Award: OCE-1737041

GEOTRACES Project Outcomes Report GP15 Rana Fine and James Happell The goal of the international GEOTRACES program is to understand distributions in the ocean of trace chemical elements and their isotopes (TEIs), which are different forms of the same element. Many of the TEIs are essential for phytoplankton growth (e.g., iron) and are micronutrients. Yet, other TEIs are pollutants (e.g., lead). Essential to understanding how TEIs change, e.g., scavenging and regeneration, and for estimating rates of biochemical processes is to identify water masses that are newly ventilated with atmospheric gases. In response to climate change, ocean waters are warming and becoming more stratified, which in turn has been affecting the ventilation of the ocean with atmospheric gases. The main objective of our grant was to measure the concentration of three man-made tracers of ocean ventilation that can be used to determine how long since a water mass was at the ocean surface in contact with the atmosphere. We use tracers for this research that are manufactured compounds found in the air. The concentrations of these tracers have been increasing in the atmosphere with time, and small amounts dissolve in surface ocean waters. An analogy would be adding a colored dye in the surface ocean, with more dye added each year, and then measuring the concentration of this dye as surface water moves into the interior ocean. The three tracers that were measured are Chlorofluorocarbon-11 (CFC-11), Chlorofluorocarbon-12 (CFC-12) and Sulfur Hexafluoride (SF6). Our specific outcomes are: 1) In 2018, we measured CFC-11, CFC-12 and SF6 on the GEOTRACES cruise GP15 along 152°W from Alaska into the South Pacific (SP). The concentration measurements of the three tracers were made at sea. Tracer ages were calculated, from the ratio of SF6 to CFCs, and for oldest waters the ratio of CFCs to each other. The most robust tracer combination for ocean processes on time scales of less than 35 years is obtained using the SF6/CFC ratios. We calculated tracer ages and identified the relative extent of ventilation and dilution of thermocline, intermediate and bottom waters within which TEIs reside - from the end member sources to downstream locations as waters age and put the GEOTRACES data in the historical context of changing ventilation due to natural and anthropogenic climate change. The age data have been available for other scientists to evaluate how scavenging and regeneration are affected as TEIs are transported downstream. 2) We used tracer ages to calculate rates of biochemically important processes - e.g., apparent oxygen utilization rates, and help to distinguish between contributions from physical and biochemical processes across the different circulation regimes. Along GP15 gases show: well ventilated North Pacific (NP) subtropical gyres (SG), and the westward extension of the oxygen minimum zones (OMZs) in NP and SP. While there is weak ventilation below ~26.8 sig-th in NP as these waters are not directly ventilated by the atmosphere. In comparison with data from 1991, the gas tracers (CFCs and oxygen) have been transported into the equatorial region and there is increased ventilation in the SP SG. In contrast, apparent oxygen utilization (AOU) shows the opposite trends as oxygen with AOU increasing in NP and OMZ and decreasing in SP SG. Tracer ages used to estimate oxygen utilization rates suggest an increase in the NP SG over the past several decades. Thus, we have shown that changes in ventilation over the past three decades vary substantially regionally, due to differences in physical forcing and oxygen demands. These data are a step towards understanding what causes changes in ocean gases due to climate change. Last Modified: 04/04/2022 Submitted by: Rana A Fine