Sea surface temperture reconstruction for ODP Site 189-1172

Relative to the present day, meridional temperature gradients in the Early Eocene age (~56-53 Myr ago) were unusually low, with slightly warmer equatorial regions (Pearson et al., 2007, doi:10.1130/G23175A.1 ) but with much warmer subtropical Arctic (Sluijs et al., 2008, doi:10.1029/2007PA001495) and mid-latitude (Sluijs et al., 2007, doi:10.1038/nature06400) climates. By the end of the Eocene epoch (~34 Myr ago), the first major Antarctic ice sheets had appeared (Zachos et al., 1992, doi:10.1130/0091-7613(1992)020<0569:EOISEO>2.3.CO;2; Barker et al., 2007, doi:10.1016/j.dsr2.2007.07.027), suggesting that major cooling had taken place. Yet the global transition into this icehouse climate remains poorly constrained, as only a few temperature records are available portraying the Cenozoic climatic evolution of the high southern latitudes. Here we present a uniquely continuous and chronostratigraphically well-calibrated TEX86 record of sea surface temperature (SST) from an ocean sediment core in the East Tasman Plateau (palaeolatitude ~65° S). We show that southwest Pacific SSTs rose above present-day tropical values (to ~34° C) during the Early Eocene age (~53 Myr ago) and had gradually decreased to about 21° C by the early Late Eocene age (~36 Myr ago). Our results imply that there was almost no latitudinal SST gradient between subequatorial and subpolar regions during the Early Eocene age (55-50 Myr ago). Thereafter, the latitudinal gradient markedly increased. In theory, if Eocene cooling was largely driven by a decrease in atmospheric greenhouse gas concentration Zachos et al. (2008, doi:10.1038/nature06588), additional processes are required to explain the relative stability of tropical SSTs given that there was more significant cooling at higher latitudes.

与现代相比，始新世早期（约5600万至5300万年前）的经向温度梯度（meridional temperature gradients）异常平缓，赤道区域温度略高于现代（Pearson等人，2007，doi:10.1130/G23175A.1），而亚热带北极地区（Sluijs等人，2008，doi:10.1029/2007PA001495）与中纬度地区（Sluijs等人，2007，doi:10.1038/nature06400）的气候则更为温暖。至始新世末期（约3400万年前），首个大型南极冰盖已然形成（Zachos等人，1992，doi:10.1130/0091-7613(1992)020<0569:EOISEO>2.3.CO;2；Barker等人，2007，doi:10.1016/j.dsr2.2007.07.027），这表明全球曾发生过显著降温。然而，全球向该冰室气候的转变机制仍未得到明确约束，目前仅有少量温度记录可用于刻画南极高纬度地区的新生代气候演化历程。在此，我们基于东塔斯曼高原（古纬度约南纬65°）的海洋沉积物岩芯，获取了一套独一无二且经年代地层学精准校准的TEX86海水表层温度（sea surface temperature, SST）记录。研究结果显示，西南太平洋的SST在始新世早期（约5300万年前）升至高于现代热带海域的水平（约34℃），至晚始新世早期（约3600万年前）时已逐渐降至约21℃。本研究结果表明，在始新世早期（5500万至5000万年前），亚赤道与亚极地区域之间几乎不存在纬向SST梯度；此后，该纬向梯度显著增大。理论上，若始新世的降温主要由大气温室气体浓度降低所驱动（Zachos等人，2008，doi:10.1038/nature06588），那么鉴于高纬度地区降温幅度更为显著，还需要额外的气候过程来解释热带SST的相对稳定性。