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Cretaceous age crust from the western North Atlantic (DSDP Sites 417/418) contains ∼4 wt% CO 2 in carbonate minerals, which is an order of magnitude more than modern ocean crust. We investigated the trace element, Sr, O and C isotope compositions, and attempted U-Pb dating, of 16 veins from Hole 417A and 12 veins from Hole 418A. The veins have δ 13 C of 0 to 3.5 ‰ (VPDB) and 87 Sr/ 86 Sr ranging from 0.7075 to 0.7072 to slightly lower values that are representative of 80–140 Ma seawater admixed with small amounts of crustal Sr. The veins have Eu/Eu* of 1.4 ± 0.1 that is intermediate of seawater and the basaltic host rock and initial 207 Pb/ 206 Pb of 0.86 ± 0.02 that is higher than ancient seawater. Calcite veins have average concentrations of U (4 to 6200 ng/g) that are broadly correlated with Sr (61 to 450 μg/g), but not Pb (10–860 ng/g) or total rare earth elements (0.4–190 μg/g). These data indicate that calcite veins record varying degrees of fluid-rock reaction that influence the U concentration and the Pb isotope composition of the alteration fluid influencing the datability of calcite. Thirteen of the investigated samples were dated and have U/Pb ages between 126 ± 3 Ma and 67 ± 26 Ma. Combined with previous studies of these sites that include a maximum age of 128 ± 5 Ma, these data suggest that about 90% of veins formed within the first ∼28 Ma after accretion, whereas carbonation continued for at least 61 ± 26 Myr. This duration is comparable to the time-scale of the global average crustal heat flow anomaly of ∼65 Ma. The new age data confirm that one reason Cretaceous and Jurassic ocean crust is more enriched in CO 2 than modern ocean crust is that carbonation occurs over extended timescales. However, the newly synthesized data are also consistent with the idea that ocean crust undergoes more rapid carbonation during periods of elevated atmospheric CO 2 , supporting the interpretation that seafloor carbonation functions as a feedback mechanism regulating atmospheric CO 2 .
Kendrick et al. (Sat,) studied this question.