M. Grant Gross

Oxygen and carbon isotopic composition of limestones and dolomites, bikini and eniwetok atolls

Aragonitic, unconsolidated sediments from the borings on the Eniwetok and Bikini atolls are isotopically identical with unaltered skeletal fragments, whereas the recrystallized limestones exhibit isotopic variations resulting from alteration in meteoric waters during periods of emergence. Dolomites and associated calcites are enriched in O18, perhaps because of interaction with hypersaline brines.

Sediment movement on the continental shelf near Washington and Oregon

The nuclides zinc-65 and cobalt-60 associated with river-borne particulate matter are incorporated in sediment on the Continental Shelf near the Colum- ia River. Changes in the relative concentrations of zinc-65 and cobalt-60 and in the ratio of the activity of zinc-65 and cobalt-60 suggest that radioactive sediment moves northward 12 to 30 kilometers per year along the shelf and 2.5 to 10 kilometers per year westward away from the coast.

Radioactivity of the Columbia River Effluent

Chromium-51 and zinc-65 were detected as far as 115 and 15 kilometers, respectively, from the mouth of the Columbia River; zinc-65 also was found at a few isolated stations. Zirconium-95 and niobium-95 from atmospheric fallout occurred in the river effluent but they were most abundant in surface waters further ofJshore. Distribution of the radionuclides is controlled largely by surface currents and by upwelling of sea water near the coast.

Carbonate Deposits on Plantagenet Bank near Bermuda

Drilling operations on Plantagenet Bank, southwest of the Bermuda Islands, penetrated approximately 20 m of carbonate sediment and dolomite without reaching the substrate. To a depth of about 13 m the subsurface consists of fragments of calcareous algae, foraminifera, mollusks, and coral (listed in order of abundance). Carbonate fragments from 8 and 13 m were diagenetically altered and may be partially consolidated. Poorly ordered and Ca-rich (57 mole per cent CaCO 3 ) dolomite, containing recognizable fragments of coralline algae, was found at 20 m. The C 13 contents of the carbonates are within the range of values observed in the sediments and calcareous skeletal fragments of marine organisms from the Bermuda Islands. The undolomitized sediment is enriched in O 18 with respect to the modern sediments near the Bermuda Islands; the dolomite is enriched in O 18 by 3 to 3.7 per mil with respect to associated carbonate minerals in the cuttings. These differences in O 18 content may be the result of reaction of the carbonate sediment with waters enriched in O 18 .