Sable isotope record and sediment composition of Cretaceous samples of Deremara Rise

We estimate tropical Atlantic upper ocean temperatures using oxygen isotope and Mg/Ca ratios in well-preserved planktonic foraminifera extracted from Albian through Santonian black shales recovered during Ocean Drilling Program Leg 207 (North Atlantic Demerara Rise). On the basis of a range of plausible assumptions regarding seawater composition at the time the data support temperatures between 33° and 42°C. In our low-resolution data set spanning ~84-100 Ma a local temperature maximum occurs in the late Turonian, and a possible minimum occurs in the mid to early late Cenomanian. The relation between single species foraminiferal d18O and Mg/Ca suggests that the ratio of magnesium to calcium in the Turonian-Coniacian ocean may have been lower than in the Albian-Cenomanian ocean, perhaps coincident with an ocean 87Sr/86Sr minimum. The carbon isotopic compositions of distinct marine algal biomarkers were measured in the same sediment samples. The d13C values of phytane, combined with foraminiferal d13C and inferred temperatures, were used to estimate atmospheric carbon dioxide concentrations through this interval. Estimates of atmospheric CO2 concentrations range between 600 and 2400 ppmv. Within the uncertainty in the various proxies, there is only a weak overall correspondence between higher (lower) tropical temperatures and more (less) atmospheric CO2. The GENESIS climate model underpredicts tropical Atlantic temperatures inferred from ODP Leg 207 foraminiferal d18O and Mg/Ca when we specify approximate CO2 concentrations estimated from the biomarker isotopes in the same samples. Possible errors in the temperature and CO2 estimates and possible deficiencies in the model are discussed. The potential for and effects of substantially higher atmospheric methane during Cretaceous anoxic events, perhaps derived from high fluxes from the oxygen minimum zone, are considered in light of recent work that shows a quadratic relation between increased methane flux and atmospheric CH4 concentrations. With 50 ppm CH4, GENESIS sea surface temperatures approximate the minimum upper ocean temperatures inferred from proxy data when CO2 concentrations specified to the model are near those inferred using the phytane d13C proxy. However, atmospheric CO2 concentrations of 3500 ppm or more are still required in the model in order to reproduce inferred maximum temperatures.

本研究通过对采自大洋钻探计划（Ocean Drilling Program）第207航次（北大西洋德梅拉拉海隆）所获阿尔布期至桑托期黑色页岩中保存完好的浮游有孔虫（planktonic foraminifera）的氧同位素（oxygen isotope）与镁钙比（Mg/Ca ratios）进行分析，估算热带大西洋上层海水温度。基于一系列合理的当时海水组分假设，本研究的数据显示海水温度介于33℃至42℃之间。在本次覆盖约84 Ma至100 Ma的低分辨率数据集里，晚土仑期出现了局部温度峰值，而中晚塞诺曼期早期可能存在温度谷值。单种有孔虫的δ¹⁸O（d18O）与Mg/Ca比值之间的关联表明，土仑期-康尼亚克期海洋的镁钙比可能低于阿尔布期-塞诺曼期海洋，这或许与海水⁸⁷Sr/⁸⁶Sr比值的最低值同期出现。研究人员对同一沉积物样品中的不同海洋藻类生物标志物（marine algal biomarkers）的碳同位素组成进行了测定。研究人员结合植烷（phytane）的δ¹³C（d13C）值、有孔虫的δ¹³C值以及推断出的海水温度，估算了该段地质时期的大气二氧化碳浓度。大气二氧化碳浓度的估算值介于600至2400体积百万分比（ppmv）之间。在各类代用指标的不确定性范围内，热带温度升高（降低）与大气二氧化碳浓度升高（降低）之间仅存在微弱的整体相关性。当我们采用同一沉积物样品中生物标志物同位素估算得到的近似二氧化碳浓度作为输入参数时，GENESIS气候模型对热带大西洋温度的预测值低于通过大洋钻探计划第207航次有孔虫δ¹⁸O与Mg/Ca比值推断出的温度。本研究还讨论了温度与二氧化碳浓度估算中可能存在的误差，以及模型本身可能存在的缺陷。结合近期一项揭示甲烷通量增加与大气甲烷（CH4）浓度之间存在二次关系的研究，本研究探讨了白垩纪缺氧事件期间大气甲烷浓度大幅升高的可能性及其影响——此类甲烷或源自氧最小带（oxygen minimum zone）的高通量排放。当模型输入的二氧化碳浓度接近通过植烷δ¹³C代用指标推断出的浓度时，若设定大气甲烷浓度为50 ppm，GENESIS模型模拟的海表温度近似等于代用数据推断出的上层海洋最低温度。但要重现代用数据推断出的最高温度，模型仍需要输入3500 ppm及以上的大气二氧化碳浓度。