Graham B. Shimmield

Upwelling intensity and ocean productivity changes off Cape Blanc (northwest Africa) during the last 70,000 years: geochemical and micropalaeontological evidence

Abstract The accumulation of biogeochemical (organic carbon, calcium carbonates, molybdenum and iodine), micropalaeontological (benthic foraminifera) and terrigenous markers (grain-size, Si/Al, Zr/Al, K/Al) over the last 70,000 years in one core (Sedorqua-11K) on the northwest African margin off Cap Blanc has been used to reconstruct past variations of local upwelling intensity and oceanic productivity. The study demonstrates that productivity in this area increased during Stage 3, particularly between 40,000 and 50,000 yr B.P., and during the last deglaciation between 6000 and 15,000 yr B.P. During most of isotopic Stage 2, and particularly during the last glacial maximum, productivity was much lower. These variations can be attributed to changes in local wind stress and seasonality that are related to variability in monsoon pressure intensity. Because of the establishment of upwelling cells over the shelf due to high sea-level, the conditions of sedimentation during the last 5–6 thousend years on the upper slope (site 11K) are largely dominated by advection from the shelf, leading to strong sorting prior to deposition. Advection seems to have been minor during the other periods of enhanced productivity. A conceptual model is proposed to link the productivity variations to atmospheric circulation, in particular to the wind stress, direction and seasonality.

Material transport from the nearshore to the basinal environment in the southern Baltic Sea I. Processes and mass estimates

Abstract Processes involved in erosion, transport and deposition of cohesive materials are studied in a transect from shallow (16 m) to deep (47 m) water of the SW Baltic Sea. The wave- and current-induced energy input to the seabed in shallow water is high with strong variability and suspended matter concentrations may double within a few hours. Primary settling fluxes (from sedimentation traps) are less than 10 g m −2 day −1 , whereas resuspension fluxes (evaluated from sedimentation flux gradients) are 15–20 times higher and the residence time for suspended matter in the water column is 1–2 days. Settling velocities of aggregates are on average six times higher than for individual particles resulting in an enhanced downward transport of organic matter. Wave-induced resuspension (four to six times per month) takes place with higher shear stresses on the bottom than current-induced resuspension (three to five times per month). The short residence time in the water column and the frequent resuspension events provide a fast operating benthic–pelagic coupling. Due to the high-energy input, the shallow water areas are nondepositional on time scales longer than 1–2 weeks. The sediment is sand partly covered by a thin fluff layer during low-energy periods. The presence of the fluff layer keeps the resuspension threshold very low ( −2 ) throughout the year. Evaluated from 3-D sediment transport modeling, transport from shallow to deep water is episodic. The net main directions are towards the Arkona Basin (5.5×10 5 t per year) and the Bornholm Basin (3.7×10 5 t per year). Energy input to the bottom in deep water is low and takes place much less frequently. Wave-induced resuspension occurs on average once per month. Residence time of particles (based on radioactive isotopes) in the water column is half a year and the sediment accumulation rate is 2.2 mm year −1 in the Arkona Basin.

Sea level impact on nutrient cycling in coastal upwelling areas during deglaciation: Evidence from nitrogen isotopes

A common feature in δ15N profiles downcore in continental margin sediments is that the heaviest δ15N values are frequently observed during the deglaciation (i.e., between 12,000 calendar years BP and the climatic optimum at 6000 yrs BP), not at the warmest stage. Using a conceptual model across the northwestern Africa margin, a region of pronounced modern upwelling, as well as data from a core in the area, we show that this feature can be explained as a consequence of postglacial sea level rise. The model is based on a simplified twodimensional physical circulation scheme orthogonal to the margin and uses the topographic profile at the latitude of the core as well as a simplified biological model for nitrate utilization and nitrogen isotope fractionation. Shore-parallel influences are ignored. The most recently published age model of sea level rise for the last deglaciation is used [Bard et al., 1996]. The trangression causes a progressive increase in the area of shallow regions where large amounts of nutrients are recycled relative to deep regions, to which a significant portion of the nutrients is exported. This causes first an increase and then a decrease in the δ15N of the organic matter accumulating at a fixed point on the upper slope. Although the deglacial δ15N maximum is more pronounced in areas where there is not a marked oxygen minimum layer [Holmes et al., 1997; this paper], it does exist in areas where an oxygen minimum layer is present in the water column [Altabet et al., 1995; Ganeshram et al., 1995]. In such areas, the major δ15N contrast between glacial and interglacial episodes is explained by higher denitrification during interglacial stages, but it is probable that transgressing sea level contributes to this effect. The model has implications for the changes of vertical oceanic nutrient fractionation [Boyle, 1988] and the respiratory dissolution of deep carbonates [Archer and Maier-Reimer, 1994] and hence could have important potential implications for the timing of global CO2 exchanges between ocean and atmosphere and their feedback to climate.

Organic‐rich sediments in ventilated deep‐sea environments: Relationship to climate, sea level, and trophic changes

Sediments on the Namibian Margin in the SE Atlantic between water depths of ∼1000 and ∼3600 m are highly enriched in hydrocarbon-prone organic matter. Such sedimentation has occurred for more than 2 million years and is geographically distributed over hundreds of kilometers along the margin, so that the sediments of this region contain a huge concentrated stock of organic carbon. It is shown here that most of the variability in organic content is due to relative dilution by buried carbonates. This reflects both export productivity and diagenetic dissolution, not differences in either water column or bottom water anoxia and related enhanced preservation of organic matter. These observations offer a new mechanism for the formation of potential source rocks in a well-ventilated open ocean, in this case the South Atlantic. The organic richness is discussed in terms of a suite of probable controls including local wind-driven productivity (upwelling), trophic conditions, transfer efficiency, diagenetic processes, and climate-related sea level and deep circulation. The probability of past occurrences of such organic-rich facies in equivalent oceanographic settings at the edge of large oceanic basins should be carefully considered in deep offshore exploration.

Spatial variations in nutrient utilization, production and diagenesis in the sediments of a coastal upwelling regime (NW Africa): Implications for the paleoceanographic record

A biogeochemical study of recent (multicores) sediments of the northwest African slope was undertaken to understand how the sediment composition varies with respect to the location of core sites relative to the centers of coastal upwelling, and how this has affected the palaeoceanographic record. Sedimentary organic carbon contents are inversely correlated with the nitrogen isotopic composition (δ 15 N), high C organic concentrations and low δ 15 N occurring at proximal (shallow) sites and the opposite at distal (deep) ones. These spatial differences are interpreted to result from higher relative nutrient utilization and a decrease in production as waters are advected offshore from the zone of upwelling. Highest C organic contents also correlate positively with highest concentrations of redox-sensitive elements (U, Mo and S) that are fixed diagenetically in the sediments. These results suggest that the sedimentary regime at a fixed position depends on the spatial location of the productive areas relatively to a given core site. Downcore records of Zr/Al, Ti/Al, mean grain size of the terrigenous fraction, δ 15 N, C organic , biogenic Ba, U, Mo and sulfur at a single site on the slope are interpreted to reflect glacial-interglacial changes in the core location relative to the coastline (sea-level effect), and hence changes in production as the area of coastal upwelling moved on- and offshore as sea-level changed, as well as undoubtedly changes in upwelling intensity through wind forcing. Further studies are needed to fully understand the interrelationships of all these processes, which are required for building more reliable paleoceanographic-paleoclimatic records.

Assessment of metal concentrations found within a North Sea drill cuttings pile

North Sea drill cuttings piles are a distinct anthropogenic legacy resulting from the exploration and production of North Sea oil reserves. The need to understand metal cycling within the piles becomes increasingly important with the imminent decommissioning of many North Sea platforms and the subsequent fate of associated cuttings piles. This paper presents results of the simultaneous analysis of geochemical carrier substances (Mn and Fe oxyhydroxides), along with dissolved (<0.2 microm) and total (>0.2 microm) metal (Ba, Co, Cr, Cu, Mo, Pb, V) concentrations from a North Sea cuttings pile and surrounding sediment. These data are examined in conjunction with in situ measured porewater oxygen and sulfide. Results show a rapid removal of oxygen within the top few millimeters of the cuttings pile along with elevated concentrations of total hydrocarbons and solid phase metal concentrations compared to the surrounding environment.

Benthic Processes and the Burial of Carbon

A major goal of the Joint Global Ocean Flux Study (JGOFS) has been to understand the export of carbon from the surface ocean to the deep sea, a process which removes carbon from the active exchange with the atmosphere for long periods of time. Deep-sea sediments are the final sink of organic matter which is not degraded in the water column nor at the water-sediment interface. This interface is a physical boundary collecting and concentrating sinking particulate organic matter from fine debris to dead whales and consequently supports a fairly active benthic community. The level of biotic activity, the rates of remineralization, and last, but not least, the amount of material buried and preserved in the sediment, all depend on the mass flux and the composition of the material reaching the sea floor. As will be shown in this chapter, the connection between the surface ocean and the seafloor is not a simple one. However, the integration of signals at the sea floor allows conclusions to be drawn about upper ocean processes which go beyond the period of direct observation.

Rates and Regulation of Microbially Mediated Aerobic and Anaerobic Carbon Oxidation Reactions in Continental Margin Sediments from the Northeastern Arabian Sea (Pakistan Margin)

Rates of microbially mediated C oxidation were measured at sites above, within, and below the oxygen minimum zone (OMZ) on the Pakistan margin of the Arabian Sea, before and after the southwest monsoon, with the goal of assessing how low bottom water O 2 concentration affects microbial C oxidation processes. Rates of C oxidation coupled to aerobic and anaerobic processes were measured at five depths: 140 m (seasonally hypoxic), 300 m (OMZ core), 940 m (OMZ transition), 1200 m (OMZ transition), and 1850 m (non-OMZ). Rates and mechanisms of C oxidation did not vary significantly between seasons. However, an exception was found at the 140-m site, which became hypoxic during the southwest monsoon. Considering both seasons, C oxidation rates ranged from 0.73 to 4.86 mmol C m ―2 d ―1 . Generally, OMZ sites and those on the OMZ transition had lower C oxidation rates (0.73―2.90 mmol C m ―2 d ―1 ) than those located below the OMZ (3.13―4.86 mmol C m ―2 d ―1 ). The relative importance of C oxidation via different terminal electron acceptors varied between sites according to the position and intensity of the OMZ. At all sites, a large proportion of measured O 2 consumption (30―100%) was coupled to the oxidation of reduced species; consequently, aerobic processes were essentially absent at low-0 2 sites. In contrast, under higher bottom water O 2 concentrations, aerobic processes accounted for 4―64% of C oxidation. Denitrification largely dominated carbon oxidation at all sites (36―99%). Rates of C oxidation coupled to microbial Mn 4+ and Fe 3+ reduction were quantitatively unimportant. Measured sulphate reduction rates at all sites across the margin were surprisingly low (0―0.45 mmol m ―2 d ―1 ) compared to rates measured on other margin environments.

Rates and regulation of microbially mediated aerobic and anaerobic carbon oxidation reactions in continental margin sediments from the Northeastern Arabian Sea (Pakistan Margin)

Rates of microbially mediated C oxidation were measured at sites above, within, and below the oxygen minimum zone (OMZ) on the Pakistan margin of the Arabian Sea, before and after the southwest monsoon, with the goal of assessing how low bottom water O 2 concentration affects microbial C oxidation processes. Rates of C oxidation coupled to aerobic and anaerobic processes were measured at five depths: 140 m (seasonally hypoxic), 300 m (OMZ core), 940 m (OMZ transition), 1200 m (OMZ transition), and 1850 m (non-OMZ). Rates and mechanisms of C oxidation did not vary significantly between seasons. However, an exception was found at the 140-m site, which became hypoxic during the southwest monsoon. Considering both seasons, C oxidation rates ranged from 0.73 to 4.86 mmol C m ―2 d ―1 . Generally, OMZ sites and those on the OMZ transition had lower C oxidation rates (0.73―2.90 mmol C m ―2 d ―1 ) than those located below the OMZ (3.13―4.86 mmol C m ―2 d ―1 ). The relative importance of C oxidation via different terminal electron acceptors varied between sites according to the position and intensity of the OMZ. At all sites, a large proportion of measured O 2 consumption (30―100%) was coupled to the oxidation of reduced species; consequently, aerobic processes were essentially absent at low-0 2 sites. In contrast, under higher bottom water O 2 concentrations, aerobic processes accounted for 4―64% of C oxidation. Denitrification largely dominated carbon oxidation at all sites (36―99%). Rates of C oxidation coupled to microbial Mn 4+ and Fe 3+ reduction were quantitatively unimportant. Measured sulphate reduction rates at all sites across the margin were surprisingly low (0―0.45 mmol m ―2 d ―1 ) compared to rates measured on other margin environments.

The use of science in understanding the marine environment of the Atlantic margin

Abstract In taking this overview, it is clearly necessary to identify some major contributions that have helped us understand the complexity of the marine environment that embodies the Atlantic Margin to the west of Scotland. Here, I attempt to provide a point of view that spans the wide spatial scale that we can obtain from satellites, right down the way to the behaviour of materials and substances as they transit through the water column and across the sediment–water interface. It is in this latter context that mans impact in this frontier region needs to be assessed and in particular, any potential impact that the exploration of oil and gas may have.