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

Orogeny builds mountains through compressional plate tectonic processes. Multiple lines of evidence indicate that the Palaeoproterozoic marks the onset of modern-style mountain building: a rapid increase in the frequency of continental orogens, greater preservation of continental crust linked to large lateral plate motions, establishment of a global subduction network around 2 Ga with predominantly steep subduction zones, and the first appearance of ophiolites near 2 Ga. Concurrently, oceanic biomass increased markedly, leading to extensive burial of organic carbon recorded in the Lomagundi-Jatuli and Shunga events. Early Palaeoproterozoic carbon burial is documented on passive margins during the emergence of plate tectonics. Prolific cyanobacteria generated globally widespread carbon-rich sediments (black shales and graphite). The study analyzes whether increased organic carbon burial facilitated deformation and orogenesis by weakening the lithosphere, thereby linking biosphere processes to lithospheric dynamics.

Background work cited indicates: (1) emergence of modern-style plate tectonics and subduction networks by ~2.0 Ga, including the earliest ophiolites; (2) major geochemical anomalies (Lomagundi-Jatuli and Shunga) reflecting increased organic carbon burial; (3) prior recognition that organic-rich shales and graphite reduce frictional strength along faults and detachments, facilitating deformation in numerous Phanerozoic settings; and (4) global distributions of black shales/platform carbonates and graphite deposits through time. These studies motivate testing a temporal link between Palaeoproterozoic high-carbon sedimentation and orogenic deformation.

The study compiles and reviews published data to evaluate the relationship between organic carbon burial and plate-margin deformation. Approach: (1) In young (Phanerozoic) settings, two case studies of detachment on organic-rich beds are used to document how carbon content, sedimentation age, and deformation age relate to fault weakening. (2) For 20 Palaeoproterozoic orogens, the authors compile ages of carbonaceous sedimentation (black shales, graphite-bearing units), carbon contents (TOC and graphite occurrences), isotopic composition of carbon to confirm biogenic origin, and ages of deformation/metamorphism to determine time lags between sedimentation and orogeny. (3) They assess structural evidence (cross-sections) for detachment and imbricate thrusting within pelitic/graphitic metasediments. (4) They synthesize spectroscopic data (Raman/XRD) on graphitic order in country rocks versus fault rocks to infer strain-induced amorphization linked to fault slip. (5) They compare pressure–temperature trajectories from published sources to characterize thermal histories from crustal thickening to exhumation. Data sources are provided in Supplementary materials and at the National Geoscience Data Centre.

• Temporal coupling: Across 20 Palaeoproterozoic orogens, the maximum interval between deposition of carbonaceous sediments and peak orogenic deformation/metamorphism is consistently less than 200 Myr, comparable to Phanerozoic orogenic timescales (Fig. 3).
• Carbon as a mechanical weakener: Sediments containing only a few percent total organic carbon (TOC) markedly reduce frictional strength and facilitate deformation. Reported frictional strength can be reduced to ~0.3 times normal stress in carbonaceous pelitic rocks.
• Empirical threshold from younger analogues: In a dataset of Devonian shales (n = 102), shearing occurred where mean TOC ≈ 2.8%, while no shearing occurred where mean TOC ≈ 1.4%.
• Global extent of carbon-rich sedimentation: Peak accumulations of black shales (organic matter) and graphite deposits occurred between ~2.1–1.8 Ga, coincident with a peak in orogenic activity (Fig. 2). Cumulative thicknesses highlighted include black shales approaching ~5000 m and graphite deposits peaking near ~180 m in compiled curves.
• Structural style: Many Palaeoproterozoic belts exhibit imbricate thrust slices detached within pelitic/graphitic sediments, often stacked near-vertically (e.g., Kimban, Wopmay, Trans-Hudson, Magondi; Fig. 4). Numerous belts host ore-grade graphite, indicating high carbon concentrations in deformed units.
• Biogenic carbon source: δ13C data for graphite/carbonaceous rocks across all twenty orogens fall in ranges characteristic of biological origin (some with minor magmatic signatures), supporting a biogenic carbon contribution to weakening (Fig. 5).
• Graphite disorder as a deformation signature: Fault rocks show decreased structural order (lower graphitization equivalent temperatures) relative to country-rock graphite, consistent with strain-induced amorphization during slip (Fig. 6), as observed in both Palaeoproterozoic belts and young subduction/collision settings.
• Thermal histories: P–T paths for pelitic granulites typically record high-temperature (>800 °C) conditions during crustal thickening followed by cooling during exhumation, with timescales ranging from rapid to >1 Ga (Fig. 7).
• Long-term legacy: Palaeoproterozoic carbon-rich sediments created mechanically weak horizons that were reactivated during younger orogenies (e.g., Grenvillian, Pan-African, Caledonian, Himalayan), focusing thrusting and shortening in 2 Ga pelites (Fig. 8).

The analysis indicates that a global episode of increased biomass and organic carbon burial around 2.1–1.8 Ga lowered the frictional strength of sedimentary sequences, enhancing detachment and thrusting in collisional settings. This mechanical lubrication facilitated crustal thickening and the construction of Palaeoproterozoic mountain belts, temporally linking biospheric productivity to lithospheric deformation. The consistent <200 Myr lag between carbonaceous sedimentation and orogenic deformation mirrors patterns in younger orogens, implying similar plate convergence rates and tectonic tempos to the present day. Isotopic evidence supports a predominantly biogenic origin for the graphite in these units, tying the weakening directly to increased organic productivity following the Great Oxidation Event. The structural, petrological, and spectroscopic observations collectively argue that carbon-rich layers localized strain at plate margins. Moreover, the inherited mechanical weakness of 2 Ga crust has been repeatedly exploited during subsequent orogenies up to the Himalayas, demonstrating a lasting coupling between early biospheric carbon burial and the deformational architecture of continents.

A pronounced rise in biomass and organic carbon burial at ~2.0 Ga produced widespread carbon-rich sediments that significantly weakened the lithosphere, enabling efficient thrusting, crustal thickening, and the rise of Palaeoproterozoic mountain belts. The temporal proximity (<200 Myr) between sedimentation and deformation across 20 orogens, combined with biogenic carbon isotopic signatures and evidence for graphite-facilitated fault slip, supports a causal link between biosphere expansion and orogenesis. This episode imprinted a persistent mechanical anisotropy in continental crust that has localized deformation during many younger orogenic cycles, including the Himalayas. Future work could expand high-resolution geochronology of sedimentation and deformation, systematically quantify TOC thresholds and frictional properties across metamorphic grades, and further develop spectroscopic proxies (graphite disorder) to fingerprint carbon-facilitated deformation in deep time.

The study synthesizes published datasets rather than generating new primary measurements; conclusions rely on the quality and completeness of compiled ages, TOC estimates, and carbon isotope data from diverse sources. Stratigraphic completeness and preservation biases may affect global curves of carbonaceous sedimentation and orogenic metrics. Detailed methods for the two Phanerozoic case studies and all twenty orogens are in supplementary materials, and uncertainties in timing and carbon contents are not fully quantified in the main text.