Exam revision chapter 6

what causes high rates of sedimentation

what causes high rates of sedimentation

reduction in river water flow - seasonally and weekly, common in summer with no rain, predominantly tidal flow, aggregation/flocculation - deposition of fine sediments, adds to sediment, high rates of biological activity - estuaries one of the most productive systems, get a sulphur smell with high sedimentation

oxidation of organic matter

a key process in marine sediments decomposition of organic matter is mainly carried out by bacteria

what are organic carbon levels in coastal waters and estuaries

high (compared to deep ocean) due to high rates of biological productivity - less in deep ocean as surface material rains out - consumed on way down which takes time (rapidly attenuated)

why is oxygen the most important oxidant in seawater

it has a high concentration and is used in microbial cellular processes - high concentration from photosynthesis as bacteria take up O2 and produce CO2

why would other oxidising agents be used

in sediments, oxygen may be used up more rapidly than it can be re-supplied by diffusion so a diagenetic sequence was established

what is the diagenetic sequence

O2 -> used first, aerobic respiration NO3- -> denitrification, becomes source of electrons Mn(IV) -> as Mn oxide, Mn reduction Fe(III) as Fe oxide (oxyhydroxides) -> Fe reduction SO42- -> close to surface so smell sulphur at riverbank, sulphate reduction CH20 -> don't want to reach this point as more potential on climate, methane fermentation all these reactions are microbially catalysed

redox boundary

sinking organic material in the oxidising estuarine water, sink to the sediments across the redox boundary

what is the idealised diagenetic sequence

based on energy - balance the equation and work out how much energy is used idealised diagenetic sequence of bacterial decomposition of organic matter in estuarine/coastal muddy sediments as a function of redox conditions and availability of oxidants oxidants are used up in order of their energy yield - DG (Gibbs free energy change) - negative DG indicates energy is released

idealised diagenetic sequence

oxic conditions: electron acceptor is oxygen, processes are aerobic respiration, nitrification and sulphide oxidation

suboxic conditions: electron acceptor is nitrate and metals, processes are denitrification, manganese oxide reduction (MnO2 is in soluble so bacteria produce 2Mn2+ so can escape from the sediment), nitrate reduction and iron oxide reduction (4Fe2+ hits surface and therefore O2 goes to Fe3+ so precipitates out)

anoxic conditions: no O2 source, electron acceptors are sulphate, CO2 selected and C1-C2 compounds, processes are sulphate reduction (fermentation) (eggs smell as H2S is soluble so comes out of sediment), acetate fermentation and CO2 reduction

organic material supply in estuaries and shelf seas

higher and oxygen is completely removed from sediment pore waters in the upper few mm-cm of sediments

organic material supply in the open ocean

oxygen penetrates the sediments to 10s of cm depth, big circulation in the open ocean = swept to depths = low sedimentation rate since takes so long, so O2 penetrates deeper in atlantic compared to pacific - constantly releases more O2 in pacific than atlantic

importance of denitrification and sulphate oxidation

they are widespread, quantitatively significant and massive sources of these elements to the water column

importance of Mn and Fe

reduction of these is less important due to limited supply of reactive metal oxides they are more prevalent and important in continental margins than in the open ocean

redox zones in estuarine and coastal sediments

compressed (short) and may even occur simultaneously e.g. due to presence of microenvironemnts, such as burrows/root rubes and organic hot spots microorganisms add O2 to the environment due to burrowing they have less O2 due to intense productivity and flooding events - flooding from rivers washes farmland and topsoil which is carried down the river, it flocculates and precipitates out of the estuary adding more to the sediments

O2 concentration with depth in an incubated marine sediment

aggregate of organic matter inserted into sediment which starts to be used by organisms a plume forms as get low O2 where wouldn't expect, an anoxic microfiche is formed over time as the aggregate is gradually decomposed goes back to normal after time

rate of exchange in sediment drivers

exchange of solutes between pore waters and overlying seawater is driven by diffusion rate of exchange is enhanced by:

• sediment re-suspension (friction of tide)

• bioirrigation (organism doing)

• dredging (anthropogenic)

importance of bioturbation

important for carbon and trace element cycling different organisms introduce niche zones dead organisms are a source of organic carbon introduce oxygen deeper than normal microniches created around food sources - almost a zonation

what is pore water sample processing

measure how much is diffusion and how much is enhanced by organisms

trace metal cycling and Fe and Mn oxyhydroxides

metals adsorbed on Fe/Mn oxyhydroxides (Cu, Ni, Zn, Cd, Co) will be released into sediment pore waters if these phases are reduced this leads to cycling of metals at the at the oxic/anoxic boundary and can lead to accumulation of metals in sediments

trace metals in different phases

pore waters (dissolved phase): no manganese, nickel or cobalt at the surface (no diffusion), peak when they become soluble in the anoxic enviro solid(particulate) phase: higher at the surface since all precipitated

• shows lots of elements locked in the sediments

how to assess estuarine sediment rates

important as deliver sources of Fe, Mn and N primordial radionuclides (derived from rocks, long half lives) can assess:

• sediment accumulation rates and mixing (234Th, 210Pb)

• dating techniques e.g. shell growth rates (228Ra, 210Pb, 210Po)

• bioturbation (how much is happening) (234Th)

how to use 210Pbxs for dating

226Ra naturally occurs in rocks and decays to 222Rn (noble gas) 222Rn escapes and decays to 210Pb (lead) 210Pb removed by precipitation and incorporated into accumulating sediment - this is the unsupported/excess activity (210Pbxs) (C14 doesn't bind to particles so isn't useful for sediments)

what is total 210Pb

the sum of excess activity and supported activity from decay of 226Ra in the sediment

how to estimate sediment accumulation

once incorporated into sediment, 210Pbxs decays at a known rate allowing sediment accumulation rate to be estimated

radioactive equilibrium

concentration at depth = concentration at surface

supported 210Pb activity

bioturbation occurring at surface up to peak point decaying occurring on the decline from the peak - work with this data supported activity occurring when there is no change in activity with depth (in radioactive equilibrium) - subtract this value to get the unsupported activity

unsupported 210Pb activity

A1 is first point of decaying values A2 is last point of decaying values A1 corresponding depth value is z1/t1 A2 corresponding depth value is z2/t2

sedimentation rate

if it is constant then s = z/t (z is depth and t is time) decrease in excess 210Pb is a function of time

radioactive decay

ln A1/A2 = lamda(t2-t1) change in conc is ln A1/A2 substituting the relationship with time and depth: ln A1/A2 = (lamda (z2-z1) / S therefore a plot of ln(A1/A2) vs depth will take the form of a straight line with gradient -S/lamda radioactive decay constant (lamda) for 210Pb is 0.0311/yr

how to calculate a sedimentation rate

identify the background (supported) activity, Ab - value of A at greater depths where it's not changing with depth subtract the background activity form the observed activities at shallower depths take the natural log to get ln(A) = ln(Ameasured - Ab) plot depth z against ln(A) ignore the points in the surface mixed region where ln(A) doesn't change with depth ignore points in the background region at depth (Ameasured = Ab) measure slope in the middle region - it will be negative multiply the minus slope by the radioactive decay constant (lamda = 0.0311/yr for 210Pb) to get the sedimentation rate

global sedimentation rates

some continental margins e.g. river depth and upper gulf of thailand have high sediment rate and high productivity too deep ocean sediments have low sediment rates

how can estuarine sediments record pollution

sediments close to Fawley oil refinery show evidence for pollution high sedimentation rates after development of refinery - takes crude ores and refines to oil, pumps up pollution have lower 206Pb/207Pb and 206Pb/208Pb ratios after development of refinery (they are all stable isotopes) lead isotopes give an indication of where rocks are formed and therefore different isotopic signatures