MANOP Site Research Articles

Molecular and isotopic abundances of long-chain n-fatty acids in open marine sediments of the western North Pacific

Molecular abundance and compound-specific carbon isotope ratios of long-chain n-fatty acids (LCFAs) ranging from C 20 to C 30 are reported for open marine sediments of the western North Pacific. A positive correlation ( r 2=0.94) is observed between total organic carbon (TOC) and LCFA concentrations, although the TOC concentrations varied from 0.34 to 2.62 (wt.% in dry sediment). The relatively uniform ratio of LCFAs to TOC (0.74±0.12 mg/gC) is similar to that of the central Pacific reported by Ohkouchi et al. [Ohkouchi, N., Kawamura, K., Kawahata, H., Taira, A., 1997. Latitudinal distributions of terrestrial biomarkers in the sediments from the Central Pacific. Geochim. Cosmochim. Acta 61, 1911–1918] and in a core sample from the tropical central Pacific by Prahl et al. [Prahl, F.G., Muehlhausen, L.A., Lyle, M., 1989. An organic geochemical assessment of oceanographic conditions at MANOP site C over the past 26,000 years. Paleoceanography 4, 495–510]. On the other hand, LCFA concentrations in riverine and estuarine sediments are apparently higher than in open marine sediments. Carbon isotopic compositions of individual LCFAs in open marine sediments are similar (−26±1‰ for n–C 24 and n–C 26), being more enriched in 13 C than bay and riverine sediments by up to 6‰. Terrestrial C3 higher plants commonly have a δ 13 C value of ∼−26‰ in bulk organic carbon, and because lipid components including fatty acids are known to be depleted in 13 C by several per mil compared to bulk organic matter, this study indicates that the open marine sedimentary LCFAs are not derived from normal terrestrial C3 higher plants. Rather, the isotopic composition is consistent with a mixed source of C4 and C3 terrestrial higher plants, and/or marine organisms. Low carbon preference index (CPI) values of the LCFAs, and the isotopic difference between the northern and southern samples suggest that the LCFAs of this study are likely to be related to marine primary productivity, even though LCFAs have often been used to infer a terrestrial higher plant input to marine sediments.

In situ measurements of calcium carbonate dissolution rates in deep-sea sediments

Benthic fluxes of alkalinity, carbon dioxide, and oxygen have been measured using an in situ incubation chamber at three sites (MANOP Sites C and S and PACFLUX Site SC) in the central equatorial north Pacific. At two carbonate-rich sites (C and SC), a budget for oxygen, alkalinity, and TCO 2 fluxes indicate a net CaCO 3 dissolution rate of approximately 0.4 mmol m −2 day −1. This rate is only 20% of previous estimates but is consistent with a dissolution rate constant predicted from laboratory experiments with deep-sea sediments and derived from in situ flux measurements in sediments of the southern California borderland. Organic matter oxidation in the sediment column provides the acid for 60–100% of the calcium carbonate dissolution occurring at these sites with the remainder derived from undersaturated bottom water. At a low-carbonate site (S), no carbonate dissolution in the sediment or on the sea floor is apparent, although sediment traps indicate a rain of CaCO 3 through 3400 m. The rates of remineralization relative to resuspension or erosion at this site must differ for organic carbon and calcium carbonate, so that carbonate grains reaching the sea floor are physically removed by erosion or resuspension before they dissolve, while organic carbon is largely oxidized before it can be physically removed.

An organic geochemical assessment of oceanographic conditions at Manop Site C over the past 26,000 years

Eleven sediment samples taken downcore and representing the past 26 kyr of deposition at MANOP site C (0°57.2′N, 138°57.3′W) were analyzed for lipid biomarker composition. Biomarkers of both terrestrial and marine sources of organic carbon were identified. In general, concentration profiles for these biomarkers and for total organic carbon (TOC) displayed three common stratigraphic features in the time series: (1) a maximum within the surface sediment mixed layer (≤4 ka); (2) a broad minimum extending throughout the interglacial deposit; and (3) a deep, pronounced maximum within the glacial deposit. Using the biomarker records, a simple binary mixing model is described that assesses the proportion of terrestrial to marine TOC in these sediments. Best estimates from this model suggest that ≈20% of the TOC is land‐derived, introduced by long‐range eolian transport, and the remainder is derived from marine productivity. The direct correlation between the records for terrestrial and marine TOC with depth in this core fits an interpretation that primary productivity at site C has been controlled by wind‐driven upwelling at least over the last glacial/interglacial cycle. The biomarker records place the greatest wind strength and highest primary productivity within the time frame of 18 to 22 kyr B.P. Diagenetic effects limit our ability to ascertain directly from the biomarker records the absolute magnitude that different types of primary productivity have changed at this ocean location over the past 26 kyr.

A theoretical study of diagenetic concentration fields near manganese nodules at the sediment‐water interface

The effects of manganese nodules on concentration fields in adjacent pore waters and sediments are investigated theoretically with a three‐dimensional (axisymmetric) diffusion‐reaction model. The nodules are idealized as impermeable and poorly permeable disks and hemispheres. The first part of this paper examines the organic matter and coupled pore water oxygen fields in order to evaluate the local redox conditions under an isolated nodule. Oxygen is allowed to disappear on a free surface that is calculated as an output of the model. In sediments that become suboxic, like those at MANOP site C, the model predicts that pore waters neighboring a nodule are markedly more depleted in oxygen than in its absence. Although these reduced conditions may remobilize metals that would be available for nodule growth, these same conditions would appear to favor nodule dissolution. Typically, the presence of a nodule raises the local position of the zero‐oxygen surface, but it is deeper than expected for large nodules on sediments containing only highly reactive organic matter. The redox changes calculated for an isolated nodule on a hypothetical, but representative, oxidized abyssal red clay appear inadequate to raise the redox boundaries enough to facilitate metal transport from reduced regions deeper in the sediment. In the later part of the paper the model is used to evaluate the influence of a nodule field on the outward flux of dissolved silica from a siliceous ooze. Although the flux attenuation is generally modest, some dense packings observed in the Pacific should produce substantial reductions (>35%), which should promote opal preservation.

Sedimentation at MANOP Site H (eastern equatorial Pacific) over the past 400,000 years: Climatically induced redox variations and their effects on transition metal cycling

Gravity cores recovered from Manganese Nodule Project site H (6°33′N, 92°49′W) show marked downcore variations in the abundance of calcium carbonate, organic carbon, opal, manganese, and other components deposited over the past 400,000 years. Variations in the downcore abundance of organic carbon, which ranges from 0.2 to 1.0%, can be used to hindcast redox conditions in the surface sediments over this time. The results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles. Downcore variations in solid phase Mn, Ni, and Cu can be produced by such changes in redox conditions. A model which predicts that solid phase Mn can be trapped and buried when the Mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and Mn records and supports the reconstructed redox variations. The history of redox variations at site H can be explained by changes with time in surface water productivity. Major productivity variations at the site occur over 100‐kyr cycles, with relatively higher productivity occurring during glacial stages. Thus Quaternary climate changes influence surface water productivity, redox conditions in sediments, and the cycling of transition metals.

Transport of 10Be and 9Be in the ocean

Beryllium isotopes ( 10Be and 9Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of 10Be and 9Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 10 9 and 6 × 10 16 atoms g −1 of 10Be and 9Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The 10Be/ 9Be ratio in trap samples varies from 3 to 20 × 10 −8. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of 10Be into the traps at about 1500 m are estimated as 9 × 10 5 and 4 × 10 5 atoms cm −2 a −1 at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 10 5 atoms cm −2 a −1 at both locations. Good correlations exist between 10Be, 9Be and 27Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.

Radon fluxes measured with the MANOP bottom lander

At five Pacific Ocean sites, radon fluxes were determined from water samples collected by the MANOP Lander, from measurements of 222Rn and 226Ra concentrations in Lander-collected box core sediments, and from measurements of excess radon in the water column. At MANOP sites H and M, fluxes (all in atoms m −2 s −1) determined with Lander water samples (2200 and 1540 ± 480) agree within the measurement uncertainty with water column standing crop measurements (2220 ± 450, 2040 ± 470). At MANOP site C, the diffusive flux calculated from measurements of 226Ra in box core sediments (550 ± 20), the integrated deficiency of 222Rn in the sediments (720 ± 90), and the water column standing crop (500 ± 160) are in agreement, but all are about twice as large as the single Lander water measurement of the radon flux (330). At MANOP site S radon fluxes from measurements of Lander water (3000 ± 260) are in agreement with the predicted diffusive flux from site S sediments (2880), and both fluxes are close to the lower end of the range of water column standing crop measurements (3000–5170). In San Clemente Basin, California, the Lander water flux measurements at four different sites vary by a factor of 3 due to variability in the sediment radium distribution, but the average (1030 ± 190) is close to the water column standing crop value (780 ± 230). Because there is excellent agreement between the fluxes measured with Lander water samples and the predicted diffusive fluxes in most cases, diffusion must be the primary process controlling benthic exchange of radon at the sites studied. The agreement between the Lander water flux estimates and the water column standing crop estimates indicates that the MANOP Lander functions as an accurate benthic flux chamber in water depths ranging from 1900 to 4900 m. In San Clemente Basin, surficial sediments are enriched in manganese and radium, due to manganese cycling near the sediment-water interface. Molecular diffusion of radon from this radium-rich surficial layer is sufficient to obscure any effect of macrofaunal irrigation on the standing crop of radon in this basin.

A steady-state diagenetic model for dissolved carbonate species and pH in the porewaters of oxic and suboxic sediments

An open-system diagenetic (transport) model is presented which accounts for the concurrent behavior of all the dissolved carbonate species (i.e. CO2(aq), HCO3−, and CO3=) as well as hydrogen and hydroxyl ions in the porewaters of marine sediments during the oxic and suboxic decay of organic-matter. The model includes interconversion between the dissolved carbonate species due to association/dissociation reactions as well as production by organic decay and CaCO3 dissolution. The existence of rapid association/dissociation reactions has important consequences. First, the transport of a dissolved carbonate species is facilitated, because it can react and diffuse as another carbonate species. This action modifies the concentration profiles which would be expected without interconversion. As a consequence, the rate of CaCO3 dissolution is increased because it is more difficult for CO3= to reach and maintain the saturation concentration. Finally, CO2(aq) and HCO3− produced by decay affect the concentration of CO3= and, therefore, the saturation state of porewaters with respect to carbonate minerals. The model is applied to the carbonate alkalinity and pH data from the Guatemala Basin and MANOP Site C. The model reproduces the sharp near-surface minimum in pH, observed at the Guatemala Basin sites; however, the carbonate alkalinity increase is underpredicted. This model result implies that there is an additional source of HCO3− that is not presently recognized, probably in the form of sulfate reduction at depth. The MANOP Site C data are well explained by using organic-matter oxidation rates in accord with the minimum O2 fluxes suggested by Reimerset al. (1984).

Processes at the sediment‐water interface, advances during 1982–1986

1982 to mid 1986 saw important advances in our understanding of processes ocurring at the sediment‐water interface. Notable were significant gains in our knowledge of the influence of bioturbation on zonation and rates of microbially‐mediated reactions, quantification of mixing processes near the interface using radiochronometric tracers, process‐oriented studies of trace metal behavior in oxic, suboxic and anoxic environments, and detailed investigations of microscale geochemical distributions and processes at the deep‐sea MANOP sites. The overriding importance of organic matter input and decomposition in driving many of these processes was noted by many investigators.

The effect of bioturbation and adsorption gradients on solid and dissolved radium profiles in sediments from the eastern equatorial Pacific

Remobilization of Mn resulting from the diagenetic oxidation of organic carbon causes steep adsorption gradients within hemipelagic sediments of the eastern equatorial Pacific. Transport of Ra along these gradients is capable of maintaining marked 226Ra- 230Th disequilibrium in the surface sediments. Pore water and solid phase 226Ra profiles measured in cores from two sites in this area are modeled by adjusting the Ra adsorption coefficient as a function of the depth-varying Mn content. At MANOP site M, the high organic carbon accumulation rate causes a sharp redox gradient within the upper 10 cm of sediment. Biological mixing is constant and rapid (−500 cm 2/kyr) in this upper layer, below which the Mn content, the Ra adsorption coefficient and the 226Ra 230Th ratio are all sharply reduced. At MANOP site H, the organic carbon accumulation rate is less and therefore the redox gradient is expanded over a greater depth scale. In the surface sediment, biological mixing is slower, decreases with depth and gradients in the adsorption coefficients and 226Ra 230Th ratio are less marked than at site M.

Enhanced scavenging of 210Pb and 210Po by processes associated with the East Pacific Rise near 8°45′N

In 1977 and 1981 a hydrothermal plume was detected at the East Pacific Rise (EPR) near 8°45′N and at MANOP Site M, 25 km east of the EPR, by anomalous 222Rn and Mn concentrations. In 1981, samples were also taken for 210Pb, 210Po and 226Ra analyses to determine if enhanced scavenging of these elements occurred in the plume. At both the ridge crest and at Site M, the 210Pb/ 226Ra ratios range from 0.09 to 0.35, which are among the lowest values ever measured. It appears that removal of the 210Pb is occurring by processes operating at or near the seafloor. There is also significant 210Po/ 210Pb disequilibrium at both locations, which appears to increase away from the seafloor towards the plume (as indicated by elevated Mn concentrations). An in-situ water column scavenging process is suggested by correlation of [ 210Pb- 210Po] and Mn concentrations in the plume. The residence time of the 210Po is only about 1 year, which is close to its residence time in surface waters but quite short compared to typical deep sea values. Thus rapid scavenging of the polonium in the plume and relatively rapid settling of particles from the plume is suggested.

The surface chemistry of sediments from the Panama Basin: The influence of Mn oxides on metal adsorption

The solid Mn content of sediments at a site in the Panama Basin (5°21′N 81°56′W) decreases from 3.9% in the interfacial sediment to 1% at 1.5 cm and <0.2% below 5 cm. These conditions provide an opportunity to examine the influence of Mn oxides on the metal adsorption characteristics of natural marine sediments. The adsorption of 14 metals on interfacial sediment and sediment from depths of 0.5 to 3 cm and 15 to 19 cm from the Panama Basin site was studied, and distribution coefficients ( K D ) were determined. A comparison of the K D values for a variety of samples containing different Mn contents ( i.e., Panama Basin sediments, MANOP site H interfacial sediment, red clay, and buserite) indicates that an increase in the solid Mn content enhances the ability of the particles to bind certain metals ( e.g., Zn, Pb, Co, Cd, and Ba) while the binding ability for other metals ( e.g., Cs, Be, Sc, Pu, Sn, and Fe) is not significantly affected. For the Panama Basin sediment, the K D values for Ni, Co, Cd, Ba, and Mn for the Mn enriched interfacial sediment are 5 to 23 times greater than the K D values for the Mn depleted deep sediment. The K D values for Cs, Be, Sc, Pu, and Fe for the two types of sediment are essentially the same. The correlation between the Mn content and the binding ability of the sediment for particular metals coincides with the Mn-metal correlations observed in bulk compositional data for ferromanganese nodules and sediments. This implies that the observed metal enrichments in nodules or hemipelagic sediments are most likely caused by preferential adsorption of the metals by Mn enriched phases.

The benthic cycle of copper: Evidence from sediment trap experiments in the eastern tropical North Pacific Ocean

The benthic fluxes of 24 elements and components are calculated from sediment trap fluxes and sediment accumulation rates at MANOP sites H and M in the eastern tropical North Pacific Ocean. In this paper, we focus our discussion on our data for copper. Benthic flux estimates for copper are 0.7 and 1.4 nmol/cm 2/yr at H and M, respectively. These results compare well with estimates from another trap deployment in the Galapagos and modeling of pprewater and water column concentration data by other authors. Partitioning of near-bottom Cu flux indicates that the two sites receive similar amounts of primary Cu flux (flux from upper water column) and resuspended Cu flux. Only the site located on the East Pacific Rise (Site M) shows any input of hydrothermal Cu. At Site M, 35% of the near-bottom flux is due to some source other than primary, resuspended, or hydrothermal flux. We speculate that dissolved Cu from the sediments is readsorbed onto particles near the bottom at this site. Sequential leaching of trap material indicates that the flux of Cu associated with organic matter, using an operational definition, ranges from 56% to 84% of the trap material. This fraction of the total Cu increases with depth in the water column, probably through scavenging.

Flux comparisons between sediments and sediment traps in the eastern tropical Pacific: Implications for atmospheric C02 variations during the Pleistocene1

Regeneration rates of labile elements and biogenic components were measured at two sites in the Guatemala Basin by comparing fluxes of material caught in sediment traps to fluxes of that preserved in the sediments. From 95 to 99% of the N, P, organic C, CaCO3, opal, and Br reaching the sea floor are regenerated at the two sites. For elements bound largely in refractory phases (e.g. Al, Sc, Ti, and Fe), the particle‐associated flux to the bottom is within 20% of the accumulation rate in the sediment.The two MANOP sites, about 800 km apart, have similar fluxes of organically associated N, P, and C, reflecting similar and relatively high primary productivity. There is a marked contrast in the opal/carbonate fluxes which leads to differences in CO2 cycling at the two locations. At least 95% of the total particulate organic carbon and &gt;97% of the total particulate phosphate are regenerated at &lt;1,500‐m depth in the oceans. In contrast, much of the CaCO3 is destroyed at the sea floor. By a simple three‐box ocean model we show that the recycling and storage depths of organic C, CaCO3, and P can be important factors in the control of the CO2 content of the atmosphere. We suggest that Pleistocene CO2 variations were due in part to global increases in upwelling during glacial periods with synchronous increases in total oceanic productivity and differences in diatom vs. carbonate productivity.

Stable carbon isotopes in marine porewaters

We present centimeter-scale porewater δ 13C profiles from four sites in the Central Equatorial Pacific Ocean, which reveal steep porewater δ 13C gradients (up to −1.0‰ cm −1) at the deep-sea sediment—water interface. We describe the processes controlling porewater carbon isotopic composition and incorporate them into a model of the deep-sea benthic carbon cycle. Our measured δ 13C profiles are then used to estimate carbon rain rates at two of these sites, MANOP sites C and S. The δ 13C data yield organic carbon rain rates of 10 μmol C cm −2 yr −1 at site C and 3 μmol C cm −2 yr −1 at site S. These carbon rain rates agree with recent carbon rain estimates based on sediment trap data and porewater nitrate profiles, but are a factor of 2 lower than estimates based on millimeter-scale porewater oxygen profiles. Our model results indicate that changes of the same magnitude in organic carbon rain rate and bottom water composition (O 2, ΣCO 2, and δ 13C) have comparable effects on the carbon isotopic composition of surficial porewaters. This suggests the need to consider the effects of benthic carbon cycling on the stable carbon isotope record preserved in deep-sea benthic foraminifera tests.

Particle mixing rates in sediments of the eastern equatorial Pacific: Evidence from 210Pb, 239,240Pu and 137Cs distributions at MANOP sites

Particle mixing rates ( D B ) calculated from excess 210Pb gradients in sediments of the east equatorial Pacific range from 0.04 to 0.5 cm 2/y, with variation of a factor of 3–4 at a single site. Diffusion of the 236Ra daughter 222Rn may affect 210Pb distributions under conditions of slow mixing and low 210Pb flux to the seafloor, as shown by a siliceous ooze-clay core which contained the fallout radionuclides 239,240Pu and 137Cs but no excess 210Pb (relative to 226Ra). There is no clear relationship between 210Pbderived mixing rates and sediment type, accumulation rate or organic carbon flux to the sediments. Comparison of 210Pb mixing rates with those calculated from 239,240Pu and 137Cs distributions reveals better agreement for a pulse input of the fallout radionuclides ( D B = 0.03−0.4 cm 2/ y) than for continuous input at a constant rate ( D B = 0.1−1.6 cm 2/ y), although the Pu and 137Cs data are better fit by the latter model. The agreement may be fortuitous because 239,240Pu and 137Cs appear significantly deeper than 210Pb in at least one core. Tracer separation could be caused by particle size-selective mixing by the benthic fauna or by chemical mobilization. If the fallout radionuclides are scavenged from surface waters by large, organic-rich particles such as fecal pellets, their release and migration may result from decomposition of the carrier in surface sediments. Either a relatively unreactive form of Pu (e.g. oxidized Pu) has been released by this process or a one-dimensional model is inadequate to explain its observed penetration into the sediments. Activity ratios of 239,240Pu 137Cs in the sediments decrease with increasing north latitude, and the trend reflects higher fluxes of 239,240Pu near the weapons test site at Christmas Island (2°N). The 239,240Pu 137Cs ratios and fluxes to the sediment (assuming constant input) at the siliceous ooze-red clay site are consistent with published sediment trap data from a nearby site. Thus if fallout radionuclide fluxes to the sea floor were higher in the past, both 239,240Pu and 137Cs have been released from sinking particles.

Organic carbon dynamics and preservation in deep-sea sediments

Three methods of estimating the particulate organic carbon fluxes to the sediment-water interface of the deep Pacific Ocean agree to within the error of the measurements at MANOP sites M, H, and C. Sediment trap experiments, pore water results, and surface sediment organic carbon data suggest that a major fraction of the particulate organic carbon raining to abyssal depths at these locations is degraded within the surface sediments rather than at the sediment-water interface or in the nephloid layer. Organic carbon rain rates at the three sites are similar—within a factor of two; however, the preservation rate of organic carbon and the chemistry of sediment pore waters are very different. A model developed to describe the pore water oxygen and sedimentary carbon distributions indicates model developed to describe the pore water oxygen and sedimentary carbon distributions indicates that changes in the rate constant for organic matter degradation and the bioturbation rate may contribute significantly to the observed differences in character of both pore water and sediment chemistry at these locations. The implication with respect to interpreting the sedimentary record is that cycles of organic carbon and redox sensitive metals (i.e., manganese) are not simply related to particulate organic carbon flux or surface water primary productivity. The residence time of organic carbon with respect to degradation in the surface sediments is on the order of 15 to 150 y.

Transport and release of transition elements during early diagenesis: Sequential leaching of sediments from MANOP Sites M and H. Part I. pH 5 acetic acid leach

Sediments from MANOP sites M and H in the eastern tropical Pacific Ocean can be partitioned into operationally defined phases by means of a sequential leach procedure. This paper reports the results of the first leach in the sequence, an acetic acid solution buffered as p H 5 with sodium acetate. This leach is designed to remove carbonate-bound and sorbed cations. The only cation bound in a consistent ratio to calcium in the carbonate is strontium. The molar ratio is 2 × 10 −3. In contrast, transition metals are sorbed onto the surfaces of other sedimentary particles. The proportions sorbed range from 2 to 10% of the total manganese, about 10% of the total nickel and copper, and less than 1% of the total iron. The pool of sorbed metals in surface sediments is sufficiently large and the rate of biological stirring is sufficiently rapid for this metal reservoir potentially to dominate the porewater reservoir in supplying metals to ferromanganese nodules. A simple model for nodule growth based upon transfer of metals through this reservoir suggests that only 1% of the sedimentary sorbed metals within a radius of 2 to 9 cm is required to support typical nodule growth rates.

Models of depth‐dependent bioturbation at MANOP Site H in the eastern equatorial Pacific

The vertical distribution of excess 226Ra sorbed to manganese‐rich sediments from a box core taken at MANOP site H (6°34′N, 92°48′W) in the eastern equatorial Pacific can be modeled only if the rate of bioturbation decreases rapidly downward from the seafloor within the upper few centimeters of sediment. Independent bioturbation rates estimated by balancing dissolved and particulate diagenetic Mn fluxes are compatible with the values obtained from the radium model. Such results are consistent with 210Pb and 230Th measurements reported here and with MANOP lander and porewater observations that rapid organic carbon degradation in these sediments consumes oxygen within a few centimeters of the sediment‐seawater interface. The modeled decrease in bioturbation (expressed as a biodiffusion coefficient) is consistent with values of more than 25 cm2/kyr at the surface falling to less than 0.4 cm2/kyr at a depth of 14 cm.

The oxidation state of manganese in marine sediments and ferromanganese nodules

Marine sediments and ferromanganese nodules from the Pacific Ocean have been analyzed for the O Mn ratio of solid manganese. We tested six chemical methods and concluded that the iodometric and oxalate methods were equivalent and were the best choice in terms of accuracy and precision on natural samples. We choose the iodometric method for most of our analyses because the oxalate procedure is a method of differences. The ferromanganese nodules that we analyzed were all from MANOP site H and had Mn Fe ratios that ranged from 5.6 to 70. These nodules were invariably highly oxidized with O Mn values ranging from 1.90 to 2.00. Our most precise analyses suggest that less than 1% of the total manganese is present as Mn(II). We also analyzed red clay and hemipelagic sediments from the eastern tropical Pacific (Baja borderland and MANOP site H) and carbonate ooze samples from the equatorial Pacific. These sediments are also highly oxidized ( O Mn = 1.90 to 2.00 ) except when Mn(II) appears in the interstitial water. As dissolved Mn(II) increases the value of the O Mn ratio in the solid phase decreases. The O Mn ratio decreases to values as low as 1.40. This decrease appears to be due to a decrease in oxidized manganese by reduction, however, an increase in reduced manganese in the solid sediments by adsorption or MnCO 3 formation can not be ruled out in all cases.