Atlantic Abyssal Plain Research Articles

Miocene deep-sea benthic foraminifera from the Atlantic and Indian oceans: diversity patterns and palaeoceanography

During the early to middle Miocene of the Atlantic and Indian Oceans there were variations in species diversity of the lower bathyal to abyssal plain (3-4.4 km) benthic foraminifcral assemblages allhough, overall, diversity remained high and comparable with that of the modern environment. The early Miocene was a period of palaeoceanographic change and this has previously been documented both through studies of stable isotopes and the rate of appearances or disappearances of taxa (whether evolutionary or ecological). The diversity oscillations appear to represent shorter period change. The lowest diversities are associated with the peak abundance of bolivinids. Other variations cannot be correlated from one site to another and are thought to represent local environmental changes. During the early-middle Miocene, the diversity of the Atlantic abyssal plain was lower than that of the Indian and Pacific Oceans possibly indicating significant diversity differences between ocean basins. On the basis of limited data, it is possible to speculate that the species diversity of the Miocene was perhaps higher than that of today.

The structure and evolution of the lithosphere–asthenosphere boundary beneath the Atlantic–Mediterranean Transition Region

The present-day thermal and compositional 3D structure of the lithosphere beneath the Atlantic–Mediterranean Transition Region and the lithosphere–asthenosphere interaction from Jurassic times to present has been studied. The Atlantic–Mediterranean Transition Region comprises the western segment of the Africa–Eurasia plate boundary, encompassing two main large-scale tectonic domains: the Gibraltar Arc System and the Atlas Mountains. An integrated and self-consistent geophysical–petrological methodology ( LitMod3D) has been applied that combines elevation, gravity, geoid, surface heat flow, and seismic data and allows modelling of compositional heterogeneities within the lithospheric mantle. Our results reveal large variations in the depth of the Moho and the lithosphere–asthenosphere boundary (LAB) as well as a lack of spatial correlation between the thicknesses of these two boundaries. The Moho essentially mimics the topography with depths ranging from ∼ 10 km beneath the oceanic domains of the Atlantic abyssal plains and the Algerian Basin to > 34 km in the Eastern Betics and Rif, the High Atlas mountains, and the Sahara Platform. In contrast, the LAB is shallower beneath the central and eastern Alboran Basin (∼ 70 km) and all along the High, Middle and Anti Atlas (< 100 km) coinciding with the loci of Cenozoic volcanism. Deeper LAB depths are found along the central and western Betics and the Moroccan Atlantic margin (> 140 km) with values exceeding 230 km beneath the Rif and the Sahara Platform. The average bulk composition of the lithospheric mantle corresponds to that of a typical Tecton (i.e. Phanerozoic) domain, with the exceptions of the Sahara Platform, the Alboran Basin, and Atlas Mountains. Distinct mantle compositions are required in these areas to make model predictions and geophysical observables compatible. It is proposed that the highly irregular LAB topography is the result of the superposition of three different geodynamic processes: i) shortening and thickening related to NW–SE Iberia–Africa convergence lasting from Late Cretaceous to Recent, ii) impingement of a baby-like mantle plume or small-scale convection beneath the High-Middle Atlas and Anti Atlas commencing in the mid Eocene, and iii) slab roll-back or mantle delamination in the Betic–Rif–Alboran realm acting from early to late Miocene.

Diversity and large-scale biogeography of Paramesochridae (Copepoda, Harpacticoida) in South Atlantic Abyssal Plains and the deep Southern Ocean

Multicorer samples for meiofaunal study were obtained within the framework of the international project CeDAMar, at 21 stations occupied during the DIVA and ANDEEP campaigns (2000, 2002 and 2005) to the southern Atlantic Ocean (Guinea, Angola and Cape Basins) and the Southern Ocean (Weddell and Scotia Seas), respectively. A total of 311 adult Paramesochridae Lang, 1944 (Copepoda, Harpacticoida) was extracted from 437 cores obtained during 83 deployments of the corer at depths between 1107 and 5655 m. All specimens were determined to species based on morphological characteristics. They belonged to 19 species and four genera ( Kliopsyllus Kunz, 1962, Leptopsyllus T. Scott, 1894, Paramesochra T. Scott, 1892 and Scottopsyllus Kunz, 1962). Eleven species were restricted to single regions, whereas the others showed a much wider distribution. For example, Kliopsyllus andeep Veit-Köhler, 2004 and Kliopsyllus diva Veit-Köhler, 2005, were both collected from Guinea, Angola and Weddell Abyssal Plains, and Kliopsyllus schminkei Veit-Köhler & Drewes, 2009 occurred in the three West-African basins. This study provides a first insight into the large-scale biogeography of deep-sea harpacticoids, represented by the Paramesochridae, and indicates that distribution ranges, at least in this family, may extend across South Atlantic and Southern Ocean Abyssal Plains.

Comparison of the nematode fauna from the Weddell Sea Abyssal Plain with two North Atlantic abyssal sites

Abstract This study provides for the first time data on the nematode community composition from the Weddell Sea Abyssal Plain. In addition, new data on the nematode communities from the Cape Verde Abyssal Plain and the Porcupine Abyssal Plain are provided. Three stations along an east–west transect in the Weddell Sea were sampled during the ANDEEP II campaign and compared with two north-east Atlantic abyssal sites, characterized by a contrasting food supply, in terms of densities, community structure and biodiversity. Local total nematode densities on the Weddell Sea Abyssal Plain (station 138) rank among the highest values recorded from abyssal plains worldwide, pointing to a locally high availability of food for meiofauna. Minor differences in the proportion of dominant genera were observed between the Weddell Sea Abyssal Plain and both North Atlantic sites. The nematode communities at the Weddell Sea Abyssal Plain were dominated by Thalassomonhystera and Acantholaimus, which is comparable to the North Atlantic abyssal plains. No endemic genera for the Southern Ocean were found. However, the higher abundance of the genera Microlaimus and Dichromadora seems to be typical for the Southern Ocean deep-sea, and might be related to the availability of fresh food.

Detection of methane plumes in the water column of Logatchev hydrothermal vent field, Mid-Atlantic Ridge

During DY105-17 cruise onboard the R/V “Da Yang Yi Hao” in 2005, methane concentrations in the water column above Logachev hydrothermal vent field were measured by applying stripping/trapping-gas chromatographic (GC) and the distinct methane plumes were detected. Results show that the background methane concentration within the Logachev area is from 1.05 nmol/L to 1.68 nmol/L, significantly higher than the background level of the Atlantic abyssal plain of 0.4–0.5 nmol/L, suggesting that hydrothermal venting is a major source of dissolved methane to the ocean. The highest anomalies of methane concentrations in the water column range from 7.14 nmol/L to 113.9 nmol/L and occur just at 180–500 m above the seafloor. The distribution of methane concentration and the structural characteristics of hydrothermal plumes are strongly influenced by the supply of underlying hydrothermal fluids, the mixing process of ocean bottom currents and the microbial oxidation. Furthermore, the differences in distribution of methane plume between the station MAR-CTD3 and the other stations indicate a probable unknown hydrothermal vent site nearby. There occurs high concentration of methane along with temperature and nephelometry anomalies, which strongly confirms that the subtle measurement of methane concentration in water column is one of the effective ways to locate active sites of hydrothermal venting.

On the behaviour of deep-ocean sediments

The behaviour of deep-ocean sediments from the North Atlantic abyssal plains is examined within the context of a recognised framework, the Sensitivity Framework, which was developed by Cotecchia & Chandler for structured terrestrial clays. The deep-ocean sediments have very high sensitivities, of the order of those of quick clays, but in volumetric compression their destructuration occurs at more moderate rates, similar to those usually found in terrestrial clays of medium sensitivities. The deep-sea sediments were retrieved using different types of sampler, which influenced greatly the strength and stiffness of the different samples. In particular, in the lower-quality samples the greater disturbance caused by sampling is responsible for changing their behaviour from brittle to ductile during shearing after compression to stresses post-gross yield, giving them an apparent insensitivity to shear strains. Results from numerical analyses using a constitutive model that was developed for structured terrestrial clays are presented. They show that such a model, which accounts for natural structure, can be used to simulate the behaviour of deep-ocean sediments provided an appropriate destructuration law is used in the model.

On the behaviour of deep-ocean sediments

The behaviour of deep-ocean sediments from the North Atlantic abyssal plains is examined within the context of a recognised framework, the Sensitivity Framework, which was developed by Cotecchia &amp; Chandler for structured terrestrial clays. The deep-ocean sediments have very high sensitivities, of the order of those of quick clays, but in volumetric compression their destructuration occurs at more moderate rates, similar to those usually found in terrestrial clays of medium sensitivities. The deep-sea sediments were retrieved using different types of sampler, which influenced greatly the strength and stiffness of the different samples. In particular, in the lower-quality samples the greater disturbance caused by sampling is responsible for changing their behaviour from brittle to ductile during shearing after compression to stresses post-gross yield, giving them an apparent insensitivity to shear strains. Results from numerical analyses using a constitutive model that was developed for structured terrestrial clays are presented. They show that such a model, which accounts for natural structure, can be used to simulate the behaviour of deep-ocean sediments provided an appropriate destructuration law is used in the model.

Abundance of smectite and the location of a plate-boundary fault, Barbados accretionary prism

Isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics. The frontal decollement zone of the northern Barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy, fluid flow, chemical interactions, and sediment physical properties affect strain localization. This plate-boundary fault is centered at a lithologic contact between Miocene claystone and fine-grained Oligocene turbidites. The fault zone is nearly 40 m thick (at Ocean Drilling Program Site 948), and its upper part passes through smectite-rich deposits. A sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement. There is a consistent increase in percent smectite with distance above the base of the decollement, but the top of the decollement is poorly defined by clay mineralogy. The intrinsic mechanical weakness of strata with abundant smectite-group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the Atlantic abyssal plain. A second inherited parameter is the local abundance of radiolarians, which contribute to higher than normal porosities. Sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic. The principal cause of excess pore pressure seems to be updip fluid advection; in theory, however, decreases in pore-fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals. The imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions. If fresher pore water migrates to the propagating tip of the decollement, its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more. The location and evolution of the decollement, therefore, are controlled by a complicated interplay of static factors inherited from the abyssal Atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry.

Patterns in polychaete abundance and diversity from the Madeira Abyssal Plain, northeast Atlantic

Polychaete abundance and diversity patterns from the Madeira Abyssal Plain (MAP) were studied together with data from three other sites on the northeastern Atlantic abyssal plains. Polychaete abundance at MAP was significantly lower than at any of the other sites, including those lying under comparable productivity regimes. Analysis of diversity, using rarefaction and species counts per unit area, suggests that MAP is extremely species poor and shows dominance by a few common species. The MAP site is characterised by a superficial layer of turbidite sediment, and the hypothesis is put forward that the unusual sediment characteristics at MAP have affected macrofaunal abundance over a vast area (>2000 km 2). Analysis of species composition indicates that the MAP site is not faunistically unique; rather it contains a high proportion of widespread, abundant, cosmopolitan species. We suggest that these are the opportunists of the abyssal benthic habitat. Differences in abundance between the other abyssal sites are the result of both productivity and local environmental conditions. Equitability at the other north Atlantic sites is not affected by productivity, although the actual number of species per unit area is affected, showing a south–north gradient.

Early diagenesis in North Atlantic abyssal plain sediments characterized by rock-magnetic and geochemical indices

Processes of early diagenesis are investigated in a sequence of late Quaternary turbiditic sediments from the Madeira Abyssal Plain (MAP), west of the NW African continental margin. In three sediment cores from this region, solid-phase geochemical indices of redox-sensitive ion mobilization are compared with rock-magnetic parameters as proxies for diagenesis characterization. The mud-dominated, distal turbidites are emplaced mainly as a result of glacio-eustatic changes in sea-level, and derive both from different sites on the NW African margin, and from the flanks of the Canary Islands. Thus, the turbidites are of variable composition, especially in terms of carbonate, detrital magnetic mineral, and organic carbon content. Within organic-rich (>0.5% Corg) turbidite horizons, remineralization of organic detritus proceeds in two stages. The initial stage involves suboxic (reductive) diagenesis, following depletion of pore-water O2, while the later stage involves oxidative diagenesis associated with the slow descent of an oxidation front through the sediment, due to diffusion of O2 from the overlying seawater. Rock-magnetic parameters of these sediments, when expressed on a carbonate-free basis, reveal that significant depletion of detrital ferrimagnetic iron (Fe2+/Fe3+) oxide grains has occurred within the organic-rich turbidites during suboxic diagenesis. Normalized quotients of magnetic parameters, and remanence-coercivity profiles of selected samples, also show that reductive diagenesis is a ferrimagnetic grain size-selective process, but it has a minimal effect on the canted-antiferromagnetic (Fe3+) oxides in the sediment. Such components, if present, therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are progressively dissolved. In all three cores studied, however, there is clear evidence for the presence of ultrafine ferrimagnetic iron oxide grains at depth within the suboxic zones of organic-rich turbidites, beneath both active and fossil oxidation fronts. These grains are probably associated with populations of live magnetotactic bacteria which inhabit such organic-rich horizons, and which form a link in the chain of bacterially mediated reactions normally associated with suboxic diagenesis. These results comply with previous observations made in studies of suboxic diagenesis in hemipelagic sediments, and demonstrate the value of rapid and non-destructive rock-magnetic measurements for illuminating redoxomorphic processes of diagenesis involving transformation of iron phases in deep-sea sediments.

Deep-sea benthic foraminiferal species diversity in the NE Atlantic and NW Arabian sea: a synthesis

We present a synthesis of species diversity data (Fishers’ alpha index, Shannon–Wiener (log 2), ES(100), Rank 1 Dominance) for “live” (stained) foraminifera from five bathyal (1340 m depth) and abyssal (4450–4950 m depth) sites in the NE Atlantic and a 3400 m-deep site in the Arabian Sea. Three Atlantic sites (Porcupine Seabight, BIOTRANS, Porcupine Abyssal Plain) are subject to seasonal phytodetritus inputs that support low diversity populations (8–17 species). In other respects the foraminifera are highly diverse. The meiofaunal fractions (>45 or >63 μm; including fragmented and phytodetritus species) of abyssal Atlantic samples yielded >110 and >170 species in the 0–1 cm and 0–10 cm layers, respectively; the Arabian Sea sample (0–1 cm layer only) yielded 232 species. In both cases, values for diversity measures were very high. Diversity was rather lower in bathyal Porcupine Seabight samples (0–1 cm layer), which yielded <100 species. The foraminiferal macrofauna (>500 μm; Porcupine and Madeira Abyssal Plains) was also speciose (113–133 species), but diversity measures were lower and dominance higher than for the meiofauna. All assemblages contained numerous undescribed species, many belonging to poorly known monothalamous, soft-bodied taxa. Sample diversity was influenced by several factors. Combining phytodetrital and sediment populations reduced diversity and increased dominance slightly; the inclusion of deeper sediment layers and finer sieve fractions had the opposite effect. The inclusion of fragments had more impact on macrofaunal than on meiofaunal diversity, although in both cases the effect was inconsistent (either positive or negative). Porcupine and Madeira Abyssal Plain multicore samples (>63 μm fraction) contained substantially more foraminiferal species than nematode species; the numbers of foraminiferal species in boxcore samples (>500 μm fraction) were comparable to, or greater than, literature values for macrofaunal taxa such as polychaetes and isopods. Few of the more abundant species at our Atlantic abyssal plain and Arabian Sea sites are endemic. This is consistent with literature evidence that many common deep-sea foraminiferal species are cosmopolitan and implies that global foraminiferal diversity may be more modest than the high sample diversity might suggest. Calcareous foraminifera, which are well-known taxonomically and have a good fossil record, may provide a model for diversity patterns among the deep-sea benthic biota in general.

Large, deep-sea agglutinated Foraminifera; two differing kinds of organization and their possible ecological significance

We consider two groups of deep-sea agglutinated foraminifera: 1) robust, tubular astrorhizaceans (Bathysiphon, Rhabdammina) and hippocrepinaceans (Hyperammina, Saccorhiza) which reach several centimeters or more in length and contain a substantial protoplasmic mass (sarcode); and 2) delicate komokiaceans with tests typically 1-5 mm in size and forming a branching tubule system containing accumulations of stercomata and sparse protoplasm, sometimes with fine sediment incorporated between the tubules to form a mudball structure. The tubular taxa occur in eutrophic settings, usually on continental margins, where the food supply originates from strong surface primary production, currents, or downslope transport. We speculate that they are opportunists, able to grow fairly quickly when food is available. The sarcode may provide a buffer against periods of food shortage, enabling these taxa to take advantage of a relatively high but fluctuating food input. Komokiaceans, on the other hand, have a large volume/low biomass organization and are most important in oligotrophic, central oceanic regions. We speculate that the combination of relatively body size and low protoplasmic content may reflect slow growth and longevity; it may also serve to reduce the food content of komokiaceans and hence their attractiveness to predators. In order to illustrate these contrasting groups, we briefly consider faunas from the Oman margin (3400 m depth) and NE Atlantic abyssal plains (4850-4950 m depth), distinguishing between: 1) oligotrophic, central oceanic regions rich in komokiaceans (Madeira Abyssal Plain); 2) central oceanic regions with seasonal phytodetritus inputs where komokiaceans coexist with small (

Ocean Drilling Bordering the South American Continent

The Ocean Drilling Program (ODP) has completed three expeditions bordering South America and is in the planning stages of a further expedition. The first cruise, Leg 110, drilled at six sites on a transect across the structural boundary between the overthrusting Barbados Ridge Complex and Atlantic abyssal plain (Caribbean/Atlantic Plate Boundary) in order to investigate structural, hydrological, and diagenetic effects of the transition from undeformed deep sea sediments to stratally disrupted melange on land. The second cruise, Leg 112, drilled ten sites to investigate the geological and paleoceanographic history of the area between the Peru Trench (which marks the suture between the Nazca and South American Plates) and the Peruvian coast. A third cruise, Leg 1 41, drilled the Chile Triple Junction, which represents the only presently active ridge-crest subduction and the physical properties and geochemistry of gas hydrates in oceanic sediments. A fourth cruise off South America is presently in the planning stage by the international science community. This proposed paleoceanographic transect is on the Ceara Rise off northern Brazil and has a proposed objective, amongst others, of studying the history of deep water flow of the Atlantic during the Cenozoic with an emphasis on the relationships between deepmore » water circulation chemistry and the Earth's climate. This paper focuses on the significant scientific results of the above cruises and discusses future plans off South America.« less

Geochemistry and provenance of north atlantic abyssal plain sediments

Geochemistry and provenance of north atlantic abyssal plain sediments

Amino acids in the interstitial waters of ESOPE long cores from two North Atlantic abyssal plains.

We here report the results of dissolved free amino acid and hydrolyzable amino acid analysis for interstitial water samples collected from two ESOPE long cores (SNAP core 48 and GME core 10). Dissolved free amino acids (DFAA) in the ESOPE samples constitute, in most cases, less than 20% of the total hydrolyzable amino acids in the pore water. Their concentration varies rather systematically with depth in correlation with that of dissolved species such as nitrate, nitrite and manganese for SNAP core 48 and silicate and phosphate for GME core 10. In contrast, the concentration of total hydrolyzable amino acids (THAA) varies with depth by as much as two orders of magnitude. This may be because the organic matter from which the THAA are freed by hydrolysis is strongly associated with sediments and hence has poor mobility through interstitial water. The various individual amino acids show no systematic changes in relative abundance with depth which suggests that the bacteria which decompose organic matter in the deep-sea sedimentary environment do not significantly discriminate among the various amino acids.

Th-230 dating of -30m long piston cores taken from two North Atlantic abyssal plains during the ESOPE Cruise.

Two sediment cores longer than 20 meters, obtained from two abyssal plains of the North Atlantic during the ESOPE cruise were analyzed for U and Th isotopes. In the Madeira Abyssal Plain sediment (ESOPE core 10), excess 230Th data do not show a simple exponential decrease with depth, being reflected by complex depositional history of the sediments, and hence the Th isotope data can only weakly constrain the rate of sediment accumulation. Relatively low 230Th concentration in the top 5 meters may be caused by rapid accumulation of turbiditic materials. That a significant excess 230Th exists even at depths of ∼20m in the core suggests that the average sedimentation rate is rapid, perhaps ∼10 of centimeters per thousand years. This high rate of sediment accumulation has resulted largely from frequent turbidites, and not from pelagic clay sedimentation. In the Nares Abyssal Plain sediment (ESOPE core 48), excess 230Th relative to 234U has been found down to ∼5m, showing its almost exponential decrease with depth. Based on the excess 230Th distribution, an average sedimention rate for the core is estimated to be 1.2 ± 0.3 cm/103 y.

Faulting in Madeira Abyssal Plain sediments

Interpretation of high-resolution seismic data revealed the occurrence of faults in the sediments of two North Atlantic abyssal plain environments, the Madeira Abyssal Plain (MAP) and the Nares Abyssal Plain (NAP). Most of the data are from the MAP, where faulting appears to be confined to particular locations. From our data individual faults cannot be traced from one seismic section to another. A positive correlation exists between the concentration of faults and the maximum sediment thickness. Although the mechanism that caused the faulting is still not understood, the faults apparently are related to processes which have been active for a long period of time.

HYDROCARBONS IN SEDIMENTS FROM THE EDGE OF THE BERMUDA PLATFORM

ABSTRACT Surficial and subsurface (10–13 cm) sediment samples were taken at 7 stations (17 cores) on the northern margin of the Bermuda seamount, remote from ship traffic, beaches, and atmospheric fallout from aircraft. Their aliphatic (pentane-extractable) hydrocarbon content was found to be very low, comparable to samples from the North Atlantic abyssal plain, and two orders of magnitude lower than for typical coastal samples. About half of the aliphatic hydrocarbons are clearly biogenic, and the remainder are characteristic of petroleum residues. Petroleum hydrocarbon concentrations are lower in subsurface (for example, 0.11 µg/g dry weight) than surface samples (0.47 µg/g), and are lower outside the reef (0.25 µg/g) than inside (0.47 µg/g). These results are quantitatively consistent with a diffusion model. Extremely rapid bioturbation or totally quiescent deposition on a stable sedimentary facies can probably be eliminated as hypotheses for the deposition and transport mechanism within the sediment. Whether degradation is important cannot be answered without further studies.

Petroleum Potential of U.S. Atlantic Slope, Rise, and Abyssal Plain

A Mesozoic shelf margin below the present continental slope is believed to have the potential for hydrocarbon accumulations. Of special interest for the petroleum geologist are reef carbonate deposits that may be associated with this paleoshelf edge. Carbonate reef trends of the Edwards Limestone of Texas and the El Abra-Tamaulipas Formation of the Mexican Golden Lane are chosen as analogs to infer the locations of potential reservoirs and the probable relations between potential reservoirs and seals. Petroleum could have been generated in Mesozoic or younger basin facies and migrated essentially updip, but possibly down section, to reservoir rocks in the Mesozoic shelf margin. Other exploration targets associated with this paleomargin complex and which could have been charged by migrating petroleum are structural highs above the predicted reef buildups and closures associated with known faults. Seaward of the Mesozoic shelf edge, potential exploration targets include stratigraphic pinchouts of Lower Cretaceous and Jurassic sediments near the base of the present slope, abandoned canyon-fan complexes, channel fills on the present rise, and inferred Cretaceous and Jurassic turbidites beneath the present rise and abyssal plain. Although current petroleum prices do not justify the development of petroleum resources in such deep-water areas, some of these targets, especially those beneath the present slope, may be of considerable interest in the near future.

Amino acids in a surface sediment core of the Atlantic abyssal plain

The amino acids in a 56 cm sphincter core (K-19-4-9, Woods Hole Oceanographie Institution) from 30°N latitude, 60°W longitude—approximately 300 miles southeast of Bermuda—at a water depth of 5454m were analyzed by gas and liquid chromatography. Amino acids decreased rapidly with depth with β-alanine and γ-aminobutyric acid predominating in deeper sections. Most of the amino acids are tightly bound in the sediment and are released only upon hydrolysis with 6N HC1. Amino acids accounted for only 0.40% of total nitrogen in the non-carbonate minerals in the top 0–8 cm and 7.9 × 10 −4 percent in the bottom 48–56 cm. The remaining nitrogen is removed from the sediment by amino acid extraction.