Carbon Sources in Fruit Carbonate of <i>Buglossoides arvensis</i> and Consequences for ¹⁴C Dating

Fruit carbonate of Buglossoides arvensis (syn. Lithospermum arvense ) is a valuable dating and paleoenvironmental proxy for late Quaternary deposits and cultural layers because CaCO 3 in fruit is assumed to be accumulated from photosynthetic carbon (C). However, considering the uptake of HCO 3 – by roots from soil solution, the estimated age could be too old depending on the source of HCO 3 – allocated in fruit carbonate. Until now, no studies have assessed the contributions of photosynthetic and soil C to the fruit carbonate. To evaluate this, the allocation of photo-assimilated carbon and root uptake of HCO 3 – was examined by radiocarbon ( 14 C) labeling and tracing. B. arvensis was grown in carbonate-free and carbonate-containing soils (sand and loess, respectively), where 14 C was provided as (1) 14 CO 2 in the atmosphere (5 times shoot pulse labeling), or (2) Na 2 14 CO 3 in soil solution (root-labeling; 5 times by injecting labeled solution into the soil) during one month of fruit development. Distinctly different patterns of 14 C distribution in plant organs after root- and shoot labeling showed the ability of B. arvensis to take up HCO 3 – from soil solution. The highest 14 C activity from root labeling was recovered in roots, followed by shoots, fruit organics, and fruit carbonate. In contrast, 14 C activity after shoot labeling was the highest in shoots, followed by fruit organics, roots and fruit carbonate. Total photo-assimilated C incorporated via shoot labeling in loess-grown plants was 1.51 mg lower than in sand, reflecting the presence of dissolved carbonate (i.e. CaCO 3 ) in loess. Loess carbonate dissolution and root-respired CO 2 in soil solution are both sources of HCO 3 – for root uptake. Considering this dilution effect by carbonates, the total incorporated HCO 3 – comprised 0.15% of C in fruit carbonate after 10 hr of shoot labeling. However, if the incorporated HCO 3 – during 10 hr of shoot labeling is extrapolated for the whole month of fruit development (i.e. 420-hr photoperiod), fruit carbonate in loess-grown plants incorporated approximately 6.3% more HCO 3 – than in sand. Therefore, fruit carbonates from plants grown on calcareous soils may yield overestimated 14 C ages around 500 yr because of a few percentage uptake of HCO 3 – by roots. However, the age overestimation because of HCO 3 – uptake becomes insignificant in fruits older than approximately 11,000 yr due to increasing uncertainties in age determination.