High Lithogenic and Micro‐Nutrient Fluxes From the West Greenland Margin Traced by Thorium in Seawater and Sediments

The flux of nutrients from continents to the oceans sustains oceanic primary productivity and is a fundamental component of the carbon cycle. In most regions of the world's oceans primary productivity is limited by the supply of nutrients. In particular, iron can become limiting in the open-ocean due to its low solubility. Glaciated continents have been suggested as an underappreciated source of iron to the high-latitude oceans. Yet, uncertainty remains regarding the magnitude and spatial variability of glacially derived nutrient fluxes, and the extent to which these nutrients impact open-ocean ecosystems. To quantify lithogenic fluxes at the West Greenland margin, we measured ²³²Th and ²³⁰Th in seawater and core-top sediments across the shelf and slope. Our results highlight a negative correlation between low-salinity waters and dissolved and particulate ²³²Th, suggesting a glacial source for this lithogenic isotope. We calculated dissolved ²³²Th fluxes 5–24 μg m⁻² yr⁻¹ (100–500 m depth), and sedimentary ²³²Th fluxes 105–711 μg m⁻² yr⁻¹, higher than typical open-ocean settings and similar to margin sites influenced by large inputs from aeolian dust and rivers. A sampling transect shows that dissolved ²³²Th fluxes increase toward Greenland, confirming that lithogenic inputs are sourced laterally from the margin. Using our ²³²Th fluxes, we estimate an elevated supply of dissolved Fe which extends over the continental slope toward the open ocean. This Fe flux is large enough to support much of the local primary productivity, highlighting the importance of lithogenic fluxes in supporting the marine ecosystem in high-latitude oceans.

Plain Language Summary: Small plant‐like creatures (algae) living in the surface ocean are an important food source for other marine creatures and affect the Earth's climate by drawing carbon dioxide into the ocean when they grow. In most parts of the oceans the algae's growth is limited because they do not have enough of certain nutrients, such as iron. It is possible that around Greenland, algae are supplied with iron that comes from mud ground‐up by ice on the continent. There are no measurements of how much, and how quickly, mud (and iron) reaches the algae in the ocean far away (around 100 km) from the coast of Greenland. We measured two types of the radioactive element thorium in seawater, and seafloor mud, to calculate the rate that mud is supplied to the ocean off West Greenland. We found that high amounts of thorium and iron are supplied to the region, enough to account for the large quantity of algae growing there. This suggests that the ice on Greenland, and maybe also on Antarctica, helps to keep the algae in the ocean growing. Changes to Earth's ice‐sheets might then impact how algae grow and absorb carbon dioxide in the future.