Biomineralization in seeds: developmental trends in isotopic signatures of hackberry

Experiments on hackberry (Celtis) fruits collected over the course of a growing season at two localities were undertaken to assess the isotopic composition of the intricate mineral (CaCO3 and SiO2 · nH2O) and organic structure of hackberry endocarps. Hackberry fruits contain biomineralized aragonite-rich endocarps and have been documented as fossils in a variety of sites in North America and Eastern Europe ranging in age from historical to millions of years old, giving them potential as paleoclimate indicators. In the modern hackberries studied, three distinct stages of development were observed: stage 1 (0–55 days) shows a rapid increase in the whole mass of the hackberry drupe as basic components differentiate; stage 2 (day 55–85) involves a short (30-day) period of slow growth overall, but is concurrent with a sharp rise in the mass of calcium carbonate mineral; stage 3 (day 85 to full maturity) involves a sharp increase in total mass due to mesocarp development. Fruit tissues were found to have lower δ13C values than stem tissues at both sites of collection; there was very little difference between the δ18O values in the endocarp carbonate at the two sites. The most variable δ13C and δ18O values of endocarp were observed in stage 1 development. After stage 2 begins, the δ18O values in endocarp carbonate stabilized. The δ13C value of opal-occluded organic carbon appeared to be constant throughout the growing season.