NOAA/WDS Paleoclimatology - Paleo-pCO2 Database ODP1264 5 Million Year Boron Isotope and CO2 Data

During the past five million yrs, benthic d18O records indicate a large range of climates, from warmer than today during the Pliocene Warm Period to considerably colder during glacials. Antarctic ice cores have revealed Pleistocene glacial-interglacial CO2 variability of 60-100 ppm, while sea level fluctuations of typically 125 m are documented by proxy data. However, in the pre-ice core period, CO2 and sea level proxy data are scarce and there is disagreement between different proxies and different records of the same proxy. This hampers comprehensive understanding of the long-term relations between CO2, sea level and climate. Here, we drive a coupled climate-ice sheet model over the past five million years, inversely forced by a stacked benthic d18O record. We obtain continuous simulations of benthic d18O, sea level and CO2 that are mutually consistent. Our model shows CO2 concentrations of 300 to 470 ppm during the Early Pliocene. Furthermore, we simulate strong CO2 variability during the Pliocene and Early Pleistocene. These features are broadly supported by existing and new d11B-based proxy CO2 data, but less by alkenone-based records. The simulated concentrations and variations therein are larger than expected from global mean temperature changes. Our findings thus suggest a smaller Earth System Sensitivity than previously thought. This is explained by a more restricted role of land ice variability in the Pliocene. The largest uncertainty in our simulation arises from the mass balance formulation of East Antarctica, which governs the variability in sea level, but only modestly affects the modeled CO2 concentrations.

在过去五百万年间，底栖δ¹⁸O（benthic d18O）记录揭示了跨度极大的气候区间：从上新世暖期（Pliocene Warm Period）较现代更为温暖的气候，到冰期（glacials）显著偏冷的气候。南极冰芯已揭示更新世冰期-间冰期期间二氧化碳（CO₂）的变幅为60~100 ppm，而代用资料（proxy data）记录显示海平面波动幅度通常可达125米。然而，在冰芯记录出现之前的时期，二氧化碳与海平面的代用资料极为匮乏，且不同代用指标之间、同一代用指标的不同记录之间均存在分歧。这一现状阻碍了我们对二氧化碳、海平面与气候之间长期关联的全面认知。本研究基于堆叠的底栖δ¹⁸O记录作为反演强迫条件，驱动耦合气候-冰盖模型（coupled climate-ice sheet model）开展过去五百万年的模拟，最终得到了彼此自洽的底栖δ¹⁸O、海平面与二氧化碳连续模拟结果。模型模拟显示，上新世早期的二氧化碳浓度介于300至470 ppm之间。此外，我们模拟得到上新世与更新世早期存在显著的二氧化碳波动。这些特征得到了现有及新发布的基于d¹¹B（d11B）的代用二氧化碳资料的广泛支持，但基于烯酮（alkenone）的记录对其支持度相对较弱。模拟得到的二氧化碳浓度及其波动幅度均大于全球平均温度变化所预期的水平。因此，本研究结果表明地球系统敏感性（Earth System Sensitivity）较此前的认知更低，这一现象可由上新世时期陆地冰盖变化的受限作用加以解释。本模拟中最大的不确定性源自东南极冰盖的质量平衡参数化方案：该方案决定了海平面的波动幅度，但仅对模拟的二氧化碳浓度产生微弱影响。