ALBERT

Description: An active seafloor hydrothermal system subjects the background sediments of the Grimsey Graben (Tjörnes Fracture Zone) to alteration that produces dissolution of the primary volcaniclastic matrix and replacement/precipitation of sulfides, sulfates, oxides, oxyhydroxides, carbonates and phyllosilicates. Three types of hydrothermal alteration of the sediment are defined on the basis of the dominant hydrothermal phyllosilicate formed: smectite, kaolinite, chlorite. The most common alteration is near‐total conversion of the volcaniclastic material to smectite (95–116°C). The dominant smectite in the deepest sediments sampled is beidellite, which is replaced by montmorillonite and an intimate mixture of di‐ and tri‐octahedral smectite up core. This gradual vertical change in smectite composition suggests an increase in the Mg supply upward, the result of sediment alteration by the ascending hydrothermal fluids mixing with descending seawater. The vertical sequence kaolinite → kaolinite‐smectite mixed‐layer → smectite from bottom to top of a core, as well as the distinct zonation across the veins (kaolinite in the central zone → kaolinite‐smectite in the rim), suggests hydrothermal transformation of the initially formed smectite to kaolinite through kaolinite‐smectite mixed‐layer (150–160°C). The cause of this transformation might have been an evolution of the fluids toward a slightly acidic pH or a relative increase in the Al concentration. Minor amounts of chamosite fill thin veins in the deepest sections of some cores. The gradual change from background clinochlore to chamosite across the veins suggests that chamosite replaces clinochlore as Fe is made available from hydrothermal dissolution of detrital Fe‐containing minerals. The internal textures, REE distribution patterns and the mode of occurrence of another magnesian phyllosilicate, kerolite, suggest that this mineral is the primary precipitate in the hydrothermal chimneys rather than an alteration product in the sediment. Kerolite precipitated after and grew on anhydrite in the chimneys. Oxygen isotope ratios are interpreted to reflect precipitation of kerolite at temperatures of 302° to 336°C. It accumulated in the hydrothermal mounds following the collapse of the chimneys and subsequent dissolution of anhydrite, thereby forming highly permeable aquifer layers underlying the vent field. Some kerolite was redeposited in the near vent field sediments by turbidity flows. The altered sediments are depleted in Mn, Rb and Sr, and enriched in U, Mo, Pb, Ba, As, Bi, Sb, Ag, Tl and Ga, as a result of leaching and precipitation, respectively. Conservative elements (Ti, Zr, Hf, Sc, Cr, Nb and Sn) are depleted or enriched in the altered sediments because of passive (precipitation or leaching of other phases) rather than active (because of their mobility) processes.

Description: Particle flux data from 27 sites in the Atlantic Ocean have been compiled in order to determine regional variations in the strength and efficiency of the biological pump and to quantify carbon fluxes over the ocean basin, thus estimating the potential oceanic sequestration of atmospheric CO2. An algorithm is derived relating annual particulate organic carbon (POC) flux to primary production and depth that yields variations in the export ratio (ER = POC flux/primary production) at 125 m of between 0.08 and 0.38 over the range of production from 50 to 400 g C m−2 yr−1. Significant regional differences in changes of the export ratio with depth are related to the temporal stability of flux. Sites with more pulsed export have higher export ratios at 125 m but show more rapid decreases of POC flux with depth, resulting in little geographic variation in fluxes below ∼3000 m. The opposing effects of organic carbon production and calcification on ΔpCO2 of surface seawater are considered to calculate an “effective carbon flux” at the depth of the euphotic zone and at the base of the winter mixed layer. POC flux at the base of the euphotic zone integrated over the Atlantic Ocean between 65°N and 65°S amounts to 3.14 Gt C yr−1. Of this, 5.7% is remineralized above the winter mixed layer and thus does not contribute to CO2 sequestration on climatically relevant timescales. The effective carbon flux, termed Jeff, amounts to 2.47 Gt C yr−1 and is a measure of the potential sequestration of atmospheric CO2 for the area considered. A shift in the composition of sedimenting particles (seen in a decrease of the opal:carbonate ratio) is seen across the entire North Atlantic, indicating a basin-wide phenomenon that may be related to large-scale changes in climatic forcing.

Description: Bottom-tethered sediment traps deployed in the deep eastern North Atlantic between 54°N 20°W and 33°N 20°W (L1, L2, L3), at the European continental margin at 49°N (OMEX) and off the Canary Islands (ESTOC) were investigated for the determination of 230Th trapping efficiencies. The ratios of 230Th flux measured in the traps (Fa) to the expected 230Th flux from the production rate of 230Th in the overlying water column (Fp) ranged between 0.09 and 1.26. For the traps with deployment periods 〉300 days the interannual variation of Fa/Fp ratios (different years but same location and water depth) were up to 10%, suggesting that the average 230Th flux to the sediment traps did not vary significantly. The influence of lateral advection on the 230Th flux was taken into account either by applying a mass balance of 230Th and 231Pa or by assuming a constant removal rate of 230Th from the water column, an assumption based on similar 230Th concentration-depth profiles observed at most locations investigated. 230Th trapping efficiencies were between 9 and 143%, showing a trend of increasing efficiencies with increasing water depth. No relation was found between current velocities and 230Th trapping efficiencies. Our investigations suggest that the observation of constant or even increasing particle flux rates with increasing water depths in several sediment trap arrays investigated may be a result of sediment trap biases. The correction for the trapping biases is important for the understanding of the regional differences in the particle flux in the eastern North Atlantic.

Description: Submarine groundwater discharge (SGD) into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical porewater and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and SGD on high frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intra‐pockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically 〈15 m across) pockmarks within the much larger, previously mapped, pockmarks. The small pockmarks, which we refer to as “intra‐pockmarks”, have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of ‘eyed’ pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic‐rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow.

Description: Lateral fluxes (i.e., outwelling) of dissolved organic (DOC) and inorganic (DIC) carbon and total alkalinity were estimated using radium isotopes at the groundwater, mangrove creek, and continental shelf scales in the Amazon region. Observations of salinity and radium isotopes in the creek indicated tidally driven groundwater exchange as the main source of carbon. Radium-derived transport rates indicate that mangrove carbon is exported out of the continental shelf on timescales of 22 ± 7 d. Bicarbonate was the main form (82% ± 11%) of total dissolved carbon in all samples, followed by DOC (13% ± 12%) and CO2 (5% ± 4%). DIC (18.7 ± 15.7 mmol m−2 d−1) exceeded DOC (3.0 ± 4.1 mmol m−2 d−1) outwelling at all spatial scales. The interpretation of outwelling across the mangrove-ocean continuum is related to the spatial and temporal scales investigated. At all scales, outwelling represented a major coastal carbon pathway driving bicarbonate storage in the ocean.