Stable carbon and oxygen isotope ratios of Cibicidoides wuellerstorfi, and CaCO3 and sand content of ODP Hole 165-999A

The Late Cenozoic closure of the seaway between the North andSouth American continents is thought to have caused extensive changes in ocean circulation and Northern Hemisphere climate (Keigwin, 1982, doi:10.1126/science.217.4557.350; Maier-Reiners et al., 1990, doi:10.1029/PA005i003p00349). But the timing and consequences of the emergence of the Isthmus of Panama, which closed the seaway, remain controversial (Keigwin, 1982, doi:10.1126/science.217.4557.350; Maier-Reiners et al., 1990, doi:10.1029/PA005i003p00349; Duque-Caro, 1990, doi:10.1016/0031-0182(90)90178-A; Keller et al., 1989, doi:10.1016/0895-9811(89)90028-X; Collins et al., 1996, doi:10.1130/0091-7613(1996)024<0687:TLMPIS>2.3.CO;2). Here we present stable-isotope and carbonate sand-fraction records from Caribbean sediments which, when compared to Atlantic and Pacific palaeoceanographic records, indicate that the closure caused a marked reorganization of ocean circulation starting 4.6 million years ago. Shallowing of the seaway intensified the Gulf Stream and introduced warm and saline water masses to high northern latitudes. These changes strengthened deep-water formation in the Labrador Sea over the next million years - as indicated by an increased deep-water ventilation and carbonate preservation in the Caribbean Sea - and favoured early Pliocene warming of the Northern Hemisphere. The evaporative cooling of surface waters during North Atlantic Deep Water formation would have introduced moisture to the Northern Hemisphere. Although the pronounced intensification of Northern Hemisphere glaciation between 3.1 and 2.5 million years ago substantially lagged the full development of North Atlantic Deep Water formation, we propose that the increased atmospheric moisture content was a necessary precondition for ice-sheet growth, which was then triggered by the incremental changes in the Earth's orbital obliquity.

晚新生代时期，南北美洲大陆之间水道的闭合被认为引发了海洋环流和北半球气候的广泛变化（Keigwin, 1982, doi:10.1126/science.217.4557.350；Maier-Reiners等, 1990, doi:10.1029/PA005i003p00349）。但关闭该水道的巴拿马地峡出现的时间与后果仍存争议（Keigwin, 1982, doi:10.1126/science.217.4557.350；Maier-Reiners等, 1990, doi:10.1029/PA005i003p00349；Duque-Caro, 1990, doi:10.1016/0031-0182(90)90178-A；Keller等, 1989, doi:10.1016/0895-9811(89)90028-X；Collins等, 1996, doi:10.1130/0091-7613(1996)024<0687:TLMPIS>2.3.CO）。本文基于加勒比海沉积物的稳定同位素与碳酸盐砂组分记录，并结合大西洋和太平洋的古海洋学记录，指出该水道闭合始于460万年前，引发了海洋环流的显著重组。水道变浅增强了墨西哥湾流，并将暖盐水团输送至北高纬度地区。这些变化在随后的百万年间增强了拉布拉多海的深层水形成——加勒比海深层水通风增强与碳酸盐保存增加可佐证这一点——并有利于北半球上新世早期的变暖。北大西洋深层水形成过程中表层水的蒸发冷却，可能为北半球带来了水汽。尽管310万至250万年前北半球冰川作用的显著增强，与北大西洋深层水形成的完全确立存在显著滞后，但我们认为大气湿度的增加是冰盖增长的必要前提，而冰盖增长随后由地球轨道倾角的增量变化所触发。