Neoproterozoic Glacial Origin of the Great Unconformity

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Neoproterozoic glacial origin of the Great Unconformity

C. Brenhin Kellera,b,1, Jon M. Hussonc, Ross N. Mitchelld, William F. Bottkee, Thomas M. Gernonf, Patrick Boehnkeg,h, Elizabeth A. Belli, Nicholas L. Swanson-Hysellb, and Shanan E. Petersj

aBerkeley Geochronology Center, Berkeley, CA 94709; bDepartment of Earth and Planetary Science, University of California, Berkeley, CA 94720; cSchool of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 2Y2, Canada; dDepartment of Applied Geology, Curtin University, Perth, WA 6845, Australia; eSouthwest Research Institute, Boulder, CO 80302; fOcean and Earth Science, University of Southampton, Southampton SO17 1BJ, United Kingdom; gDepartment of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637; hChicago Center for Cosmochemistry, Chicago, IL 60637; iDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095; and jDepartment of Geoscience, University of Wisconsin–Madison, Madison, WI 53706

Edited by Paul F. Hoffman, University of Victoria, Victoria, BC, Canada, and approved November 29, 2018 (received for review March 21, 2018)

The Great Unconformity, a profound gap in Earth’s stratigraphic A Discontinuous Global Unconformity record often evident below the base of the Cambrian system, The extent and magnitude of secular variation in preserved sed- has remained among the most enigmatic field observations in iment abundance across the Proterozoic–Phanerozoic boundary Earth science for over a century. While long associated directly or were first quantified by Ronov et al. (ref. 4 and Dataset S2), esti- indirectly with the occurrence of the earliest complex animal fos- mating preserved sediment volume flux over the past 1.6 Gy from sils, a conclusive explanation for the formation and global extent mapped sedimentary basin areas and stratigraphic thicknesses. of the Great Unconformity has remained elusive. Here we show The resulting temporal pattern has been subsequently refined that the Great Unconformity is associated with a set of large in Laurentia by the Macrostrat database (7–9) which (within global oxygen and hafnium isotope excursions in magmatic zircon North America) provides higher-resolution temporal and spa- that suggest a late Neoproterozoic crustal erosion and sediment tial constraints. Together these records corroborate the presence subduction event of unprecedented scale. These excursions, the of a large global shift in preserved continental sediment abun- Great Unconformity, preservational irregularities in the terrestrial dance near the base of the Cambrian (Fig. 1A and SI Appendix, bolide impact record, and the first-order pattern of Phanerozoic Figs. S1–S3). sedimentation can together be explained by spatially heteroge- The observed increase from roughly 0.2 km3/y of preserved neous Neoproterozoic glacial erosion totaling a global average sedimentary rock in the Proterozoic to ∼1 km3/y in the Phanero- of 3–5 vertical kilometers, along with the subsequent thermal zoic (Fig. 1A) might be attributed in principle to either con- and isostatic consequences of this erosion for global continental structive (faster sediment accumulation in the Phanerozoic) or freeboard. destructive (erosion of Proterozoic strata) processes. However, the abrupt nature of the observed transition presents difficulties

Great Unconformity | snowball Earth | glacial erosion | zircon | Cambrian explosion Signiﬁcance

It has long been observed that the sequence of sedimentary arth’s sedimentary cover necessarily rests at depth upon rocks deposited in the past half-billion years often sharply Eigneous or metamorphic crystalline basement. This contact overlies older igneous or metamorphic basement at an ero- need not be abrupt, since accumulating sediments gradually sional surface known as the Great Unconformity. We provide recrystallize and metamorphose under increasing heat and pres- evidence that this unconformity may record rapid erosion dur- sure. Where observed, however, this transition often takes the ing Neoproterozoic “snowball Earth” glaciations. We show form of a spatially abrupt and temporally correlated expo- that the extent of Phanerozoic sedimentation in shallow con- sure surface known as the Great Unconformity, a lacuna of tinental seas can be accurately reproduced by modeling the both time and mass (1–5). While often deeply buried, the accommodation space produced by the proposed glacial ero- Great Unconformity is exposed in areas of relief such as the sion, underlining the importance of glaciation as a means Grand Canyon of the southwestern United States, where it for lowering erosional base level. These results provide con- was first recognized by Powell et al. (1), most dramatically at straints on the sedimentary and geochemical environment in the sharp nonconformity between the Paleoproterozoic Vishnu which the first multicellular animals evolved and diversified in Schist and Cambrian Tapeats Sandstone (6). The ubiquity of the “Cambrian explosion” following the unconformity. this pattern—undeformed clastic sediments deposited directly and unconformably atop Precambrian basement—was subse- Author contributions: C.B.K., J.M.H., and S.E.P. designed research; C.B.K. and J.M.H. per- quently recognized by Walcott (2). Observing a dearth of con- formed research; C.B.K., J.M.H., R.N.M., W.F.B., T.M.G., P.B., E.A.B., N.L.S.-H., and S.E.P. analyzed data; R.N.M., W.F.B., and T.M.G. contributed the impact record; P.B. and E.A.B. formable sections spanning the lower boundary of the Cam- contributed zircon Hf and O isotope data; N.L.S.-H. provided geological context; and brian, Walcott proposed a “Lipalian” interval of continental C.B.K. wrote the paper.y exposure and erosion, which would have restricted any fossil Conflict of interest statement: The editor, P.F.H., is an adjunct professor in the same precursors of the Cambrian fauna to the deep ocean basins. department as J.M.H.y Subsequent investigation has revealed a more complete Pro- This article is a PNAS Direct Submission.y terozoic, including fossiliferous strata and conformable bound- This open access article is distributed under Creative Commons Attribution License 4.0 ary sections; yet the observation of a profound and extensive (CC BY).y (if discontinuous) pre-Cambrian unconformity remains (refs. Data deposition: Code for this article has been deposited in Github, https://github. 4 and 5 and Dataset S1). Here we attempt to unite dis- com/brenhinkeller/GreatUnconformity.y parate evidence including the zircon Hf and O isotope records, 1 To whom correspondence should be addressed. Email: [email protected] the terrestrial bolide impact record, and the record of con- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tinental sediment coverage in the context of this widespread 1073/pnas.1804350116/-/DCSupplemental.y unconformity. Published online December 31, 2018.

1136–1145 | PNAS | January 22, 2019 | vol. 116 | no. 4 www.pnas.org/cgi/doi/10.1073/pnas.1804350116 Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 elre al. et Keller the 1B, Fig. in shown as Meso- instance, great the upon For directly at deposited is formed crust. quartzite Ignacio was the Cambrian that in basement depth crystalline fluvial with juxtaposing sediment characteristically nature, the in pro- erosional and/or is throughout foundly Unconformity supply Great sustained observed sediment the However, space, Phanerozoic. in increase accommodation fivefold continental roughly a require (11). ), S4 the kilometers Fig. vertical of 14 Appendix , 80% as (SI much some as cover of totaling erosion sedimentary the of the Proterozoic base involve original the near would step or 1A this at the Fig. erosion Cambrian, Were concentrated in of (7–9). observed result abundance a history sediment purely sediment Earth’s marine preserved in in epicratonic times function on most at erosion age—suggesting survival from with the abundance influence constant Instead, roughly little sediment (10). individually preserved are model records Phanerozoic survivorship and would standard as Proterozoic curve, a abundance pre- glob- from exponential and an of result America follow volume not North does in estimated ally) (both The sediment model. continental served endmember either for sharp CO. a Mountains, at Needle granite the Eolus in Ignacio Ga) nonconformity Cambrian to peneplanar (∼1.35 The according Mesoproterozoic (B) of 6.1) the alluvium. of ratio overlies recent of factor quartzite excluding area (a scaling (8), the area global Peters land by a and American Husson database and North to (4) Macrostrat area al. the land et global from Ronov units of American estimate North the Phanerozoic–Proterozoic the both across 5 in of factor boundary a than more by increases ume 1. Fig. B A

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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES arc zircon εHf observed in more recent zircons (SI Appendix, While the timing of the observed negative Hf isotope anomaly Fig. S8). is potentially consistent with erosion and subduction of crust To quantify crustal average εHf evolution over the past ∼4.4 elided by the Great Unconformity, the required volume of Gy, we study a dataset of 29,523 zircon U-Pb age and Hf and/or O isotopic analyses using the weighted bootstrap resampling method of Keller and Schoene (13). While sampling and preservation biases are inescapable in the geologic record, this approach accurately propagates uncertainty in age and composition of each sample, while mitigating sampling bias via resampling weights inversely proportional to temporal sample density A (13, 16, 28). The result is a continuous record of mean εHfi in zircon and 2 SE uncertainty of the mean for 90-My age bins between 0 Ga and 4.35 Ga (Fig. 2A). Average initial zircon εHf remains broadly near zero throughout all of geological history (Fig. 2A), close to the isotopic composition of a reservoir with chondritic Lu/Hf. Variations in zircon εHf at the global scale have been traditionally attributed to the supercontinent cycle (29–31). Indeed, moderate fluctua- tions in this global mean zircon εHf occur throughout Earth’s history on plate tectonic timescales, with significant spectral power at Wilson cycle periods of ∼500–700 My (SI Appendix, Fig. S10). However, all other variations are eclipsed in magnitude by a single negative anomaly which begins in the earliest B Cryogenian and persists into the Paleozoic, representing by far the most dramatic excursion in the preserved zircon Hf isotope record. Alone, this Hf isotope anomaly requires the recycling of old, felsic crust. There are many potential mechanisms through which this may occur, but if such remelting is to represent a significant fraction of the global magmatic flux, thermal constraints C favor a lower crustal or mantle setting; in this context we consider two endmember scenarios. If recycling were to occur by, e.g., remelting of hot deep crust by basalt pooling near the crust–mantle boundary, the oxygen isotope composition of the resulting partial melt should largely reflect that of the preexisting igneous continental crust. If, however, recycled crust has instead been exposed at or near Earth’s surface, subjected to hydrothermal alteration, or processed through the hydrosphere D (as in the case of subducting eroded crust), a positive oxygen isotope anomaly reflecting low-temperature aqueous alteration may coincide with the observed Hf excursion. Fig. 2 reveals just such a correlation; a moving-window covariance estimate confirms the visually evident correlation between the Cryogenian and Edi- acaran zircon O and Hf isotope records. In principle, such a correlation is independent of the geologic process by which sediment is recycled into new magmas. However, nonarc magmas produced by sediment melting are a small proportion of global magmatism (Himalayan leucogranites, for instance (32)—but these represent a very small proportion of Cenozoic magmatism, and even here, sedimentary material is transported only to depths and temperatures conducive to anatexis by the subduction and underplating of the Indian continent under Eurasia). Considering sediment subduction to be the dominant mechanism of recycling sediment into new magmas, as suggested by E crustal mass balance (21), a more specific indicator of sediment subduction is provided by the product of the calculated εHf-δ18O covariance with the average slope of the standardized Hf and O isotope records. This product may be considered crudely analo- Fig. 2. Zircon isotope variability and continental sediment coverage gous to the derivative of sediment subduction rate (Materials and throughout Earth’s history. (A) Average zircon εHf. (B) Average zircon δ18O. Methods and SI Appendix, Fig. S9), highlighting intervals where (C) The covariance between standardized zircon εHf and δ18O. Positive both isotope systems indicate consistently increasing (positive covariance indicates times where average zircon oxygen and hafnium isotopes both indicate either increasing or decreasing crustal recycling in new product) or decreasing (negative product) recycling of surficially 18 altered felsic crust. The results (Fig. 2D) reveal a distinct pair- magmas. (D) The product of standardized εHf - δ O covariance with standardized average slope. Large positive values indicate high covariance and wise anomaly near the time of the Great Unconformity across increasing crustal reworking. Large negative values indicate high covariance the Proterozoic–Phanerozoic boundary, with an unprecedented and decreasing crustal reworking. (E) Fraction of North American continen- increase in the recycling of continental crust into new magmas tal area covered by marine sediment (age uncertainty represented by σ = 10 in the late Neoproterozoic, followed by a largely Phanerozoic My Gaussian kernel) from Macrostrat (7–9), along with the corresponding recovery. global Phanerozoic record of Ronov (23).

1138 | www.pnas.org/cgi/doi/10.1073/pnas.1804350116 Keller et al. Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 elre al. et Keller diamictites uncon- glacial basement Neoproterozoic a superposes where found directly be formity may sections other surface, some (48). continents most Neoproterozoic on to found not be Archean may Although basement and crystalline 47). sediments glacial 46, Neo- between proterozoic (36, contact unconformable direct diamictites exposed, well always glacial dropstones, preserved and clasts exotic and and striated evidence includ- pavements, erosional, field striated unmistakably ing enhance characteristic is further glaciation the Neoproterozoic might of for (45)] Much state for cycling. snowball sink hydrological a notable [a in ponds radiation poten- cryoconite solar erosive from consequent evaporation and (40); velocities tial sliding streams basal ice high wet-based localized with of development appear the alone for sublimation 44). sufficient by (41, driven controversial rates is precipitation glaciation However, global such during cycling of cal erosion sheet. vertical ice 2-km of a of while km by crust kilometer 12 erosion, continental than typical per drive large more to level a permitting gradient provide sea isostatically would energy below configuration potential level a km gravitational Such base 0.89 thickness. ice to sheet temperature, oth- ice up not water if extend or Moreover, air may of (39). by averages episode constrained global glacial erwise each with for positive, m but Neoprotero- 400–650 variable ice-covered is for continents zoic freeboard modeled gravitational Antarc- local by adjustments, and level in isostatic sea After found lower adjustment. isostatic would currently before thickness) that average km to (∼2 glaciation analogous tica (39–41), (38). continents km) all event (0–6 con- on heterogeneous pan-glacial, imperfectly is likely not Earth and snowball but a strained on extensive, thickness and sheet 37) an ice While (36, as events “snowball” Ma)— Gaskiers global (∼580 the as Gaskiers envisioned and two (717– first Ma), the Sturtian (641–635 Marinoan the Ma), 660 intervals: Neoproterozoic well-established million per kilometers of outstrip- order the evidently (35). on years height, rates uplift gra- mountain tectonic topographic erosive global ping sufficiently large limit proved a effectively has of to erosion presence subglacial the modern in dient variable, While are erosion. subglacial rates ero- through to directly continents alter and the erosion) (exposing to subaerial level sea ability global lowering Glaciers their both by denudation indirectly record. in continental promotes agents Glaciation sedimentary level: base erosive the sive among of unique constraints subduc- are sediment the mechanism and with erosion simple global tion a rapid Neo- provides reconcile explana- 2). may (34) (Fig. which tectonic recycling erosion preserved crustal local glacial or in proterozoic any 1) variations (Fig. global how abundance observed sediment unclear the produce is could Unconformity tion it Great the However, of exposures (6). specific could same for The locally cause. said tectonic be plausible a geologic have the often and throughout record common are unconformities Erosional Erosion and Glaciation Neoproterozoic crust, subducted (33)— of greater. fluids volumes or slab larger in less even Hf likely suggest of but would immobility known the conti- poorly considering subducted is the 50%, of magmas—which than across efficiency arc evenly new recycling into low distributed Hf the if for globally Accounting crust nents. of km 1.6 10 stp xuso ol ugs h eyln fsome of recycling the Hf suggest observed the would that excursion Methods ) and isotope (Materials mantle calculate and we flux, crust estimates average conservative generally for Using large. be would sediment hl h ra nofriysraei i.1B Fig. in surface Unconformity Great the While hydrologi- sustain to available ocean ice-free of extent The three in paleolatitudes low to extended glaciation Continental 8 km . yfrehmto fcytliebsmn othe to basement crystalline of exhumation for Gy ∼0.9 3 faeaecut orsodn oteeoinof erosion the to corresponding crust, average of fadcnietlmagmatic continental and εHf . km ∼3.2 8 m ∼787 allows 2.4 × rso ol stemldfuinajsst h e relative existing of new range the A condition. to boundary adjusts surficial diffusion the numerous of thermal position large-scale as by in exhumed cools crust erosion resulting instance, proposed For the erosion, predictions. as testable nonuniformitarian Cryogenian as event kilometer-scale geological a 2). (Fig. ering records as isotope subducted—just O and ultimately Hf is observed it the and by where shelves suggested basins, continental ocean the most the off glaciation, in into snowball entirely a flour transported during continen- level is glacial all base sediment when ice fine below terms, of is simplistic area more settling tal In of (iii) basins. edge ocean and the deep often shelf; to are extend continental day that the present fjords the overdeepened in with which associated for (47)], responsible paleovalley those as Chuos eroded [such result the of glaciers a transport outlet direct as erosive (ii) by waters level; sediment deeper base to erosional lowered shift (i) should of conditions, deposition for of pan-glacial predictions locus with During the consistent erosion. is glacial and Neoproterozoic continents) Great else- found the observed not on is the crust where eroded of the of production much (where the Unconformity for from requirement rebound critical isostatic by S11). Fig. thwarted Appendix, tec- (SI or be erosion thermal regional may glaciation, Local subsidence between erosion: global competition tonic upland of a regional context as and the considered subsidence be In syndeposi- must 47). local accommodation (46, by tectonism accommodated kilometer-scale tional directly reach where may below thicknesses diamictites basins accommodation-limited Neoproterozoic ocean an the with model, in Consistent accu- rather level. not base but erosional will continents, sediment the accommodation, on such mulate (56); of timescales absence My) likely the (>5 in applicable supply—is at sediment variable rate-limiting or the 3–10 accommodation process 56), rates depositional (45, equivalents space—not accumulation modern and than evidence slower Sturtian physi- times deposits while of Moreover, glacial range erosion. the pro- Marinoan glacial within for reversible well feasibility nonetheless (if cal is estimates rate dependence, required timescale such for the corrected some be Greenland must involved) while modern are the cesses (55); for 64 magnitude sheet estimates of over direct ice orders erosion recent two mm/y—nearly than of ero- 0.0625 slower average only km an of require 4 rate would sion comparison, from glaciation Neoproterozoic magnitude For of Ma of (54). orders mm/y four ∼100 some span to subsi- 53). tectonic ref. glacial ice (e.g., appreciable of lacking erosion outlet areas sections complete of in relatively erosional (often erosion of dence) preservation localized negligible the the and the ice, streams, the cold-based considering of produce erosion potential to expect glacial uniform we spatially should uniform signature—nor be subduction Neoproterozoic sediment not concentrated Critically, observed need be topography. erosion may preexisting glacial basement of young areas of erosion in glacial erosion rapid glacial facilitates (35), gradient energy potential clasts tional basement 755 granitic as contain young Group as Rapitan glacial ero- the Sturtian instance, of glacial For deposits direct basement: crystalline for young evidence of sion additional Neoproterozoic with provide in sections may clasts In episode. diamict juvenile glacial preservation, the exceptional during less igneous surface phaneritic the syn-Sturtian to of rocks exhumation of rang- possibility ages the with complex Stur- basement from instance, ing crystalline for juvenile unconformably Oman, a sediments of overlie syn-glacial and region diamictites Mirbat million glacial of the tian hundreds In to older. tens some years only basement crystalline with ietadidrc mlctosaewdsra hnconsid- when widespread are implications indirect and Direct a is basins ocean deep the to sediment eroded of Delivery estimated variable, highly are rates erosion glacial Modern 1 at syuga 696.7 as young as to Ma ∼810 ± PNAS 8M 5) ic xliaino gravita- a of exploitation Since (52). 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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES thermochronologic inversions, although geographically variable, two craters matching the criteria of Fig. 3 predate the onset appear permissively consistent with ∼100–300 ◦C(∼3–9 km at a of Sturtian glaciation, both deeply eroded remnants of massive 33 ◦C/km geothermal gradient) of potentially rapid Neoprotero- craters: Sudbury and Vredefort, eroded to depths of 4.2–5.8 km zoic crustal exhumation (57–60). Further analyses are required and 8–11 km, respectively (61, 62). This trend is particularly strik- to conduct a systematic global survey of the long-term thermal ing when considered as a function of crater density per unit area history of the continents, since a large proportion of existing ther- (Fig. 3A and SI Appendix, Fig. S12), with an abrupt truncation of mochronologic data is focused on areas of more recent tectonic <100-km diameter craters before 700 Ma and <10-km diameter activity that are unlikely to preserve a record of Neoproterozoic craters before 600 Ma—temporally consistent with progressive exhumation. Neoproterozoic glacial erosion. One specific testable prediction concerns the terrestrial bolide More qualitatively, we may extend our analysis of preserva- impact record: Impact craters are surficial features, subject to tional bias from the bolide impact record to consider a wide destruction by exhumation and erosion. Since impact craters range of geological features with an affinity for the shallow are shallow relative to their diameter, kilometer-scale Neopro- crust. For instance, we may predict that any mineral assemblage terozoic erosion, if widespread, should significantly reduce the which cannot survive prolonged low-grade metamorphism in a preservation potential of all but the largest impact craters. Fig normal continental geotherm should be less abundant before 3A shows the record of known terrestrial impact craters larger the Sturtian. This prediction appears consistent with the noted than 10 km diameter with ages known within ±75 My, updated absence of thermodynamically fragile (U)HP/LT assemblages from the Planetary and Space Science Center (PASSC) com- such as jadeitites and glaucophane eclogites before ∼700 Ma pilation (42). While the abundance of >10-km impact craters (63, 64), although not uniquely so (65, 66). The same predic- closely follows exposed bedrock area for the past 700 My, only tion appears likewise consistent with the strong (and apparently stepwise) “preservational bias toward [mineral] deposits of the Phanerozoic Eon” reported by Liu et al. (page 2 in ref. 19). A 25 0.5 Consequences of Rapid Crustal Erosion / yr)

Diameter >10 km 2 The timing of Neoproterozoic glaciation is remarkably consistent >20 km with both the observed zircon isotopic excursions and conti- 20 0.4 >100 km nental sediment coverage history at the scale of Fig. 2. This discontinuous record is an expected consequence of the step- Sturtian onset wise preservation potential imposed by focusing extensive, if 15 0.3 nonuniform, kilometer-scale continental denudation into a few discrete episodes of intense glacial erosion amid a background of comparatively negligible (<2.5 m/My) cratonic exhumation (67). 10 0.2 Consequently, the observed sediment coverage record may be considered in part a discretization of the exponential survivorship curve (10) that would result from continuous erosion (e.g., 5 0.1 SI Appendix, Fig. S4). Impact craters per 100 Myr In this discretization, each glacial epoch acts as a ﬁlter in the crustal record, removing some proportion of older sediments via 0 0 erosion. Since erosional surfaces are subject to capture by sub-

2500 2000 1500 1000 500 0 Presently exposed bedrock area (km sequent erosion, the most dramatic unconformity (and largest step in preserved sediment abundance) may be inherited by the B most recent glaciation, consistent with Fig. 2E. However, such

2 40 erosion does not preclude a constructive contribution to the Gaskiers Great Unconformity; to the contrary, it requires one. Continen- km Diameter

8 Marinoan > tal thinning through erosion directly decreases continental free- 2 km Sturtian >10 km board, raising relative sea level and providing accommodation 30 >20 km space for sediment accumulation. While this new accommoda- >100 km tion space may be temporarily moderated by thermal buoyancy given erosional advection of the continental geotherm, continental erosion nonetheless inevitably leads to increased continental 20 sediment storage, as proposed by ref. 8. To quantify the depositional consequences of rapid Neopro-

yr per cumulative 10 terozoic erosion, we constructed a 1D model of continental free- 8 board, combining the effects of erosion, isostasy, thermal subsi- 10 dence, and sediment accumulation over the past 800 My. Using either the Phanerozoic net sedimentation rate from Fig. 1A or a constant assumed rate of 0.9 km3/y, varying the model magnitude of Neoproterozoic erosion directly influences initial freeboard 0 via mass balance (SI Appendix, Fig. S13). Near-modern free- Craters per 10 1600 1200 800 400 0 board at 750 Ma is reproduced with 3.4–4.5 km Neoproterozoic Age (Ma) glacial erosion, producing in each case a nearly 250-m isostatic excursion in relative sea level (Fig. 4A). Using a modern hypso- Fig. 3. The record of impact craters preserved in Earth’s continental crust metric profile (SI Appendix, Fig. S15) to convert from sea level with formation ages known to within ±75 My (1-σ) from the PASSC B database (42). (A) Absolute crater counts (left axis) for several size ranges to continental submergence fraction as illustrated in Fig. 4 , this tallied in 100-My bins over the past 2.5 Ga, plotted alongside global exposed 250-m excursion corresponds remarkably well with the observed bedrock area in km2/y (right axis) (43). (B) Apparent impact cratering rate macrostratigraphic record of marine sediment coverage. per unit bedrock area area tallied in 100-My bins for crater diameters from The first-order success of this 1D freeboard model prediction 2 km to >100 km. is particularly remarkable considering that the model includes no

1140 | www.pnas.org/cgi/doi/10.1073/pnas.1804350116 Keller et al. Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 bnac.Ptnilcue o hsmstmyfl nothree into fall may misfit this Neoprotero- categories: for broad of causes sediment preserved end Potential in increase the abundance. Cambrian between the and delay glaciation remains zoic feature time one the However, problematic: tectonics. local of consideration of record global (23) Ronov’s coverage. and sediment (7–9) marine each Phanerozoic Macrostrat of from North duration observed record fraction the the American with coverage compared profile, to continental hypsometric constant proportion modeled a assuming Corresponding in (B) distributed interval. is erosion, glacial Neoprotero- erosion by accumulation. glacial sediment driven zoic and freeboard subsidence, continental thermal average subsequent in evolution Temporal 4. Fig. elre al. et Keller odpsto eutn rmsdmn trainmybe may starvation sediment from resulting Nondeposition ii) rsoa oso h daaa eod lca rotherwise, or glacial record, Ediacaran the of loss Erosional i) B Relative continental freeboard (m) A ntcoi ieclsutlooeei rvdsarenewed a provides orogenesis until timescales persist tectonic could on supply sediment reduced the topography; reduces available sufficiently (71) peneplanation glacial if expected replicated. been not have (70)] diamictite Ediacaran an [e.g., observations are key constraints and been geochronological lacking has precise glaciation 69), Ediacaran for (68, late propose suggested a we While trivial erosion base Cryogenian. be kilometer-scale the the would the to means with Gaskiers erosional compared the through from Cambrian) My the of 39 Ediacaran the the of particularly pre- My 92 (and low the Maintaining over abundance mechanism. sediment served direct most the provides

Continental coverage fraction -250 -200 -150 -100 -50 0.1 0.2 0.3 0.4 50 0 800 0 ssai lblsalvladcnietlcvrg oe.(A) model. coverage continental and level sea global Isostatic

Sturtian Marinoan 600 Gaskiers A of sedimentsupply erosion orlack Lost to Isostatic freeboardmodel g e (Ma) 400 3.4 kmglacialerosion Macrostrat sedimentation 4.5 kmglacialerosion 0.9 km Cloudina 3 /yr sedimentation Ronov,1994 (global) Macrostrat (N.Am.) 200 ntemti of matrix the in Cretaceous seaway interior { Model 0 hoooia ismyrsl na neetmto nthe in underestimation an in result may bias Chronological iii) bcr h lblsgicneo epoeoocgailerosion. glacial Neoproterozoic to of significance helped global (6). has the diachronous obscure unconformities highly amalgamated is of it diachroneity continents, This all on ubiquitous is ment oceanic ice-free below thus erosion may level. glacial base by 76) enabled (9, anomaly Phanerozoic an be the throughout emergence) recovery subsequent (i.e., and coverage or continental which delamination extensive freeboard enabled by continental negative limited The positive otherwise collapse. slightly gravitational not or if zero reaches freeboard, it average until oroge- sedimentation by and thicken must nesis buoyant freeboard any (80); negative crust, with Earth continental mass active continental felsic tectonically and orogenesis (SI a active on conse- level given direct erosion sea a subaerial rather global of but and coincidental, quence not continents is S15) the Fig. of Appendix, that elevation suggest modal We between correspondence the 79). The (78, as reevaluated. be persisted emergence must has paradigm continental this cooling global monotonic tecton- of of plate result by paradigm a obviated the been (77), has ics original record the this While 4B. of Fig. interpretation in Egyed as coverage), Phanerozoic by marine the declining throughout (i.e., submergence emergence continental dramatic indicated of (76) quantification first The Conclusions and Inferences better be may that predictions work. testable future of with range understood a imply all tory, 7) oetesria fPaeooccvrudrteiedvd near divide ice the under al. et cover low. Licciardi are Phanerozoic by velocities estimated sliding the of basal as and where survival ka Bay, (74) Hudson’s 18 the America at North Note sheet of basement ice (75). Map Laurentide Precambrian the Geologic of between the extent in (73) mapped correspondence as exposure the by illustrated 5. Fig. USGS GMNA(2009) hl ocnomt ewe eietadcytliebase- crystalline and sediment between nonconformity While satisfac- entirely is alone hypotheses above the of none While hskonpolm(2 a enlreycretdover corrected largely been has that hope (72) decades. recent we problem However, known system. this Cambrian crystalline the above sediments to the first basement to the testifies of cur- basement” association residual “Precambrian historical phrase The the Cambrian. of the rency to assigned units mistakenly ambiguous if are sediments Ediacaran of extent and volume are which starved. basins sediment Ediacaran not are there However, input. clastic bedrock Precambrian atr fteGetUcnomt yLuetd lca erosion, glacial Laurentide by Unconformity Great the of Capture PNAS | aur 2 2019 22, January | o.116 vol. Laurentide IceSheet thickness at18ka | Licciardi etal. o 4 no. (1998) |

1141 0 1000 2000 3000 m

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Proterozoic or Archean basement is commonly exposed at the where n is the number of samples in the dataset and t is sample age. surface even today (Fig. 5)—an ongoing Great Unconformity. Subsequently, the dataset is resampled with replacement, with sampling However, exhumation at such sites likely results from multiple probability proportional to sample weight. This weighting produces a more ancient (e.g., Neoproterozoic) unconformities collapsed, cap- even temporal distribution (SI Appendix, Fig. S7) and obviates the manual tured, and deepened by more recent erosion. A remarkable elimination of, e.g., duplicate analyses. Throughout resampling, each geo- correspondence has been noted between Precambrian bedrock chemical measurement (e.g., a single zircon Hf isotope ratio) is represented as a Gaussian random variable with a known mean and SD such that a new exposure and glaciation (Fig. 5); virtually all nonorogenic expo- value is drawn from this distribution each time the dataset is resampled, sures of Precambrian basement have been subject to glaciation thereby fully representing analytical uncertainty. Average results through- during either the Late Paleozoic Ice Age or the Quaternary (73, out Earth’s history are presented as an average and 2 SE of the mean for 81) (SI Appendix, Fig. S16). In this context, we suggest that the overlapping 90-Ma windows between 0 Ma and 4,350 Ma (e.g., Fig. 2). present icehouse epoch may display comparatively high conti- The global average zircon Hf and O isotope timeseries both record the nental erosion rates (82) relative to the Phanerozoic background, recycling of preexisting crust into new magmas. Positive O isotope excur- reconciling unsustainable modern erosion rates of 0.05–0.5 mm/y sions above the mantle baseline (∼5.5/mil) reflect the recycling of silicate (i.e., 50–500 km/Gy) with the survival of Archean crust and crust that has undergone low-temperature aqueous alteration at Earth’s surface (i.e., sediment), while negative Hf isotope excursions reflect the recy- lithosphere (67). 176 Considering the glacigenic model for the Great Unconfor- cling of old, felsic crust that has undergone less Hf ingrowth than the convecting mantle. Zircon Hf and O isotope averages vary throughout the mity proposed here, zircon Hf and O isotopes may represent supercontinent cycle as the proportion and preservation of arc, rift, and the first paleoerosion proxy preserved in Earth’s igneous record, collisional magmatism vary (29–31); such normal variations are observed preserving a signal of surface earth processes over billion-year throughout the entirety of the preserved record, with roughly the expected timescales. In this context, we note that a set of smaller but corre- periodicity (SI Appendix, Fig. S10). Compared with this normal tectonic back- lated Paleoproterozoic excursions in the zircon Hf and O isotope ground, the Neoproterozoic excursions are notable both in magnitude and records circa 2.2 Ga appears following a known period of Paleo- in the covariance between Hf and O isotope records. While atypical O and Hf proterozoic glaciation (83). Given the lack of geologic evidence isotope characteristics of Neoproterozoic zircon have been previously noted for glacial deposits between the ∼2.2-Ga Rietfontein (83) and (30, 31, 90), their systematic global covariance and the broader implications ∼0.72-Ga Sturtian (37) glaciations, Earth may have experienced thereof have not been previously explored. a prolonged period of weathering and regolith development To assess the importance of sediment subduction, we examined the covariance between the zircon Hf isotope signature of felsic crustal recy- (84) with comparatively little marine sediment accumulation on cling and the zircon O isotope signature of sediment recycling, following the continents due to a lack of glaciation-derived accommo- a procedure illustrated in SI Appendix, Fig. S9. First, to remove any scale dation space. Thus, Neoproterozoic global glaciation may have dependence or extraneous covariance from long-term secular crustal evolu- been responsible for initiating a Phanerozoic cycle of continen- tion (as opposed to distinct crustal recycling episodes), both isotopic records tal sedimentation with enhanced Paleozoic continental inunda- are detrended and normalized to unit variance, with the εHf isotopic sig- tion and sediment accumulation relative to the preceding late nal inverted such that increasing recycling is positive for both systems (SI Proterozoic. We conclude that the Phanerozoic sedimentary Appendix, Fig. S9 A and B). The resulting covariance is illustrated in SI record is best explained by a Great Unconformity of inherently Appendix, Fig. S9C. This raw covariance is positive where the Hf and O coupled erosive and constructive genesis, with Neoproterozoic signals either increase or decrease in concert: Both the excursion and recovery of the Neoproterozoic isotope anomaly yield large positive covariance glacial erosion governing the subsequent history of continental peaks. Since we wish to distinguish between excursion (increasing sedi- freeboard and sediment accumulation (Fig. 4B). As such, the ment subduction) and recovery (decreasing sediment subduction back to environmental and geochemical changes that led to the diver- baseline), we additionally examine the product of this covariance with the sification of multicellular animals (5) may be considered a direct average slope of the two Hf and O isotope signals (SI Appendix, Fig. S9D). consequence of Neoproterozoic glaciation. Since the average slope tends to zero in the case of negative covariance, the covariance–slope product (SI Appendix, Fig. S9E) emphasizes large positive Materials and Methods covariance co-occurring with either increasing or decreasing sediment sub- To investigate anomalies in the continental rock record near the Proterozoic– duction; individual subduction events thus appear as characteristic pairwise Phanerozoic boundary, we assemble a range of stratigraphic, geochemical, features with a positive excursion peak immediately followed by a nega- and geological datasets. Stratigraphic data for North America are obtained tive recovery peak. Two such events are evident: a Paleoproterozoic pair from the Macrostrat database (macrostrat.org), originally produced by Peters with an excursion beginning circa 2,200 Ma and a much larger Neoprotero- (7) by digitization of the the American Association of Petroleum Geologists zoic pair with an initial excursion coincident with the onset of the Sturtian Correlation of Stratigraphic Units of North America (COSUNA) charts (85). glaciation [∼717 Ma (37, 91)], a nadir at ∼560 Ma, and an ∼220-My recov- This stratigraphic record of the Great Unconformity is interpreted alongside ery that is complete by ∼340 Ma. Notably, the essentially immediate (on compiled zircon Hf and O isotope geochemistry, as well as terrestrial and gigayear scales) onset of the excursion following Sturtian glaciation is con- lunar bolide impact datasets. Finally, stratigraphic and geochemical results sistent with the fast recycling of sediment into new magmas (<7–9 Ma from are integrated and interpreted in the context of an isostatic and thermal erosion to eruption) suggested by cosmogenic 10Be anomalies in modern model of continental freeboard. Computational source code and data are arc magmas (92)—while the timescale of recovery is entirely consistent with freely available at https://github.com/brenhinkeller/GreatUnconformity. the ∼200-My characteristic timescale for complete turnover of the oceanic crust (and thus complete subduction of any accumulated sediments into the Zircon Isotope Systematics and Monte Carlo Analysis. We compiled zircon Hf ocean basins). and O isotopic compositions along with U-Pb ages for igneous and detrital Given the observed magnitude of the global Hf isotope excursion, we zircons from the preexisting datasets of Belousova et al. (86), Dhuime et may estimate the minimum required volume of subducted crust. Taking the al. (87), and Spencer et al. (88)/Payne et al. (89), augmented by some further compiled zircon εHf datastet as an estimate of average εHf of new igneous compilation of literature data, resulting in a dataset of 35,368 analyses from crust throughout Earth’s history, we may calculate the average crustal εHf at all continents (SI Appendix, Figs. S5 and S6), of which 29,523 are unique. any subsequent time accounting for Hf ingrowth in accordance with Lu/Hf To obtain a maximally representative temporal record of zircon Hf and O ratios for each whole-rock sample in the dataset of Keller and Schoene (13), isotopic composition, we applied weighted-bootstrap resampling following obtaining Neoproterozoic values ranging from −33.7 ε at 717 Ma to −34.9 the approach of Keller and Schoene (13, 16). While ages are known directly ε at 635 Ma. Since a more negative crustal endmember will result in lower for each analysis, geographic locations are largely absent from the dataset. estimated volume of subducted crust, we choose −35ε as a minimum value. Consequently, sample weights wi for each sample i are assigned inversely This estimate is conservative since the zircon record samples only zircon- proportional to temporal sample density following the relation bearing magmas, which are predominantly felsic (26) and may exhibit more negative initial εHf than average crust due to a greater contribution from n assimilation of preexisting crust than, e.g., a primitive basalt. Meanwhile, as X 1 wi = 1 , the most positive reservoir in εHf space, the evolution of the depleted man- (t − t )2 + 1 j=1 i j tle may be traced as the upper limit of the compiled εHf field through time,

1142 | www.pnas.org/cgi/doi/10.1073/pnas.1804350116 Keller et al. 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Sedimentary the and Freeboard Continental glaciation. Neoproterozoic predating craters impact for suggested between C intermediate S12C in likely Fig. is discontinuity that Appendix, signal dramatic at (SI preservation Cryogenian true more impacts the bolide even deposition across for an rate of cratering target in time preserved more a results vol- the as normalization is or available at This been sedimentary exposed resolution) time. have been for binning would have thus natural some must and more within which considered bedrock, impact, be canic given may but a aggressive as age at impact same an record to be unable may thus bedrock and 1-Ga buried time. deeply instance, that but for Ga, because, age 0.5 extent of at maximum impact extant an a preserve is age could This of that Ma. bedrock area surface area exposed exposed (SI maximum presently crater the the impact that 3B given in Fig. a tive in as seen age as same area, the (ii of and S12 age today Fig. impact Appendix, exposed given crust a of than older area is at that exposed today again exposed now is and past the in time available surface. resolved was some the that better at area cratering is continental impact raw preservation the to for of the record signal impact by the the confirmed normalizing 3A), by broadly (Fig. of is alone class prediction record largest impact this the but While all impacts. for crater terrestrial Cryogenian impact the in decrease across glacial dramatic potential Neoproterozoic a preservation expect the we If correct, is erosion. hypothesis Con- susceptible glacial erosion (95). be Neoproterozoic deep should by km craters 4 erasure impact only terrestrial to be largest may the diameter but their km all that 100 the sequently, such of diameter of crater km character- impact 1/10th 15 Lunar above are around decreasing a 95), depth craters (94, less initial impact or an diameter Hypervelocity original with depth. features, crater shallow of istically erosion to function resistance a their surface, Impact bolide is Earth’s at Since Earth occur applicable. PASSC necessarily where the craters updated impact in constraints compiled age with as (42), record Database crater examined have impact we terrestrial erosion, Neoproterozoic the of magnitude and timing the Record. on Impact Bolide Terrestrial The 1.489 the over evenly some distributed of if recycling the kilometers require cal would My mantle 400 a over 2.4 from source magmatism crustal continental a of to 12% source shifting we case, Consequently, this destruction. 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Systematic and introduced sea My. (99) be global rate 500 may average spreading misfit changing past oceanic thus the change height, may ridge over flow midocean wildly heat mantle varied in basin have and variation orogenesis not as such may processes local formation relevant of rates global context, the this after that In misfit model. 1D low simple relatively our in the included are events orogenic or tonic before apparent: coverage (ii are and expectations range misfit model exceeds wide of tially the intervals given particular good two (i quite involved, is uncertainties trends of coverage fills observed between agreement sedimentation and general erosion continued model the While Neoproterozoic as space. background accommodation of available to the aftermath cover- declining slowly the continental then with in and curve, dramatically extent increasing coverage age continental model observed where the unsurprisingly perhaps is which My), present lower. 500 the is misfit to past closer the supportable (i.e., more is day assump- hypsometry The glaciation. near-modern Neoproterozoic Conse- of before of tion (71). aftermath both immediate constrained conditions the poorly in is different and gradient hypsometric under global contrast the produce quently, topographic either of to destroy absence potential signif- the or an may hypsometry—with hypsome- given Glaciation continental hypsometry. present-day alter unavoidable global icantly of presently past on assumption but constraints imperfect, The independent notably S15). is translated Fig. try was Appendix, hypsometric curve (SI present-day a freeboard curve using extent, resulting coverage continental the expected into sedimentation, and gence Appendix , (SI results the on impact little S14). has Fig. erosion glaciations distributing three equally all in instead intervals between However, glacial duration. Neoproterozoic their the three to the proportion For between rate. partitioned sedimentation was 1A) sion km and Fig. 4 (0.9 in Fig. kilome- continuous of as several purposes either derived, with (Macrostrat by perturbed variable sedimentation, followed or then erosion and Neoproterozoic was erosion of model of ters This scenarios thicknesses. (98) various thermal lithospheric by expected (96) litho- between mantle elastic intermediate + generally and (crust km, thickness 133 lithospheric of total sphere) a for thickness, 100-km of kg/m 3,300 of sity kg/m 2,818 of mm density 1 average 3 of of diffusivity sedimen- expansion thermal thermal and of a erosion coefficient a of assumes effects model into isostatic our back tation, direct To decays the geotherm protracted. with advected along more the equilibrium, as be subsidence may thermal ther- erosion for the account kilometer-scale However, of timestep. consequences model likely mal single-million-year 96) a (45, rebound within mantle effec- complete viscous be postglacial to with assumed approximately instantaneous, is On tively adjustment and lithosphere. isostatic thermal and timescales, 1D crust simulation continental gigayear a the constructed of we model 3.1-km isostatic accumulation, (now) sediment entire and the freeboard capture to fol- sediment crust, of of m km Gaskiers the 100 3 unconformity. 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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES rate. While controversial, increased mantle heat ﬂow in the Cretaceous has ACKNOWLEDGMENTS. A.C. Maloof, M.J. Keller, and B. Schoene provided been proposed in conjunction with the Cretaceous Long Normal Superchron valuable discussion. Francis Macdonald, Peter Cawood, and anonymous and the Kerguelen and Ontong–Java oceanic ﬂood basalt plateau (103– reviewers greatly improved the manuscript. C.B.K. acknowledges sup- 105), potentially consistent with high average ocean ridge elevation and port from the Ann and Gordon Getty Foundation (through Berkeley Geochronology Center) and from the US Department of Energy Compu- increased sea level for much of the Cretaceous. tational Science Graduate fellowship under Contract DE-FG02-97ER25308. Macrostrat development was supported by NSF Grants EAR-1150082 and Code and Data Availability. Macrostratigraphic data are accessible via ICER-1440312. R.N.M. was supported by Australia Research Council Grant https://macrostrat.org/api. All compiled datasets and computational source FL150100133. T.M.G. acknowledges funding from the United Kingdom code are available at https://github.com/brenhinkeller/GreatUnconformity. Natural Environment Research Council Grant NE/R004978/1.

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