Maria C. Marcano

True polar wander during the Permo^Triassic

The apparent polar wander path for the Pangea supercontinent is about 35° long for the interval of 295-205Ma, which means that in that interval Pangea rotated over an angle of 35° with respect to the rotation axis about an Euler pole located at the equator. If the rest of the world, largely comprised of the Panthalassa Ocean, rotated about the same Euler pole in the same sense as Pangea, then a good case can be made that true polar wander occurred during the Permo-Triassic. In contrast, if the Panthalassa Ocean moved in a sense opposite to that of Pangea, then true polar wander is not likely to have occured. In the latter case, convergence between Panthalassa and Pangea would have led to subduction of large amounts of ocean crust under the leading edge of Pangea. We have examined the geology of Pangeas leading edge for evidence of such subduction, in the form of Permo-Triassic plutonism, andesitic volcanism and deformation. No such evidence was generally found, except for areas very close to the Euler pole in the western U.S.A. and in displaced terranes that also were in near-equatorial paleolatitudes at the time. We conclude that true polar wander has most likely occurred during the Permo^Triassic at a rate of about 0.4 per million years. 1999 Elsevier Science Ltd. All rights reserved.

Diagenetic incorporation of Sr into aragonitic bivalve shells: implications for chronostratigraphic and palaeoenvironmental interpretations

Aragonite is easily altered during diagenesis, therefore presumed pristine when present. In effect, beyond polymorphic transformation to calcite, alteration paths of aragonite remain poorly understood despite heavy reliance on such material to produce palaeoenvironmental and chronostratigraphic interpretations. Previous work on core material from Southern McMurdo Sound, Antarctica, showed that unlike their calcitic counterparts, seemingly unaltered aragonite shell fragments invariably produced older than expected 87Sr/86Sr ages. In this study, we pursued additional analyses of these aragonite shells and of the porewater of the core to understand this discrepancy. Aragonite mineralogy was reconfirmed and elemental mapping of shell fragments revealed growth lines within the middle layer suggestive of good preservation. The outer layer, however, showed anomalously high Sr concentrations (average 4·5 ± 0·6 mole% SrCO3; ca 25 mmol mol−1 Sr/Ca) and was depleted in 18O and 13C compared to the middle layer, both features inconsistent with pristine material. The δ18O values and Sr concentrations of the porewater were used to model outer layer compositions reasonably well. Coincidentally, porewater Sr isotope composition was in general agreement with the age model of the core only at the aragonite‐bearing interval suggesting that Sr‐isotopic disequilibrium between porewater and the carbonates was the rule rather than the exception in the core. The Sr isotope compositions of the aragonite shells are most likely the result of early diagenesis as suggested by the inconsistent O and C isotope compositions between shell layers and the anomalously high Sr concentrations. We conclude that knowledge of Sr concentration and distribution in shells is critical to determine the viability of Sr stratigraphy and the scale at which it may be applied. Reliance on traditional indicators of lack of alteration, such as cathodoluminescence, Mn‐Fe concentration, and the presence of labile mineralogies to assert chronostratigraphic and palaeoenvironmental questions may produce erroneous conclusions due to obscurely altered material.