Isotopic composition of recent shark teeth as a proxy for environmental conditions


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Isotopic composition of recent shark teeth as a proxy for environmental conditions
Geochimica et Cosmochimica Acta
Vennemann T.W., Hegner E., Cliff G., Benz G.W.
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The O, C, and Sr isotope compositions of teeth from ten species,
belonging to five families, and three orders of sharks were measured to
determine the influence of habitat. dirt. and possible species-specific
fractionation effects on the isotopic composition of biogenic phosphate
from fish. The sharks were recently caught in subtropical waters off the
KwaZulu-Natal (KZN) coast of South Africa, as well as from cold waters
in Prince William Sound (PWS), Alaska, and Victor Bag (VB), Nunavut.
delta O-18 values of tooth phosphate (delta O-18(p), range from 20.9 to
23.5 parts per thousand, for the KZN sharks. For most species the range
in measured delta O-18(p), values is about 0.6 parts per thousand but it
may be as high as 1.1 parts per thousand for different teeth from a
single shark. Dentine and enameloid within individual teeth have no
apparent differences in delta O-18(p), values. The delta O-18(p), values
of the KZN shark teeth reflect the typical habitat of the studied
species, primarily the thermal structure of the water column off KZN at
depths between 20 and 280 m. The delta O-18(p), values of teeth from
different Greenland sharks from VB and Pacific sleeper sharks from PWS
are very homogeneous. averaging 25.8 and 24.7 parts per thousand.
respectively. These values appear to he in equilibrium with deep (>500
m) ocean waters in each case at temperatures of about -0.30 degreesC or
less. There is little discernable evidence for species-specific
fractionation effects for the oxygen isotope composition of phosphate in
the studied marine fish.
The oxygen isotope composition of carbonate in apatite averages about
9.17 parts per thousand higher than corresponding delta O-18(p), values.
in agreement with equilibrium fractionation between carbonate and
phosphate, but with a large variance ( 1 sigma = +/- 1.5 parts per
thousand). delta O-18(c), values also vary by up to 1% between
enameloid and dentine within ingle teeth, but in a non-systematic way.
Differences in delta C-13 values between carbonate in enameloid and
dentine is also large (up to 8 parts per thousand) but the delta C-13
values vary systematically. Enameloid is always enriched in C-13
compared to dentine and the C-13 content increases with developmental
stage of the teeth. delta C-13 values measured for enameloid (1.6 to 4.8
parts per thousand) appear to approach equilibrium with dissolved ed
inorganic carbon in seawater. In contrast, delta C-13 values for dentine
range from -6.4 to -2.3 parts per thousand for KZN sharks. and -9.0 to
-10.87 parts per thousand for the cold-water sleeper sharks. and are
compatible with a predominantly dietary carbon source. The Sr-87/Sr-86
ratios of teeth from KZN sharks as well as those from PWS and VB are
uniform, averaging 0.709167. Sr content varies from 1270 to 2100 ppm, a
range that is similar to that in well preserved fossilized teeth.
Seawater Sr is thus clearly incorporated in vivo. Concentrations of Sm
and Nd are in the ppb range and contrast the ppm range in fossilized
teeth, indicating a postmortem incorporation of rare earth elements in
apatite of the teeth. Copyright (C) 2001 Elsevier Science Ltd.
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29/09/2012 16:23
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20/08/2019 16:13
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