Inconsistencies between14C and short-lived radionuclides-based sediment accumulation rates: Effects of long-term remineralization.
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 AMS14C-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 using239,240Pu,137Cs and excess210Pb (210Pbxs). The measured sediment inventories of210Pbxs,137Cs and239,240Pu 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 of239,240Pu indicates very little sediment mixing. The penetration depths of137Cs and239,240Pu were found to be much deeper than expected and this is attributed to their post-depositional mobility. MAR calculated using14C-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 × 10-4y-1and propose that over the long-term, remineralization of organic-rich sediment affected the long-term MAR, but not the ratio of14C/12C. Thus, the MAR and linear sedimentation rate obtained using14C (and other isotope-based methods) could be erroneous, although14C 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.
Department of Geology, Wayne State University, Detroit, MI 48202, USA. Electronic address: Baskaran@wayne.edu.
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Baskaran
Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA.
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Bianchi
Department of Earth, Atmospheric, and Planetary Sciences and the Purdue Climate Change Research Center, Purdue University, West Lafayette, IN 47907, USA.
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Filley
D002250:Carbon Radioisotopes / Q000032:analysis*; D002588:Cesium Radioisotopes / Q000032:analysis; D019015:Geologic Sediments / Q000737:chemistry*; D060106:Lakes; D007856:Lead Radioisotopes / Q000032:analysis; D014875:Water Pollutants, Radioactive / Q000032:analysis*
D002250:Carbon Radioisotopes; D002588:Cesium Radioisotopes; D007856:Lead Radioisotopes; D014875:Water Pollutants, Radioactive
AMS C-14 dating; Cs-137 dating of lake sediments; Intercomparison of C-14 and short-lived radionuclides-based sediment accumulation rates; Pb-210 dating of lake sediments; Pu-239, 240 dating of lake sediments; Remineralization of organic sediments