G.J. de Lange

Redistribution and geochemical behaviour of redox-sensitive elements around S1, the most recent eastern Mediterranean sapropel

The youngest eastern Mediterranean sapropel (S1) is generally believed to have formed 7–9 14C ky B.P. The distribution of redox-sensitive elements above, in and below the sharply defined, organic-rich unit reveal that S 1 has already suffered heavy diagenetic alteration. Its present thickness, based on its dark colour, is only a fraction of its original thickness. In one core, visual evidence of S1 with an inferred initial thickness of 6 cm has been destroyed, and a similar thickness appears to have been lost from the tops of another two examples. This is caused by oxidation of the upper reaches of S 1 by a progressive oxidation front which commenced 5.2–5.4 14C ky ago, and still appears to be active in the three cores studied. Diagenetic relocation of many redox-sensitive elements has occurred. Most elements expected to be enriched in sapropels or sulphide-rich sediments already have maximum concentrations above or below, rather than in, the present S 1 layers. Similarities are observed in the geochemical behaviour of As, Fe, and P; of Co, Cu, Mn, Ni, and Zn; and of Cd, Mo, Sb, TI, U, and V. The behaviour of Ba is distinctive, and its immobility as barite makes it an indicator for palaeoproductivity and the original high S1 Corg content. The elements Se and I exhibit particularly large, sharp concentration peaks, and together are proposed as useful indicators of the present boundary between oxic and post-oxic conditions.

Universal Dynamical Decoupling of a Single Solid-State Spin from a Spin Bath

Avoiding Loss in a Quantum System Single electron spins in solid-state environments have been explored as candidates for quantum information storage and computation; however, they often interact strongly with their surroundings and lose the stored information on the time scale of pico- to milliseconds. Dynamical decoupling schemes have been introduced to “undo” the effects of this interaction by applying a sequence of control pulses that reverse the undesirable evolution of the system. De Lange et al. (p. 60, published online 9 September) tested several decoupling schemes on a nitrogen vacancy center in diamond and found that a scheme with evenly spaced pulses with double-axis decoupling could prolong the coherence time of an arbitrary spin state up to 25-fold. The coherence time of single spins is extended by a sequence of microwave pulses. Controlling the interaction of a single quantum system with its environment is a fundamental challenge in quantum science and technology. We strongly suppressed the coupling of a single spin in diamond with the surrounding spin bath by using double-axis dynamical decoupling. The coherence was preserved for arbitrary quantum states, as verified by quantum process tomography. The resulting coherence time enhancement followed a general scaling with the number of decoupling pulses. No limit was observed for the decoupling action up to 136 pulses, for which the coherence time was enhanced more than 25 times compared to that obtained with spin echo. These results uncover a new regime for experimental quantum science and allow us to overcome a major hurdle for implementing quantum information protocols.

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.

Palaeoproductivity and post-depositional aerobic organic matter decay reflected by dinoflagellate cyst assemblages of the Eastern Mediterranean S1 sapropel

Abstract In the reconstruction of bioproductivity in surface waters the extent to which a proxy has been diagenetically altered is often a matter of debate. Here we investigate how organic- and calcareous-walled dinoflagellate cysts can be used for separately estimating bioproductivity and oxygen related diagenesis. This is achieved by studying the cyst content of the most recent Eastern Mediterranean sapropel S1, that is thought to have been deposited under conditions of increased primary production in surface waters and possible anoxia in the bottom waters. Based on chemical evidence, it has been shown that the visible sapropelic layer represents only the residual lower part of what was initially a much thicker sapropel, as a result of post-depositional decay of organic matter related to oxygen penetration into the sediments. The effect of aerobic organic matter decay on the cyst associations is studied through the comparison of the unaffected, lower part of the initial sapropel and the ‘oxidised’ upper part. Comparing the unaffected sapropelic sediments with pre- and post-sapropelic material gives insight into the relationship between fossil cysts assemblages and palaeoproductivity. Impagidinium aculeatum, Impagidinium patulum, Operculodinium israelianum, Polysphaeridium zoharyi and probably Impagidinium spp., Impagidinium paradoxum and Nematosphaeropsis labyrinthus are very resistant against aerobic decay and their accumulation rates appear to be primarily related to productivity in surface waters. Protoperidinium and Echinidinium species, on the other hand, are shown to be very sensitive and can be used to recognise oxygen-related decay. The calcareous-walled dinoflagellate cysts seem to be unaffected by oxic organic matter decay in Mediterranean sediments.

Elemental and major biochemical changes across an oxidation front in a relict turbidite: An oxygen effect

Abstract The elemental and major biochemical compositions of the relict f-turbidite sampled in two cores from the Madeira Abyssal Plain were determined. This fine-grained, distal sequence occurs at ca. 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change. Oxygen diffused downward through sediments above this interface and in ca. 10 kyr destroyed 80% of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca. 140 kyr. These deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates. In both cores, a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers. The unoxidized sequences of both turbidities contained 0.93–1.02 wt% organic carbon (OC) and 0.10–0.11 wt% total nitrogen (TN). Approximately 20% of the initial OC and 40% of the initial TN remained in the oxidized horizons, with a consequent decrease in atomic C/N ratio from ca. 11 to ca. 5. All samples gave very low yields of lignin phenols and comparable OC-normalized yields of total aldoses and amino acids, and indicated predominantly marine organic matter (OM) and nonselective oxic degradation of these biochemical classes. Compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments, except that the oxidized horizons yielded markedly elevated (3–5X) percentages of nonprotein amino acids. This study clearly demonstrates that prolonged exposure to OZ can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone. In comparison to early diagenesis, however, alteration of the measured biochemicals was largely nonselective. Such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep-ocean environments.

Stratified prokaryote network in the oxic-anoxic transition of a deep-sea halocline

The chemical composition of the Bannock basin has been studied in some detail. We recently showed that unusual microbial populations, including a new division of Archaea (MSBL1), inhabit the NaCl-rich hypersaline brine. High salinities tend to reduce biodiversity, but when brines come into contact with fresher water the natural haloclines formed frequently contain gradients of other chemicals, including permutations of electron donors and acceptors, that may enhance microbial diversity, activity and biogeochemical cycling. Here we report a 2.5-m-thick chemocline with a steep NaCl gradient at 3.3 km within the water column betweeen Bannock anoxic hypersaline brine and overlying sea water. The chemocline supports some of the most biomass-rich and active microbial communities in the deep sea, dominated by Bacteria rather than Archaea, and including four major new divisions of Bacteria. Significantly higher metabolic activities were measured in the chemocline than in the overlying sea water and underlying brine; functional analyses indicate that a range of biological processes is likely to occur in the chemocline. Many prokaryotic taxa, including the phylogenetically new groups, were confined to defined salinities, and collectively formed a diverse, sharply stratified, deep-sea ecosystem with sufficient biomass to potentially contribute to organic geological deposits.

Fluid–sediment interactions at Eastern Mediterranean mud volcanoes: a stable isotope study from ODP Leg 160

Abstract Pore fluids from two ODP sites at Eastern Mediterranean mud volcanoes have been analyzed for their Cl concentration and their δ18O and δD isotopic composition. The Cl data span a wide range of concentrations, from extremely depleted with respect to seawater (as low as 60 mM) at the crest of Milano dome (site 970) to strongly enriched (up to 5.4 M) at Napoli dome (site 971). Chloride enrichment is known to be due to dissolving Messinian evaporites, whereas the source of the low-Cl fluid is deduced from stable isotope data presented here. The isotopic composition of the endmember fluid is found to be +10‰ for δ18O and −32‰ for δD for low- as well as for high-Cl waters. From this signature it can be concluded that neither gas hydrates nor meteoric water play a significant role in the freshening of the pore water. Several other processes altering the δ18O/δD composition of pore waters are discussed and considered to be of only negligible influence. The process characterizing the isotopic composition of the fluid is found to be clay mineral dehydration (mainly smectite–illite transformation), corresponding to a depth range of 3.5–7 km and an elevated temperature of about 120–165°C. A quantitative estimate shows that this reaction is capable of producing the observed extreme Cl depletion.

Sulphidic Mediterranean surface waters during Pliocene sapropel formation

Sapropels—organic-matter rich layers—are common in Neogene sediments of the eastern Mediterranean Sea. The formation of these layers has been attributed to climate-related increases in organic-matter production and increased organic-matter preservation due to oxygen depletion in more stagnant bottom waters,. Here we report that eastern Mediterranean Pliocene sapropels contain molecular fossils of a compound (isorenieratene) known to be synthesized by photosynthetic green sulphur bacteria, suggesting that sulphidic (euxinic)—and therefore anoxic—conditions prevailed in the photic zone of the water column. These sapropels also have a high trace-metal content, which is probably due to the efficient scavenging of these metals by precipitating sulphides in a euxinic water column. The abundance and sulphur-isotope composition of pyrite are consistent with iron sulphide formation in the water column. We conclude that basin-wide water-column euxinia occurred over substantial periods during Pliocene sapropel formation in the eastern Mediterranean Sea, and that the ultimate degradation of the increased organic-matter production was strongly influential in generating and sustaining the euxinic conditions.

The formation of Pliocene sapropels and carbonate cycles in the Mediterranean: diagenesis, dilution, and productivity

High-resolution micropaleontological (planktonic foraminifera and calcareous nannofossils) and geochemical (stable isotopes, organic carbon, Fe, P, S, Ca, Ba, Mn, and Al) records are presented for the first sapropel-containing carbonate cycle in the Pliocene of Sicily. The carbonate cycle is characterized by a gray to white to beige to white color layering typical of the marls of the Trubi formation. A faintly laminated sapropel is intercalated in the gray-colored bed of the carbonate cycle. CaCO3 content varies from 40% in the beige to 45-50% in the white layers. Lowest CaCO3 content of 25–30% is found in the gray layer and sapropel. Variations in carbonate and organic matter percentages can best be explained by changes in paleoproductivity rather than by variations in dilution and dissolution. Total productivity was highest during deposition of the gray layer and sapropel, as indicated by high organic carbon and Ba contents and high abundance of Globorotalia puncticulata. Carbonate production reached its highest values, however, during deposition of the white layers, as evidenced by enhanced abundances of planktonic foraminifera and nannofossils. The low carbonate content in the gray layer and sapropel is explained in terms of a collapse in carbonate production caused by extreme changes in the physical and biochemical properties of the water column, which in turn resulted in siliceous plankton and opportunistic foraminifers such as Globorotalia puncticulata outcompeting most calcareous organisms. The beige layer represents a low-productivity environment similar to the present-day eastern Mediterranean basin. Several mechanisms have previously been proposed to explain variations in productivity in the eastern Mediterranean. Both sapropels and gray layers were deposited at times when perihelion occurred in northern hemisphere summer. We envisage that the increase in seasonal contrast resulting from this orbital configuration enhanced winter mixing and stabilization of the water column during summer, both leading to favorable conditions for intensification of the spring bloom. In addition, a decrease in excess evaporation, as can be deduced from the δ18O record, led to shoaling of the pycnocline and reduced circulation, thus enhancing the availability of nutrients in the photic zone. Finally, enhanced precipitation and associated runoff should have caused an increase in river-borne nutrients.

Enhanced regeneration of phosphorus during formation of the most recent eastern Mediterranean sapropel (S1)

Phosphorus regeneration and burial fluxes during and after formation of the most recent sapropel S1 were determined for two deep-basin, low-sedimentation sites in the eastern Mediterranean Sea. Organic C/P ratios and burial fluxes indicate enhanced regeneration of P relative to C during deposition of sapropel S1. This is largely due to the enhanced release of P from organic matter during sulfate reduction. Release of P from Fe-bound P also increased, but this was only a relatively minor source of dissolved P. Pore-water HPO4 concentrations remained too low for carbonate fluorapatite formation. An increased burial of biogenic Ca-P (i.e., fish debris) was observed for one site. Estimated benthic fluxes of P during sapropel formation were elevated relative to the present day (900 to 2800 vs. 70 to 120 mol m 2 yr 1 ). The present-day sedimentary P cycle in the deep-basin sediments is characterized by two major zones of reaction: (1) the zone near the sediment-water interface where substantial release of HPO4 from organic matter takes place, and (2) the oxidation front at the top of the S1 where upward-diffusing HPO4 from below the sapropel is sorbed to Fe-oxides. The efficiency of aerobic organisms in retaining P is reflected in the low organic C/P ratios in the oxidized part of the sapropel. Burial efficiencies for reactive P were significantly lower during S1 times compared with the present day (7 to 15% vs. 64 to 77%). Budget calculations for the eastern Mediterranean Sea demonstrate that the weakening of the antiestuarine circulation and the enhanced regeneration of P both contributed to a significant increase in deep-water HPO 4 concentrations during sapropel S1 times. Provided that sufficient vertical mixing occurred, enhanced regeneration of P at the seafloor may have played a key role in maintaining increased productivity during sapropel S1 formation. Copyright