Geochronology of sediments in the Sabine-Neches estuary, Texas, U.S.A.

The accumulation rate of sediments in the Sabine-Neches estuary, near Beaumont, Texas, was studied in four sediment cores using radioisotopes 210Pb and 239,240Pu. Due to very low and variable activities of unsupported 210Pb in the sediments, the sedimentation rates could not be reliably established using the 210Pb dating method. However, 239,240Pu profiles showed distinct peaks, corresponding to the maximum fallout in 1963, and using these Pu peaks the average sedimentation rates were estimated at 4–5 mm yr−1 in the upper and lower estuary. s assumptions that are commonly used in the 210Pb dating method were tested. It was found that the 226Ra activities were in equilibrium with 238U, and the possible loss of 222Rn was judged to be insignificant. The lack of an exponential profile of excess 210Pb with depth did not appear to be a direct result of sediment mixing, as the post-depositional mixing rates estimated using a numerical mixing model were low compared to other coastal areas. A significant positive correlation between the amount of < 63-μm fraction sediments and the total 210Pb activity was observed in the surface sediments. While the hydraulic residence time was relativelt short in this estuary (∼ 10 days), the average total residence time of 210Pb was ∼ 10 days. Between 50% and 94% of the total 210Pb in the water column was found in the < 0.5-μm fraction, resulting in low Kd-values. The measured inventories of excess 210Pb and 239,240Pu in estuarine sediments were only ∼ 10–34% and ∼ 19–50%, respectively, of the total expected inventories. It appears that the estuarine retention and sediment inventory of these radionuclides, and by analogy, other particle reactive metals, are a function of the hydraulic residence time, grain size, and possibly complexation of these nuclides with DOC.