NOAA/WDS Paleoclimatology - Paleo-pCO2 Database Atlantic ODP925 40 Million Year Alkenone and CO2 Data

The alkenone-pCO2 methodology has been used to reconstruct the partial pressure of ancient atmospheric carbon dioxide (pCO2) for the past 45 million years of Earth's history (Middle Eocene to Pleistocene epochs). The present long-term CO2 record is a composite of data from multiple ocean localities that express a wide range of oceanographic and algal growth conditions that potentially bias CO2 results. In this study, we present a pCO2 record spanning the past 40 million years from a single marine locality, Ocean Drilling Program Site 925 located in the western equatorial Atlantic Ocean. The trends and absolute values of our new CO2 record site are broadly consistent with previously published multi-site alkenone-CO2 results. However, new pCO2 estimates for the Middle Miocene are notably higher than published records, with average pCO2 concentrations in the range of 400-500 ppm. Our results are generally consistent with recent pCO2 estimates based on boron isotope-pH data and stomatal index records, and suggest that CO2 levels were highest during a period of global warmth associated with the Middle Miocene Climatic Optimum (17-14 million years ago, Ma), followed by a decline in CO2 during the Middle Miocene Climate Transition (approx. 14 Ma). Several relationships remain contrary to expectations. For example, benthic foraminiferal d18O records suggest a period of deglaciation and/or high-latitude warming during the latest Oligocene (27-23 Ma) that, based on our results, occurred concurrently with a long-term decrease in CO2 levels. Additionally, a large positive d18O excursion near the Oligocene-Miocene boundary (the Mi-1 event, approx. 23 Ma), assumed to represent a period of glacial advance and retreat on Antarctica, is difficult to explain by our CO2 record alone given what is known of Antarctic ice sheet history and the strong hysteresis of the East Antarctic Ice Sheet once it has grown to continental dimensions. We also demonstrate that in the Neogene with low CO2 levels, algal carbon concentrating mechanisms and spontaneous biocarbonate-CO2 conversions are likely to play a more important role in algal carbon fixation, which provides a potential bias to the alkenone-pCO2 method.

烯酮-pCO2法（alkenone-pCO2 methodology）已被用于重建地球过去4500万年（始新世中期至更新世）的古大气二氧化碳分压（pCO2）。当前的长期CO2记录为多海洋站位数据的整合结果，这些站位涵盖了广泛的海洋学与藻类生长条件，而这些条件可能会对CO2反演结果产生偏差。本研究基于西赤道大西洋海域的单一海洋钻探站位——大洋钻探计划（Ocean Drilling Program, ODP）925号站位，构建了跨度达4000万年的pCO2记录。本研究获得的该站位CO2记录的变化趋势与绝对值，与已发表的多站位烯酮-pCO2研究结果大体一致。不过，中中新世时期的新pCO2估算值显著高于已公开的记录，平均pCO2浓度介于400~500 ppm之间。我们的结果总体上与近期基于硼同位素-pH数据及气孔指数记录得到的pCO2估算值相符，表明二氧化碳浓度在与中中新世气候适宜期（17~14百万年前，Ma）相关的全球温暖期达到峰值，随后在中中新世气候转型期（约14 Ma）出现下降。部分观测结果仍与预期相悖：例如，底栖有孔虫δ¹⁸O记录显示，渐新世晚期（27~23 Ma）存在一段冰消期和/或高纬度变暖期，而根据本研究结果，该时期恰好与CO₂浓度的长期下降同步。此外，渐新世-中新世边界附近的显著δ¹⁸O正偏移（Mi-1事件，约23 Ma）通常被认为代表了南极冰川的进退过程，但结合已知的南极冰盖演化历史，以及东南极冰盖发育至大陆规模后极强的滞后性，仅依靠本研究的CO₂记录难以解释该事件。本研究还证实，在CO₂浓度较低的新近纪（Neogene），藻类碳浓缩机制与自发的碳酸氢盐-CO₂转化过程可能在藻类固碳过程中发挥更为重要的作用，这为烯酮-pCO2法带来了潜在的偏差来源。