Thorium distributions in high‐ and low‐dust regions and the significance for iron supply

Abstract

Thorium and uranium isotopes (232Th, 230Th, 238U, and 234U) were investigated to refine their use for estimating mineral dust deposition and Fe delivery to the ocean. U concentrations and isotope ratios were consistent with conservative behavior and can safely be described using published U‐salinity relationships and global average seawater isotopic composition. Near Barbados, waters affected by the Amazon outflow contained elevated 232Th. This signals one region where the thorium‐dust method is inaccurate because of a confounding continental input. Dissolved 232Th fluxes in this region suggest that Amazonian Fe supply to the adjacent open ocean is much larger than local atmospheric deposition. The colloidal content of dissolved Th south of Bermuda was found to be quite small (2–6%), similar to that found north of Hawaii, despite the order of magnitude higher dust deposition in the Atlantic. This finding supports the assumption that dissolved 232Th and 230Th are scavenged at the same rate despite their different sources and also sheds light on the increase of dissolved 232Th fluxes with integrated depth. Outside the region influenced by Amazon River waters, dissolved 232Th fluxes are compared with Bermudan aerosol Fe deposition to estimate that fractional Th solubility is around 20% in this region. Finally, new dissolved and soluble Fe, Mn, and Cr data from the subtropical North Pacific support the idea that Fe concentrations in the remote ocean are highly buffered, whereas 232Th has a larger dynamic range between high‐ and low‐dust regions.

Changes in Antarctic Bottom Water Formation During Interglacial Periods

“…Samples were taken at~10 cm intervals, allowing us to construct a record with an age resolution on the scale of hundreds of years. The U concentrations of the sediment samples were processed by published methods using strong acid digestion and column chromatography (Hayes et al, 2017) and analyzed on a Thermo Scientific Element XR inductively-coupled plasma mass spectrometer (ICP-MS).…”

“…Typically, this is done by measuring the amount of 232 Th, a Th isotope only present in detrital material, and making a correction based on the 238 U/ 232 Th ratio of the detritus (Henderson & Anderson, 2003). Concentrations of 232 Th were determined by processing samples according to published methods using strong acid digestion and column chromatography (Hayes et al, 2017) for a select group of samples. With a known concentration of 232 Th, the detrital U concentration can be calculated using Equation 2:…”

Replacement Times of a Spectrum of Elements in the North Atlantic Based on Thorium Supply

“…There are few studies that determine the relative solubility of all the elements of interest here. In particular, oceanic Th fractional solubility data are rather limited and range from about 1–29%, in published work (Anderson et al, ; Arraes‐Mescoff et al, ; Hayes et al, ; Robinson et al, ; Rowland et al, ; Roy‐Barman et al, ). Based on time series measurements of Th and Fe at station ALOHA (Hayes, Fitzsimmons, et al, ), it is likely that S Fe / S Th is close to 1, but this may vary regionally and episodically.…”

Aerosol trace metal leaching and impacts on marine microorganisms

“…GEOTRACES extensive intercalibration efforts have led to standardized techniques to minimize contamination and ensure that multiple laboratories achieve the same accurate results, regardless of method 139 . GEOTRACES is also providing large-scale measurements of iron-binding ligands 69 , 140 , the trace element content and stoichiometry in different plankton groups 141 , novel methods to assess the depositional flux of metals to the ocean 142 – 144 . An important complement to the large-scale surveys from GEOTRACES are timeseries measurements from a single location, which can help constrain trace metal sources and sinks 145 , 146 and process studies to help constrain trace metal inputs and the biotic and abiotic ocean processes 5 .…”