Sensing the sea bed

Nature Geoscience 2, E11 (2009). doi:10.1038/ngeo566

William Wilcock and a team of scientists and engineers drilled holes in the sea floor, and inadvertently provided a breeding ground for octopuses, in their attempt to understand deep-ocean hydrothermal venting.

Nature Geoscience 2, E12 (2009). doi:10.1038/ngeo569

Paolo Gabrielli and colleagues dug deeply — and extremely cautiously — into Antarctic ice, to see whether the poles acted as a sink for mercury in the geological past.

Nature Geoscience 2, 447 (2009). doi:10.1038/ngeo576

Humans have been responding to fluctuating sea levels for millennia. Adapting to future change will require a swift start on developing innovative infrastructure while keeping the option to adjust in the long term.

Nature Geoscience 2, 450 (2009). doi:10.1038/ngeo572

Authors: Pavel Kabat, Louise O. Fresco, Marcel J. F. Stive, Cees P. Veerman, Jos S. L. J. van Alphen, Bart W. A. H. Parmet, Wilco Hazeleger & Caroline A. Katsman

The Netherlands has a long and varied history of coastal and river flood management. The anticipation of sea-level rise during the twenty-first century has renewed the push for sustainable solutions to coastal vulnerability.

Nature Geoscience 2, 452 (2009). doi:10.1038/ngeo567

Authors: Dennis P. Lettenmaier & P. C. D. Milly

Changes in continental water stores, largely human-induced, affect sea level. Better hydrological models and observations could clarify the land's role in sea-level variations.

Nature Geoscience 2, 455 (2009). doi:10.1038/ngeo562

Author: John R. Underhill

Homer's Ithaca had been viewed as a work of poetic licence and imprecise geography. However, as recent research shows the island's form may have been disguised over the past two millennia by catastrophic rockfalls, co-seismic uplift events and relative sea-level change.

Nature Geoscience 2, 461 (2009). doi:10.1038/ngeo565

Author: La Toya Myles

Ammonia is a significant atmospheric pollutant whose global distribution is poorly understood. Satellite measurements highlight ammonia hotspots across the globe and indicate that current inventories may underestimate emissions in the Northern Hemisphere.

Nature Geoscience 2, 462 (2009). doi:10.1038/ngeo560

Author: Rolf Weller

It is unclear whether the modern processes of mercury cycling — such as mercury deposition in polar regions — operated before anthropogenic emissions. Ice-core records from Antarctica now reveal strikingly high mercury concentrations during the coldest glacial periods.

Nature Geoscience 2, 463 (2009). doi:10.1038/ngeo564

Author: Maya Tolstoy

Seafloor vents spewing mineral-rich plumes of hydrothermal fluid — termed black smokers — can persist at mid-ocean ridges for decades or longer. Earthquake data indicate that ongoing magma injection may determine their locations.

Nature Geoscience 2, 465 (2009). doi:10.1038/ngeo563

Author: Kenneth G. Miller

Large and rapid global sea-level changes indicate that polar ice sheets may have ephemerally existed during the Cretaceous greenhouse climate. Two oxygen isotopic studies provide evidence for and against this conclusion.

Nature Geoscience 2, 467 (2009). doi:10.1038/ngeo570

Author: Gian-Kasper Plattner

Some components of the climate system continue to adjust long after atmospheric greenhouse-gas levels have stopped changing. A coupled climate–vegetation model shows that forests can be committed to die-back or expansion before change is observed.

Nature Geoscience 2, 468 (2009). doi:10.1038/ngeo559

Author: Johann H. Jungclaus

Understanding millennial-scale climate variability provides context for present and future climate change. It now emerges that temperatures were spatially and seasonally more heterogeneous over the past 1,000 years than previously thought.

Nature Geoscience 2, 479 (2009). doi:10.1038/ngeo551

Authors: Lieven Clarisse, Cathy Clerbaux, Frank Dentener, Daniel Hurtmans & Pierre-François Coheur

Global ammonia emissions have more than doubled since pre-industrial times, largely owing to agricultural intensification and widespread fertilizer use. In the atmosphere, ammonia accelerates particulate matter formation, thereby reducing air quality. When deposited in nitrogen-limited ecosystems, ammonia can act as a fertilizer. This can lead to biodiversity reductions in terrestrial ecosystems, and algal blooms in aqueous environments. Despite its ecological significance, there are large uncertainties in the magnitude of ammonia emissions, mainly owing to a paucity of ground-based observations and a virtual absence of atmospheric measurements. Here we use infrared spectra, obtained by the IASI/MetOp satellite, to map global ammonia concentrations from space over the course of 2008. We identify several ammonia hotspots in middle–low latitudes across the globe. In general, we find a good qualitative agreement between our satellite measurements and simulations made using a global atmospheric chemistry transport model. However, the satellite data reveal substantially higher concentrations of ammonia north of 30∘ N, compared with model projections. We conclude that ammonia emissions could have been significantly underestimated in the Northern Hemisphere, and suggest that satellite monitoring of ammonia from space will improve our understanding of the global nitrogen cycle.

Nature Geoscience 2, 484 (2009). doi:10.1038/ngeo555

Authors: Chris Jones, Jason Lowe, Spencer Liddicoat & Richard Betts

Targets for stabilizing climate change are often based on considerations of the impacts of different levels of global warming, usually assessing the time of reaching a particular level of warming. However, some aspects of the Earth system, such as global mean temperatures and sea level rise due to thermal expansion or the melting of large ice sheets, continue to respond long after the stabilization of radiative forcing. Here we use a coupled climate–vegetation model to show that in turn the terrestrial biosphere shows significant inertia in its response to climate change. We demonstrate that the global terrestrial biosphere can continue to change for decades after climate stabilization. We suggest that ecosystems can be committed to long-term change long before any response is observable: for example, we find that the risk of significant loss of forest cover in Amazonia rises rapidly for a global mean temperature rise above 2 ∘C. We conclude that such committed ecosystem changes must be considered in the definition of dangerous climate change, and subsequent policy development to avoid it.

Nature Geoscience 2, 488 (2009). doi:10.1038/ngeo553

Authors: Michael D. Blum & Harry H. Roberts

Over the past few centuries, 25% of the deltaic wetlands associated with the Mississippi Delta have been lost to the ocean. Plans to protect and restore the coast call for diversions of the Mississippi River, and its associated sediment, to sustain and build new land. However, the sediment load of the Mississippi River has been reduced by 50% through dam construction in the Mississippi Basin, which could affect the effectiveness of diversion plans. Here we calculate the amount of sediment stored on the delta plain for the past 12,000 years, and find that mean storage rates necessary to construct the flood plain and delta over this period exceed modern Mississippi River sediment loads. We estimate that, in the absence of sediment input, an additional 10,000–13,500 km2 will be submerged by the year 2100 owing to subsidence and sea-level rise. Sustaining existing delta surface area would require 18–24 billion tons of sediment, which is significantly more than can be drawn from the Mississippi River in its current state. We conclude that significant drowning is inevitable, even if sediment loads are restored, because sea level is now rising at least three times faster than during delta-plain construction.

Nature Geoscience 2, 492 (2009). doi:10.1038/ngeo552

Authors: Casey Saenger, Anne L. Cohen, Delia W. Oppo, Robert B. Halley & Jessica E. Carilli

Sea surface temperature variability in the North Atlantic Ocean recorded since about 1850 has been ascribed to a natural multidecadal oscillation superimposed on a background warming trend. It has been suggested that the multidecadal variability may be a persistent feature, raising the possibility that the associated climate impacts may be predictable. However, our understanding of the multidecadal ocean variability before the instrumental record is based on interpretations of high-latitude terrestrial proxy records. Here we present an absolutely dated and annually resolved record of sea surface temperature from the Bahamas, based on a 440-year time series of coral growth rates. The reconstruction indicates that temperatures were as warm as today from about 1552 to 1570, then cooled by about 1 ∘C from 1650 to 1730 before warming until the present. Our estimates of background variability suggest that much of the warming since 1900 was driven by anthropogenic forcing. Interdecadal variability with a period of 15–25 years is superimposed on most of the record, but multidecadal variability becomes significant only after 1730. We conclude that the multidecadal variability in sea surface temperatures in the low-latitude western Atlantic Ocean may not be persistent, potentially making accurate decadal climate forecasts more difficult to achieve.

Nature Geoscience 2, 496 (2009). doi:10.1038/ngeo556

Authors: Jason P. Briner, Aaron C. Bini & Robert S. Anderson

Ice-sheet behaviour is disproportionately controlled by the dynamics of outlet glaciers that terminate in the ocean. However, outlet-glacier dynamics—particularly over timescales longer than the observational record—are not well understood, leading to uncertainties in our models of ice-sheet response to climate change. Here we use 10Be exposure ages and radiocarbon dating from the Sam Ford Fjord in the Canadian Arctic to reconstruct the retreat chronology of an outlet glacier of the Laurentide ice sheet, following the last glacial termination. We find that Sam Ford Fjord, which has a similar morphology to the troughs holding many outlet glaciers of the Greenland ice sheet, was rapidly deglaciated about 9,500 years ago, with retreat rates ranging from 5 to 58 m yr−1. The highest rates occurred in the deepest part of the fjord (900 m), whereas regions beyond the fjord mouth and up-valley of the head of the fjord experienced the lowest rates of retreat. We conclude that in such a fjord setting, there is a strong bathymetric control on the retreat of marine outlet glaciers: once the terminus of the outlet glacier retreated into deeper waters, increasing calving rates and basal sliding speeds caused the glacier to rapidly thin and retreat, stabilizing only when it reached the shallow inland head of the fjord.

Nature Geoscience 2, 500 (2009). doi:10.1038/ngeo557

Authors: E. J. Rohling, K. Grant, M. Bolshaw, A. P. Roberts, M. Siddall, Ch. Hemleben & M. Kucera

Ice cores from Antarctica record temperature and atmospheric carbon dioxide variations over the past six glacial cycles. Yet concomitant records of sea-level fluctuations—needed to reveal rates and magnitudes of ice-volume change that provide context to projections for the future—remain elusive. Reconstructions indicate fast rates of sea-level rise up to 5 cm yr−1 during glacial terminations, and 1–2 cm yr−1 during interglacials and within the past glacial cycle. However, little is known about the total long-term sea-level rise in equilibration to warming. Here we present a sea-level record for the past 520,000 years based on stable oxygen isotope analyses of planktonic foraminifera and bulk sediments from the Red Sea. Our record reveals a strong correlation on multi-millennial timescales between global sea level and Antarctic temperature, which is related to global temperature. On the basis of this correlation, we estimate sea level for the Middle Pliocene epoch (3.0–3.5 Myr ago)—a period with near-modern CO2 levels—at 25±5 m above present, which is validated by independent sea-level data. Our results imply that even stabilization at today’s CO2 levels may cause sea-level rise over several millennia that by far exceeds existing long-term projections.

Nature Geoscience 2, 505 (2009). doi:10.1038/ngeo549

Authors: Petru Jitaru, Paolo Gabrielli, Alexandrine Marteel, John M. C. Plane, Fréderic A. M. Planchon, Pierre-Alexis Gauchard, Christophe P. Ferrari, Claude F. Boutron, Freddy C. Adams, Sungmin Hong, Paolo Cescon & Carlo Barbante

Mercury is a globally dispersed toxic metal that affects even remote polar areas. During seasonal atmospheric mercury depletion events in polar areas, mercury is removed from the atmosphere and subsequently deposited in the surface snows. However, it is unknown whether these events, which have been documented for the past two decades, have occurred in the past. Here we show that over the past 670,000 years, atmospheric mercury deposition in surface snows was greater during the coldest climatic stages, coincident with the highest atmospheric dust loads. A probable explanation for this increased scavenging is that the oxidation of gaseous mercury by sea-salt-derived halogens occurred in the cold atmosphere. The oxidized mercury compounds were then transferred to the abundant mineral dust particles and deposited on the snowpack, leading to the depletion of gaseous mercury in the Antarctic atmosphere. We conclude that polar regions acted as a mercury sink during the coldest climatic stages, and that substantial polar deposition of atmospheric mercury is therefore not an exclusively recent phenomenon.

Nature Geoscience 2, 509 (2009). doi:10.1038/ngeo550

Authors: William S. D. Wilcock, Emilie E. E. Hooft, Douglas R. Toomey, Paul R. McGill, Andrew H. Barclay, Debra S. Stakes & Tony M. Ramirez

Black-smoker hydrothermal systems at mid-ocean ridges are often driven by heat loss from a crustal magma chamber, but black-smoker systems have not been detected above all magma chambers. The high fluxes of heat recorded at black-smoker systems require a thin, metre-scale conductive boundary layer at the top of the magma chamber, separating hydrothermal fluids from magma. Extensive seismicity above the magma chambers has been attributed to stresses from hydrothermal cooling. The presence of high-temperature hydrothermal systems has previously been linked with episodes of magma chamber inflation, but the exact mechanism remained to be clarified. Here we analyse seismic data for the Endeavour segment of the Juan de Fuca ridge—a site of long-lived hydrothermal activity—recorded by seismometers located beneath the sea floor. Earthquake focal mechanisms derived from the data reveal a transition from normal faulting above the mid-crustal magma chamber to reverse faulting on either flank. This pattern of faulting is consistent with stress perturbations resulting from the emplacement of pressurized magma that forms a thin sill. We suggest that the ongoing recharge of magma into a crustal magma chamber not only replenishes the heat source, but also helps maintain a thin conductive boundary layer that would otherwise thicken owing to water–rock interactions and crystallization at the chamber roof.

Nature Geoscience 2, 514 (2009). doi:10.1038/ngeo546

Authors: James R. Rustad & Qing-Zhu Yin

Understanding the distribution of iron isotopes within planetary bodies can help constrain their histories of accretion and differentiation. A large fraction of the iron in the silicate Earth is dissolved in ferropericlase and ferroperovskite—mineral phases that make up the bulk of the lower mantle. These phases have distinct crystallographic structures and iron resides in them in multiple spin states; they are therefore likely to partition iron isotopes differently. Here we use density functional methods to calculate the equilibrium iron isotope composition of these phases at a range of temperatures and pressures, including those thought to exist near the core–mantle boundary. We find that the iron isotopic composition of ferropericlase strongly depends on the spin state of iron. At pressures near the base of the mantle, the low-spin state is enriched in heavy isotopes relative to the high-spin state. In contrast, for ferroperovskite, our calculations suggest that the isotopic composition is almost independent of spin state. Our results warrant a careful search for a pressure-dependent isotopic signature in samples brought up by mantle plumes and in materials subjected to lower mantle pressures and temperatures in the laboratory.

Nature Geoscience 2, 528 (2009). doi:10.1038/ngeo568

Hermann M. Fritz and colleagues travelled by cargo boat through the Ayeyarwady delta in Myanmar to document the damage after cyclone Nargis.