The Indian Ocean, its supra-subduction history, and implications for ophiolites

ABSTRACT Ophiolite complexes represent fragments of ocean crust and mantle formed at spreading centers and emplaced on land. The setting of their origin, whether at midocean ridges, back-arc basins, or forearc basins has been debated. Geochemical classification of many ophiolite extrusive rocks reflect an approach interpreting their tectonic environment as the same as rocks with similar compositions formed in various modern oceanic settings. This approach has pointed to the formation of many ophiolitic extrusive rocks in a supra-subduction zone (SSZ) environment. Paradoxically, structural and stratigraphic evidence suggests that many apparent SSZ-produced ophiolite complexes are more consistent with mid-ocean ridge settings. Compositions of lavas in the southeastern Indian Ocean resemble those of modern SSZ environments and SSZ ophiolites, although Indian Ocean lavas clearly formed in a mid-ocean ridge setting. These facts suggest that an interpretation of the tectonic environment of ophiolite formation based solely on their geochemistry may be unwarranted. New seismic images revealing extensive Mesozoic subduction zones beneath the southern Indian Ocean provide one mechanism to explain this apparent paradox. Cenozoic mid-ocean-ridge–derived ocean floor throughout the southern Indian Ocean apparently formed above former sites of subduction. Compositional remnants of previously subducted mantle in the upper mantle were involved in generation of mid-ocean ridge lavas. The concept of historical contingency may help resolve the ambiguity on understanding the environment of origin of ophiolites. Many ophiolites with “SSZ” compositions may have formed in a mid-ocean ridge setting such as the southeastern Indian Ocean.

Quantifying the buffering of oceanic oxygen isotopes at ancient midocean ridges

To quantify the intensity of oceanic oxygen isotope buffering through hydrothermal alteration of the oceanic crust, a 2D hydrothermal circulation model was coupled with a 2D reactive transport model of oxygen isotopes. The coupled model calculates steady-state distributions of temperature, water flow and oxygen isotopes of solid rock and porewater given the physicochemical conditions of oceanic crust alteration and seawater δ18O. Using the present-day seawater δ18O under plausible modern alteration conditions, the model yields δ18O profiles for solid rock and porewater and fluxes of heat, water and 18O that are consistent with modern observations, confirming the model's validity. The model was then run with different assumed seawater δ18O values to evaluate oxygen isotopic buffering at the midocean ridges. The buffering intensity shown by the model is significantly weaker than previously assumed, and calculated δ18O profiles of oceanic crust are consistently relatively insensitive to seawater δ18O. These results are attributed to the fact that isotope exchange at shallow depths does not reach equilibrium due to the relatively low temperatures, and 18O supply via spreading solid rocks overwhelms that through water flow at deeper depths. Further model simulations under plausible alteration conditions during the Precambrian showed essentially the same results. Therefore, δ18O records of ophiolites that are invariant at different Earth ages can be explained by the relative insensitivity of oceanic rocks to seawater δ18O and do not require constant seawater δ18O through time.

A Regional Sn Magnitude Scale mb(Sn) and Estimates of Moment Magnitude for Earthquakes along the Northern Mid-Atlantic Ridge

ABSTRACT We present a regional short-period Sn magnitude scale mb(Sn) for small earthquakes along the northern Mid-Atlantic Ridge. Surface-wave magnitudes, teleseismic body-wave magnitudes, and seismic moments cannot be reliably determined for small earthquakes along this and other midocean ridges. Local magnitudes that rely on Lg waves are likewise not generally useful due to the substantial oceanic paths for earthquakes along midocean ridges. In contrast, Pn and Sn arrivals for earthquakes along the northern Mid-Atlantic Ridge are generally well recorded by the existing seismographic networks, and, in fact, Sn arrivals are larger than Pn arrivals for about one-third of the ridge events. For this reason, we have developed a new regional Sn magnitude scale that is tied to Mw, so that seismic moments can be readily approximated. In our least-squares fit of peak amplitudes from 120 earthquakes having a published moment magnitude, we solved for the attenuation curve for paths in the oceanic mantle lid, for event magnitude adjustments (EMAs) to account for differences between long-period moment magnitude Mw and short-period Sn magnitude, and for station corrections. We find regional EMAs that are well correlated with the style of faulting: they are positive for normal-faulting earthquakes along spreading ridges and negative for strike-slip earthquakes along transform faults. These source-specific EMAs are approximately +0.11 magnitude units for normal-fault earthquakes and −0.26 magnitude units for strike-slip earthquakes on transform faults, and are consistent with previously reported apparent stresses from these regions. The amplitude distance curve determined for Sn for the northern Atlantic Ocean is similar to that determined for Pn in the northern Atlantic out to a distance of about 500 km, but at larger distances is more similar to the western U.S. Pn curve, likely reflective of the warmer temperatures at greater upper-mantle depths.

Transformation and Upwelling of Bottom Water in Fracture Zone Valleys

Closing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.

Interdependence of Internal Tide and Lee Wave Generation at Abyssal Hills: Global Calculations

The generation of internal waves at abyssal hills has been proposed as an important source of bottom-intensified mixing and a sink of geostrophic momentum. Using the theory of Bell, previous authors have calculated either the generation of lee waves by geostrophic flow or the generation of the internal tide by the barotropic tide, but never both together. However, the Bell theory shows that the two are interdependent: that is, the presence of a barotropic tide modifies the generation of lee waves, and the presence of a geostrophic (time mean) flow modifies the generation of the internal tide. Here we extend the theory of Bell to incorporate multiple tidal constituents. Using this extended theory, we recalculate global wave fluxes of energy and momentum using the abyssal-hill spectra, model-derived abyssal ocean stratification and geostrophic flow estimates, and the TPX08 tidal velocities for the eight major constituents. The energy flux into lee waves is suppressed by 13%–19% as a result of the inclusion of tides. The generated wave flux is dominated by the principal lunar semidiurnal tide (M2), and its harmonics and combinations, with the strongest fluxes occurring along midocean ridges. The internal tide generation is strongly asymmetric because of Doppler shifting by the geostrophic abyssal flow, with 55%–63% of the wave energy flux (and stress) directed upstream, against the geostrophic flow. As a consequence, there is a net wave stress associated with generation of the internal tide that reaches magnitudes of 0.01–0.1 N m−2 in the vicinity of midocean ridges.

Quantifying the buffering of oceanic oxygen isotopes at ancient midocean ridges

To quantify the intensity of oceanic oxygen-isotope buffering through hydrothermal alteration of oceanic crust, a two-dimensional hydrothermal circulation model was coupled with a two-dimensional reactive transport model of oxygen isotopes. The coupled model calculates steady-state distributions of temperature, water flow and oxygen isotopes of solid rock and porewater given physicochemical conditions of oceanic crust alteration and seawater δ18O. Using the present-day seawater δ18O under plausible modern alteration conditions, the model yields δ18O profiles for solid rock and porewater and fluxes of heat, water and 18O that are consistent with modern observations, confirming the model's validity. The model was then run with different assumed seawater δ18O values to evaluate oxygen isotopic buffering at the midocean ridges. The buffering intensity shown by the model is significantly weaker than previously assumed and, consistently, calculated δ18O profiles of oceanic crust are relatively insensitive to seawater δ18O. These results are attributed to the fact that isotope exchange at shallow depths does not reach equilibrium due to the relatively low temperatures, and 18O supply via spreading solid rocks overwhelms that through water flow at deeper depths. Further model simulations under plausible alteration conditions during the Precambrian showed essentially the same results. Therefore, δ18O records of ophiolites that are invariant at different Earth's ages can be explained by the relative insensitivity of oceanic rocks to seawater δ18O and do not require constant seawater δ18O through time.

Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions

The conditions of methane (CH4) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H2) and CH4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH4(g) and H2(g) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ∼400 °C. When temperatures decrease below ∼340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H2O. The generation of molecular H2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH4. Once formed, CH4(g) and H2(g) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH4 and H2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.

Ridges, Seamounts, Troughs, and Bowls: Topographic Control of the Dianeutral Circulation in the Abyssal Ocean

In situ observations obtained over the last several decades have shown that the intensity of turbulent mixing in the abyssal ocean is enhanced toward the seafloor. Consequently, a new paradigm has emerged whereby dianeutral downwelling dominates in the ocean interior and dianeutral upwelling only occurs within thin bottom boundary layers. This study shows that when mixing is bottom intensified the net abyssal dianeutral transports and the stratification can depend on subtle features of the seafloor geometry. Under an assumption of depth-independent net dianeutral upwelling, small changes in the curvature of the seafloor can result in interior stratification that is bottom intensified, uniform, or surface intensified. Further, when the net dianeutral transport is allowed to vary in the vertical, changes in the seafloor slope and bathymetric contour length with height can drive lateral exchange between the boundary layer and interior, with particularly strong lateral outflows predicted at the crests of midocean ridges. Finally, using a realistic neutral density climatology the authors suggest that the increase in the perimeter of abyssal neutral density surfaces with height drives much of the dianeutral upwelling at depths greater than 4 km, while the increase in the slope of the seafloor at shallower depths acts to oppose upwelling. These results add to a growing body of literature highlighting the key control of seafloor geometry on the abyssal overturning circulation.