Corg Fluxes Research Articles

Suspended Matter and Hydrocarbons Fluxes in the Kara and Laptev Seas

The distribution of suspended particulate matter (SPM) by filtration and the magnitude of its fluxs using sediment traps in the Kara and Laptev Seas (cruise 72 of the R/V Akademik Mstislav Keldysh, August–September 2018) was studied. The composition of the SPM was determined by the material composition using an electron microscope, the organic component (Corg and hydrocarbons) and the inorganic component (amorphous silica and calcium carbonate). It was found that the SPM content in surface waters varied from 0.2 to 5.9 mg/L (on average 0.90 mg/L) with a maximum in the Blagopolychia Bay and aliphatic hydrocarbons from 10 to 49 µg/L (average 22 µg/L) with a maximum in the area of the removals of the river Ob. The SPM flux decreased from the surface to the near-bottom horizon only into the Blagopolychia Bay (from 13,127 to 11,900 mg/m2/day), but in most samples the flux increased in the near-bottom horizon with a maximum of 6920 mg/m2/day in the Ob’s discharge water area and correlated with the maximum of the Corg flux 695 mg/m2/day. In the composition of SPM, as well as in organic compounds, including hydrocarbons, allochthonous basically prevailed over autochthonous.

Exports and inputs of organic carbon on agricultural soils in Germany

The quantity and quality of organic carbon (Corg) input drive soil Corg stocks and thus fertility and climate mitigation potential of soils. To estimate fluxes of Corg as net primary production (NPP), exports, and inputs on German arable and grassland soils, we used field management data surveyed within the Agricultural Soil Inventory (n = 27.404 cases of sites multiplied by years). Further, we refined the concept of yield-based Corg allocation coefficients and delivered a new regionalized method applicable for agricultural soils in Central Europe. Mean total NPP calculated for arable and grassland soils was 6.9 ± 2.3 and 5.9 ± 2.9 Mg Corg ha−1 yr−1, respectively, of which approximately half was exported. On average, total Corg input calculated did not differ between arable (3.7 ± 1.8 Mg ha−1 yr−1) and grassland soils (3.7 ± 1.3 Mg ha−1 yr−1) but Corg sources were different: Grasslands received 1.4 times more Corg from root material than arable soils and we suggest that this difference in quality rather than quantity drives differences in soil Corg stocks between land use systems. On arable soils, side products were exported in 43% of the site * years. Cover crops were cultivated in 11% of site * years and contributed on average 3% of the mean annual total NPP. Across arable crops, total NPP drove Corg input (R2 = 0.47) stronger than organic fertilization (R2 = 0.11). Thus, maximizing plant growth enhances Corg input to soil. Our results are reliable estimates of management related Corg fluxes on agricultural soils in Germany.

Fluxes of Sedimentary Material in the Lofoten Basin of the Norwegian Sea: Seasonal Dynamics and the Role of Zooplankton

We studied sedimentation processes in the Lofoten Basin of the Norwegian Sea based on data of the Automatic Deep-Sea Sedimentation Observatory (ADSO) deployed for a year (August 2017–July 2018). The article presents estimates of the vertical fluxes of total sedimentary material, organic carbon (Corg), calcium carbonate (CaCO3), biogenic silica (SiO2bio), aluminum, and the particles of planktonic origin (pteropods, foraminifera, fecal pellets, necrozooplankton). The seasonal pattern of total vertical flux is characterized by a unimodal distribution with a maximum in September 2017 (191 mg dry weight m–2 day–1) in the 0–500 m surface water layer. The total flux decreases markedly from November and reaches the minimum (2.6 mg m–2 day–1) in March–April 2018. The maximum Corg flux values (5.2 mg C m–2 day–1), CaCO3 (178 mg CaCO3 m–2 day–1), and SiO2bio (9.1 mg SiO2bio m–2 day–1) are observed in autumn. The value of fluxes and the portion of lithogenic material in it are higher at a depth of 2950 m due to redeposition from the nepheloid layer. The amplitude of flux variations during the year is noticeably smaller at this depth. The highest fluxes are related to the maass sedimentation of juvenile pteropods Limacina retroversa. The contribution of pteropods to vertical fluxes in autumn is 60–100% for CaCO3 and 56–73% for Corg. The contribution of foraminifera to the total CaCO3 flux does not exceed 6% and only reaches 60% at a depth of 550 m in October. The flux of fecal pellets, where ostracod pellets dominate, varies from <0.01 to 2.1 mgC m–2 day–1, making up from 0.1 to 90% of the Corg flux. The flux of zooplankton carcasses (ZCs) in the 0–550 m water layer varies from 0 to 0.45 mgC m–2 day–1, reaching the maximum value in February. The share of ZCs flux was 0–33% in the summer–autumn season, 12–52% in winter, and 23–41% in spring relative to the total (taking into account the ZCs) Corg flux. The ZCs flux varied from 0.04 to 0.22 mgC m–2 day–1 in the near-bottom water layer and amounted to 0.7–2% of the total Corg flux. The total contribution of all studied planktonic particles to the annual CaCO3 flux was 16–47%, and their contribution to the annual Corg flux was 37–45%. Comparative analysis of our results and published data obtained in the 1990s demonstrates that the observed climatic changes (increasing temperature and salinity, as well as an increase in the thickness of the Atlantic water layer) in the Lofoten Basin did not result in significant changes in the quantitative characteristics of sedimentation process.

Bathymetric trends in biomass size spectra, carbon demand, and production of Arctic benthos (76-5561 m, Fram Strait)

This is the first study presents the patterns and environmental controls of benthic biomass size spectra, carbon demand, and production along the entire bathymetric gradient from the shelf to the abyssal depths in the transition zone between the northern North-Atlantic and the central Arctic Ocean. The material was collected at 17 stations (76-5561 m) in the eastern Fram Strait, in the vicinity of the productive Marginal Ice Zone, with increased food availability from phytodetritus sedimentation compared to other deep-sea localities at similar depths. Meiobenthic and macrobenthic individuals were measured using image analysis to assess their biovolume, biomass, annual production, and carbon demand. Benthic biomass in the study area was clearly higher than that in High Arctic locations and comparable to that in the lower-latitude North Atlantic. Biomass and annual production were significantly negatively correlated with water depth, with stronger bathymetric clines in macrofauna than in meiofauna and the increasing dominance of meiofauna with increasing depth. The entire range of the spectra (i.e., the number of size classes) decreased with increasing depth, especially in the macrofaunal part of the spectrum. Similar slope values in the normalized spectra indicated that the distribution of the biomass across the present size classes was consistent from the shelf to the abyssal depths, irrespective of the decreasing amount of food availability. The fragmented macrofaunal size spectra documented at two specific stations were probably due to physical disturbances at the sediment-water interface (e.g., intense bioturbation of holothurians and strong near-bottom currents). Benthic carbon demand declined from 50.7 gC m−2 y−1 at the shelf to 11.5 gC m−2 y−1 at the slope to 2.2 gC m−2 y−1 at the abyssal depths, and its partitioning among meiofauna and macrofauna changed with water depth, with meiofauna contributions increasing from 50% at the shelf to over 90% at the deepest station. The estimated total benthic carbon demand exceeded the vertical Corg fluxes, suggesting that the studied system can be particularly sensitive to future changes in productivity regimes and associated organic matter fluxes.

Changes in the Dust‐Influenced Biological Carbon Pump in the Canary Current System: Implications From a Coastal and an Offshore Sediment Trap Record Off Cape Blanc, Mauritania

AbstractLong‐term data characterizing the oceans' biological carbon pump are essential for understanding impacts of climate variability on marine ecosystems. The “Bakun upwelling intensification hypothesis” suggests intensified coastal upwelling due to a greater land‐sea temperature gradient influenced by global warming. We present long time series of bathypelagic (approximately 1,200–3,600 m) particle fluxes from a coastal (CBeu: 2003–2016) and an offshore (CBmeso: 1988–2016) sediment trap setting located in the Canary Current upwelling. Organic carbon (Corg) and biogenic opal (BSi, diatoms) fluxes were twofold to threefold higher at the coastal upwelling site compared to the offshore site, respectively, and showed higher seasonality with flux maxima in spring. A relationship between winter and spring BSi fluxes to the North Atlantic Oscillation index was best expressed at the offshore site CBmeso. Lithogenic (dust) fluxes regularly peaked in winter when frequent low‐altitude dust storms and deposition occurred, decreasing offshore by about threefold. We obtained a high temporal match of short‐term peaks of BSi and dust fluxes in winter to spring at the inner site CBeu. We found synchronous flux variations at both sites and an anomalous year 2005, characterized by high BSi and Corg fluxes under a low North Atlantic Oscillation. Corg and BSi fluxes revealed a decreasing trend from 2006 to 2016 at the coastal site CBeu, pointing to coastal upwelling relaxation during the last two decades. The permanent offshore upwelling zone of the deflected Canary Current represented by the flux record of CBmeso showed no signs of increasing upwelling as well which contradicts the Bakun hypothesis.

Hypoxia in the Holocene Baltic Sea: Comparing modern versus past intervals using sedimentary trace metals

Anthropogenic nutrient input has caused a rapid expansion of bottom water hypoxia in the Baltic Sea over the past century. Two earlier intervals of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTMHI; 8–4 ka before present; BP) and the Medieval Climate Anomaly (MCAHI; ~1200–750 years BP), have been identified from Baltic Sea sediments. Here we present sediment records from two sites in the Baltic Sea, and compare the trace metal (As, Ba, Cd, Cu, Mo, Ni, Pb, Re, Sb, Tl, U, V, Zn) enrichments during all three hypoxic intervals. Distinct differences are observed between the intervals and the various elements, highlighting the much stronger perturbation of trace metal cycles during the modern hypoxic interval. Both Mo and U show a strong correlation with Corg and very high absolute concentrations, indicative of frequently euxinic bottom waters during hypoxic intervals. During the modern hypoxic interval (ModernHI) comparatively less Mo is sequestered relative to Corg than in earlier intervals. This suggests partial drawdown of the water column Mo inventory in the modern water column due to persistent euxinia and only partial replenishment of Mo through North Sea inflows. Molybdenum contents in modern sediments are likely also affected by the recent slowdown in input of Mo in association with deposition of Fe and Mn oxides. Strong enrichments of U in recent sediments confirm that the ModernHI is more intense than past intervals. These results suggest that U is a more reliable indicator for the intensity of bottom water deoxygenation in the Baltic Sea than Mo. Sedimentary Re enrichment commences under mildly reducing conditions, but this element is not further enriched under more reducing conditions. Enrichments of V are relatively minor for the MCAHI and ModernHI, possibly due to strong reservoir effects on V in the water column, indicating that V is unreliable as an indicator for the intensity of bottom water hypoxia in this setting. Furthermore, Ba profiles are strongly influenced by post-depositional remobilization throughout the Holocene. The strong relationship between Corg and Ni, Tl and particularly Cu suggests that these trace metals can be used to reconstruct the Corg flux into the sediments. Profiles of As, Sb and Cd and especially Pb and Zn are strongly influenced by anthropogenic pollution.

Zooplankton impact on the organic matter flux of Siberian Arctic seas.

Role of zooplankton in the organic matter flux was studied in the Kara and Laptev seas using sediment traps in the course of voyage 63 of the Academician Mstislav Keldysh research vessel in August-October 2015. The values of the total flux and that of organic matter were at least an order of magnitude higher than those obtained before the 2000s. A hypothesis is proposed on an increasing influence of the river runoff on the Kara Sea area under climatic changes. Zooplankton contribution into Corg flux varied broadly, averaging 50%. The highest flux values and contribution of zooplankton in them (up to 96%) were recorded in the frontal (gradient) zones.

Stable carbon isotope gradients in benthic foraminifera as proxy for organic carbon fluxes in the Mediterranean Sea

Abstract. We have determined stable carbon isotope ratios of epifaunal and shallow infaunal benthic foraminifera in the Mediterranean Sea to relate the inferred gradient of pore water δ13CDIC to varying trophic conditions. This is a prerequisite for developing this difference into a potential transfer function for organic matter flux rates. The data set is based on samples retrieved from a well-defined bathymetric range (400–1500 m water depth) of sub-basins in the western, central, and eastern Mediterranean Sea. Regional contrasts in organic matter fluxes and associated δ13CDIC of pore water are recorded by the δ13C difference (Δδ13CUmed-Epi) between the shallow infaunal Uvigerina mediterranea and epifaunal species (Planulina ariminensis, Cibicidoides pachydermus, Cibicides lobatulus). Within epifaunal taxa, the highest δ13C values are recorded for P. ariminensis, providing the best indicator for bottom water δ13CDIC. In contrast, C. pachydermus reveals minor pore water effects at the more eutrophic sites. Because of ontogenetic trends in the δ13C signal of U. mediterranea of up to 1.04 ‰, only tests larger than 600 µm were used for the development of the transfer function. The recorded differences in the δ13C values of U. mediterranea and epifaunal taxa (Δδ13CUmed-Epi) range from −0.46 to −2.13 ‰, with generally higher offsets at more eutrophic sites. The measured δ13C differences are related to site-specific differences in microhabitat, depth of the principal sedimentary redox boundary, and TOC content of the ambient sediment. The Δδ13CUmed-Epi values reveal a consistent relation to Corg fluxes estimated from satellite-derived surface water primary production in open-marine settings of the Alboran Sea, Mallorca Channel, Strait of Sicily, and southern Aegean Sea. In contrast, Δδ13CUmed-Epi values in areas affected by intense resuspension and riverine organic matter sources of the northern to central Aegean Sea and the canyon systems of the Gulf of Lion suggest higher Corg fluxes compared to the values based on recent primary production. Taking regional biases and uncertainties into account, we establish a first Δδ13CUmed-Epi-based transfer function for Corg fluxes for the Mediterranean Sea.

Eastern-Mediterranean ventilation variability during sapropel S1 formation, evaluated at two sites influenced by deep-water formation from Adriatic and Aegean Seas

Present-day bottom-water ventilation in the Eastern Mediterranean basin occurs through deep-water convection originating from the two marginal basins, i.e. Adriatic and Aegean Seas. In the paleo record, long periods of enhanced deep-water formation have been alternating with shorter periods of reduced deep-water formation. The latter is related mainly to low-latitude humid climate conditions and the enhanced deposition and preservation of organic-rich sediment units (sapropels). This study focuses on sedimentary archives of the most-recent sapropel S1, retrieved from two sites under the direct influence of the two deep-water formation areas. Restricted oxygen conditions have developed rapidly at the beginning of S1 deposition in the Adriatic site, but bottom-water conditions have not persistently remained anoxic during the full interval of sapropel deposition. In fact, the variability in intensity and persistence of sedimentary redox conditions at the two deep-water formation sites is shown to be related to brief episodes of climate cooling. In the Adriatic site, sapropel deposition appears to have been interrupted twice. The 8.2 ka event, only recovered at the Adria site, is characterized by gradually increasing suboxic to possibly intermittently oxic conditions and decreasing Corg fluxes, followed by an abrupt re-establishment of anoxic conditions. Another important event that disrupted sapropel S1 formation, has taken place at ca. 7.4 cal ka BP. The latter event has been recovered at both sites. In the Adriatic site it is followed by a period of sedimentary conditions that gradually change from suboxic to more permanently oxic, as deduced from the Mn/Al pattern. Using the same proxy for suboxic/oxic sedimentary redox conditions, we observe that conditions in the Aegean Sea site shift to more permanently oxic from the 7.4 ka event onwards. However, at both sites the accumulation and preservation of enhanced amounts of organic matter have continued under these suboxic to intermittently oxic sedimentary conditions. It seems thus, that after 7.4 cal ka BP sapropel-like surface or deep-chlorophyll-maximum conditions including enhanced productivity continued, whereas bottom-water conditions were at least intermittently oxic. The latter is related to decreasing precipitation, i.e. run-off, and thus a progressive development and deepening of deep-water formation. The shallower Aegean site, would be affected earlier by such deepening ventilation than the slightly deeper Adriatic site. Finally, termination of sapropel S1 formation as deduced from diminished organic matter contents and Ba/Al, appears to have occurred almost simultaneously in the two areas, namely at 6.6 ± 0.3 and 6.3 ± 0.5 cal ka BP in Adriatic and Aegean sites, respectively.

Changes in biological productivity along the northwest African margin over the past 20,000 years

The intertropical convergence zone and the African monsoon system are highly sensitive to climate forcing at orbital and millennial timescales. Both systems influence the strength and direction of the trade winds along northwest Africa and thus directly impact coastal upwelling. Sediment cores from the northwest African margin record upwelling-related changes in biological productivity connected to changes in regional and hemispheric climate. We present records of 230Th-normalized biogenic opal and Corg fluxes using a meridional transect of four cores from 19°N–31°N along the northwest African margin to examine changes in paleoproductivity since the last glacial maximum. We find large changes in biogenic fluxes synchronous with changes in eolian fluxes calculated using end-member modeling, suggesting that paleoproductivity and dust fluxes were strongly coupled, likely linked by changes in wind strength. Opal and Corg fluxes increase at all sites during Heinrich Stadial 1 and the Younger Dryas, consistent with an overall intensification of the trade winds, and changes in the meridional flux gradient indicate a southward wind shift at these times. Biogenic fluxes were lowest, and the meridional flux gradients were weakest during the African Humid Period when the monsoon was invigorated due to precessional changes, with greater rainfall and weaker trade winds over northwest Africa. These results expand the spatial coverage of previous paleoproxy studies showing similar changes, and they provide support for modeling studies showing changes in wind strength and direction consistent with increased upwelling during abrupt coolings and decreased upwelling during the African Humid Period.

Multiple early Eocene benthic foraminiferal assemblage and δ13C fluctuations at DSDP Site 401 (Bay of Biscay — NE Atlantic)

Abstract Within the last decade, several early Eocene hyperthermals have been detected globally. These transient warming events have mainly been characterized geochemically – using stable isotopes, carbonate content measurements or XRF core scanning – yet detailed micropaleontological records are sparse, limiting our understanding of the driving forces behind hyperthermals and of the contemporaneous paleoceanography. Here, detailed geochemical and quantitative benthic foraminiferal records are presented from lower Eocene pelagic sediments of Deep Sea Drilling Project Site 401 (Bay of Biscay, northeast Atlantic). In calcareous nannofossil zone NP11, several clay-enriched levels correspond to negative δ13C and δ18O bulk-rock excursions with amplitudes of up to ~ 0.75‰, suggesting that significant injections of 12C-enriched greenhouse gasses and small temperature rises took place. Coeval with several of these hyperthermal events, the benthic foraminiferal record reveals increased relative abundances of oligotrophic taxa (e.g. Nuttallides umbonifera) and a reduction in the abundance of buliminid species followed by an increase of opportunistic taxa (e.g. Globocassidulina subglobosa and Gyroidinoides spp.). These short-lived faunal perturbations are thought to be caused by reduced seasonality of productivity resulting in a decreased Corg flux to the seafloor. Moreover, the sedimentological record suggests that an enhanced influx of terrigenous material occurred during these events. Additionally, the most intense δ13C decline (here called level δ) gives rise to a small, yet pronounced long-term shift in the benthic foraminiferal composition at this site, possibly due to the reappraisal of upwelling and the intensification of bottom water currents. These observations imply that environmental changes during (smaller) hyperthermal events are also reflected in the composition of deep-sea benthic communities on both short (

Benthic foraminifer palaeoecology of the Late Quaternary continental outer shelf of a landlocked marine basin in central Aegean Sea, Greece

North Evoikos Gulf, Greece, constitutes a landlocked, epicontinental and semi-enclosed basin, connected to the central-west Aegean Sea through a strait, probably not allowing full exchange of intermediate and deep water masses. Benthic foraminifer analyses were carried out on a Late Quaternary core from the outer shelf of the basin with the aim of tracing palaeoceanographic changes.The benthic foraminifer species percentage per sample was used to calculate two main biological proxies: the Benthic Foraminifer Oxygen Index (BFOI) and Benthic Foraminifer High Productivity (BFHP) proxy estimated by grouping benthic foraminifer taxa which are known to flourish under high Corg flux. The estimated values of BFHP and BFOI were controlled mainly by the abundance of bolivinid species which are generally considered as opportunistic and shallow infaunal species, tolerant to low oxygen conditions. The statistical analysis (Q-mode cluster analysis) singled out four time intervals, each characterized by certain levels of oxygenation and food availability. Time interval 1 (8513–8300calBP) is characterized by a mixed fauna of oxyphilic taxa and taxa that live in suboxic environment, indicating the existence of a relatively eutrophic system with a well-oxygenated sediment–water interface. Time interval 2 (8300–8100calBP) records a deterioration of conditions on sea floor, testified by significant oxygen and biotic diversity decrease. Benthic foraminifer composition in Time interval 3 (8100–7000calBP) depicts environmental conditions still highly variable, yet with faunal indication of more stable conditions than during the previous interval. Finally, the benthic foraminifer assemblage of Time interval 4 (7000–3500calBP) is related to the high food supply and good bottom circulation.The data suggest a strong linkage between the quantity and quality of the organic matter supplies, and the diversity and composition of the benthic foraminifer faunas. Moreover, this study clearly demonstrates that vertical tectonic movements need to be taken into account when reconstructing the late Quaternary palaeoceanography of this region.

210Pb-derived Sedimentation Rates and Corg Fluxes in Soledad Lagoon (Cispatá Lagoon System, NW Caribbean Coast of Colombia)

With the aim of evaluating temporal changes in sedimentation and organic carbon (Corg) supplied over the last ~100 years, a sediment core was collected at Soledad Lagoon, a costal ecosystem surrounded by mangroves, located in the Cispata Estuary (Caribbean coast of Colombia). The core sediments were characterized by low concentrations of calcium carbonate (0.2–2.9%), organic matter (3–8%), total nitrogen (0.11–0.38%), and total phosphorus (0.19–0.65 mg g−1). Fe and Al concentrations ranged from 4% to 5%, and Mn from 356 to 1,047 μg g−1. The 210Pb-derived sediment and mass accumulation rates were 1.54 ± 0.18 mm year−1 and 0.08 ± 0.01 g cm−2 year−1, respectively. The sediment core did not provide evidence of human impact, such as enhancement of primary production or nutrient enrichment, which may result from recent land uses changes or climate change. The Corg fluxes estimated for Soledad Lagoon core lay in the higher side of carbon fluxes to coastal ecosystems (314–409 g m−2 year−1) and the relatively high Corg preservation observed (~45%) indicate that these lagoon sediments has been a net and efficient sink of Corg during the last century, which corroborate the importance of mangrove areas as important sites for carbon burial and therefore, long-term sequestration of Corg.

Settling barium fluxes in the Arabian Sea: Critical evaluation of relationship with export production

Abstract Time series measurement of biogenic and barium fluxes was made using nine sediment traps deployed in the western, central and eastern part of the Arabian Sea with the objective of evaluating barium as a proxy for surface ocean productivity. Our observations show a strong linear correlation between Baexcess fluxes and biogenic opal and organic carbon (Corg) fluxes, indicating a biogenic origin of particulate Ba. However, the correlation between biogenic and Baexcess fluxes is remarkably strong in areas of uniform productivity like the central Arabian Sea, when compared to regions of episodic productivity. The processes that precipitate particulate Ba appear to be less active during periods of high biogenic flux. A large discrepancy is documented between the export flux calculated from the barium-based algorithm [Francois, R., Honjo, S., Manganini, S.J., Ravizza, G.E., 1995. Biogenic barium fluxes to the deep sea: implications for paleoproductivity reconstruction. Global Biogeochemical Cycles 9, 289–303] and from Corg fluxes [Sarnthein, M., Winn, K., Duplessy, J.C., Fontugne, M.R., 1988. Global variations of surface ocean productivity in low and mid latitudes: influence on CO2 reservoirs of the deep ocean and atmosphere during the last 21,000 years. Paleoceanography 3, 361–399] at different depths. However, ∼35% increase in Ba fluxes and a concomitant increase in Ba/Corg ratios are documented between 919 and 2002 m water depth in the western Arabian Sea. This increase may be due either to barite formation within the fecal pellets of mesozooplankton thriving at deeper levels or to scavenging of Ba by Mn oxyhydroxides. Manganese oxyhydroxides seem to act as a prominent scavenging phase for particulate Ba when the Mnexcess content in the settling particles exceeds ∼200 ppm, especially in the western and central Arabian Sea where in situ precipitation of Mn oxides is reported. The estimated preservation efficiency of Ba in the Arabian Sea sediments ranges between 50% and 61%. This is two-fold higher than the global average, suggesting that Ba may be a promising proxy for paleoproductivity estimation.

Bias in the paleoceanographic time series: Tests with a numerical model of U, Corg, and Al burial

[1] We have constructed a nonsteady state numerical model of organic carbon, aluminum, and uranium burial to examine the dynamic response of sediments to variations on thousand years scale. The three components are defined by distinct behaviors in the sedimentary system. The aluminum is a conservative component, the organic carbon is a nonconservative component degraded in superficial sediment, and the uranium is a component affected by redox conditions. By introducing a simultaneous variation of incoming fluxes of Corg and Al, geochemical profiles of Al, Corg, and U clearly show separate distributions after burial. Each component record the synchronous variation of the incoming fluxes, but the position in the sedimentary pile of the recording differs from a component to another. We show that a minimum of U could be linked to a rapid ascent of the oxic front during an increase in Corg flux.

Oxidation and origin of organic matter in surficial eastern Mediterranean hemipelagic sediments

Aerobic mineralisation of Corg in surface sediments of the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailed box core Corg concentration versus depth profiles and the modelling environment for early diagenetic problems MEDIA. The reactive fraction comprises 60-80% of the total Corg reaching the sediments and is largely oxidised within the surficial 10 cm. A non-reactive Corg fraction (GNR) dominates at depths >10 cm, and makes up 20-40% of the total Corg flux to the sediments. First-order rate constants for decomposition of the reactive fraction calculated from the Corg profiles range from 5.4 × 10 −3 to 8.0 × 10 −3 y −1 to 8.0 × 10 −3 y −1 . Total mineralization rates in the surface sediment are between 1.7 and 2.6 µmol Cc m −2 y −1 and thus are typical for oligotrophic, deep-sea environments. The low fluxes and rapid remineralisation of Corg are accompanied by 210 Pbexcess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments. Model results indicate a mis- match between the Corg profiles and O2 microprofiles which were measured onboard ship. This can be attributed to a combination of decompression artefacts affecting onboard measurement of the O2 profiles or the leakage of oxygen into the core during handling on deck. Furthermore, the used Db values, based on 210 Pb, may not be fully appropriate; calculations with higher Db values improve the O2 fits. The surficial sediment δ 13 Corg values of ∼− 22 become less negative with increasing depth and decreasing Corg concentrations. The major δ 13 C change occurs in the top 3 to 4 cm and coincides with the interval were most of the organic carbon oxidation takes place. This indicates that the reactive fraction of organic matter, commonly assumed to be marine, has a more negative δ 13 Corg than the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from the sediment data by means of literature algorithms yield low surface pro- ductivities (12-88 gC m −2 y −1 ), which are in good agreement with field measurements of primary productivity in other studies. Such values are, however, significantly lower than those indicated by recent productivity maps of the area derived from satellite imagery (>100 gC m −2 y −1 ).

A geochemical model of the Peru Basin deep-sea floor—and the response of the system to technical impacts

A geochemical model of the Peru Basin deep-sea floor, based on an extensive set of field data as well as on numerical simulations, is presented. The model takes into account the vertical oscillations of the redox zonation that occur in response to both long-term (glacial/interglacial) and short-term (El Nino Southern Oscillation (ENSO) time scale) variations in the depositional flux of organic matter. Field evidence of reaction between the pore water NO3− and an oxidizable fraction of the structural Fe(II) in the clay mineral content of the deep-sea sediments is provided. The conditions of formation and destruction of reactive clay Fe(II) layers in the sea floor are defined, whereby a new paleo-redox proxy is established. Transitional NO3− profile shapes are explained by periodic contractions and expansions of the oxic zone (ocean bottom respiration) on the ENSO time scale. The near-surface oscillations of the oxic–suboxic boundary constitute a redox pump mechanism of major importance with respect to diagenetic trace metal enrichments and manganese nodule formation, which may account for the particularly high nodule growth rates in this ocean basin. These conditions are due to the similar depth ranges of both the O2 penetration in the sea floor and the bioturbated high reactivity surface layer (HRSL), all against the background of ENSO-related large variations in depositional Corg flux. Removal of the HRSL in the course of deep-sea mining would result in a massive expansion of the oxic surface layer and, thus, the shut down of the near-surface redox pump for centuries, which is demonstrated by numerical modeling.

Lateral Transport of Lithogenic Particles from the Continental Margin of the Southern East China Sea

Geochemical compositions and fluxes of suspended and trapped (settling) particulate matter were studied to elucidate particle sources and transports from the continental margin of southern East China Sea (ECS) to the Okinawa Trough. Experimental results indicate that the abundance of suspended particulate matter (SPM) and lithogenic elements (Al, Fe and Mn) decreases with an increase of distance from the land, but increases with water depth, implying the resuspension and lateral transport of SPM over the southern ECS shelf and upper slope. This transport process is further demonstrated by a decrease of mass flux from the Mien-Hwa Canyon (MHC) to the Okinawa Trough, but an increase of mass flux with depth for trapped particulate matter (TPM). In addition, mass fluxes are anomalously high and substantially greater within the MHC (6·64–71·5gm−2day−1) than nearby slope areas (0·77–15·7gm−2day−1) and other globally analogous environments. MHC appears to function as an essential conduit for particle transport. Both SPM and TPM collected below the surface layer are mainly lithogenic. Approximately 76·2–89·3% of TPM are lithogenic materials regardless of trap sites, strongly indicating that laterally lithogenic sources must add to vertical mass fluxes across the margin. Biogenic contributions are relatively small, ranging from 1·05 to 6·54% for organic matter, from 6·33 to 14·3% for carbonate and from 0·67 to 3·50% for opal, resulting from dilution by lithogenic materials. Vertical fluxes may just contribute 8–34% of Corg fluxes collected from bottom traps. Statistical correlations among Al, K, Mn, Mg and Fe contents are highly significant, also reflecting that TPM is primarily lithogenic. However, only 40–73% of collected TPM may be regarded as recent inputs from the Yangtze River. Particles resuspended and advected from shelf and/or slope may contribute significantly to TPM collected between the upper slope and the Okinawa Trough. Furthermore, short-term variabilities of mass and lithogenic fluxes on all trap occupations may arise from episodic events of resuspension/lateral transport, rather than from temporal variations of riverine inputs and local production.

A 36 kyr geochemical record from the Sea of Japan of organic matter flux variations and changes in intermediate water oxygen concentrations

Intervals of organic C‐ and carbonate‐rich laminated sediments occur in the Sea of Japan with roughly the same frequency as temperature changes observed in Greenland ice cores, providing clear evidence of rapid oceanographic change during the past 36 kyr. Planktonic foraminiferal δ18O data suggest that only the laminated sediments deposited during the Last Glacial Maximum (LGM), and perhaps one other interval formed during a period of increased water column stratification. Sedimentary Re and Mo data are consistent with bottom waters that were sulfidic during the LGM and suboxic during other laminated intervals. Results of a numerical model of Corg and Re burial are consistent with a mechanism whereby an increased Corg flux to the seafloor drove oxygen concentrations toward depletion during times of deposition of the suboxic laminated intervals. Such a process could have resulted from increased upwelling driven either by increased deep water formation due to colder and/or more saline surface waters or by stronger northeasterly monsoonal winds.

Role of filtering and biodeposition by Adamussium colbecki in circulation of organic matter in Terra Nova Bay (Ross Sea, Antarctica)

At Terra Nova Bay, the scallop Adamussium colbecki (Smith, 1902) characterises the soft and hard bottoms from 20 to 80 m depth, constituting large beds and reaching high values of density (50–60 individuals/m 2) and biomass (120 g/m 2 DW soft tissues). To assess its role in the organic matter recycling in the coastal ecosystem, its filtering and biodeposition rates were evaluated in laboratory experiments during the austral summer 1993/94. Filtration rates, measured in a flow-through system, were calculated from the difference in particulate organic carbon (POC), nitrogen (PON) and chlorophyll- a (Chl- a) concentration in inflow and outflow water. Experiments were performed using natural sea water with POC, PON and Chl- a concentrations of about 450 μg/l, 90 μg/l and 2 μg/l, respectively. The biodeposition rate and the biochemical composition of the biodeposits were studied in order to detect how the organic matter is transformed through feeding activity of A. colbecki. At +1°C temperature, the average filtering rate was about 1 l h −1 g −1 (DW soft tissues) in specimens ranging in body mass from 2 to 3 g (DW soft tissues) and 6–7 cm long. The biodeposition rate in 3–8 cm long specimens, ranging from 0.4 to 5.7 g (DW soft tissues), was about 5.65 mg DW/g DW/day, leading to an estimate of Corg flux, through biodeposition by A. colbecki, of about 21 mg C m −2 day −1 at in situ conditions. Comparison between the biochemical composition of seston and biodeposits shows a decrease of the labile compounds, of the Chl- a/phaeopigments ratio in the biodeposits. The recorded C/N ratio decrease suggests a microbial colonisation in the biodeposits. This study suggests that Adamussium colbecki plays an important role in coupling the material fluxes from the water column to the sea bed, processing about 14% of total Carbon flux from the water column to the sediments, with an assimilation efficiency of 36%.