Sclerosponges as a new potential recorder of environmental changes: Lead in Ceratoporella nicholsoni
Claire E. Lazareth et al., p. 515

The authors test the potential of using the massive basal skeleton of sclerosponges as a new paleoenvironmental recorder. Lead concentrations on a 223-yr-long profile through the calcareous skeleton of a Caribbean Ceratoporella nicholsoni were analyzed using the high-resolution laser-ablation microprobe technique. The lead trend in the specimen displays a general increase from 1760 to 1984, with a major threefold increase after 1930. This lead profile is analogous to results acquired from ice or coral cores and clearly highlights the potential of sclerosponges as a new proxy of environmental changes under tropical latitudes, for time series extending over several centuries.

Can slab melting be caused by flat subduction?
Marc-Andr Gutscher et al., p. 535

Current models of plate subduction (where oceanic crust descends into Earth's mantle at deep sea trenches, as in the circum-Pacific "ring of fire" region) and formation of volcanic chains parallel to the trench are based on a steep angle of descent. Magma generation is considered to occur at depths of roughly 100 km as fluids (mostly water) are released from the lower plate and migrate into the upper mantle "wedge." Here partial melting occurs and the magma ascends upward to form a "volcanic arc." Direct melting of the subducting oceanic crust, once favored in the early 1980s to explain arc magmatism, is now no longer considered to be possible, except in the rare cases of subduction of very hot, young oceanic crust (< 5 million years old). The magmas thus generated have a particular geochemical signature (typically a high silica content and a depletion in heavy rare earth elements) and are called "adakit
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Contact: Ann Cairns
acairns@geosociety.org
303-447-2020 x 156
Geological Society of America
31-May-2000