Alkenone stable carbon isotopes, planktic foraminfera stable carbon isotopes, alkenone unsaturation indices and calculated sea surface temperatures and atmospheric pCO2 for ODP Site 165-999

Temperature reconstructions indicate that the Pliocene was ~3 °C warmer globally than today, and several recent reconstructions of Pliocene atmospheric CO2 indicate that it was above pre-industrial levels and similar to those likely to be seen this century. However, many of these reconstructions have been of relatively low temporal resolution, meaning that these records may have failed to capture variations associated with the 41 Kyr glacial-interglacial cycles thought to operate in the Pliocene. Here we present a new, high temporal resolution alkenone carbon isotope based record of pCO2 spanning 2.8 to 3.3 million years ago from ODP Site 999. Our record is of high enough resolution (~19 Kyrs) to resolve glacial-interglacial changes beyond the intrinsic uncertainty of the proxy method. The record suggests that Pliocene CO2 levels were relatively stable, exhibiting variation less than 55 ppm. We perform sensitivity studies to investigate the possible effect of changing sea surface temperature, which highlights the importance of accurate and precise SST reconstructions for alkenone palaeobarometry, but demonstrate that these uncertainties do not affect our conclusions of relatively stable pCO2 levels during this interval.

温度重建结果表明，上新世（Pliocene）全球平均气温比现今高约3°C；近期多项上新世大气二氧化碳（CO₂）重建研究显示，当时的CO₂浓度已超过工业化前水平，且与本世纪可能出现的浓度相近。然而，这些重建研究中多数的时间分辨率相对较低，这意味着相关记录可能未能捕捉到与上新世期间存在的4.1万年冰期-间冰期旋回（glacial-interglacial cycles）相关的变化。本文基于海洋钻探计划（Ocean Drilling Program, ODP）999站位的烯酮（alkenone）碳同位素数据，提出了一项新的、高时间分辨率的古大气二氧化碳分压（pCO₂）记录，其时间跨度为280万至330万年前。该记录的分辨率（约1.9万年）足够高，能够解析冰期-间冰期变化，且不受代用指标方法（proxy method）固有不确定性的影响。记录显示，上新世CO₂浓度相对稳定，波动幅度小于55 ppm。我们通过敏感性研究（sensitivity studies）探讨了海表温度（sea surface temperature, SST）变化的潜在影响，结果凸显了准确且精确的SST重建对烯酮古气压法（alkenone palaeobarometry）的重要性；但同时表明，这些不确定性并未影响我们关于该时期pCO₂浓度相对稳定的结论。