Radionuclide activities, geochemistry, and accumulation rates of sediments in the Gulf of Thailand

Abstract

Downcore concentration profiles of 210Pb, U, and Th isotopes, Al, Fe, Ti, Mn and Sc were measured in sediment box cores collected at 22 stations (16–70 m water depth) covering most of the Thai zone of the Gulf of Thailand. Distributions of excess 210Pb and the detrital elements were used to study spatial variations in sedimentary processes, mineralogy, and geochemistry between different regions of the gulf. Steady-state depositional concentrations and fluxes of excess 210Pb are 3–10 times lower in Gulf of Thailand sediments than in sediments from mid-latitudes in the northern hemisphere, reflecting lower 210Pb inputs from atmospheric fallout at 6–13°N latitude and from lower production of 210Pb from 226Ra in the shallower waters of the Gulf. U and Th concentrations are approximately 2–3 times higher than those in shelf sediments from mid-latitudes of North America, consistent with a higher proportion of granitic source rocks in the Thai environment. Downcore variations in 228Th/232Th activity ratios and in U activities reveal that exchange of interstitial and overlying waters and their dissolved chemicals occurs down to 20 cm in 8 of 10 cores. This benthic exchange may be important in budgets of fluxes of other soluble chemicals in this shallow shelf sea. A net flux of U isotopes from overlying water into Gulf of Thailand sediments occurs, in contrast to their release from sediments of the tropical Amazon shelf. Detectable levels of 137Cs were found only in sediments near the mouth of the largest river, the Chao Phraya. The detrital elements 232Th, 230Th, Al, Ti, and Sc all show relatively uniform downcore concentration profiles. This supports a key assumption in calculations of sediment accumulation rates from downcore profiles of 210Pb activity, that steady-state depositional conditions exist and that basic sediment mineralogy and grain size does not change. 210Pb model derived mass accumulation rates vary between 270 and 490 mg/cm2 per year in the upper Gulf and between 64 and 190 mg/cm2 per year in the central basin. They indicate that suspended sediments and sediment-associated contaminants from the Bangkok area delivered to the upper Gulf are not efficiently transported south into the central Gulf. Al, Ti, Sc, Mn, and Fe sediment concentrations are generally lower than those observed in average shales and crustal materials. This is attributed to greater solubility of these metals during chemical weathering in the hot, humid, organic carbon rich drainage basins of the Gulf. Enrichment factors show that Fe is less and Mn is more efficiently mobilized from the drainage basin than Al, Ti, or Sc. The strong covariance of Al, Ti, and Sc concentrations suggests that they should be equivalently useful normalizing elements to correct for effects of changing grain size and mineralogy on concentrations of other metals with possible anthropogenic inputs. © 1997 Elsevier Science Ltd