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

C-14 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 C-14-based long-term mass accumulation rate (MAR) of an organic rich (>70%) sediment core from Mud Lake, Florida to be similar to 5 times lower than the short-term MAR obtained using (239,240)pb, Cs-137 and excess (210)pb (Pb-210(xs)). The measured sediment inventories of Pb-210(xs), Cs-137 and (PU)-P-239,240 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 Pu-239,Pu-240 indicates very little sediment mixing. The penetration depths of Cs-137 and Pt-239,Pt-240 were found to be much deeper than expected and this is attributed to their post-depositional mobility. MAR calculated using C-14-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 x 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 C-14/C-12. Thus, the MAR and linear sedimentation rate obtained using C-14 (and other isotope-based methods) could be erroneous, although C-14 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. (C) 2016 Elsevier Ltd. All rights reserved.