Benthic carbon mineralization in hadal trenches:assessment by in situ O₂ microprofile measurements

Hadal trenches are considered to act as depo-centers for organic material at the trench axis and host unique and elevated biomasses of living organisms as compared to adjacent abyssal plains. To explore the diagenetic activity in hadal trench environments we quantified in situ benthic O2 consumption rates and sediment characteristics from the trench axis of two contrasting trench systems in the Pacific Ocean; the Izu-Bonin Trench underlying mesotrophic waters and the Tonga Trench underlying oligotrophic waters. In situ oxygen consumption at the Izu-Bonin Trench axis site (9200 m; 746±103 µmol m−2 d−1; n=27) was 3-times higher than at the Tonga Trench axis site (10800 m; 225±50 µmol m−2 d−1; n=7) presumably reflecting the higher surface water productivity in the Northern Pacific. Comparing benthic O2 consumption rates measured in the central hadal Tonga Trench to that of nearby (60 km distance) abyssal settings (6250 m; 92±44 µmol m−2 d−1; n=16) revealed a 2.5 higher activity at the trench bottom. Onboard investigations on recovered sediment furthermore revealed that the prokaryotic abundance and concentrations of phytopigments followed this overall trend (i.e minimum values at the abyssal site followed by higher values from the Tonga and Izu-Bonin Trenches axis, respectively). Excess 210Pb profiles suggested that mass-wasting events contributed to the deposition of material enhancing the concentration of organic matter in the central trench as compared to the abyssal settings. Our results complement recent findings from the Challenger deep in the Mariana Trench area, which also revealed elevated diagenetic activity in the central trench underpinning the importance of hadal ecosystems for the deep sea carbon cycling

An abyssal hill fractionates organic and inorganic matter in deep-sea surface sediments

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

Species replacement dominates megabenthos beta diversity in a remote seamount setting

Seamounts are proposed to be hotspots of deep-sea biodiversity, a pattern potentially arising from increased productivity in a heterogeneous landscape leading to either high species co-existence or species turnover (beta diversity). However, studies on individual seamounts remain rare, hindering our understanding of the underlying causes of local changes in beta diversity. Here, we investigated processes behind beta diversity using ROV video, coupled with oceanographic and quantitative terrain parameters, over a depth gradient in Annan Seamount, Equatorial Atlantic. By applying recently developed beta diversity analyses, we identified ecologically unique sites and distinguished between two beta diversity processes: species replacement and changes in species richness. The total beta diversity was high with an index of 0.92 out of 1 and was dominated by species replacement (68%). Species replacement was affected by depth-related variables, including temperature and water mass in addition to the aspect and local elevation of the seabed. In contrast, changes in species richness component were affected only by the water mass. Water mass, along with substrate also affected differences in species abundance. This study identified, for the first time on seamount megabenthos, the different beta diversity components and drivers, which can contribute towards understanding and protecting regional deep-sea biodiversity

Bioturbation in the abyssal Arabian Sea: influence of fauna and food supply

In order to evaluate bioturbation in abyssal Arabian-Sea sediments of the Indus fan profiles of (half-life: 22.3 yr) and (half-life: 24.1 d) were measured in cores collected during September and October 1995 and April 1997, respectively. The density and composition of epibenthic megafauna and lebensspuren were determined in vertical seafloor photographs during April 1997. Mean eddy-diffusive mixing coefficients according to the distribution of excess ( -DB) were 0.072±0.028, 0.068±0.055, 0.373±0.119, 0.037±0.009 and 0.079±0.119 cm2 yr−1 in the northern, western, central, eastern and southern abyssal Arabian sea, respectively. Mean eddy-diffusive mixing coefficients according to the distribution of excess (-DB) were 0.53, 1.64 and 0.47 cm2 yr−1 in the northern, western and central abyssal Arabian Sea, respectively. Mobile epibenthic megafauna at the western, northern, central and southern study sites were dominated by ophiuroids, holothurians, ophiuroids and natant decapods (the respective densities were 100, 82, 29 and 6 individuals 1000 m−2). The northern study site was characterized by a high abundance of spoke traces and fecal casts. The central site showed spoke traces and many tracks. The southern site displayed the highest abundance of spoke traces, whereas at the western site hardly any lebensspuren were observed. There is evidence for at least two functional endmember communities in the Arabian Sea. In the northwestern Arabian Sea (WAST) vertical particle displacement seems to be dominated by macrofauna and primarily eddy-diffusive. In the southern Arabian Sea (SAST) non-local and `incidental’ mixing due to spoke-trace producers might become more important and superimpose reduced eddy-diffusive mixing. With respect to biological data CAST is an intermediate location. Given the biological data, average -DB is higher and decimeter-scale variability of -DB smaller at CAST than expected. These findings indicate that in a mixture of both endmember communities the organisms may interact in way that increases values of biodiffusivity, as reflected by -DB, and reduces decimeter-scale -DB heterogeneity in comparison to the simple sum of the isolated effects of the endmembers. For time scales <100 years there was no evidence for a relationship between food supply (POC flux) and bioturbation intensity, as reflected by -DB and -DB. Bioturbation intensity should be controlled primarily by the composition of the benthic fauna, its specific adaptation to the environmental setting, and the abundance of each species of the benthic community. Food supply can have only an indirect influence on bioturbation intensity. In certain parts of the ocean the a priori overall positive relationship between POC flux and biodiffusivity might include restricted intervals displaying no or even negative relations

Experimental evidence for an effect of early-diagenetic interaction between labile and refractory marine sedimentary organic matter on nitrogen dynamics

In most natural sedimentary systems labile and refractory organic material (OM) occur concomitantly. Little, however, is known on how different kinds of OM interact and how such interactions affect early diagenesis in sediments. In a simple sediment experiment, we investigated how interactions of OM substrates of different degradability affect benthic nitrogen (N) dynamics. Temporal evolution of a set of selected biogeochemical parameters was monitored in sandy sediment over 116 days in three experimental set-ups spiked with labile OM (tissue of Mytilus edulis), refractory OM (mostly aged Zostera marina and macroalgae), and a 1:1 mixture of labile and refractory OM. The initial amounts of particulate organic carbon (POC) were identical in the three set-ups. To check for non-linear interactions between labile and refractory OM, the evolution of the mixture system was compared with the evolution of the simple sum of the labile and refractory systems, divided by two. The sum system is the experimental control where labile and refractory OM are virtually combined but not allowed to interact. During the first 30 days there was evidence for net dissolved-inorganic-nitrogen (DIN) production followed by net DIN consumption. (Here ‘DIN’ is the sum of ammonium, nitrite and nitrate.) After 30 days a quasi steady state was reached. Non-linear interactions between the two types of OM were reflected by three main differences between the early-diagenetic evolutions of nitrogen dynamics of the mixture and sum (control) systems: (1) In the mixture system the phases of net DIN production and consumption commenced more rapidly and were more intense. (2) The mixture system was shifted towards a more oxidised state of DIN products [as indicated by increased (nitrite + nitrate)/(ammonium) ratios]. (3) There was some evidence that more OM, POC and particulate nitrogen were preserved in the mixture system. That is, in the mixture system more particulate OM was preserved while a higher proportion of the decomposed particulate N was converted into inorganic N. It can be concluded that during the first days and weeks of early diagenesis the magnitude and composition of the flux of decompositional dissolved N-compounds from sediments into the overlying water was influenced by non-linear interactions of OM substrates of different degradability. Given these experimental results it is likely that the relative spatial distributions of OM of differing degradability in sediments control the magnitude and composition of the return flux of dissolved N-bearing compounds from sediments into the overlying water column

234Th-derived particulate organic carbon export from an island-induced phytoplankton bloom in the Southern Ocean

It has long been recognised that some oceanic regions have persistently low-chlorophyll levels, even though there are abundant inorganic nutrients. Studies have shown that these high-nutrient low-chlorophyll (HNLC) areas are depleted in iron, an essential micronutrient. In these regions biological production can be enhanced with artificial mesoscale iron fertilisation. However, the ability of iron-induced blooms to efficiently sequester carbon to mesopelagic depths is still an open question. It is hypothesised that sub-Antarctic islands in the HNLC Southern Ocean are also a source of iron and thus fuel the natural phytoplankton blooms observed in their proximity, thereby enhancing levels of particulate organic carbon (POC) export. To test the third part of this hypothesis, POC export was measured in the Southern Ocean region of the Crozet Islands (52°E, 46°S) during the austral summer of 2004/2005 as part of the CROZEX project. Based on satellite imagery, a high-chlorophyll region (maximum concentration=4 ?g l?1) north and downstream of the islands was distinguished from a low-chlorophyll region (typical concentration=0.3 ?g l?1) south and upstream of the islands. POC export estimates were obtained by using the naturally occurring particle-reactive radionuclide tracer 234Th. POC export was initially 15 mmol C m?2 d?1 in the high-chlorophyll bloom region, compared with 5 mmol C m?2 d?1 in the low-chlorophyll, non-bloom region. After a moderately small bloom at the southern control stations (max concentration=0.7 ?g l?1) the spatial variability in POC export was lost, resulting in equally high levels of POC export (ca. 20 mmol C m?2 d?1) throughout the study region. Comparison of 234Th-derived POC export with estimates of new production, calculated from nitrate budgets, revealed evidence for a decoupling of new and export production, with this effect most apparent within the northern bloom area. In addition to methodological issues this apparent decoupling of new and export production could be due to a buildup of dissolved organic nitrogen within the bloom region, thus reducing the amount of POC available for export to mesopelagic depths

Particulate organic carbon export from the North and South Atlantic gyres: the 234Th/238U disequilibrium approach

Subtropical ocean gyres are believed to be characterized by low carbon export from the surface into the deep ocean. However, due to their large areas, even relatively small average export could be of significance for the global carbon cycle. To better constrain carbon export from the surface ocean in such regions, radioactive disequilibria between the particle-reactive, short-lived radionuclide 234Th (half-life 24.1 d) and its parent 238U were used to estimate fluxes of 234Th and particulate organic carbon (POC) from surface waters of the North and South Atlantic subtropical gyres and their fringes. Samples were collected between 50°S and 50°N as part of the Atlantic Meridional Transect (AMT) programme during April/May 2004 (AMT14). Application of a steady-state model to the 234Th data revealed particle export from the surface (234Th deficit) and, in one instance, some evidence for shallow particle remineralisation at depth (234Th excess). Export fluxes of POC were calculated from water column 234Th /238U disequilibria and the POC to 234Th ratios on large rapidly sinking particles (&gt;50 ?m). Based on latitudinal distributions of selected hydrographic and biological parameters within the topmost 300 m of the water column, the transect was divided into six regions: ‘temperate’ (35°–50°N and 35°–50°S), ‘oligotrophic’ (20°–35°N and 5°–35°S), ‘equatorial’ (5°S–5°N), and ‘upwelling’ (5°–20°N). The lowest 234Th-derived POC export fluxes were found in the oligotrophic gyres and ranged from 0 in the northern to 6 mmol C m?2 d?1 in the southern oligotrophic, indicating a tightly coupled food web. Enhanced POC export was associated with the equatorial region (25 mmol C m?2 d?1) and the upwelling region north of the equator (15 mmol C m?2 d?1). POC export in the temperate regions ranged from 7 mmol C m?2 d?1 to a maximum of 41 mmol C m?2 d?1. High fluxes at the poleward edges of the oligotrophic gyres probably result from episodic nutrient-loading processes associated with submesoscale features. Estimates of instantaneous primary production (PP) were compared with 234Th-derived POC export, the latter bearing information from the past few weeks. Most export efficiencies that were calculated based on this comparison were high (10 s%), suggesting uncoupling of PP and export estimates due to the different time scales of the approaches. Moreover, this uncoupling points to the occurrence of pulsed high-export events that could be easily missed by instantaneous sampling but traced by temporally quasi-integrating tracers such as 234Th. Results from this study suggest that although carbon export in the oligotrophic centres of the gyres may be low, carbon sequestration in the temperate fringes of the gyres as well as in the equatorial and upwelling regions can be substantial, and that spatio–temporal variability in these areas of the world's oceans needs to be considered more fully in the context of global oceanic carbon sequestration. <br/

High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth

Microbes control the decomposition of organic matter in marine sediments. Decomposition, in turn, contributes to oceanic nutrient regeneration and influences the preservation of organic carbon(1). Generally, rates of benthic decomposition decline with increasing water depth, although given the vast extent of the abyss, deep-sea sediments are quantitatively important for the global carbon cycle(2,3). However, the deepest regions of the ocean have remained virtually unexplored(4). Here, we present observations of microbial activity in sediments at Challenger Deep in the Mariana Trench in the central west Pacific, which at almost 11,000 m depth represents the deepest oceanic site on Earth. We used an autonomous micro-profiling system to assess benthic oxygen consumption rates. We show that although the presence of macrofauna is restricted at Challenger Deep, rates of biological consumption of oxygen are high, exceeding rates at a nearby 6,000-m-deep site by a factor of two. Consistently, analyses of sediments collected from the two sites reveal higher concentrations of microbial cells at Challenger Deep. Furthermore, analyses of sediment Pb-210 profiles reveal relatively high sediment deposition in the trench. We conclude that the elevated deposition of organic matter at Challenger Deep maintains intensified microbial activity at the extreme pressures that characterize this environment

Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of 210Pb fluxes, calculated from inventories of sedimentary excess 210Pb, with 210Pb input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers ≥0.23, a fractional seamount height of ~0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived 210Pb fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived 210Pb flux is lower than the water-column-derived 210Pb flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years (~5 210Pb half lives) northward currents prevailed within the lowest ~1000 m of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean