A diatom record of CO2 decline since the late Miocene@en

Extratropical sea surface temperature records from alkenones record a dramatic cooling of up to 17°C over the last ~14 Ma, but the relationship between this cooling and greenhouse gas forcing has been elusive due to sparse and contrasting reconstructions of atmospheric CO2 for the time period. Alkenone carbon isotopic fractionation during photosynthesis has previously been used to estimate changes in pCO2 over this interval, but is complicated by significant changes in cell size of the alkenone-producing coccolithophorids over this time period. In this study, we reconstruct carbon isotopic fractionation during photosynthesis (epsilon p) using organic compounds trapped within the frustules of pennate diatoms in sediments from the Eastern Equatorial Pacific Ocean at Ocean Drilling Program Site 846 over the last ~13 Ma. Physical separation of pennate diatoms prior to measuring carbon isotopic fractionation enables us to obtain a record with constant cell geometry, eliminating this factor of uncertainty in our pCO2 reconstruction. In the past ~11Ma, epsilon p declines from 15.5 to 10.3 per mil. Using the classic diffusive model and taking into account variations in opal content, alkenone concentration and coccolith Sr/Ca as indicators of past productivity and growth rate, and sea surface temperature records from the site, we estimate a decline in pCO2 from 454 (+/-41) to 250 (+/-15) ppmv between ~11 and 6 Ma. Models accounting for changing the significance of active carbon uptake for photosynthesis, which likely produce more accurate CO2 estimates, suggest a significant larger pCO2 decline of up to twice that shown by the classic diffusive model (in average from 794 (+/-233) ppmv at ~11 Ma to 288 (+/-25) ppmv at ~6 Ma, considering growth rates varying between 0.5 and 1.7 day-1). Large uncertainties in the pCO2 estimated between ~8 and 11 Ma using the active uptake model are related to the growth rate used for calculations. Together, these results suggest CO2 forcing for this period of steep decline in temperatures.基于烯酮类化合物（alkenones）重建的 extratropical海表温度（extratropical sea surface temperature）记录显示，过去约14百万年（Ma）间存在幅度高达17℃的显著降温，但由于该时段大气二氧化碳（atmospheric CO₂）重建数据稀少且存在矛盾，此次降温与温室气体强迫（greenhouse gas forcing）之间的关联始终难以厘清。 此前学界曾利用光合作用过程中的碳同位素分馏（carbon isotopic fractionation）估算该时段的分压二氧化碳（pCO₂）变化，但受产烯酮颗石藻（coccolithophorids）细胞尺寸在该时期的显著改变影响，该方法存在较大复杂性。 本研究针对东赤道太平洋大洋钻探计划（Ocean Drilling Program）846站位的沉积物样品，基于其中羽纹硅藻（pennate diatoms）硅质壳（frustules）内捕获的有机化合物，重建了过去约13百万年间光合作用过程中的碳同位素分馏值（εₚ，即epsilon p）。 通过在碳同位素分馏测量前对羽纹硅藻进行物理分选，我们获得了细胞几何形态恒定的记录，从而消除了pCO₂重建中的这一不确定性来源。 在过去约11百万年间，εₚ从15.5‰下降至10.3‰。结合经典扩散模型（diffusive model），并以蛋白石含量、烯酮类化合物浓度及颗石锶钙比作为古初级生产力与生长速率的代用指标，同时整合该站位的海表温度记录，我们估算得到pCO₂在约11 Ma至6 Ma间从454（±41）ppmv下降至250（±15）ppmv。 考虑到光合作用活性碳摄取的重要性发生变化的模型（该模型或可提供更精准的二氧化碳估算结果）显示，pCO₂的下降幅度较经典扩散模型高出近一倍：当生长速率介于0.5~1.7 d⁻¹时，pCO₂平均值从约11 Ma时的794（±233）ppmv降至约6 Ma时的288（±25）ppmv。 利用活性摄取模型估算的约8~11 Ma时段的pCO₂存在较大不确定性，这与计算中采用的生长速率参数相关。 综合上述结果，本研究表明该时段的显著降温可归因于二氧化碳强迫作用。