An outreach presentation on the uses for measuring thorium in the marine environment, modeled after a children's book and presented at AGU Fall Meeting 2022 Up-Goer Five Challenge: Making Big Ideas Simpler by Talking About Them in Words We Use a Lot.Pictures by Neil Redmond and Midjourney AI

North African dust is known to be deposited in the Gulf of Mexico, but its deposition rate and associated supply of lithogenic dissolved metals, such as the abiotic metal thorium or the micronutrient metal iron, have not been well-quantified. 232Th is an isotope with similar sources as iron and its input can be quantified using radiogenic 230Th. By comparing dissolved 232Th fluxes at three sites in the northern Gulf of Mexico with upwind sites in the North Atlantic, we place an upper bound on North African dust contributions to 232Th and Fe in the Gulf of Mexico, which is about 30% of the total input. Precision on this bound is hindered by uncertainty in the relative rates of dust deposition in the North Atlantic and the northern Gulf of Mexico. Based on available radium data, shelf sources, including rivers, submarine groundwater discharge and benthic sedimentary releases are likely as important if not more important than dust in the budget of lithogenic metals in the Gulf of Mexico. In other words, it is likely there is no one dominant source of Th and Fe in the Gulf of Mexico. Finally, our estimated Fe input in the northern Gulf of Mexico implies an Fe residence time of less than 6 months, similar to that in the North Atlantic despite significantly higher supply rates in the Gulf of Mexico.

Changes in the circulation of the Southern Ocean are known to have impacted global nutrient, heat, and carbon cycles during the glacial and interglacial periods of the late Pleistocene. Proxy-based records of these changes deserve continued scrutiny as the implications may be important for constraining future change. A record of authigenic uranium from the South Atlantic has been used to infer changes in deep-sea oxygenation and organic matter export over the past 0.5 million years. Since sedimentary uranium has the possible complication of remobilization, it is prudent to investigate the behavior of other redox-sensitive trace metals to confidently interpret temporal changes in oxygenation. Focusing here on the exceptionally long interglacial warm period, Marine Isotope Stage (MIS) 11, we found concurrent authigenic enrichments of uranium and rhenium throughout MIS 12 to 10, overall supporting prior interpretations of low-oxygen periods. However, there are differential responses of Re and U to oxygen changes and some evidence of small-scale Re remobilization, which may involve differences in molecular-level reduction mechanisms. Peaks in authigenic manganese intervening with peaks in Re and U indicate increases in porewater oxygenation which likely relate to increased Antarctic Bottom Water circulation at the onset of MIS11c and during the peak warmth of the interglacial around 400 ka.

Quantitative knowledge about the burial of sedimentary components at the seafloor has wide-ranging implications in ocean science, from global climate to continental weathering. The use of 230 Th-normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global dataset of 230 Th-normalized fluxes with an updated database of seafloor surface sediment composition to derive global maps of the burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), non-biogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of burial of the major components are mainly consistent with prior work, but the new quantitative estimates allow evaluations of global deep-sea burial. Our integrated deep-sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo-productivity proxies (TOC, biogenic opal, and Baxs) are not well-correlated geographically with satellite-based productivity estimates. Our new compilation of sedimentary fluxes provides more detailed information on burial fluxes, which should lead to improvements in the understanding of how preservation affects these paleoproxies.