Radium isotopes have been used to trace nutrient, carbon, and trace metal fluxes inputs from ocean margins. However, these approaches require a full accounting of radium sources to the coastal ocean including rivers. Here, we aim to quantify river radium inputs into the Arctic Ocean for the first time for226Ra and to refine the estimates for228Ra. Using new and existing data, we find that the estimated combined (dissolved plus desorbed) annual226Ra and228Ra fluxes to the Arctic Ocean are [7.0–9.4] × 1014dpm y−1and [15–18] × 1014dpm y−1, respectively. Of these totals, 44% and 60% of the river226Ra and228Ra, respectively are from suspended sediment desorption, which were estimated from laboratory incubation experiments. Using Ra isotope data from 20 major rivers around the world, we derived global annual226Ra and228Ra fluxes of [7.4–17] × 1015and [15–27] × 1015dpm y−1, respectively. As climate change spurs rapid Arctic warming, hydrological cycles are intensifying and coastal ice cover and permafrost are diminishing. These river radium inputs to the Arctic Ocean will serve as a valuable baseline as we attempt to understand the changes that warming temperatures are having on fluxes of biogeochemically important elements to the Arctic coastal zone.
Submarine groundwater discharge (SGD) plays a critical role in coastal and ocean biogeochemistry. Elucidating spatially and temporally heterogeneous SGD fluxes is difficult. Here we use radium isotopes to explore the external sources and mixing regime along the eastern coast of South Africa. We demonstrate that the long‐lived radium isotope compositions are controlled by low inputs of low‐ and high‐salinity terrestrial groundwater. While activities of228Ra and226Ra in beach porewaters are similar to coastal waters,224Ra is enriched by inputs of228Th from coastal seawater. Porewater ages, based on the production of224Ra from228Th, range from 0.3 to 2.3 days, indicating rapid flushing of the beach system. Unlike radium, however, nutrients follow a more complex pathway. We hypothesize that high total dissolved nitrogen (TDN) and phosphorus concentrations in beach porewaters (TDN ranges from 1 to >700 μM) and the coastal ocean (TDN ranges from 1 to >40 μM) are derived from a source not enriched in radium. We speculate that this source is terrestrial water flowing below the dune barrier at depths exceeding our beach sampling depths. This water likely flows upward through breaches in the confining layer into the beach or enters the ocean directly through paleochannels. The presence of high nutrient concentrations in the coastal ocean unaccompanied by high228Ra activities leads to the hypothesis of this additional nutrient source. These combined inputs may be of considerable importance to the coastal ecology of southeastern Africa, an oligotrophic ecosystem dominated by the nutrient‐poor Agulhas Current.
more » « less- PAR ID:
- 10458913
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 124
- Issue:
- 3
- ISSN:
- 2169-9275
- Page Range / eLocation ID:
- p. 2010-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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