Description: Seamounts are amongst the most common physiographic structures of the deep-ocean landscape, but remoteness and geographic complexity have limited the systematic collection of integrated and multidisciplinary data in the past. Consequently, important aspects of seamount ecology and dynamics remain poorly studied. We present a data collection of ocean currents and raw acoustic backscatter from shipboard Acoustic Doppler Current Profiler (ADCP) measurements during six cruises between 2004 and 2015 in the tropical and subtropical Northeast Atlantic to narrow this gap. Measurements were conducted at seamount locations between the island of Madeira and the Portuguese mainland (Ampère, Seine Seamount), as well as east of the Cape Verde archipelago (Senghor Seamount). The dataset includes two-minute ensemble averaged continuous velocity and backscatter profiles, supplemented by spatially gridded maps for each velocity component, error velocity and local bathymetry.

Description: Current estimates suggest that more than 60% of the global seafloor are covered by millions of abyssal hills and mountains. These features introduce spatial fluid-dynamic granularity whose influence on deep-ocean sediment biogeochemistry is unknown. Here we compare biogeochemical surface-sediment properties from a fluid-dynamically well-characterized abyssal hill and upstream plain: (1) In hill sediments, organic-carbon and -nitrogen contents are only about half as high as on the plain while proteinaceous material displays less degradation; (2) on the hill, more coarse-grained sediments (reducing particle surface area) and very variable calcite contents (influencing particle surface charge) are proposed to reduce the extent, and influence compound-specificity, of sorptive organic-matter preservation. Further studies are needed to estimate the representativeness of the results in a global context. Given millions of abyssal hills and mountains, their integrative influence on formation and composition of deep-sea sediments warrants more attention.

Description: Residual flow, barotropic tides and internal (baroclinic) tides interact in a number of ways with kilometer-scale seafloor topography such as abyssal hills and seamounts. Because of their likely impact on vertical mixing such interactions are potentially important for ocean circulation and the mechanisms and the geometry of these interactions are a matter of ongoing studies. In addition, very little is known about how these interactions are reflected in the sedimentary record. This multi-year study investigates if flow/topography interactions are reflected in distributional patterns of the natural short-lived (half-life: 24.1 d) particulate-matter tracer 234Th relative to its conservative (non-particle-reactive) and very long-lived parent nuclide 238U. The sampling sites were downstream of, or surrounded by, fields of short seamounts and, therefore, very likely to be influenced by nearby flow/topography interactions. At the sampling sites between about 200 and 1000 m above the seafloor recurrent ‘fossil’ disequilibria were detected. ‘Fossil’ disequilibria are defined by clearly detectable 234Th/238U disequilibria (total 234Th radioactivity 〈238U radioactivity, indicating a history of intense particulate 234Th scavenging and particulate-matter settling from the sampled parcel of water) and conspicuously low particle-associated 234Th activities. ‘Fossil’ disequilibria were centered at levels in the water column that correspond to the average height of the short seamounts near the sampling sites. This suggests the ‘fossil’ disequilibria are formed on the seamount slopes. Moreover, the magnitude of the ‘fossil’ disequilibria suggests that the slopes of the short seamounts in the study region are characterized by particularly vigorous fluid dynamics. Since ‘fossil’ disequilibria already occurred at ∼O(1–10 km) away from the seamount slopes it is likely that these vigorous fluid dynamics rapidly decay away from the slopes on scales of O(1–10 km). These conclusions are supported by the horizontal distribution and magnitude of the modeled total (barotropic+baroclinic) tidal current velocities of the predominating tidal M2 constituent: on (near-)critical seamount slopes baroclinic tides lead to localized [∼O(1 km)] increases of the overall tidal current velocity by a factor of ∼ 2, thereby pushing the total current velocity well above the threshold for sediment erosion. The results of this and a previous study [Turnewitsch, R., Reyss, J.-L., Chapman, D.C., Thomson, J., Lampitt, R.S., 2004. Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount. Earth and Planetary Science Letters 222(3–4), 1023–1036] show that kilometer-scale flow/topography interactions leave a marine geochemical imprint. This imprint may help develop new sediment proxies for the reconstruction of past changes of fluid dynamics in the deep sea, including residual and tidal flow. Sedimentary records controlled by kilometer-scale seafloor elevations are promising systems for the reconstruction of paleo-changes of deep-ocean fluid dynamics. For the sediment-based reconstruction of paleo-parameters other than physical oceanographic ones it may be advisable to avoid kilometer-scale topography altogether.

Description: Particulate matter in aquatic systems is an important vehicle for the transport of particulate organic carbon (POC). Its accurate measurement is of central importance for the understanding of marine carbon cycling. Previous work has shown that GF/F-filter-based bottle-sample-derived concentration estimates of POC are generally close to or higher than large-volume in-situ-pump-derived values (and in some rare cases in subzero waters are up to two orders of magnitude higher). To further investigate this phenomenon, water samples from the surface and mid-water Northeast Atlantic and the Baltic Sea were analyzed. Our data support a bias of POC concentration estimates caused by adsorption of nitrogen-rich dissolved organic material onto GF/F filters. For surface-ocean samples the mass per unit area of exposed filter and composition of adsorbed material depended on the filtered volume. Amounts of adsorbed OC were enhanced in the surface ocean (typically 0.5 μmol cm− 2 of exposed filter) as compared to the deep ocean (typically 0.2 μmol cm− 2 of exposed filter). These dependencies should be taken into account for future POC methodologies. Bottle/pump differences of samples that were not corrected for adsorption were higher in the deep ocean than in the surface ocean. This discrepancy increased in summer. It is shown that POC concentration estimates that were not corrected for adsorption depend not only on the filtered volume, true POC concentration and mass of adsorbed OC, but also on the filter area. However, in all cases we studied, correction for adsorption was important, but not sufficient, to explain bottle/pump differences. Artificial formation of filterable particles and/or processes leading to filterable material being lost from and/or missed by sample-processing procedures must be considered. It can be deduced that the maximum amounts of POC and particulate organic nitrogen (PON) that can be artificially formed per liter of filtered ocean water are ∼ 3–4 μM OC (5–10% of dissolved OC) and ∼ 0.2–0.5 μM ON (2–10% of dissolved ON), respectively. The relative sensitivities of bottle and pump procedures, and of surface- and deep-ocean material, to artificial particle formation and the missing/losing of material are evaluated. As present procedures do not exist to correct for all possible biasing effects due to artificial particle formation and/or miss/loss of filterable material, uncertainties of filtration-based estimates of POC concentrations need further testing. The challenge now is to further constrain the magnitude of the biasing effects that add to the adsorption effect to reduce the uncertainties of estimates of POC concentrations, inventories and fluxes in the ocean.

Description: General aspects of particle transport within the abyssal near-bottom water column [ up to 1000 meters above bottom (mab)] and within abyssal surface sediments of the Northeast Atlantic (Porcupine Abyssal Plain) and of the Arabian Sea have been investigated using natural particle-reactive radiotracers (234Th and 210Pb). lt is assumed that food supply controls the composition of the benthic community which, in turn, determines bioturbation intensity. Bioturbation intensity was investigated on different time scales using 234Th and 210Pb (234Th: 100 d; 210Pb: 100 yr). The intensity of biogenic particle mixing of surface sediments in the Arabian Sea was positively and negatively related to increasing food supply on the time-scales of 100 d and 100 yr, respectively. On a time-scale of 100 yr a negative relationship between macrofauna abundance and bioturbation intensity was observed. There was a positive relationship between the density of spoke traces and bioturbation intensity. Low food supply in the southeastern Arabian Sea seems to promote the selection of large infaunal benthic organisms with long particle-transport step lengths (spoke-trace producers) whereas high food supply in the northwestern Arabian Sea seems to promote the selection of infaunal macrofauna with short step lengths and presumably long rest periods. Thus, a lower abundance of large organisms may cause more rapid mixing than a higher abundance of small organisms. These results indicate that single components of the benthic community may dominate the distribution of bioturbation intensity in a given region. There is no a priori reason to assume that there is a general positive relationship between particulate-organic-carbon fluxes (food supply) and bioturbation intensity in all parts of the ocean. For the investigation of the near-bottom water column it was subdivided into the first meter above bottom, the benthic mixed layer (BML) and the layer above the benthic mixed layer up to the upper boundary of the bottom nepheloid layer (BNL). This subdivision was based on the distributions of total particulate matter, transmission, potential temperature and 234Th. The interplay of disaggregation and aggregation within the first meter above bottom and subsequent resuspension of rebound particles at the sediment-water interface are proposed to be the principal reasons for high particle-mass concentrations and increased freshness of particulate matter (PM) within the first meter above bottom. The BML, exhibiting thicknesses of ≈ 10 - 65 m, was characterized by more or less uniform distributions of transmission and/or potential temperature. In the BNL above the mixed layer the composition and distribution of PM is governed by lateral advection, settling, turbulent diffusion and presumably decomposition of PM (see below). A one-dimensional steady state box model was developed to investigate 234Th and particle cycling within the abyssal BNL and surface sediment. Mean particle residence times in the BML and in the resuspension zone of the surface sediment with respect to net transports out of these compartments suggest the BML and the adjoining compartments (upper BNL and resuspension zone of the surface sediment) to be a highly dynamic system with respect to particle cycling and sorptive reactions on time scales of several days up to a few weeks. Modelling results and comparison of the measured downward flux of settling particles with the calculated turbulent-diffusive upward flux of suspended PM indicate virtually simultaneous net upward and downward fluxes of 234Thpart and PM across the upper and lower boundaries of the BML, respectively. This finding requires PM to feed the BML presumably during sedimentation pulses which cause a transient non-steady state situation. An alternative source of PM is adsorption of dissolved organic matter (DOM) onto particles. Moreover, significant net decomposition / dissolution of PM in the upper BNL is implied. This result suggests that decomposition/dissolution of PM in the BNL should be considered in future studies of biogeochemical cycles in addition to particle decomposition/dissolution in the sediment. The BML is proposed to be a decisive filter for the exchange of matter and biogeochemical information between the ocean 's interior and the sediment. The roportion of the BML within the BNL and the thickness of the BNL are important controls of this exchange.

Description: Mit Hilfe eines auf dem G-Modell (Berner, 1980; Westrich & Berner, 1984) beruhenden Verfahren wurde in einem Laborexperiment untersucht, ob sich ein natürliches Gemisch labiler partikulärer organischer Substanz (Gewebe von Mytilus edulis) und ein natürliches Gemisch refraktärer partikulärer organischer Substanz (Spülsaummaterial, fast ausschlieBlich Zostera marina) in ihrem Abbauverhalten beeinflussen, so daß ein beschleunigter Gesamtabbau des organischen Materials erfolgt. Das labile Material stellt einen Nahrungseintrag in das Sediment dar. Das refraktäre Material repräsentiert das sedimentäre Material, das bereits gealtert und frühdiagenetisch geprägt war. Die Bezugsgröße für das Verfahren war partikulärer organischer Kohlenstoff (POC). Es wurden drei Versuchsansätze benötigt: (1) Ein Ansatz, der nur labiles Material enthielt (labiler Einzelansatz), (2) ein Ansatz, der nur refraktares Material enthielt (refraktärer Einzelansatz) und (3) ein Gemischansatz, der labilen und refraktären partikulären organischen Kohlenstoff im Verhältnis 1:1 enthielt. Die für den labilen und den refraktären Ansatz ermittelten Abbaukurven wurden addiert (Summenkurve) und mit der Abbaukurve des Gemischansatzes, die mit 2 multipliziert wurde (Gemischkurve x 2), verglichen. Die Summenkurve dient als Bezugssituation. Mit diesem Verfahren war es möglich, eine Aussage über Änderungen im Gesamtabbau des organischen Materials zu machen. Der Vergleich der Summenkurve mit der Gemischkurve x 2 ergab für partikulären organischen Kohlenstoff, partikulären Stickstoff und aschefreies Trockengewicht keinen Unterschied. Die Hypothese, daß ein beschleunigter Gesamtabbau des organischen Materials einsetzt, wenn labiles Material mit refraktärem gemischt wird, wurde daher abgelehnt. Dieses Ergebnis schließt nicht aus, daß eine der beiden Substanzfraktionen schneller und die andere langsamer abgebaut wurde als im betreffenden Einzelansatz, so daß sich beide Effekte gerade kompensierten und der Gesamtabbau unverändert blieb. Das beschriebene Verfahren läßt sich auch auf Zeitreihen anderer Parameter anwenden. Für die gelösten anorganische Stickstoffverbindungen Ammonium, Nitrit und Nitrat ergaben sich dabei Unterschiede zwischen den Summenkurven und Gemischkurven x 2. Bei Versuchsbeginn setzten remineralisierende und nitrifizierende Aktvitäten im Gemisch der beiden Fraktionen organischer Substanz schneller ein als fOr die Summenkurve (Bezugssituation), wodurch die Ammonium- bzw. Nitrit- und Nitratkonzentration kurz nach dem Nahrungseintrag anstieg. Das Verhältnis von Ammonium zu Nitrit und Nitrat und die Summe der Ammonium-, Nitrit- und Nitratkonzentration war im Gemisch höher. Der Abbau des partikulären organischen Materials verlief stets in zwei Phasen. Während der ersten Phase dominierte der Abbau durch das Auslaugen von gelöstem, stickstoffreichem organischen Material. Dieser Vorgang war durch hohe Zerfallskonstanten des partikulären organischen Materials (labiler Ansatz und Gemischansatz ≥ 100 yr-1 ; refraktärer Ansatz: 1 - 35 yr-1) und gesteigerte Teilungsaktivität der Bakterien gekennzeichnet. Da das refraktäre Material bereits gealtert war, wirkte sich die Auslaugung im labilen und im Gemischanstz am stärksten aus. Die zweite Phase, in der ausschlieBlich mikrobieller Abbau stattfand, läßt sich durch kleinere Zerfallskonstanten charakterisieren, die für organisches Material, das nach dem Abbau der labilen Fraktion zuruckbleibt, typisch sind (0,4 - 4,4 yr-1 ). In dieser Phase blieben die Bakterienzellzahlen konstant und geringer als in der ersten Phase. Nur ein kleiner Anteil des über den gesamten Versuchszeitraum abgebaut organischen Materials wurde in bakterieller Biomasse festgelegt (Kohlenstoff: 〈 1,8%; Stickstoff: 〈 1,1%). Der größte Teil des abgebauten partikulären organischen Materiales wurde zum einen remineralisiert und zum anderen in gelöstes organisches Material umgewandelt.

Description: Author Posting. © Association for the Sciences of Limnology and Oceanography, 2012. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography: Methods 10 (2012): 631-644, doi:10.4319/lom.2012.10.631.

Description: Intercomparision of 234Th measurements in both water and particulate samples was carried out between 15 laboratories worldwide, as a part of GEOTRACES inter-calibration program. Particulate samples from four different stations namely BATS (both shallow and deep) and shelf station (shallow) in Atlantic and SAFE (both shallow and deep) and Santa Barbara station (shallow) in Pacific were used in the effort. Particulate intercalibration results indicate good agreement between all the participating labs with data from all labs falling within the 95% confidence interval around the mean for most instances. Filter type experiments indicate no significant differences in 234Th activities between filter types and pore sizes (0.2-0.8 μm). The only exception are the quartz filters, which are associated with 10% to 20% higher 234Th activities attributed to sorption of dissolved 234Th. Flow rate experiments showed a trend of decreasing 234Th activities with increasing flow rates (2-9 L min-1) for 〉 51 μm size particles, indicating particle loss during the pumping process. No change in 234Th activities on small particles was observed with increasing flow-rates. 234Th intercalibration results from deep water samples at SAFe station indicate a variability of 〈 3% amongst labs while dissolved 234Th data from surface water at Santa Barbara Station show a less robust agreement, possibly due to the loss of 234Th from decay and large in-growth corrections as a result of long gap between sample collection and processing.

Description: This research is funded by NSF Chemical Oceanography program. LM will like to thank Fisheries and Oceans Canada for support. PM is supported through ICREA Academia funded by Generalitat de Catalunya. The International Atomic Energy Agency is grateful to the Government of the Principality of Monaco for the support provided to its Environment Laboratories.