Stable isotope record of Cretaceous samples of DSDP Hole 71-511

Oxygen isotope data for upper Turonian planktonic foraminifera at Deep Sea Drilling Project Site 511 (Falkland Plateau, 60°S paleolatitude) exhibit an ~2 per mil excursion to values as low as -4.66 per mil (Vienna Peedee belemnite standard; PDB) coincident with the warmest tropical temperature estimates yet obtained for the open ocean. The lowest planktonic foraminifer d18O values suggest that the upper ocean was as warm as 30-32°C. This is an extraordinary temperature for 60°S latitude but is consistent with temperatures estimated from apparently coeval mollusc d18O from nearby James Ross Island (65°S paleolatitude). Glassy textural preservation, a well-defined depth distribution in Site 511 planktonics, low sediment burial temperature (~32°C), and lack of evidence of highly depleted pore waters argue against diagenesis (even solid state diffusion) as the cause of the very depleted planktonic values. The lack of change in benthic foraminifer d18O suggests brackish water capping as the mechanism for the low planktonic d18O values. However, mixing ratio calculations show that the amount of freshwater required to produce a 2 per mil shift in ambient water would drive a 7 psu decrease in salinity. The abundance and diversity of planktonic foraminifera and nannofossils, high planktonic:benthic ratios, and the appearance of keeled foraminifera argue against lower-than-normal marine salinities. Isotope calculations and climate models indicate that we cannot call upon more depleted freshwater d18O to explain this record. Without more late Turonian data, especially from outside the South Atlantic basin, we can currently only speculate on possible causes of this paradoxical record from the core of the Cretaceous greenhouse.深海钻探计划（Deep Sea Drilling Project）511站位（福克兰高原，古纬度60°S）的土仑阶上部浮游有孔虫氧同位素数据显示，存在约2‰的同位素偏移，最低值可达-4.66‰（维也纳佩德贝lemnite标准，即PDB），这与目前公开海域测得的最高热带温度估算值相吻合。最低的浮游有孔虫δ¹⁸O值表明，上层海水温度可达30~32℃。这一温度在60°S纬度下极为异常，但与邻近詹姆斯罗斯岛（古纬度65°S）同期软体动物δ¹⁸O估算的温度一致。该站位浮游有孔虫具有玻璃质结构保存特征、明确的深度分布特征，沉积物埋藏温度较低（约32℃），且未发现孔隙水极度贫化的证据，这些均排除了成岩作用（乃至固态扩散）是导致浮游有孔虫同位素值极度贫化的原因。底栖有孔虫δ¹⁸O未发生变化，这表明淡水帽层是浮游有孔虫δ¹⁸O值偏低的成因机制。然而，混合比计算显示，若要使环境水体的同位素值产生2‰的偏移，所需的淡水补给量会导致盐度降低7实用盐度单位（Practical Salinity Unit，psu）。浮游有孔虫与超微化石的丰度和多样性、较高的浮游/底栖生物比值，以及具壳缘脊有孔虫的出现，均不支持海水盐度低于正常水平的假说。同位素计算与气候模型均表明，无法通过更贫化的淡水δ¹⁸O来解释这一记录。在获取更多土仑阶晚期的数据（尤其是南大西洋盆地以外的采样数据）之前，目前我们仅能对这一白垩纪温室气候核心区域出现的矛盾记录的潜在成因进行推测。