Marine mammals through time: when less is more in studying palaeodiversity

The validity of biological explanations of patterns of palaeodiversity has been called into question owing to an apparent correlation of diversity with the amount of sedimentary rock preserved. However, this claim has largely been based on comprehensive estimates of global marine Phanerozoic diversity, thus raising the question of whether a similar bias applies to the records of smaller, well-defined taxonomic groups. Here, new data on European Caenozoic marine sedimentary rock outcrop area are presented and compared with European occurrences of three groups of marine mammals (cetaceans, pinnipedimorphs and sirenians). Limited evidence was found for a correlation of outcrop area with marine mammal palaeodiversity. In addition, similar patterns were identified in the cetacean and pinnipedimorph diversity data. This may point to the preservation of a genuine biological signal not overwhelmed by geological biases in the marine mammal diversity data, and opens the door to further analyses of both marine mammal evolution and geological bias in other small and well-defined groups of taxa.

Temporal patterns of barren intervals in the Phanerozoic

It has recently been argued that barren intervals of marine sedimentary rock are less common in the Cenozoic than in the Paleozoic, and that this arises as a direct consequence of widespread epeiric seas and the prevalence of dysaerobic conditions at such times. We show, using an independent and more direct measure of rock outcrop through time in western Europe, that barren marine sedimentary rocks do become less frequent toward the present, but that this is not linked to any epeiric-seas effect. The proportion of barren to fossiliferous rock outcrop correlates well with the inferred Phanerozoic marine diversity curve (although more so in the Paleozoic than in the post-Paleozoic), and shows no correlation or only a weak negative correlation with area over which the sediments have been deposited. We therefore concluded that the Phanerozoic trend in fossiliferousness most likely records the degree to which space is occupied in the shallow marine realm.

Cyclicity in the fossil record mirrors rock outcrop area

In a recent article, Rohde & Muller (Rohde & Muller 2005 Nature 434 , 208–210) identified a strong 62 Myr cyclicity in the history of marine diversity through the Phanerozoic. The data they presented were highly convincing, yet they were unable to explain what process might have generated this pattern. A significant correlation between observed genus-level diversity (after removal of long-term trends) and the amount of marine sedimentary rock measured at a surface outcrop in Western Europe is demonstrated. This suggests that cyclicity originates from long-term changes in sedimentary depositional and erosional regimes, and raises the strong possibility that the cyclicity apparent in the record of marine fossils is not a biological signal but a sampling signal.

Origin of radioactive barite sinter, Flybye springs, Northwest Territories, Canada

Cold springs emerging a long the contact between Devonian limestone and shale units in the northwestern Canadian Cordillera are presently depositing a radium-enriched barite sinter. A geological cross section through the springs area shows that groundwaters could circulate through a mainly limestone aquifer to depths of approximately 2 km. Some shales and volcanic rocks associated with the aquifer contain: barium, bound in feldspars; barite, pyrite, and organic matter hosted in shale; and radium in feldspars or produced by the radioactive decay of uranium associated with organic matter hosted in shale. Spring waters are of the [Formula: see text] type characteristic of water that has equilibrated with clay minerals. A subsurface equilibration temperature of 34 °C was determined by silica geothermometry, and 31 °C by magnesium-corrected Na+–K+–Ca2+ geothermometry. Emerging waters are partly mixed with surface runoff and therefore these temperatures represent only minimum values. Assuming a normal geothermal gradient these temperatures indicate minimum groundwater percolation depths of 1 km. The δ34S values of barite sinter samples and one sample of aqueous sulphide range from + 15 to + 23‰, indicating a marine sedimentary rock source for sulphur. The corresponding δ18O values are negative, implying that the bulk of the sulphate oxygen is derived from groundwater during sulphide oxidation. These data suggest that the springs are fed by groundwaters that have percolated to depths of as much as 2 km, passing through an aquifer of Paleozoic marine sedimentary rocks and volcanic rocks. At depth these waters were reducing and probably weakly acidic. They dissolved barium, sulphur, and radium, which were transported to the surface where the water quickly oxidized and precipitated Ba(Ra)SO4.