Late Pliocene stable oxygen isotope record of the Mediterranean Sea

Detailed pollen analyses and oxygen isotope records of three foraminiferal species, Globigerina bulloides, Uvigerina peregrina and Cibicides pachyderma, from the Semaforo and Vrica composite sections (Crotone, southern Italy) have been compared to the global climatic changes depicted by late Pliocene-early Pleistocene foraminiferal d18O records of Site 607 in the North Atlantic, and Hole 653A in the Tyrrhenian basin, West Mediterranean. Major overturns in the mid-altitude vegetation are shown near isotopic stages 82, 60, 58 and 50, at about 2.03 Ma, 1.6 Ma and 1.37 Ma according to the Raymo et al. (1989, doi:10.1029/PA004i004p00413) and Ruddiman et al. (1989, doi:10.1029/PA004i004p00353) timescales. At the same dates, glacial 18O maxima either became higher or display step increases in the western Mediterranean or in the open ocean as well. This suggests that size increases of Northern Hemisphere ice sheets were the driving factor for regional or local marine and continental environmental changes within the Mediterranean basin. Near isotopic stages 62-60, close to the conventional Plio-Pleistocene boundary, the climatic conditions severed enough within the Mediterranean basin to modify the continental environment, as depicted by a sudden increase of Artemisia percentages, while the first significant southward migration of the North Polar Front may have been recorded by an influx of left coiling Neogloboquadrina pachyderma in the central Mediterranean. It also appears that 'Boreal Guests' entered the Mediterranean during phases of 18O enrichment of foraminiferal calcite. There does not seem to be any discrepancy between the climatic concept of the Pliocene-Pleistocene boundary and its chronostratigraphic definition.对来自意大利南部克罗托内（Crotone）地区的塞马福（Semaforo）与弗里卡（Vrica）复合剖面的3种有孔虫——球室球虫（Globigerina bulloides）、远游拟抱球虫（Uvigerina peregrina）及厚壳圆片虫（Cibicides pachyderma）——开展的详细孢粉分析与氧同位素记录，已与北大西洋607站位及西地中海第勒尼安海盆653A钻孔的上新世晚期-更新世早期有孔虫δ¹⁸O记录所揭示的全球气候变化进行了对比。根据Raymo等人（1989，DOI:10.1029/PA004i004p00413）与Ruddiman等人（1989，DOI:10.1029/PA004i004p00353）的年代标尺，中海拔植被的重大更替事件出现在同位素阶段82、60、58和50附近，对应年代约为2.03 Ma、1.6 Ma与1.37 Ma。在上述对应年代，地中海西部与远洋海域的冰期δ¹⁸O最大值要么进一步升高，要么呈现阶梯式增长。这表明北半球冰盖的规模扩张，是地中海盆地内区域性或局地海洋与陆地环境变化的驱动因素。在接近传统上新世-更新世边界的同位素阶段62-60附近，地中海盆地内的气候条件已足够严酷，足以改变陆地环境——这一点可从蒿属（Artemisia）百分含量的突然升高得到佐证；而中地中海海域左旋厚壳新抱球虫（Neogloboquadrina pachyderma）的大量出现，可能记录了北极高纬锋面首次大规模向南迁移的事件。此外，在有孔虫方解石δ¹⁸O富集阶段，“北方来客（Boreal Guests）”也曾侵入地中海海域。上新世-更新世边界的气候概念与其年代地层定义之间，似乎并无任何矛盾之处。