GEOCHEMISTRY OF SEDIMENTS IN JOHOR STRAIT BETWEEN MALAYSIA AND SINGAPORE

We report the results of a study of the geochemistry of sediments of t he Strait if Johor between the southern tip of peninsular Malaysia and the island nation of Singapore. This shallow channel in the continent al shelf connects the South China Sea to the east with the Strait of M elaka to the west. We analyzed sediment layers collected at 25 locatio ns along the east-west axis of the Strait, from the estuary of the Joh or river at its eastern end to the estuary of the Pulai river at its w estern end, using a box corer to minimize disturbance of surficial lay ers. Both trace metal concentrations and bulk sediment properties chan ge drastically between the relatively pristine open ends of the Strait and the dam-like causeway near the east-west center of the Strait, wh ere the greatest anthropogenic discharges occur and water flows become too sluggish to disperse them. Reasonably ''normal'' muddy sediments with oxygenated surface layers and apparently healthy benthic communit ies are found at both open ends of the Strait. Anoxic, watery, black m uds, smelling of H2S and with no macrobenthic organisms? are found in a zone extending several kilometers on either side of the causeway bui lt in 1924 to facilitate commerce between the island of Singapore and peninsular Malaysia. This zone of anoxic sediment has developed since the causeway was built, due primarily to discharges of sewage wastes w ith their high oxygen demand, and to greatly reduced water dispersion. Total concentrations of V, Cr, Mn, Ni, Cu, Co and possibly Cd and Sc in Johor Strait sediments are lower than concentrations estimated for average shales and mean crustal materials. This is attributed to incre ased solubility of all these elements during intense chemical weatheri ng in the hot, humid tropical climate of the drainage basin. Total con centrations of U and Th are greater in Johor sediments than in both ty pes of average materials, due to greater abundance of granites and mon azites in the Johor drainage basin. Greater than global average concen trations of Pb and Zn are found in surface sediments near the causeway , due principally to additional inputs associated with large volumes o f vehicular traffic across the causeway. Concentrations of As are high er in Johor sediment than in global average materials, for unknown rea sons. Johor Strait sediments have rare earth concentrations less than or equal to those in average shales, and no evidence of enrichments of light rare earth elements from wastes from the large oil refineries s outh of Singapore Island. Excess Pb-210 activities and inventories in sediments of the shallow, low latitude Johor Strait are similar to tho se in nearby Gulf of Thailand sediments. They are several times lower than those in sediments underlying deeper waters from mid-latitude con tinental shelves, due to less in situ production of Pb-210 from decay of parent Ra-226 in the shallower Johor Strait water column, lower inp ut of Pb-210 from atmospheric fallout at lower latitudes, and less riv erine input of particulate Pb-210 to Johor Strait. Johor Strait sedime nt activities and inventories of this natural radionuclide are limited by its supply rather than by particulate scavenging reactions. Sedime nts on the tropical Amazon shelf have similar Pb-210 activities, but i nventories over ten times those in Johor Strait sediments. The differe nces are because of much faster sediment accumulation rates on the Ama zon shelf, and because lateral water transport brings more offshore wa ters with dissolved excess Pb-210 produced in situ on to the Amazon sh elf than into Johor Strait. The low excess Pb-210 activities in Johor Strait sediments often show erratic changes with depth that cannot be reliably modeled by assuming steady stale; constant deposition rate of particles of uniform bulk chemistry, mineralogy and initial unsupport ed Pb-210 activity; and that mixing is limited to a recognizable surfa ce layer. The Pb-210 profiles are still essential in explaining differ ences in As/Sc and Zn, Pb, and Cr concentration profiles in the cores. (C) 1997 Elsevier Science Ltd.