Reconstruction of vertical temperature gradients in past oceans - Proxy data from the Hauterivian-early Barremian (Early Cretaceous) of the Boreal Realm

A total of 25 mudstone samples from two sections in northwest Germany and northeast England (palaeolatitude ~ 40°N), respectively, of Hauterivian and early Barremian age were analysed using TEX86 palaeothermometry. In addition, the stable isotope ratio (δ18O, δ13C) and trace element content (Mg, Sr, Fe, and Mn) of 138 belemnite guards from the same two outcrops were determined to reconstruct past sea water temperatures.The TEX86 based sea surface temperatures were constantly warm (24-26 °C) throughout the entire Hauterivian for both sections and even warmer for the early Barremian (27-30 °C). Stable water temperatures prevailed over a period of ~ 6.4 Ma in the southernmost part of the Boreal Realm. These findings clearly support the view of warm equable conditions for the Hauterivian at least for mid latitudinal settings. A constant off-set between higher TEX86-derived temperatures and lower δ18OBel-derived temperatures was observed. This may be explained by either increased sea water salinities, a deep dwelling habitat of belemnites or a combination of both. A by 3‰ higher salinity of the sea water below the thermocline, which would at least partly explain the by ~ 2‰ too positive δ18OBel values, cannot be ruled out in these epicontinental settings. A habitat of the belemnites below the thermocline in perhaps 50-100 m water depth may have contributed to the positive δ18OBel data as well. The temperature difference between the TEX86 and the δ18OBel findings might therefore reflect a vertical temperature gradient of the water column of 4-5 °C and a salinity of the subsurface waters of 38‰.

► TEX86 data record warm T [°C] for the Hauterivian for a palaeolatitude of ~ 40°N. ► A constant off-set of warmer TEX86 and colder δ18OBel data has been observed. ► Causes include: salinity, deep water habitat of belemnites, and gradual changes of δ18O seawater.