Sediments Off Research Articles

Distribution patterns of particle-reactive radionuclides in sediments off eastern Hainan Island, China: Implications for source and transport pathways

The study of sediment sources and transport processes from land to ocean can help in predicting the fate of the pollutants released from land or the potential change in sediment delivery to coastal areas and/or open oceans. The activities of 7Be, excess 210Pb (210Pbxs), excess 234Th (234Thxs) and 137Cs in surface sediments collected offshore of eastern Hainan Island, China, in August of 2008 were measured by an HPGe γ-spectrometer to evaluate the sediment source and transport processes. The results showed that all the surface sediments were silt or sand, and the mean grain sizes of the northern locations were higher than those in the other regions. The ranges of activities of 7Be, 210Pbxs, 234Thxs and 137Cs in surface sediment were 0.14–12.7, 37.4–199, 2.24–176 and 0.02–1.06Bqkg−1, with averages of 3.78±4.77, 110±8.1, 66.7±8.9 and 0.52±0.22Bqkg−1, respectively. The activities of the radionuclides increased from coast to offshore in the northern section. The upwelling may cause high particle fluxes with high activities of 210Pbxs and 234Thxs. A comparison of the source and transport of the suspended sediments with river discharge along the coast shows that the coastal current and offshore upwelling are the dominant factors for the transport and sources of surface sediment in the study region. The sediment was transported from south to north by the coastal current, and sediments with a large grain size may be deposited via the north loop current. The ratios of the nuclide activities indicated that the suspended particles need approximately one year to be removed from the water column into the seabed and that the main source of the sediments off eastern Hainan Island in the study regions was terrigenous deposits.

Persistent organic pollutants in coastal sediment off South China in relation to the importance of anthropogenic inputs

Surface sediments collected from the coastal region off South China were analyzed for persistent organic pollutants (POPs), including polybrominated diphenyl ethers (PBDEs), polycyclic aromatic hydrocarbons (PAHs), and organochlorine pesticides (OCPs). The concentrations of BDE-209, Σ(12)PBDE, Σ(15)PAH, Σ(7)PAH, and Σ(11)OCP were 0.22 to 26.3, 0.01 to 0.77, 13.9 to 271, 6 to 133, and 0.9 to 104 ng/g, respectively. The spatial distribution patterns of PBDEs and PAHs suggested that the eastern coastal region was slightly more contaminated than the western coast. In addition, the concentrations of dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDXs) were highly variable, with the highest level found at a site in Zhanjiang Bay, which might have been reflective of the impact of antifouling paints mainly used in boat maintenance in harbor areas. The predominance of BDE-209 in the study region was consistent with the usage pattern of penta-, octa-, and deca-BDEs in China, whereas sediment PAHs appeared to have been derived largely from coal or wood and petroleum combustion. Preliminary assessments indicated that terrestrial inputs, such as atmospheric transport and riverine runoff, may have been the major input pathways for PBDEs and PAHs, respectively, to accumulate in coastal sediment off South China. Conversely, residues of DDT-containing antifouling paints associated with shipping activities and boat maintenance accounted for most of the accumulated sediment DDTs.

Geotechnical Aspects of Clayey Sediments Off Badagara on the Kerala Coast, India

Offshore geotechnical surveys form part of an integrated investigation to rejuvenate a decrepit minor port at Badagara, Kerala on the southwestern coast of India. The sediments typify a fluvio-marine milieu ranging from silty clay, sand, silty sand, sandy silt and clayey silt. Geotechnical and sedimentological studies of shallow cores reveal the geotechnical aspects besides the depositional history of the sediments. Downcore geotechnical variations and regressive coefficients based on their inter-relationships highlight diverse factorial inferences. X-Ray Diffraction data indicate the prominent clay type. A comparative evaluation of the geotechnical characteristics of clayey sediments off Badagara, with similar studies along various sectors of the Kerala coast, both on land as well as in the near shore, is broadly attempted. Geotechnical studies carried out earlier on the uplifted Cochin marine clays provide comparative data for evaluating the possible variations between present day marine clayey sediments occurring along the Kerala coast and uplifted marine clays which, besides their gross variations in levels with respect to the present sea-level, also obviously relate to a much older depositional environment and provenance during probable Holocene times.

The meridional temperature gradient in the eastern North Atlantic during MIS 11 and its link to the ocean–atmosphere system

Temporal and spatial patterns in eastern North Atlantic sea-surface temperatures (SST) were reconstructed for marine isotope stage (MIS) 11c using a submeridional transect of five sediment cores. The SST reconstructions are based on planktic foraminiferal abundances and alkenone indices, and are supported by benthic and planktic stable isotope measurements, as well as by ice-rafted debris content in polar and middle latitudes. Additionally, the larger-scale dynamics of the precipitation regime over northern Africa and the western Mediterranean region was evaluated from iron concentrations in marine sediments off NW Africa and planktic δ13C in combination with analysis of planktic foraminiferal abundances down to the species level in the Mediterranean Sea. Compared to the modern situation, it is revealed that during entire MIS 11c sensu stricto (ss), i.e., between 420 and 398ka according to our age models, a cold SST anomaly in the Nordic seas co-existed with a warm SST anomaly in the middle latitudes and the subtropics, resulting in steeper meridional SST gradients than during the Holocene. Such a SST pattern correlates well with a prevalence of a negative mode of the modern North Atlantic Oscillation. We suggest that our scenario might partly explain the longer duration of wet conditions in the northern Africa during MIS 11c compared to the Holocene.

Effects of estuarine and fluvial processes on sediment transport over deltaic tidal flats

Tidal flats at a river mouth feature estuarine and fluvial processes that distinguish them from tidal flats without river discharge. We combine field observations and a numerical model to investigate hydrodynamics and sediment transport on deltaic tidal flats at the mouth of the Skagit River, in Puget Sound, WA, during the spring freshet. River discharge over tidal flats supplies a mean volume flux, freshwater buoyancy, and suspended sediment. Despite the shallow water depths, strong horizontal density fronts and stratification develop, resulting in a baroclinic pressure gradient and tidal variability in stratification that favor flood-directed bottom stresses. In addition to these estuarine processes, the river discharge during periods of low tide drains through a network of distributary channels on the exposed tidal flats, with strongly ebb-directed stresses. The net sediment transport depends on the balance between estuarine and fluvial processes, and is modulated on a spring-neap time scale by the tides of Puget Sound. We find that the baroclinic pressure gradient and periodic stratification enhance trapping of sediment delivered by the river on the tidal flats, particularly during neap tides, and that sediment trapping also depends on settling and scour lags, particularly for finer particles. The primary means of moving sediment off of the tidal flats are the high velocities and stresses in the distributary channels during late stages of ebbs and around low tides, with sediment export predominantly occurring during spring low tides that expose a greater portion of the flats. The 3-d finite volume numerical model was evaluated against observations and had good skill overall, particularly for velocity and salinity. The model performed poorly at simulating the shallow flows around low tides as the flats drained and river discharge was confined to distributary channels, due in part to limitations in grid resolution, seabed sediment and bathymetric data, and the wetting-and-drying scheme. Consequently, the model predicted greater sediment retention on the flats than was observed.

Variability in the marine radiocarbon reservoir effect in Muscat (Sultanate of Oman) during the 4th millennium BC: reflection of taphonomy or environment?

Radiocarbon (14C) dating of archaeological coastal sites in arid areas is challenging because of the relative rarity of datable terrestrial artefacts. Marine artefacts provide an abundant source of carbon, which is technically easy to date but radiocarbon dates need to be corrected for local variations from the global average marine 14C reservoir age (ΔR). In archaeological sites, this is usually done by the comparison of the 14C age measured between coeval pairs of terrestrial and marine samples that have no inbuilt age. Here, we present the results of a study that aims to determine the local 14C marine reservoir effect (MRE) along the Eastern coast of the Arabic Peninsula during the 4th millennium BC, a period of important cultural, demographic and social changes in this region. Previous estimates of the MRE based on associated shells and charcoal vary by about 400 14C yr and this is a serious limitation to the establishment of a precise chronology in the area based on marine material. In order to maximize the chances to obtain contemporaneous terrestrial–marine pairs for a new assessment of the MRE in this region we dated charcoal, shells, fish and turtle bones as well as human bones found in three recently and carefully excavated graves from Ra’s al-Hamra RH5. A large variability was found between charcoals (up to 240 14C yr) and marine specimens (up to 785 14C yr) present in the same grave. This variability was attributed to inbuilt ages, diagenesis and possibly environmental factors although taphonomic processes cannot be excluded. We discarded the oldest charcoals and used the 14C ages obtained on human apatite as a cut-off value to remove the marine outliers. Two out of the three graves provided terrestrial and marine assemblages which did not differ statistically and were used to calculate an average MRE of 645 ± 40 14C yr (ΔR = 255 ± 55 14C yr). This value is consistent with previous estimates based on the 14C dating of marine sediments off Pakistan and suggests stability of the MRE in the Arabian Sea since the 4th millennium BC.

Identification and definition of primary and reworked tephra in Late Glacial and Holocene marine shelf sediments off North Iceland

AbstractNine tephra layers in marine sediment cores (MD99‐2271 and MD99‐2275) from the North Icelandic shelf, spanning the Late Glacial and the Holocene, have been investigated to evaluate the effectiveness of methods to detect tephra layers in marine environments, to pinpoint the stratigraphic level of the time signal the tephra layers provide, and to discriminate between primary and reworked tephra layers in a marine environment. These nine tephra layers are the Borrobol‐like tephra, Vedde Ash, Askja S tephra, Saksunarvatn ash, and Hekla 5, Hekla 4, Hekla 3, Hekla 1104 and V1477 tephras. The methods used were visual inspection, magnetic susceptibility, X‐ray photography, mineralogical counts, grain size and morphological measurements, and microprobe analysis. The results demonstrate that grain size measurements and mineralogical counts are the most effective methods to detect tephra layers in this environment, revealing all nine tephra layers in question. Definition of the tephra layers revealed a 2–3 cm diffuse upper boundary in eight of the nine tephra layers and 2–3 cm diffuse lower boundary in two tephra layers. Using a multi‐parameter approach the stratigraphic position of a tephra layer was determined where the rate of change of the parameters tested was the greatest compared with background values below the tephra. The first attempt to use grain morphology to distinguish between primary and reworked tephra in a marine environment suggests that this method can be effective in verifying whether a tephra layer is primary or reworked. Morphological measurements and microprobe analyses in combination with other methods can be used to identify primary tephra layers securely. The study shows that there is a need to apply a combination of methods to detect, define (the time signal) and discriminate between primary and reworked tephra in marine environments. Copyright © 2011 John Wiley & Sons, Ltd.

Evaluation of sediment trace metal records as paleoproductivity and paleoxygenation proxies in the upwelling center off Concepción, Chile (36°S)

This study analyzes the records of several trace metals sensitive to redox conditions in continental shelf sediments off Concepción, Chile (36°S). The continental margin off Concepción (36°S; 73°W) lies beneath an important upwelling center characterized by high primary production rates and, consequently, high fluxes of organic matter. In spring and summer, this material settles to the seafloor where it decays, producing periods of very low oxygen content in bottom waters (<1 mL L −1). In addition, an oxygen minimum zone develops at ∼100–400 m water depth, where dissolved oxygen levels are <0.5 mL L −1. This situation changes during strong El Niño events, when dissolved oxygen at the bottom increases drastically (>1 mL L −1). The goals of this study were to determine the input of trace metals to the sediment and to decipher how this information can be used to reveal variations in primary productivity or bottom oxygenation. Gravity cores collected at two stations – VG06-2 over the mid-shelf station (88 m water depth, upper boundary of the oxygen minimum zone) and VG06-3 over the outer shelf (120 m water depth, within the oxygen minimum zone) – were sampled for high resolution profiles (1 cm) of trace metals, biogenic opal, stable isotopes, and total organic carbon. The results suggest that the variability in the trace metal distribution on the continental shelf off Concepción is determined by redox conditions and the organic carbon flux to the bottom. Some sections of the sediment cores from the outer shelf showed appreciable authigenic enrichment of U, Cd, and Mo (EF: 5–10, 2–5, and 10–16 respectively) along with heavier values of δ 15N, suggesting periods of suboxic conditions. During these periods, fluxes of organic material to the bottom were higher, as indicated by elevated TOC and opal contents. Alternating periods of higher and lower trace metal contents were not observed mid-shelf as they were on the outer shelf. Rather, the mid-shelf samples showed authigenic enrichment of U, Cd, and Mo (EF: 1–6, 4–5, and 10–20, respectively) throughout the core except in a 10-cm-thick gray layer. In general, authigenic enrichment of U, Mo, and Cd occurred at both sites, coincident with olive green layers in the cores. These layers were associated with periods of elevated primary productivity and suboxic conditions. Such periods did not seem to last as long as the oxygenated periods, which had higher inputs of refractive detrital material, coincident with the occurrence of distinct gray sediment layers.

Influence of Deglaciation on Microbial Communities in Marine Sediments Off the Coast of Svalbard, Arctic Circle

Increases in global temperatures have been shown to enhance glacier melting in the Arctic region. Here, we have evaluated the effects of meltwater runoff on the microbial communities of coastal marine sediment located along a transect of Temelfjorden, in Svalbard. As close to the glacier front, the sediment properties were clearly influenced by deglaciation. Denaturing gradient gel electrophoresis profiles showed that the sediment microbial communities of the stations of glacier front (stations 188-178) were distinguishable from that of outer fjord region (station 176). Canonical correspondence analysis indicated that total carbon and calcium carbonate in sediment and chlorophyll a in bottom water were key factors driving the change of microbial communities. Analysis of 16S rRNA gene clone libraries suggested that microbial diversity was higher within the glacier-proximal zone (station 188) directly affected by the runoffs than in the outer fjord region. While the crenarchaeotal group I.1a dominated at station 176 (62%), Marine Benthic Group-B and other Crenarchaeota groups were proportionally abundant. With regard to the bacterial community, alpha-Proteobacteria and Flavobacteria lineages prevailed (60%) at station 188, whereas delta-Proteobacteria (largely sulfate-reducers) predominated (32%) at station 176. Considering no clone sequences related to sulfate-reducers, station 188 may be more oxic compared to station 176. The distance-wise compositional variation in the microbial communities is attributable to their adaptations to the sediment environments which are differentially affected by melting glaciers.

Studies on the variations of heavy metals in the marine sediments off Kalpakkam, East Coast of India

During the last two decades, the coastal environment of southeast India has experienced intense developments in industry, urbanization and aquaculture. Moreover, the 2004 mega tsunami has devastated this coast, thus affecting the coastal sediment characteristics. These two phenomena prompted a study to characterize the sediment, to understand the mechanisms influencing the distribution of heavy metals and to create baseline data for future impact assessment. Results showed that the coastal sediment was carpeted with a mosaic of sand and silty sand with a minor amount of clay. Heavy metal values showed maximum variation for Fe and minimum for Cd. Their average values showed the following decreasing trend: Fe > Cu > Zn > Pb > Cr > Ni > Cd. This study shows that the major source of metals at Kalpakkam coast are land-based anthropogenic ones, such as, discharge from industrial waste, agricultural waste, urban, municipal and slum sewage into the Buckingham canal, which in turn discharges into the sea through backwaters, particularly during northeast monsoon period. A clear signature of the role of backwater discharge increasing the concentration of a few metals in the coastal sediments during monsoon period was observed. Assessments of the degree of pollution, concentration factor (CF), geoaccumulation index (Igeo) and pollution load index (PLI) have been calculated. CF values and Igeo indicated that the coastal sediment is moderately polluted by Cu and Cd. Increase in Cu, Pb and Zn concentration during the monsoon period (October–January) compared to the rest of the year was noticed. Factor analysis and correlation among the heavy metals concluded that Cr, Ni, Cd and Fe are of crustal origin, whereas, Cu, Pb and Zn are from anthropogenic sources. Organic carbon content in the sediment increased during monsoon period, pointing to the role of land runoff and backwater discharge in enhancing its content. The study also elucidates the impact of the recent tsunami in depleting metal content in the coastal sediment as compared to the pre-tsunami period.

Biochemical characteristics of surface sediments on the eastern Weddell Sea continental shelf, Antarctica: is there any evidence of seasonal patterns?

Biochemical characteristics of seafloor sediment off Austasen in the southeastern Weddell Sea were assayed in samples recovered in the early autumn and late spring of 2000 and 2003, respectively. Sediment was separated in the grain-size fractions >200 μm and <200 μm to distinguish biochemical characteristics in the fraction available for benthic suspension feeders (<200 μm). In the bulk sediment, the lipid (LPD) and carbohydrate (CHO) contents were significantly different between seasons with higher LPD content in the early autumn and higher CHO content in the late spring. In the grain-size fractions <200 μm, the LPD and protein (PRT) contents were significantly higher in the early autumn meaning that in this season the fraction available for benthic suspension feeders presented higher nutritive value. The relatively higher CHO concentrations observed in each fraction in the late spring were attributed to refractory matter, whereas the higher PRT and LPD concentrations found during the early autumn were associated with planktonic material settled after the summer phytoplankton bloom. Our results suggest that there is seasonal variation in the composition of organic matter in the sediment, with better nutritive quality in the early autumn, especially in the grain-size fraction available for benthic suspension feeders. These variations also suggest that the benthic community exploits the fresh organic matter accumulated after the summer throughout the Antarctic dark months leaving the sediment almost exhaust of LPD and with higher CHO contents, presumably of refractory nature, at the onset of the seasonal phytoplankton bloom of the following year.

Cenozoic post-rift sedimentation off northwest Britain: Recording the detritus of episodic uplift on a passive continental margin

The Cenozoic sedimentary basins on the Atlantic margin of NW Britain contain a remarkable record of tectonically influenced post-breakup sedimentation. We have mapped the distribution and quantified the solid grain volume of four unconformity-bound successions in the region: the Eocene (~6–8 × 104 km3), Oligocene (~2 × 104 km3), MioceneLower Pliocene (~4–5 × 104 km3) and Lower PlioceneHolocene (~4–5 × 104 km3) complementing previous work on the Paleocene succession. Approximately 80% of the total Cenozoic sediment volume on the Atlantic margin of NW Britain was deposited in Eocene and later times. The relative volumes of the Cenozoic succession do not support previous claims that the Paleocene was the main period of Cenozoic uplift and erosion of sediment source areas. Rather, the Cenozoic sedimentary basins on the Atlantic margin of NW Britain record the detritus of four major episodes of Cenozoic uplift of the British Isles (Paleocene, Eocene–Oligocene, Miocene and Pliocene–Pleistocene).

Fine-Scale Volume Heterogeneity in a Mixed Sand/Mud Sediment off Fort Walton Beach, FL

As part of the effort to characterize the acoustic and physical properties of the seafloor during the high-frequency 2004 Sediment Acoustics Experiment (SAX04), fine-scale variability of sediment sound speed and density was measured in a medium quartz sand using diver cores and an in situ conductivity probe. This study has a goal of providing environmental input to high-frequency backscatter modeling efforts. Because the experiment was conducted immediately following exposure of the site to Hurricane Ivan, measurements revealed storm-generated sedimentary structure that included mud deposits and trapped sand pockets suspended in the mud. Fluctuations of sediment sound speed and density were measured downcore at 1- and 2-cm increments, respectively, with standard laboratory techniques. Sediment density was also measured on a very fine scale with an in situ conductivity probe [in situ measurement of porosity (IMP2)] and by means of computed tomography (CT) imaging of a diver core. Overlap between the locations of the diver cores and the conductivity probe measurements allowed an examination of multiple scales of sediment heterogeneity and a comparison of techniques. Sediment heterogeneity was characterized using estimates of covariance corresponding to an algebraic form for the power spectrum of fluctuations obtained from core, conductivity, and CT measurements. Correcting for sampling brings the power spectra for density fluctuations determined from the various measurements into agreement, and supports description of heterogeneity at the site over a wide range of scales by a power spectrum having a simple algebraic form.

Diversity of bathyal macrofauna on the northeastern Pacific margin: the influence of methane seeps and oxygen minimum zones

AbstractThe upper continental slope in the northeastern Pacific Ocean is intercepted by a deep oxygen minimum zone (OMZ; 650–1100 m) and punctuated by conduits of methane seepage. We examined the effects of these two dominant sources of heterogeneity on the density, composition and diversity of heterotrophic macrofauna off Hydrate Ridge, Oregon (OR; 800 m water depth), where the seeps co‐occur within an OMZ, and off the Eel River, Northern California (CA; 500 m), where seeps are overlain by better oxygenated waters. We hypothesized that seeps (containing clam beds and microbial mats) should contribute a suite of distinct species to the regional margin species pool but that OMZ‐associated hypoxia would dampen seep‐related heterogeneity. Macrofaunal densities were highest (23,000–33,510 ind·m−2) in the CA seep sediments and in the OR near‐seep samples, intermediate in the OR seep, CA near seep and CA and OR 500‐m margin sediments (10,054–19,777 ind·m−2), and lowest in the CA and OR OMZ habitats at 800 m (4269–7847 ind·m−2). Annelids constituted over 50% of the taxa in all but the CA clam bed and OR microbial mat sediments, where mollusks were abundant. Approximately 50% of seep species appeared to be habitat endemic; species present in microbial mats largely formed a subset of those present in the clam beds. Dorvilleid and ampharetid polychaetes were dominant in the seep sediments; non‐seep margin sediments at 500 and 800 m were populated heavily by branchiate polychaetes including cossurids and paraonids. Alpha diversity (Es[20] calculated per core) was lowest and rank 1 dominance was highest in the CA and OR microbial mat habitats. Pooled analyses of Es[100] revealed highest species richness in the CA clam bed and near‐seep habitats (30.3 and 29.6, respectively), and lowest species richness in the OR microbial mat and near‐seep habitats (16.5 and 17.9, respectively). Non‐seep sediments (500 and 800 m) off both CA and OR were more homogeneous (55–57% within‐habitat similarity) than clam bed and microbial mat sediments (only 32–37% within‐habitat similarity). CA sediment macrofauna generally exhibit higher alpha diversity, and as habitats are combined, a higher rate of increase in the slope of the species accumulation curves than do OR margin macrofauna. Methane seeps in the NE Pacific introduce significant heterogeneity that increases margin biodiversity at multiple spatial scales. However, our hypothesis that the OMZ would lessen the seep contributions to diversity was not supported. The better oxygenated CA seeps at 500 m shared more of the background margin fauna (at 500 m) than did the OR seeps at 800 m (with OMZ fauna at 800 m). Geographical differences in the fluxes of methane‐rich fluids and the increased reliance on chemosynthetic food sources with increased depth could explain these results.

Spatial Distribution of Δ14C Values of Organic Matter in Surface Sediments Off Saru River in Northern Japan, One Year After a Flood Event in 2006

Dispersion and deposition of terrestrial organic matter by flooding on the inner shelf were studied using C/N ratios, δ13C, and Δ14C values of sedimentary organic matter. Surface sediment samples (top 2 cm) were collected from coastal areas near the Saru River in southwestern Hokkaido, northern Japan, 1 yr after a flood event in 2006. Riverine suspended solids were also collected at a fixed station downstream during 2006–2008. Sandy sediments were located at the front of the river mouth and the western part of the sampling area, with the δ13C of organic matter ranging from −23.8‰ to −22.0‰, Δ14C of –655‰ to –388‰, and an organic carbon/total nitrogen (C/N) ratio of 5.9–7.7. On the other hand, silt and clay sediments were distributed in a restricted area 11–16 km from the river mouth, with lighter δ13C (–26.7‰ to −24.1‰) and higher Δ14C (–240‰ to –77‰) of organic matter and C/N ratio (7.8–13.3). From end-member analysis, the apparently younger and less degraded organic matter in the silt and clay sediments consists mainly of terrestrial organic matter released by flood events. They remain in the depression, although most flood deposits were moved to deep-sea environments.

A new species of the genus Paramesochra (Copepoda: Harpacticoida) from Korean waters

We describe a new species of the genus Paramesochra that was found in sandy sediments off Taean on the west coast of Korea during a study of interstitial harpacticoid copepods. Paramesochra taeana, sp. nov. superficially resembles P. acutata hawaiiensis Kunz, 1981, in the structure of the antenna, shape of the caudal rami, and seta formulae of thoracopods P1–P4. However, the new species was clearly distinguishable from P. acutata hawaiiensis by a combination of characteristics. With the exception of the last two somites, the new species is the only species in the genus that has well-developed hyaline frills on all of the body segments. The female displays eight-segmented antennules, and the median incision between both plates of its baseoendopod reaches about to the proximal line of the exopod in P5. Lastly, in males the baseoendopodal plate has a deeper median incision in P5. An updated key for species of the genus Paramesochra is provided.

Molecular evidence for anaerobic ammonium‐oxidizing (anammox) bacteria in continental shelf and slope sediments off northwest Africa

The presence and abundance of anaerobic ammonium‐oxidizing (anammox) bacteria was investigated in continental shelf and slope sediments (300‐3000 m water depth) off northwest Africa in a combined approach applying quantitative polymerase chain reaction (q‐PCR) analysis of anammox‐specific 16S rRNA genes and anammox‐specific ladderane biomarker lipids. We used the presence of an intact ladderane monoether lipid with a phosphocholine (PC) headgroup as a direct indicator for living anammox bacteria and compared it with the abundance of ladderane core lipids derived from both living and dead bacterial biomass. All investigated sediments contained ladderane lipids, both intact and core lipids, in agreement with the presence of anammoxspecific 16S rRNA gene copies, indicating that anammox occurs at all sites. Concentrations of ladderane core lipids in core top sediments varied between 0.3 and 97 ng g−1 sediment, with the highest concentrations detected at the sites located on the shelf at shallower water depths between 300 and 500 m. In contrast, the C20 [3]‐ ladderane monoether‐PC lipid was most abundant in a core top sediment from 1500 m water depth. Both anammox‐specific 16S rRNA gene copy numbers and the concentration of the C20 [3]‐ladderane monoether‐PC lipid increased downcore in sediments located at greater water depths, showing highest concentrations of 1.2 × 108 copies g−1 sediment and 30 pg g−1 sediment, respectively, at the deepest station of 3000 m water depth. This suggests that the relative abundance of anammox bacteria is higher in sediments at intermediate to deep water depths where carbon mineralization rates are lower but where anammox is probably more important than denitrification.

Characterization of organic matter in the Oligocene (Chattian) turbiditic fine grained deposits, offshore Angola

Variations in the abundance and gross composition of the organic matter (OM) content of fine grained turbiditic Chattian sediments off Angola were explored with Rock-Eval pyrolysis of 216 core samples from four wells. The study revealed that the OM in the sediments is immature but is present in sufficient abundance [>1% total organic carbon (TOC)] and with a good enough quality [hydrogen index (HI) around 185 mg hydrocarbon/g TOC on average] to constitute a potential source rock. A deeper insight into the composition of the OM was provided by the extraction and analysis of the kerogen from 15 samples. These samples were submitted to visual inspection and the counting of organic particles under the microscope (palynofacies analysis), elemental analysis and pyrolysis-gas chromatography–mass spectrometry (GC–MS). Although individually the various observations did not provide evidence clearly diagnostic of the origin of the OM, they confirm that it has a mixed marine–terrestrial origin, a notable part of the terrestrial contribution very probably originating from leaves. The good buoyancy of the OM and its richness in lipid components might explain the spreading of terrestrial OM over a large area and its relatively good oil potential.

Rapid reorganization in ocean biogeochemistry off Peru towards the end of the Little Ice Age

Abstract. Climate and ocean ecosystem variability has been well recognized during the twentieth century but it is unclear if modern ocean biogeochemistry is susceptible to the large, abrupt shifts that characterized the Late Quaternary. Time series from marine sediments off Peru show an abrupt centennial-scale biogeochemical regime shift in the early nineteenth century, of much greater magnitude and duration than present day multi-decadal variability. A rapid expansion of the subsurface nutrient-rich, oxygen-depleted waters resulted in the present-day higher biological productivity, including pelagic fish. The shift was likely driven by a northward migration of the Intertropical Convergence Zone and the South Pacific Subtropical High to their present day locations, coupled with a strengthening of Walker circulation, towards the end of the Little Ice Age. These findings reveal the potential for large reorganizations in tropical Pacific climate with immediate effects on ocean biogeochemical cycling and ecosystem structure.

Accumulation of prokaryotic remains during organic matter diagenesis in surface sediments off Peru

Bacterial biomarkers (D‐amino acids and muramic acid) were investigated in surface sediments (0–1 cm) at 22 stations in the Peru margin. Concentrations were used to quantitatively estimate the relative importance of peptidoglycan in the preservation of prokaryotic remains during diagenesis. The bacterial imprint in organic matter was also evident from low molar ratios (average of 1.6) of glucosamine and galactosamine. Estimates of the fraction of biomarkers associated with intact peptidoglycan showed that peptidoglycan became a progressively less important component of prokaryotic remains with ongoing diagenesis, whereas other prokaryotic biomolecules increased in importance. The exact nature of these biomolecules remains unknown, but constant ratios between individual D‐amino acids and amino sugars observed in this study provide novel information on their molecular composition. Examination of total hydrolyzable amino acids revealed systematic compositional changes with increasing water depth and age of the sediment, reflecting increased diagenetic alteration of the organic matter. The time scale integrated in the upper 1 cm of the sediment was ca. 2 yr at the shallow sites and up to 20 yr at the deeper stations. The alteration stages of organic matter ranged from coastal and ocean margin sediments at shallow water depth to organic matter as degraded as hemipelagic surface sediments at greater water depths.