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eight new species in the South Atlantic Ocean over 80,000 years before the species turn up in the northern Pacific Ocean (Fig. 1).

Support for the hemispheric asymmetry proposed by Jiang and colleagues comes in the form of studies of carbon cycling at the K/T boundary3. Evidence for Strangelove oceans is clearly seen in an equatorial setting, but is less defined in a core from 38° S, implying only partial loss of plankton productivity in the mid-southern latitudes. Any mechanism to explain the extinction of the nannoplankton must therefore also account for a hemispheric selectivity.

Global darkness is ruled out by Jiang and colleagues, at least as the primary cause of the extinction: experiments on modern phytoplankton indicate significant survival rates even after several months of darkness6. And some of the species of calcareous nannoplankton that are most sensitive to changing seawater pH survived the extinction, excluding ocean acidification as the cause. Having eliminated these contenders, Jiang and colleagues offer the new alternative of trace-metal poisoning from the debris ejected from the impact. As the debris fell back to the surface, it brought trace metals such as copper, chromium, aluminium and mercury to the oceans’ surface.

Under normal circumstances these trace metals are rapidly removed as they stick

to the sinking remains of dead plankton. However, the Strangelove ocean of the post-impact interval was not normal, and lacked an efficient biological pump. Jiang and colleagues suggest that this would have significantly reduced the number of sinking particles and prolonged the residence time of the toxic metals in the surface oceans, slowing the recovery of the phytoplankton. And, intriguingly, there is independent evidence for a northward-directed blast from the impact. Jiang and colleagues suggest that this would have made this kill mechanism especially lethal in the northern oceans.

Definitive proof for the idea may prove to be hard to come by: absolute concentrations of trace metals in sediments are controlled by a plethora of factors, primarily local seafloor chemistry. Additional evidence of a hemispheric selectivity may come from radiolarians, another group of plankton that survived the K/T extinction relatively unscathed in the Southern Ocean2. Of course, given the gaps in the radiolarian fossil record, it is possible that they survived equally well in other locations. And unlike the plankton, the extinction of molluscs at the sea floor shows no signs of hemispheric differences4. Further tests could come from a type of larger calcareous plankton well represented in the fossil record — the foraminifera — which should respond to

metal poisoning in a similar manner to the nannoplankton.

And, as with many K/T extinction studies, it remains to be seen what, if any, role the contemporaneous eruptions of Deccan Trap flood basalts in India may have had in the crisis. Reconstructions of the nannoplankton extinction in the Indian Ocean are needed to isolate any local effects that may have arisen from this volcanic activity.

Overall, Jiang and colleagues1 propose an interesting mechanism for the demise of calcareous nannoplankton, and have shown that for some calcareous nannoplankton, residence in the Southern Hemisphere oceans may have been the key to surviving the mass extinction at the Cretaceous/Tertiary boundary. ❐

Paul B. Wignall is at the School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. e‑mail: p.wignall@see.leeds.ac.uk

References1. Jiang, S.-J., Bralower, T. J., Patzkowsky, M. E., Kump, L. R. &

Schueth, J. D. Nature Geosci. 3, 280–285 (2010).2. Hollis, C. J. in Cretaceous‑Tertiary Mass

Extinctions: Biotic and Environmental Changes. (eds Macleod, N. & Keller, G.) 173–204 (W. W. Norton, 1996).

3. Coxall, H. K., D’Hondt, S. & Zachos, J. C. Geology 34, 297–300 (2006).

4. Raup, D. M. & Jablonski, D. Science 260, 971–973 (1993).

5. Zachos, J. C., Arthur, M. A. & Dean, W. E. Nature 337, 61–64 (1989).

6. Antia, N. J. & Cheng, J. Y. Phycologia 9, 179–183 (1970).

Mid-ocean ridges are underwater mountain ranges that form where two tectonic plates separate.

As the oceanic crust drifts apart along a central spreading axis, the eruption of seafloor lava flows creates a long volcanic chain. New magma is continually added along the plate boundary, generating new crust that creates the floor of ocean basins. The magmatic system at ocean ridges supplies energy to drive seafloor hydrothermal systems, which accommodate one quarter of the heat loss from the solid Earth and control ocean chemistry. Writing in Nature Geoscience, Standish and Sims1 report age measurements of volcanic rocks at the ultraslow-spreading

Southwest Indian Ridge that indicate an unusually wide dispersal of freshly erupted volcanic material throughout the rift valley, possibly fed by magma flow along rift-bounding faults.

Six decades of surveying and sampling have led to models of magma supply and crystallization at mid-ocean ridges. However, geological mapping of seafloor eruptions is extremely difficult. On relatively young volcanic islands such as Hawaii or Iceland, individual lava flows can be paced out and sampled with a sledgehammer. The time of eruption of each flow can often be determined from historical accounts or by radiocarbon dating of suitable material. These methods do not work at submerged

volcanoes. Only exceptional circumstances allow seafloor samples to be identified as the products of individual eruptions of known age2

.Text-book cartoon models of mid-

ocean ridges often show a narrow zone of volcanic activity, with magmatism tightly focused at the ridge crest, known as the axial volcanic ridge (Fig. 1). In this model, almost all eruptions emanate from fissures within a few hundred metres of the spreading axis where the plates diverge. With continued plate separation, the lava flows are transported away from the axial volcanic ridge and replaced in their wake with younger volcanic rock. This axial-centric view stems from observations of the

Mid-oceAn Ridges

Widening the goal-postsOceanic crust forms through the addition of volcanic rock to mid-ocean ridges. Widely dispersed, young lavas observed at an ultraslow-spreading ridge provide impetus for the redevelopment of models of oceanic magmatism.

John Maclennan

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morphology of the sea floor at ridges that are classified as fast spreading, where plates are separating at more than 75 mm yr–1.

It has recently become clear that the structure of ultraslow-spreading ridges, where plates separate at less than 20 mm yr–

1, is different from that of the text-book ridge. Surprisingly, long stretches of the ultraslow-spreading ridges were discovered to be almost devoid of volcanoes3. These stretches of the rift-valley floor are instead composed of solid mantle rocks, which are exhumed by the tectonics of plate separation4,5. Where the floor is covered by volcanic rock along ultraslow-spreading ridges, it forms a deep, wide, axial valley bound by large faults.

Standish and Sims1 focus on the ultraslow-spreading Southwest Indian Ridge, which accommodates the separation of the African and Antarctic plates at a rate of 14 mm yr–1. They collected lava samples from across the rift valley floor and conducted geochemical measurements of the imbalance between radioactive production and decay of thorium and radium isotopes to constrain the age of eruption of the volcanic rocks. Whereas the simple axial-centric model of plate separation predicts that the sample age should increase away from the axial volcanic ridge, the lava samples analysed by Standish and Sims1 do not always conform to this model. Instead, samples with young eruption ages are found widely dispersed across the valley floor. They interpret this

mismatch to result from broadly distributed volcanism, with eruptions fed not only from the inferred central spreading axis, but also from volcanoes situated elsewhere within the 20-km-wide axial valley. Tectonic faults that bound the mantle blocks may provide conduits for the lava, promoting dispersal of young magma across the width of the valley floor.

Some uncertainty in the conclusion of distributed volcanism comes from ambiguities in the determination of the exact location of the spreading centre. The position of the spreading axis is estimated from bathymetry, morphology and sonar images of the sea floor, and the visual appearance of the recovered rock samples. These estimates are qualitative and may be confounded by localized faulting, small-scale recent shifts in the position of the spreading axis or heterogeneous seafloor alteration of the samples. Some uncertainty must therefore be associated with the picking of this preferred ridge axis. Indeed, it would be possible to define a single, narrow, ridge axis that passes very close to all of the samples that have been shown to be young by geochemical dating, removing the requirement for across-axis distributed volcanism. This problem reflects the challenge of working with submerged mid-ocean ridges where detailed sampling and geological mapping are extremely difficult.

To settle this doubt, future studies should aim to obtain radioisotopic ages from a sampling line that runs perpendicular to

the ridge axis. The transect should start at the young-looking axial volcanic ridge and traverse the adjacent rift valley floor to the bounding faults. If samples collected from such a clearly defined transect also showed young ages, then the case for dispersed volcanism at ultraslow-spreading ridges would be greatly strengthened. Improved geological mapping of the sea floor could then help to understand the volumes of dispersed volcanic rock, and this knowledge could then be incorporated into physical models of ocean-crust generation.

The observations reported by Standish and Sims1 indicate that dispersed volcanism may be important at the ultraslow-spreading Southwest Indian Ridge. Further detailed geological, geophysical and geochemical studies at ridges are required to refine our models of magmatism, not only for ultraslow ridges but also for all other parts of the global spreading-ridge system6. ❐

John Maclennan is at the Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK. e‑mail: jcm1004@cam.ac.uk

References1. Standish, J. J. & Sims, K. W. W. Nature Geosci. 3, 286–292 (2010).2. Soule, S. A. et al. Geology 35, 1079–1082 (2007).3. Michael, P. J. et al. Nature 423, 956–961 (2003).4. Dick, H., Lin, J. & Schouten, H. Nature 426, 405–412 (2003).5. Cannat, M. et al. Geology 34, 605–608 (2006).6. Sims, K. W. W. et al. Geochem. Geophy. Geosy. 4, 8621 (2003).

Published online: 28 March 2010

Text–book view of fast–spreading ridge New model of ultraslow–spreading ridge

3 km

2 km

a b

Focusedvolcanic activity

Crust

Mantle

Crust

~15 km wide, broadly dispersed volcanism

Mantle

Figure 1 | Schematic models of magmatism at spreading ridges. Red lenses and channels in the crust are positions of crust magma chambers and conduits. These feed young volcanic eruptions, shown in dark grey on the surface. The dashed black line shows the location of the axis of spreading. a, The axial-centric view of magmatism at fast-spreading ridges that has lead the thinking on crustal generation at ridges. b, A new model of dispersed volcanism at ultraslow-spreading ridges, with some magma supplied to volcanoes along rift-bounding faults.

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