Ralf R. Haese

Iron species determination to investigate early diagenetic reactivity in marine sediments

Iron speciation was determined in hemiplegic sediments from a high productivity area to investigate systematically the early diagenetic reactivity of Fe. A combination of various leaching agents (1 M HCI, dithionite buffered in citrate/acetic acid, HF/H2SO4, acetic Cr(II)) was applied to sediment and extracted more than 80% of total Fe. Subsequent Fe species determination defined specific mineral fractions that are available for Fe reduction and fractions formed as products of Fe diagenesis. To determine the Fe speciation of (sheet) silicates we explored an extraction procedure (HF/H2SO4) and verified the procedure by application to standard rocks. Variations of Fe speciation of (sheet) silicates reflect the possible formation of Fe-bearing silicates in near surface sediments. The same fraction indicates a change in the primary input at greater depth, which is supported by other parameters. The Fe(II)/ Fe(III) -ratio of total sediment determined by extractions was compared with Mossbauer-spectroscopy ] at room temperature and showed agreement within 10%. M6ssbauer-spectroscopy indicates the occurrence of siderite in the presence of free sulfide and pyrite, supporting the importance of microenvironments during mineral formation. The occurrence of other Fe(II) bearing minerals such as ankerite (Ca-, Fe-, Mg-carbonate) can be presumed but remains speculative.

Significance of the sedimentary Al∶Ti ratio as an indicator for variations in the circulation patterns of the equatorial North Atlantic

This paper presents results of geochemical investigations conducted on three gravity cores recovered along an E-W transect (3°–5°N) in the equatorial Atlantic. Considering the high terrigenous content of the sediments (approximately 12–74 wt%), Al and Ti concentrations are predominantly associated with the input of lithogenous phases. The terrigenous fraction of western equatorial Atlantic sediments is more influenced by material derived from the Amazon River than by deposition of African dust particles. As a possible route of transport, the North Equatorial Countercurrent is considered. Fluctuations in Al/Ti records generally reflect changes in the low-latitude insolation cycle (19–23 kyr) and thus indicate a close coupling to variations of the trade-wind systems and continental climate changes. However, characteristics of the terrigenous signals in the eastern and western equatorial Atlantic differ significantly. While the precessional signal in the Al/Ti record and low latitude portion in the δ18O record are in phase in the eastern equatorial Atlantic, variations in the terrigenous fraction westward of the Mid-Atlantic Ridge precede the marine signal by 2–3 kyr, which we ascribe to a shift between trade-wind systems of the Northern and Southern Hemispheres.

Carbon geochemistry of cold seeps: Methane fluxes and transformation in sediments from Kazan mud volcano, eastern Mediterranean Sea

Abstract Despite growing concerns about potential enhancement of global warming and slope failure by methane produced by gas hydrate dissociation, much uncertainty surrounds estimates of gas hydrate reservoir sizes, as well as methane fluxes and oxidation rates at the sea floor. For cold seep sediments of the eastern Mediterranean Sea, depth-dependent methane concentrations and rates of anaerobic oxidation of methane (AOM) are constrained by modeling the measured pore-water sulfate profile. The calculated dissolved methane distribution and flux are sensitive to the advective flow velocity, which is estimated from the depth distributions of conservative pore-water constituents (Na, B). Near-complete anaerobic oxidation of the upward methane flux of ∼6.0 mol m−2 yr−1 is supported by the depth distributions of indicative biomarkers, and the carbon isotopic compositions of organic matter and dissolved inorganic carbon. Pore-water and solid-phase data are consistent with a narrow depth interval of AOM, 14–18 cm below the sediment–water interface. Based on an isotopic mass balance, the biomass of the microbial population carrying out oxidation of methane coupled to sulfate reduction at the given methane flux represents ∼20% of the total organic carbon, which is a significant pool of in situ formed organic matter. Model results indicate that the asymptotic methane concentration is reached a few meters below the sediment surface. The predicted asymptotic concentration is close to the in situ saturation value with respect to gas hydrate, suggesting that the rate of shallow gas hydrate formation is controlled by the ascending methane flux. The proposed model approach can be used to predict the formation of gas hydrate, and to quantify methane fluxes plus transformation rates in surface sediments where fluid advection is an important transport mechanism.

Barium peaks at glacial terminations in sediments of the equatorial Atlantic Ocean—relicts of deglacial productivity pulses?

In sediment cores recovered from oligotrophic settings of the western and eastern north-equatorial Atlantic Ocean, Ceara Rise and Sierra Leone Rise, distinct peaks of excess barium were detected at glacial/interglacial transitions. These Ba maxima are unrelated to any other potential productivity proxy, e.g. organic carbon, carbonate or opal. Moreover, they coincide with minima in organic carbon contents. Despite the lack of correlation between excess barium and organic carbon, we ascribe these Ba spikes to pulses of productivity that occurred during glacial/interglacial transitions. The discrepancy between barium and organic carbon in these transitional sediment intervals is attributed to the action of downward-progressing oxidation fronts during deglacial nonsteady-state depositional conditions. The oxidation fronts were initiated due to the overall low sedimentation rates prevailing in the study areas and an increase in bottom water oxygen concentrations at the onset of enhanced NADW production during interglacial periods. The fronts led to a very efficient oxidation of the organic carbon initially present leaving peaks in solid phase barium as relicts of these short-time productivity events. This assumption is supported by the depth arrangement of the solid phase peaks of Ba, Fe, and Mn along the 6/5 oxygen isotope stage boundary in sediments of the Ceara Rise which show striking similarities to the distribution of these elements in oxidized sapropel intervals in eastern Mediterranean sediments. It remains unclear whether productivity was proportional to the magnitude of the Baexcess maxima. Possibly, higher dissolved Ba concentrations in intermediate and deep waters during glacial/interglacial transitions caused the precipitation of barite to increase out of proportion of the (postulated) deglacial productivity pulses.

The early diagenesis of iron in pelagic sediments: a multidisciplinary approach

Abstract Biogeochemical reactions of iron within pelagic sediments from the eastern and western equatorial Atlantic are investigated by means of pore water chemistry, chemical leaching experiments and total elemental determinations, color reflectance spectroscopy, rock magnetic measurements and TEM observations of the magnetic fraction. Results indicate that in the presence of nitrate, ascorbate (a weak reducing agent) leachable iron decreased with depth from the sediment surface. Within this upper sediment region, iron assimilation by bacteria is indicated as magnetosomes were found by TEM observations throughout the whole core. Extractions with a strong reducing agent (dithionite), representing iron bound to iron oxides, correlated linearly with the concentration of highly coercive magnetic minerals and with the reflection intensity of red color within the same iron redox zone. The abrupt decrease of red color reflection intensity, relative to the amount of iron oxides below the iron redox boundary, is the result of a decrease in the specific surface area of iron oxyhydroxide/oxides. Magnetic parameters imply a smaller average grain size of the magnetic fraction below the iron redox boundary, arising from an increase in biogenic magnetite formed by magnetotactic (assimilatory) bacteria, dissolution of very fine-grained magnetite, and the gradual decrease of coarser grained terrigenous (titano-) magnetite with depth. The early diagenetically formed magnetic fraction withstands subsequent dissolution and gives pronounced peaks of magnetic parameters within sediments with high carbonate and low terrigenous matter content.

Changes in the deep subsurface microbial biosphere resulting from a field-scale CO2 geosequestration experiment

Subsurface microorganisms may respond to increased CO2 levels in ways that significantly affect pore fluid chemistry. Changes in CO2 concentration or speciation may result from the injection of supercritical CO2 (scCO2) into deep aquifers. Therefore, understanding subsurface microbial responses to scCO2, or unnaturally high levels of dissolved CO2, will help to evaluate the use of geosequestration to reduce atmospheric CO2 emissions. This study characterized microbial community changes at the 16S rRNA gene level during a scCO2 geosequestration experiment in the 1.4 km-deep Paaratte Formation of the Otway Basin, Australia. One hundred and fifty tons of mixed scCO2 and groundwater was pumped into the sandstone Paaratte aquifer over 4 days. A novel U-tube sampling system was used to obtain groundwater samples under in situ pressure conditions for geochemical analyses and DNA extraction. Decreases in pH and temperature of 2.6 log units and 5.8°C, respectively, were observed. Polyethylene glycols (PEGs) were detected in the groundwater prior to scCO2 injection and were interpreted as residual from drilling fluid used during the emplacement of the CO2 injection well. Changes in microbial community structure prior to scCO2 injection revealed a general shift from Firmicutes to Proteobacteria concurrent with the disappearance of PEGs. However, the scCO2 injection event, including changes in response to the associated variables (e.g., pH, temperature and salinity), resulted in increases in the relative abundances of Comamonadaceae and Sphingomonadaceae suggesting the potential for enhanced scCO2 tolerance of these groups. This study demonstrates a successful new in situ sampling approach for detecting microbial community changes associated with an scCO2 geosequestration event.

Macrobenthic Activity and its Effects on Biogeochemical Reactions and Fluxes

The impact of macrobenthic activity on the geochemistry of surface sediments is reviewed to provide conceptual insights on animal-sediment relations for benthic ecologists, paleoceanographers applying paleo-redox proxies and geochemists interested in the broad area of early diagenesis. It is pointed out that conceptual models for the geochemical implications of macrobenthic activity are relatively well understood but that quantitative approaches are largely lacking. Consequently, particular attention is directed to in situ and ex situ methods to derive rates of macrobenthic activity. From this literature study it becomes clear that benthic fauna studies and geochemical studies have rarely been integrated. However, this is essential to fully understand the impact of the temporal and spatial variable benthic assemblages on important issues such as organic matter mineralization and metal mobilization in ocean margin sediments. The effects of macrobenthic activity are highly diverse and concern dissolved and solid phase distributions. With respect to nutrient cycling and organic matter mineralization the most important effects arise from bioirrigation. Burrows and tubes are flushed with oxic bottom water which increases the total surface area for aerobic respiration, nitrification and denitrification. In addition, active pumping increases the efflux of dissolved species and creates radial diffusion which is not accounted for when fluxes are quantified from (vertical) pore water profiles by means of molecular diffusion. Since metal diagenesis is ultimately related to solid phase redistributions, e.g. across redox boundaries, bioturbation plays an important role. The depth distribution of bioturbatory activity depends on the feeding strategy of the prevailing fauna which varies significantly.

High-Magnesium Calcite Dissolution in Tropical Continental Shelf Sediments Controlled by Ocean Acidification

Increases in atmospheric CO2 cause the oceanic surface water to continuously acidify, which has multiple and profound impacts on coastal and continental shelf environments. Here we present the carbonate mineral composition in surface sediments from a range of continental shelf seabed environments and their current and predicted stability under ocean acidifying conditions. Samples come from the following four tropical Australian regions: (1) Capricorn Reef (southern end of the Great Barrier Reef), (2) the Great Barrier Reef Lagoon, (3) Torres Strait, and (4) the eastern Joseph Bonaparte Gulf. Beyond the near-shore zone, these regions typically have a carbonate content in surface sediments of 80 wt % or more. The abundance of high-magnesium calcites (HMC) dominates over aragonite (Arag) and low-magnesium calcite (LMC) and constitutes between 36% and 50% of all carbonate. HMC, with a magnesium content larger than 8-12 mol %, is more soluble than both Arag and LMC, and the solubility of HMC positively correlates with its magnesium concentration. From the solubility data of Plummer and Mackenzie ( Am. J. Sci. 1974 , 274 , 61 - 83 ), 95% of HMC in the four regions is presently in metastable equilibrium relative to global mean tropical sea surface water. HMC is predicted to become destabilized in the four regions between 2040 and 2080 AD, with typical HMC decline rates between 2% and 5% per year. The range of respective estimated carbonate dissolution rates is expected to exceed current continental shelf carbonate accumulation rates, leading to net dissolution of carbonate during the period of HMC decline. In a geological context, the decline in HMC in tropical continental shelf environments is a global event triggered by reaching below-equilibrium conditions. The characteristic change in carbonate mineral composition in continental shelf sediments will serve as a geological marker for the proposed Anthropocene Epoch.

Subsurface Fluid Flow and Material Transport

Submarine subsurface fluid flow is ubiquituous with rates varying enormously with space and time. Therefore, fluid flow most probably is of paramount importance for the transport of matter and heat as well as to control fluxes between the subsurface and the ocean. However, rates to date still are only very poorly understood. In this study we therefore first identify major fluid flow systems including fault transport at active and passive margins, gas hydrate affected flow systems, or submarine groundwater discharge. Then, we describe geophysical and geochemical methods which are capable to better and quantitatively understand interacting submarine fluid flow systems and lastly, propose strategies for future research.