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We use time series analysis to compare the impact histories of the Earth and Moon with the record of mantle plume activity. We use events with errors in their ages of ≤150 Ma. The terrestrial and lunar impact records, when smoothed at a 45-Ma interval, correlate at a 97% confidence level. This high confidence level suggests that we have an adequate sampling of most of the major impact events on the Earth. We then test the idea that existing mantle plumes may be strengthened by impacts. When smoothed at a 45-Ma interval, strong plumes correlate with the terrestrial impact record at better than a 99% confidence level. No time lag is discernible between the data sets, which is expected given their present error level. When the time series are smoothed at a 30-Ma interval, there are 10 major peaks in impact activity. Nine out of ten of these peaks have a counterpart in either or both of the strong mantle plume or the mantle plume time series. As a result, the strong mantle plume and the impact time series correlate at the 97% confidence level. The mantle plume and the impact time series correlate at the 90% confidence level. Finally, the Deccan plume showed greatly increased activity immediately after the Chixculub impact. The results of our analysis suggest that large meteorite and cometary impacts may well increase the amount of volcanism from already active mantle plumes.

We define the characteristics of plume events that can be called superplumes. Using the surface area of the smallest oceanic plateau generated during the Cretaceous superplume era, we define a cutoff surface area for superplume flood basalts of ⩾410,000 km 2 . We show that the maximum widths of feeder dikes of plume lavas are linearly related to the square root of the surface area covered by their flood basalts. From this we derive a cutoff: the widest feeder dikes of a superplume event must be ⩾70±4 m wide. All high Mg rocks as defined by Isley and Abbott [J. Geol. (2001)] are superplume rocks. Layered intrusions formed by superplumes have either high abundances of platinum group elements and/or chromium. We use all of the data from the superplume proxies: flood basalts, dike swarms, high Mg rocks, and layered intrusions to define the duration of superplume eras over Earth history. Over two thirds of the superplume eras last less than 8 million years. We find no significant difference between the average duration of Archean (13±7 Ma) and Phanerozoic (12±3Ma) superplume eras. Finally, we use our data on maximum dike widths and flood basalt surface area to construct estimates of the overall surface area covered by lava during individual superplume events over the last 2.9 Ga. We find that the largest Precambrian superplume events erupted at least 10 times more lava than the largest Phanerozoic superplume event, covering a minimum of 14–18% of the planet. Between 1.7 and 2.9 Ga, there were enough large Precambrian superplume events to completely resurface the planet. We also find evidence for many superplume events earlier than 2.9 Ga, but due to a lack of data on maximum widths of feeder dikes, we cannot estimate the relative sizes of most of these events.

We have compiled a record of the geochronology of mantle plume activity between 3.8 and 1.6 Ga. Over this time period, the ages of komatiites, and those of global plumes, correlate strongly, at the 99% confidence level, with the ages of banded iron formations (BIFs). The ages of continental plumes correlate more weakly, at an overall 85% confidence level. Using the geochronological records of these events, we can define four periods characterized by mantle superplume activity. Three of these periods are also times of enhanced BIF deposition. The fourth mantle plume period may similarly be coeval with increased BIF accumulation, but the BIF chronostratigraphic resolution is not accurate enough to test this rigorously. Mantle superplume volcanism may promote BIF deposition by increasing the Fe flux to the global oceans through continental weathering and/or through submarine hydrothermal processes. It may also be enhanced by increasing the number of paleotectonic environments appropriate for BIF deposition (particularly plume-induced ocean plateaus, seamounts, and intracratonic rifts) and by promoting global anoxic, Fe-rich hydrothermal plumes in the shallow to intermediate marine water column.

Most of the iron in banded iron formation (BIF) was deposited during the period 2.7-1.8 Ga. Within this period, Fe deposition rates were at least as high as $10^{12} gm/y$. According to new age models for the largest BIFs, peak deposition occurred between 2.4-2.3 Ga. It is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment. Estimates of global heat loss for the Late Archean-Early Proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present, so hydrothermal water-rock exchanges were a more important source of Fe to the oceans than today. Hydro-thermal plumes, depleted in $O_{2}$ and enriched in Fe, were generated at mid-ocean ridge crests with substantially shallower depths than present. Assuming hydrothermal effluent dilution factors of $10^{4}$, plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to...

Turbulent boundary shear stress depends on a roughness length scale which characterizes momentum transfer to the seabed. The effective roughness length is due to physical roughness geometry such as sediment ripples, grain size and processes affecting momentum transfer, such as bedload transport and wave motions. During storms on the California shelf, wave motions dominate the turbulent boundary layer, although feedback through wave-induced bed forms and sediment transport are important. Several intense storms on the Northern California Shelf were monitored with pressure sensors, acoustic current meters and an optical backscatter sensor within 5 m of the bed at 90 m depth. Wave spectra, velocity profiles, turbulent kinetic energy and suspended sediment concentrations were obtained. At 90 m depth, waves were measured in the band of 0.05-0.08 Hz, with nearbed wave velocities of 5–30 cm s −1 . In spite of wave-induced currents of up to three times the mean speed, 30 min average velocities yielded typical logarithmic profiles. Roughness, as indicated by the zero intercept of the logarithmic profiles, z 0 , varied by a factor of 25 throughout the storms, with a maximum of 18 cm, when mean currents were above 5 cm s −1 and the wave amplitude was maximal. Such large increases in z 0 are predicted by models of wave-current interaction. Effects of sediment transport and bed forms are not easily extracted from the data during storms. But, the fairly large non-storm values of ∼0.5cm indicate the effect of bed ripples.

Benthic turbid events, with particle concentrations of up to 1.5 mg l−1, occupy about 20% of a two-year record of near-bottom light attenuation made above the Northern Hatteras Abyssal Plain. Theyare most often associated with deep-sea mesocle eddies, although rare events occur during periods when southward contour flow predominates. In a temporal sense, the presence of deep-sea mesoscale eddies corresponds strongly to periods when cold-core Gulf Stream rings propagate through surface waters of the study area. Additionaly, over half of the benthic turbid events observed are initiated within a day of the passage of atmospheric storms with propagation speeds in excess of 800 km day−1.

We compile a 3.7-b.yr.-long time series of ultramafic and mafic rocks including extrusives and shallow intrusives (dikes and sills). We infer that peaks in the time series represent mantle plume events. Rocks erupted from plumes are becoming more Ti rich through time, and several rock types having 118 wt % MgO are Phanerozoic analogs for komatiites. These include meimechites, ankaramites, and rocks previously called “picrites.” Spectral analysis reveals the time series is driven by periods of ∼800 and ∼273 m.yr. Two 256-m.yr.-long data subsets, one sampling the Archean and one sampling the Phanerozoic, are driven by periods of and m.yr., respectively. The 26 3 34.5 4.5 ∼800-m.yr.- long energy may reflect changes in the rate of impacts of extraterrestrial objects, tectonic slab cascades into the mesosphere, or resonance between free-core nutations and those forced by solar torques. We suggest that the 273 m.yr. period reflects the cosmic year. The latter modulates fluctuation in cometary impacts that occur with a 30–35 m.yr. period (Matese et al. 1996). Thus, there may be more than one driving force for mantle plume volcanism, including forces endogenic and exogenic to Earth.

Today, the global lead emissions into the atmosphere from anthropogenic sources are about 200 times higher than in the past1 and dominate other lead pathways to the ocean. Exhaust from leaded gasoline is the major source of pollutant lead in the atmosphere2, and subsequently in the marine environment due to atmospheric deposition by wet and dry removal processes3,4. This pollution has already perturbed the biogeochemical cycle of lead in the open ocean2,3,5–7 especially in the north Atlantic. In surface waters, where lead is introduced mainly in the dissolved state, its residence time is relatively short (less than 5 years), as it is rapidly scavenged by, and transported with, participate matter5,7. The mean residence time of lead in the ocean is estimated to be ∼100 yr5,8. One can therefore expect to see some pollutant lead in the deep ocean surficial sediments. Here, we report lead concentrations in two short cores from the north-east Atlantic. Lead concentrations in surficial sediments (21.0 and 15.0 p.p.m. in the top cm) are higher than in the uppermost 10 cm of the cores (6.0 and 2.8 p.p.m.), and are likely to be enhanced from anthropogenic sources. The quantity of lead stored in these surficial sediments (5.7 and 2.5 µg cm−2) is of the same order as the amount of pollutant lead present in the dissolved state in the water column.