The impact of leaching on radiocarbon ages of small-sized foraminifera samples

Reliable radiocarbon (C-14) ages of foraminifera are a prerequisite to generate robust high-resolution age depth models or to obtain precise understanding of past carbon cycle dynamics. With the advance of small-scale C-14 measurements, instrumental precision and levels of contamination (extraneous carbon introduced during sample pretreatment or analysis) became increasingly important to consider. To reduce the effect of carbon contamination, an attempt can be made to remove it by leaching the surface with weak acids. Alternatively, mathematical corrections (e.g., subtraction) based on processing blanks can be applied. We report on C-14 analyses of monospecific foraminifera samples compared between different blank corrections (correction against C-14-free CO2, IAEA-C1 and foraminifera) and sample treatments (i) to examine whether chemical pretreatment and mathematical blank subtraction are comparable, and (ii) to determine limitations hindering reliable C-14 dating with ever smaller sample sizes. The data show that chemical pretreatment of foraminifera corrected against IAEA-C1 does remove surface contamination and that the same effect can be achieved for untreated samples that were mathematically corrected for blank values determined from sample size-matched C-14-free foraminifera. Leaching only has a beneficial effect on C-14 data for older samples, where the isotopic difference between untreated and chemically pretreated samples exceeds the analytical precision.