Abyssal Circulation Driven by Near-Boundary Mixing: Water Mass Transformations and Interior Stratification

The emerging view of the abyssal circulation is that it is associated with bottom-enhanced mixing, which results in downwelling in the stratified ocean interior and upwelling in a bottom boundary layer along the insulating and sloping seafloor. In the limit of slowly varying vertical stratification and topography, however, boundary layer theory predicts that these upslope and downslope flows largely compensate, such that net water mass transformations along the slope are vanishingly small. Using a planetary geostrophic circulation model that resolves both the boundary layer dynamics and the large-scale overturning in an idealized basin with bottom-enhanced mixing along a midocean ridge, we show that vertical variations in stratification become sufficiently large at equilibrium to reduce the degree of compensation along the midocean ridge flanks. The resulting large net transformations are similar to estimates for the abyssal ocean and span the vertical extent of the ridge. These results suggest that boundary flows generated by mixing play a crucial role in setting the global ocean stratification and overturning circulation, requiring a revision of abyssal ocean theories.

Polymerization during melting of ortho- and meta-silicates: Effects on Q species stability, heats of fusion, and redox state of mid-ocean range basalts (MORBs)

29Si NMR and Raman spectroscopic studies demonstrate that fusion of crystalline orthosilicates and metasilicates produces melts more polymerized than their precursor crystals. Forsterite, for example, consists of 100% Q0 species, whereas its melt consists of ~50 mol% of Q1 species (Q = a Si tetrahedron and the superscript indicates the number of bridging oxygen atoms in the tetrahedron). Polymerization during melting can be rationalized from an energetics perspective. Si-NBO-M moieties of Q species are more susceptible to librational, rotational, and vibrational modes than are Si-BO-Si moieties (NBO = non-bridging oxygen; BO = bridging oxygen; M = counter cation). Thermal agitation activates these additional modes, thus increasing the CP and free energy of melts. The reaction of Qn to Qn+1 species during melting eliminates Si-NBO-M moieties and produces Si-O-Si moieties that are less susceptible to the additional modes, thereby minimizing the CP of melts. By decreasing the abundances of Q0, Q1, and Q2 species in favor of Q3 and Q4 species, melts become more stable. In the absence of polymerization, melting temperatures of minerals would be appreciably greater than observed. Polymerization involves formation of Si-O bonds, which are strongly endothermic (Si-O bond dissociation is ~798 kJ/mol). The large heats of fusion (ΔHf) of orthosilicates result primarily from polymerization reactions during melting (ΔHf of forsterite, fayalite, and tephroite are ~142, ~92, and ~90 kJ/mol). The fusion of metasilicates and sorosilicates (e.g., pyroxenes and melilites) involves endothermic polymerization and exothermic depolymerization reactions, although the former dominates. These reactions tend to negate each other during melting, yielding less positive ΔHf values than observed for orthosilicate fusion (e.g., ΔHf of enstatite, diopside, pseudowollastonite, and åkermanite are ~73, ~69, ~57, and ~62 kJ/mol). Where polymerization and depolymerization reactions are absent ΔHf is low and is due mostly to disordering during melting (e.g., ΔHf of cristobalite iŝ8.9 kJ/mol). Experimental evidence indicates that ferric iron is present as a negatively charged oxy-anionic complex in melts (e.g., [FeO2]1–) so that oxidation of Fe2+ should proceed according to: 4Femelt2+ + 1O2 + 6Omelt2−→4[FeO2]melt1−. Free oxygen (O2–), a by-product of polymerization reactions, drives the reaction to the right. Midocean ridge basalts (MORBs) consequently should be more oxidized than their source (e.g., lherzolites) or their residues (e.g., harzburgites). Extraction of melt from the upper mantle and deposition in the crust should produce a crust more oxidized than its upper mantle source. Production of O2– during melting and its presence in alkali-rich magmas also explains the alkali-ferric iron effect.

The Contrasting Dynamics of the Buoyancy-Forced Lofoten and Greenland Basins

The Nordic seas are commonly described as a single basin to investigate their dynamics and sensitivity to environmental changes when using a theoretical framework. Here, we introduce a conceptual model for a two-basin marginal sea that better represents the Nordic seas geometry. In our conceptual model, the marginal sea is characterized by both a cyclonic boundary current and a front current as a result of different hydrographic properties east and west of the midocean ridge. The theory is compared to idealized model simulations and shows good agreement over a wide range of parameter settings, indicating that the physics in the two-basin marginal sea is well captured by the conceptual model. The balances between the atmospheric buoyancy forcing and the lateral eddy heat fluxes from the boundary current and the front current differ between the Lofoten and the Greenland Basins, since the Lofoten Basin is more strongly eddy dominated. Results show that this asymmetric sensitivity leads to opposing responses depending on the strength of the atmospheric buoyancy forcing. Additionally, the front current plays an essential role for the heat and volume budget of the two basins, by providing an additional pathway for heat toward the interior of both basins via lateral eddy heat fluxes. The variability of the temperature difference between east and west influences the strength of the different flow branches through the marginal sea and provides a dynamical explanation for the observed correlation between the front current and the slope current of the Norwegian Atlantic Current in the Nordic seas.

Neoproterozoic to early Phanerozoic rise in island arc redox state due to deep ocean oxygenation and increased marine sulfate levels

A rise in atmospheric O2levels between 800 and 400 Ma is thought to have oxygenated the deep oceans, ushered in modern biogeochemical cycles, and led to the diversification of animals. Over the same time interval, marine sulfate concentrations are also thought to have increased to near-modern levels. We present compiled data that indicate Phanerozoic island arc igneous rocks are more oxidized (Fe3+/ΣFe ratios are elevated by 0.12) vs. Precambrian equivalents. We propose this elevation is due to increases in deep-ocean O2and marine sulfate concentrations between 800 and 400 Ma, which oxidized oceanic crust on the seafloor. Once subducted, this material oxidized the subarc mantle, increasing the redox state of island arc parental melts, and thus igneous island arc rocks. We test this using independently compiled V/Sc ratios, which are also an igneous oxybarometer. Average V/Sc ratios of Phanerozoic island arc rocks are elevated (by +1.1) compared with Precambrian equivalents, consistent with our proposal for an increase in the redox state of the subarc mantle between 800 and 400 Ma based on Fe3+/ΣFe ratios. This work provides evidence that the more oxidized nature of island arc vs. midocean-ridge basalts is related to the subduction of material oxidized at the Earth’s surface to the subarc mantle. It also indicates that the rise of atmospheric O2and marine sulfate to near-modern levels by the late Paleozoic influenced not only surface biogeochemical cycles and animal diversification but also influenced the redox state of island arc rocks, which are building blocks of continental crust.

Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth’s oxygenation

The behavior of C, H, and S in the solid Earth depends on their oxidation states, which are related to oxygen fugacity (fO2). Volcanic degassing is a source of these elements to Earth’s surface; therefore, variations in mantle fO2 may influence the fO2 at Earth’s surface. However, degassing can impact magmatic fO2 before or during eruption, potentially obscuring relationships between the fO2 of the solid Earth and of emitted gases and their impact on surface fO2. We show that low-pressure degassing resulted in reduction of the fO2 of Mauna Kea magmas by more than an order of magnitude. The least degassed magmas from Mauna Kea are more oxidized than midocean ridge basalt (MORB) magmas, suggesting that the upper mantle sources of Hawaiian magmas have higher fO2 than MORB sources. One explanation for this difference is recycling of material from the oxidized surface to the deep mantle, which is then returned to the surface as a component of buoyant plumes. It has been proposed that a decreasing pressure of volcanic eruptions led to the oxygenation of the atmosphere. Extension of our findings via modeling of degassing trends suggests that a decrease in eruption pressure would not produce this effect. If degassing of basalts were responsible for the rise in oxygen, it requires that Archean magmas had at least two orders of magnitude lower fO2 than modern magmas. Estimates of fO2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere.

Microbial Sulfur Cycle in Two Hydrothermal Chimneys on the Southwest Indian Ridge

ABSTRACT Sulfur is an important element in sustaining microbial communities present in hydrothermal vents. Sulfur oxidation has been extensively studied due to its importance in chemosynthetic pathways in hydrothermal fields; however, less is known about sulfate reduction. Here, the metagenomes of hydrothermal chimneys located on the ultraslow-spreading Southwest Indian Ridge (SWIR) were pyrosequenced to elucidate the associated microbial sulfur cycle. A taxonomic summary of known genes revealed a few dominant bacteria that participated in the microbial sulfur cycle, particularly sulfate-reducing Deltaproteobacteria. The metagenomes studied contained highly abundant genes related to sulfur oxidation and reduction. Several carbon metabolic pathways, in particular the Calvin-Benson-Bassham pathway and the reductive tricarboxylic acid cycles for CO2 fixation, were identified in sulfur-oxidizing autotrophic bacteria. In contrast, highly abundant genes related to the oxidation of short-chain alkanes were grouped with sulfate-reducing bacteria, suggesting an important role for short-chain alkanes in the sulfur cycle. Furthermore, sulfur-oxidizing bacteria were associated with enrichment for genes involved in the denitrification pathway, while sulfate-reducing bacteria displayed enrichment for genes responsible for hydrogen utilization. In conclusion, this study provides insights regarding major microbial metabolic activities that are driven by the sulfur cycle in low-temperature hydrothermal chimneys present on an ultraslow midocean ridge. IMPORTANCE There have been limited studies on chimney sulfides located at ultraslow-spreading ridges. The analysis of metagenomes of hydrothermal chimneys on the ultraslow-spreading Southwest Indian Ridge suggests the presence of a microbial sulfur cycle. The sulfur cycle should be centralized within a microbial community that displays enrichment for sulfur metabolism-related genes. The present study elucidated a significant role of the microbial sulfur cycle in sustaining an entire microbial community in low-temperature hydrothermal chimneys on an ultraslow spreading midocean ridge, which has characteristics distinct from those of other types of hydrothermal fields.

Impacts of Localized Mixing and Topography on the Stationary Abyssal Circulation

Stommel and coworkers calculated the stationary, geostrophic circulation in the abyssal ocean driven by prescribed sources (representing convective downwelling sites) and sinks (slow, widespread upwelling through the thermocline). The applied basin geometries were highly idealized with nearly uniform upwelling and gradual bottom slopes. In this paper, the classical Stommel–Arons theory for the abyssal circulation is extended by introducing pronounced bathymetry in the form of a midocean ridge and strongly enhanced upwelling in the vicinity of this ridge, modeled after direct observations of diapycnal mixing rates in the deep ocean. Locally enhanced upwelling over a midocean ridge drives a β-plume circulation that is modified by topographic stretching. The dynamics of this abyssal circulation pattern are explained by analyzing the combined impacts of the upwelling pattern and the bathymetry on the stationary circulation, building on their well-known separate impacts. On the western flank of the ridge, the effects of topographic stretching and upwelling oppose, and the direction of the local flow depends on their relative size. In this paper, a simple theoretical estimate is derived that can predict the direction of the flow along the ridge based on the geometry of the basin and the upwelling region. Its applicability is demonstrated for both the idealized model configurations applied in this study and for more realistic model simulations.