STABLE ISOTOPIC COMPOSITION (DELTA18OCO32-, DELTA13C ) OF EARLY EOCENE FISH-APATITE FROM HOLE 913B: AN INDICATOR OF THE EARLY NORWEGIAN-GREENLAND SEA PALEOSALINITY

Fredrik P. Andreasson, Birger Schmitz, and Dorothee Spiegler

ABSTRACT

  Fish-apatite (teeth and bone fragments) of early Eocene age from Ocean Drilling Program Hole 913B (Greenland Basin) was analyzed, in the absence of biogenic calcite, for stable isotopic (delta18OCO32-, delta13C) composition to determine paleosalinity. Comparisons are made with isotopic results for early Eocene fish-apatite from Deep Sea Drilling Project (DSDP) Hole 550 (northeastern Atlantic) and the Røsnæs Clay Formation (Denmark). These two sites represent fully marine and semimarine conditions, respectively. The delta18OCO32- values of the fish-apatite from Hole 913B are 4.3‰ to 6.1‰ more negative than the values from the Røsnæs Clay Formation. The results indicate reduced salinity in the early Eocene Greenland Basin relative to the open ocean. Using the present salinity/delta18O relationship in the North Atlantic, the salinity in the Greenland Basin was 22 ppt to 28 ppt. The reduced salinity is in agreement with an isolated early Eocene Greenland Basin, as suggested in earlier geophysical and paleontological studies. It is also likely that other parts of the Norwegian-Greenland Sea, such as the Lofoten Basin, exhibited brackish water conditions.
  Because of similar oxygen-isotopic composition of fish-apatite and excellently preserved foraminifer tests in the samples from the Røsnæs Clay Formation as well as DSDP Hole 550, we consider the fish-apatite delta18OCO32- to be a reliable instrument for paleosalinity determination. It is possible that the fish-apatite was exposed to diagenesis and isotopic reequilibration shortly after deposition on the seafloor. This should not, however, reduce the possibility of using delta18OCO32- as an indicator of paleosalinity, because both delta18O and salinity usually are very similar in the uppermost pore water and the overlying water mass. The fish-apatite delta18O values may support that early diagenesis has affected the original isotopic signal. The values seem to be related to the organic carbon content of the sediment as the lowest delta18O values, -4.6‰ to -10.5‰, are found in the fish-apatite from the very dark sediment of Hole 913B, whereas the highest delta18O values, +06‰ to -1.7‰, are found in the pale, oxidized sediment of DSDP Hole 550.

Date of initial receipt: 31 July 1995
Date of acceptance: 31 January 1996


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