DataSheet1_Temperature-dependent carbon isotope fractionation in coccolithophores.ZIP

Introduction: The stable carbon isotope ratio of long-chain alkenones produced by marine haptophyte phytoplankton has often been used to estimate past variations in atmospheric CO2 throughout the Cenozoic. However, previous experimental studies and surveys of alkenones from surface sediment and suspended particulate matter document additional environmental and physiological influences on carbon isotopic fractionation in alkenones.
Methods: To clarify the non-CO2 effects on the alkenone carbon isotope fractionations, an important alkenone producer, Gephyrocapsa oceanica, was cultured in laboratory. To separate effects of different environment parameters, G. oceanica was grown in continuous cultures under a matrix of environmental conditions in order to explore the influence of temperature independently of CO2(aq). Through careful manipulation of the media carbon system, we can control the variation of the media CO2(aq) independently of temperature solubility. Carbon isotope fractionations from alkenones, coccolith, and particulate organic carbon were measured from this steady state system.
Results and Discussion: We find εp in alkenones and particulate organic carbon inversely correlates with temperature, and temperature affects εp more strongly than CO2(aq). The magnitude of the temperature effect can be explained by higher growth rates at warmer temperatures with a similar growth rate dependence as observed in previous cultures in which growth rate was regulated by other factors. Where the past temperature influence on growth rate could be constrained using the UK’37 alkenone index in the same samples, our finding offers an approach to deconvolve an important physiological factor affecting ancient alkenones εp, and may therefore improve past pCO2 estimates.

引言：海洋定鞭藻门浮游植物（marine haptophyte phytoplankton）所产生的长链烯酮（long-chain alkenones）的稳定碳同位素比值，常被用于估算新生代（Cenozoic）以来大气二氧化碳的古浓度变化。然而，以往针对表层沉积物和悬浮颗粒物中烯酮的实验研究与调查显示，烯酮的碳同位素分馏还受到额外的环境与生理因素影响。
方法：为明确非二氧化碳因素对烯酮碳同位素分馏的影响，我们以重要的烯酮生产物种——大洋桥石藻（Gephyrocapsa oceanica）为研究对象开展室内培养实验。为分离不同环境参数的作用，我们设置多组环境条件进行连续培养，以探究温度对分馏的独立影响（排除溶解态二氧化碳（CO₂(aq)）的干扰）。通过精准调控培养基碳体系，我们可在不改变温度溶解度的前提下，独立控制培养基中溶解态二氧化碳的含量。在本稳态培养系统中，我们测定了烯酮、颗石粒（coccolith）与颗粒有机碳（particulate organic carbon）的碳同位素分馏值。
结果与讨论：我们发现，烯酮与颗粒有机碳的碳同位素分馏值εp与温度呈负相关，且温度对εp的影响强度高于溶解态二氧化碳。该温度效应的幅度可通过升温条件下生长速率提升来解释，其生长速率依赖特征与以往研究中以其他因素调控生长速率的培养实验结果一致。若在同一样品中利用UK’37烯酮指数（UK’37 alkenone index）约束古温度对生长速率的影响，本研究结果可为分离影响古烯酮εp的关键生理因素提供可行路径，进而有望提升古大气二氧化碳分压（pCO₂）估算的准确性。