Ocean Temperatures through the Phanerozoic

The oxygen isotope compositions of carbonate and phosphatic fossils hold the key to understanding Earth system evolution during the last 500 million years. However, the veracity and interpretation of this record remain unsettled. To resolve this problem, we have compiled Phanerozoic 18O data for carbonate (N = 23,815) and phosphate (N = 4225) fossils and microfossils from shallow marine environments1. In calculating paleotemperatures, we assert a constant hydrosphere 18O and correct seawater 18O values for ice volume and paleolatitude. Furthermore, we correct belemnite 18O values for 18O enrichment relative to traditional 18O-temperature relations. Our compilation shows rapid shifts best explained by climate change. Similar paleotemperature trends for carbonates and phosphates (conodonts) for the Paleozoic indicate retention of original isotopic signatures for hundreds of millions of years. Low-latitude (0 30) sea-surface temperatures (SSTs) were mostly warm (32-40 C) during the Late Ordovician through Devonian (458-359 Ma) and cool (20-30 C) from the Carboniferous through Permian (359-252 Ma). After the Permian, low-latitude oceans returned to hothouse temperatures (35 C) in the Early Triassic (~252 Ma), Cenomanian-Turonian (94 Ma), Paleocene-Eocene (56 Ma) and early Eocene (~51 Ma). Because pre-Cretaceous samples are restricted to continental margins and epeiric seas, these temperatures may be 2 C higher than open-ocean temperatures2. Extreme warmth (>40 C) in the early to Middle Ordovician (490-465 Ma) remains a conundrum as these temperatures are well above the thermal limits of most modern multi-cellular marine organisms. Changes in solar forcing due to pCO2 doubling, corrected for increasing solar radiation with time, correlate significantly (p < 0.0001) with low-latitude SST change for the Paleozoic (R2 = 0.37), Mesozoic (R2 = 0.07), and Cenozoic (R2 = 0.15), though the correlations are poor for the Mesozoic due to insufficient data. Estimates of low-latitude climate sensitivity for the Cenozoic and Paleozoic (0.34 and 1.6°C per W/m2 respectively) suggest that climate sensitivity was higher in the Paleozoic compared with the Cenozoic. 1http://stabisodb.org 2Judd EJ, Bhattacharya T, Ivany LC. Geophysical Research Letters 47, e2020GL089044 (2020).