Atmospheric 14C record from Wellington, New Zealand

Trays containing ~2 L of 5 normal NaOH carbonate-free solution are typically exposed for intervals of 1-2 weeks, and the atmospheric CO2 absorbed during that time is recovered by acid evolution. Considerable fractionation occurs during absorption into the NaOH solution, and the standard fractionation correction (Stuiver and Polach 1977) is used to determine a delta 14C value corrected to delta 13C = -25 per mil.
Some samples reported here were taken using BaOH solution or with extended tray exposure times. These variations in procedure do not appear to affect the results (Manning et al. 1990). A few early measurements were made by bubbling air through columns of NaOH for several hours. These samples have higher delta 13C values. Greater details on the sampling methods are provided in Manning et al. (1990) and Rafter and Fergusson (1959).

These measurements of 14C in atmospheric CO2 at Wellington constitute one of the longest isotopic records available, particularly for the Southern Hemisphere, and support the overall decline in atmospheric 14CO2 reported by others. The 14C record for Wellington in 1965 shows a peak occurring roughly 1 year later than that observed in the Northern Hemisphere (Nydal and Lovseth 1983; Levin et al. 1985).

Manning et al. (1990) reported a seasonal component for the Wellington record and a cycle of decreasing amplitude for the seasonal component. Manning et al. (1990) reported that up to 1980, the cycle had a maximum in March and a minimum in August along with a negative anomaly in December. The amplitude of the cycle for this period decreased steadily from a peak-to-peak range of 20 per mil in 1966 to 3 per mil in 1980. From 1980 onwards, Manning et al. (1990) found that a different cycle emerged: the new cycle had an amplitude of ~5 per mil, a maximum in July-August, and a minimum in January.

通常将盛有约2升5当量无碳酸盐氢氧化钠（NaOH）溶液的托盘暴露于空气中1至2周，期间空气中的二氧化碳（CO₂）会被溶液吸收，随后通过酸解析法回收所吸收的CO₂。氢氧化钠溶液吸收CO₂的过程中会发生显著的同位素分馏，因此需采用标准分馏校正方法（Stuiver与Polach，1977），将测得的δ¹⁴C（delta 14C）值校正至δ¹³C（delta 13C） = -25‰的基准。
本文报道的部分样品采用氢氧化钡（BaOH）溶液进行采样，或延长了托盘暴露时长。曼宁等（Manning et al.，1990）指出，这些采样流程的差异并未对实验结果产生显著影响。少数早期测量采用将空气鼓泡通过氢氧化钠柱的方式完成，耗时数小时，此类样品的δ¹³C值偏高。关于采样方法的更多细节可参见Manning et al.（1990）与Rafter和Fergusson（1959）的研究。
此次针对惠灵顿地区大气CO₂中¹⁴C的测量，构建了目前可获取的最长同位素记录之一，尤其针对南半球而言，该记录也佐证了其他研究报道的大气¹⁴CO₂整体下降趋势。1965年惠灵顿地区的¹⁴C记录显示，其浓度峰值出现的时间较北半球观测到的峰值晚约1年（Nydal与Lovseth，1983；Levin et al.，1985）。
Manning等（1990）在惠灵顿的记录中发现了季节性分量，且该季节性分量的周期振幅呈逐渐衰减趋势。研究表明，在1980年之前，该循环的峰值出现在3月，谷值出现在8月，12月还存在一个负异常。此阶段的循环振幅从1966年的峰-峰差值20‰持续下降至1980年的3‰。1980年之后，Manning等（1990）发现出现了新的循环模式：新循环的振幅约为5‰，峰值出现在7-8月，谷值出现在1月。