Stable Isotopic Composition (d18OCO32–, d13C) of Early Eocene Fish-Apatite from Hole 913B: An Indicator of the Early Norwegian-Greenland Sea Paleosalinity

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 (δ 18 θco, 2~, δ 13 C) 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 R0snaεs Clay Formation (Denmark). These two sites represent fully marine and semimarine conditions, respectively. The δ^Ocα, 2-values of the fish-apatite from Hole 913B are 4.3%o to 8.1%c more negative than the fish-apatite values from DSDP Hole 550, and 1.9%o to 6.1%o more negative than the values from the R0snses Clay Formation. The results indicate reduced salinity in the early Eocene Greenland Basin relative to the open ocean. Using the present salinity/ δ' 8 O 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 R0snaεs Clay Formation as well as DSDP Hole 550, we consider the fish-apatite δ 18 θco, 2-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 δ 18 θco, 2-as an indicator of paleosalinity, because both δ 18 θ and salinity usually are very similar in the uppermost pore water and the overlying water mass. The fish-apatite δ 13 C 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 δ 13 C values,-4.6%c to-10.5%o, are found in the fish-apatite from the very dark sediment of Hole 913B, whereas the highest δ 13 C values, +0.6‰ to-1.7%o, are found in the pale, oxidized sediment of DSDP Hole 550.

Salinity of the Eocene Arctic Ocean from oxygen isotope analysis of fish bone carbonate

“…Overall, the δ 18 O CO3 2− values obtained in this study for the Eocene Arctic Ocean, which range from −6.84‰ to −2.96‰, are higher than those obtained by Andreasson et al [1996] from ODP Hole 913B for the Eocene Norwegian‐Greenland Sea (−6.70‰ to −8.23‰). Analysis of untreated Eocene fish debris from Hole 913B and fully marine northeastern Atlantic ODP Hole 550 in this study yielded stable isotope values very similar to those obtained by Andreasson et al [1996] (Table 5).…”

“…Overall, the δ 18 O CO3 2− values obtained in this study for the Eocene Arctic Ocean, which range from −6.84‰ to −2.96‰, are higher than those obtained by Andreasson et al [1996] from ODP Hole 913B for the Eocene Norwegian‐Greenland Sea (−6.70‰ to −8.23‰). Analysis of untreated Eocene fish debris from Hole 913B and fully marine northeastern Atlantic ODP Hole 550 in this study yielded stable isotope values very similar to those obtained by Andreasson et al [1996] (Table 5). Thus, differences in interlaboratory methods can be ruled out, and although the Arctic Ocean samples were chemically treated prior to analysis while the Hole 913B samples were not, the increases in δ 18 O expected from chemical treatment are too small to account for the differences in δ 18 O observed between the two localities.…”

“…The lowest Eocene δ 13 C values of −16.98‰ and −16.96‰ were recorded in the PETM samples M0004A‐31X‐CC and 30X‐CC, respectively, and the highest value of −8.90‰ was from sample 11X‐3W 46–48 cm, which corresponds to the Azolla event. With high TOC contents (1.5 to >3 wt %) in the Eocene Arctic sediments as compared to the Miocene and younger sediments (<0.4 wt %) [ Expedition 302 Scientists , 2005], it appears that bones buried in sediment with a higher TOC content tend to exhibit lower δ 13 C values than those buried in a more oxidizing environment [ Andreasson et al , 1996].…”

“…The bones analyzed in this study were buried in the upper pore waters of Lomonosov Ridge, which had a water depth no greater than a few hundred meters in the Eocene [ Moore and Expedition 302 Scientists , 2006], and should thereby provide a reasonable approximation of Arctic near‐surface water salinity. Andreasson et al [1996], for example, found that the carbonate of early Eocene fish apatite from the semimarine Røsnæs Clay formation and the fully marine DSDP Hole 550 gave δ 18 O values similar to those of well‐preserved foraminifera from the same samples. An added benefit of performing stable isotope analysis on the structurally bound carbonate of bone rather than phosphate is the acquisition of δ 13 C values.…”

“…In many studies δ 13 C data are not discussed. However, Andreasson et al [1996] found that the organic carbon content of the sediment seemed to have a significant influence on the 13 C content of apatite, with bones found in dark organic‐rich sediment tending to be more enriched in 12 C than those found in pale oxidized sediment.…”