Sedimentary Diagenetic Processes Research Articles

Early diagenesis in an organic-rich turbidite and pelagic clay sequence from the Cape Verde Abyssal Plain, NE Atlantic: magnetic and geochemical signals

Abstract Processes of early diagenesis in sediments from the Cape Verde Abyssal Plain are investigated using the complimentary techniques of environmental magnetism and solid-phase geochemistry. The sequence captured by a 2 m-long box core comprises an organic-rich turbidite derived from the NW African continental margin (emplaced ca. 330 ka BP), overlain by brown, pelagic clay. Diagenesis in the pelagic clay mainly involves slow, in-situ oxidation of the iron oxide phases present, which Mossbauer-effect data suggest are detrital haematite and magnetite (possibly maghaemitised). In the turbidite, diagenesis has proceeded in two stages: the first involving suboxic (reductive) processes, following depletion of porewater O2, and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of O2 from the overlying seawater. Carbonate-free magnetic parameters, interparametric quotients, and remanence acquisition, demagnetisation and coercivity measurements of the core, reveal that significant depletion of detrital ferrimagnetic iron (Fe2+/Fe3+) oxide grains like (titano)magnetite has occurred within the turbidite during suboxic diagenesis, and confirm previous suggestions that reductive dissolution of such components is a grain size-selective process. However, the higher oxidation state (Fe3+), canted-antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis. Remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic Fe/Mn micronodules in a distinct layer immediately overlying the front, and above this, a further interval of Mn-enriched, “diagenetic laminae”. However, within the suboxic, organic-rich zone of the turbidite, well below the oxidation front, there is clear evidence for the presence of a significant concentration of fine (single-domain) ferrimagnetic iron oxide grains, which are most probably associated with a population of live magnetotactic bacteria. Between this zone and the oxidation front, there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly, and fossil magnetosomes in the sediment are being reductively dissolved progressively up-sequence. These results are broadly similar, though differing in some respects, to magnetic and geochemical studies of hemipelagic and high-productivity pelagic sediments in which Mn- and Fe-redox boundaries have been recognised, and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind.

Early diagenesis of organic matter from sediments of the eastern subtropical Atlantic: evidence from stable nitrogen and carbon isotopes

Stable isotopes of sedimentary nitrogen and organic carbon are widely used as proxy variables for biogeochemical parameters and processes in the water column. In order to investigate alterations of the primary isotopic signal by sedimentary diagenetic processes, we determined concentrations and isotopic compositions of inorganic nitrogen (IN), organic nitrogen (ON), total nitrogen (TN), and total organic carbon (TOC) on one short core recovered from sediments of the eastern subtropical Atlantic, between the Canary Islands and the Moroccan coast. Changes with depth in concentration and isotopic composition of the different fractions were related to early diagenetic conditions indicated by pore water concentrations of oxygen, nitrate, and ammonium. Additionally, the nature of the organic matter was investigated by Rock-Eval pyrolysis and microscopic analysis. A decrease in ON during aerobic organic matter degradation is accompanied by an increase of the 15N/ 14N ratio. Changes in the isotopic composition of ON can be described by Rayleigh fractionation kinetics which are probably related to microbial metabolism. The influence of IN depleted in 15N on the bulk sedimentary (TN) isotope signal increases due to organic matter degradation, compensating partly the isotopic changes in ON. In anoxic sediments, fixation of ammonium between clay lattices results in a decrease of stable nitrogen isotope ratio of IN and TN. Changes in the carbon isotopic composition of TOC have to be explained by Rayleigh fractionation in combination with different remineralization kinetics of organic compounds with different isotopic composition. We have found no evidence for preferential preservation of terrestrial organic carbon. Instead, both TOC and refractory organic carbon are dominated by marine organic matter. Refractory organic carbon is depleted in 13C compared to TOC.

Simulation of reactive processes related to biodegradation in aquifers: 1. Structure of the three-dimensional reactive transport model

The reactive transport model TBC (transport, biochemistry, and chemistry) numerically solves the equations for reactive transport in three-dimensional saturated groundwater flow. A finite element approximation and a standard Galerkin method are used. Solute transport is coupled to microbially mediated organic carbon degradation. Microbial growth is assumed to follow Monod-type kinetics. Substrate consumption and release of metabolic products is coupled to microbial growth via yield coefficients and stoichiometric relations. Additionally, the effects of microbial activity on selected inorganic chemical species in the aquifer can be considered. TBC allows the user to specify a wide range of possible biochemical and chemical reactions in the input file. This makes TBC a powerful and flexible simulation tool. It was developed to simulate reactive processes related to in situ bioremediation, but further fields of application are laboratory column studies on redox processes coupled to organic carbon degradation, field cases of intrinsic biodegradation, and early diagenetic processes in sediments.

An experimental and modeling study of pH and related solutes in an irrigated anoxic coastal sediment

Macrofaunal irrigation is an important process in nearshore sediments, facilitating greater exchange between sediments and seawater and imparting significant lateral heterogeneity to the porewater profiles of many constituents. Like many macrofaunal activities, irrigation is a transient behavior, i.e. tubes and burrows are flushed periodically, at frequencies that generally are species-specific. As a result, transient concentrations within the dwelling arise, potentially impacting gradients, fluxes and reaction rates in the vicinity of the dwelling. We investigated the impact of periodic burrow irrigation on the distribution of several diagenetically important porewater constituents. Laboratory experiments evaluated irrigation periodicity using artificially irrigated tubes embedded in nearshore organic-rich sediments, and microdistributions of oxygen and pH in laboratory experiments were measured with microelectrodes. To help interpret our results, we also constructed a simplified time and space-dependent transport-reaction model for oxygen, pH and sulfide in irrigated sediments. Laboratory results show substantial differences in the pH field of sediments surrounding an irrigated tube as a function of irrigation frequency. Higher pH values, indicative of an overlying water signature, were observed in the vicinity of the tube wall with increasing duration of irrigation. Conversely, oxygen concentrations did not vary significantly with the amount of irrigation, most likely a result of extremely high sediment oxygen demand. Model results are consistent with laboratory findings in predicting differences in the measured variables as a function of irrigation frequency. However, the nature and extent of the model-predicted differences are often at variance with the experimental data. Overall, experimental and modeling results both suggest irrigation periodicity can substantially influence porewater distributions and diagenetic processes in sediments. Future studies should examine the influence of irrigation periodicity on the types and rates of reactions, and the attendant biological features, in the environment encompassing the tube or burrow wall.

FERROMANGANESE NODULES ON THE UNDERWATER SLOPES OF THE USKHAN ISLANDS, LAKE BAYKAL

Although ferromanganese crusts from shallow depths in Lake Baykal have been described previously, the presence of nodules at depths of about 500 m are a new discovery made possible by exploration using manned submersibles. The nodules are comparable to oceanic nodules in composition, but have some significant differences, including a more rapid rate of growth. Sedimentary-diagenetic processes are mainly responsible for their formation, but there is reason to believe hydrothermal fluids play some role.

The distribution of aluminium in the Greenland Sea and its relationship to ventilation processes

During two cruises to the Greenland Sea in 1988 and 1989 a total of 690 on‐board aluminium (Al) determinations were performed on samples collected from profiles located along four transects in the Greenland Sea, as part of the Greenland Sea Project. Al concentrations range from extremely low values (0.4 nM) in the Polar waters of the East Greenland Current draining the Arctic Ocean, to 17.9 nM in the newly formed Norwegian Sea Deep Water (NSDW) at the periphery of the Greenland gyre. The Al content in the component water masses serves to constrain the make‐up of these water masses from the various mixing products of the region. The enrichment of Al in the NSDW both at its site of formation within the Greenland Sea (up to 17.9 nM) and in the Lofoten Basin of the Norwegian Sea (up to 40.3 nM) leads to the conclusion that there must be an additional, external, source of Al to these water masses. The apparent inability of water column or sedimentary diagenetic processes to provide this enrichment and the observation of enriched Al values associated with a high‐salinity shelf brine leads us to the conclusion that these brines may be responsible for the observed enrichments. The high concentration of Al in the Greenland Sea Deep Water compared with the water masses that produce it, suggests that Al may be acting as an inverse tracer of the processes that ventilate this basin by convective overturn, i.e. the Al concentration which increases with time, as a result of continuous brine or other external inputs, is reduced during convective events. It is also argued that this flushing of elevated Al water during convective events may have a similar counterpart in the Iceland Sea and that this mechanism could go some way toward solving the Al deficit that exists between the enriched North Atlantic Deep Water and its depleted precursor water masses observed in the intermediate waters of the Greenland and Iceland seas.

Effects of Diagenesis on Rock Magnetic Properties of Sediments and Sedimentary Rocks

Rock magnetic methods are sensitive indicators of the state of iron in a sediment, and changes in rock magnetic properties can provide information about diagenetic processes in sediments. One means by which this can be done is to compare the alternating field demagnetization of the natural remanent magnetization (NRM) with the alternating field demagnetization of a laboratory induced saturation isothermal remanent magnetization (SIRM). Data collected from various types of sediments and sedimentary rocks that have been studied in the Paleomagnetism Laboratory of the University of California-Davis indicate that unlithified lake sediments of Holocene or Pleistocene age have NRM-SIRM intensity ratios that range from 5 {times} 10{sup {minus}2} to 5 {times} 10{sup {minus}3}, and that on log-log plots, the intensity ratios are concave upward slightly. For Neogene and Late Cretaceous sedimentary rocks, the ratios are closer to 1 {times} 10{sup {minus}3}, and the curvature is considerably less systematic. These results imply that lithification may lead to the destruction or demagnetization of a significant fraction of the magnetic carriers as well as to changes in the grain size distribution of the remaining carriers. On the other hand, chemical alteration of sediments appears to produce NRM:SIRM ratios that are three to five timesmore » higher than those of equivalent unaltered sediments. This result is corroborated by studies of the rock magnetic properties of individual tephra layers that can be found in both altered and unaltered states in large, drained lacustrine basins.« less

Diagenetic behavior of manganese in Tokyo Bay sediment

To understand the behavior of manganese in diagenetic processes in sediments of an enclosed bay which is similar to those of an estuary, chemical analyses have been carried out on both sediment and interstitial water of a core sample collected from Tokyo Bay. The results suggest that redistribution of manganese takes place within the sediment as a result of the dissolution of buried manganese oxides and hydroxides under reducing condition, the downward diffusion of Mn2+ through the interstitial water toward lower layers and then the precipitation of carbonate. The carbonate formed in the sediment contains managanese carbonate probably as a solid solution between calcitic calcium carbonate and manganese carbonate.

Early diagenesis in transitional sedimentary environments of the White Oak River Estuary, North Carolina 1

Changes in sedimentary diagenetic processes resulting from the transition from a terrestrial to a marine environment were studied through a series of riverbed cores collected from the White Oak River Estuary. Analyses of interstitial water and associated solid phases reveal systematic geochemical and mineralogical trends over an 18‐km stretch of estuary and river channel related to the transition from a dominantly marine to a dominantly terrestrial setting. The most striking of these trends involves the nature of the metabolic pathways by which bacterial decomposition of organic matter occurs and probable differences in the composition of organic matter. In the riverbed stretches under periodic marine influence, sulfate reduction is a major process, owing to the availability of seawater sulfate. Upstream, however, methane generation becomes more important. Consistent with this trend is a decrease in sedimentary pyrite in methane‐rich sediments. Total inorganic carbon decreases by nearly an order of magnitude upstream, and pH is significantly lower in the terrestrial environment. Chemically reactive products of organic decomposition, such as ammonia and phosphate, are considerably less concentrated in the cores taken farthest from marine influence.