Celestine in a sulfidic spring barite deposit - A potential biomarker?

Abstract

We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulphidic spring samples and lithified tufa sample, celestine was detected in finegrained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratorybased precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulphidic springs.

Controls on Barite Crystal Morphology during Abiotic Precipitation

“…Therefore, barite might have incorporated strontium in its structure. The XRD pattern of celestine is very similar to the pattern of the barite, and they can be identified together because celestine may precipitate locally within the barite crystal [44]. This presence of minor quantities of celestine within barite minerals may occur due to temperature variations during barite precipitation, which would influence the thermodynamic stability of the barite-celestine solid solution [45].…”

Barium bioaccumulation by bacterial biofilms and implications for Ba cycling and use of Ba proxies

“…Experimental work also demonstrated that phytoplankton may play a crucial role in bioconcentrating Ba 18 , and since sinking organic matter serves as a substrate for heterotrophic bacteria, the link between bacterial abundance and export productivity may be a key factor for Ba bioaccumulation in microenvironments in the ocean water column. Indeed, in natural environments, Ba enrichment associated with microbial biomass has been described in diverse settings, including thermal springs 15 , 16 , 20 , biofilms in the Roman Catacombs 21 , bacterial EPS at fumaroles in Solfatara Crater, Italy 22 , and in filaments of sulfide-oxidizing bacteria in marine cold seep 17 . Likewise, culture experiments demonstrated the capability of bacteria to promote barite precipitation under laboratory conditions 10 , 11 , 23 .…”

“…Although the microbial role in precipitation of sulfates has been poorly investigated in comparison to the microbial involvement in carbonates formation, microbial barite precipitation has been described in hot springs environments, where bacteria play a role either in oxidizing sulfur compounds to generate sulfate or in accumulating Ba in microbial mats 15 , 16 . In marine environments, bacterial involvement in barite formation has recently been reported for a cold seep where barite precipitated on filaments of sulfide-oxidizing bacteria.…”

Organic–mineral interfacial chemistry drives heterogeneous nucleation of Sr-rich (Ba x , Sr 1− x )SO 4 from undersaturated solution

“…Despite the geochemical importance of marine (Ba x , Sr 1−x )SO 4 formation, two paradoxes still exist. First, the global oceans are undersaturated with respect to barite, while barite is extensively present in seawater column and marine sediments (5,7,8). Second, Sr-rich marine barite (>50 mol% strontium) formation (7,9) is not thermodynamically favorable (7,10), as the solubility of SrSO 4 (K sp,SrSO4 = 10 −6.63 ) is three orders of magnitude higher than BaSO 4 (K sp,BaSO4 = 10 −9.98 ) (3,10).…”