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Lecture 4: Sediments

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Lecture 4: Sediments Announcements • First problem set due next Tuesday. • Review for first exam next Thursday. • Quiz on Booth (1994) after break today. Intertidal, Lowes Cove, ME Marine Sediments: Clues to the ocean’s past There is more to mud than meets the eye Learning goals: 1: Determine factors that control distributions of sediment types 2: Interpret oceans history using sediment tracers 550’ Casco Bay, ME 1 Small-scale biological components of sediment Biogenous sediments - primarily pelagic Classifying mud (large-scale perspective) Major sediment classes based on sediment source: Oozes (> 30% biogenic material) terrigenous, biogenous, hydrogenous, cosmogenous Calcareous ooze Composed of the remains of Formaminifera, coccolithophores, pteropods, other calcareous organisms Terrigenous sediments - primarily coastal Siliceous ooze Origin: Composed of the remains of diatoms, radiolarians Weathering of continental crust: Physical weathering → physical breakup of minerals into grains Chemical weathering → dissolution of minerals, produces clays Transport: Rivers, wind (pelagic clays), glaciers Diatoms (centric and pennate) Radiolaria Foraminifera Coccolithophore 2 Non-biogenic sediment of open ocean (deep, low productivity) Sediments of slope and continental rise: - Abyssal clay - Mixture of neritic (terrigenous) and pelagic sediments Plate-shaped, often negatively charged. Nearshore, generated by chemical weathering or glaciers, transported by rivers. Offshore, transported by wind NASA Manganese nodules: hydrogenous sediment Fe/Mn crusts found near ridges Mn nodules found in areas of with low rates of sedimentation Growth rate: <1 cm / 106 years Contains: Mostly Mn and Fe, but also Co, Ni, Cu, Zn, Cr Continental rise and abyssal plain turbidite deposits: Alternating layers of coarse and fine-grained terrigenous & biogenous sediment 3 Large-scale sediment distributions: Supply > dilution + dissolution Large-scale sediment distributions: Supply > dilution + dissolution 4 Distribution of calcareous oozes = f(water depth) Calcite compensation depth (CCD) (Calcium carbonate compensation depth) Factors controlling CCD (zcc): (pressure, flux of CaCO , deep [CO -2]) Geochemical definition: The depth at which the rate of supply of CaCO3 = 3 3 the rate of dissolution at the sediment surface Geological definition: The depth where CaCO3 drops to 10-20% by mass 2+ 2- CaCO3 ↔ Ca + CO3 Dependent upon pressure (depth) and pH ) solubility) Blue line is CCD + - + 2- 3 CO2 + H20 ↔ H2CO3 ↔ H + HCO3 ↔ 2H + CO3 Increasing pressure and increasing CO2 at depth shifts equilibrium, Solubility + 2- increasing H (lowering pH) and reducing CO3 . 3x higher at 5000 m Calcite (CaCO Calcite Little calcareous or (saturationCCD) Depth ooze below ~4,500 m ( (~5 km in Atlantic Current conditions (avg CCD ~ 4500 m) ~4 km in Pacific) Zsat = saturation depth Zcc = CCD Figure from Boudreau et al. 2010 Sediment distributions: Calcareous ooze, siliceous ooze, abyssal clay, and terrigenous sediment Rules of sediment distribution • Rate of supply vs. rate of dissolution or dilution • Nearshore, terrigenous sediment dilutes all others • Offshore and shallower than CCD, calcareous ooze dilutes all others • Deeper than CCD, under areas of high productivity, siliceous ooze dilutes all others • Deeper than CCD, under areas of low productivity, abyssal clay predominates (not diluted by other types) 5 Rates of sediment accumulation River deltas: 10 - 100 mm/year (or greater in some cases) Nearshore ~ 1-2 mm/year (same rate as sea level rise) But, this varies depending upon local uplift or subsidence Deep sea, 1-10 mm/103 years (Rule of thumb: < 1cm / 1000 y) Question: How to manganese nodules stay on the surface? Mn-nodule growth rate: 1-10 mm/ 106 years (Rule of thumb: < 1 cm / 1,000,000 y) Size-selective Feeding by Cirriformia grandis 70 Sediment 0 Worm Gut 60 50 Direction of Particle Movement 40 5 Burrow Construction 30 Feeding Mode of Cirriformia grandis 20 10 10 0 0 50 100 150 200 Bead Size (mm) 6 Relative Bead Concentration Relative Bead Concentration 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0 0 5 5 10 Small particles 10 are transported 15 to depth 15 Size: 16 - 32 mm Size: 32 - 64 mm 20 20 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0.5 0 0 5 5 10 10 Big particles stay near the surface 15 15 Size: 64 - 128 mm Size: 128 - 256 mm 20 20 Sediments and paleoceanography Requirements: Means of dating sediment (radionuclide dating, paleomagnetism) Tracers for past environmental conditions Paleothermometers: Oxygen isotopes ratios (d18O) in carbonate shells of foraminifera or in glacial ice: 18O:16O (Temperature + Global Ice Volume) Relative abundance of different species of foraminifera (Temp) Mg/Ca ratios in foraminifera shells (Temp) Alkenones found in marine organic material (Temp) If you want to look deep into the earth’s past (~100 MY) Where would you collect sediment cores? 7 Effect of temperature on Mg/Ca ratio in Paleotemperature comparisons foraminfera CaCO3 Species of Globigerinoides Winter Summer Fauna Temperature (°C) Elderfield and Ganssen, 2000, Nature Elderfield and Ganssen, 2000, Nature Summary Temperature change in southern Sediments are combinations of terrigenous particles, organism parts, authigenic ocean over 0.5 million years (hydrogenous) and cosmogenic particles Sediment distributions result from differences in supply, dilution, and dissolution of sediment from different sources Nearshore sediments: terrestrial origin (terrigenous) Mg/Ca temperature Continental rise: may be composed of turbidites (alternating layers of coarser terrigenous sediments and finer terrigenous & biogenous sediment) δ18O temperature Open ocean: Biogenous sediments: Calcareous oozes widespread in areas shallower than CCD. Siliceous oozes found only in productive waters deeper than the CCD. Clays: Found deeper than the CCD in areas of relatively low productivity, and downwind of large deserts. Sediments record ocean history, read by measuring tracers to determine the age Elderfield et al. 2009 of each stratum and indicators of past environmental conditions. 8.
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