Abstract We examine four fields of undulating sediment in the Bismarck Volcanic Arc, Papua New Guinea, to assess causal mechanisms. The possible mechanisms include deformation, episodic turbidity currents, and continuous bottom currents. Two of the fields, one off the coast of Dakataua caldera and one in Kimbe Bay, display an arcuate and irregular morphology similar to one another in multibeam imagery. In sidescan imagery, each of these fields is proximally associated with downslope scour features and other evidence of turbidity-current activity. There is no evidence of significant bottom-current activity in these regions. We suggest that these two fields were formed by a combination of extensional deformation and repeated turbidity currents, based on a quantitative analysis of their morphologies and the evidence for turbidity currents in each location. In particular, the Kimbe Bay and Dakataua fields are morphologically distinct from a field of turbidity-current sediment waves mapped nearby in Hixon Bay (Torkoro Trough field). Also in Hixon Bay, an irregular depression north of Lolobau Island that may be a slide scar appears to have provided the initial topography for a small turbidity-current sediment-wave field growing within it. Although flows that formed the sediment-wave fields in the study area are not specifically defined as mass-transport processes, the processes that preconditioned all of these field sites for subsequent sediment-wave formation clearly are. In the Dakataua and Kimbe Bay fields, the mass-transport process of sediment creep in conjunction with turbidity currents is the necessary combination of mechanisms to generate the morphology of the sediment waves observed. Likewise in the Hixon Bay fields, a slide scar exhumed by an earlier extensive mass-failure event was required to create the necessary seafloor morphology to appropriately funnel the subsequent sediment-wave-forming turbidity currents.

In the Bismarck Volcanic Arc in Papua New Guinea, six fields of sediment waves were imaged with sonar. Sediment structures observed in seismic data and swath bathymetry are not unique and can result from predominantly continuous (bottom) currents, or episodic (turbidity) currents, or from deformation of sediment. Two of these wave fields overlap and appear to be of turbidity-current origin and modified by bottom currents, with one field unconformably overlying the other field. A field off the coast of Dakataua caldera displays an arcuate morphology, and a series of enclosed depressions within the field suggests creation by extensional deformation of rapidly deposited sediment. Scour features in side-scan imagery suggest turbidity-current activity, which also likely modifies the sediment waves. The wave field is isolated from hyperpycnal currents, however, suggesting that in the absence of a shelf, coastal erosion and small landslides can produce semiregular gravity-driven sediment flows that deposit in deep (>1400 m) water. In Kimbe Bay a fourth sediment-wave field also displays arcuate morphology and enclosed depressions within the field. This wave field is found within a bay >40 km from shore and also appears to have been formed by a combination of extensional deformation of sediment and energetic current activity. Two additional fields in Hixon Bay are fed by small and medium rivers (<∼450 m 3 /s mean annual discharge) draining volcanoes and mountainous regions. One small field appears within a slide scar, suggesting that the initial topography of the scar provided the conditions for early sediment-wave growth. A much larger field is best explained by repeated hyperpycnal currents originating from the Pandi River. We cored a series of upward-fining, graded sequences consistent with a turbidity-current origin. Ages from these cores and measurements of relative thickness in sub-bottom imagery of the field constrain deposition rates for the field and suggest that a large part of the Pandi River discharge must be bypassing the shelf and depositing on the sediment-wave field in deep water (>1200 m). These findings suggest that the sedimentary record in arc collision zones will be dominated by mass-wasting deposits very close to volcanoes, and by river discharge depositing in select, extent regions far from shore. Because sedimentation rates can vary by a factor of 2 between the two flanks of a sediment wave, care must be taken when comparing bed thickness across an entire sedimentary section.

We present a mechanism linking large impacts, such as Chicxulub, to significant continental sedimentary slope failures and gas hydrate releases, and hence to carbon isotope excursions. Extensive continental margin failures and seabed sediment liquefaction at the Cretaceous-Tertiary boundary up to thousands of kilometers from Chicxulub have been linked to this impact in previous studies: here we analyze the implied seismic shaking and explore its effects in terms of gas hydrate release from failed continental margin sediments. Paleoseismic analysis of published studies of liquefaction and slope failure at the Cretaceous-Tertiary boundary in North America and adjacent regions suggests that, due to low seismic attenuation in plate interior rocks, there was a sufficient seismic forcing to cause the observed widespread sediment liquefaction and failure along tens of thousands of kilometers of continental slope. The implied magnitude of the impact-related seismicity (equivalent to an earthquake with moment magnitude ≈11) is shown to be broadly consistent with the characteristics of the Chicxulub impact structure. An extended period of post-impact liquefaction and slope failure may account for the observed complexity of Cretaceous-Tertiary boundary sequences in Mexico and North America. We favor a seismic shaking model for the triggering of slope failure over previous models implicating impact generated tsunamis, because the shallow-water Chicxulub impact itself is now recognized as an inefficient tsunami source. We have calculated the potential storage of gas hydrates based on known environmental conditions during the Cretaceous. This suggests that slope failures caused by the Chicxulub impact could have released between 300 and 1300 GtC (best estimate ∼700 GtC) of methane from the destabilization of gas hydrates. This would produce a global carbon isotopic excursion of between −0.5‰ and −2‰ (best estimate ∼−1‰). This compares well with the observed carbon isotopic excursion of −1‰ in the planktonic foraminifera records across the K-T boundary. This large release of methane may also account for the recently reconstructed very high atmospheric pCO 2 levels after the Cretaceous-Tertiary boundary as our estimated gas hydrate releases could have increased atmospheric carbon dioxide by a maximum of 600–2300 ppm (best estimate ∼1200 ppm). This mechanism linking impacts to carbon isotope excursions may apply to other significant excursions, such as that at the Paleocene-Eocene Thermal Maximum. The difficulty in identifying impact craters means that many of the other abrupt carbon isotope excursions found in the geological record could be related to impacts and not to climatic changes.