B/Ca ratios in planktonic foraminifera@en

Boron isotope systematics indicate that boron incorporation into foraminiferal CaCO3 is determined by the partition coefficient, KD = [B/Ca](CaCO3)/[B(OH)4**-/HCO3**-](seawater), and [B(OH)4-/HCO3-](seawater), providing, in principle, a method to estimate seawater pH and PCO2. We have measured B/Ca ratios in Globigerina bulloides and Globorotaliainflata for a series of core tops from the North Atlantic and the Southern Ocean and in Globigerinoides ruber (white) from Ocean Drilling Program (ODP) site 668B on the Sierra Leone Rise in the eastern equatorial Atlantic. B/Ca ratios in these species of planktonic foraminifera seem unaffected by dissolution on the seafloor. KD shows a strong species-specific dependence on calcification temperature, which can be corrected for using the Mg/Ca temperature proxy. A preliminary study of G. inflata from Southern Ocean sediment core CHAT 16K suggests that temperature-corrected B/Ca was ~30% higher during the last glacial. Correspondingly, pH was 0.15 units higher and aqueous PCO2 was 95 matm lower at this site at the Last Glacial Maximum. The covariation between reconstructed PCO2 and the atmospheric pCO2 from the Vostok ice core demonstrates the feasibility of using B/Ca in planktonic foraminifera for reconstructing past variations in pH and PCO2.硼同位素系统学（Boron isotope systematics）研究表明，有孔虫碳酸钙（foraminiferal CaCO3）中硼的赋存受控于分配系数（partition coefficient）KD，其中KD = [B/Ca](碳酸钙)/[四羟基合硼酸根/碳酸氢根(B(OH)₄⁻/HCO₃⁻)](海水)，而海水的[B(OH)₄⁻/HCO₃⁻]比值原则上为海水pH值与二氧化碳分压（PCO2）的估算提供了可行方法。我们针对北大西洋与南大洋的多组表层岩芯，测定了布氏球房虫（Globigerina bulloides）及膨胀拟抱球虫（Globorotalia inflata）的B/Ca比值；同时对东赤道大西洋塞拉利昂海隆大洋钻探计划（Ocean Drilling Program, ODP）668B站位的白色红拟抱球虫（Globigerinoides ruber, white）开展了相关测试。这类浮游有孔虫（planktonic foraminifera）的B/Ca比值似乎不受海底溶解作用的影响。分配系数KD对钙化温度（calcification temperature）存在显著的种属特异性依赖，该效应可通过Mg/Ca温度代用指标（Mg/Ca temperature proxy）进行校正。对南大洋沉积岩芯CHAT 16K中膨胀拟抱球虫的初步研究显示，末次冰期时经温度校正的B/Ca比值较现代高出约30%。对应地，该站位末次冰盛期的海水pH值升高0.15个单位，水体二氧化碳分压降低95毫大气压（matm）。重建得到的水体二氧化碳分压与沃斯托克冰芯（Vostok ice core）记录的大气pCO2之间的协同变化，证实了利用浮游有孔虫的B/Ca比值重建过去海水pH与二氧化碳分压变化的可行性。