Even though it is long known that micro-organisms play an important role in the dissolution of synthetic glass, their impact on volcanic glass alteration was ignored until recently (Thorseth et al., 1992). Yet, biologically mediated alteration of volcanic glass is potentially a major process in low temperature geochemistry: Large quantities of thermodynamically unstable volcanic ash are deposited into the marine and terrestrial environments where microbes may take an active part in nearly all low temperature geochemical processes. Colonizing bacteria may locally change the pH enhancing dissolution of glass which then provides nutrients that further enhance biological growth. It is important to understand the kinetics of these processes as well as their geochemical fluxes and their potential feedbacks.
We carried out a series of controlled experiments to determine the types and reaction rates of biologically mediated chemical exchange between volcanic glass, biological materials and sea water. Starting materials include a technical borosilicate glass (nuclear waste glass) and glass from a natural oceanic tholeiitic basalt. These glasses were exposed to sand-filtered surface seawater (5 µmols/l Si), in open and in closed chemical sytems. Most biologically active experiments included the microbe populations introduced with the non-sterilized seawater. One experiment was carried out with a well defined marine cyanobacteria culture. We also carried out sterile control experiments. In closed system exchange experiments, we used 1-6 mm sized crushed glass particles to minimize the risk of clogging and removal of fine particles, while keeping the chemically active surface area relatively high. Polished glass plates were introduced in all experiments for SEM studies of microbes and etch morphology of corrosion features.
None of our sterile dissolution experiments displayed any visible signs of glass surface pitting, while all biologically active experiments displayed etch marks on the glass surfaces and development of biofilms. Biofilms are thickest and most complex in experiments with the longest duration and exposure to the largest quantities of seawater. Corrosion pits in >100 day exposure experiments were generally smaller than 1 µm in size, while >300 day experiments display pits up to several µm in size, depending on the duration of an experiment and the types of microbes.
Sterile flow-through experiments of seawater through 1-6mm glass particles (50 l water/approx. 300 g glass) show rapidly increasing Si from 3.5 µmoles/l (=surface seawater), increasing to about 75 µmoles/l after 270 days, asymptotically approximating a saturation limit of about 100 µmoles/l. Sea water in the parallel, biologially active experiment, over the same time period maintained Si concentrations below 6.5 µmoles/l. In this experiment we identified large numbers of siliceous microbes, mostly diatoms but also siliceous dinoflagellates and a Si-rich sediment (80 wt% SiO2). Mass balance considerations suggest that most of the Si in diatoms and the sediment has to be derived from the dissolution of the glass. The Si inventory in the water remains low becuase of effective Si utilization by siliceous algae. These obervations rejuvenate the long discussion about the biogenic versus volcanic origin of Si in cherts, where amorphous Si is commonly distinguished from biogenic Si with diatom morphologies. Our observations suggest that much of this "biogenic" chert may also have (indirectly) received its Si from the alteration of volcanic ash. This is supported by the very common association of chert with volcanic ash.
Trace element abundance patters were determined for fresh glass, and glass that has been allowed to exchange with 50 l sewater for 314 days under non-sterile conditions. Trace element abundance patterns are overall quite similar except for the enrichment of some elements in the altered glass, in particular Ba (enriched by 85 %), Sr (17 %) La (15 %) Ce (18 %) Sr (17 %) and U (48 %). These results show that interaction of seawater and basalt glass is very effective in the removal of these elements from seawater.
These data show that the earliest diagenesis of basalt, and the settling of volcanic ash thorugh the water column are very effective in supplying seawater and the biosphere with Si while removing in particular Ba, U, La and Ce.
Thorseth, I.H. et al., Geochim. Cosmochim. Acta 56: 845-850 (1992).