Michael D. Krom

The diagenesis of phosphorus in a nearshore marine sediment

Abstract Chemical analyses have been made for total inorganic phosphorus, total organic phosphorus, and organic carbon in plankton and in anoxic sediments from Long Island Sound, U.S.A. When combined with laboratory experiments, the measurements of other studies on the same sediments, and mathematical modelling, the results indicate that: 1. (1) A large proportion of dissolved phosphate in the upper ~10 cm of sediment (zone of bioturbation) is provided by the release of adsorbed phosphate during the reduction of ferric oxyhydroxides; 2. (2) Below the zone of bioturbation phosphate is liberated to solution solely via organic matter decomposition and the release is stoichiometrically coupled to bacterial sulfate reduction and ammonia formation, 3. (3) A steady state diagenetic model can be used to predict the profile of organic phosphorus from that of dissolved phosphate at depths below the zone of bioturbation; 4. (4) The flux of dissolved phosphate out of the sediment must arise largely from phosphate liberation occurring very close ( 5. (5) There is a large preferential loss of phosphorus relative to carbon from organic matter at the time of or prior to burial, whereas after burial phosphorus is not lost preferentially at this location.

Active post-depositional oxidation of the most recent sapropel (S1) in sediments of the eastern Mediterranean Sea

Abstract Over a wide area of the eastern Mediterranean basin, two Mn-rich layers have been observed above the most recent sapropel (S1), one immediately above the sapropel top and one a few centimetres closer to the sediment surface. Different mechanisms have been proposed to explain the occurrence of these two Mn peaks: either both peaks have a diagenetic origin in which case the upper Mn peak is actively forming, or the lower peak is actively forming and the upper peak has a different formation mechanism. High-resolution porewater, including a gel sampler used for the first time in marine sediments, and solid phase data are now used to demonstrate that the oxidation front is located at the level of the lower Mn peak which, therefore, is presently being formed. A barium-organic carbon relationship is used to calculate the initial organic carbon profile of the S1 sapropel. The palaeoproductivity profiles generated by this method demonstrate that the original sapropel unit was bounded by the upper Mn peak. This implies that the interval between the two Mn peaks, where a low organic carbon content is now observed, was originally part of the sapropel. The initially deposited organic carbon has been oxidised by a progressive downwards-moving oxidation front. The penetration depth of this oxidation front, i.e., the distance between the two Mn peaks, is mainly determined by the organic carbon content, the sediment accumulation rate, and the bioturbation depth. The upper Mn peak appears to have formed as a result of either Mn precipitation upon oxygenation of previously anoxic eastern Mediterranean deep water, or preservation of a surficial Mn peak at the end of the high productivity episode. In either case the upper Mn peak marks the end of sapropel formation as indicated by the Ba profiles. This means that formation of the S1 sapropel ceased more recently than is indicated by radiocarbon dating of the visible top of S1.

Seaweed biofilters as regulators of water quality in integrated fish-seaweed culture units

Abstract The water-quality characteristics of a new system for the integrated culture of fish ( Sparus aurata L.) and seaweed ( Ulva lactuca L.) were examined. Seawater was recirculated between intensive fishponds and seaweed ponds. The seaweed removed most of the ammonia excreted by the fish and oxygenated the water. A model consisting of several tanks and a pilot consisting of two 100-m 3 , 100-m 2 ponds were studied. In both, the metabolically dependent water-quality parameters (dissolved oxygen, NH 4 + -N, oxidized-N, pH and phosphate) remained stable and within safe limits for the fish during over 2 years of operation. The design allowed significant increases in overall water residence time (4.9 days), compared with conventional intensive ponds, and produced a high yield of seaweed in addition to the fish. The design provides a practical solution to major management and environmental problems of land-based mariculture.

Nutrient dynamics and new production in a warm-core eddy from the Eastern Mediterranean Sea

The nutrient dynamics of a warm-core eddy from the Eastern Mediterranean was studied through a complete annual cycle. Winter mixing of the upper 450 m of the eddy core resulted in phytoplankton biomass build-up (70 mg Chl a m−2) of stock well above that present in the boundary (44 mg m−2). During this biomass build-up phosphate was reduced to undetectable levels while excess nitrate (0.6 μM) remained which, taken with other evidence, showed that this area of the Eastern Mediterranean is phosphorus limited. After stratification in late March, nitrate and phosphate concentrations were at undetectable levels in the upper 120 m, a deep chlorophyll maximum had developed (90–120 m), and a gradual systematic accumulation of nutrients occurred in the zone from 120 to 450 m. The permanent nutricline lay between 450 and 650 m. The net supply of nutrients to the euphotic zone (0–120 m), which is the annual new production, was 300 mmoles N m−2 y−1 and 14.9 mmoles P m−2 y−1. Most of this new production (∼90%) was supplied by deep winter mixing and occurred over a limited period of time (days to a few weeks). Previous estimates for new production in the Eastern Mediterranean by Dugdale and Wilkerson (1988, Oceanologica Acta, 9, 170–184), based on bottle incubations, were low because the major input of nutrients by deep winter mixing was not included in the calculation. Most of the nutrients supplied to the euphotic zone in the eddy (60–70%) were derived from the decomposition of the previous years productivity.

Nile River sediment fluctuations over the past 7000 yr and their key role in sapropel development

The provenance pattern of Nile River sediments can be used as a proxy for paleoclimatic changes in East Africa. The 87Sr/86Sr ratios are particularly appropriate for such provenance investigations, because the White Nile drains predominantly crystalline basement rocks, whereas the Blue Nile and Atbara flow off the Ethiopian Highlands, which consist of Tertiary volcanic rocks. A high-resolution profile of 87Sr/86Sr and Ti/Al ratios from a well-dated core in the Nile Delta shows a close correspondence with known changes in Nile flow over the past 7000 yr. At times of higher river flow there was markedly decreased input of Blue Nile–derived and total sediment. This change was caused by northward movement of the Inter Tropical Convergence Zone, resulting in increased vegetative cover in the Ethiopian Highlands due to higher rainfall and a longer wet season. This inverse relationship between Nile River flow and sediment flux may have had important implications in the development of agricultural technology in ancient Egypt. The marked minimum in 87Sr/86Sr at 4200–4500 yr B.P. is coincident with the end of the Old Kingdom in Egypt and provides independent evidence that demise of the Old Kingdom might have been associated with an extended period of catastrophic low floods. During the Quaternary and late Neogene, there was periodic deposition of organic-rich sediments (sapropels) in the eastern Mediterranean that represent important indicators of major environmental change. Evidence from the Ti/Al ratio suggests that the pattern of erosion and sediment supply from the Nile catchment observed in this study also occurred throughout much of the Neogene and Quaternary. The reduced inputs of Blue Nile sediment during times of sapropel formation contributed to the increased primary productivity by reducing the amount of phosphate removed on particles and to the observed change to N limitation in the eastern Mediterranean, which are important characteristics of sapropel deposition.

Dry atmospheric inputs of trace metals at the Mediterranean coast of Israel (SE Mediterranean) : sources and fluxes

Abstract This study presents the first detailed data on aerosol concentrations of trace metals (Cd, Pb, Cu, Zn, Cr, Mn, Fe and Al) at the SE Mediterranean coast of Israel, and assesses their sources and fluxes. Aerosol samples were collected at two sampling stations (Tel-Shikmona and Maagan Michael) along the coast between 1994 and 1997. Two broad categories of aerosol trace metal sources were defined; anthropogenic (Cd, Cu, Pb and Zn) and naturally derived elements (Al, Fe, Mn and Cr). The extent of the anthropogenic contribution was estimated by the degree of enrichment of these elements compared to the average crustal composition (EFcrust). High values (median >100) were calculated for Cd, Pb and Zn, minor values for Cu and relatively low values (

Biological productivity during sapropel S5 formation in the Eastern Mediterranean Sea: Evidence from stable isotopes of nitrogen and carbon

We determined 15N/14N ratios in modern surface and sapropel S5 sediments of the Mediterranean Sea to clarify differences in the nutrient regime associated with sapropel formation. In the modern situation, high δ15N of unused nitrate (15–20 ‰) remaining in the surface waters during the winter phytoplankton bloom evidences P-limitation of biological production in winter. δ15N of surface sediments decrease towards the east of the basin (5 to >2.5‰). This is a consequence of either eastward increasing nitrogen fixation during the summer months, or of particulate matter being supplied predominantly by the P-limited winter bloom. Very low (−1–1‰) δ15N values in sapropel S-5 from four locations require a very light source of nutrient-N assimilated at a minimum of ten times the modern export flux. Because the isochronous records show no spatial gradient in δ15N, we exclude both Ekman-type upwelling and direct riverine discharge as likely sources of nutrients. Our data are consistent with an anti-estuarine thermohaline circulation in the upper 500m during S5 time, allowing for the trapping of nutrients in the eastern basin. The most likely scenario for S5 is that phosphorus release from a relatively shallow redox boundary resulted in an imbalanced supply of N:P (<16:1) to the photic zone. The result was a slow assimilation of carbon during summer stratification and extensive N2-fixation providing the majority of the export flux from a N-limited system.

Localised remobilization of metals in a marine sediment

Trace metals and Fe and Mn were measured at vertical spatial resolutions of 2.5 and 5 mm in the top 35 cm of the profundal sediment of a Scottish sea-loch using DGT (diffusive gradients in thin films) technique. DGT probes lower adjacent metal concentrations in pore waters and induce a flux of metal from the solid phase to porewater. The concentrations of metals in porewaters at the interface of the probe were measured during its deployment in a box core. These measurements reflect porewater concentrations of metals and their rates of resupply from the local solid phase of a very small volume (25 microl) of sediment. There was pronounced horizontal and vertical structure in the interfacial concentrations. Horizontal variations were shown by results from adjacent DGT assemblies being markedly different in detail, while vertical structure was measured directly by the DGT-depth profiles. Iron and Mn varied systematically with depth, with both broad and detailed features of Co aligning with those of Mn. There was, however, evidence of additional localised sources of Co that were apparently unrelated to the redox behaviour that Mn typifies, but associated with the remobilization of Ni, possibly from mineral dissolution. Arsenic(III) was remobilized in well-defined zones. Detailed correspondence of As(II) with some Fe features suggest that its release is mechanistically-related to iron oxide dissolution, but the 3 orders of magnitude higher concentrations of Fe may sometimes obscure the association. These results demonstrate that, within sediments, metals may be released in discrete locations that are not measured by conventional porewater sampling techniques due to their horizontal averaging.

The characterisation of Saharan dusts and Nile particulate matter in surface sediments from the Levantine basin using Sr isotopes

Abstract The provenance of sediments within the Levantine basin of the eastern Mediterranean was studied using 87 Sr/ 86 Sr isotopic ratios, together with major elements. Measurements were made on the detrital fraction of surface sediments, and the two most important sources of detrital matter to the region. Saharan dust was characterised by an 87 Sr/ 86 Sr isotopic ratio in the range 0.7160–0.7192. There was a small systematic decrease in 87 Sr/ 86 Sr from west to east which is interpreted as due to a change in the balance of aeolian source material. The Nile particulate matter had a 87 Sr/ 86 Sr isotopic ratio in the range 0.7057–0.7071. 87 Sr/ 86 Sr isotopic signatures together with Sr concentration were used to calculate the contributions made by Saharan dust and Nile particulate matter to the surface sediments of the Levantine basin. It was shown that Nile-derived sediment was dispersed widely across the Levantine basin enabling the area affected by retention of sediment behind the Aswan dam to be determined.

Trace metal distribution in sediments and benthic fauna of Haifa Bay, Israel

Abstract Haifa Bay, although adjacent to major chemical and other industries, showed no significant contamination of Cu, Cd, Pb and Zn in the sediments or benthic biota except for one station opposite the polluted Kishon river estuary. There was moderate to low Hg contamination throughout the bay, derived from the outfall of a chlor-alkali plant. The levels of Hg in the surficial sediment and biota have not changed significantly over the last 7 years, a period when effluent treatment was operating at the chlor-alkali plant. Using pollutant laden particles from recognized point sources as tracers for sediment transport, it was concluded that the southern part of Haifa Bay is an area of net sedimentation. No sediment from this area was detected further north in the bay. Sediments introduced at the north of the bay spread both north and south. Within most of the bay there has been no major sediment accumulation or erosion over the past 7 years.