Paleocirculation Proxies

1. Paleocirculation Proxies Sortable Silt Bianca, Laura, Tania, Michael

2. Basic Premise Sediment enters the ocean through various processes: Volcanic ash, aeolian dust, ice rafted debris, river runoff, etc. Sediment that is silt sized, between 10-63 microns, “displays size sorting in response to hydrodynamic processes and can be used to infer current speed” Sediment is moved around based on silt size and speed of current The stronger the current, the larger the particle that can be transported Eventually, the sediment will settle out and what deposits on the ocean floor can be examined to determine properties of the ocean currents For 2nd point- use analogy of an m&m to a boulder. Which would take more energy to move?For 2nd point- use analogy of an m&m to a boulder. Which would take more energy to move?

3. Just an idea of how sediment is broken down into size. *note: settling velocityJust an idea of how sediment is broken down into size. *note: settling velocity

4. Basic Premise Linear relationship between current speed and sediment size Fig. 1- Linear relationship between current speed and sediment size Fig. 2- Population distributions based on current speedsFig. 1- Linear relationship between current speed and sediment size Fig. 2- Population distributions based on current speeds

5. How does it work? Some important concepts that are interrelated: Settling velocity is a function of grain size Shear stress is a function of current velocity As currents strengthen, shear stress increases along the seafloor and particles can become resuspended. As currents weaken, particles will settle out and redeposit on the seafloor. Particles get trapped in a turbulent boundary layer above the seafloor and get sorted based on settling velocity and can either settle out or be transported downcurrent. This sorting in the sediment cores tells us the relative current speed. in the viscous sublayer, that’s what were looking at (from bed to sublayer) In the buffer layer, the aggregates get broken up and then fall into the sublayer where they can settle out. in the viscous sublayer, that’s what were looking at (from bed to sublayer) In the buffer layer, the aggregates get broken up and then fall into the sublayer where they can settle out.

6. How does it work? SS is often paired with other proxies. Can you think of why? SS only gives you current speed based on phi size (and ultimately settling velocity). To study changes in water masses, like salinity, you want to use other proxies like 18O. Together, you can reconstruct currents during glacial and interglacial times.SS only gives you current speed based on phi size (and ultimately settling velocity). To study changes in water masses, like salinity, you want to use other proxies like 18O. Together, you can reconstruct currents during glacial and interglacial times.

7. What is analyzed? Fine fraction of the sediment recovered in core samples. Between 10-63 µm (sortable silt) The biogenic material is not considered because its size distribution may respond to characteristic of the species

8. What is analyzed? There are different methods used to measure fine grain sizes: Measurements using sedimentation principle Electrical sensing zone counters Laser diffraction sizers There are different methods used to measure fine grain sizes: Measurements using sedimentation principle: sense the total amount of material and gives an accurate measurement of the 1-100 mm size distribution. Electrical sensing zone counters: grain size in the 0.5-100 mm. It records only SSaverage but not SS%. Laser diffraction sizers: grain size over 30 mm, but “doesn't see” lower SS range (10-30mm). Also, overestimate the size of platey minerals. There are different methods used to measure fine grain sizes: Measurements using sedimentation principle: sense the total amount of material and gives an accurate measurement of the 1-100 mm size distribution. Electrical sensing zone counters: grain size in the 0.5-100 mm. It records only SSaverage but not SS%. Laser diffraction sizers: grain size over 30 mm, but “doesn't see” lower SS range (10-30mm). Also, overestimate the size of platey minerals.

9. What is analyzed? Sedigraph (settling velocity principle) Pretreatment of the sample (dry, digest of org. mat.) Sedigraph #1 Carbonate removal (to get lithogenic material) Sedigraph #2 Result: fine fraction size distribution and recovery of the sample for further studies (ex: XRD).

10. Assumptions Below 10 um, the sediment will flocculate Above 63 um, the grain size is too big to be removed by ocean currents Sedimentation rates and bioturbation are constant and measurable Events that could have affected the sedimentary sequence (e.g. turbidites) are previously recognized

11. Problems and Complications Flocculation and aggregation Particles below the range (10-63 ?m) Charge imbalances Van der Waals forces Particles below 10 microns tend to form aggregates because of the charge imbalances in clay particles and van der waals forces(attraction between platelets in clays)Particles below 10 microns tend to form aggregates because of the charge imbalances in clay particles and van der waals forces(attraction between platelets in clays)

12. Problems and complications Shear stress non-linear relationship because of cohesionShear stress non-linear relationship because of cohesion

13. Problems and complications As flow speed increases, accumulation increases until you reach a critical point After that point, the flow speed becomes too great and it begins to erode the sediment instead of depositing it As flow speed increases, accumulation increases until you reach a critical point After that point, the flow speed becomes too great and it begins to erode the sediment instead of depositing it

14. Problems and Complications Topography strongly influences flow near bed Flow varies on upstream and downstream faces Flow topographically steered Stronger flow on one side and weaker on the other Must understand topography and make transects Flow slows on upstream face=higher deposition Flow increases on downstream face=slower deposition or even erosionFlow slows on upstream face=higher deposition Flow increases on downstream face=slower deposition or even erosion

15. Problems and Complications Transient perturbation Turbidite tails of finer grains can cause confusion Bioturbation Eddies Turb. Tails of fine grains are often hard to identify as belonging to a turbidite, and can be confused as being depositied under normal deposition conditions (to be used for flow speed inferences) Western boundary currents are faster and carry more sediment Currents also change thru time High energy in eddies stir things upTurb. Tails of fine grains are often hard to identify as belonging to a turbidite, and can be confused as being depositied under normal deposition conditions (to be used for flow speed inferences) Western boundary currents are faster and carry more sediment Currents also change thru time High energy in eddies stir things up

16. Problems and complications Eddies (from McCave et al., 1995). “Purely on the basis of sedimentary properties we cannot distinguish whether a change in size is due to an increase in the mean speed or an increase in the variability of the speed”. This is why we have to avoid areas with high eddy activity to conduct this kind of studies.

17. Advantages Sediment highly available No chemistry involved Ideal when combined with other proxies Relatively available and inexpensive equipment Most proxies use a lot of chemsitry but this proxy is a purely physical process and only requires settling velocities If you use sortable silt along with tracers, like isotope ratios, you can know a lot about not only the flow speed, but the age of the water mass and other chemical characterisitics Most proxies use a lot of chemsitry but this proxy is a purely physical process and only requires settling velocities If you use sortable silt along with tracers, like isotope ratios, you can know a lot about not only the flow speed, but the age of the water mass and other chemical characterisitics

18. Favorable Conditions You need: high sedimentation rates low bioturbation western boundaries (up to a certain speed) high mean kinetic energy and low eddy kinetic energy