Natural Variations in Dissolved Silicon Isotopes Across the Arctic Ocean From the Pacific to the Atlantic

We present measurements of the silicon isotopic composition of silicic acid (delta Si-30-Si(OH)(4)) from seawater collected along a transect following one of the main flow paths of Pacific-origin waters through the Arctic Ocean during the Canadian Arctic GEOTRACES and Distributed Biological Observatory expeditions in the summer of 2015. The delta Si-30-Si(OH)(4) signals track the modification of Pacific-origin waters as they transit from west to east, and reflect the distribution of water masses, the dissolution of biogenic silica (bSiO(2)), and the biological utilization of Si in surface waters. Pacific-origin waters have lower delta Si-30-Si(OH)(4) values than the surrounding water masses and are closely linked with a tongue of nutrient-rich cold water that can be traced back to the Bering Strait. The delta Si-30-Si(OH)(4) measurements indicate that the high [Si(OH)(4)] in the deep waters of Baffin Bay are driven entirely by dissolution of exported bSiO(2). Using an open-system model, we estimate the biogenic Si isotope effect ((30)epsilon) for the Bering and Chukchi Seas, and the Canadian Arctic Archipelago to be (30)epsilon = -1.18 +/- 0.02 parts per thousand (1SE), which agrees very well with laboratory and field estimates. In contrast, the isotope systematics in the Canada Basin and Baffin Bay are better represented by a closed-system model. We also present an approach to estimate production by sea-ice algae using delta Si-30-Si(OH)(4) signals, though a better understanding of the sea-ice/water column Si systematics in the Arctic is needed to confirm the viability of this approach.