Organic Paleotemperature Reconstructions from the Early Icehouse North Atlantic

The early icehouse climate of the Oligocene ( 33-23 Ma) was characterized by a number of unique features including sustained extratropical warmth and extreme sensitivity to orbital forcing. A new alkenone (U^k'₃₇) paleotemperature record from the Newfoundland Margin (IODP U1404) has recently expanded this list, by showing a transient asymmetry in Northern and Southern hemisphere cooling with the establishment of permanent Antarctic glaciers and sustained surface ocean warmth on the Newfoundland margin, more than 10 °C warmer than existing terrestrial temperatures through the Oligocene, both features attributed to ocean circulation. Here we use sediments from IODP U1406, to reconstruct terrestrial temperatures in the high North Atlantic and to examine changes in terrestrial versus marine conditions through the Oligocene to lowest Miocene (30-22 Ma), to test whether local terrestrial temperatures are consistent with this mechanism. Branched and isoprenoid tetraether (BIT) indices are high, 0.62 to 0.94, suggesting significant input of terrestrial organic matter to U1406 sediments. Mean annual air temperatures (MAATs), derived using the MBT^'_5me soil temperature proxy (based on terrestrially-sourced branched glycerol dialkyl glycerol tetraethers) varied between 15.5 °C and 18.1 °C between 30-22 Ma. From 26.4-24.7 Ma, MAATs rose by 1.5 °C, likely reflecting late Oligocene warming, as documented in benthic oxygen isotope and paleotemperature proxy records from several global locations (including the U1404 alkenone paleotemperature record). MAATs then remained approximately stable, 17 °C, before briefly decreasing by 0.6 °C and then recovering in the earliest Miocene, 23-22 Ma. Overall, our U1406 MAAT record is significantly cooler, 5-9 °C, than U^k'₃₇-SST estimates from the same site and also U1404. We shall discuss the implications of our terrestrial temperature record alongside other geochemical data, including %TOC, bulk carbon isotopes and n-alkanes abundances, in order to elucidate the paleoclimatic and paleoceanographic evolution of the early icehouse North Atlantic.