This Week in the Deep

New and noteworthy publications in deep-sea science for the week of January 21, 2013.

Geobiology: Did life originate from a global chemical reactor?

Many decades of experimental and theoretical research on the origin of life have yielded important discoveries regarding the chemical and physical conditions under which organic compounds can be synthesized and polymerized. However, such conditions often seem mutually exclusive, because they are rarely encountered in a single environmental setting. As such, no convincing models explain how living cells formed from abiotic constituents. Here, we propose a new approach that considers the origin of life within the global context of the Hadean Earth. We review previous ideas and synthesize them in four central hypotheses: (i) Multiple microenvironments contributed to the building blocks of life, and these niches were not necessarily inhabitable by the first organisms; (ii) Mineral catalysts were the backbone of prebiotic reaction networks that led to modern metabolism; (iii) Multiple local and global transport processes were essential for linking reactions occurring in separate locations; (iv) Global diversity and local selection of reactants and products provided mechanisms for the generation of most of the diverse building blocks necessary for life. We conclude that no single environmental setting can offer enough chemical and physical diversity for life to originate. Instead, any plausible model for the origin of life must acknowledge the geological complexity and diversity of the Hadean Earth. Future research may therefore benefit from identifying further linkages between organic precursors, minerals, and fluids in various environmental contexts.

Geochemistry, Geophysics, Geosystems: Trophic regions of a hydrothermal plume dispersing away from an ultramafic-hosted vent-system: Von Damm vent-site, Mid-Cayman Rise

Deep-sea ultramafic-hosted vent systems have the potential to provide large amounts of metabolic energy to both auto- and heterotrophic microorganisms in their dispersing hydrothermal plumes. Such vent-systems release large quantities of hydrogen and methane to the water column, both of which can be exploited by autotrophic microorganisms. Carbon cycling in these hydrothermal plumes may, therefore, have an important influence on open-ocean biogeochemistry. In this study, we investigated an ultramafic-hosted system on the Mid-Cayman Rise, emitting metal-poor and hydrogen sulfide-, methane-, and hydrogen-rich hydrothermal fluids. Total organic carbon concentrations in the plume ranged between 42.1 and 51.1 μM (background = 43.2 ± 0.7 μM (n = 5)) and near-field plume samples with elevated methane concentrations imply the presence of chemoautotrophic primary production and in particular methanotrophy. In parts of the plume characterized by persistent potential temperature anomalies but lacking elevated methane concentrations, we found elevated organic carbon concentrations of up to 51.1 μM, most likely resulting from the presence of heterotrophic communities, their extracellular products and vent larvae. Elevated carbon concentrations up to 47.4 μM were detected even in far-field plume samples. Within the Von Damm hydrothermal plume, we have used our data to hypothesize a microbial food web in which chemoautotrophy supports a heterotrophic community of microorganisms. Such an active microbial food web would provide a source of labile organic carbon to the deep ocean that should be considered in any future studies evaluating sources and sinks of carbon from hydrothermal venting to the deep ocean.

Paleoceanography: Reconstructing 7000 years of North Atlantic hurricane variability using deep-sea sediment cores from the western Great Bahama Bank

Available overwash records from coastal barrier systems document significant variability in North Atlantic hurricane activity during the late Holocene. The same climate forcings that may have controlled cyclone activity over this interval (e.g. the West African Monsoon, ENSO) show abrupt changes around 6000 yrs BP but most coastal sedimentary records do not span this time period. Establishing longer records is essential for understanding mid-Holocene patterns of storminess and their climatic drivers, which will lead to better forecasting of how climate change over the next century may effect tropical cyclone frequency and intensity. Storms are thought to be an important mechanism for transporting coarse sediment from shallow carbonate platforms to the deep-sea and bank-edge sediments may offer an unexplored archive of long-term hurricane activity.

Here we develop this new approach, reconstructing more than 7000 years of North Atlantic hurricane variability using coarse-grained deposits in sediment cores from the leeward margin of the Great Bahama Bank. High energy event layers within the resulting archive are: (1) broadly correlated throughout an offbank transect of multi-cores, (2) closely matched with historic hurricane events, and (3) synchronous with previous intervals of heightened North Atlantic hurricane activity in overwash reconstructions from Puerto Rico and elsewhere in the Bahamas. Lower storm frequency prior to 4400 yrs BP in our records suggests that precession and increased NH summer insolation may have greatly limited hurricane potential intensity, outweighing weakened ENSO and a stronger West African Monsoon—factors thought to be favorable for hurricane development.

Marine Geosciences and Geotechnology: Deep-sea Fe-Mn Crusts from the Northeast Atlantic Ocean: Composition and Resource Considerations

Eighteen deep-sea ferromanganese crusts (Fe-Mn crusts) from 10 seamounts in the northeast Atlantic were studied. Samples were recovered from water depths of 1,200 to 4,600 m from seamounts near Madeira, the Canary and Azores islands, and one sample from the western Mediterranean Sea.

The mineralogical and chemical compositions of the samples indicate that the crusts are typical continental margin, hydrogenetic Fe-Mn crusts. The Fe-Mn crusts exhibit a Co + Cu + Ni maximum of 0.96 wt%. Platinum-group element contents analyzed for five samples showed Pt contents from 153 to 512 ppb.

The resource potential of Fe-Mn crusts within and adjacent to the Portuguese Exclusive Economic Zone (EEZ) is evaluated to be comparable to that of crusts in the central Pacific, indicating that these Atlantic deposits may be an important future resource.

Journal of Molluscan Studies: Progenetic dwarf males in the deep-sea wood-boring genus Xylophaga (Bivalvia: Pholadoidea)

Sunken plant debris (sunken wood, hereafter) in deep-sea environments harbours an idiosyncratic fauna that is based directly or indirectly on wood decomposition (Turner, 1973, 1978). This resource is ecologically comparable with deep-sea whale-falls, because of its ephemeral nature (Distel et al., 2000). The obligate wood-boring and wood-consuming (xylophagous) bivalve genera Xylophaga, Xylopholas and Xyloredo, all belonging to the family Xylophagaidae (Turner, 2002; we here regard it as an independent family based on unpublished molecular phylogenetic data of TH), occur primarily in the deep sea, extending down to the hadal zone, from polar to tropical regions (Knudsen, 1961; Schiøtte, 2005). They have been recognized as the most important organisms that convert refractory sunken wood into a food source available to other members of the community (Turner, 1973; Distel & Roberts, 1997; Distel, 2003). However, Xylophagaidae remain little studied, as they are difficult to find due to their patchy distribution at great depths. Therefore, many species are known only from the type localities (see Voight, 2007).