Groundwater discharge into the sea occurs along many coastlines around the world in different geological settings and constitutes an important component of global water and matter budget. Estimates of how much water flows into the sea worldwide vary widely and are largely based on onshore studies and hydrological or hydrogeological modeling. In this study, we propose an approach to quantify a deep submarine groundwater outflow from the seafloor by using autonomously measured ocean surface data, i.e., 222Rn as groundwater tracer, in combination with numerical modeling of plume transport. The model and field data suggest that groundwater outflows from a water depth of ∼100 m can reach the sea surface implying that several cubic meters per second of freshwater are discharged into the sea. We postulate an extreme rainfall event 6 months earlier as the likely trigger for the groundwater discharge. This study shows that measurements at the sea surface, which are much easier to conduct than discharge measurements at the seafloor, can be used not only to localize submarine groundwater discharges but, in combination with plume modeling, also to estimate the magnitude of the release flow rate.

North African greening phases, during which large rivers ran through the Sahara Desert, occurred repeatedly during the Quaternary and are regarded as key periods for the development of past human populations. However, the timing and mechanisms responsible for the reactivation of the presently dormant fluvial systems remain highly uncertain. Here we present hydroclimate changes over the past 160,000 years, reconstructed from analyses of the provenance of terrestrial sediments in a marine sediment record from the Gulf of Sirte (offshore Libya). By combining high-resolution proxy data with transient Earth system model simulations, we are able to identify the various drivers that led to the observed shifts in hydroclimate and landscapes. We show that river runoff occurred during warm interglacial phases of Marine Isotope Stages 1 and 5 due to precession-forced enhancements in the summer and autumn rainfall over the entire watershed, which fed presently dry river systems and intermittent coastal streams. In contrast, shorter-lasting and less-intense humid events during glacial Marine Isotope Stages 3 and 4 were related to autumn and winter precipitation over the Libyan coastal regions driven by Mediterranean storms. Our results reveal large shifts in hydroclimate environments during the last glacial cycle, which probably exerted a strong evolutionary and structural control on past human populations, potentially pacing their dispersal across northern Africa. The presence of large rivers in North Africa critical for Quaternary human migrations were controlled by a combination of orbital forcing and Mediterranean storminess, according to terrestrial proxy records from a marine core off Libya integrated with paleoclimate modelling.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Sediments deposited on deep-sea fans are an excellent geological archive to reconstruct past changes in fluvial discharge. Here we present a reconstruction of changes in the regime of the Nile River during the Holocene obtained using bulk elemental composition, grain-size analyses and radiogenic strontium (Sr) and neodymium (Nd) isotopes from a sediment core collected on the Nile deep-sea fan. This 6-m long core was retrieved at ∼700m water-depth and is characterized by the presence of a 5-m thick section of finely laminated sediments, which were deposited between 9.5 and 7.3ka BP and correspond to the African Humid Period (AHP). The data show distinct changes in eolian dust inputs as well as variations in discharge of the Blue Nile and White Nile. Sedimentation was mainly controlled by changes in fluvial discharge during the Holocene, which was predominantly forced by low-latitude summer insolation and by the location of the eastern African Rain Belt. The changes in relative contribution from the Blue Nile and White Nile followed changes in low-latitude spring/autumn insolation, which highlights the role of changes in seasonality of the precipitation on the Nile River regime. The relative intensity of the Blue Nile discharge was enhanced during the early and late Holocene at times of higher spring insolation (with massive erosion and runoff during the AHP at times of high summer insolation), while it was reduced between 8 and 4ka at times of high autumn insolation. The gradual insolation-paced changes in fluvial regime were interrupted by a short-term arid event at 8.5–7.3ka BP (also associated with rejuvenation of bottom-water ventilation above the Nile fan), which was likely related to northern hemisphere cooling events. Another arid event at 4.5–3.7ka BP occurred as the apex of a gradually drier phase in NE Africa and marks the end of the AHP.

Cold seep ecosystems can support enormous biomasses of free-living and symbiotic chemoautotrophic organisms that get their energy from the oxidation of methane or sulfide. Most of this biomass derives from animals that are associated with bacterial symbionts, which are able to metabolize the chemical resources provided by the seeping fluids. Often these systems also harbor dense accumulations of non-symbiotic megafauna, which can be relevant in exporting chemosynthetically fixed carbon from seeps to the surrounding deep sea. Here we investigated the carbon sources of lithodid crabs (Paralomis sp.) feeding on thiotrophic bacterial mats at an active mud volcano at the Costa Rica subduction zone. To evaluate the dietary carbon source of the crabs, we compared the microbial community in stomach contents with surface sediments covered by microbial mats. The stomach content analyses revealed a dominance of epsilonproteobacterial 16S rRNA gene sequences related to the free-living and epibiotic sulfur oxidiser Sulfurovum sp. We also found Sulfurovum sp. as well as members of the genera Arcobacter and Sulfurimonas in mat-covered surface sediments where Epsilonproteobacteria were highly abundant constituting 10% of total cells. Furthermore, we detected substantial amounts of bacterial fatty acids such as i-C15∶0 and C17∶1ω6c with stable carbon isotope compositions as low as -53‰ in the stomach and muscle tissue. These results indicate that the white microbial mats at Mound 12 are comprised of Epsilonproteobacteria and that microbial mat-derived carbon provides an important contribution to the crab's nutrition. In addition, our lipid analyses also suggest that the crabs feed on other (13)C-depleted organic matter sources, possibly symbiotic megafauna as well as on photosynthetic carbon sources such as sedimentary detritus.

The Pacific Margin of Costa Rica is an area of intense tectonic deformation and repeated slope instability. Several processes are assumed to affect the long-term stability of the slope. Convergence-related seamount subduction and erosion are the primary reasons for large-scale failures at the lower slope. A second type of smaller slope failures occurs on the upper slope coincident with the landward termination of the regional Bottom Simulating Reflector (BSR). The coincidence of the BSR boundary and the position of slide headwalls might suggest a close genetic relationship. In this paper we present a detailed analysis of the so called “BGR Slide”, a small-type submarine landslide located on the upper slope offshore the Golfo de Nicoya. The 60 m-high headwall is embedded in an area characterized by small canyon structures on the continental slope in water depths where the regional BSR reaches the sea floor. The slide with a volume of ∼0.3 km³ affected an area of ∼8 km². Our acoustic data set suggest faults and migrating fluids playing key roles as preconditioning factors for slope failure, while there is no clear indication for the occurrence of gas hydrates in the headwall region. Furthermore, we assume that an external trigger, e.g., an earthquake, finally initialized the sediment mobilization.

Projected increases in demand and thus increasing metal prices have brought the exploration and exploitation of marine mineral resources back into focus. The Atlantis II Deep, located in the central Red Sea between Saudi Arabia and Sudan, is one of the largest marine sulfide deposits known, with high concentrations of metals such as zinc, copper, silver and gold. However, little is known about the economic potential of marine minerals as well as the legal constraints. Our geological assessment shows that the deep is similar in grades and scale to large land-based deposits. Its economic potential is far from negligible. The total present value of possible gross revenues for the four metals zinc, copper, silver and gold ranges from 3.03 to 5.29 billion US$, depending on the assumptions made concerning future price development, mass calculation and discount rate. From a legal perspective, a general duty to cooperate in the exploration and exploitation of non-living resources located in disputed maritime areas is identified in both customary international law and in UNCLOS. It is submitted that a joint development agreement is one means of ensuring compliance with this duty in general and in the case of the Atlantis II Deep in particular.

We report total dissolved inorganic carbon (DIC) abundances and isotope ratios, as well as helium isotope ratios (3He/4He), of cold seep fluids sampled at the Costa Rica fore arc in order to evaluate the extent of carbon loss from the submarine segment of the Central America convergent margin. Seep fluids were collected over a 12 month period at Mound 11, Mound 12, and Jaco Scar using copper tubing attached to submarine flux meters operating in continuous pumping mode. The fluids show minimum 3He/4He ratios of 1.3 RA (where RA is air 3He/4He), consistent with a small but discernable contribution of mantle‐derived helium. At Mound 11, δ13C∑CO2 values between −23.9‰ and −11.6‰ indicate that DIC is predominantly derived from deep methanogenesis and is carried to the surface by fluids derived from sediments of the subducting slab. In contrast, at Mound 12, most of the ascending dissolved methane is oxidized due to lower flow rates, giving extremely low δ13C∑CO2 values ranging from −68.2‰ to −60.3‰. We estimate that the carbon flux (CO2 plus methane) through submarine fluid venting at the outer fore arc is 8.0 × 105 g C km−1 yr−1, which is virtually negligible compared to the total sedimentary carbon input to the margin and the output at the volcanic front. Unless there is a significant but hitherto unidentified carbon flux at the inner fore arc, the implication is that most of the carbon being subducted in Costa Rica must be transferred to the (deeper) mantle, i.e., beyond the depth of arc magma generation.

Submarine slope failures occur at all continental margins, but the processes generating different mass wasting phenomena remain poorly understood. Multibeam bathymetry mapping of the Middle America Trench reveals numerous continental slope failures of different dimensions and origin. For example, large rotational slumps have been interpreted to be caused by slope collapse in the wake of subducting seamounts. In contrast, the mechanisms generating translational slides have not yet been described. Lithology, shear strength measurements, density, and pore water alkalinity from a sediment core across a slide plane indicate that a few centimeters thick intercalated volcanic tephra layer marks the detachment surface. The ash layer can be correlated to the San Antonio tephra, emplaced by the 6000 year old caldera‐forming eruption from Masaya‐Caldera, Nicaragua. The distal deposits of this eruption are widespread along the continental slope and ocean plate offshore Nicaragua. Grain size measurements permit us to estimate the reconstruction of the original ash layer thickness at the investigated slide. Direct shear test experiments on Middle American ashes show a high volume reduction during shearing. This indicates that marine tephra layers have the highest hydraulic conductivity of the different types of slope sediment, enabling significant volume reduction to take place under undrained conditions. This makes ash layers mechanically distinct within slope sediment sequences. Here we propose a mechanism by which ash layers may become weak planes that promote translational sliding. The mechanism implies that ground shaking by large earthquakes induces rearrangement of ash shards causing their compaction (volume reduction) and produces a rapid accumulation of water in the upper part of the layer that is capped by impermeable clay. The water‐rich veneer abruptly reduces shear strength, creating a detachment plane for translational sliding. Tephra layers might act as slide detachment planes at convergent margins of subducting zones, at submarine slopes of volcanic islands, and at submerged volcano slopes in lakes.