Frank Wenzhöfer

Early diagenesis of organic matter from sediments of the eastern subtropical Atlantic: evidence from stable nitrogen and carbon isotopes

Stable isotopes of sedimentary nitrogen and organic carbon are widely used as proxy variables for biogeochemical parameters and processes in the water column. In order to investigate alterations of the primary isotopic signal by sedimentary diagenetic processes, we determined concentrations and isotopic compositions of inorganic nitrogen (IN), organic nitrogen (ON), total nitrogen (TN), and total organic carbon (TOC) on one short core recovered from sediments of the eastern subtropical Atlantic, between the Canary Islands and the Moroccan coast. Changes with depth in concentration and isotopic composition of the different fractions were related to early diagenetic conditions indicated by pore water concentrations of oxygen, nitrate, and ammonium. Additionally, the nature of the organic matter was investigated by Rock-Eval pyrolysis and microscopic analysis. A decrease in ON during aerobic organic matter degradation is accompanied by an increase of the 15 N/ 14 N ratio. Changes in the isotopic composition of ON can be described by Rayleigh fractionation kinetics which are probably related to microbial metabolism. The influence of IN depleted in 15 N on the bulk sedimentary (TN) isotope signal increases due to organic matter degradation, compensating partly the isotopic changes in ON. In anoxic sediments, fixation of ammonium between clay lattices results in a decrease of stable nitrogen isotope ratio of IN and TN. Changes in the carbon isotopic composition of TOC have to be explained by Rayleigh fractionation in combination with different remineralization kinetics of organic compounds with different isotopic composition. We have found no evidence for preferential preser- vation of terrestrial organic carbon. Instead, both TOC and refractory organic carbon are dominated by marine organic matter. Refractory organic carbon is depleted in 13 C compared to TOC. Copyright

Export of algal biomass from the melting Arctic Sea ice

Diatom Fall 2012 saw the greatest Arctic ice minimum ever recorded. This allowed unprecedented access for research vessels deep into the Arctic Ocean to make high-latitude observations of ice melt and associated phenomena. From the RV Polarstern between 84° to 89° North, Boetius et al. (p. 1430, published online 14 February; see the cover) observed large-scale algal aggregates of the diatom Melosira arctica hanging beneath multiyear and seasonal ice across a wide range of latitudes. The strands of algae were readily dislodged and formed aggregates on the seabed up to 4400 meters below, where the algae are consumed by large mobile invertebrates, such as sea cucumbers and brittle stars. Although Nansen observed sub-ice algae in the Arctic 100 years ago, the extent of this bloom phenomenon was unknown. The dynamics of such blooms must impinge on global carbon budgets, but how the dynamics will change as ice melt becomes more extensive remains unclear. As polar ice retreated in 2012, it left evidence of large algal deposits in its wake. In the Arctic, under-ice primary production is limited to summer months and is restricted not only by ice thickness and snow cover but also by the stratification of the water column, which constrains nutrient supply for algal growth. Research Vessel Polarstern visited the ice-covered eastern-central basins between 82° to 89°N and 30° to 130°E in summer 2012, when Arctic sea ice declined to a record minimum. During this cruise, we observed a widespread deposition of ice algal biomass of on average 9 grams of carbon per square meter to the deep-sea floor of the central Arctic basins. Data from this cruise will contribute to assessing the effect of current climate change on Arctic productivity, biodiversity, and ecological function.

Benthic carbon mineralization in the Atlantic: a synthesis based on in situ data from the last decade

Benthic oxygen uptake rates quantified by the use of microsensors and flux chambers over a period of approx. 10 yr were compiled and used to assess the organic carbon mineralization in the central and South Atlantic (351N–501S). Measurements were performed in situ and in the laboratory on recovered sediment cores. In contrast to the laboratory data, both the in situ diffusive (DOU) and total oxygen uptake (TOU) decreased with increasing water depth. The data demonstrated that sediment recovery alter the O2 microdistribution and affect the measured O2 uptake rates. The ratio between TOU and DOU, a measure of the benthic fauna-mediated oxygen uptake, decreased from 3 to 4 in shallow and productive areas to around 1 at the deeper sites. The in situ oxygen uptake rates (both diffusive and total) also correlated with the oceanic primary production. Based on the compiled in situ measurements an empirical relation between the surface water primary production (PP, g C m � 2 yr � 1 ), water depth (z; m) and benthic mineralization deduced from the TOU and DOU was established (C-DOU=PP 0:7358 z � 0:3306 (g C m � 2 yr � 1 ); C-TOU=PP 1:0466 z � 0:4922 (g C m � 2 yr � 1 )). These equations were extrapolated to the entire investigated area of the Atlantic. The mineralization mimicked the surface water primary production, with high consumption rates in the upwelling areas. For the entire area (water depthX1000 m) the benthic carbon mineralization was between 134 and 168 � 10 12 gC yr � 1 (from C-DOU and C-TOU, respectively), which equals 1.7–2.1% of the surface water primary production. These rates are higher than previous estimates of benthic carbon mineralization in deep-sea sediments. Integrated for the investigated area of the Atlantic the benthic fauna-mediated carbon mineralization accounted for 35 � 10 12 gC yr � 1 (or 21% of the total mineralization rate). Using our relations to calculate the organic carbon flux through the 1000 m depth horizon revealed that between 212 and 333 � 10 12 gC yr � 1 sink below this depth horizon, of which 63% and 51% is remineralized in the sediments. Particulate organic carbon fluxes obtained from sediment trap data cannot support either the measured or extrapolated benthic mineralization. The areal distribution of the oxygen penetration depth (OPD) for the investigated area of the Atlantic was estimated from the relation between the in situ C-DOU and OPD measurements

Towards a greater understanding of pattern, scale and process in marine benthic systems: a picture is worth a thousand worms

Historically, advances in our knowledge of benthic community structure and functioning have necessarily relied upon destructive sampling devices (grabs, cores, anchor dredges, etc.) that lose valuable contextual information in the process of sampling. In the last 40 years, instrumentation capable of measuring dynamic events and/or processes within and immediately above the seafloor has been developed that facilitates the collection of ecological information. Of these, both acoustic and optical imaging devices have played a significant role in revealing much about the physiology and behaviour of, and interactions between benthic species, and the sedimentary habitat in which they reside. While a number of reviews have separately considered the methodological and technical aspects of imaging technologies, the collective contribution that imaging has made to benthic ecology has received less attention. In this short review, we attempt to highlight key instances over the last 40 years where either acoustic or optical-based imaging techniques have provided new ecological insights and information about fine-grained sedimentary environments. In so doing, we focus on the ecological advances that have formed the precursor to current research efforts and introduce some of the latest revelations from appropriate and emerging imaging applications.

Benthic biogeochemistry: state of the art technologies and guidelines for the future of in situ survey

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists. Our understanding of sediment biogeochemistry and exchange fluxes was revolutionized by the introduction of benthic chambers and in situ micro-electrode profilers that allow for the direct measurement of chemical fluxes between sediment and water at the sea floor and for porewater composition. Since then, rapid progress in the technology of in situ sensors and benthic chambers (such as the introduction of gel probes, voltammetric electrodes or one- and two-dimensional optodes) have yielded major breakthroughs in the scientific understanding of benthic biogeochemistry. This paper is a synthesis of discussions held during the workshop on sediment biogeochemistry at the “Benthic Dynamics: in situ surveillance of the sediment–water interface” international conference (Aberdeen, UK—March 25–29, 2002). We present a review of existing in situ technologies for the study of benthic biogeochemistry dynamics and related scientific applications. Limitations and possible improvement (e.g., technology coupling) of these technologies and future development of new sensors are discussed. There are countless important scientific and technical issues that lend themselves to investigation using in situ benthic biogeochemical assessment. While the increasing availability of these tools will lead research in yet unanticipated directions, a few emerging issues include greater insight into the controls on organic matter (OM) mineralization, better models for the understanding of benthic fluxes to reconcile microelectrode and larger-scale chamber measurements, insight into the impacts of redox changes on trace metal behavior, new insights into geochemical reaction pathways in surface sediments, and a better understanding of contaminant fate in nearshore sediments.

COMMUNITIES AND MICROHABITATS OF LIVING BENTHIC FORAMINIFERA FROM THE TROPICAL EAST ATLANTIC: IMPACT OF DIFFERENT PRODUCTIVITY REGIMES

Living (Rose Bengal stained) benthic foraminifera were collected with a multicorer from six stations between 2°N and 12°S off West Africa. The foraminiferal communities in the investigated area reflect the direct influence of different productivity regimes, and are characterized by spatially and seasonally varying upwelling activity. At five stations, foraminiferal abundance coincides well with the gradient of surface productivity. However, at one station off the Congo River, the influence of strong fresh water discharge is documented. Although this station lies directly in the center of an upwelling area, foraminiferal standing stocks are surprisingly low. It is suggested that the Congo discharge may induce a fractionation of the organic matter into small and light particles of low nutritional content, by contrast to the relatively fast-sinking aggregates found in the centers of high productivity areas. Quality and quantity of the organic matter seem to influence the distribution of microhabitats as well. The flux of organic carbon to the sea-floor controls the sequence of degradation of organic matter in sediment and the position of different redox fronts. The vertical foraminiferal stratification within sediment closely parallels the distribution of oxygen and nitrate in porewater, and reflects different nutritive strategies and adaptation to different types of organic matter. The epifauna and shallow infauna colonize oxygenated sediments where labile organic matter is available. The intermediate infauna ( M. barleeanum ) is linked to the zone of nitrate reduction in sediments where epifaunal and shallow infaunal species are not competitive anymore, and must feed on bacterial biomass or on metabolizable nutritious particles produced by bacterial degradation of more refractory organic matter. The deep infauna shows its maximum distribution in anoxic sediments, where no easily metabolizable organic matter is available.

In situ microsensor studies of a shallow water hydrothermal vent at Milos, Greece

Abstract The microenvironment and microcirculation of a shallow water hydrothermal vent system was studied together with the benthic primary production at Milos, Greece. In situ microprofiles of O 2 , pH, H 2 S and temperature were obtained using a miniaturised version of a profiling instrument. The sediment temperature increased toward the centre of the vent system, reaching a surface maximum of 100°C in the central yellow coloured sulfidic area. The oxygen penetration depth decreased from the unaffected sediment surrounding the vent system towards the vent centre; however, at the inner vent area the O 2 penetration increased again. Similar results were obtained during laboratory measurements. H 2 S concentrations increased rapidly beneath the oxygenated zone in the different vent areas and reached values of approximately 900 μM at sediment depths of 7–17 mm in the central vent areas. The microprofiles resolved a microcirculative pattern where local pressure differences caused by outflowing seep fluids induced a downward transport of oxygenated water, creating small convective cells which efficiently reoxidised H 2 S of the seep fluid. Patches of benthic diatoms covered the sediment surface in the areas surrounding the vent system. The net photosynthesis of this community increased from 25 to 41.8 mmol O 2 m −2 d −1 from early morning to midday. The amount of carbon fixed daily, as calculated from the in situ oxygen microprofiles, accounted for 0.67 mmol C m −2 d −1 . Laboratory incubations indicated that photosynthesis was not carbon limited and consequently the excess dissolved inorganic carbon contained in the vent fluids presumably had no effect on benthic primary production.

Calcite dissolution driven by benthic mineralization in the deep-sea: in situ measurements of Ca2+, pH, pCO2 and O2

Abstract In situ measured microprofiles of Ca 2+ , pCO 2 , pH and O 2 were performed to quantify the CaCO 3 dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m −2 d −1 and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m −2 a −1 . The metabolic CO 2 , released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca 2+ microprofiles were 0.6 mmol m −2 d −1 for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO 2 produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O 2 , pH and Ca 2+ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw −1 a −1 (mol per kg water and year), which equals 95% d −1 . This rate constant is at the upper end of reported in situ values.

Niche differentiation among mat-forming, sulfide-oxidizing bacteria at cold seeps of the Nile Deep Sea Fan (Eastern Mediterranean Sea)

Sulfidic muds of cold seeps on the Nile Deep Sea Fan (NDSF) are populated by different types of mat-forming sulfide-oxidizing bacteria. The predominant sulfide oxidizers of three different mats were identified by microscopic and phylogenetic analyses as (i) Arcobacter species producing cotton-ball-like sulfur precipitates, (ii) large filamentous sulfur bacteria including Beggiatoa species, and (iii) single, spherical Thiomargarita species. High resolution in situ microprofiles revealed different geochemical settings selecting for the different mat types. Arcobacter mats occurred where oxygen and sulfide overlapped above the seafloor in the bottom water interface. Filamentous sulfide oxidizers were associated with steep gradients of oxygen and sulfide in the sediment. A dense population of Thiomargarita was favored by temporarily changing supplies of oxygen and sulfide in the bottom water. These results indicate that the decisive factors in selecting for different mat-forming bacteria within one deep-sea province are spatial or temporal variations in energy supply. Furthermore, the occurrence of Arcobacter spp.-related 16S rRNA genes in the sediments below all three types of mats, as well as on top of brine lakes of the NDSF, indicates that this group of sulfide oxidizers can switch between different life modes depending on the geobiochemical habitat setting.

How deep-sea wood falls sustain chemosynthetic life.

Large organic food falls to the deep sea – such as whale carcasses and wood logs – are known to serve as stepping stones for the dispersal of highly adapted chemosynthetic organisms inhabiting hot vents and cold seeps. Here we investigated the biogeochemical and microbiological processes leading to the development of sulfidic niches by deploying wood colonization experiments at a depth of 1690 m in the Eastern Mediterranean for one year. Wood-boring bivalves of the genus Xylophaga played a key role in the degradation of the wood logs, facilitating the development of anoxic zones and anaerobic microbial processes such as sulfate reduction. Fauna and bacteria associated with the wood included types reported from other deep-sea habitats including chemosynthetic ecosystems, confirming the potential role of large organic food falls as biodiversity hot spots and stepping stones for vent and seep communities. Specific bacterial communities developed on and around the wood falls within one year and were distinct from freshly submerged wood and background sediments. These included sulfate-reducing and cellulolytic bacterial taxa, which are likely to play an important role in the utilization of wood by chemosynthetic life and other deep-sea animals.