Microfossils Reveal the Workings of a Water Planet

863 Knowing past changes in surface ocean salinity would provide a powerful tracer of the hydrologic cycle, yielding climatic information such as the amount of water vapor in the atmosphere or meridional shifts in moisture sources. Water vapor itself is a potent greenhouse gas, with the potential to further heat an already warm atmosphere. Its equator-topole distribution on the globe affects Earth’s geographic climate zones, ranging from deserts to rainforests, with obvious implications for world populations. Holbourn et al. (2010, p. 783 in this issue of Geology) show how geochemical records of surface ocean−dwelling foraminifera can be used to reconstruct changes in one aspect of the hydrologic cycle, the tropical-subtropical rain belt during the middle Miocene. By combining different tracers preserved by the same zooplankton microfossil, Holbourn et al. are able to determine elusive aspects of the hydrologic cycle such as surface water salinity. Tracking past changes in the hydrologic cycle is a diffi cult task because evaporation versus precipitation patterns (or simply put, surface ocean salinity) do not leave a unique trace in the geologic record. We can, however, indirectly assess large-scale changes in the hydrologic cycle, such as the amount of fresh water locked up in polar ice sheets, using the oxygen isotopic composition of the calcium carbonate tests of foraminifera. Polar ice sheets concentrate water molecules with the lighter of the oxygen isotopes, 16 O, and their growth and decay brings about changes in the oxygen isotopic composition of seawater. The effect is ultimately due to a slight tendency of water molecules tagged with