Stable carbon and oxygen isotope record of benthic foraminifera from Paleocene-Eocene sediments

A major change in Cenozoic deep-sea benthic foraminifera occurred in the Atlantic, Indian, and Pacific oceans near the Paleocene/Eocene boundary. Benthic foraminiferal abundance changes began at about 61.5 Ma at Pacific Deep Sea Drilling Project (DSDP) Site 577. A major extinction event followed at 58-57 Ma (between Zones P6a and P6b), and a series of first appearances continued until circa 55.5 Ma (Zone P6c). These faunal changes occurred during a 6°C warming of Pacific bottom water and may indicate that the primary cause was changing temperature. Other potential causes of the faunal turnover include global changes in surface ocean productivity and changing bottom water source regions. Comparison of benthic and planktonic delta13C records requires no change in the ratio of oceanic phosphorous to carbon during the late Paleocene to early Eocene, which weakens the case for (but does not disprove) a change in surface ocean productivity at this time. Interbasinal comparisons of benthic foraminiferal delta13C records document that water with high delta13C values filled the Cape Basin during the late Paleocene and possibly the early Eocene (circa 61-57 Ma), but apparently did not extend into the western basins of the Atlantic. This pattern suggests a supply of Antarctic source water for the Cape Basin and possible tectonic isolation of the western Atlantic basins during at least part of the late Paleocene. Carbon isotope comparisons show that bottom water supply to the Cape Basin was reduced in the early Eocene. Eolian grain size data suggest that a decrease in zonal wind intensity occurred at the end of the Paleocene. These late Paleocene climatic changes (bottom water warming and decreased wind intensity) correspond with evidence for an important global tectonic reorganization and extensive subaerial volcanism, which may have contributed to climatic warming through increased supply of CO2.大西洋、印度洋与太平洋海域中，新生代深海底栖有孔虫（deep-sea benthic foraminifera）在古新世/始新世边界附近发生了重大更替事件。太平洋深海钻探计划（Deep Sea Drilling Project, DSDP）577号站位的底栖有孔虫丰度变化始于约61.5 Ma。58~57 Ma（P6a与P6b带之间）发生了一次大规模灭绝事件，此后一系列新类群的首次出现直至约55.5 Ma（P6c带）才陆续结束。上述动物群变化发生在太平洋底层水升温6℃的时段内，这表明温度变化可能是其主要成因。该动物群更替的其他潜在诱因还包括表层海洋生产力的全球变化以及底层水源区的改变。对底栖与浮游有孔虫的碳同位素δ¹³C（delta13C）记录进行对比后发现，古新世晚期至始新世早期的海洋磷碳比并未发生改变，这削弱了（但并未否定）彼时表层海洋生产力发生变化的可能性。针对底栖有孔虫碳同位素δ¹³C记录的盆地间对比显示，古新世晚期乃至可能的始新世早期（约61~57 Ma），碳同位素δ¹³C值较高的水体充斥于开普盆地（Cape Basin），但显然未延伸至大西洋西部盆地。该分布格局表明，至少在古新世晚期的部分时段，开普盆地的底层水补给源自南极源水，且大西洋西部盆地可能存在构造隔离。碳同位素对比结果显示，始新世早期开普盆地的底层水补给有所减少。风成粒度数据表明，古新世末期纬向风强度出现减弱。这些古新世晚期的气候变化（底层水升温与纬向风强度减弱）与一项重要的全球构造重组及广泛陆上火山活动的证据相吻合，而后者或可通过增加二氧化碳（CO₂）的排放量加剧气候变暖。