Cycles of Andean mountain building archived in the Amazon Fan.

Cordilleran orogenic systems have complex, polycyclic magmatic and deformation histories, and the timescales and mechanisms of episodic orogenesis are still debated. Here, we show that detrital zircons (DZs) in terrigenous sediment from the late Pleistocene Amazon Fan, found at the terminus of the continent-scale Amazon River-fan system, record multiple, distinct modes of U-Pb crystallization ages and U-Th/He (ZHe) cooling ages that correlate to known South American magmatic and tectonic events. The youngest ZHe ages delineate two recent phases of Andean orogenesis; one in the Late Cretaceous - Paleogene, and another in the Miocene. Frequency analyses of the deep-time Phanerozoic record of DZ U-Pb and ZHe ages demonstrate a strong 72 Myr period in magmatic events, and 92 Myr and 57 Myr periods in crustal cooling. We interpret these results as evidence of changes in upper and lower plate coupling, associated with multiple episodes of magmatism and crustal deformation along the subduction-dominated western margin of South America.

Potentially bioavailable iron produced through benthic cycling in glaciated Arctic fjords of Svalbard.

The Arctic has the highest warming rates on Earth. Glaciated fjord ecosystems, which are hotspots of carbon cycling and burial, are extremely sensitive to this warming. Glaciers are important for the transport of iron from land to sea and supply this essential nutrient to phytoplankton in high-latitude marine ecosystems. However, up to 95% of the glacially-sourced iron settles to sediments close to the glacial source. Our data show that while 0.6-12% of the total glacially-sourced iron is potentially bioavailable, biogeochemical cycling in Arctic fjord sediments converts the glacially-derived iron into more labile phases, generating up to a 9-fold increase in the amount of potentially bioavailable iron. Arctic fjord sediments are thus an important source of potentially bioavailable iron. However, our data suggests that as glaciers retreat onto land the flux of iron to the sediment-water interface may be reduced. Glacial retreat therefore likely impacts iron cycling in coastal marine ecosystems.