Wednesday, January 26, 2011

Did silica prevent the phosphate crisis in the Archaean ocean?

**Please note: I wrote this for a class 2 yrs ago, and I know it needs updated with more recent references. It will be, but I wanted to get something other than a placeholder up for you in the meantime.**

Controversy is a defining characteristic of Precambrian geologic studies, in part as a result of the lack of preservation of many rocks and environmental indicators as a result of rapid tectonic cycling during this earliest period of Earth’s history. This controversy extends to debates over the nature of formation of banded iron formations (BIFs), which were deposited during two main time periods ca. 3.8–2.2 Ga and ca. 1.8 Ga and have no modern analogs. At the heart of this debate is the question of what, if any, role biota played in the formation of BIFs. Bejerrum and Canfield (2002) have argued, based on modeling of phosphate concentrations preserved in banded BIFs, that the deposition of iron oxides preserved in BIFs would have created a phosphate crisis in the Archaean and early Proterozoic oceans. Because phosphate strongly adsorbs onto iron oxides, the high iron content of the oceans would have actively scavenged phosphate from the water column. This would lead to an overall reduction in biota because phosphate is a limiting nutrient; this suggests that perhaps abiological models for the formation of BIFs have more standing. However, Konhauser et al. (2007) present contrasting evidence that silica out-competes phosphate adsorption, forming stable complexes that inhibit anion adsorption under conditions similar to the Archaean ocean. They argue that rather than a phosphate crisis, phosphate availability was similar to today’s oceanic concentrations, giving the argument for biologically mediated BIF deposition standing. 

Inherent within these hypotheses regarding the debate over the microbial role in BIF deposition is the subsidiary debate over the timing of the rise of atmospheric oxygen. Brocks et al.’s (1999) publication of biomarker evidence for the presence of cyanobacteria  ca. 2.7 Ga has given strength to the argument that free oxygen must have been present in limited quantities at or near the sea surface during BIF deposition. This has been used to suggest that photo-oxidation of iron in conjunction with the production of free oxygen by cyanobacteria could create a large enough pool of oxidized iron to form BIFs. However, Rasmussen et al. (2008) called this into question based on analysis of the Pilbara Craton kerogen from which the Brocks’ biomarker was extracted. At present, the rise of cyanobacteria and the oxygenation of the oceans appears to have occurred later. In fact, while cyanobacteria were likely operant before the rise of atmospheric oxygen  ca. 2.45 Ga, a stratified, largely anoxic ocean is recorded by biomarkers (Brocks et al., 2005), as well as molybdenum (Anbar et al., 2004) and sulfur (Farquhar and Wing, 2003) isotopic evidence at 1.64 Ga, after the majority of BIF deposition had ceased. These data have been used to infer that anoxygenic photoferrotrophs may have been responsible for oxidized iron present in BIFs, which was supported by Kappler et al.’s (2005) experimental model that showed that purple sulfur bacteria oxidize iron under anoxic conditions similar to those present in the Archaean ocean. More recently, field data from Lake Matano, Indonesia, suggests that green sulfur bacteria may oxidize iron photoferrotrophically under anoxic conditions (Crowe et al., 2008). This provides additional evidence that suggests a causal link between microbial metabolisms and the presence of large quantities of oxidized iron found in Precambrian BIFs.

Johnson et al. (2008) attempted to distinguish between microbial oxidation in the near surface environment and microbial reduction of FeOH3 at depth, which was proposed to have occurred based on carbon isotope data (Walker, 1984) and is believed to be a necessary step in the formation of BIFs based on the oxidation state of iron in the deposits (e.g. Fe2.4+). However, because the fractionation of iron is so small, these different processes appear to have ranges of fractionation values that overlap (Johnson et al., 2008; Severmann et al., 2008), making their use problematic in modern as well as ancient environments.

The biological origin question was further addressed by Posth et al. (2008) who sought to go beyond simply discussing the nature of iron oxidation in the Archaean, but to also couple microbial activity to the alternating iron-silica banding that is a defining characteristic of many BIFs. Their experiment suggests that temperature fluctuations in an iron and silica rich ocean could cause the banding observed. At low temperatures microbial oxidation of iron is inhibited, but it resumes when temperatures increase. While this study may be useful for the rare BIFs associated with Proterozoic glaciations, the relevance of this study to Archaean BIFs is limited because it is unlikely that the Archaean ocean reached such low temperatures. Nevertheless, the cyclic deposition of silica and iron raises questions about the source, roles, and relationships of silica and phosphate in the depositional system, a question not addressed by Posth et al. (2008). Thus, the origin and mechanism of formation of BIFs remains unknown.

BIFs have been divided into two types of deposits (Algoma and Superior). These groups are determined by what the geological relationships infer about the putative origin of iron . However, positive ╬ÁNd and negative Eu anomalies reported by Jacobsen and Pimentel-Klose (1988), as along with oxygen isotope data, have been used to suggest that the source of iron for all BIFs is related to mid-ocean ridges (Hayashi et al., 2007). This is of interest and importance because much of the silica (and shale in the case of the Hamersley Basin, Western Australia) has a suggested continental source, although the continental signature decreases upsection (Hayashi et al., 2007). Since the majority of phosphate input in to the oceans is controlled by continental weathering (Schlesinger, 1997), and the source of silica has also been suggested to originated from a continental source, it is likely that their input to the system would be coeval. These different sources for the compositional bands in one way complicate the story of the depositional environment of BIFs, but on the other hand provide a potential rationale for why phosphates, while found in BIFs (Bejerrum and Canfield, 2002), may have been effectively out-competed by silica on a large scale, especially if silica and phosphate inputs into the system were not entirely coeval with iron inputs. Moreover, these different component sources suggest that a nascent oceanic rift may be a probable primary tectonic setting of formation for BIFs, because iron mobilized from hydrothermal alteration is not present in the water column in large concentrations at great distances from the ridge axis.

While ridge axes in the Precambrian may have been broader structures and may have been associated with greater hydrothermal fluxes to higher heat flows than those of modern fast-spreading ridges, like the East Pacific Rise, it is difficult to imagine high iron concentrations persisting many kilometers away from the ridge axes. Moreover, the association of Algoma BIFs with greenstone belts suggests that preservation of at least some BIFs is directly associated with ophiolite emplacement mechanisms, because most ophiolites are now considered to have formed in marginal basins in the upper plate of subduction zones (Phillips-Lander and Dilek (2008)Robinson et al., 2008). These settings would provide broad continental slopes due to crustal loading as a result of the intrusion of mafic dikes during rifting, which would in turn provide ample area for anoxygenic phototrophs to thrive within the photic zone.

Nascent rift settings would put microorganisms in direct association with a light energy source, phosphate nutrient sources, as well as iron and silica sources—all the components believed to be necessary for BIF deposition during the anoxic environment during the Archaean and early Proterozoic. This creates a plausible circumstantial case for the mode, nature, and setting of BIF deposition; however, significant additional research needs to be done in order to prove that the iron-rich oceans present during BIF deposition did not create a phosphate crisis and that BIF deposition was inherently biologically mediated.

Future research should include (1) investigations into the rate at which anoxygenic photoferrotrophic microbial metabolisms mobilize and oxidize iron in analogous field settings; and (2) the relationship of this metabolism to nutrient availability—including phosphate—a point well articulated by Crowe et al.(2008). This should provide a means for better estimating the minimum amount of phosphate required for microbially mediated BIF deposition, which can be correlated with existing data regarding phosphate concentrations preserved in BIFs. Phosphate concentrations preserved in BIFs likely record minimum phosphate concentration for the Precambrian oceans; these data may shed additional light on whether or what extent there was a phosphate crisis in the Archaean ocean.

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Thanks to @Colo_Kea for editing help.