(Table 2) Stable isotopes in foraminifers at DSDP Leg 78

Analyses of stable isotopes of monospecific planktonic foraminifers (G. quadrilobatus group) and monogeneric benthic foraminifers (Cibicidoides spp.) from late Neogene Atlantic Site 502 and Pacific Site 503 were conducted in order to determine the paleoceanographic changes resulting from the late Neogene uplift of the Panama Isthmus and from climatic cooling.
In general, results at each site are similar to those from previous studies for the late Miocene and late Pliocene time interval, documenting the late Miocene (6 Ma) shift in carbon isotopes and the inferred growth of permanent Northern Hemisphere continental ice sheets beginning about 3.2 Ma. Comparison of Atlantic-Pacific planktonic-benthic isotope data for four stratigraphic intervals (~6-8, ~5-6, ~3-5, and ~2-3 Ma) suggests that increasing isolation of Atlantic and Pacific low-latitude waters may be related to the emergence of the Panama Isthmus. The contrast between Atlantic and Pacific benthic foraminiferal d13C increased in two steps from 0.60 per mil to 1 per mil (the modern contrast) at about 6 Ma and 3 Ma. The first increase (0.15 per mil) may represent the end of previously limited deep-water communication between the Atlantic and Pacific at the present location of Panama. The second increase (0.25 per mil) may be due to increased production of North Atlantic Deep Water. This probably reflects the development of modern deep-sea circulation.
The d18O of planktonic foraminifers begins to increase in Atlantic Site 502 at 4.2 Ma and may reflect the increasing salinity of the North Atlantic Ocean arising from diminishing surface-water exchange across Panama. This increase is clearly shown by contrasting the d18O of Atlantic and Pacific planktonic foraminifers, as well as the d18O of planktonic and benthic foraminifers at Site 502. This inferred increase in surface-water salinity begins at the time of increasing provinciality of Atlantic and Pacific planktonic foraminifers.

本研究针对大西洋502站位与太平洋503站位的新近纪晚期（late Neogene）样品，开展单种浮游有孔虫（planktonic foraminifers，G. quadrilobatus类群）以及单属底栖有孔虫（benthic foraminifers，卷包虫属（Cibicidoides）物种）的稳定同位素（stable isotopes）分析，旨在探究巴拿马地峡（Panama Isthmus）新近纪晚期隆升与气候冷却所引发的古海洋变化。

总体而言，两个站位的研究结果与此前针对晚中新世（late Miocene）至晚上新世（late Pliocene）时段的相关研究结果高度一致，证实了晚中新世（6 Ma）碳同位素（carbon isotopes）偏移现象，以及约3.2 Ma起北半球永久大陆冰盖（Northern Hemisphere continental ice sheets）开始发育的推论。通过对比4个地层间隔（stratigraphic intervals，约6~8 Ma、5~6 Ma、3~5 Ma及2~3 Ma）的大西洋-太平洋浮游-底栖同位素数据，结果显示大西洋与太平洋低纬度水域（low-latitude waters）的隔离程度持续升高，这或与巴拿马地峡的形成有关。大西洋与太平洋底栖有孔虫的d13C差值以两步式增长，在约6 Ma和3 Ma时分别达到0.60‰与1‰（即现代差值水平）。第一次增量（0.15‰）或代表此前局限于巴拿马现今区域的大西洋-太平洋深水连通性宣告终结；第二次增量（0.25‰）则可能源于北大西洋深层水（North Atlantic Deep Water）生成量的增加，这一变化或反映了现代深海环流（deep-sea circulation）系统的形成。

大西洋502站位的浮游有孔虫d18O值自4.2 Ma起开始升高，这或许反映了巴拿马区域表层水交换（surface-water exchange）减弱所导致的北大西洋表层水盐度（salinity）上升。通过对比大西洋与太平洋浮游有孔虫的d18O值，以及502站位浮游与底栖有孔虫的d18O值，可以清晰观测到该盐度升高现象。该推断的表层水盐度升高时间，与大西洋和太平洋浮游有孔虫的生物分区性（provinciality）不断增强的时间相吻合。