Geochemistry of labile organic matter in sediments and interstitial waters of ODP Sites 107-651 and 107-653

Sediment and interstitial water from Sites 651 and 653 (ODP Leg 107) were investigated by organic geochemical methods to characterize labile organic compound classes (amino compounds and carbohydrates) and to evaluate their progressive diagenetic and thermal degradation in deep-sea sediments. Downhole distribution of dissolved organic carbon (DOC) appears related to redox zones associated with bacterial activity and of diagenetic recrystallization of biogenic tests and not so much to organic matter concentrations in ambient sediments. DOC ranges from 250 to 8300 µmol/L (3-100.1 ppm). Amino acids contribute 10%-0.3% of DOC; carbohydrates range from 78 to 5 µmol/L. Rate of degradation of amino acids by thermal effects and/or bacterial activity at both sites (significantly different in sedimentation rates: average 41 cm/1000 yr in the top 300 m at Site 651, average 3.9 cm/1000 yr in the Pliocene/Quaternary sequence at Site 653 to 220 mbsf) is more dependent on exposure time rather than on the depth within the sediment column. Variability in neutral, acidic, and basic amino acid fractions of total amino acids (with a range of 1.1-0.02 µmol/g sediment; up to 2.5% of organic carbon) varies with carbonate content and by differences in thermal stability of amino acids. Distribution patterns of monosaccharides are interpreted to result from differences in organic matter sources, sedimentation rates, and the degree of organic matter decomposition prior to and subsequent to burial. Total particulate carbohydrates range from 1.82 to 0.21 µmol/g sediment and contribute about 8% to the sedimentary organic matter. Investigation of trace metals in the interstitial waters did not show any correlation of either DOC, amino compounds, or carbohydrates.

Accumulation rates and composition of organic matter in sediments off Peru

Deep-sea sediment samples from three Ocean Drilling Program (ODP) Leg 112 sites on the Peru continental margin were investigated, using a number of organic geochemical and organic petrographic techniques, for amounts and compositions of the organic matter preserved. Preliminary results include mass accumulation rates of organic carbon at Site 679 and characteristics of the organic facies for sediments from Sites 679, 681, and 684. Organic-carbon contents are high, with few exceptions. Particularly high values were determined in the Pliocene interval at Site 684 (4%-7.5%) and in the early Pliocene to Quaternary section of Hole 679D (2%-9%). Older sediments at this site have distinctively lower organic-carbon contents (0.2%-2.5%). Mass accumulation rates of organic matter at Site 679 are 0.02 to 0.07 g carbon/cm**2/k.y. for late Miocene to early Pliocene sediments and higher by a factor of 5 to 10 in the Quaternary sediments. The organic matter in all samples has a predominantly marine planktonic and bacterial origin, with minor terrigenous contribution. Organic particle sizes are strikingly small, so that only a minor portion is covered by visual maceral analysis. Molecular organic-geochemical data were obtained for nonaromatic hydrocarbons, aromatic hydrocarbons (including sulfur compounds), alcohols, ketones, esters, and carboxylic acids. Among the total extractable lipids, long-chain unsaturated ketones from Prymnesiophyte algae strongly predominate among the gas chromatography (GC) amenable components. Steroids are major constituents of the ketone and free- and bound-alcohol fractions. Perylene is the most abundant aromatic hydrocarbon, whereas in the nonaromatic hydrocarbon fractions, long-chain n-alkanes from higher land plants predominate, although the total terrigenous organic matter proportion in the sediments is small.

(Table 1) Minor and trace element concentrations in interstitial waters from ODP Leg 166 sites

Concentrations of minor and trace elements (Li, Rb, Sr, Ba, Fe, and Mn) in interstitial water (IW) were found in samples collected during Ocean Drilling Program (ODP) Leg 166 from Sites 1005, 1006, and 1007 on the western flank of the Great Bahama Bank (GBB). Concentrations of Li range from near-seawater values immediately below the sediment/water interface to a maximum of 250 µM deep in Site 1007. Concentrations determined during shore-based studies are substantially lower than the shipboard data presented in the Leg 166 Initial Reports volume (range of 28-439 µM) because of broad-band interferences from high dissolved Sr concentrations in the shipboard analyses. Rubidium concentrations of 1.3-1.7 µM were measured in IW from Site 1006 when salinity was less than 40 psu. A maximum of 2.5 µM is reached downhole at a salinity of 50 psu. Shipboard and shore-based concentrations of Sr2+ are in excellent agreement and vary from 0.15 mM near the sediment water interface to 6.8 mM at depth. The latter represent the highest dissolved Sr2+ concentrations observed to date in sediments cored during the Deep Sea Drilling Project (DSDP) or ODP. Concentrations of Ba2+ span three orders of magnitude (0.1-227µM). Concentrations of Fe (<0.1-14 µM) and Mn (0.1-2 µM) exhibit substantially greater fluctuations than other constituents. The concentrations of minor and trace metals in pore fluids from the GBB transect sites are mediated principally by changes in pore-water properties resulting from early diagenesis of carbonates associated with microbial degradation of organic matter, and by the abundance of detrital materials that serve as a source of these elements. Downcore variations in the abundance of detrital matter reflect differences in carbonate production during various sea-level stands and are more evident at the more proximal Site 1005 than at the more pelagic Site 1006. The more continuous delivery of detrital matter deep in Site 1007 and throughout all of Site 1006 is reflected in a greater propensity to provide trace elements to solution. Concentrations of dissolved Li+ derive principally from (1) release during dissolution of biogenic carbonates and subsequent exclusion during recrystallization and (2) release from partial dissolution of Li-bearing detrital phases, especially ion-exchange reactions with clay minerals. A third but potentially less important source of Li+ is a high-salinity brine hypothesized to exist in Jurassic age (unsampled) sediments underlying those sampled during Leg 166. The source of dissolved Sr2+ is almost exclusively biogenic carbonate, particularly aragonite. Concentrations of dissolved Sr2+ and Ba2+ are mediated by the solubility of their sulfates. Barite and detrital minerals appear to be the more important source of dissolved Ba2+. Concentrations of Fe and Mn2+ in anoxic pore fluids are mediated by the relative insolubility of pyrite and incorporation into diagenetic carbonates. The principal sources of these elements are easily reduced Fe-Mn-rich phases including Fe-rich clays found in lateritic soils and aoelian dust.

Soil characteristics and decomposition of organic matter on Anchorage, Signy and Falkland Islands

Antarctic terrestrial ecosystems have poorly developed soils and currently experience one of the greatest rates of climate warming on the globe. We investigated the responsiveness of organic matter decomposition in Maritime Antarctic terrestrial ecosystems to climate change, using two study sites in the Antarctic Peninsula region (Anchorage Island, 67°S; Signy Island, 61°S), and contrasted the responses found with those at the cool temperate Falkland Islands (52°S). Our approach consisted of two complementary methods: (1) Laboratory measurements of decomposition at different temperatures (2, 6 and 10 °C) of plant material and soil organic matter from all three locations. (2) Field measurements at all three locations on the decomposition of soil organic matter, plant material and cellulose, both under natural conditions and under experimental warming (about 0.8 °C) achieved using open top chambers. Higher temperatures led to higher organic matter breakdown in the laboratory studies, indicating that decomposition in Maritime Antarctic terrestrial ecosystems is likely to increase with increasing soil temperatures. However, both laboratory and field studies showed that decomposition was more strongly influenced by local substratum characteristics (especially soil N availability) and plant functional type composition than by large-scale temperature differences. The very small responsiveness of organic matter decomposition in the field (experimental temperature increase <1 °C) compared with the laboratory (experimental increases of 4 or 8 °C) shows that substantial warming is required before significant effects can be detected.

Detailed analysis of the organic carbon content in sediment cores M0002 and M0004

During IODP Expedition 302 (Arctic Coring Expedition-ACEX), the first scientific drilling campaign in the permantly ice-covered central Arctic Ocean, a 430 m thick sequence of upper Cretaceaous to Quaternary sediments has been drilled. The lower half of this sequence is composed of organic-carbon-rich (black shale-type) sediments with total organic carbon contents of about 1-14%. Significant amounts of the organic matter preserved in these sediments is of algae-type origin and accumulated under anoxic/euxinic conditions. Here, for the first time detailed data on the source-rock potential of these black shales are presented, indicating that most of the Eocene sediments have a (fair to) good source-rock potential, prone to generate a gas/oil mixture. The source-rock potential of the Campanian and upper Paleocene sediments, on the other hand, is rather low. The presence of oil or gas already generated in situ, however, can be ruled out due to the immaturity of the ACEX sediments.

Molecular dissolved organic matter composition and environmental parameters from a laboratory incubation study

Dissolved organic matter (DOM) in the oceans constitutes a major carbon pool involved in global biogeochemical cycles. More than 96% of the marine DOM resists microbial degradation for thousands of years. The composition of this refractory DOM (RDOM) exhibits a molecular signature which is ubiquitously detected in the deep oceans. Surprisingly efficient microbial transformation of labile into RDOM was shown experimentally, implying that microorganisms produce far more RDOM than needed to sustain the global pool. By assessing the microbial formation and transformation of DOM in unprecedented molecular detail for 3 years, we show that most of the newly formed RDOM is molecularly different from deep sea RDOM. Only <0.4% of the net community production was channeled into RDOM molecularly undistinguishable from deep sea DOM. Our study provides novel experimentally derived molecular evidence and data for global models on the production, turnover and accumulation of marine DOM.

Dissolved and particulate organic carbon in the Sea of Okhotsk

Data on distribution of dissolved and particulate organic matter obtained during Cruises 21 and 24 of R/V Akademik A. Nesmeyanov in June-August 1992 and 1993 are presented. In general a remarkable heterogeneity in distributions of both dissolved and particulate organic carbon is revealed. Concentrations of dissolved organic carbon vary from 98 to 700 µmol/l and those of particulate organic carbon vary from 3 to 50 µmol/l. Maximum concentrations are commonly observed in the shelf region while minimum concentrations - in the central basin. Run-off of the Amur River raises dissolved matter concentration in the Sakhalin Bay, while oil exploitation at the Sakhalin shelf maximizes particulate organic carbon concentration and minimizes dissolved one. Concentrations of dissolved and particulate organic carbon in the surface microlayer were estimated for the first time and are shown to be 1.5-2.0 times higher than in surface waters.

Contents of organic compounds in sediments from DSDP Legs 1 and 4

Fifteen sediment samples were studied from five drill sites recovered by the Glomar Challenger on Legs I and IV in the Gulf of Mexico and western Atlantic. This study concentrated on compounds derived from biogenic precursors, namely: (1) hydrocarbons, (2) fatty acids, (3) pigments and (4) amino acids. Carbon isotope (dC13) data [values <(-26)?, relative to PDB], long-chain n-alkyl hydrocarbons (>>C27) with odd carbon numbered molecules dominating even carbon numbered species, and presence of perylene proved useful as possible indicators for terrigenous contributions to the organic matter in some samples. Apparently land-derived organic matter can be transported for distances over 1000 km into the ocean and their source still recognized. The study was primarily designed to investigate: (i) the sources of the organic matter present in the sediment, (ii) their stability with time of accumulation and (iii) the conditions necessary for in situ formation of new compounds.

Minerals of bulk and fine-grained (2 µm) samples from DSDP Leg 60

A bulk-sediment and clay-fraction X-ray diffraction study of samples from Deep Sea Drilling Project Leg 60 shows an abundance of the following minerals: plagioclase feldspar, zeolite, smectite, Fe-Mg chlorite, attapulgite, and serpentine. Amorphous compounds are also abundant. The variations in abundance of the different components correspond to episodes of volcanic activity through time. Deposits from periods of great activity are composed of sediments very rich in amorphous matter and in "primary" minerals (e.g., plagioclase feldspars). During relatively quiet periods, clay minerals and zeolites predominate.

Benthic foraminifera in surface sediments of the Gulf of Lions

Surface sediment was sampled at two bathyal sites in the southwestern Gulf of Lions in the western Mediterranean Sea in February and August 1997 to study the distribution and microhabitat of living (Rose Bengal stained) deep sea benthic foraminifera. Both standing stock and diversity of the faunas, and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites. Our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments. In the axial channel of the Lacaze-Duthiers Canyon, organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer. The corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity, and a well-developed deep infauna. In addition to freshly deposited phytodetritus, more degraded organic matter seems to be an important food source. In contrast, at the open slope, organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon. The corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow-infaunal species with much lower standing stocks and clear differences between February and August. In August standing stocks are higher and the average living depths of most species shift towards the sediment surface. These differences can be attributed to patchiness or represent a seasonal trophic signal.

Stable isotope record and distribution of foraminifera in sediment core GeoB3004-1

Fluctuations in the abundance of selected foraminiferal indicator species and diversity allowed the reconstruction of changes in deepwater oxygenation and monsoon-driven organic matter fluxes in the deep western Arabian Sea during the last 190 kyr. Times of maximum surface production coincide with periods of intensified SW monsoon as shown by the abundance of Globigerina bulloides and enhanced carbonate corrosion. Benthic ecosystem variability in the deep Arabian Sea is not exclusively driven by variations in monsoonal upwelling and related organic matter supply to the seafloor but also by changes in deepwater ventilation. Deepening of the base of the oxygen minimum zone (OMZ) below 1800 m water depth is strongly coherent on the precessional band but lags proxies of SW monsoon strength by 4 to 6 kyr. The "out-of-phase" relationship between OMZ deepening and maximum SW monsoon strength is explained by temporal changes in the advection of oxygen-rich deepwater masses of North Atlantic and Antarctic origin. This process affected the remineralization and burial efficiency of organic matter in the deep Arabian Sea, resulting in the observed phase lag between maximum monsoon strength and organic carbon preservation.

Geochemistry and pyrolysis at DSDP Hole 77-535

There are substantial differences in the character of organic matter contained in the Pleistocene and Cretaceous sedimentary sequences of DSDP Site 535. The argillaceous Pleistocene section contains type III, gas-prone organic matter whereas the calcareous Cretaceous section is dominated by type II, oil-prone organic matter. A more detailed investigation of the Cretaceous section reveals that the finely laminated limestones of Valanginian to Barremian age are of good to excellent source quality. The indigenous organic matter contained within this organically rich section is thermally immature, not having undergone sufficient thermal diagenesis for the generation and expulsion of hydrocarbons. Within this stratigraphic section, however, staining by mature hydrocarbons was detected. These stains are associated with a fractured interval. These fractures may in turn represent potential migration pathways.

Organic geochemistry of lower Cretaceous sediments at DSDP Hole 77-535

At Site 535, the four lithologic units of Cretaceous age are controlled by two types of sedimentologic facies: (1) the massive light-colored limestones or marly limestones in which the total organic carbon (TOC) content is low and the organic matter more or less oxidized and (2) laminated dark facies in which the TOC content is higher and associated with a well-preserved organic matter of Type II origin. Very little typical Type III organic matter occurs in the whole series from late Berriasian to Aptian and Cenomanian. Fluctuations from oxidizing to reducing environments of deposition are proposed to account for the variations in properties of the Type II organic matter between the different facies. Dark laminated layers are good but immature potential source rocks: petroleum potential is often higher than 2 kg HC/t of rock.

Element and Sr isotope composition of interstitial waters in sediments of ODP Leg 129

Interstitial water samples from Leg 129, Sites 800, 801, and 802 in the Pigafetta and Mariana basins (central western Pacific), have been analyzed for major elements, B, Li, Mn, Sr, and 87Sr/86Sr. At all sites waters show enrichment in Ca and Sr and are depleted in Mg, K, Na, SO4, B, alkalinity, and 87Sr compared to seawater. These changes are related to alteration of basaltic material into secondary smectite and zeolite and recrystallization of biogenic carbonate. Water concentration depth profiles are characterized by breaks due to the presence of barriers to diffusion such as chert layers at Sites 800 and 801 and highly cemented volcanic ash at Site 802. In Site 800, below a chert layer, concentration depth profiles are vertical and reflect slight alteration of volcanic matter, either in situ or in the upper basaltic crust. Release of interlayer water from clay minerals is likely to induce observed Cl depletions. At Site 801, two units act as diffusion barrier and isolate the volcaniclastic sediments from ocean and basement. Diagenetic alteration of volcanic matter generates a chemical signature similar to that at Site 800. Just above the basaltic crust, interstitial waters are less evolved and reflect low alteration of the crust, probably because of the presence in the sediments of layers with low diffusivities. At Site 802, in Miocene tuffs, the chemical evolution generated by diagenetic alteration is extreme (Ca = 130 mmol, 87Sr/86Sr = 0.7042 at 83 meters below seafloor) and is accompanied by an increase of the Cl content (630 mmol) due to water uptake in secondary hydrous phases. Factors that enhance this evolution are a high sediment accumulation rate, high cementation preventing diffusive exchange and the reactive composition of the sediment (basaltic glass). The chemical variation is estimated to result in the alteration of more than 20% of the volcanic matter in a nearly closed system.