Lesson II Biological components of sediments and nutrient ratio considerations Sediment classification Silica cycle The steady state concept Feedbacks
Controls on ocean sedimentation Sediment classification? Material supply vs. material export Primary production Transport and aggregation Dissolution processes The control of ocean depth Lateral transport Alteration/formation at the seafloor
The distribution of oceanic sediments
Sediment thickness
The age of the seafloor
Bathymetry of the word oceans
Silicious oozes Mostly in highly productive areas Mostly in relatively deep waters (carbonate imprint) High latitude silicious oozes formed mostly by diatoms Equatorial upwelling silicates formed mostly by Radiolarians
Primary production Highest production in The northern Atlantic in spring/summer In the southern circumpolar current in winter In areas of coastal and equatorial upwelling Limitations Light Nutrients Cofactors (Iron etc)
The different types of sediment Ice rafted material contains pebbles, boulders etc. Terrigenous sediments: large fraction of land-derived material from chemical weathering and riverine transport Red Clays: Mostly fine-grained terrigenous material far from land in the deep ocean Carbonate: large fraction of skeletal material from carbonate-building phyto- and zooplankton Silicates: large fraction of skeletal material from silicate-building phytoand zooplancton
Opal preservation Diatomaceous earth, the pinkish white outcrop shown above (near Lovelock, Nevada), is a mineral of plant origin. It represents the accumulation of an enormous number of fossil diatoms (single-celled plants ). It's also known as diatomite or kieselguhr, and it's referred to as white dirt by miners -- in bright sunlight it can be almost as white as new snow. Diatomite has several unique characteristics. Because of its lightness, porosity, and its honeycombed structure, it's an ideal filtering medium. In addition, it's inherently stable and devoid of most trace elements. Thus, diatomite is ideal for use by processors who have very high purity requirements such as the food, beverage and pharmaceutical industries. epod.usra.edu/archive
major component of clays aluminosilicates but, limited availability of dissolved Si thus, can control growth of primary (diatoms) and secondary (radiolarian) producers
Silicates Quartz Alumosilicates (clays etc.)
Silica Secreting Organisms include: Diatoms which are unicellular alga a few micron to 200 micron in size. They secrete frustules from amorphous hydrated silica (opal). They are abundant in high productivity areas such as coastal upwelling, equatorial regions and high latitude areas. There are some benthic diatoms that are restricted to shallow waters. epod.usra.edu/archive
Radiolarians are large zooplankton in the range of 50 to 300 micron. They secrete very intricate shells structures. They are usually abundant in low latitudes. Radiolarian a protozoan
The key players Diatoms Radiolarians Silicoflagellates
Silica cycle is important because: It is thought that diatoms are one of the dominant organisms responsible for export production from the surface ocean (Dugdale et al., 1995). Silicates may play an important role in the global carbon cycle. Opal accumulation can tell us where export production has occurred on time scales of hundreds to millions of yeas: paleoproductivity information (Ragueneau et al., 2000). But careful: see feedback tale at end of lesson Mean ocean residence time for silicate is ~ 10-15 kyr (Treguer et al. 1995), short enough that concentrations and fluxes can respond to glacial/interglacial perturbations.
Sources rivers (~80% total)- but careful: - non conservative behavior eolian seafloor weathering hydrothermal
What s the source of dissolved Si in the river water? High River Flow dissolved Si Low River Flow What is the Si removal process? Diatom Production
Seasonal variation often observed due to changes in: weathering intensity and/or source of water discharge within drainage basin diatom production Columbia River Have Dams Made An Impact?
Biogenic opale in sediments Caution with %- may depend on the rate of accumulation of nonopal components.
Rain rate Higher nutrients = higher biogenic silica production Ecological factors determine that when nutrients are high, diatoms flourish
Preservation of biogenic silica FSi: Flux of particulate biogenic opale through the water column Water column Dissolution in part speciesdependent In the water column, the distribution in the ocean can be only modelled with T-dependent opale dissolution kinetics Z: Water depth k: first order kinetic constant for the dissolution of opale wf: Particle settling velocity (50m/day) T a : absolute temperature Gnanadesikan, GBC 13, 199 220, 1999.
Deep waters of Pacific are more silica-enriched than Atlantic. Hence, surface waters influenced by upwelling in Pacific host greater diatom (and radiolarian) production than those in the Atlantic.
Pacific deep waters more silica enriched than Atlantic Pacific surface waters influenced by upwelling host greater diatom (and radiolarian) production than those in Atlantic
Thermodynamic driving force For biogenic silica (@25 o C), K eq = {H 4 SiO 4 }/({SiO 2 }{H 2 O} 2 } {H 4 SiO 4 } = 2.0x10-3 M (2000 μm) Measured silicate concentrations in seawater are: 1-2 x 10-5 M (10-20 μm) in surface waters 1-2 x 10-4 M (100-200 μm) in deep waters Ocean is undersaturated everywhere with respect to biosi
Solubility of quartz and amorphous silica
SiO 2 + H 2 O Si(OH) 4 K eq = [Si(OH) 4 ] [Si(OH) 4 ] < 200 μm everywhere in the ocean Biogenic SiO 2 wants to dissolve everywhere in the ocean aka Si(OH) 4
Opal accumulation rates are greatest in sediments beneath high productivity regions. Diatom and radiolarian productivity are maximal in nutrient-rich, upwelling regions. Sediments with highest concentration of opal coincide with these regions high in: Arabian Sea Peruvian coast and westward along the equator North African coast and westward along the equator Southern Ocean Bering Sea
Factors controlling this distribution Rain rate of skeletal debris Thermodynamic driving force for dissolution of this rain, i.e., the degree of undersaturation of bottom waters: (C sat C bottom ) Remember that C sat in turn depends on temperature and pressure (K sp ). Burial efficiency -degree to which this biogenic phase is diluted on the ocean floor by nonbiogenic silica phases
Preservation of biogenic silica Sediments Thermodynamic driving force: T, P, ph, ΔC Rainrate of opal Total rain rate => high opaline production enhances the fraction buried in the sediments => high total production enhances the fraction of opal buried in the sediments The mechanisms of silicate preservation enhance the contrast of the production pattern! Nice quantitative approach in Broecker and Peng: Tracers in the sea 1982
Why are opal deposits observed anywhere? Burial efficiency = A opal /R opal For a given thermodynamic driving force, the solution rate for opal will be proportional to the fraction of opal in the sediment
R C bottom S C saturation [Si] in porewater set by: C pore D d 2 C/dz 2 = -k (C sat C pore ) A = R - S A Dissolved Si Profile in Sediment
The silicate cycle (Intro) Tréguer et al., Science, 268, 375-379 (1995) Dissolved silica mostly (>95%) as monomeric silicid acid Si(OH) 4 Total content about 10 17 mol Surface waters concentrations from > 2μM (central gyres) to >80 μm in Antarctic winter waters Deep waters from 10-40 μm in the North Atlantic to 100-160μM in the Pacific Ocean generally saturated for most lithogenic materials (100μM for quarz, 220μM for montmorillonite, but undersaturated with respect to biogenic silica (roughly 1000μM at deep ocean temperatures
The silicate cycle (Inputs of silicic acid) Particulate silicates from rivers settle fast and do not contribute Rivers and groundwaters contain silicic acid from weathering reactions of silicates with kinetics covering several orders of magnitude (see Lasaga et al., GCA 58, 2361-2386 (1994) Average river water silicic acid content about 150μm, but varies with latitude (how, why?) Biological uptake / remineralization lead to loss of about 10 % of river supply. Riverine input of 5.6±0.6 Tmol/yr 0.6 ±0.5 Tmol/yr Aeolian input about 10Tmol/yr, 5% dissolved (Direct measurements + Si/Geratios Seafloor weathering of basalts about 0.4 ±0.5 Tmol/yr, but estimates vary by an order of magnitude and situation of leaching from clays is unclear. Tréguer et al., Science, 268, 375-379 (1995)
Production and burial of biogenic silica Tréguer et al., Science, 268, 375-379 (1995) Large variations of estimates of opal production based on 14 C productivity and gross C/Si-ratios Tréguer estimates 200-280 Tg based on PP of 60 Gton C, diatom contribution to PP of 35% for oligotrophic gyres and 75% for coastal and Antarctic ocean, and 80% of oligotrophic contribution to global PP, and Si/C of 0.13 in well-fed diatoms Internal cycling by mass balances and marine Si distribution Burial rates of 6.1 ± 1.8 Tmol/yr, based on sed. accumulation rates, dry densities, and biogenic sediment content. About 70% of this in the Antarctic ocean (Why?). Residence times of 15.000yrs for (burial) or 400 yrs (biological uptake) What does that tell us?
averages ~150 μm Total content ~ 10 5 Tmol τ ocean = 15,000 yrs τ surface = 400 yrs Average Ocean [Si] ~ 70μM Si supplied to surface waters is cycled ~40x before permanent removal in sediments Steady State: Accounted: 6.1 Tmol/yr (x10 12 mol)
What we just learned. Steady state concept Reservoir size(s) For each reservoir, total losses match total inputs Residence time concept External (input, sink) Internal (process turnover) Tréguer et al., Science, 268, 375-379 (1995)
Feedbacks pco 2 -T surface -CO 2 solubility f (T) pco 2 -T surface - Ocean circulation pco 2 => weathering (slow) Some more sophisticated controls Here: Change in major nutrient ratios
Advanced studies: Silicate removal by damming Humborg et al., Nature 386, 385-388, 1997. Danube river accounts for about 70% of the riverine runoff into the Black Sea Was dammed by the iron gates about 1000 km upstream in 1972 Mean silicate concentration fell from about 140μM (in 1959-1960) to less then 60μM today, and shows unusal dependence on (artificial) flow regime Artificial lake effect Effect more than compensated for nitrate and phosphate by eutrophication Silicate concentrations in the Black sea: a: winter silicate concentrations about 60 nm from the Danube river; b Silicate concentrations in the central Black Sea during nitrate-depleted conditions
Advanced studies: Silicate removal by damming Humborg et al., Nature 386, 385-388, 1997. Si:N ratios decreased from 42 to 2.8, also driven by rise in median nitrate concentrations from 1.3mM in the early 60th to 7.9mM in the 1980th. Increase in phytoplancton blooms, stronger increase in non-silica forming phytoplancton ( in particular coccolithophores) Change in biological assemblages as well as the ratio of silicate forming to carbonate-forming species
HNLC-Areas
Advanced studies: changing productivity in the ocean by iron fertilization Martin et al., 1994
Advanced studies: changing productivity in the ocean by iron fertilization Martin et al., 1994
Advanced studies: The effect of iron availability on nutrient consumption ratio of diatoms Takeda, Nature 393, 774-777, 1998. In the subarctic Pacific, the Equatorial Pacific, and the Southern Oceans, all major nutrients are available (HNLCareas) Growth limited by availablility of iron Iron fertilization considered to increase the uptake of fossil-fuel CO2 Availability of iron fosters production, but also changes Si:N and Si:P ratios
A possible player in glacialinterglacial CO 2 fluctuations