Chemical composition of interstitial solutions from sediments of DSDP Legs 6-8, 11, 12, 14, 15, and 22

Through the Deep Sea Drilling Project samples of interstitial solutions of deeply buried marine sediments throughout the World Ocean have been obtained and analyzed. The studies have shown that in all but the most slowly deposited sediments pore fluids exhibit changes in composition upon burial. These changes can be grouped into a few consistent patterns that facilitate identification of the diagenetic reactions occurring in the sediments. Pelagic clays and slowly deposited (<1 cm/1000 yr) biogenic sediments are the only types that exhibit little evidence of reaction in the pore waters. In most biogenic sediments sea water undergoes considerable alteration. In sediments deposited at rates up to a few cm/1000 yr the changes chiefly involve gains of Ca(2+) and Sr(2+) and losses of Mg(2+) which balance the Ca(2+) enrichment. The Ca-Mg substitution may often reach 30 mM/kg while Sr(2+) may be enriched 15-fold over sea water. These changes reflect recrystallization of biogenic calcite and the substitution of Mg(2+) for Ca(2+) during this reaction. The Ca-Mg-carbonate formed is most likely a dolomitic phase. A related but more complex pattern is found in carbonate sediments deposited at somewhat greater rates. Ca(2+) and Sr(2+) enrichment is again characteristic, but Mg(2+) losses exceed Ca(2+) gains with the excess being balanced by SO4(post staggered 2-) losses. The data indicate that the reactions are similar to those noted above, except that the Ca(2+) released is not kept in solution but is precipitated by the HCO3(post staggered -) produced in SO4(post staggered 2-) reduction. In both these types of pore waters Na(+) is usually conservative, but K(+) depletions are frequent. In several partly consolidated sediment sections approaching igneous basement contact, very marked interstitial calcium enrichment has been found (to 5.5 g/kg). These phenomena are marked by pronounced depletion in Na(+), Si and CO2, and slight enhancement in Cl(-). The changes are attributed to exchange of Na(+) for Ca(2+) in silicate minerals forming from submarine weathering of igneous rocks such as basalts. Water is also consumed in these reactions, accounting for minor increases in total interstitial salinity. Terrigenous, organic-rich sediments deposited rapidly along continental margins also exhibit significant evidences of alteration. Microbial reactions involving organic matter lead to complete removal of SO4(post staggered 2-), strong HCO3(post staggered -) enrichment, formation of NH4(post staggered +), and methane synthesis from H2 and CO2 once SO4(post staggered 2-) is eliminated. K+ and often Na+ (slightly) are depleted in the interstitial waters. Ca(2+) depletion may occur owing to precipitation of CaCO3. In most cases interstitial Cl- remains relatively constant, but increases are noted over evaporitic strata, and decreases in interstitial Cl- are observed in some sediments adjacent to continents.

Strontium and rubidium isotopes in serpentinites from ODP Site 195-1200

Subduction of the Pacific plate beneath the Mariana forearc releases fluids to the overlying mantle wedge that ascend, producing serpentinite "mud" that discharges on the ocean floor. As part of Leg 195 of the Ocean Drilling Program cores were obtained from drill-holes into the mud volcanoes. We report the isotopic composition of Sr in water squeezed from intervals of the cores, in the serpentinite mud, in leaches of the serpentinite mud, and in entrained small harzburgitic clasts. Except in the upper few meters below the seawater-mud interface, where pore water approaches seawater Sr concentration and isotopic ratio, Sr concentration and isotopic composition remain constant at 3-6 µmol/kg and ~0.7054. Because the elemental chemistry of the pore water is unlike seawater, this isotopic composition reflects fluids derived from the subducted slab, probably modified by reaction with mantle material during ascent. Higher Sr isotopic ratios, up to 0.7087, - but not with higher Sr concentrations in pore water - occur superimposed on an advection profile at 13-16 mbsf surrounding a thin layer of foraminiferal sand. Since the upward seepage velocity of slab fluids in the mud volcano vents is a few cm/yr, exchange of Sr between these carbonates and the rising fluids must have occurred within a maximum of a few hundred years, essentially instantaneously given the millions, or tens of millions, of years the mud volcanoes have been in existence. In contrast, the strontium isotopic compositions of leached serpentinite mud, and of small harzburgite clasts entrained in the mud, are always significantly greater than that of the pore water. In small harzburgite clasts the ratio reaches 0.7088, almost as high as the seawater value of 0.7092 and much higher than the value of typical mantle-derived strontium of ~0.704. The serpentinite muds and harzburgite clasts clearly equilibrated with seawater Sr when they were initially deposited at the surface of the seamount, but following burial they have not fully equilibrated with strontium in the pore water now discharging through the vents. These variations in the strontium isotopic composition of solids and pore waters are more consistent with episodic expulsion of fluids in the subduction zone than steady state flow. Whereas strontium in carbonates equilibrates isotopically within a few hundred years, strontium in buried harzburgite clasts does not equilibrate in the same time, assuming steady state rates of upward fluid flow. By inference, the harzburgite clasts and associated serpentinite mud must have been near the seafloor, unburied, for a yet undetermined but much longer period of time to have equilibrated from ~0.704 to 0.709 prior to subsequent burial. It may be possible to characterize at least the periodicity of fluid release in the mud volcano setting by investigating the zonation of strontium isotopic composition of hartzburgite clasts throughout the 60-meter deep composite cores.

(Table 1) Oxygen and carbon isotope compositions of altered carbonates from DSDP Hole 6-53 from the western Pacific Ocean

In the lower part of DSDP core 53.0, partly recrystallized carbonate sediments and well recrystallized limestone breccias of Oligo-Miocene age are associated with altered volcanic flows, lithified tuffs, and tuff breccias, suggesting that carbonate alteration was the result of thermal metamorphism. However, the oxygen isotope compositions of these carbonates (-3.4 to +0.6 per mil rel. PDB) are not compatible with recrystallization and isotope exchange with sea water at high temperatures. Evaluating the effects of the composition of the water which exchanged with the carbonates and of carbonate-water isotope exchange in closed systems yields the following approximate maximum temperature of recrystallization: limestone breccias, 100°C; calcite veins rimming breccia clasts, 30°C; and unconsolidated sediments overlying the breccias, 20°C. Therefore, the volcanics at site 53.0 must have been emplaced into the primary carbonate sediments at relatively low temperatures. Subsequent carbonate alteration was probably a consequence of chemical changes in ambient pore waters resulting from the submarine weathering of volcanic material.

Interstitial water chemistry at DSDP Leg 67 Holes

Interstitial water chemistry has proved to be a sensitive indicator for early diagenetic reactions, particularly those related to organic matter oxidation. Downhole chemical variations in the pore waters from Deep Sea Drilling Project Holes 496 and 497 on the Middle America Trench slope off Guatemala are anomalous because both salinity and chlorinity show strong decreases to half the values of seawater, and d18O values become positive (maximum of about +2.5% at the bottom of the holes). These observations are explained in terms of dilution of pore waters after retrieval as a result of decomposition of the gas hydrates before removal of pore waters by shipboard squeezing techniques. In all holes, except Hole 495 (drilled in pelagic sediments), decomposition of organic matter leads to rapid sulfate depletion and subsequent methane generation. Associated with methane generation are large increases in alkalinity and dissolved ammonia. The latter component causes ion exchange reactions with clay minerals, which results in maxima in magnesium and perhaps potassium. At greater depths, as yet unidentified reactions cause the removal of magnesium. Especially in the deeper Trench Sites 499 and 500, rapid variations in calcium, magnesium, and alkalinity occur in turbidite sequences.

Strontium isotope geochemistry of ODP Leg 113 sites

The concentration of dissolved Sr and the distribution of 87Sr/86Sr isotope ratios in Leg 113 interstitial waters may be interpreted in terms of mixing of Sr from four different reservoirs: indigenous seawater, marine carbonate minerals, and basaltic and siliceous detrital material. The input to the pore water from these reservoirs is determined by the reactivity of the reservoir rather than its size. The presence of strontium derived from siliceous detrital material is unequivocally demonstrated in the pore waters of the hemipelagic deposits, and is also significant in the calcareous Maud Rise sediments due to the unusually low degree of carbonate recrystallization. Also, alteration of basic volcanic material is important at several sites.

Concentration of the rare-earth elements in metalliferous sediments from DSDP Leg 92 holes

DSDP Leg 92 drilled at four sites along an east-west transect at 19°S on the western flank of the East Pacific Rise (EPR), in an area where sediments are essentially a mixture of hydrothermal and biogenic components, with only a minimal contribution of clastic material. Rare-earth element (REE) data on the metalliferous (non-carbonate) fraction of samples ranging in age from ~2 to ~27 Ma indicate the existence of two distinct groups of patterns corresponding to two broad age groups, one <=8 Ma, the other >=10 Ma. Within each group, REE patterns have characteristics which are near-uniform, despite large variations in total REE abundances. Sediments of the younger group are enriched in light REE (LREE) relative to deep bottom waters influenced by the hydrothermal plume extending west from the EPR at 19°S. Sediments of the older groups show further relative LREE enrichment and/or heavy REE (HREE) depletion. Surficial sediments deposited beneath the lysocline have high Sum REE concentrations resulting from slow accumulation rates, and patterns resembling older sediments due to early diagenetic effects. A correlation between the mass accumulation rates (MAR) of Sum REE and Fe + Mn suggests that ferromanganese particulate matter supplied by the hydrothermal plume scavenges REE; during this process the LREE are preferentially removed from plume seawater. The MAR of Fe + Mn shows a general decrease with age above basement, whereas Sum REE concentrations in the metalliferous component increase with age above basement. This supports the Ruhlin and Owen model wherein limited scavenging of REE, due to rapid burial of sediment near the palaeo-axis, leads to low concentrations (but high MAR-values) for the REE. Following deposition and burial of the hydrothermal component, further relative flattening of the REE pattern takes place, probably the result of diagenetic reactions over several million years. Phase partitioning data indicate that the proportion of REE residing in more poorly crystalline phases tends to increase with age (from ~45% to 90% of Sum REE). This suggests that as initial ferromanganese precipitates undergo diagenetic recrystallization, REE are transferred to the poorly crystalline phases, and/or are scavenged from pore waters by these phases. Because of the various modifications to REE patterns apparently produced both in the water column and post-depositional settings, the REE patterns of metalliferous sediments will not reflect fine-scale REE variations in associated oceanic water masses.

Macroturbidites of ODP Hole 103-641C

Barremian through uppermost Aptian strata from ODP Hole 641C, located upslope of a tilted fault block on the Galicia margin (northwest Spain), are syn-rift sediments deposited in the bathyal realm and are characterized by rapid sedimentation from turbidity currents and debris flows. Calcarenite and calcirudite turbidites contain shallow-water carbonate, terrigenous, and pelagic debris, in complete or partial Bouma sequences. These deposits contain abraded micritized bioclasts of reefal debris, including rudist fragments. The youngest turbidite containing shallow-water carbonate debris at Site 641 defines the boundary between syn-rift and post-rift sediments; this is also the boundary between Aptian and Albian sediments. Some Aptian turbidites are partially silicified, with pore-filling chalcedony and megaquartz. Adjacent layers of length-fast and -slow chalcedony are succeeded by megaquartz as the final pore-filling stage within carbonate reef debris. Temperatures of formation, calculated from the oxygen isotopic composition of the authigenic quartz, are relatively low for formation of quartz but are relatively warm for shallow burial depths. This quartz cement may be interpreted as a rift-associated precipitate from seawater-derived epithermal fluids that migrated along a fault associated with the tilted block and were injected into the porous turbidite beds. These warm fluids may have cooled rapidly and precipitated silica at the boundaries of the turbidite beds as a result of contact with cooler pore waters. The color pattern in the quartz cement, observed by cathodoluminescence and fluorescence techniques, and changes in the trace lement geochemistry mimic the textural change of the different quartz layers and indicates growth synchronism of the different quartz phases. Fluorescence petrography of neomorphosed low-Mg-calcite bioclasts in the silicified turbidites shows extensive zonation and details of replacive crystal growth in the bioclasts that are not observed by cathodoluminescence. Fluorescence microscopy also reveals a competitive growth history during neomorphism of the adjacent crystals in an altered carbonate bioclast. Barremian-Aptian background pelagic sediments from Hole 641C have characteristics similar to pelagic sediments from the Blake-Bahama Formation described by Jansa et al. (1979) from the western North Atlantic. Sediments at this site differ from the Blake-Bahama Formation type locality in that the Barremian-Aptian pelagic sediments have a higher percentage of dark calcareous claystone and some turbidites are silicified at Site 641. The stable isotopic composition of the pelagic marlstones from Site 641 is similar to those of other Berriasian-Aptian pelagic sediments from the Atlantic.

Geochemical analysis of sediments and interstitial water in sediment cores from the North-West African continental margin and from the Central Pacific

A. Continental slope sediments off Spanish-Sahara and Senegal contain up to 4% organic carbon and up to 0.4% total nitrogen. The highest concentrations were found in sediments from water depths between 1000 and 2000 m. The regional and vertical distribution of organic matter differs significantly. Off Spanish-Sahara the organic matter content of sediment deposited during glacial times (Wuerm, Late Riss) is high whereas sediments deposited during interglacial times (Recent, Eem) are low in organic matter. Opposite distribution was found in sediments off Senegal. The sediments contain 30 to 130 ppm of fixed nitrogen. In most sediments this corresponds to 2-8 % of the total nitrogen. Only in sediments deposited during interglacial times off Spanish-Sahara up to 20 % of the total nitrogen is contained as inorganically bound nitrogen. Positive correlations of the fixed nitrogen concentrations to the amounts of clay, alumina, and potassium suggest that it is primarily fixed to illites. The amino acid nitrogen and hexosamine nitrogen account for 17 to 26 % and 1.3 to 2.4 %, respectively of the total nitrogen content of the sediments. The concentrations vary between 200 and 850 ppm amino acid nitrogen and 20 to 70 ppm hexosamine nitrogen, both parallel the fluctiations of organic matter in the sediment. Fulvic acids, humic acids, and the total organic matter of the sediments may be clearly differentiated from one another and their amino acid and hexosamine contents and their amino acid composition: a) Fulvic acids contain only half as much amino acids as humic acids b) The molar amino acid/hexosamine ratios of the fulvic acids are half those of the humic acids and the total organic matter of the sediment c) The amino acid spectra of fulvic acids are characterized by an enrichment of aspartic acid, alanine, and methionine sulfoxide and a depletion of glycine, valine, isoleucine, leucine, tyrosine, phenylalanine, lysine, and arginine compared to the spectra of the humic acids and those of the total organic matter fraction of the sediment. d) The amino acid spectra of the humic acids and those of the total organic matter fraction of the sediments are about the same with the exception that arginine is clearly enriched in the total organic matter. In general, as indicated by the amino compounds humic acids resemble closer the total organic matter composition than the low molecular fulvic acids do. This supports the general idea that during the course of diagenesis in reducing sediments organic matter stabilizes from a fulvic-like structure to humic-like structure and finally to kerogen. The decomposition rates of single aminio acids differ significantly from one another. Generally amino acids which are preferentially contained in humic acids and the total organic matter fraction show a smaller loss with time than those preferably well documented in case of the basic amino acids lysine and arginine which- although thermally unstable- are the most stable amino acids in the sediments. A favoured incorporation of these compounds into high molecular substances as well as into clay minerals may explain their relatively high "stability" in the sediment. The nitrogen loss from the sediments due to the activity of sulphate-reducing bacteria amounts to 20-40 % of the total organic nitrogen now present. At least 40 % of the organic nitrogen which is liberated by sulphate-reducing bacteria can be explained ny decomposition of amino acids alone. B. Deep-sea sediments from the Central Pacific The deep-seas sediments contain 1 to 2 orders of magnitude less organic matter than the continental slope sediments off NW Africa, i.e. 0.04 to 0.3 % organic carbon. The fixed nitrogen content of the deep-sea sediments ranges from 60 to 270 ppm or from 20 to 45 % of the total nitrogen content. While ammonia is the prevailing inorganic nitrogen compound in anoxic pore waters, nitrate predominates in the oxic environment of the deep-sea sediments. Near the sediment/water interface interstital nitrate concentrations of around 30 µg-at. N/l were recorded. These generally increase with sediment depth by 10 to 15 µg-at. NO3- N/l. This suggests the presence of free oxygen and the activity of nitrifying bacteria in the interstitial waters. The ammonia content of the interstitial water of the oxic deep-sea sediments ranges from 2 to 60 µg-at. N/l and thus is several orders of magnitude less than in anoxic sediments. In contrast to recorded nitrate gradients towards the sediments/water interface, there are no ammonia concentration gradients. However, ammonia concentrations appear to be characteristic for certain regional areas. It is suggested that this regional differentiation is caused by ion exchange reactions involving potassium and ammonium ions rather than by different decomposition rates of organic matter. C. C/N ratios All estimated C/N ratios of surface sediments vary between 3 and 9 in the deep-sea and the continental margin, respectively. Whereas the C/N ratios generally increase with depth in the sediment cores off NW Africa they decrease in the deep-sea cores. The lowest values of around 1.3 were found in the deeper sections of the deep-sea cores, the highest of around 10 in the sediments off NW Africa. The wide range of the C/N ratios as well as their opposite behaviour with increasing sediment depth in both the deep-sea and continental margin sediment cores, can be attributed mainly to the combination of the following three factors: 1. Inorganic and organic substances bound within the latticed of clay minerals tend to decrease the C/N ratios. 2. Organic matter not protected by absorption on the clay minerals tends to increase C/N ratios 3. Diagenetic alteration of organic matter by micro-organisms tends to increase C/N ratios through preferential loss of nitrogen The diagenetic changes of the microbially decomposable organic matter results in both oxic and anoxic environments in a preferential loss of nitrogen and hence in higher C/N ratios of the organic fraction. This holds true for most of the continental margin sediments off NW Africa which contain relatively high amounts of organic matter so that factors 2 and 3 predominate there. The relative low C/N ratios of the sediments deposited during interglacial times off Spanish-Sahara, which are low in organic carbon, show the increasing influence of factor 1 - the nitrogen-rich organic substances bound to clay minerals. In the deep-sea sediments from the Central Pacific this factor completely predominates so that the C/N rations of the sediments approach that of the substance absorbed to clay minerals with decreasing organic matter content. In the deeper core sections the unprotected organic matter has been completely destroyed so that the C/N ratios of the total sediments eventually fall into the same range as those of the pure clay mineral fraction.

Ikaite of sediment cores from the Zaire deep-sea fan

Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at many locations, which has been attributed to anoxic sulfide oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, about 10 m thick, consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate (ikaite: CaCO3·6H2O). Ikaite delta13C values are indicative of a methane carbon contribution to the CO2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate-methane transition zone within the turbiditic sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resulting from this slide emplacement, and the development towards a new steady state are fully sufficient to explain all features related to the pore-water profiles, particularly, [SO4]2- and dissolved inorganic carbon (DIC). The model shows that the downslope transport took place about 300 yr ago.

(Table 1) Experimental results of Kd at various conditions from ODP Hole 131-808C

A comprehensive experimental study, utilizing a rocking autoclave hydrothermal apparatus with isotope tracers, was applied to evaluate the temperature of squeezing artifacts on B contents and isotopic compositions in pore waters. The partition coefficient (KD) was determined at temperatures from 25 ° to 350 °C, at 800 bars, and this information was applied to reconstruct pore water B and d11B in ODP drill sites, where pH, T, and porosity are known. The partition coefficient of B is a function of temperature, pH, and sediment mineralogy. The solution pH exerts a dominant control at low temperatures; however, KD decreases to a value of essentially zero (compared to that of KD = ~3.5 at 25 °C) at high temperatures indicating no adsorption. Two empirical equations were derived to represent most of the available experimental results. For pelagic clay rich sediments, a KD = -3.84-0.020T + 0.88pH (R = 0.84; 1sigma = 0.25) is established. For sediments that have experienced progressive metamorphism, a KD = -1.38-0.008T + 0.59pH (R = 0.81; 1sigma = 0.37) can be applied. Similarly the effect on pore water d11B can be corrected if the fractionation factors at different temperatures are assumed. The corrected B and d11B in ODP Sites 671, 672, and 808 indicate significant mobilization of bulk B in sediment (exchangeable + lattice bound) at depth, especially near the décollement zone or other potential flow conduits. Tectonically expelled fluids from mud diapirs of Barbados Ridge Complex, hot springs of Rumsey Hills, California, and mud pot waters of the Salton Sea geothermal field, are enriched in B (up to 20 mM) with lower d11B, supporting the argument of B mobilization as a result of fluid expulsion in accretionary prisms.

Major, trace element and XRF data for sediments and fish tooth samples, from a shallow marine setting with varying redox conditions (Site U1356, Wilkes Land Margin, E.Antarctica)

Fossil fish teeth from pelagic open ocean settings are considered a robust archive for preserving the neodymium (Nd) isotopic composition of ancient seawater. However, using fossil fish teeth as an archive to reconstruct seawater Nd isotopic compositions in different sedimentary redox environments and in terrigenous-dominated, shallow marine settings is less proven. To address these uncertainties, fish tooth and sediment samples from a middle Eocene section deposited proximal to the East Antarctic margin at Integrated Ocean Drilling Program Site U1356 were analyzed for major and trace element geochemistry, and Nd isotopes. Major and trace element analyses of the sediments reveal changing redox conditions throughout deposition in a shallow marine environment. However, variations in the Nd isotopic composition and rare earth element (REE) patterns of the associated fish teeth do not correspond to redox changes in the sediments. REE patterns in fish teeth at Site U1356 carry a typical mid-REE-enriched signature. However, a consistently positive Ce anomaly marks a deviation from a pure authigenic origin of REEs to the fish tooth. Neodymium isotopic compositions of cleaned and uncleaned fish teeth fall between modern seawater and local sediments and hence could be authigenic in nature, but could also be influenced by sedimentary fluxes. We conclude that the fossil fish tooth Nd isotope proxy is not sensitive to moderate changes in pore water oxygenation. However, combined studies on sediments, pore waters, fish teeth and seawater are needed to fully understand processes driving the reconstructed signature from shallow marine sections in proximity to continental sources. This article is protected by copyright. All rights reserved.

(Table 1) Radiolarians in the Oligocene-Miocene at DSDP Hole 72-515B

The Neogene of the southwestern Atlantic is virtually barren of biogenic silica. Of the four sites drilled on Leg 72, only two contained identifiable radiolarian specimens. In the southwestern Brazil Basin (Site 515), radiolarians are present only from the upper Oligocene (Anomaly 8, about 28 Ma) to the middle Miocene (Zone NN8, about 11.5 Ma). On the Rio Grande Rise (Site 516), radiolarians are present only within a short interval of the lower Miocene (Zones N5-N6, about 18-20 Ma). The abrupt cessation of silica deposition in the upper middle Miocene is characteristic of many drill sites in the tropical and temperate Atlantic and implies that a major oceanographic "threshold" was exceeded at this time, allowing the Atlantic waters to become either less productive or relatively silica deficient. Siliceous microfossils are notably more abundant in Oligocene-Miocene sediments of deep regions where carbonate preservation is poor (Site 515) than in equivalent carbonate-rich strata nearby (Site 516). This discrepancy suggests that the presence of calcareous microfossils may act to enhance post-depositional dissolution of biogenic silica tests by elevating the pH of the surrounding pore waters. Carbonate-free clays, by contrast, may provide a more favorable chemical environment for silica preservation.

Benthic foraminiferal denitrification rates and nitrate storage

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.