapid climate fluctuations and as a reservoir to facilitate pronounced carbon isotope excursions.