MIT OpenCourseWare · Video summary
Session 17: Sinking Particles and Remineralization (2)
Understanding particle sinking and remineralization reveals how nutrients cycle in the ocean, using radionuclides like thorium and protactinium as tracers to quantify these processes and their impact on carbon export.
Watch on YouTube ↗Key takeaways
- 🎯The flux of organic matter decreases with depth due to remineralization, with particles needing to be larger than ~100 microns to sink effectively according to Stokes' law.00:31
- 💡Large sinking particles include zooplankton carcasses, fecal pellets, and marine snow (organic aggregates), which can grow by accreting smaller particles, increasing their sinking speed.02:24
- 📈Radionuclides like thorium-230 are used to track particle dynamics. Its production from uranium decay and subsequent scavenging onto sinking particles creates a flux that increases with depth, unlike a simple irreversible scavenging model.10:13
- 🔍A reversible scavenging model for thorium-230, allowing exchange between dissolved and particulate phases, better matches observational data showing dissolved thorium increasing with depth.34:25
- 🚀Consider using thorium isotopes as a calibration tool for sediment traps by comparing measured flux to expected flux based on overlying uranium activity.43:27
- 🚀The protactinium-to-thorium ratio in sediments acts as a proxy for ocean scavenging intensity, with higher ratios indicating more scavenging, particularly in productive margin regions.51:39
Chapters
- 0:20🔄 Introduction to Sinking Particles and Remineralization
- 1:52🔬 Composition of Sinking Particles
- 4:25🌊 Marine Snow and Aggregates
- 7:45📊 Limitations of Sediment Traps
- 10:13☢️ Using Radionuclides to Study Particle Dynamics
- 10:51📈 Thorium-230 vs. Uranium-238/234
- 15:06🧮 Irreversible Scavenging Model for Thorium-230
- 19:23❓ Questioning the Irreversible Model
- 34:04📉 Discrepancies with Observations
- 34:25🔄 Reversible Scavenging Model
- 35:19🧪 Equations for Reversible Scavenging
- 39:25📈 Model Predictions vs. Reality (Reversible)
- 40:18⏳ Thorium Residence Time
- 43:27📏 Thorium-230 as a Sediment Trap Calibrator
- 46:42⚖️ Protactinium-231 vs. Thorium-230
- 48:20↔️ Lateral Transport and Scavenging Differences
- 51:44📊 Protactinium-Thorium Ratios as a Proxy
- 53:46💧 Introduction to Dissolved Organic Matter (DOM)
- 55:49📈 DOM Concentration Profiles
- 58:23📉 DOC Gradients in Deep Water
- 1:00:15⬆️⬇️ DOM vs. POC Concentrations
- 1:01:11⚛️ DON and DOP Measurements
- 1:12:43⏳ DOM Pools by Timescale
- 1:14:50🔄 DOM Production and Respiration
- 1:16:18☢️ Radiocarbon Measurements of DOC
- 1:22:15🕰️ Age of Deep Ocean DOC
- 1:23:16🏁 Conclusion and Future Topics
The digest
Radionuclide Tracers for Particle Dynamics and Dissolved Organic Matter in the Ocean
This lecture explores how radionuclides are used to understand the sinking of particles and the fate of dissolved organic matter in the ocean. It details the use of thorium isotopes to quantify particle scavenging and remineralization rates, contrasting irreversible and reversible scavenging models. The discussion then shifts to dissolved organic matter (DOM), examining its distribution, composition, and residence time, and how radiocarbon measurements illuminate its age and potential role in oceanic carbon cycling.
Keep reading in The Digest
The full 5-minute article, 4 more sections, and every new episode from MIT OpenCourseWare and the creators you follow.
- Sinking Particles and Remineralization
- Radionuclides as Tracers: Thorium Isotopes
- Radionuclides as Tracers: Protactinium-231
- Dissolved Organic Matter (DOM)
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