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

摘要

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 TEX 86 palaeothermometry. In addition, the stable isotope ratio(δ ~(18)O, δ ~(13)C) 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 TEX 86 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 ~6.4Ma 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 TEX 86-derived temperatures and lower δ ~(18)O _(Bel)-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 δ ~(18)O _(Bel) values, cannot be ruled out in these epicontinental settings. A habitat of the belemnites below the thermocline in perhaps 50-100m water depth may have contributed to the positive δ ~(18)O _(Bel) data as well. The temperature difference between the TEX _(86) and the δ ~(18)O _(Bel) 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‰.