Yishai Weinstein

Submarine groundwater discharge as a major source of nutrients to the Mediterranean Sea

Significance The Mediterranean Sea (MS) is one of the most oligotrophic seas in the world, and external inputs of nutrients are especially relevant to sustaining primary productivity in this basin. Here we evaluate the role of submarine groundwater discharge (SGD) as a source of nutrients to the entire MS, a pathway that has been largely overlooked. This study demonstrates that SGD is a volumetrically important process in the MS, is of a larger magnitude than riverine discharge, and also represents a major source of dissolved inorganic nitrogen, phosphorous, and silica to the MS. The Mediterranean Sea (MS) is a semienclosed basin that is considered one of the most oligotrophic seas in the world. In such an environment, inputs of allochthonous nutrients and micronutrients play an important role in sustaining primary productivity. Atmospheric deposition and riverine runoff have been traditionally considered the main external sources of nutrients to the MS, whereas the role of submarine groundwater discharge (SGD) has been largely ignored. However, given the large Mediterranean shore length relative to its surface area, SGD may be a major conveyor of dissolved compounds to the MS. Here, we used a 228Ra mass balance to demonstrate that the total SGD contributes up to (0.3–4.8)⋅1012 m3⋅y−1 to the MS, which appears to be equal or larger by a factor of 16 to the riverine discharge. SGD is also a major source of dissolved inorganic nutrients to the MS, with median annual fluxes of 190⋅109, 0.7⋅109, and 110⋅109 mol for nitrogen, phosphorous, and silica, respectively, which are comparable to riverine and atmospheric inputs. This corroborates the profound implications that SGD may have for the biogeochemical cycles of the MS. Inputs of other dissolved compounds (e.g., iron, carbon) via SGD could also be significant and should be investigated.

What Is the Role of Fresh Groundwater and Recirculated Seawater in Conveying Nutrients to the Coastal Ocean

Submarine groundwater discharge (SGD) is a major process operating at the land-sea interface. Quantifying the SGD nutrient loads and the marine/terrestrial controls of this transport is of high importance, especially in oligotrophic seas such as the eastern Mediterranean. The fluxes of nutrients in groundwater discharging from the seafloor at Dor Bay (southeastern Mediterranean) were studied in detail using seepage meters. Our main finding is that the terrestrial, fresh groundwater is the main conveyor of DIN and silica to the coastal water, with loads of 500 and 560 mol/yr, respectively, per 1 m shoreline. Conversely, recirculated seawater is nutrient-poor, and its role is mainly as a dilution agent. The nutrient loads regenerated in the subterranean estuary (sub-bay sediment) are relatively small, consisting mostly of ammonium (24 mol/yr). On the other hand, the subterranean estuary at Dor Bay sequesters as much as 100 mol N/yr per 1 m shoreline, mainly via denitrification processes. These, and observations from other SGD sites, imply that the subterranean estuary at some coastal systems may function more as a sink for nitrogen than a source. This further questions the extent of nutrient contributions to the coastal water by some subterranean estuaries and warrants systematic evaluation of this process in various hydrological and marine trophic conditions.

Implications of carbon flux from the Cascadia accretionary prism: results from long-term, in situ measurements at ODP Site 892B

A 403-day in situ field experiment at Ocean Drilling Program Site 892B sought to quantify the flux of methane along a fluid-active fault and to experimentally determine rates of methane hydrate and authigenic carbonate deposition associated with fluid expulsion from the borehole. An instrument package was deployed that osmotically sampled fluid, measured borehole pressure and flow rates, and contained reaction chambers in which deposition of gas hydrates and carbonates was anticipated, and from which microbial communities might be extracted. Flow is highly variable in the three-phase water–methane system that exists at Site 892B. Flow rates fluctuate over two orders of magnitude in response to tidally induced pressure variations and gas hydrate formation and dissociation. Hydrate formation began 45 days into the experiment and reduced the initial flow (∼2 l/day) to 20 ml/day. Unexpectedly, the hydrate destabilized after about 125 days. Tidally induced flow reversals are common (∼25% of time) in this setting characterized by ‘overpressured’ pore waters. These reversals pump sulfate-rich bottom water into near-surface sediments where Archaea anaerobically oxidize CH4 and induce carbonate precipitation. At the sediment–water interface, authigenic carbonates are undergoing dissolution. Methanotrophs dominated the microbial community where fluid is discharged to ambient seawater. All expelled methane is apparently oxidized in the water column.

The Dead Sea Transform and the Volcanism in Northwestern Arabia

Volcanism is common along the northern segments of the Dead Sea Transform (DST). In this paper we review its distribution and composition and conclude that this tectono-magmatic association has mainly to do with the magma migration toward the surface and less with magma generation, namely: some volcanic activity concentrated along the DST due to better magma channeling and not due to an enhanced mantle partial melting along this lineament. The volcanism along the DST is clearly part of the western Arabia magmatism, and the early phases of this volcanism probably have to do with Red Sea-related extension during the Early to Middle Miocene. Nevertheless, the DST does play a role in the emplacement of lithospheric mantle domains with different compositions next to each other, which is reflected in the derived lavas.

Melt instabilities in an intraplate lithosphere and implications for volcanism in the Harrat Ash-Shaam volcanic field (NW Arabia)

We investigate melt generation in a slowly extending lithosphere with the aim of understanding the spatial and temporal relationships between magmatism and preexisting rift systems. We present numerical models that consider feedback between melt generation and lithospheric deformation, and we incorporate three different damage mechanisms: brittle damage, creep damage, and melt damage. Melt conditions are calculated with a Helmholtz free energy minimization method, and the energy equation is solved self-consistently for latent heat and shear heating effects. Using a case of a slowly extending (1-1.5mm/yr) continental lithosphere with a relatively low surface heat flow (similar to 50mW/m(2)), we show that melt-rich shear bands are nucleated at the bottom of the lithosphere as a result of shear heating and damage mechanisms. Upon further deformation, melt zones intersect creep damage zones, thus forming channels that may be used for the melt to migrate upward. If a preexisting structure resides only in the brittle crust, it does not control the path of melt migration to the surface, and melt-filled channels propagate from the bottom upwards, independently of upper crustal structures. In contrast, a preexisting weak structure that reaches a critical depth of 20km allows fast (similar to 2Ma) propagation of melt-filled channels that link melt damage from the bottom of the lithosphere to near-surface structures. Our model results may explain the short time scale, volume, and magma extraction from the asthenosphere through a low surface heat flow lithosphere, such as observed, for example, in the Harrat Ash-Shaam volcanic field (northwestern Arabia), which developed in the Arabian Plate and is spatially linked to the Azraq-Sirhan Graben.