12.2: Microbial influence on marine sediments

Microorganisms can significantly influence the composition of sediments accumulating in depositional environments. To provide an example, this section considers how microorganisms affect the composition of marine sediments. Major sources of sediments to the Earth’s oceans include detrital inputs from terrestrial environments, biogenic material that forms in the water column, hydrogenous precipitates, and authigenic phases that form within sediment following deposition. Microorganisms play significant roles in generating these sediment phases and transforming them following deposition, as discussed in the paragraphs below. Content presented here draws heavily from Aplin (2000), which offers a wider and more detailed discussion of the mineralogy of modern marine sediments.

Detrital material is generated by weathering in terrestrial environments and transported to the oceans by water (including ice), wind, and gravity. Any mineral can be supplied to the oceans as detritus but the most common are clay minerals and quartz (Aplin, 2000). Organic detritus also enters the oceans from terrestrial areas, and consists mostly of living biomass, plant litter, and highly-degraded soil organic matter (Burdige, 2007). Most river sediment is deposited in nearshore environments but fine-grained detritus reaches the open ocean (Aplin, 2000). Some detrital phases persist beyond early diagenesis and become deeply buried, whereas others are consumed as reactants in diagenetic reactions. Microorganisms help supply these sediments by driving weathering reactions in terrestrial environments (Section 11.4). Moreover, microorganisms also induce formation of some of the clay minerals in freshwater systems (Konhauser and Urrutia, 1999) (Section 12.1.5).

Biogenic material that forms in the marine water column is mostly skeletal biominerals and organic matter from planktonic microorganisms (Berner, 1979; Honjo et al., 1982). Regarding biominerals, the main phases are carbonates formed by foraminifera and coccolithophorids as well as amorphous silica produced by radiolarians and diatoms. The abundance of these biogenic mineral inputs in marine sediment reflects the balance between biological productivity in the water column, the amount of dissolution that occurs between the water column and sediment-water interface, and the extent to which they are diluted by non-biogenic phases (e.g., detritus) (Aplin, 2000). Some portion of these phases dissolve as they settle because seawater is undersaturated with respect to amorphous silica and, except in surface waters, undersaturated with respect to calcite (Aplin, 2000). Roles of microorganisms in generating these sediments are obvious, given that their production is biologically controlled by eukaryotic microorganisms (Sections 12.1.1 and 12.1.2).

Regarding organic biogenic material from the water column, most is considered to be phytoplankton debris (Burdige, 2007). Phytoplankton are also known as microalgae. They are phototrophic microorganisms, and major groups include dinoflagellates, diatoms, coccoliths, and cyanobacteria. Organic matter inputs to the ocean, both terrestrial organics and that produced by phytoplankton, are mostly degraded before they can be buried in sediment. Specifically, less than 0.2% of marine primary production and less than 20% of terrestrial inputs are preserved in marine sediments (Hedges, 1992). Factors that influence what proportion of organic matter is preserved include rates of primary production, the composition of the organic matter, and the speed with which it can be buried and shielded from aerobic degradation (Berner, 1979; Hedges, 1992).

Hydrogenous components of marine sediments are those that precipitate from dissolved reactants in the water column. The most common examples of hydrogenous sediments are iron and manganese (oxyhydr)oxides (Aplin, 2000). They can form where metal-rich anoxic hydrothermal waters discharge and mix with oxic seawater. Discharge of reduced hydrothermal waters into oxic seawater provides a rich energy source for chemolithotrophic microorganisms (Jannasch and Mottl, 1985) (Section 7.5.3). Oxidation of reduced metals within those solutions can produce (oxyhydr)oxide phases that settle onto the seafloor. Metal (oxyhydr)oxides also form in response to upward diffusion of dissolved iron and manganese produced by metal reduction in marine sediment. For both pathways, microorganisms can help drive (oxyhydr)oxide production by catalyzing the associated redox reactions.

Authigenic phases form within the sediment as products of reactions between mineral inputs, organic matter, and pore water. Authigenic clays are generated by reactions involving sediment inputs of amorphous silica, degraded aluminosilicates, and metal (oxyhydr)oxides (Aplin, 2000), as noted above (Section 12.1.5). Other minerals form in response to organic matter degradation. Specifically, microbes couple oxidation of organic matter within the sediments with reduction of available electron acceptors. Oxygen can be depleted from pore water within centimeters of the sediment-water interface, followed by zones where microorganisms reduce nitrate, manganese(IV), iron(III), sulfate, and carbon dioxide as they work their way along the thermodynamic ladder (Section 8.5). Section 12.1 includes descriptions of how such microbial reactions can lead to formation of sulfide and carbonate minerals (12.1.1 and 12.1.4). Among other possibilities, phosphate minerals may also form. Microbial reactions can induce supersaturation of carbonate fluorapatite \(\left(\text{~Ca}_{10} \left(\text{PO}_{4}\right)_{5.74} \left(\text{CO}_{3}\right)_{0.26} \text{F}_{2.26}\right)\) by increasing the phosphate and alkalinity content of pore water (Aplin, 2000). Phosphate necessary to form the mineral is supplied from breakdown of organic matter and reductive dissolution of metal (oxyhydr)oxide, which contain sorbed phosphorus. Microbial reactions can also help increase alkalinity by consuming hydrogen ions and producing bicarbonate.

Taken together this section illustrates that microorganism play major roles marine depositional environments, as suppliers of reactive inputs, drivers of redox reactions, and contributors to terrestrial weathering. It should be emphasized here that not all geochemical reactions are microbial, and many major sedimentary processes do not involve microorganisms. Nonetheless, the contributions of microorganisms are clearly enormous. Microorganisms play significant roles in the supply of each major category of marine sediment and in the transformation of that sediment following deposition.

Figure \(12.8\): Schematic illustration of major inputs and reactions that guide marine sediment composition. Terrestrial sediment inputs include dust and river sediment. Hydrothermal inputs are fluids with reduced elements that can serve as energy sources for vent microbial communities. Redox zones develop in marine sediment as microorganisms couple oxidation of organic matter with available electron acceptors. Authigenic minerals form in response to those redox reactions and reactions between reactive sediment inputs, including organic matter \(\left(\text{C}_{\text{org}}\right)\), amorphous silica \(\left(\text{SiO}_{2} \ (am)\right)\), and calcium carbonate \(\left(\text{CaCO}_{3}\right)\).
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