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| chapter 4 |
| chapter 5 |
| chapter 6 |
| chapter 7 |
| chapter 8 |
| chapter 9 |
| chapter 10 |
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| chapter 12 |
| chapter 13 |
| chapter 14 |
| chapter 15 |
| chapter 16 |
| chapter 17 |
| chapter 18 |
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3.1. What is Raleigh fractionation, or distillation, and how does it work?
3.2. What is the difference between “equilibrium” versus “kinetic” isotope effects?
3.3. What factors control the isotopic composition of clay minerals found in marine sediments?
3.4. In coastal sediments why might one observe the “apparent” fractionation of the sediment organic matter (SOM) during early diagenesis? Based on your answer, would you expect the SOM to become heavier or lighter? Why?
3.5. Why does methanogenesis produce extremely light methane?
3.6. During sulfate reduction, bacteria preferentially use light sulfate (32SO4-2) versus heavy sulfate (34SO4-2). How do you think this fractionation would vary as:
a. the rate of sulfate reduction decreases
b. the concentration of sulfate decreases.
b. the concentration of sulfate decreases.
3.7. Seagrasses (unlike other marine plants) obtain the majority of the carbon they fix from CO2(aq) rather than dissolved bicarbonate. As a result of this, they are often carbon limited. Based on this observation:
a. explain why seagrasses generally have heavier δ13C values (greater than ca. -10 to -15‰) than typical marine phytoplankton (roughly -23 to -26‰),
b. explain why seagrasses become isotopically lighter as light levels decrease.
3.8. Imagine that a fixed amount of 226Ra is placed in a sealed vessel and undergoes radioactive decay according to eqn. (3.17). Using the radioactive decay equation (3.14) derive the mathematical relationship for secular equilibrium between 226Ra and 222Rn. Recall that secular equilibrium occurs when the activity of 226Ra (= λRaNRa) equals that of 222Rn (= λRnNRn).
3.9. Discuss two different ways (in a broad sense) that radiocarbon analyses can be used to place age constraints on natural materials?
| Chapter 4 |
4.1. Calculate the bulk sediment density (ρb) for sediments with a porosity (ϕ) of 0.5, 0.7 and 0.9. Assume a dry sediment density of 2.7 g/cm3.
4.2. Fine-grained sediments have small particle sizes as compared to sandy sediments; however, they have higher porosities. Why might one view this as counter-intuitive? What is, however, an explanation for these observations?
4.3. Along these same lines, why do fine-grained sediments undergo compaction with sediment depth/burial?
4.4 Why is advective flow in fine-grained muds less important than it is in sandy sediments?
4.5 Using equation (4.8) and figure 4.5, estimate the minimum sediment grain size for which advective flow will be significant.
Chapter 5
5.1 Describe the difference between steady-state and non-steady state conditions?
5.2 In some works the process of bioturbation is sometimes referred to as “biodiffusion”, while bioirrigation is sometimes referred to as “bioadvection”. Please explain the reasons for this.
5.3 Based on eqn. (5.2) what is the diffusion length scale for seasonal time scales? Given this observation, develop an explanation for why pore water profiles of organic matter remineralization end-products show seasonal variations over much larger depth scales. Is this somehow related to seasonal variations in the production (or consumption) rates of these constituents?
5.4 For a river such as the Amazon, why do the associated “estuarine” process occur on the continental shelf, rather than within a more physically confined estuary?
Chapter 6
6.1. What is sediment tortuosity and how does it impact diffusion in sediments?
6.2. In a sediment undergoing compaction, why are solid phase compaction and sediment accumulation uncoupled?
6.3. Looking at Fig. 6.4 (p. 93), why is it reasonable to assume that ks, kn and kc are approximately equal to one another?
6.4. How are sediment burial rate, sediment accumulation rate and sedimentation rate related? What is their relationship to cumulative mass-depth?
| Chapter 7 |
7.1 What is the difference between autotophic and heterotropic metabolism?
7.2 How does respiration differ from fermentation?
7.3 What are the strengths and weaknesses of the biogeochemical zonation model?
7.4 In examining the occurrence of certain microbial processes in sediments, why is it important to look at the ∆G° and the ∆G of a reaction?
7.5 What factors make metal oxide reduction different than other forms of microbial respiration?
7.6 What controls the occurrence of microbial Mn and Fe reduction (coupled to organic matter oxidation) in marine sediments?
7.7 How does “reverse methanogenesis” appear to work (as a mechanism to explain anaerobic methane oxidation)?
7.7 Why is aerobic H2 oxidation not apparently important in most marine sediments?
7.8 How do the linkages between chemolithotrophic reactions (or their abiotic equivalents) and respiratory processes work?
7.9 Sediment oxygen uptake often provides a reasonably good estimate of the overall (depth-integrated) rate of sediment carbon oxidation (i.e., oxic plus anoxic remineralization), despite the fact that aerobic respiration is only a fraction of the total sediment oxygen uptake and the depth-integrated rate of sediment carbon oxidation. Why is this the case?
7.11 What is meant by the phrase an “elastic (or variable) depth scale” at the beginning of section 7.5.1?
7.12 What factors control the trends in the individual panels in Fig. 7.7?
7.13 In many global models of sediment diagenesis (e.g., Middelburg et al., Deep-Sea Res. 1997), water column depth is used as the master variable in the relationships used to define sediment biogeochemical rate parameters. Why does this appear to be a reasonable approach?
7.14 How do the relative and absolute rates of sediment aerobic respiration vary as one moves from deep-sea to coastal sediments?
7.15 What are redox oscillations?
7.16 Why is it thought that the initial depolymerization of sediment organic matter is somehow involved in the possible controls of O2 in sediment carbon remineralization/preservation?
7.17 What factors may lead to enhanced organic carbon remineralization under mixed redox conditions?
| Chapter 8 |
8.1. Why is the concept of sedimentary organic matter “reactivity” related to the time scales of observation?
8. 2. What is the significance of eqn. (8.13), which indicates that solute bethic fluxes across the sediment water interface equal the depth-integrated rates of solute production/consumption? Why does the relationship break down under non-steady-state conditions?
8.3. The continental margins are less than ~20% of the total surface area of the oceans, yet they represent the region where >80% of the marine sediment carbon burial occurs. Why do you think this is the case?
8.4. How are recent numerical models (e.g., CANDI) “better” than past steady-state models (e.g., Burdige and Gieskes, 1983)?
8.5. Compare these numerical models with the fitting procedures described by Berg et al. (Limnol. Ocean. 43:1500, 1998) with particular reference to the types of information they give us about sediment processes.
8.6. What are the empirical relationships that have to be established to build global sediment diagenesis models?
Chapter 9
9.1 Define diagenetic maturity.
9.2 Why are amino acids not particularly good biomarkers for organic matter sources?
9.3 Why are D-amino acids potentially good tracers for bacterial biomass?
9.4 Besides using them to determine the general presence/absence of bacterial biomass, what would you need to know about the biogeochemistry of D-amino acids to use them as quantitative indicators of bacterial biomass in sediments?
9.5 What are some of the weakenesses of the Dauwe degradation index in quantifying different types of natural organic matter?
9.6 How do concentrations of neutral sugars provide information on the degradation state of natural organic matter?
9.7 Why is lignin “different” than other biopolymers such as proteins or carbohydrates?
9.8 How are “free” versus “bound” lipids different?
9.9 What is one fundamental difference between the lipids found in archaea versus those found in true bacteria?
9.10 Branched-chain fatty acids are common biomarkers of bacterial sources and show differing degrees of reactivity depending on the time scale over which their diagenesis is examined. Why is this the case?
9.11 We often say that MUOM simply represent organic matter that escapes our “analytical window.” Explain what that means.
9.12 What are the strengths and the weaknesses of the techniques used to define and isolate humic substances (i.e., humic acid, fulvic acid, and humin)?
9.13 What are the weaknesses of the geopolymerization model?
9.14 What other ways might abiotic polymerization processes “produce” MUOM?
9.15 What are refractory biomacromolecules and why might they be considered as MUOM?
9.16 What are the necessary conditions for physical protection of organic matter to cause it to “become” MUOM?
9.17 What is the dominant form of nitrogen in living organisms? In ancient organic matter (kerogen, coal, fossil fuels)? If the former is the precursor of the latter, why are they different?
Chapter 10
10.1 Why do we think that the DOM accumulating in sediments (with depth) is refractory?
10.2 What factors may control the asymptotic concentration of DOC (or DON) observed in many sediments?
10.3 Describe the pore water size-reactivity (PWSR) model.
10.4 How is the PWSR model similar to the “biodegradation” model for humic substance formation?
10.5 Describe the material found in each of these pools in the the PWSR model: HMW-DOM; pLMW-DOM; mLMW-DOM.
10.6 Why is there interest in studying short chain organic acids (SCOAs), such as acetate, in sediment pore waters?
10.7 The chemically-measured concentrations of compounds such as SCOAs may be less than the biologically-available concentration. Why may this be the case?
10.8 In sediments, why is net DOM production less than gross DOM production?
10.9 Much of the DOM adsorption in sediments appears to be largely irreversible. What are the implications of this observation in terms of the factors controlling sediment carbon preservation?
Chapter 11
11.1 How can we differentiate between marine and terrestrial organic matter in sediments, and what are the strengths and weaknesses of the different approaches used here?
11.2 How and why does the sediment C/N ratio break down as a tracer of terrestrial versus marine organic matter?
11.3 Why does terrestrial organic matter (TOM) appear to be more heterogeneous than marine organic matter?
11.4 Why does this heterogeneity of TOM appear to vary with particle size?
11.5 Why is recycled kerogen difficult to determine in marine sediments?
11.6 What role does recycled kerogen play in terms of the relationship between carbon burial in marine sediments and the addition of O2 to the atmosphere?
11.7 What role does the production of bacterial biomass play in sediment carbon cycling?
11.8 What evidence do we have that the majority of the organic matter remineralized in marine sediments is marine organic matter?
11.9 How do we reconcile the observation discussed in question 11.8 with the fact that terrestrial organic matter is also deposited in marine sediments?
11.10 What is referred to by “remineralization constancy”, and what might lead to its occurrence?
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12.1 What controls the thickness of the diffusive sub-layer?
12.2 You have a fine-grained sediment in which diffusion is the predominant transport process across the sediment-water interface. What role does the thickness of the diffusive sub-layer play in controlling the flux across the interface to the overlying waters?
12.3 What are the characteristics that differentiate permeable sands from non- (or less) permeable clastic sediments?
12.4 How does permeability in sandy sediments modify the "normal" diagenetic pathways seen in less permeable silts and muds?
12.5 What is the evidence for transport of bottom water into permeable sediments, and to what depth if any does this phenomenon occur? What controls this depth?
12.6 What is the evidence for the input of reactive particulate organic matter in permeable sediments by these same advective processes, and how does this express itself in sediment processes?
12.7 What are the difficulties one encounters when using benthic flux chambers to estimate fluxes from permeable sediments?
12.8 How well do lab (flume) studies of processes in permeable sediments reproduce field observations in similar natural sediments? What are the “weak” links between these lab and field studies?
12.9 To what degree are permeable sediments important on a global scale? What is their imprint on ocean chemical properties? How important are these advective processes as compared to bio-advective processes such as bioturbation and bioirrigation on local and global scales?
12.10 When using a radiotracer to examine sediment processes such as bioturbation, why is it important to “match” the time scale of radioactive decay to the time scale of the process?
12.11 Why is bioturbation generally of greatest importance in affecting sediment solids (versus pore waters)?
12.12 How does one differentiate local from non-local sediment mixing?
12.13 Estimates of bioturbation rates appear to be tracer-dependent (i.e., short-lived radioisotopes often yield faster bioturbation rates). Why is this the case?
12.14 What is “age-dependent” mixing?
12.15 How does bioturbation affect the depth-distribution of reaction rates in sediments? How might this affect solute distributions in sediments?
12.16 How does bioirrigation change the “reaction geometry” of marine sediments?
12.17 Why might the α values calculated with eqn. (12.24) vary for different solutes?
12.18 In a particular bioirrigated sediment differences in reaction geometries for different solutes are likely to result in different α values for the solutes. Please explain why this is the case.
12.19 How does the diffusive “openness” of a marine sediment affect sediment processes?
12.20 Is the concept of diffusive openness an experimental artifact of sediment incubation studies (for details, see Valdemarsen, T., and Kristensen, E., 2005, Diffusion scale dependent change in anaerobic carbon and nitrogen mineralization: True effect or experimental artifact? J. Mar. Res., v. 63, p. 645-669).
12.21 What are the problems with extrapolating BFE values estimated with one solute to other solutes?
12.22 Why do BFE values for O2 show a much smaller dynamic range than values for other solutes (e.g., nitrate or silica)?
Chapter 13
13.1 How do the three examples of nitrate profiles in Fig. 13.1 relate to the profiles shown in Fig. 7.8? What makes the 3-layer model in Fig. 13.1 different than the other two models shown here?
13.2 Why is trace metal cycling in sediments strongly linked to Fe and Mn redox cycling?
13.3 An examination of Table 13.2 indicates that many trace metals are strongly enriched in deep-sea sediments as compared to the concentrations in “average” shale. Why is this the case?
13.4 Sinking biogenic material is likely the dominant carrier phase for trace metals to deep-sea sediments. However, metal oxides appear to be the major burial phase (with sediment depth). How (and why) doe this transformation occur?
13.5 Why does Mn redox cycling in sediments often lead to a “peak” in solid phase Mn near the sediment redox boundary?
13.6 What factors may lead to a benthic flux of Mn2+ out of marine sediments?
13.7 Why do we generally assume that the rate of silica dissolution decreases with sediment depth?
13.8 In a given sediment, the asymptotic concentration of silica in many pore water profiles (i.e., C∞ as discussed in the text) is generally less than the solubility of biogenic silica in that sediment (Csat). What factors may lead to this difference?
13.9 Why do pore water silica profiles provide evidence in support of authigenic clay mineral formation?
13.10 Why is it difficult to use solid phase biogenic silica deposition in sediments as a proxy for paleo-productivity?
13.11 Describe the factors controlling the distribution of biogenic silica in marine sediments. In particular be certain to explain how silica accumulates in any marine sediment given the fact that the oceans are everywhere undersaturated with respect to biogenic silica.
13.12 The distribution of silica in surface sediments is primarily controlled by productivity in the surface waters while that of calcite is primarily controlled by bottom water chemistry. Please briefly discuss the reasons for this observation.
13.13 What is the difference between the lysocline, calcite saturation horizon and the CCD?
13.14 What is the role of metabolic CO2 in promoting CaCO3 dissolution in sediments overlain by supersaturated bottom waters?
13.15 Why is the depth-dependence of metabolic CO2 production important in controlling the occurrence of CaCO3 dissolution in sediments? How does this relate to the concept of metabolic dissolution efficiency (MDE)?
13.16 Recent papers have suggested that carbonate dissolution in seawater may be first order with respect to the degree of undersaturation (versus the 4th to 5th order as previously thought). What is the evidence in favor of first order kinetics? What are its implications in terms of understanding carbonate dissolution in deep-sea sediments? In shallow water sediments?
13.17 What are some of the considerations that lead to uncertainties in the oceanic alkalinity budget?
13.18 On glacial-interglacial time scales, what factors may contribute to non-steady-state condition in the oceanic calcium carbonate budget?
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14.1 Why does diffusion damp out the expression of high frequency events in pore water profiles?
14.2 How does bioturbation (in the sediment mixed layer) act as a similar “filter” for high frequency events?
14.3 In a related sense, why is the derivation of eqn.(14.5) incomplete?
14.4 Explain why the k for organic matter remineralization must be greater than ~1 yr-1 to observe seasonality in deep-sea sediment remineralization processes?
14.5 How do such considerations impact the occurrence of such seasonality in remineralization processes in estuarine and coastal sediments?
14.6 What other factors control the occurrence of seasonality in remineralization processes in estuarine and coastal sediments?
14.7 In sediments that show seasonality in remineralization processes, why is eqn. (14.7) valid?
14.8 Sediments that show seasonality in remineralization processes may appear to be in steady-state over longer time scales. Please explain this observation
14.9 In light of this observation, how can one obtain estimates of the steady-state rates of sediment remineralization processes over these longer time scales?
14.10 Describe the process of turbidite “burn-down”?
14.11 Why does turbidite burn-down act as a negative feedback on further downward movement of the redox boundary?
14.12 How does the burn-down process affect a redox sensitive element such as manganese, or uranium?
14.13 What does turbidite burn-down tell us about the factors controlling organic matter remineralization in marine sediments?
14.14 What are the factors the control the steady-state formation of a Mn-peak in marine sediments? Why do the same factors sometimes lead to a surface sediment layer enriched in Mn?
14.15 Why do opposite changes in the carbon rain rate or the bottom water oxygen concentration have similar impacts on the non-steady-state diagenesis of Mn in marine sediments?
14.16 What factors lead to the occurrence of multiple Mn-peaks in marine sediments?
14.17 What factors control the preservation of relict Mn peaks in marine sediments?
Chapter 15
15.1 Why does the relationship between carbon preservation and remineralization appear to vary with the time scale of the observations?
15.2 What are the five broad levels of carbon loading observed on sediment particles, and what controls these differences?
15.3 Describe the surface adsorption/mesopore protection hypothesis? How has this model evolved since Mayer originally described it in 1994?
15.4 Why is bottom water oxygen concentration not a very good proxy for the role oxygen may play in sediment carbon preservation?
15.5 How can benthic macrofaunal processes affect sediment carbon remineralization?
15.6 While sediment oxygen “exposure” may play an important role in sediment carbon preservation, oxygen exposure time (as described in Fig. 15.4) may not always be a good way to quantify this effect. Why is this so?
15.7 Why does the “mineral conveyor belt” in Fig. 15.6 provide negative feedback control on atmospheric O2 levels?
15.8 Why is most terrestrial organic matter buried in deltaic continental margin sediments?
15.9 The preservation of organic matter in marine sediments can be examined from two broad perspectives: factors that more directly enhance preservation and therefore indirectly inhibit remineralization, and factors that specifically inhibit remineralization and therefore indirectly enhance preservation. What sorts of processes/phenomena fall into the later case? What sorts fall into the former case?
15.10 What are the different ways that oxygen exposure may specifically enhance organic matter remineralization?
15.11 What are the different ways that oxygen exposure may specifically impact organic matter preservation?
15.12 The factors that control organic matter preservation in marine sediments are not necessarily mutually exclusive, and almost certainly operate together and/or in succession. Discuss how this might occur.
15.13 Does remineralization ever “stop” in marine sediments?
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16.1 Compare and contrast aerobic respiration and sub-oxic respiration in terms of their impact on carbonate mineral saturation state in pore waters.
16.2 Why does sulfate reduction initially lead to carbonate mineral undersaturation in sediment pore waters?
16.3 Why does the fate of the sulfide (H2S) produced during sulfate reduction play a role in impacting the carbonate mineral saturation state of anoxic sediment pore waters?
16.4 Why does ammonium production in sub-oxic and anoxic sediments represent an alkalinity source to the pore waters?
16.5 What other acid-generating processes can promote CaCO3 dissolution?
16.6 How might bioturbation or bioirrigation affect CaCO3 dissolution?
16.7 Describe the similarities and the differences in the factors that favor the formation of Mn and Fe carbonate phases in sediments.
16.8 Describe the causes of seasonal cycles in carbonate mineral saturation state in coastal sediments.
16.9 What are the different sources of nitrate to fuel sediment denitrification, and how does their importance vary as one moves from deep-sea to continental margin and eventually coastal sediments?
16.10 What is the biogeochemical evidence for the anoxic oxidation of ammonium in marine sediments (i.e., ammonium oxidation that does not use O2 as the oxidant)?
16.11 Describe how bioirrigation impacts nitrogen redox cycling in sediments.
16.12 Describe the factors that will eventually lead to ammonium fluxes from sediments being a significant component of sediment nitrogen cycling.
16.13 As a remineralization process denitrification appears to be most important in sediment regions between the deep-sea and the continental shelf. Why is this the case?
16.14 What are the major forms of phosphorus found in marine sediments?
16.15 How are iron and phosphorus cycling in sediments linked?
16.16 Describe the process of “sink-switching” as it relates to phosphorus burial in sediments.
16.17 Although phosphate and ammonium are both produced during anoxic remineralization in coastal sediments, the seasonal cycles of their benthic fluxes appears to be uncoupled. Why is this the case?
Chapter 17
17.1 What are the main sources and sinks for sulfur in the oceans?
17.2 What is sulfur disproprtionation?
17.3 What is the fate(s) of sulfide produced during bacterial sulfate reduction?
17.4 How does sulfide oxidation affect the δ34S of pyrite formed in sediments?
17.5 How does diffusive transport in the pore waters affect the δ34S of pyrite formed in sediments?
17.6 Describe the three possible mechanisms of pyrite formation in marine sediments.
17.7 Why is sulfur burial efficiency generally low in many marine sediments?
17.8 What factors control (or impact) the C/S ratio of marine sediments?
17.9 How do shifts in the “equilibrium” of eqn. (17.6) impact atmospheric O2?
17.10 What is the significance of anaerobic methane oxidation?
17.11 How is methanogenesis in continental margin sediments different than that in shallow marine sediments?
17.12 How is methanogenesis in continental margin sediments related to the formation of methane gas hydrates?
17.13 How does methane production occur in deeply-buried marine sediments, and how does it relate to what is referred to as the deep marine biosphere?
17.14 Why is trace metal cycling in shallow marine sediments strongly tied to sulfur redox cycling?
17.15 What factors will control whether a shallow marine sediment is a source or a sink for dissolved metals to the overlying waters?
17.16 How do benthic macrofaunal processes affect trace metal cycling in shallow marine sediments?
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18.1 Any reverse weathering processes (whether associated with hydrothermal vents or authigenic clay mineral formation) must have certain “properties” to achieve geochemical mass balance in the oceans. Briefly describe what these properties must be.
18.2 What factors may have contributed to the failure to see a definitive “signal” of authigenic clay mineral formation in studies prior to the 1970’s?
18.3 How are the studies of silica diagenesis discussed in ch. 13 consistent with the occurrence of authigenic clay mineral formation?
18.4 How does authigenic clay mineral formation appear to occur in Amazon delta sediments?
18.5 Describe the properties of sedimentary environments in which Al or Fe versus silica represent the limiting reactants in authigenic clay mineral formation.
18.6 How could these observations of authigenic clay mineral formation impact estimates of silica burial efficiency?
18.7 If authigenic clay mineral formation is indeed an important process in sediments, how is it similar to the concept of “sink-switching” as was discussed in ch. 16 in terms of sediment phosphorus burial?