Coastal sediments behind fringing and barrier reefs in Guam and Saipan were analyzed for carbonate, ash, organic matter, and concentrations of HCl-extractable iron, phosphorous, aluminum, silica, and ammonium. Stable C isotope ratios were obtained from samples of sedimentary organic and inorganic matter and macrophytes. The data are used as a first order estimate of the amount and distribution of terrestrial material in variety of sites that were chosen to represent a decreasing terrestrial impact gradient from intertidal discharge of aquifer waters and a true river estuary of an offshore lagoon.

The presence of very small amounts of terrestrial material (particularly iron, ash, and relatively isotopically light C) can be easily detected with simple methods. Muddy estuarine sediments contain less P, isotopically lighter C, and ca. 250 times more Fe than marine sediments. Offshore lagoons within barrier reefs may sequester some materials of apparently terrestrial origin due accumulation from chemically dilute ocean waters in areas of high biological productivity, but this process may be distinguished with the other analyses used here. Between these, an easily resolved gradient is observed that has both geochemical and biochemical causes.

Average accumulation factors (annual terrestrial flux/surface sediment contents) for individual terrigenous materials range from 0.03 to 3.8 for ash and Fe at sites not expected to be impacted by natural erosion of terrestrial material. The data are useful for engineers, developers, and regulatory personnel who need to know existing levels of terrestrial "contaminants" in coastal waters and sediments prior to major changes in land use of watersheds.