1. Types and origins of marine sediments 2. Distribution of sediments: controls and patterns 3. Sedimentary diagenesis: (a) Sedimentary and organic matter burial (b) Aerobic and anaerobic decomposition (c) Sulfate reduction 4. Biogenic sediment sources: (a) Siliceous sediments (b) Distribution of siliceous sediments (c) Carbonate sediments (d) Distribution of carbonate sediments 5. Sedimentary records 6. Summary 1
Volcanogenic particles are derived from volcanic eruptions, and range from boulders to dust Lithogenic particles are derived from preexisting rocks by weathering (disruption of rocks by wind, temperature, water), and are deposited by wind (aeolian) or rivers (fluvial) Glacially derived particles are ground from rock beds by moving glaciers Biogenic particles are shells or skeletons of organisms that sink to the sea floor after the organisms death; made of silicate or carbonate minerals Distribution of sediments is controlled by "the 3 Ds": 1) Delivery: Without delivery of sediments into the ocean, they expectedly will never be found there 2) Dilution: Many different types of sediment particles reach the seafloor. If too much of one type reaches a given place, the other types will become unimportant 3) Destruction: Certain chemical, physical, and biological processes destroy sedimentary particles, removing them from the seafloor sediment We have already seen how NPP affects the concentration of organic matter in sediments (lecture of Oct 23; see next slide). What types of sediments do we expect at regions of high OC concentration? Think of the role of biomineralizing photosynthesizers, of fluvial and aeolian inputs, of seasonal ice, etc. 2
The distribution of OM in ocean sediments is similar to that of NPP in surface waters, except that a greater fraction of total burial (83%) occurs on the shelf (from Oct 23 lecture) SCOPE 35 Scales and Global Change (1988) http://www.scopenvironment.org/downloadpubs/scope35/chapter14.html Sedimentary diagenesis: the changes that occur in sediments as a result of biogeochemical diagenetic reactions Most diagenetic reactions are microbially mediated, and follow the same energetic hierarchy also seen in soils, lakes and estuaries From Sep 25 lecture 3
Typically, the higher the sedimentation rate, the higher the sediment burial rate The organic matter (OM) burial rate correlates well with the sediment burial rate High burial rates are observed in near-shore environments, where: NPP is higher, and, consequently: The flux of OM to sediments is high Burial of OM is rapid due to high sedimentation rates Pelagic sediments underlying areas of lower NPP, have the lowest sedimentation rates Oxygen from the overlying water gets consumed relatively rapidly by sedimentary microbiota during aerobic OM decomposition Consequently, the oxygen penetration depth in near-shore sediments, where OM supply is highest, is only a few mm Pelagic sediments have long O 2 exposure times Approximately 14 % of sedimentary OM is oxidized through anaerobic respiration, especially sulfate reduction Sulfate reduction is an important pathway in marine sediment diagenesis because of the high concentrations of SO 4 2- in seawater. Sulfate reduction in sediments leads to production of reduced S and pyrite formation: 2H + + SO 4 2- + 2CH 2 O 2CO 2 + H 2 S + H 2 O 2Fe(OH) 3 + 2H 2 S + 2H + FeS 2 + 6H 2 O + Fe 2+ 4
Pyrite S and organic C are positively correlated in sediments Sulfate reduction rates generally increase with overall rates of sedimentation Pyrite formation is often limited by available Fe. Most of the excess H 2 S escapes to upper layers of sediment where it is reoxidized to SO 4 2- Enhanced burial of reduced compounds (OC, FeS 2 ) results in enhanced accumulation of O 2 to the atmosphere: CH 2 O + O 2 CO 2 + H 2 O 4FeS 2 + 15O 2 + 4H 2 O 4Fe 3+ + 8SO 4 2- + 4H + + 2H 2 O Throughout geologic time, burial of pyrite may account for 20% of the O 2 in the atmosphere Atmospheric O 2 levels are regulated by the burial of reduced substances throughout geologic time During periods of rapid continental uplift, erosion and sedimentation, large quantities of OM were buried, and the O 2 content of the atmosphere increased Rising atmospheric O 2 increased aerobic decomposition in marine sediments, consuming O 2 and preventing further increases in atmospheric O 2 5
Sulfate-reducers out-compete methanogens for reduced C substrates (recall the hierarchy of oxidants: the zone of methanogenesis underlies the zone of sulfate reduction) Because the oceans have high SO 4 2- concentrations, methanogenesis is uncommon Most methane released from sediments is oxidized in the water column, and never makes it to the surface ocean As a consequence, anthropogenic methane sources dictate the current atmospheric methane concentrations: The natural flux to the atmosphere is < 10 x 10 12 g y -1 In comparison, anthropogenic sources release 376 x 10 12 g y -1 Davis and Gorsline (1976) (on-line by Henry Herms) 6
Siliceous Diatoms: unicellular microalgae with cell walls made of silica; 2 µm to 2 mm Radiolaria: protozoans; 50 µm to several mm; feed on bacteria, small phyto- and zooplankton; cold water and deep-sea http://www.eol.org, http://www.tolweb.org Davis and Gorsline (1976) 11
Carbonate Foraminifera: Protozoa; shell composed of a series of chambers; 30 µm to 2 mm); most abundant 40 N 40 S Coccolithophorids: algae; produce platelets called "coccoliths ; 2-50 µm http://serc.carleton.edu National Oceans Office Australia NOC Southampton, UK Pteropods: planktonic molluscs; 300 µm to 10 mm 12
The CO 2 produced in deeper ocean waters as a result of OM decomposition reacts with CO 3 2- which drives dissolution of CaCO 3 : CaCO 3 + H 2 CO 3 Ca 2+ + 2HCO 3 - Libes (1992) Berger (1976) 13
The saturation horizon occurs where Ω = 1 The lysocline is where dissolution effects first appear The Carbonate Compensation Depth (CCD) is where the deposition rate of carbonate is equal to the dissolution rate (i.e., no net accumulation of carbonates on the seafloor). Carbonate dissolution is complete below the CCD Dissolution may be affected by the geometry of fragments, the presence of organic coatings and other substances, etc. (on-line by Henry Herms) Broecker and Peng (1982) 14
Broecker and Peng (1982) Marine sediments record oceanic conditions in the geologic past The isotopic signature of O in CaCO 3 is used as a record of temperature, because it originates from H 2 O, whose isotopic signature is affected by a balance of evaporation and precipitation: the higher the relative 18 O content (δ 18 O), the more the ice and/or the lower the ocean volume. The isotopic signature of strontium is indicative of weathering on land: the more the relative 87 Sr content in CaCO 3 (they coprecipitate due to the similarity between Ca and Sr) the greater the rate of rock weathering Reminder from Sep 4 lecture: plant material slightly enriched in 12 C relative to 13 C. If organic matter storage is accelerated, atmospheric and oceanic CO 2 will be relatively enriched in 13 C, and consequently so will CaCO 3. Elevated O 2 concentrations are inferred during such time periods. 15
Oceanic sediments originate from volcanic activities (volcanogenic), weathering (lithogenic), glacial motions, and from biomineralization (biogenic) Their global distribution is determined by delivery rates, dilution with other types and materials, and destruction by biotic and abiotic processes High sedimentation rates are accompanied by high burial rates for sediment as well as organic matter High burial rates are observed in near-shore environments, where NPP is higher, while pelagic sediments have the lowest sedimentation rates Aerobic respiration is limited by oxygen availability, with sulfate reduction accounting for an important fraction of anaerobic respiration of organic matter (most significant in the near-shore Anthropogenic sources dominate the methane signal in the atmosphere The major types of biogenic sediments are siliceous and carbonate sediments Siliceous sediment distribution is dictated primarily by diatom productivity Carbonate sediment distribution is dictated in part by bathymetry and the CCD The succession of sedimentary types down the sediment column at any one location can be explained by the processes of delivery, dilution and destruction Marine sediments record oceanic conditions in the geologic past Examples relating to CaCO 3 in sediments include the isotopic signatures of O (temperature), Sr (weathering) and C (organic matter burial) 16