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要旨:
The nitrogen isotopes (δ15N) of the organic matter preserved in
fossil otoliths are a promising tool for reconstructing past
changes in environmental conditions and trophic level. We
analyzed the δ15N of fossil otolith-bound organic matter in Late
Cretaceous Eutawichthys maastrichtiensis, E. zideki and
Pterothrissus sp. otoliths from three different localities along the
U.S. east coast. Despite morphological signs of alteration of the
otolith biomineral material, there is no significant difference in
otolith-bound organic matter δ15N between well-preserved and
poorly preserved otoliths. Both genera showed a ~4‰ increase in
otolith-bound organic matter δ15N from Campanian (10.89 ±
1.02‰ in Eutawichthys spp. and 11.08 ± 0.79‰ in Pterothrissus
sp.) to Maastrichtian (14.87 ± 1.25‰ and 14.78 ± 0.80‰). The
similarity of the change in two separate genera argues against a
trophic level change. Moreover, the proximity of the deposits and
other geographic considerations argue against the locations of the
host sedimentary deposits as the cause. Rather, the N isotopic
change is best interpreted as an environmental (i.e., “baseline”)
signal at the regional scale or greater. The lower δ15N in the
Campanian may be a consequence of its warmer climate,
consistent with warming-correlated declines in foraminifera-
bound δ15N during the Cenozoic. More specifically, our otolith
data are consistent with the foraminifera-based finding that
warmer conditions are associated with a lower global rate of
water column denitrification. This interpretation is speculative,
and δ15N measurements from other fossil types and other regions
are called for. Nevertheless, the large, coherent signal observed
in this first study suggests that the nitrogen isotopes of fossil
otoliths hold important information about the ocean and marine
ecosystems over the Earth history.
