Sediment Geochemistry: Understanding Marine Sediments in Earth's Cycles

1. 12.743 Sediment Geochemistry - 2005 • Split the lectures about evenly; both attend all. • Work will include: • - Reading papers and participating in classroom discussions. • - Problem sets - hopefully 3 from each of us. • - An in-class midterm exam March 17th. • A 2-3 hour written final exam. • Text - Early Diagenesis (Robert Berner, 1980)

2. Why would anyone study sediment geochemistry? Understand role of marine sediments in modern geochemical cycles Solid phase - primary fluxes to sea floor - recycling efficiency / burial efficiency Dissolved species - reactions and rates - source / sink to overlying water column

3. Interpret downcore records of sediment composition and accumulation rate - Major components (carbonate, opal, clay) - Minor components (organic C/N/P) - Trace elements and authigenic minerals - Paleoceanographic proxies (faunal patterns, foraminiferal shell chemistry) Main focus of the course will be on modern processes – pore water and solid phase studies of relatively shallow sediments (mm to cm to…)

4. Today: Global bulk sedimentation rates Broad patterns in distributions of major sediment types. (composition vs. accumulation rate) (supply, dilution, preservation) A few examples of temporal variability (difficulty in comparing across time scales snapshot sampling vs. “integrative” sediments)

5. Age of basement (Kennett) Long-term sediment accumulation rates

6. Sediment thickness to basement (Berger) Long-term sediment accumulation rates

7. Long-term sediment accumulation rates: North Pacific gyre < 100 m / 100 x 106 y ( = < 0.1 cm / 103 y ) South Atlantic 100 to > 1000 m / 100 x 106 y ( = 0.1 to > 1 cm / 103 y ) High sedimentation rates are essential for high resolution paleoceanographic studies – where can we find them? Drift deposits, continental margins, nearshore deposits 10s to 100s x higher sed rates than global average. ( = up to meters / 103 y ) Long-term sediment accumulation rates

8. Global sediment load in rivers = 140 x 1014 g / yr [Milliman, ’91; Milliman and Syvitshi, ‘92] Global sedimentation rate = Load / {Area x sediment density x (1-porosity)} = (140 x 1014 g / yr) x (103 y) / {(361 x 106 km2) x (1010 cm2 / km2) x (2.5 g / cm3 sed) x (0.2 cm3 sed/ cm3 bulk)} = 7.7 cm / 103 y (much of Pacific < 0.1 cm/ky, much of deep sea ~ 1 cm / ky (and much of that is biogenic, supported by dissolved inputs.)) Most of river sediment load winds up in estuary / delta / shelf / slope sediments.

9. Biogenic phases in sediments: CaCO3 (calcite, aragonite) SiO2 (“opal”) Organic matter (organic carbon) Easy to measure distributions and concentrations; typically want to know the accumulation rates. (sediment accumulation rate and mixing rate estimates)

10. Interpret concentrations and rates in terms of: supply (wind or river input; production) preservation (dissolution, decomposition) dilution (by other sediment components) To understand these three factors, construct benthic budgets: particle input fluxes (sediment trap rain rates ) solute fluxes (flux chambers, pore water profiles) burial fluxes (sediment concentration x sed rate)

11. Biogenic sediments – the main players: CaCO3 Cocolithophorids and foraminifera (calcite), pteropods (aragonite) SiO2 Diatoms, radiolaria Organic matter Phytoplankton cells, zooplankton fecal pellets, marine snow. Cocoliths and diatoms need light to photosynthesize. Planktonic foraminifera and radiolaria are not restricted to the euphotic zone, but are most abundant there.

13. % CaCO3 vs. water depth “lysocline” – onset of dissolution “calcite compensation depth” – dissolution rate = rain rate

14. Carbonate-rich sediments Broecker and Peng Global carbonate accumulation balances continental weathering. Carbonate distribution reflects preservation (water chemistry, and calcite solubility (depth))

15. But can’t ignore possibility of dilution… Keigwin and Jones, downcore carbonate cycles in Bermuda Rise sediments.

16. Percent opal (carbonate-free) Broecker and Peng Opal production – localized near upwelling (diatom pri. prod.) Preservation – Only where flux to sea floor is high (oceans are everywhere undersaturated w.r.t. opal) So – preservation follows production

17. <0.25 % > 2.0 % Premuzic – percent organic C highest on margins

18. Dramatic attenuation of organic carbon content. C (organic) / C (CaCO3 ) typically ~1 in sediment traps; often only 0.01 in sediments. Organic C efficiently recycled by decomposers ( = poorly preserved). Organic C content clearly highest on margins (where sed rates are also highest.) Preservation efficiency, or supply?

19. In general: The vertical particle flux is dominated by large, fast-sinking particles (100 m / day) Biogenic components dominate the open-ocean particle flux. CaCO3 dominates the biogenic flux. C (organic) / C (CaCO3 ) typically ~1 Short-term temporal variability from sediment trap studies. How do we avoid aliasing in benthic process “snapshots”?

20. Biogenic fluxes in top 2 km fairly well correlated. SiO2 CaCO3 CaCO3 + SiO2 Organic C Lyle et al., 1988

21. Seasonality in the high-latitude North Atlantic. Honjo

22. Ca:Si flux ratio varies in time and by location. High SiO2 fluxes High CaCO3 fluxes Honjo

23. And what actually makes it to the sea floor? Honjo and Manganini