Increased biomass and carbon burial 2 billion years ago triggered mountain building

The geological record following the c. 2.3 billion years old Great Oxidation Event includes evidence for anomalously high burial of organic carbon and the emergence of widespread mountain building. Both carbon burial and orogeny occurred globally over the period 2.1 to 1.8 billion years ago. Prolific cyanobacteria were preserved as peak black shale sedimentation and abundant graphite. In numerous orogens, the exceptionally carbonaceous sediments were strongly deformed by thrusting, folding, and shearing. Here an assessment of the timing of Palaeoproterozoic carbon burial and peak deformation/metamorphism in 20 orogens shows that orogeny consistently occurred less than 200 million years after sedimentation, in a time frame comparable to that of orogens through the Phanerozoic. This implies that the high carbon burial played a critical role in reducing frictional strength and lubricating compressive deformation, which allowed crustal thickening to build Palaeoproterozoic mountain belts. Further, this episode left a legacy of weakening and deformation in 2 billion year-old crust which has supported subsequent orogenies up to the building of the Himalayas today. The link between Palaeoproterozoic biomass and long-term deformation of the Earth’s crust demonstrates the integral relationship between biosphere and lithosphere. High burial of organic carbon in sediments around 2 billion years ago acted to enhance crustal deformation and led to intensified mountain building both in the Paleoproterozoic and since, suggests an assessment of the timing of carbon burial and deformation

O rogeny involves the building of mountains by plate tectonic processes in which the lithosphere is deformed under compression. Several lines of evidence mark the Palaeoproterozoic as the time when mountain building commenced in the forms which have continued to the present day. This is evident in a rapid increase in the frequency of collisional orogens, an increase in the preservation of continental crust as recorded by the earliest large lateral plate motions, the earliest global subduction network at 2 Ga, the predominance of steep subduction zones, and the first appearance of ophiolites about 2 Ga 1-6 . The Palaeoproterozoic also saw a marked increase in the biomass in the oceans, and consequent burial of organic carbon in the crust, which are recorded in the geochemical anomalies of the Lomagundi-Jatuli and Shunga events [7][8][9] . The earliest Palaeoproterozoic carbon burial occurred at the passive margins of fragmenting supercratons as plate tectonics emerged 10 . The increase in abundance may have been a unique response to the c. 2.3 Ga Great Oxidation Event (GOE), and/or a series of regional responses to widespread igneous activity that provided increased nutrients 7,11 . The result was prolific oxygentolerant cyanobacteria that left globally widespread carbon-rich sediment. An analysis of the time interval between the burial of organic carbon and orogeny indicates that the developments of high biomass and plate tectonics are linked. The additional carbon allowed easier deformation of the crust, in a manner that built mountain belts, and thereby plate margins characteristic of modern plate tectonics.

Results and discussion
Lubricating the growth of orogens: organic matter and mountain building. The formation of plate margins, where orogens develop, is enhanced by sediment, which confers low strength. Wet sediment and metasediment increase the lubrication of subduction 12,13 . Sediment containing carbonaceous matter is particularly effective in allowing deformation to occur, due to the ductile nature of organic-rich shales, overpressure weakening due to maturation of organic matter, and metamorphism to low-friction graphite 14 . Sediments rich in organic matter are relatively common in collisional orogens due to the incorporation of thrust platform sequences 14 . Consequently, numerous studies identify organic material as a critical control on the weakening of faulted rocks in orogens [15][16][17][18] . Only small amounts (few%) of carbon are required to dominate the frictional strength of the rock and allow seismic slip 19 . Upon graphitization of the carbon, the frictional strength is further reduced to about 0.3 times the normal stress for carbonaceous pelitic rocks 20 . Critically, low frictional strength enhances thruststacking in collisional orogens 21 , thus promoting crustal thickening during mountain building 22 .
Direct evidence for the deformation potential of highly carbonaceous rocks in orogens is found in an examination of major detachment surfaces in the younger Phanerozoic record. Twenty examples of thrust detachments that supported orogenic deformation (10 s to 100 s km extent), aged Cambrian to Paleogene, are consistently facilitated by shales containing a few percent TOC (Supplementary Table 1). The critical importance of this high level of organic carbon concentration to allow deformation is evident in a data set of Devonian shales (n = 102), where shearing occurred in shales with mean TOC 2.8%, while no shearing occurred in shales with mean TOC 1.4 % 23 . When highly carbonaceous rocks occur on a global basis, their contribution to orogenic deformation is similarly widespread. Following the oceanic anoxic events of the Cretaceous, the resultant black shales became detachment surfaces in numerous orogens including in the Rockies, the Andes, Svalbard, central Europe, Indonesia, and Japan (Supplementary Table 1).
Given the link between organic matter and deformation evident in younger rocks, the scene was set for collisional orogenesis at~2 Ga (Fig. 1) by the exceptional accumulation of Fig. 1 Palaeoproterozoic orogens, distributed globally, exhibiting significant deformation within highly carbonaceous rocks. Orogenies all commenced 2.2 to 1.8 Ga. Details in Table 1. Orogens shown relative to distribution of Proterozoic shields 47 . organic carbon during the mid-Palaeoproterozoic (Figs. 1, 2). Abundant carbon is evident from a peak in black shale deposition 24 , and from the record of the world's richest (highest % TOC) graphite deposits 25 . The maximum carbon contents in carbonaceous rocks in the Palaeoproterozoic orogens are consistently above 10%, and many are above 20% (Table 1). These values are much greater than typical values for carbonaceous rocks in Phanerozoic orogens (Supplementary Table 1). Most of the richest graphite deposits are Palaeoproterozoic 25 .
The marine cyanobacteria that are assumed to be responsible for enhanced carbon burial underwent several important developments following the GOE. Cyanobacteria became larger after the GOE, up to 50 µm diameter from <3 µm previously, and then developed sheaths 26 , potentially increasing the mass of cellular carbon that was buried. A 10 µm diameter cell has about 25 times as much carbon as a 2.5 µm cell, based on established relationships 27 . At this range of cell sizes, larger phytoplankton cells would also settle through the water column significantly faster, and would be more likely to flocculate 8 . This would enhance the burial of carbon by restricting the time when oxidation could occur 28 .
In many cases, the carbonaceous sediments were accompanied by platform limestones. The stromatolite record shows a peak at 1.9 Ga, consistent with widespread sedimentation on broad shallow platforms 29 . The passage of anomalous quantities of carbonate into subduction zones at this time is recorded by widely distributed carbonatites in Palaeoproterozoic orogens 30,31 . The abundant carbonate would have further reduced friction in the Palaeoproterozoic crust 32 . Supracrustal sediments engender thin-skinned tectonics, which is characterized by high degrees of crustal shortening 22,33 . The wide distribution of supracrustal sediments deposited at~2.0 Ga was therefore available to confer a high degree of shortening to the immediately subsequent orogens.
Palaeoproterozoic orogens: widespread mountain building. A peak in orogenesis during the Palaeoproterozoic at~2 Ga is marked by a large number of individual orogens 34 , the overall preserved orogen length 34 , and a high incidence of metamorphism 35 . The first widespread formation of high mountains is inferred at this time, from a peak in the production of S-type granites from 1.95-1.65 Ga and a high incidence of metamorphic rocks 36 .
Global deformation of the Palaeoproterozoic lithosphere is shown by the response to abundant carbon in numerous individual orogens. Twenty orogens with very highly carbonaceous rocks, of Palaeoproterozoic age distributed worldwide ( Fig. 1), each record deformation focussed in pelitic/graphitic metasediments, including thrusting, imbrication, isoclinal folding, and shear zones that characterise crustal shortening (Table 1, Supplementary Note 1). The maximum period between sedimentation and deformation in the Palaeoproterozoic data set (measured from the onset of sedimentation to end of orogeny) was mostly less than 200 Myr, notwithstanding uncertainties in dating ( Fig. 3; Table 1). The period ranges from 60 to 120 Myr for five orogens in North America, where more age data is available, suggesting that this is a good guide to typical times for consumption of oceanic lithosphere in the Palaeoproterozoic. Most subducting lithosphere today is Late Cretaceous or younger, with a mean age of about 65 Ma 37 , and the time interval between Phanerozoic shale sedimentation and orogeny is usually less than 200 Myr and often less than 100 Myr (Supplementary Table 1). The consumption of seafloor sediment in the Palaeoproterozoic was, thus, at a similar rate to today, which has several implications. The similarity is consistent with the establishment of global subduction, and cycling of carbon via subduction zones, Fig. 2 Occurrence of carbonaceous sediment and orogeny through time. Sediments (curves coloured blue) were recorded by graphite deposits weighted by mean carbon content 25 , the abundance of black shales 24 , and stromatolite occurrences as a measure of carbonate platform abundance 29 . Orogeny (curves coloured red) was recorded by the incidence of metamorphism 35 , abundance of orogens dated by the onset of deformation 34 , and preserved length of orogens 34 . Peak occurrence in sediment input coincides with a peak in orogenic activity, emphasized by the toned bar at 2.0 to 1.8 Ga.   Table 1. The maximum interval between carbonaceous rocks (blue colour) and orogenic deformation (purple colour) is consistently <200 Myr, comparable to Phanerozoic orogens.
at~2 Ga 3 . As the carbon contents of the sediment were anomalously high in the Palaeoproterozoic, the flux of carbon into subduction zones was greater, and hence deformation could take place more readily than had been possible hitherto. As inferred above, an anomalous episode of global carbon concentration on the ocean floor can support deformation by decollement in multiple orogens within 100 Myr. The availability of abundant carbon on the Palaeoproterozoic seafloor therefore allowed widespread orogenesis, as occurred subsequently in the Phanerozoic. The increasing database of ages implies a consistent speed of subduction, which supports the inference 2 that Palaeoproterozoic plate convergence occurred at a similar rate to the present day.
The 20 examples chosen are all collisional orogens, involving continent-continent collision, or they have a mixed accretionary and collisional history 34 . The role played by carbonaceous rocks in contractional deformation is evident in cross-sections through Palaeoproterozoic orogenic belts, which show imbricate thrust slices detached in pelitic/graphitic sediments, often stacked nearvertically (Fig. 4).
The role of graphitic sediments in mountain building. In at least fifteen of the twenty orogens in Fig. 1, the deformed rocks contain graphite ore bodies. In ore-grade graphite, the organic carbon content is very high, enriched above the levels in unaltered carbonaceous sediments. In some cases, enrichment is a consequence of the mobilization of organic carbon during deformation and metamorphism 38 . In some other Precambrian graphite deposits, carbon is added from mantle carbon dioxide or due to the metamorphic decarbonation of marbles 39 . These alternatives can be distinguished by the carbon isotopic composition of the graphite, which would be relatively light or heavy, respectively 39 . All of the twenty orogens have known isotopic compositions for their graphitic sediments, which are consistently light (Supplementary Note 1, Fig. 5), indicating a biological origin for the carbon in the orogens rather than from an abiotic source.  Evidence that the graphitic sediments localized and enhanced deformation takes two forms. Firstly, the concentration of graphite on shear surfaces is a direct consequence of carbon mobility during fault slip [15][16][17][18] . In some cases (Fig. 4), there is evidence for structural thickening of the graphite due to local redistribution (e.g. in Kimban and Ketilidian orogens), and    Table 2. graphite is a noted component in subduction zones (e.g. in Trans-North China and Penokean orogens). Secondly, data from relatively young plate margins, including the Pacific rim 40 , show that fault slip in graphitic rocks can cause a decrease in order in the graphite structure. This so-called strain-induced amorphization of graphite 19,40 is an exception to the assumed irreversibility of the graphitisation process. The anomalous disorder indicates a potential signature for graphite that facilitated deformation in the deep geological record. Graphite that had been involved in deformation could be less ordered (lower maturation and inferred palaeotemperature) than the background graphitic metasediment.
Spectroscopic data from graphite in several Palaeoproterozoic orogens 38,[41][42][43] , including the Pine Creek Orogen (Australia), Birimian Orogen (Ghana), Minas Orogen (Brazil), and Svecofennian Orogen (Finland), distinguish between a component in the country rock representing metamorphosed organic matter, and a component in fault rocks which was mobilized from the country rock (Fig. 6). Exceptionally, the mobilized graphite in all four orogens is less ordered than the country rock, as deduced from standard measurements using Raman spectroscopy and/or X-ray diffraction, expressed as palaeotemperature-equivalents for comparison. The consistent pattern of disorder in mobilized carbon in fault rocks in the Palaeoproterozoic orogens strongly indicates that the carbon was directly involved in the Palaeoproterozoic deformation.
The long-term legacy of the Palaeoproterozoic. As the consequence of carbon burial in the Palaeoproterozoic seas was the incorporation of refractory graphite during orogenesis, much carbon became locked into the geological cycle. This left a legacy for mountain building through subsequent geological history. The properties that enhanced deformation and crustal shortening during the Palaeoproterozoic are just as likely to do so where 2 Ga sediments are present in much younger orogens. Accordingly, degrees of crustal shortening in Phanerozoic orogens are high where they are rooted in Palaeoproterozoic crust 22 . In the Himalayas, Palaeogene thrusting was focussed in the graphitic sediments of the~2 Ga Lesser Himalaya Sequences, which had already focussed crustal shortening during the Palaeoproterozoic 44 . The intervening 2 Gyr record includes, for example, detachment on Palaeoproterozoic shale units during the 1.0 Ga Grenvillian Orogeny in Labrador 45 and the 0.43 Ga Caledonian Orogeny in Norway 46 . Pressure-temperature (P-T) trajectories for Palaeoproterozoic granulitic metapelites (Fig. 7) reflect reactivation involving compressional uplift of the granulites, either during normal exhumation or during a younger orogenic event. Palaeoproterozoic pelites were reactivated during the main periods of orogenesis over the last 2 Gyr (Fig. 8, Table 2). The exceptional Palaeoproterozoic biomass was thus reflected not only in mountain building during the Palaeoproterozoic but also in mountain building ever since.

Methods
The relationship between carbon burial and plate margin deformation was evidenced by the review of data available in the published literature.
The link was demonstrated in young (Phanerozoic) rocks using 20 case studies where detachment on organic-rich beds is documented. The case studies were characterised using the age of sedimentation, carbon contents of sediment, and age of deformation. The characterization was repeated for 20 Palaeoproterozoic orogens, with the additional documentation of organic carbon isotope composition to prove biogenicity. The periods between sedimentation and deformation were then compared between the Phanerozoic and Palaeoproterozoic data sets.
Strain composition of graphite was inferred using published spectroscopy data for carbonaceous matter in 4 Palaeoproterozoic orogens where components in the country rocks and remobilized in fault rocks were both available.

Data availability
The data tables used to produce the results of this article can be accessed at the National Geoscience Data Centre (https://doi.org/10.5285/d10a1c96-55ff-4a35-be6b-92a4eb766b38), entitled 'Data tables for global Palaeoproterozoic black shales', linked to NERC grant NE/M010953/1.  Table 2 Examples of reworking of Palaeoproterozoic metapelites during deformation in younger orogenies (Fig. 6).