ALBERT

Description: The discovery that foraminifera are able to use nitrate instead of oxygen as energy source for their metabolism has challenged our understanding of nitrogen cycling in the ocean. It was evident before that only prokaryotes and fungi are able to denitrify. Rate estimates of foraminiferal denitrification were very sparse on a regional scale. Here, we present estimates of benthic foraminiferal denitrification rates from six stations at intermediate water depths in and below the Peruvian oxygen minimum zone (OMZ). Foraminiferal denitrification rates were calculated from abundance and assemblage composition of the total living fauna in both, surface and subsurface sediments, as well as from individual species specific denitrification rates. A comparison with total benthic denitrification rates as inferred by biogeochemical models revealed that benthic foraminifera account for the total denitrification on the shelf between 80 and 250 m water depth. They are still important denitrifiers in the centre of the OMZ around 320 m (29–56% of the benthic denitrification) but play only a minor role at the lower OMZ boundary and below the OMZ between 465 and 700 m (3–7% of total benthic denitrification). Furthermore, foraminiferal denitrification was compared to the total benthic nitrate loss measured during benthic chamber experiments. Foraminiferal denitrification contributes 1 to 50% to the total nitrate loss across a depth transect from 80 to 700 m, respectively. Flux rate estimates ranged from 0.01 to 1.3 mmol m−2 d−1. Furthermore we show that the amount of nitrate stored in living benthic foraminifera (3 to 705 µmol L−1) can be higher by three orders of magnitude as compared to the ambient pore waters in near surface sediments sustaining an important nitrate reservoir in Peruvian OMZ sediments. The substantial contribution of foraminiferal nitrate respiration to total benthic nitrate loss at the Peruvian margin, which is one of the main nitrate sink regions in the world oceans, underpins the importance of previously underestimated role of benthic foraminifera in global biochemical cycles.

Description: A simple prognostic tool for gas hydrate (GH) quantification in marine sediments is presented based on a diagenetic transport-reaction model approach. One of the most crucial factors for the application of diagenetic models is the accurate formulation of microbial degradation rates of particulate organic carbon (POC) and the coupled biogenic CH4 formation. Wallmann et al. (2006) suggested a kinetic formulation considering the ageing effects of POC and accumulation of reaction products (CH4, CO2) in the pore water. This model is applied to data sets of several ODP sites in order to test its general validity. Based on a thorough parameter analysis considering a wide range of environmental conditions, the POC accumulation rate (POCar in g/cm2/yr) and the thickness of the gas hydrate stability zone (GHSZ in m) were identified as the most important and independent controls for biogenic GH formation. Hence, depth-integrated GH inventories in marine sediments (GHI in g of CH4 per cm2 seafloor area) can be estimated as: GHI = a · POCar · GHSZb · exp (−GHSZc/POCar/d) + e with a = 0.00214, b = 1.234, c = −3.339, d = 0.3148, e = −10.265. Several tests indicate that the transfer function gives a realistic approximation of the minimum potential GH inventory of low gas flux (LGF) systems. The overall advantage of the presented function is its simplicity compared to complex numerical models: only two easily accessible parameters are needed.

Description: Testing order to assess their potential as a proxy for redox conditions the element/Ca ratios of the redox sensitive elements Mn and Fe were determined in tests of benthic foraminifera from the Peruvian oxygen minimum zone (OMZ). Prior to the determination of the element/Ca ratios the distributions of Ca, Mn, Fe, Mg, Ba, Al, Si, P and S in tests of the shallow infaunal species Uvigerina peregrina and Bolivina spissa were mapped with an electron microprobe (EMP). An Fe rich phase which is also enriched in Al, Si, P and S was found on the inner test surface of U. peregrina. The element distributions of a specimen treated with an oxidative cleaning procedure show the absence of this phase. EMP maps of B. spissa also identified a similar phase which too could be removed with oxidative cleaning. Neither in B. spissa nor in U. peregrina were any hints for diagenetic (oxyhydr)oxide or carbonate coatings found. Mn/Ca and Fe/Ca ratios of single specimens of B. spissa from different locations have been determined by secondary ion mass spectrometry (SIMS). Bulk analyses using solution ICP-MS of several samples were compared to the SIMS data. The difference between SIMS analyses on single specimens and ICP-MS bulk analyses from the same sampling sites was 14.0–134.8 μmol mol−1 for the Fe/Ca and 1.68 μmol mol−1 for the Mn/Ca ratios. This amounts to 3–29 % for the Fe/Ca and 21.5 % for the Mn/Ca ratios of the overall variability between the samples of the different sampling sites. The Mn/Ca ratios in the calcite were generally relatively low (2.21–9.93 μmol mol−1) but of the same magnitude as in the pore waters (1.37–6.67 μmol mol−1). Comparison with sediment pore water data showed that Mn/Ca in the foraminiferal calcite is proportional to the Mn/Ca ratio in the top cm of the pore water. The lowest Fe/Ca ratio in tests of B. spissa (87.0 μmol mol−1) has been found at a sampling site which was strongly depleted in oxygen and showed a high, sharp iron peak in the top interval of the pore water. This, and the fact that at this location many dead but no living specimens were found during sampling time, hints that the specimens already were dead before the Fe flux started and the sampling site just recently turned anoxic due to fluctuations of the lower boundary of the OMZ where the sampling site is located (465 m water depth).

Description: Subduction of the oceanic Cocos plate offshore Costa Rica causes strong advection of methane-charged fluids. Presented here are the first direct measurements of microbial anaerobic oxidation of methane (AOM) and sulfate reduction (SR) rates in sediments from the two mounds, applying radiotracer techniques in combination with numerical modeling. In addition, analysis of carbonate δ18O, δ13C, and 87Sr / 86Sr signatures constrain the origin of the carbonate-precipitating fluid. Average rates of microbial activities showed differences with a factor of 4.8 to 6.3 between Mound 11 [AOM 140.71 (±40.84 SD); SR 117.25 (±82.06 SD) mmol m−2 d−1, respectively] and Mound 12 [AOM 22.37 (±0.85 SD); SR 23.99 (±5.79 SD) mmol m−2 d−1, respectively]. Modeling results yielded flow velocities of 50 cm a−1 at Mound 11 and 8–15 cm a−1 at Mound 12. Analysis of oxygen and carbon isotope variations of authigenic carbonates from the two locations revealed higher values for Mound 11 (δ18O: 4.7 to 5.9‰, δ13C: −21.0 to −29.6‰), compared to Mound 12 (δ18O: 4.1 to 4.5‰, δ13C: −45.7 to −48.9‰). Analysis of carbonates 87Sr / 86Sr indicated temporal changes of deep-source fluid admixture at Mound 12. The present study is in accordance with previous work supporting considerable differences of methane flux between the two Mounds. It also strengthens the hypothesis of a predominantly deep fluid source for Mound 11 versus a rather shallow source of biogenic methane for Mound 12. The results demonstrate that methane-driven microbial activity is a valid ground truthing tool for geophysical measurements of fluid advection and constraining of recent methane fluxes in the study area. The study further shows that the combination of microbial rate measurements, numerical modeling, and authigenic carbonate analysis provide a suitable approach to constrain temporal and spatial variations of methane charged fluid flow at the Pacific Costa Rican margin.

Description: Carbon cycling in Peruvian margin sediments (11° S and 12° S) was examined at 16 stations from 74 m on the inner shelf down to 1024 m water depth by means of in situ flux measurements, sedimentary geochemistry and modeling. Bottom water oxygen was below detection limit down to ca. 400 m and increased to 53 μM at the deepest station. Sediment accumulation rates and benthic dissolved inorganic carbon fluxes decreased rapidly with water depth. Particulate organic carbon (POC) content was lowest on the inner shelf and at the deep oxygenated stations (〈 5%) and highest between 200 and 400 m in the oxygen minimum zone (OMZ, 15–20%). The organic carbon burial efficiency (CBE) was unexpectedly low on the inner shelf (〈 20%) when compared to a global database, for reasons which may be linked to the frequent ventilation of the shelf by oceanographic anomalies. CBE at the deeper oxygenated sites was much higher than expected (max. 81%). Elsewhere, CBEs were mostly above the range expected for sediments underlying normal oxic bottom waters, with an average of 51 and 58% for the 11° S and 12° S transects, respectively. Organic carbon rain rates calculated from the benthic fluxes alluded to a very efficient mineralization of organic matter in the water column, with a Martin curve exponent typical of normal oxic waters (0.88 ± 0.09). Yet, mean POC burial rates were 2–5 times higher than the global average for continental margins. The observations at the Peruvian margin suggest that a lack of oxygen does not affect the degradation of organic matter in the water column but promotes the preservation of organic matter in marine sediments.

Description: The accumulation of gas hydrates in marine sediments is essentially controlled by the accumulation of particulate organic carbon (POCar) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Marquardt et al. (2010) developed a transfer function to predict the gas hydrate inventory in diffusion-controlled geological systems based on POCar and GHSZ. We present a new parameterization of this function and apply it to global datasets of bathymetry, heat flow, seafloor temperature and organic carbon accumulation estimating a global mass of only 91 Gt of carbon (GtC) stored in marine methane hydrates. Seepage of methane-rich fluids is known to have a pronounced effect on gas hydrate accumulation. Therefore, we carried out a set of systematic model runs with the transport-reaction code in order to derive an extended transfer function explicitly considering upward fluid advection. Using averaged fluid velocities for active and passive margins, which were derived from mass balance considerations, this extended transfer function predicts the formation of gas hydrates along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of 400–1100 GtC. Overall, our systematic approach allows to clearly and quantitatively distinguish between the effect of biogenic methane generation from POC and fluid advection on the accumulation of gas hydrate and hence, provides a simple prognostic tool for the estimation of large-scale and global gas hydrate inventories in marine sediments.

Description: Large amounts of methane are delivered by fluids through the erosive forearc of the convergent margin offshore Costa Rica and lead to the formation of cold seeps at the sediment surface. Besides mud extrusion, numerous cold seeps are created by landslides induced by seamount subduction or fluid migration along major faults. Most of the dissolved methane reaching the seafloor at cold seeps is oxidized within the benthic microbial methane filter by anaerobic oxidation of methane (AOM). Measurements of AOM and sulfate reduction as well as numerical modeling of porewater profiles revealed a highly active and efficient benthic methane filter at Quepos Slide site; a landslide on the continental slope between the Nicoya and Osa Peninsula. Integrated areal rates of AOM ranged from 12.9 ± 6.0 to 45.2 ± 11.5 mmol m-2 d-1, with only 1 to 2.5% of the upward methane flux being released into the water column. Additionally, two parallel sediment cores from Quepos Slide were used for in vitro experiments in a recently developed Sediment-F low-Through (SLOT) system to simulate an increased fluid and methane flux from the bottom of the sediment core. The benthic methane filter revealed a high adaptability whereby the methane oxidation efficiency responded to the increased fluid flow within 150–170 days. To our knowledge, this study provides the first estimation of the natural biogeochemical response of seep sediments to changes in fluid flow.

Description: Oxygen minimum zones (OMZs) that impinge on continental margins favor the release of phosphorus (P) from the sediments to the water column, enhancing primary productivity and the maintenance or expansion of low-oxygen waters. A comprehensive field program in the Peruvian OMZ was undertaken to identify the sources of benthic P, including the analysis of particles from the water column, surface sediments and pore fluids as well as in situ benthic flux measurements. A major fraction of solid phase P was bound as particulate inorganic P (PIP) both in the water column and in sediments. Sedimentary PIP increased with depth in the sediment at the expense of particulate organic P (POP). The ratio of particulate organic carbon (POC) to POP exceeded the Redfield Ratio both in the water column (202 ± 29) and in surface sediments (303 ± 77). However, the POC to total particulate P (TPP = POP + PIP) ratio was close to Redfield in the water column (103 ± 9) and in sediment samples (102 ± 15) taken from the core of the OMZ. This observation suggests that the burial efficiencies of POC and TPP are similar under the low oxygen conditions prevailing in the Peruvian OMZ. Benthic fluxes of dissolved P were extremely high (up to 1.04 ± 0.31 mmol m−2 d−1) and exceeded the fluxes resulting from the degradation of particulate organic matter raining to the seabed. Most of the excess P may have been released by bacterial mats that had stored P during previous periods when bottom waters were less reducing. At one station located at the lower rim of the OMZ, dissolved P was taken up by the sediments indicating recent phosphorite formation.

Description: The accumulation of gas hydrates in marine sediments is essentially controlled by the accumulation of particulate organic carbon (POC) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, the sedimentation rate (SR) that controls the time that POC and the generated methane stays within the GHSZ, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Wallmann et al. (2012) presented transfer functions to predict the gas hydrate inventory in diffusion-controlled geological systems based on SR, POC and GHSZ thickness for two different scenarios: normal and full compacting sediments. We apply these functions to global data sets of bathymetry, heat flow, seafloor temperature, POC input and SR, estimating a global mass of carbon stored in marine methane hydrates from 3 to 455 Gt of carbon (GtC) depending on the sedimentation and compaction conditions. The global sediment volume of the GHSZ in continental margins is estimated to be 60–67 × 1015 m3, with a total of 7 × 1015 m3 of pore volume (available for GH accumulation). However, seepage of methane-rich fluids is known to have a pronounced effect on gas hydrate accumulation. Therefore, we carried out a set of systematic model runs with the transport-reaction code in order to derive an extended transfer function explicitly considering upward fluid advection. Using averaged fluid velocities for active margins, which were derived from mass balance considerations, this extended transfer function predicts the enhanced gas hydrate accumulation along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of ~ 550 GtC. Overall, our systematic approach allows to clearly and quantitatively distinguish between the effect of biogenic methane generation from POC and fluid advection on the accumulation of gas hydrate, and hence, provides a simple prognostic tool for the estimation of large-scale and global gas hydrate inventories in marine sediments.