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Radium‐226(226Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full‐depth ocean section of 226Ra activities (T1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of 226Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher 226Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with 226Ra inputs primarily resulting from the decay of 230Th in sediments. Further, dissolved 226Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non‐conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non‐conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of 226Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non‐conservative processes that shape their respective distributions. Key Points: Water mass analysis shows significant shallow deficits and deep excesses of dissolved 226Ra and Ba in the Pacific Ocean along the GP15Shallow deficits seem driven by BaSO4 precipitation, deep excesses reflect benthic inputs that are more significant for 226Ra than for BaBudgets suggest that regional barium fluxes nearly balance, and the Pacific is likely a significant source of radium‐226 to the global ocean [ABSTRACT FROM AUTHOR]

The half‐life of 228Ra (5.7 years) aligns well with near‐surface and near‐bottom ocean mixing timescales. Because 228Ra is sourced from sediments, regions of enhanced activity represent water that has recently interacted with sediments on the continental margin or seabed. The GP15 meridional transect from Alaska to Tahiti along152°W encountered several regions in the upper ocean where 228Ra was enriched. These enrichments follow surface and subsurface ocean current patterns and pair with earlier measurements of 228Ra and transient radionuclides to reveal the origins of these enriched regions. An enriched region at Alaska margin stations 1–3 was sourced locally but did not extend to the Alaskan trench at station 4. A large shallow region between 47° and 32°N. was sourced from the west by the North Pacific Current; another shallow enriched region between 11° and 5° N was also sourced from the west by the North Equatorial Countercurrent. Subsurface enrichments (100–400 m) between 18 and 47°N were associated with Central Mode Water and North Pacific Intermediate Water. The 228Ra activities in the upper Pacific were six times lower than activities in the Atlantic. In deep waters the primary enrichment was 27°–47°N. Two stations (32° and 37°N) were especially enriched, having near‐bottom inventories several times greater than other stations. With these two exceptions the remaining Pacific stations exhibited averaged inventories lower than those in the Atlantic. There was one region of enriched 223Ra (half‐life = 11 days) above the Puna Ridge near Hawaii. Plain Language Summary: The international GEOTRACES program aims to identify processes and quantify fluxes that control the distribution of trace elements and isotopes (TEIs) in the ocean. Some TEIs are important micronutrients, which may control biological productivity. Others may become concentrated as they pass up the food web and reach potentially harmful levels in some seafood. But measurements of these TEIs are not adequate to determine controlling processes or quantify their fluxes in the marine environment. Radionuclide tracers provide a means to link TEI measurements with fluxes and processes. Here we employ measurements of 228Radium (half‐life = 5.75 years) to trace water movements and current speeds in the upper Pacific and to determine regions near the seabed where sediment‐water interactions are most intense. The upper ocean measurements confirmed a broad circulation pattern that brought water from the Asian margin to the northeast Pacific at speeds of 2–8 cm/sec, in agreement with other estimates of these current speeds. Near the seabed we discovered a region of enhanced 228Ra activity indicating intense sediment‐water interactions. Such regions may control the release of TEIs from the seabed to the water column. We anticipate others will utilize these data to elucidate aspects of other TEI distributions. Key Points: Several open‐ocean regions of enriched 228Ra were documented in the North and Equatorial Pacific at 152°WThe upper ocean 228Ra‐enriched regions correspond to major surface and subsurface currents transporting water from the Asian marginOne seabed region near 32°N was especially enriched in 228Ra [ABSTRACT FROM AUTHOR]

Northern lakes are a source of greenhouse gases to the atmosphere and contribute substantially to the global carbon budget. However, the sources of methane (CH4) to northern lakes are poorly constrained limiting our ability to the assess impacts of future Arctic change. Here we present measurements of the natural groundwater tracer, radon, and CH4 in a shallow lake on the Yukon-Kuskokwim Delta, AK and quantify groundwater discharge rates and fluxes of groundwater-derived CH4. We found that groundwater was significantly enriched (2000%) in radon and CH4 relative to lake water. Using a mass balance approach, we calculated average groundwater fluxes of 1.2 ± 0.6 and 4.3 ± 2.0 cm day−1, respectively as conservative and upper limit estimates. Groundwater CH4 fluxes were 7—24 mmol m−2 day−1 and significantly exceeded diffusive air–water CH4 fluxes (1.3–2.3 mmol m−2 day−1) from the lake to the atmosphere, suggesting that groundwater is an important source of CH4 to Arctic lakes and may drive observed CH4 emissions. Isotopic signatures of CH4 were depleted in groundwaters, consistent with microbial production. Higher methane concentrations in groundwater compared to other high latitude lakes were likely the source of the comparatively higher CH4 diffusive fluxes, as compared to those reported previously in high latitude lakes. These findings indicate that deltaic lakes across warmer permafrost regions may act as important hotspots for CH4 release across Arctic landscapes. [ABSTRACT FROM AUTHOR]

Five sediment cores were collected along a cruise tract from Hawaii to Alaska in August 2017 (C-Disk-IV cruise) with the objective of characterizing the behavior of 227Ac, 228Ra, and 226Ra and their fluxes into the overlying water column, information that is essential to the interpretation of the distribution of these tracers in the ocean, for example, as measured on GEOTRACES cruises. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporated molecular diffusion, bioturbation, sedimentation, distribution coefficients (k d), and the fraction of each isotope released to pore water by parent decay (called F). Fits to these profiles used k d values determined in lab experiments for C-Disk-IV sediments. Ra k d values (1000–3000 mL g−1) agreed with previous estimates for deep-sea sediments, and Ac k d values (3500–22,000 mL g−1) correlated with those for Ra but were about 7 times greater. Two independent approaches were used to quantify the benthic fluxes of 227Ac and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agreed within uncertainty, and the average 227Ac and 228Ra sediment fluxes for the Northeast Pacific are 90 ± 20 and 600 ± 200 dpm m−2-yr−1, respectively. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific, averaging 1300 ± 200 dpm m−2-yr−1. This is over 2× higher than the water column inventory of 226Ra in this region (600 dpm m−2-yr−1), and indicates the importance of lateral 226Ra export from the N. Pacific. The largest 227Ac and Ra isotope fluxes in the study area are near the center of the Northeast Pacific (∼37°N). Smaller 227Ac, 228Ra and 226Ra fluxes occur north of 40°N, primarily due to dilution of their Pa and Th ancestors by higher sediment accumulation rates. • First study to measure 227Ac sediment profiles along a cruise tract in Northeast Pacific. • The distribution coefficient (k d) for Ac in these sediments is about 7 times higher than that for Ra. These are the first measurements of k d for Ac. • Benthic fluxes of 227Ac and 228Ra determined by core incubation are comparable to transport models applied to sediment profiles. • Average benthic fluxes of 227Ac, 228Ra, and 226Ra in the Northeast Pacific are 90 ± 20, 600 ± 200, and 1300 ± 200 dpm m−2-yr−1. [ABSTRACT FROM AUTHOR]