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

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 ef fects on the isotopic composition of biogenic phosphate from fish. The shar ks were recently caught in subtropical waters off the KwaZulu-Natal (KZN) c oast of South Africa, as well as from cold waters in Prince William Sound ( PWS), Alaska, and Victor Bag (VB), Nunavut. Canada. 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 mea sured delta O-18(p), values is about 0.6 parts per thousand but it may be a s high as 1.1 parts per thousand for different teeth from a single shark. D entine and enameloid within individual teeth have no apparent differences i n delta O-18(p), values. The delta O-18(p), values of the KZN shark teeth r eflect the typical habitat of the studied species, primarily the thermal st ructure of the water column off KZN at depths between 20 and 280 m. The del ta O-18(p), values of teeth from different Greenland sharks from VB and Pac ific 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-specifi c 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 agre ement with equilibrium fractionation between carbonate and phosphate, but w ith 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 t eeth, 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 thousa nd) but the delta C-13 values vary systematically. Enameloid is always enri ched in C-13 compared to dentine and the C-13 content increases with develo pmental stage of the teeth. delta C-13 values measured for enameloid (1.6 t o 4.8 parts per thousand) appear to approach equilibrium with dissolved ed inorganic carbon in seawater. In contrast, delta C-13 values for dentine ra nge 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 w ith 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 0.7 09167. 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 incorp orated in vivo. Concentrations of Sm and Nd are in the ppb range and contra st the ppm range in fossilized teeth, indicating a postmortem incorporation of rare earth elements in apatite of the teeth. Copyright (C) 2001 Elsevie r Science Ltd.