High Molybdenum availability for evolution in a Mesoproterozoic lacustrine environment

Trace metal data for Proterozoic marine euxinic sediments imply that the expansion of nitrogen-fixing cyanobacteria and diversification of eukaryotes were delayed while the availability of bioessential metals such as molybdenum in the ocean was limited. However, there is increasing recognition that the Mesoproterozoic evolution of nitrogen fixation and eukaryotic life may have been promoted in marginal marine and terrestrial environments, including lakes, rather than in the deep ocean. Molybdenum availability is critical to life in lakes, just as it is in the oceans. It is, therefore, important to assess molybdenum availability to the lacustrine environment in the Mesoproterozoic. Here we show that the flux of molybdenum to a Mesoproterozoic lake was 1 to 2 orders of magnitude greater than typical fluxes in the modern and ancient marine environment. Thus, there was no barrier to availability to prevent evolution in the terrestrial environment, in contrast to the nutrient-limited Mesoproterozoic oceans. Complex life forms began to emerge during the Precambrian. Here, the authors tie this evolution to an increase in trace metal availability, namely the Mo content of lacustrine shales, suggesting that life evolved in terrestrial and marginal marine environments rather than the Mo-limited deep ocean.

T he content of Molybdenum (Mo) in ancient carbonaceous shales is a valuable indicator of palaeo-redox conditions. Both the bulk concentration of Mo and the ratio of Mo to total organic carbon (% TOC) have been used to track the concentration of Mo in sea water through geologic time [1][2][3][4][5][6][7] . The data have been used particularly to infer the record of availability of nutrients in the ocean to allow the expansion of N-fixing cyanobacteria and the diversification of eukaryotic life 1,5,[8][9][10] . However, growing evidence for the importance of shallow marine or terrestrial environments to the development of both N fixers and eukaryotes [11][12][13][14] requires an assessment of nutrient availability to the Mesoproterozoic surficial environment. Molybdenum is a key element limiting life in lakes, especially through control of N-fixation [15][16][17][18] .
Most of the Mo supplied to the oceans is derived from weathering of the continents 6,19,20 . Lakes and other terrestrial environments are important reservoirs for the Mo between weathering and entry to the marine environment 21,22 . Lake sediments can exhibit significant enrichment in Mo relative to the surrounding bedrock composition 23 . For example, lake sediments in Canada and Sweden show concentrations of Mo 1 to 2 orders of magnitude greater than in parent granites 21,24 . Thus, ancient lake sediments are a good archive of Mo availability.
Well-preserved Mesoproterozoic lacustrine sediments of 1.18 Ga age occur in the Stoer Group, Torridonian Supergroup, UK 25 . The 2-km-thick Stoer Group was deposited in an extensional setting on the passive margin of the palaeocontinent Laurentia [26][27][28] . The basin was a few hundred kilometres from the ocean, unconformable upon a basement of Lewisian (Archaean) gneisses. Provenance, palaeocurrent and sedimentological studies all indicate that the Stoer Group sediments were derived from the Lewisian basement. Rapid lateral and vertical fluctuations in coarse to fine siliciclastic facies (breccias, conglomerates, sandstones, mudrocks) and variability/reversals in palaeocurrent data are regarded as characteristically continental 26,28 . Almost all of the rocks are red-coloured, also characteristically continental. Depositional environments are interpreted as alluvial fans, fluviatile, aeolian and lacustrine, interfingering with each other. Laminated mudrocks are interpreted as lacustrine. Numerous horizons of desiccation cracks indicate emergence from shallow water in most lacustrine rocks. A single unit of sulphide-bearing black shale/laminated limestone is interpreted as a permanent lake deposit. Outcrop over 40 km suggests a minimum area of B1,000 km 2 for the lake basin, but possibly only 10% (100 km 2 ) may have deposits of sulphidic black shale. Clay geochemistry data confirm that this unit is lacustrine, not marine 29 . Pseudomorphs after gypsum occur in both the black shale and red beds. The climate was semi-arid, in palaeo-latitudes between 20 and 30°N (refs 30,31). The high sedimentation rate of the Stoer Group, characteristic of a continental basin, means that the carbon burial rate in lake sediments was much higher than in contemporary marine sediments 32 . A single marker horizon consisting of fine red sediment mixed with melt fragments (Stac Fada Member) is interpreted as an impact ejecta deposit 33 . Ar-Ar analysis of authigenic K-feldspar in the impact deposit yields a date of 1.18 Ga (ref. 25). The rocks have experienced mild metamorphism, but all sedimentary features are well preserved 26 and the sedimentary geochemistry is undisturbed 34 .The Stoer Group sediments present an unparalleled opportunity for measurement of Mo flux because the timescale of sedimentation can be measured accurately. By analogue with recent lake deposits, it has been demonstrated that the lacustrine lamination was annual 35 . This interpretation was augmented by the discovery of solar cycles within the laminated succession 35 . Therefore, the rate of deposition and the Mo concentration can be measured to yield a burial flux.
The calculated Mo flux for the Stoer Group black shales is 1 to 2 orders of magnitude greater than typical Mo fluxes in the modern and ancient marine environment. Both Mo levels and Mo/TOC ratios are more comparable to values in much younger Phanerozoic marine black shales than Proterozoic marine black shales. The Mo is resident in both organic matter and sulphides, as in modern lakes. The Mesoproterozoic lacustrine environment, therefore, represented an important reservoir for Mo. The high Mo levels indicate that there was no barrier to the availability of Mo to prevent evolution in the terrestrial environment, in contrast to the nutrient-limited Mesoproterozoic oceans.

Results
Burial rates. The Stoer Group includes a single unit of finely laminated, pyritic black shale within the Poll a'Mhuilt Member at Bay of Stoer ( Fig. 1), interpreted as the deposit of a stratified, sulphate-rich lake 26 . All samples were collected from this section (for precise locations see Supplementary Fig. 1), at National Grid Reference NC 032285. This unit has yielded a microfossil assemblage 26 , a record of solar cyclicity 35 and sulphur isotope data indicative of microbial activity 36 . The lamination indicates a burial rate averaging 0.27 mm per year (see Supplementary  Table 1). This rate, in common with lacustrine sedimentation rates in general 37 , greatly exceeds rates in the deep ocean.  Table 1). The background samples average 0.7 p.p.m. The mean total organic carbon (TOC) content of the shale is 0.27%. Mo/TOC ratios are up to 859, with a mean of 304. Re/Mo ratios are in the range 10 À 5 to 10 À 3 (Fig. 3), consistent with euxinic conditions of deposition 38 .
Sulphur contents. The Stoer Group samples have a mean sulphur content of 2.2%, and S/C ratios averaging 8.5, indicating deposition in a sulphidic setting, most commonly characteristic of marine rocks but also sulphate-rich lacustrine basins 39 . Mo is readily sequestered in sulphidic environments, where hydrogen sulphide is generated in the water column or pore waters by microbial sulphate reduction, and the Mo is partially incorporated in authigenic sulphide minerals, either directly or via thiomolybdates 1,3,18 . Accordingly, in the Stoer Group, the Mo is a trace component of pyrite, which precipitated early, before compaction of the host shale, due to microbial reduction of sulphate in the lake waters 36 . Mo in modern euxinic stratified lake waters is similarly sequestered rapidly into iron sulphides 40,41 . A broad correlation between Mo content and S content ( Supplementary Fig. 2) reflects this residence of some Mo in the Stoer Group pyrite. However, element maps show that much Mo is also resident in the organic matter-rich laminae ( Supplementary Fig. 3). Laser ablation-inductively coupled plasma-mass spectrometry analysis of the pyrite confirms that some Mo is resident there, at concentrations of up to 1,000 p.p.m. In addition to pyrite, the Stoer Group shale contains authigenic cadmium sulphide (greenockite), which suggests precipitation from a sulphidic water column 42 .

Discussion
The mean Mo content of the Stoer Group shale is over three times the average Mo content in Precambrian euxinic shales, which is B25 p.p.m. (ref. 1). The concentration of Mo is especially remarkable given the high sedimentation rate of the Stoer Group sediments, as other studies suggest that a high sedimentation rate dilutes the authigenic enrichment of trace metals including Mo 3,43 . The Mo/TOC data are much higher than the Proterozoic average of 6.4 p.p.m. per wt% (ref. 2). Mo/ TOC ratios in marine euxinic sediments 1,4,6 remained o25 until a marked increase in atmospheric oxidation at the end of the Proterozoic. The Mo enrichments and elevated Mo/TOC ratios are more similar to those of Phanerozoic marine black shales than Proterozoic black shales (Fig. 2), reflecting the high degree of sulphidation in the Stoer Group shales. The variability in the Mo/ TOC results in some scatter on a cross-plot of Mo versus TOC ( Supplementary Fig. 4), contrary to the more coherent trends normally exhibited by these components 44 , which further indicates that the Mo content is controlled by both the S content and TOC content. Possibly the Mo was first associated with organic matter as a molybdate, as in modern anoxic lakes 45 , then partially reacted with sulphide to precipitate out. The Mo/ TOC ratios are particularly high because the carbon content of the Stoer Group shales is comparatively low. The carbon content would have been higher before thermal maturation and liberation of hydrocarbons, but still probably o1% TOC. Nevertheless, at the relatively low levels of oxygenation in the Mesoproterozoic, lake stratification would have occurred readily and engendered the euxinic conditions, which precipitated sulphide minerals and sequestered trace metals including Mo.
The combination of sedimentation rate and molybdenum content for individual samples allows the determination of Mo burial flux data. Sixteen samples yield a range of Mo fluxes from 2.7 to 14 Â 10 À 4 mol Mom À 2 a À 1 (Supplementary Fig. 5). The mean values for burial rate and Mo content give a Mo flux of 6.5 Â 10 À 4 mol Mom À 2 a À 1 . These values are comparable with those in some modern lakes 22,23 , and in modern anoxic marine sediments 44 , despite much lower carbon contents ( Fig. 4; Supplementary Table 2). The values are 1 to 3 orders of magnitude higher than in modern continental margin sediments 19 (Fig. 4). Comparison can be made with fluxes from episodes of anoxic marine sedimentation in the Phanerozoic geological record. Black shales from the Cambrian Alum Shale  Table 2).
Data from the Stoer Group indicate that Mo was being sequestered before it reached the open ocean, in this case in a euxinic lake. In lakes that were not euxinic, Mo would not be sequestered to the same degree. However, in a separate study, a more limited contrast between Mo levels in epicratonic (higher contents) and craton margin (lower contents) marine sediments of 1.1 Ga age 6 similarly suggests an ocean-ward gradient of declining Mo availability. An important control on high Mo levels Nevertheless, the high Mo levels in the sediments imply that Mo input to the lake was not a limiting factor. The critical advantage to the availability of Mo in the lacustrine environment compared with the marine environment may be in the speciation of Mo, and the associated bioavailability. Mo occurs in sea water as the relatively unreactive molybdate oxyanion in the VI oxidation state, whereas in lakes up to 50% should be in the V oxidation state 16 . The anoxic conditions in a stratified lake, as envisaged for Stoer Group deposition 26,35 , would enhance the relative abundance of the Mo (V) form. Not only is the Mo(V) form more bioavailable 60,61 , but it is not inhibited by dissolved sulphate in the way that Mo (VI) is inhibited, hence the lacustrine environment strongly favours the stimulation of nitrogen fixation by its dissolved Mo content 16 . A high Mo content in the underlying lacustrine sediments may also be significant to productivity, despite low levels of Mo in lake waters, due to diffusion of Mo upwards into the water from the sediments. This additional source of Mo would allow biological uptake higher than predicted 61 .
Several studies of modern lakes show that their Mo levels are adequate to support N-fixing cyanobacteria [16][17][18] , so it is possible that the Stoer Group lake was similarly adequate to host N fixers. In the particular case of a sulphate-rich lake, high availability of Mo helps to mitigate the inhibition of N fixation by the sulphate 17 . As the dating and palaeogeography of rocks of this age become better constrained 62 , it is becoming clear that sulphate deposits in continental and shallow marine deposits were more prevalent in the late Mesoproterozoic 63 than hitherto appreciated (Supplementary Table 3). This implies a high potential for the sequestration of metals as sulphides, a possibility which merits further research. The Stoer Group data are also consistent with evidence for a greater diversity of eukaryotes in marginal marine environments compared with offshore marine environments, which was hypothesized to reflect the scarcity of Mo in offshore waters distant from continental run-off 13 .The availability of trace metals was a key influence on the development of multicellular life 57,64 , and Mo-dependent enzymes in particular are essential to eukaryotic cell biology 57,58,[64][65][66] . The evolution of Mo usage is regarded as a fundamental aspect of the diversification of eukaryotes, probably from 1.5 to 1.0 Ga (ref. 58). The data reported here show that Mo was readily available to support such a diversification in the terrestrial environment at this critical time.

Methods
Measurement of burial rates in Stoer Group samples. The laminites of the Poll a'Mhuilt Member were deposited in a lacustrine setting 35 . Individual laminae comprise silt-clay, carbonate and organic components, which reflect seasonal variations in sedimentation and are comparable to annual laminae from more recent and present day lacustrine settings 35 . These annually deposited laminae allow depositional rates to be calculated with high precision from thin sections or cut and etched hand specimens. In thin section, measurement of individual laminae was possible to a precision of 10 mm using equipment originally developed for the measurement of tree rings (digital positionometer under a petrographic microscope). Depositional rates were calculated from cut and etched hand specimens by dividing the total thickness of the laminae present by the number of laminae counted under magnification. Many hundreds of laminae were measured to give high precision statistics 35 .
Measurement of molybdenum contents. Molybdenum and rhenium contents were measured using inductively coupled plasma-mass spectrometry (ICP-MS) using an Agilent 7,700 instrument. Samples of 100 g rock were milled and homogenized, and 0.25 g digested with perchloric, nitric, hydrofluoric and hydrochloric acids to near dryness. The residue was topped up with dilute hydrochloric acid and analysed by inductively coupled plasma-emission spectroscopy using a Varian 725 instrument. Samples with high concentrations were diluted with hydrochloric acid to make a solution of 12. For mean Mo content of 82 p.p.m., this equates to Mo flux of 6.5 Â 10 À 4 mol Mom À 2 a À 1 . The range of fluxes calculated is shown in Supplementary Fig. 2.
Calculations for other modern and ancient systems assume mean Mo and maximum sedimentation rates from the cited literature to give upper limits to fluxes. Where a range of TOC is cited rather than a mean value, the upper limit is used in Fig. 4. The data plotted in Fig. 4 is shown in Supplementary Table 2.
Element mapping. Mapping was undertaken using a Bruker M4 Tornado XRF Mapper, based at the CSIRO Advanced Characterization Facility, Perth, Australia. The mapping used a pixel size of 25 mm, with data acquired over 10 ms per pixel. The MCBM 50-0.6B Rh X-ray tube voltage was 50 kV and the anode current was 500 mA.