Fruit carbonate in Lithospermeae: 14C, stable isotope composition and potential as a paleoenvrionmental proxy

The tribe Lithospermeae (fam. Boraginaceae) represents one of very few taxa vascular plants that accumulate appreciable amounts of calcium carbonate in their tissues. The CaCO₃ is localized in the pericarp sclerenchyma, which makes their small fruits (nutlets) mechanically durable and provides their good preservation in sediments and cultural layers. Fossil Lithospermeae fruits appear as whitish, slightly elongated entities, 3-5 mm in length.  At archaeological sites, the nutlets can be of diverse origin: in most contexts they represent carpological evidence for weed flora of the past, however, some findings suggest that they were used for decorative purposes (beads etc.).  

Here we overview the potential use of fruit carbonate of Lithospermeae in paleoecological research.   

¹⁴C-dating: Fruit carbonate of the taxon can be successfully dated with radiocarbon.  

The ¹⁴C concentration in the CaCO₃ fraction of modern nutlets is well-correlated to the recent atmospheric ¹⁴C levels. Radiocarbon ages of old nutlets are in good correspondence with the age ranges of archaeological contexts. Obviously, fruit carbonate can represent a geochemically closed system for millennia in sediment environments.               

δ¹⁸O values: Our data based on an array of herbarium exemplars of Lithospermeae, suggest that the δ¹⁸O of fruit carbonate is distinctively sensitive to the amount of atmospheric precipitation during the warm season. The degree of correlation between δ¹⁸O and local air temperatures is lower.

We further performed an experiment on gromwell (Buglossoides arvensis (L.) I.M.Johnst), irrigated by water with different oxygen isotope signatures. The δ¹⁸O values of fruit CaCO₃ showed correlation to the δ¹⁸O of irrigation water. The oxygen isotope fractionation in fruit carbonate turned out to be surprisingly low with 1000lnα = 4.72±3.49, which is relatively close to foraminiferal CaCO₃.

 δ¹³C values: In contrast to the oxygen isotope signature, we did not find a strong correlation of the δ¹³C values of fruit carbonate to precipitation and temperature.  However, the photosynthetic origin of carbon in fruit CaCO₃ admits a possibility of some links of δ¹³C to ambient factors.