Tiny fossils tell a long(ish) story
Boulder, Colo., USA – The impact of an asteroid at the end of the Cretaceous caused mass extinctions in the oceans, as well as killing the dinosaurs on land. The carbon isotope difference between surface and seabed organisms (foraminifera) also collapsed due to these extinctions, suggesting that organic matter from surface waters did not reach the seafloor for up to 3 million years. However, seafloor organisms, which are dependent on food from surface waters, did not die off, suggesting some food must have reached the seabed.
In their open-access paper for Geology, Heather S. Birch and colleagues investigate this paradox by looking at carefully selected foraminiferal isotopes from a well-dated deep-sea core in the South Atlantic.
By taking into account the likely ecology of the foraminifera studied and whether any water mass changes were occurring at the time, they can better assess the carbon isotope record and transfer of organic matter to the seafloor. Birch and colleagues find that the flux of organic matter was reduced for a much shorter time (1.7 million instead of 3 million years). The authors note that ecology and water mass changes likely did have a small effect on the carbon isotope record, but they cannot explain the full reduction in carbon isotopes on their own.
Partial collapse of the marine carbon pump after the Cretaceous-Paleogene boundary
Heather S. Birch et al., School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK. This article is Open Access online at http://geology.gsapubs.org/content/early/2016/03/07/G37581.1.abstract.
Other recently posted GEOLOGY articles are highlighted below:
The oldest Pinus and its preservation by fire
Howard J. Falcon-Lang et al., Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37526.1.abstract.
Scientists have found the oldest fossils of the familiar pine tree that dominates Northern Hemisphere forests today. The 140-million-year-old fossils are exquisitely preserved as charcoal, the result of burning in wildfires. The fossils suggest that pines co-evolved with fire at a time when oxygen levels in the atmosphere were much higher and forests were especially flammable. Dr. Howard Falcon-Lang (Royal Holloway, University of London), who discovered the fossils in Nova Scotia, Canada, said, "Pines are well adapted to fire today. The fossils show that wildfires raged through the earliest pine forests and probably shaped the evolution of this important tree." Modern pines store flammable deadwood on the tree making them prone to lethal fires; however, they also produce abundant cones that will only germinate after a fire, ensuring a new cohort of trees is seeded after the fire has passed by.
Can CO2 trigger a thermal geyser eruption?
Bethany S. Ladd and M. Cathryn Ryan, Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37588.1.abstract.
Geyser periodicity has fascinated researchers for over two centuries. Our work investigated the effect of dissolved carbon dioxide (CO2) on the geyser eruption mechanism at Spouter Geyser, a thermal geyser in Black Sand Basin of Yellowstone National Park. While it is convenient to think of geysers as pure water systems, the reality is that geyser waters contain various dissolved gases, and particularly CO2 in volcanic regions such as Yellowstone. Dissolved CO2 concentrations sampled with time in the near-surface discharge from Spouter Geyser were highest immediately before eruptions. These dissolved CO2 concentrations, which are heavily degassed by bubble formation in the geyser conduit, were extrapolated to the deep, source-zone conditions to determine whether CO2 partial pressure could contribute to eruption triggering. It was found that water vapor pressure alone was too low to exceed bubbling pressure in the source-zone and initiate eruption. However, added partial pressure from dissolved CO2 contributed about 50-90 kPa to the total dissolved gas pressure in the source-zone immediately before eruptions, enough to induce bubble formation and trigger an eruption.
Eocene Neo-Tethyan slab breakoff constrained by 45 Ma oceanic island basalt-type magmatism in southern Tibet
Wei-Qiang Ji et al., State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37612.1.abstract.
This article reported identification of Langshan oceanic island basalt (OIB)-type gabbros, dated at 45.0 plus or minus 1.4 million years ago, from eastern Tethyan Himalaya, southern Tibet. This is the first time to identify OIB-type magmatism in the continental collision belt. Based on the geological and geochemical characters, these rocks are best explained by partial melting of the asthenosphere, and the resultant melts underwent negligible crustal contamination during magma ascent and intrusion. The most possible genetic mechanism for the OIB-type magmatism of this study is slab breakoff of Neo-Tethyan oceanic lithosphere. And this is the first time to find a directly related evidence of slab breakoff in the eastern segment of Himalaya along the Indus-Yarlung suture zone. Together with other results from relevant studies along the suture zone, the slab breakoff is sudden and full-scale at great depth. The slab breakoff model at about 45 million years ago can account for many coeval geological events in southern Tibet.
Cellular dissolution at hypha- and spore-mineral interfaces revealing unrecognized mechanisms and scales of fungal weathering
Zibo Li et al., Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, PR China. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37561.1.abstract.
Most "rock-eating" microorganisms digest rocks using corrosive chemicals excreted by the cells, but fungi do it differently. This is because fungal cells, long and thread-like filaments called hyphae, can exert very high pressure at their tips during growth. As high as 10-20 MPa, equivalent approximately to 100 times that of a typical car tire and strong enough for fungi to penetrate mineral grains, has been measured in association with hyphal extension. However, how such physical forces partake in fungus-mineral interaction is largely unknown. In this study, researchers discovered that fungi attack rocks by first lowering the cells' pH upon attaching to minerals to initiate the dissolution, followed by secreting special biochemical compounds to lock up released nutrients, and finally using the pressure to obliterate the remaining waste rock material to restart the cycle and to continue the destruction process. The amount of minerals consumed by this cycle appears to be substantially larger than previously estimated for cell-promoted weathering. This finding by Li et al. has important implications for environmental geochemistry and ecology because fungal weathering is a major pathway for plants to obtain nutrients from soils.
Shelfal sediment transport by an undercurrent forces turbidity-current activity during high sea level along the Chile continental margin
Anne Bernhardt et al., Earth and Environmental Sciences, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37594.1.abstract.
To estimate past and future fluxes of terrestrial sediment and associated organic carbon, nutrients, and pollutants to the ocean, we need to understand the relationship between sea-level changes and marine sedimentation. Delivery of terrestrial sediment from the continent to the deep sea generally occurs during periods of low sea level. If sea level is high, such as during the past ~10,000 years, sediment export to the ocean can occur when a submarine canyon is connected to a terrestrial sediment source, such as a river mouth. Such connections are facilitated when the continental shelf is narrow. Here, we present evidence for pervasive sediment export to the Pacific Ocean offshore the wide shelf of south-central Chile during high sea level. Sediment is transported by an oceanic current that flows at 200-300 m water depth along the continental shelf, where it sweeps sediment onto the continental slope, from where the sediment gets distributed to the deep ocean by sediment-gravity flows. Hence, export of terrestrial sediment to the deep ocean via such undercurrents may be especially effective during periods of high sea level when currents flow along flooded, sediment-covered shelves. Shelf undercurrents may play an important role in sediment supply to the deep sea.
Very long-term stability of passive margin escarpment constrained by 40Ar/39Ar dating of K-Mn oxides
Anicet Beauvais et al., Aix-Marseille Université (AMU), IRD, CNRS, CEREGE UM34, BP 80, 13545 Aix-en-Provence, Cedex 4, France. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37303.1.abstract.
In this paper, Beauvais and co-authors determine and interpret the ages of oxides formed in manganese ore deposits and tropical soils on either side of the Great escarpment following the southwestern continental margin of India, known as the Western Ghats. The authors argue that the formation and preservation of tropical soils as old as 47 million years at the foot of the escarpment attests for the early installation of such a topographic barrier, which may even be as old as approx. 60 million years. Their results further indicate very slow rate of erosion by rivers at the foot of the escarpment after formation of the dated soils (less than 5 meters per million years), challenging previous erosion models of the southwestern margin of India, and more generally arguing against relief rejuvenation of continental margins and shields by recent vertical movements. Finally, the authors show that studying the remnants of ancient paleo-soil-capped landscapes is promising for quantifying erosion and sedimentary exports from continents to marine basins on geological timescales.
Tethyan suturing in Southeast Asia: Zircon U-Pb and Hf-O isotopic constraints from Myanmar ophiolites
Chuan-Zhou Liu et al., State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37342.1.abstract.
The Tethys Ocean existed between the continents of Gondwana and Laurasia supercontinents during much of the Mesozoic, which was closed by accretion of a series of terranes rifted from the Gondwana to the Laurisia. Evolution of the Tethys Ocean has been subdivided into three successive Tethys oceans, i.e., the Paleo-Tethys, Meso-Tethys and Neo-Tethys, since the Paleozoic. Relics of the different Tethys Ocean have been preserved as ophiolites in sutures. In this study, we provided high precision zircon U-Pb ages and other geochemical data to constrain the ages of two ophiolites from both western and eastern sutures in Myanmar. These data provide effective constraints on the spatial relationship between these two suture and their relationships with two different sutures that crop out in the Tibetan Plateau. Our data provide critical new constraints not only on formation of the Myanmar ophiolites but also on Tethyan suturing in Southeast Asia, which is essential to our understanding opening and closure of the Tethys Ocean that plays a key role in global plate tectonics during the Mesozoic.
Nearshore along-strike variability: Is the concept of the systems tract unhinged?
Andrew S. Madof et al., Chevron Energy Technology Company, Houston, Texas 77002-7308, USA. This paper is online at http://geology.gsapubs.org/content/early/2016/03/07/G37613.1.abstract.
In the late 1970s, sequence stratigraphy revolutionized the field of sedimentary geology by providing a systemic framework to correlate bodies of rock across sedimentary basins, and to understand their evolution through time. Sequence stratigraphic models do quite well with depositional systems that are relatively uniform in 2 and 3 dimensions, but they do not adequately explain depositional patterns with significant spatial variability. A simple geometric model of nearshore deposits corrects for this deficiency through simple geometrical constructs, called hinge zones, which explain along-strike variations in depositional patterns in nearshore systems. Examples of fixed and moving hinges illustrate a range of behaviors that will improve outcrop and subsurface predictions of deposits, and will benefit interpretations of ancient environments and the locations of petroleum and groundwater reservoirs.
GEOLOGY articles are online http://geology.gsapubs.org/. Representatives of the media may obtain complimentary articles by contacting Kea Giles at the e-mail address above. Please discuss articles of interest with the authors before publishing stories on their work, and please refer to GEOLOGY in articles published. Non-media requests for articles may be directed to GSA Sales and Service, [email protected]
Contact: Kea Giles