Extreme winds and waves in theaftermath of a Neoproterozoic glaciationPhilip A. Allen1& Paul F. Hoffman21Department of Earth Sciences, ETH-Zu¨rich, Sonneggstrasse 5, CH-8092 Zu¨rich, Switzerland2Department of Earth & Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138-2902, USA...........................................................................................................................................................................................................................The most severe excursions in the Earth’s climatic history are thought to be associated with Proterozoic glaciations. According tothe ‘Snowball Earth’ hypothesis, the Marinoan glaciation, which ended about 635 million years ago, involved global or nearly globalice cover. At the termination of this glacial period, rapid melting of continental ice sheets must have caused a large rise in sea level.Here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant waveripples. These structures occur at homologous stratigraphic levels in Australia, Brazil, Canada, Namibia and Svalbard. Ourhydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds, significantly longer than thosetypical for today’s oceans. The reconstructed wave conditions could only have been generated under sustained high windvelocities exceeding 20 metres per second in fetch-unlimited ocean basins. We propose that these extraordinary wind and waveconditions were characteristic of the climatic transit, and provide observational targets for atmospheric circulation models.Glacial deposits from the Neoproterozoic era are widespread onvirtually every continent, and palaeomagnetic data indicate thatice sheets poured directly into the tropical ocean in at least twodiscrete glacial episodes—the Sturtian (,710 Myr ago; ref. 1) andthe Marinoan (,635 Myr ago; ref. 2)3. Globally, both episodesterminated abruptly with the deposition of distinctive carbonatesediments, called cap carbonates, contemporaneous with a majorsea level rise4–7. Almost all authors of published studies relate the sealevel rise to the melting of continental ice sheets, implying atimescaleoftheorderof2,000yr(ref.8)forcapcarbonatesedimentation. Sedimentary bedforms consistently observed incap carbonates contain information concerning wind-generatedwave conditions in Neoproterozoic oceans during periods ofextreme climatic change.Marinoan cap carbonates present a panoply of unusual sedimen-tary structures, which occur in broadly the same stratigraphicorder on widely separated palaeocontinental margins (Fig. 1). Acontinuous basal unit of exceptionally pale-coloured dolostone(Ca0.5Mg0.5CO3), typically 3–20 m thick, is conspicuously lami-nated. In shelf and upper slope settings, each lamina is defined by areverse-graded set of peloids (sand-sized pellet-like carbonateaggregates) and a fine micropeloidal drape (Fig. 2d)5,9. Some ofthe larger macropeloids (3 mm in diameter) are broken or faceted5,and small-scale, low-angle, cross-lamination (Fig. 2d) is ubiquitous.Large microbial bioherms (stromatolites) occur sporadically in thelower part of the dolostone unit; they host peculiar tubular or sheet-like infillings of micropeloidal sediment and/or cement, orientedplumb, the origin of which is controversial10–13. Structures inter-preted as giant wave ripples, the principal focus of this Article, areconcentrated in the upper half of the dolostone unit (Fig. 1). Thedolostone unit passes upward, with no significant break in sedi-mentation, into marly limestone (CaCO3) rhythmites with thindolostone turbidites. Sea-floor cements (crystal fans) of formeraragonite (orthorhombic CaCO3) are variably abundant in thelimestone5,14–18. The dolostone was clearly deposited within thezone of agitation by storm waves and the limestone in deeperwater beyond their reach. The giant wave ripples formed preferen-tially near the maximum depth of storm wave agitation, where onlythe longest-period waves feel the sea bed.Although giant wave ripples are widespread in Marinoan capdolostones, they are so exceptional relative to common experiencethat their origin was previously unknown. In the literature, they aredescribed as “tepee” structures5,15,17, but it has long been recognizedthat they do not exhibit the planform polygonal pattern andbrecciated and contemporaneously cemented crestal zones typicalof conventional tepee structures19, which result from volumetricexpansion, like the salt crust of a playa. Instead, their crestlines areparallel (Fig. 2b) and oriented sub-normal (408–908) to the ancientshelf break. Origination by sliding on the sea floor is inconsistentwith their orientation, systematic distribution (Fig. 1) and lack ofthrough-going slip surfaces. Their strongly cylindrical shape(Figs 2b, 3b) is difficult to reconcile with deformation by sedimentloading18. Where over-steepened flanks of the structures collapsed,leaving truncation surfaces, continued sedimentation re-establishedthe characteristic steep ripple profile. The structures are clearlyaccretionary and developed in situ on the sea bed.Giant wave ripples are well developed in the northern CanadianCordillera5,6,15, where their maximum synoptic relief is typically20–40 cm and their wavelength 1.5–2.0 m (Table 1). They built uprapidly on the sea bed by aggradation rather than by lateralmigration, individual wave ripples maintaining their identityfor 1.0–1.5 m vertically (Fig. 3). Individual laminae traverse thechevron-like crestal zone, but they thicken markedly on one flank orthe other (Fig. 2a, c, e). The ripple trains develop in a characteristicmanner in all areas (Fig. 3b). Initially, the crestlines ‘drift’ sidewaysthrough asymmetric aggradation (Fig. 2e). In the main stage, theyaggrade vertically with laminae alternating flank-to-flank (Fig. 2c).At ‘senility’, the crestlines drift non-uniformly, and are progressivelyburied by onlapping lamina-sets (Fig. 3b). Crestal angles during themain stage are remarkably small (,1108), and ripple flanks are steep,commonly ranging from 208–458, which accounts for the common,localized flank failure. We use only wave ripples unaffected bysecondary modification and tectonic deformation in the analysisthat follows.Generation of giant ripples by surface gravity wavesA number of features (Fig. 2) demonstrate that the giant ripplesdescribed