Award: OCE-1436666

This project was part of a large, multi-institution, multi-national program, Arctic GEOTRACES (AGT), which had as its goal the measurement of a wide range of trace substances and their isotope ratios in the Arctic Ocean. The main focus is on ?Rare Earths and Elements?, which live at the high-end of the Periodic Table. These appear in miniscule quantities in the ocean (often in picomol concentrations: trillionths of a mol per kilogram of water). This makes them difficult to sample for and to measure accurately. The 2015 AGT cruises were the first time that these elements have been sampled together, and on a transect across the whole Arctic. Our role was to measure chemical properties of the Arctic Ocean water that would indicate where the water in our samples originated and how long they had traveled from the last time they were at the ocean surface to the place we drew a sample. (We call these ?tracer ages?, or ?transit times?.) Since water mixes as it flows, a single sample is typically decomposed into water from several sources (in the Arctic these might be Pacific inflow; Atlantic inflow; river runoff; water from different continental shelf areas). So our methods yield ?water mass fractions? and ?transit time distributions?. We sampled to measure the isotopes of water (i.e.: 18O/16O or 2H/1H in the water molecule), tritium (3H), helium and its isotope ratios, neon, SF6 (a manmade industrial chemical), and 14C (the radioactive isotope of carbon). Water isotopes give us information about how water has been transformed by evaporation and freezing, which indicate the influence of river runoff or sea-ice formation and melting. We combined these data with other measurements to determine ?water mass fractions?. The helium, tritium, SF6 and 14C all contribute to knowing the tracer age/transit time of the water. Each water mass will have traveled a different route to our sample site, so we determine ?transit time distributions?. These data establish the approximate travel pathways of water in the GEOTRACES samples, and establish a physical context in which the chemical results of other GEOTRACES researchers can interpret their measurements. Comparing our data with data on similar tracks from 2005 and 1994, we showed that between North America and the Lomonosov Ridge, the Canadian Basin has been freshening. Our finding was supportive of other work showing the impact of freshening in the Pacific on the Arctic. However, we added detail, showing that the freshening is quite uneven, and that in some places it is from increased river runoff or changes in sea-ice meltwater distributions. We brought together data from 25 years of cruises to make maps of the tracer age distributions in three layers of the Arctic waters: the ?Fram Strait Branch Water?, the ?Barent?s Sea Branch Water?, and the ?Halocline?. Using the relative tracer ages of these waters, we were able to confirm that Atlantic water entering the Arctic flows relatively quickly along topographic features in the sea floor, and the spreads more slowly over the ocean basins. This is somewhat surprising because the ocean ridges ?guiding? these waters are hundreds, even thousands, of meters below. We were able to assign approximate speeds to most of these topographically guided currents, and showed that where the current splits, for example where the Lomonosov Ridge branches off the Siberian continental shelf, the currents slow down by about a factor of two. We showed that the age of the deep waters below about 2000 meters in the Canadian Basin are at least 450 years old; that they are essentially relict; but that it is likely they were ?ventilated? (renovated with surface water) episodically since the last ice age. In the Arctic Ocean, water density is mainly determined by salinity, and the transition from light, relatively fresh, surface water and dense, salty, water below is called the ?halocline?. The halocline itself is layered (upper and lower haloclines), which can be seen, for example, in the nutrient concentrations or in temperature profiles. We used estimates of transit times to show that the lower halocline is closely tied to the (topographically steered) flow below, but the upper halocline is linked to the surface layer, which is set in motion mainly by the drag of sea-ice drift and winds. Thus, the halocline has very complicated dynamics, and likely includes ?pockets? of very slowly moving waters where there are more vigorous motions both above and below. Our data are contributing to analyses by other GEOTRACES projects, including: demonstration of the utility of gallium as a tracer for Atlantic inflow in the Arctic; determining the composition of water in the Trans-Polar Drift between Siberia and Greenland; study of a large, persistent eddy near the Canadian continental shelf; and how rare elements are distributed in the halocline, and what their likely transit pathways are. Last Modified: 05/20/2021 Submitted by: Robert Newton