New digital seafloor map provides answers and more questions

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Dutkiewicz et al., <em>Geology</em>, Aug. 5, 2015

Ocean sediments cover 70% of our planet's surface, forming the substrate for the largest ecosystem on Earth and its largest carbon reservoir -- but the most recent map of seafloor geology was drawn by hand more than 40 years ago. Now Adriana Dutkiewicz and her colleagues from the University of Sydney have carefully analyzed and categorized 15,000 seafloor sediment samples to reveal that deep ocean basins are much more complex than previously thought. The team has created a new digital seafloor geologic map using an artificial intelligence method designed to learn how different types of deep marine sediments are juxtaposed. Combined with sea surface observations, the map reveals that diatom accumulations on the seafloor are nearly entirely decoupled from diatom blooms in surface waters in the Southern Ocean.

Diatoms are tiny planktonic organisms thriving in sunlit surface waters, producing about a quarter of the oxygen we breathe, and making a major contribution to fighting global warming as their dead remains sink to the bottom of the ocean, locking away their carbon. However, the new seafloor geology map demonstrates that geoscientists don't yet understand how carbon sources in surface ocean waters are linked to sinks on the seafloor.

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Census of seafloor sediments in the world's ocean Adriana Dutkiewicz et al., EarthByte Group, School of Geosciences, University of Sydney, Sydney, Australia. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/G36883.1. This article is OPEN ACCESS.

Other GEOLOGY articles posted online ahead of print on 5 Aug. 2015 are highlighted below.

Reconciling diverse lacustrine and terrestrial system response to penultimate deglacial warming in southern Europe Graham P. Wilson et al., University of Chester, Chester, UK. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/G36807.1. This article is OPEN ACCESS.

During the penultimate deglaciation (about 129,000 to 136,000 years) global climate shifted from one of the most extreme glaciations to one of the warmest interglacials, yet the regional expression of climate changes during this transition remains understudied. In this paper, Wilson, Reed, Frogley, Hughes, and Tzedakis present the first high-resolution southern European diatom record alongside existing pollen and oxygen isotope data, from Lake Ioannina sediment core I-284 (northwest Greece), to assess the character and timing of terrestrial and aquatic ecosystem response to rapid climate warming. Their study finds a complex deglaciation, and provides firm evidence for changes in lake conditions and regional forest populations in response to abrupt changes in North Atlantic conditions associated with Heinrich Stadial 11. This study also finds marked diachroneity in lake and terrestrial ecosystem response to warming at the onset of the last interglacial. Snowmelt and glacial meltwater transfer are identified as primary driving mechanisms for an abrupt increase in lake level 2.7 thousand years prior to regional forest expansion and associated peak precipitation. This finding has wider implications in demonstrating that, under certain boundary conditions, lakes in mountainous karstic environments are highly sensitive to past climate warming because of the role of local glaciers in controlling regional groundwater levels.

First seismic evidence for continental subduction beneath the Western Alps Liang Zhao et al., State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/G36833.1.

The first discovery of ultra-high pressure minerals in the Alps 30 years ago led to the inference that a less dense continental crust can sink (subduct) to a denser mantle and return to the surface later. The subduction of the continental lithosphere in the mantle, however, has once long been considered as not likely because of the floating force of the continental crust. The China-Italy-France Alps Seismic survey (CIFALPS) deployed the first passive seismic array (recording earthquake signals) that cross-cuts the entire western Alps. Using CIFALPS data, for the first time, this paper by Liang Zhao and colleagues provides geophysical images for continental crust at 75 km in depth that is clearly connected with the European crust exactly along the transect where ultra-high pressure minerals were found at the surface. The paper also provides evidence for a thick boundary zone beneath which the seismic velocity decrease with depth. This reverse anomaly of velocity demonstrates that the European lower crust sunk into the Adriatic mantle. These findings in the Alps (one of the best preserved and long studied ultra-high pressure orogens worldwide), shed decisive new light on geodynamic processes along collisional continental margins.

Late-stage formation of Martian chloride salts through ponding and evaporation Brian M. Hynek et al., Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado, USA. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/G36895.1.

Chloride salts have been found in hundreds of locations on Mars; identified from orbiter and rover data. They are most likely sodium chloride (table salt) and are indicators of past water. Yet little is known about how or when these deposits formed. In this work, chloride deposits near the NASA rover Opportunity were examined with remote sensing data. These materials provide strong evidence that the chlorides formed in a river-fed lake environment. The lake developed, breached the basin rim and flowed out onto surrounding volcanic plains. Eventually, the lake became a closed system and evaporated, precipitating the chlorides at the bottom of the basin. The age of the lake was a minimum of 3.6 billion years and therefore represents some of the last vestiges of surface water on Mars that resulted in mineral deposits identifiable from orbit. Digital terrain models were constructed to determine the volume of the lake and chlorides, and for the first time the salinity (saltiness) of a past Martian lake was determined. The salinity was well below that of Earth's oceans and thus habitable by this standard. If life ever existed on Mars, it could have been harbored in a lake system like this one.

Lava lake level as a gauge of magma reservoir pressure and eruptive hazard Matthew R. Patrick et al., U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii, USA. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/G36896.1.

Tracking magma reservoir pressure is a fundamental part of forecasting volcanic activity. Existing techniques, such as measuring ground deformation and earthquake activity, are limited in that they are proxies for magma pressure. Lava lakes provide an open "window" into a volcano's magmatic system, and can be used to directly measure pressure changes by using the changing lake levels - essentially using the lake as a liquid pressure gauge of the underlying magma reservoir. The lava lake at the summit of Kilauea Volcano, Hawai'i, shows a strong correlation between lake level and ground deformation, confirming this relationship. Furthermore, we show that by tracking magmatic pressure in this manner, it is possible to forecast the eruption rate at Kilauea's flank eruption, on its East Rift Zone, which recently threatened the town of Pahoa.

Modes and rates of horizontal deformation from rotated river basins: Application to the Dead Sea fault system in Lebanon Liran Goren et al., Ben-Gurion University of the Negev, Beer She-va, Israel. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/G36841.1.

When the relative motion between two tectonic plates is of oblique convergence, the deformation in the plate boundary area shows different modes, where part of the deformation is localized along discrete faults and the remaining deformation is distributed along a wide band. Quantifying the relative portions of localized and distributed deformation is, in most cases, not straightforward. Goren and colleagues study the geometry of river basins along the Sinai-Arabia plate boundary in Lebanon, with the aim of quantifying how deformation is partitioned along this oblique convergence plate boundary. The authors identify a group of basins that are consistently rotated with respect to the typical transverse drainage orientation of linear mountain ranges. Furthermore, basin drainage area distribution is shown to be indicative of a disequilibrium and transient basin shape. A kinematic model reveals that while the majority of the relative plate motion is accommodated by the seismogenic Yammouneh fault, about a quarter of the deformation is distributed along a wide band to the west of the fault. A landscape evolution model reveals a competition between river basin rotation that records crustal distributed deformation, and minor drainage network reorganization that acts to amend basin shape back to their conventional form.

Source: Geological Society of America