Volker Ratmeyer

Carbon isotope analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic

Atmospheric dust samples collected along a transect off the West African coast have been investigated for their lipid content and compound-specific stable carbon isotope compositions. The saturated hydrocarbon fractions of the organic solvent extracts consist mainly of long-chain n-alkanes derived from epicuticular wax coatings of terrestrial plants. Backward trajectories for each sampling day and location were calculated using a global atmospheric circulation model. The main atmospheric transport took place in the low-level trade-wind layer, except in the southern region, where long-range transport in the mid-troposphere occurred. Changes in the chain length distributions of the n-alkane homologous series are probably related to aridity, rather than temperature or vegetation type. The carbon preference of the leaf-wax n-alkanes shows significant variation, attributed to a variable contribution of fossil fuel- or marine-derived lipids. The effect of this nonwax contribution on the 13 C values of the two dominant n-alkanes in the aerosols, n-C29 and n-C31 alkane, is, however, insignificant. Their 13 C values were translated into a percentage of C4 vs. C3 plant type contribution, using a two-component mixing equation with isotopic end-member values from the literature. The data indicate that only regions with a predominant C4 type vegetation, i.e. the Sahara, the Sahel, and Gabon, supply C4 plant-derived lipids to dust organic matter. The stable carbon isotopic compositions of leaf-wax lipids in aerosols mainly reflect the modern vegetation type along their transport pathway. Wind abrasion of wax particles from leaf surfaces, enhanced by a sandblasting effect, is most probably the dominant process of terrigenous lipid contribution to aerosols. Copyright

The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems

In order to understand current and future patterns of climate variability and change, we need to know how climate varied in the past and which physical forcing mechanisms led to these climatic changes. Direct observations of climate indicators such as temperature and rainfall reach back about 150 years. To extend this observational record, we rely on information from environmental paleoclimatic proxy records that have been extracted from natural archives of past climate variability.

Lithogenic particle fluxes and grain size distributions in the deep ocean off northwest africa : Implications for seasonal changes of aeolian dust input and downward transport

Between 1988 and 1994, twenty time-series sediment traps were deployed at different water depths in the Canary Island region, off Cape Blanc (Mauritania), and off Cape Verde (Senegal). Lithogenic particle fluxes and grain size distributions of the carbonate-free fraction of the trapped material show a high impact of dust transported either in the northeast trade winds or the Saharan Air Layer (SAL). Highest annual mean lithogenic fluxes (31.2–56.1 mg m-2 d-1) were observed at the Cape Blanc site, and largest annual mean diameters (>6 μm) were found off Cape Verde (14.5–16.9 μm) and off Cape Blanc (15.2–16.7 μm). Lowest annual lithogenic fluxes (11.4–21.2 mg m-2 d-1 ) and smallest mean diameters (13.5–13.7 μm) occurred in the Canary Island region. A significant correlation of organic carbon and lithogenic fluxes was observed at all sites. Off Cape Blanc, fluxes and mean diameters correlated well between upper (around 1000 m depth) and lower traps (around 3500 m depth), indicating a fast and mostly undisturbed downward transport of particulate matter. In contrast, a major correlation of fluxes without correlating mean diameters occurred in the Canary Island region, which translates into a fast vertical transport plus scavenging of laterally advected material with depth at this site. The seasonality of lithogenic fluxes was highest in the Canary Island region and off Cape Verde, reflecting strong seasonal patterns of atmospheric circulation, with highest occurrence of continental winds in the trade wind layer during winter. In addition, grain size statistics reflect a dominant change of dust transport in the trade winds during winter/spring and transport in the SAL during summer 1993 at the Cape Verde site. Highest lithogenic fluxes during winter were correlated with mean diameters around 10–13 μm, whereas lower fluxes during summer consisted of coarse grains around 20 μm. Annual mean dust input wascalculated from lithogenic fluxes in the range 0.7×106–1.4×106 t yr-1, roughly confirming both sediment accumulation rates and atmospheric model calculations reported previously from this area.

Deep water particle flux in the Canary Island region: Seasonal trends in relation to long-term satellite derived pigment data and lateral sources

We present a 3 year record of deep water particle flux at the recently initiated ESTOC (European Station for Time-series in the Ocean, Canary Islands) located in the eastern subtropical North Atlantic gyre. Particle flux was highly seasonal, with flux maxima occurring in late winter-early spring. A comparison with historic CZCS (Coastal Zone Colour Scanner) data shows that these flux maxima occurred about 1 month after maximum chlorophyll was observed in surface waters in a presumed primary source region 100 km × 100 km northeast of the trap location. The main components of the particles collected with the traps were mineral particles and carbonate, both correlating strongly with organic matter sedimentation. Mineral particles in the sinking matter are indicative of the high aeolian input from the African desert regions. Comparing particle fluxes at 1 km and 3 km depth, we find that particle sedimentation increased substantially with depth. Yearly organic carbon sedimentation was 0.6 g m−2 at 1 km depth compared with 0.8 g m−2 at 3 km. We hypothesize that higher phytoplankton biomass observed further north could be a source of laterally advecting particles that interact with fast sinking particles originating from the primary source region. This hypothesis is also supported by the differences in size distribution of lithogenic matter found at the two trap depths.

Organic carbon fluxes in the Atlantic and the Southern Ocean : relationship to primary production compiled from satellite radiometer data

Abstract Fluxes of organic carbon normalised to a depth of 1000 m from 18 sites in the Atlantic and the Southern Ocean are presented, comprising nine biogeochemical provinces as defined by Longhurst et al. (1995. Journal of Plankton Research 17, 1245–1271). For comparison with primary production, we used a recent compilation of primary production values derived from CZCS data (Antoine et al., 1996. Global Biogeochemical Cycles 10, 57–69). In most cases, the seasonal patterns stood reasonably well in accordance with the carbon fluxes. Particularly, organic carbon flux records from two coastal sites off northwest and southwest Africa displayed a more distinct correlation to the primary production in sectors (1×1°) which are situated closer to the coastal environments. This was primarily caused by large upwelling filaments streaming far offshore, resulting in a cross-shelf carbon transport. With respect to primary production, organic carbon export to a water depth of 1000 m, and the fraction of primary production exported to a depth of 1000 m (export fraction=EF1000), we were able to distinguish between: (1) the coastal environments with highest values (EF1000=1.75–2.0%), (2) the eastern equatorial upwelling area with moderately high values (EF1000=0.8–1.1%), (3) and the subtropical oligotrophic gyres that yielded lowest values (EF1000=0.6%). Carbon export in the Southern Ocean was low to moderate, and the EF1000 value seems to be quite low in general. Annual organic carbon fluxes were proportional to primary production, and the export fraction EF1000 increased with primary production up to 350 gC m −2 yr −1 . Latitudinal variations in primary production were reflected in the carbon flux pattern. A high temporal variability of primary production rates and a pronounced seasonality of carbon export were observed in the polar environments, in particular in coastal domains, although primary production (according to Antoine et al., 1996. Global Biogeochemical Cycles 10, 57–69), carbon fluxes, and the export fraction remained at low.

Distinct year-to-year particle flux variations off Cape Blanc during 1988–1991: Relation to δ 18 O-deduced sea-surface temperatures and trade winds

Particle fluxes measured from 1988 to 1991 adjacent to a coastal upwelling site off Cape Blanc showed significant interannual variability of fluxes and sea-surface temperatures (SST) deduced from stable oxygen isotope analysis of the planktonic foraminifera Globigerinoides ruber and, partly, of the pteropod Limacina inflata. For the duration of the study period, a decrease in the seasonality of SSTs was observed, as well as a significant decrease in the average annual SST from 24.4° to 20.8°C. This cooling trend was mainly the effect of a drastic decrease in the summer to fall SST (from 27.2° to 21.8°C). In comparison, the winter-spring SST decreased only slightly from 20.3° in 1988 to 19.8°C in 1991. Concomitantly, we measured decreasing annual total, carbonate, biogenic opal and lithogenic fluxes and, in contrast, increasing marine organic carbon fluxes. During 1991, when cold SSTs prevailed and the trade winds were rather high throughout, annual biogenic and lithogenic fluxes (except organic carbon) were lower by approximately a factor of two compared to the other years. Colder SSTs, generally corresponding to stronger trade winds and upwelling intensity, did not result in increased biogenic opal and lithogenic matter sedimentation ; but higher marine organic carbon fluxes were recorded. Decreasing summer-fall SST from 1988 to 1991 coincided with decreased carbonate sedimentation maxima which generally occurred during the warm summer season. In the summer of 1989, when SSTs were the highest of the four-year sampling period and upwelling was less intense due to weak spring-summer trades, a large sedimentation pulse of pteropod shells was observed. Our data set does not yet provide conclusive evidence that the observed year-to-year flux and SST variations represent larger-scale, periodically occurring climatic variations in the eastern Atlantic but it offers insight into the prevailing large variability in biochemical cycles and processes in the eastern Atlantic.

A high resolution camera system (ParCa) for imaging particles in the ocean : System design and results from profiles and a three-month deployment

For direct optical measurement of abundance, concentration and size distribution of marine particles, a high-resolution camera system (ParCa) was designed to improve on similar systems used by Honjo et al. (1984), Asper (1987) and others. Imaging a probe volume of up to 37 1, smallest particles with diameters of 50 μm can be counted. The images provide information on particle size, shape and abundance either during profiling through the water column or while moored in a certain depth over time. Depth profiles were acquired between fall 1992 and late spring 1993 on R. V. Meteor cruises M22-1 and M23-3 at 6 stations in the equatorial Atlantic Ocean and off the west African shelf. The images show variable particle and aggregate concentrations through 550 m of the water column, with highest concentrations in the upper 80 m. A distinctive change in the depth of the upper chlorophyll maximum from about 75 m in the Brazil Basin to about 50 m in the Guinea Basin was measured with the attached INFLUX current meter (Krause and Ohm, 1996) and is as well represented in the particle abundances of two selected profiles. In contrast, both profiles show a second particle abundance maximum between 100 and 250 m, which is not visible in the chlorophyll-a and backscatter signal of the INFLUX sensors. Total particle abundance maxima raise from 677 counts per liter in the central Brazil Basin to 991 counts in the Guinea Basin, corresponding to marine snow abundances of 57 and 127 counts per liter, respectively. In order to compare high-resolution data on particle concentration and flux through time, ParCa was also deployed on a sediment-trap mooring at 995 m depth in the Canary Basin between June and September 1994. First results show similar trends in sediment-trap derived fluxes of particulate matter from 2.8 to 67.2 mg m -2 d -1 and equivalent spherical volumes of particles with diameters > 0.5 mm from 0.98 to 4.13 mm 3 l -1 .

HYBRID-ROV - Development of a new underwater vehicle for high-risk areas

Recently, the Center for Marine Environmental Sciences MARUM (University of Bremen, Germany) received national funds to design and build a new remote operated underwater vehicle - the HYBRID-ROV (H-ROV). Within the 3 year project the basis of a solely battery powered underwater vehicle capable of working as ROV and AUV shall be developed in the MARUM institute. Predominantly, the proposed use of the H-ROV shall be scientific operations in more risky environmental areas like under-ice operation, near-bottom-work in harsh topography, in between cold water corals or hydrothermal vent fields. Basically, due to higher operational risks in these areas, the vehicle predominately should be operated fully remotely and video controlled via a thin high bandwidth glass fiber. In event of a critical situation, e.g., tangled or blocked fiber underneath the ice, the glass fiber will be disconnected and the vehicle should switch automatically into the autonomous mode, passing back to a save rendezvous-point far out of critical area. If appropriate, it should be possible as well to run the H-ROV in a pre-programmed autonomous AUV mission mode from beginning. In basic configuration it is envisaged to design a flat shaped vehicle of approx. 5×2 m foot print dimensions, capable of standing still and hovering on site. In addition, developers are encouraged to reach 4000 m design-depth and/or rather 4000 m operation range with attached glass fiber. With regards to configuration of lights, cameras and manipulator, a challenge for vehicle design will be the approach to work, as well at the sea floor, as underneath the ice (means over head) without vehicle frame modifications and within one dive. The overall vehicle concept should be modular in order to use several already existing scientific payloads from other underwater vehicles. Finally, as an operational aspect due to its smaller size and system weight of approx. 1.5 metric ton or less, operation of the H-ROV from mid-size vessel or vessel of opportunity is an overall design necessity as well. Despite ongoing design iterations it is already decided to build the complete H-ROV software system on the Open Robot Control Software (OROCOS), an open source project based on C++ framework, in direct cooperation with the German Research Center for Artificial Intelligence (DFKI, RIC, University of Bremen, Germany). In addition, the main telemetry will be based on Schilling Robotics DTS to be consistent with already existing MARUM vehicles like QUEST ROV and the MEBO remotely operated deep sea drill. The whole battery system will be designed on a complete pressure-neutral underwater Li-Ion battery stack, because of an effective weight-/ form-factor and overall energy content of battery stack, due to unnecessary pressure housings. The paper summarizes the initial design of basic components like vehicle, propulsion, batteries, telemetry concept and sensor suite.

Transfer of Particles into the Deep Atlantic and the Global Ocean: Control of Nutrient Supply and Ballast Production

Particle fluxes from 20 trap sites in the Atlantic/Southern Ocean have been compiled to study the regional variations in comparison with important environmental variables. In turn, these results have been compared to other study sites from the world ocean, mainly regarding the relations-hip between bulk fluxes/various flux ratios to nutrient supply. It is shown that the supply of dissolved silicic acid to the surface waters (the ‘silicate pump’, Dugdale et al. 1995) plays a central role in opal fluxes, BSi:Corg ratios, BSi:carbonate ratios, and thus carbon rain ratios. The mean annual BSi:Corg ratio (mol/mol) normalized to 1000 m was 0.05 in the Atlantic, 0.4 in the Indian, 0.5 in the Pacific, and 0.1–3 in the Southern Ocean and follow s the general path of the conveyor belt (Ragueneau et al. 2000). A shift in the primary producer community from coccolithophorids to diatoms, reflected by an exponential increase of the annual BSi:carbonate flux ratios, occurs above a molar Si:N(250m) nutrient threshold of about 1.7. The surface sediment opal:carbonate ratios (%) versus the Si:N(250m) nutrient values produce a threshold of 2–2.5, however, this value may be biased by opal dissolution during early diagenesis. We also tested the most recent findings about particle ballast which presume that carbonate is most important for the rapid downward transport of organic particles to bathypelagic depths. Our compilation of global flux data confirms such a general relationship. However, at certain sites and in particular years /seasons, other minerals may serve as ballast for organic carbon. Off NW Africa, for instance, lithogenic components were the major particle carriers. There, relationships between carbonate/lithogenic/total ballast fluxes versus daily organic carbon fluxes may even vary from year to year. Off Cape Blanc, the carbonate-Corg-relationship is highly significant during a strong coccolithophorid bloom in 1991, probably resulting in an efficient downward transfer of organic carbon. Interannual variation of fluxes was highest in high production systems combined with high seasonality of fluxes. We obtained ca. 20% variability in oligotrophic regions and up to 100% in the Southern Ocean where seasonality is most pronounced.

In-situ sinking speed measurements of marine snow aggregates acquired with a settling chamber mounted to the Cherokee ROV

Marine snow plays a key role in the global carbon cycle because it transfers huge amounts of carbon dioxide (around 1–2 Gt per year) from the ocean surface to the deep-sea, thus removing it from the global system. It is of major field of study for several decades to quantify the amount of particulate matter settling through the water column. A central parameter for ocean mass flux estimates is the settling velocity of larger particles. Most of the few available datasets have been acquired by Scuba divers but are they are limited to a diving depth of a few tenth of meters. Particle settling speeds for the deeper water column may be estimated with the help of sediment trap recordings, having the disadvantage to integrate settling speeds over a long period of time and for the entire particle population settling through the water column. In situ sinking speed measurements of individual aggregates however, are rare and difficult to obtain. We present results from a settling chamber constructed for in situ sinking speed measurements of marine snow. The settling chamber was mounted to the MARUM Cherokee ROV during RV Poseidon Cruise 365 in 2008 off Cape Blanc, Mauritania. It was constructed in consideration of a similar device used by the MBARI ROV Ventana. It is a simple plexiglas box which can be opened and closed to allow an infinite number of measurements with little disturbance inside. A collimated light source illuminates a defined sample volume in which aggregates can be observed after the box has been closed. We sampled a total of 51 aggregates at four depth levels, between 50m and 400m water depth. The depths were chosen after collecting a vertical particle profile acquired by a deep-sea still image camera system before the deployment of the ROV. Sinking speeds ranged from 10m d−1to 287 m d−1 with a mean value of 57 m d−1. No clear relationsship between the size of the particles and their sinking speed was found. Furthermore we could not observe increasing particle sinking speeds with increasing water depth as found by other authors. This underlines the complexity of such studies and implies more deployments during upcoming cruises and comparison of in situ measurements with additional methods in the future.