Inconsistencies between ¹⁴C and short-lived radionuclides-based sediment accumulation rates: Effects of long-term...

¹⁴C is the most widely utilized geochronometer to investigate geological, geochemical and geophysical problems over the past 5 decades. Establishment of precise sedimentation rates is crucial for the reconstruction of paleo-climate, -ecological and - environmental studies when extrapolation of sedimentation rates is utilized for time scales beyond the dating range. However, agreement between short-term and long-term sedimentation rates in anthropogenically unperturbed sediment cores has not been shown. Here we show that the AMS ¹⁴C-based long-term mass accumulation rate (MAR) of an organic-rich (>70%) sediment core from Mud Lake, Florida to be ∼5 times lower than the short-term MAR obtained using ^239,240Pu, ¹³⁷Cs and excess ²¹⁰Pb (²¹⁰Pb_xs). The measured sediment inventories of ²¹⁰Pb_xs, ¹³⁷Cs and ^239,240Pu are comparable to the atmospheric fallout for the sampling site, indicating very little accelerated sediment erosion over the past several decades. Presence of sharp fallout peaks of ^239,240Pu indicates very little sediment mixing. The penetration depths of ¹³⁷Cs and ^239,240Pu were found to be much deeper than expected and this is attributed to their post-depositional mobility. MAR calculated using ¹⁴C-ages in successive layers also indicated decreasing MARs with depth, and was reflective of progressive remineralization. Using first-order kinetics, the sediment remineralization rate was found to be 4.4 × 10^-4 y^-1 and propose that over the long-term, remineralization of organic-rich sediment affected the long-term MAR, but not the ratio of ¹⁴C/¹²C. Thus, the MAR and linear sedimentation rate obtained using ¹⁴C (and other isotope-based methods) could be erroneous, although ¹⁴C ages may not be affected by such remineralization. Long-term remineralization rates of organic matter has a direct bearing on the biogeochemical cycling of elements in aqueous systems and mass balance of elements needs to be taken into consideration.