Award: OCE-1737318

Neodymium isotopes(Nd)and rare earth elements(REE)are widely used as tracers of water mass sources and mixing (hence transport) and are diagnostic of the sources and processes that control the distribution of trace elements and isotopes(TEIs)in the oceans.The US GEOTRACES Pacific Meridional Transect (GP15, PMT) has generated Nd isotope data between Alaska and Tahiti, generally close to the 150W meridian. The North Pacific plays a critical role in the ocean REE and Nd cycle, and in the global overturning ocean circulation, as a voluminous endmember that mixes with the rest of the oceans. Prior to GEOTRACES, there was a surprisingly small amount of deep Pacific Nd isotope and rare earth element data. These data indicated that much of the intermediate and deep North Pacific has a nearly uniform Nd isotope ratio (given as Nd, the deviation in parts per 10,000 from chondrites, which we assume are the same as the average for the Earth). The Nd value of Pacific Deep Water (PDW) is mainly between ~-3 and -4, and is much higher than the North Atlantic end-member (~ -13). This large range of ~10 Nd-units between the two global endmembers is what makes Nd isotopes a highly effective tracer of water mass mixing. This Pacific Nd Mystery has been a major open question in finding the answer to the sources and sinks of the REE cycle. An important goal of GP15 was to generate a meridional dataset that fills in a major data gap in the North Pacific that could play an important role in solving it. In fact, we believe that we have solved the Pacific Nd Mystery.We believe it will be a major contribution to understanding the Nd-REE cycle, and the subject of much community discussion topic over the next few years. The GP15 cruise path includes some of the oldest deep waters in the oceans. It includes the Alaskan coastal margins, which may be an important source of REE to the oceans, and it crosses major gradients in biological productivity and dust sources. Along different parts of the transect, the dusts originate from volcanic arcs, Pacific islands, and Asia, and North, Central, and South America, reflecting a very large range of Nd isotope ratios. Upon interaction with water, the changing dust compositions may impart distinct chemical and isotopic signatures. The most important result is that the Nd isotope ratios, by themselves, clearly distinguish the major water masses within the transect, with PDW predominating in the north with Nd values of -3 to -4, and more negative values farther to the south associated with the water masses originating in the Southern Ocean. This indicates that Nd isotopes are, to a first order, behaving conservatively, within the intermediate and deep Pacific. This is an important observation, as the Pacific, until how, has been used as the example of Nd isotopes showing non-conservative behavior. Moreover, the data confirm the existence of the large mass of PDW that shows the typical Pacific Nd value of -3 to -4, the observation leading to the Pacific Nd Mystery. W believe that we have solved the Pacific Nd Mystery, that is, why is the Nd value between ~-3 and -4, and why has it been stable over at least the last 2 Ma? The key to the solution to this puzzle is the combination of the large region in the Pacific that shows the nearly uniform Nd values and extremely high Nd concentration, and the flow of deep water from the Southern Ocean to the North Pacific relative to the GP15 transect. The details are beyond the scope of this report, and a work in progress, but here is the summary. It reflects the circulation path of the deep water, and the mechanism by which PDW obtains its REE and Nd isotope fingerprint. Deep water flows from the Southern Ocean towards the northwest Pacific and eventually ends up in the GP15 region in the north-central and northeast Pacific. It leaves the Southern Ocean with Nd ~ -8 to -9, and as it flows to the northwest Pacific it interacts with high Nd products of western Pacific volcanism, becoming more positive over its transport path, reaching values of ~-3 to -4 near Japan and Kamchatka. At the same time [Nd] increases. By the time the water reaches the northern part of GP15, it has the characteristic North Pacific Nd fingerprint and by far the highest [Nd] in the oceans. This appears to be the Nd-REE end product of the global ocean circulation. Its high [Nd] has the effect of stabilizing (buffering) the Nd value and dispersing it over a large region of the North Pacific, which has resulted in the North Pacific showing stability as a global Nd end-member over the last 2 Ma. Last Modified: 03/26/2024 Submitted by: RobertFAnderson