Reinhard Hesse

Gas hydrates (clathrates) causing pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections of terrigenous continental margins

The occurrence of gas hydrates in deep-water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep-sea drilling results. On the Pacific continental slope off Guatemala gas hydrates were brought up for the first time from two holes (497, 498A) drilled during Leg 67 of the DSDP in water depths of 2360 and 5500 m, respectively. The hydrates occur in organic matter-rich Pleistocene to Miocene terrigenous sediments. In the hydrate-bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10–20 m subbottom depth. Pore waters at the bottom of the holes (near 400 m subbottom) have as little as half the chlorinity of seawater (i.e. 9‰). Similar, but less pronounced, trends were observed during previous legs of the DSDP in other hydrate-prone segments of the continental margins where recharge of fresh water from the continent can be excluded (e.g. Leg 11). The crystallization of hydrates, like ice, excludes salt ions from the crystal structure. During burial the dissolved salts are separated from the solids. Subsidence results in a downward motion of the solids (including hydrates) relative to the pore fluids. Thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation. The volume of the latter decreases downhole thus causing downward decreasing salinity (chlorinity). Hydrate formation is responsible for oxygen isotope fractionation with18O-enrichment in the hydrate explaining increasingly more positive δ18O values in the pore fluids recovered (after hydrate dissociation) with depth.

Silica diagenesis: origin of inorganic and replacement cherts

Abstract Silicification of originally non-siliceous sediments affects a wide variety of rock-types and materials and ranges from minor to pervasive. Partial and minor chertification occur mostly in Phanerozoic carbonates, carbonate-bearing sandstones, evaporites, and fossil wood. The source of the silica is predominantly biogenic. In petrified wood the silicification mechanism is a permeation or void-filling process, not a replacement. In this example, the sequence of silica-phase transformation is the same as that in deep-sea siliceous sediments. In many silicified rocks, particularly in certain carbonates, the transformation sequence is different from that in radiolarites or diatomites. The chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of Knauth (1979), but remain unknown for most other types. An exception are the flint nodules and bands of the English Chalk. A detailed geochemical study of the paramoudra flint structures by Clayton (1986) provided remarkable insight into the replacement process. Seven different recurring silica fabrics have been recognized in chert-replaced carbonates including equigranular (microcrystalline quartz or microquartz and megaquartz) and fibrous types (chalcedony, quartzine or length-slow chalcedony, lutecite, zebraic chalcedony and microflamboyant quartz). Among the latter, quartzine and microflamboyant quartz are common in, but by no means restricted to chert-replaced evaporites, for which Milliken (1979) recognized a sequence of seven quartz-fabrics. As a single criterion, only anhydrite inclusions in megaquartz, quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor. The relative timing between silicification and well-established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur. Chertification of carbonate host sediment thus may take place during early, intermediate or late diagenesis, and even during anchimetamorphism. Pervasive to complete silicification has been described in lacustrine, pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds, members or entire formations. It may affect some of the aforementioned rock types too, for example carbonates. The source of the silica is predominantly inorganic. Chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage. Magadi-type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in East African rift valley lakes such as Lake Magadi in Late Pleistocene time. They are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes. Silcretes originate from weathering and soil-forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites. In more humid climates, however, non-weathering-profile silcretes occur which have been distinguished from weathering-profile silcretes. Four to five different silcrete fabric types have been established, including the (1) grain-supported (or ‘quartzitic’), (2) floating (terrazzo), (3) matrix (Albertinia and opaline), and (4) conglomeratic types. In volcanic edifices, silicification related to hydrothermal activity occurs (1) along the ascent-routes of the fluids (vents) within the volcanic complexes, (2) in isolated ponds and depressions on the sea-floor where the fluids discharge, mostly in the median rift valley of mid-ocean ridges, and (3) in geothermal areas on land associated with spreading lineaments, volcanic islands arcs, transform faults or continental hot spots. Attention has focussed on the associated base-metal concentrations and less on the silicification and accompanying iron enrichment processes. The Precambrian Banded Iron Formations may be indirectly or directly related to hydrothermal-volcanic processes. The origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it.

Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface : What have we learned in the past decade?

Abstract The most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the Ocean Drilling Program (ODP), notably legs 112, 141, 146 and 164, and to a lesser extent legs 131, 160, 170 and 186. Leg 164, in particular, was dedicated to gas-hydrate drilling in the Blake Ridge gas-hydrate field in the West Atlantic in an unprecedented multidisciplinary research effort [Paull et al., 1996. Proc. Ocean Drill. Program, Initial Rep. 164, 623 pp.; Paull C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), 2000a. Proc. Ocean Drill. Program Sci. Results 164, Ocean Drilling Program, College Station, TX, 459 pp.]. Most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion-modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones. This started with qualitative evidence for advective flow from drilling the (hydrate-bearing) Peru and (hydrate-free) Barbados active margins, continued with the Nankai Trough accretionary prism, the Japan Trench Slope and the Cascadia and Costa Rica active margins and culminated in the quantitative advection-diffusion model of Egeberg and Dickens [Chem. Geol. 153 (1999) 53.] for the passive margin setting of the Blake Ridge. Advective flow regimes are different at active and passive margins, as there is a tendency for the flow at active margins to be focussed along landward-dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step-pattern of the pore water profiles. For passive margins, but also for some active margin sites, advection is from sources below the drilled section, either through subvertical faults (passive margins) or from the decollement zone (active margins). We have learned that the well-known coupled pore water anomalies that are ascribed to hydrate dissociation—downward chlorinity decrease combined with δ 18 O increase [Hesse and Harrison, 1981, Earth Planet. Sci. Lett. 55 (1981) 453.]—need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low-δ 18 O fluids. However, if the anomalies show up, hydrates are present almost invariably (with the exception of advected low-Cl − /high-δ 18 O waters). Coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate-affected pore water profiles at passive margins. Instrumental for modeling are chlorine isotopes, which provide an effective tool to assess advection rates. The Egeberg and Dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection, diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions. Modeling the more complex pore water profiles of active margins is a challenge for the future. Compared to geophysical methods to estimate hydrate concentration, the geochemical method gives minimum amounts.

Oxygen-18 enrichment in the water of a clathrate hydrate☆

Abstract The equilibrium constants for the fractionation of H 2 18 O and H 2 16 O between liquid and solid phases were determined by slow freezing of ice and by slow formation of the clathrate hydrate of tetrahydrofuran from liquid solution. Both systems gave α = 1.0026 8 . It is likely that oxygen-18 enrichment of the water in clathrate hydrates generally is essentially the same as for ice and that the relatively high oxygen-18 content observed in pore waters from some deep-sea sediments arises from the recent presence of methane hydrate.

Depositional facies of late Pleistocene Heinrich events in the Labrador Sea

Late Pleistocene Heinrich ice-rafting events produced layers rich in ice-rafted debris in major parts of the North Atlantic north of 40°N. A high detrital carbonate content points to the Hudson Strait outlet of the Laurentide ice sheet as a dominant source of the icebergs. Heinrich events were coupled with short-term climate fluctuations during the last and penultimate glaciations and provide evidence for cryosphere-hydrosphere-atmosphere interaction in Pleistocene climate change. An unsolved problem with Heinrich layers has been their high concentration of fine-grained detrital carbonate (>80% of the total detrital carbonate), which cannot have been delivered by icebergs alone. We propose combinations of different processes that deposited four sedimentologically different types of Heinrich layers: ice rafting alone for the coarser, sand- to gravel-sized fractions and the fine fractions in distal regions (type IV Heinrich layers), whereas nepheloid flows deposited the bulk of the fine sediment in regions proximal to the Hudson Strait (type I Heinrich layers). On the Labrador slope, turbidity currents spilling over from canyons were also involved in transporting the fine-grained carbonate-rich material, causing an alternation of mud-turbidites and thin laminae of ice-rafted debris in type II Heinrich layers. On the levees of the Northwest Atlantic Mid-Ocean Channel, the thickness relationship is reversed: mud-turbidites deposited by occasional spillover of currents from the channel are thin and alternate with thicker laminae of ice-rafted debris (type III Heinrich layers).

Continental slope sedimentation adjacent to an ice-margin. II. Glaciomarine depositional facies on labrador slope and glacial cycles

Eight depositional facies have been identified in Labrador Slope and Basin cores on the basis of sedimentary structures and textures. These are combined into three genetic groups: (1) hemipelagic facies HI with ice-rafted debris (IRD) and H without IRD (with a combined thickness of about 53% of the total core length); (2) debris-flow facies D (7%) and three turbidite facies (34%) T (thin, fine-grained bodyspill turbidites), TH (headspill turbidites) and TI (turbidites interlayered with laminae of IRD); and (3) contourite (<3%) and nepheloid-layer deposits (3%) of facies C and N. The latter and facies TI are characteristic of proximal Heinrich layers. The intercanyon regions of the slope are mainly covered by hemipelagic sediment with intercalated spillover turbidites. The canyons, if filled, contain debris-flow deposits and turbidites. The dominant sediment-delivery and transport mechanisms change from vertical settling and bedload transport by glacial stream-discharge on the upper slope to down-slope mass-flow processes, which predominate over along-slope processes on the lower slope and rise. Up to 15 lithostratigraphic units have been differentiated in the slope cores, based on the recognition of various combinations of the 8 facies types and short-distance correlations of AMS 14C-dated intervals. Very dark hemipelagic muds with low detrital carbonate content, little ice-rafted debris, a low degree of bioturbation and abundant sinistral-coiling, cold-water foraminifera (Facies H of units 3,5,7,9,11 and 13) are tentatively ascribed to tunes of the Mid-to Late Wisconsinan glaciation with extensive seasonal sea-ice cover in the Labrador Sea. Turbidites and debris-flow deposits cannot be related to specific phases of the glacial cycle and result largely from redeposition of upper and mid-slope sediments. Ice-rafted debris dispersed in nepheloid-flow deposits or interlayered as separate laminae between thin mud-turbidites are related to short-lived Heinrich events during times of partial ice-cap collapse, but are not restricted to these events. The youngest slope sediments, which were deposited since the early part of marine oxygenisotope stage 2 (<32 ka) correlate chronologically and lithologically with adjacent shelf, rise and basin sediments. The oldest AMS 14C date of 54,530 ± 2090 yr B.P. was obtained for the unit 1112 boundary at 1008 cm subsurface depth in core 90-26.

Gigantism in unique biogenic magnetite at the Paleocene–Eocene Thermal Maximum

We report the discovery of exceptionally large biogenic magnetite crystals in clay-rich sediments spanning the Paleocene–Eocene Thermal Maximum (PETM) in a borehole at Ancora, NJ. Aside from previously described abundant bacterial magnetofossils, electron microscopy reveals novel spearhead-like and spindle-like magnetite up to 4 μm long and hexaoctahedral prisms up to 1.4 μm long. Similar to magnetite produced by magnetotactic bacteria, these single-crystal particles exhibit chemical composition, lattice perfection, and oxygen isotopes consistent with an aquatic origin. Electron holography indicates single-domain magnetization despite their large crystal size. We suggest that the development of a thick suboxic zone with high iron bioavailability—a product of dramatic changes in weathering and sedimentation patterns driven by severe global warming—drove diversification of magnetite-forming organisms, likely including eukaryotes.

Fine-grained sediment lofting from meltwater-generated turbidity currents during Heinrich events

Turbidity currents generated from sediment-carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater. From experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal. This ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows. Evidence for large-scale lofting of suspended sediment from the top of giant sand- and gravel-carrying turbidity currents in the Labrador Sea comes from two independent lines of observations: (1) The first is a distinct sedimentary facies consisting of stacked, centimeter-thick graded mud layers that contain grains of ice-rafted debris (IRD) supported by the mud. Deposition of these unusual layers requires a gradedlayer‐forming process that is slow enough to allow the incorporation of IRD; this is not possible with normal mud-carrying turbidity currents. (2) The second observation is the presence of a huge abyssal sand and gravel plain in the central Labrador Sea that received its sediment from bed-load‐rich meltwater discharges from the Hudson Strait outlet of the Pleistocene Laurentide Ice Sheet. These discharges turned into turbidity currents that released rising columns of freshwater that carried fine-grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater. Deposition from these slow turbid interflows would allow the incorporation of IRD in the accumulating graded mud deposits. The IRD-spiked graded mud facies is restricted to Heinrich layers within 300 km radius of the Hudson Strait ice-stream terminus, tying the sand-carrying turbidity currents via fine-grained sediment lofting to Heinrich events. Estimated total discharge volumes of individual currents are on the order of 10 3 km 3 , supporting the notion that Heinrich ice-rafting events were times of maximum meltwater generation.

CONTINENTAL SLOPE SEDIMENTATION ADJACENT TO AN ICE MARGIN. III. THE UPPER LABRADOR SLOPE

Abstract The upper Labrador Slope is the key area for sediment transfer from the northeastern margin of the Pleistocene Laurentide ice sheets (LIS) to the deep basin. It is subdivided from north to south into 8 sectors based on relief differences and echo-character on 750 line-km of continuous sleeve-gun and 3.5 kHz seismic profiles. High- to moderate-relief sectors 2 and 4 and, in part, 6 and 8 are seismically transparent and well stratified with continuous individual high-amplitude reflections and deep penetration; low-relief sectors 1, 3, 5 and 7 show strong bottom reflections and, with the exception of sector 1, low seismic penetration, poor stratification, and low-amplitude, if any, subbottom reflections. The lateral distribution of these alternating high- to moderate- and low-relief sectors reflects fundamental differences in the sediment transfer mechanisms through outlets from the LIS onto the slope. Low-relief slope sectors represent debrite and turbidite slopes and are located in front of ice-outlets on the slope and adjacent regions to the north. They originate from mass wasting on the upper slope of glacial detritus with a significant coarse component originally delivered as englacial or subglacial material and deposited in (end-) moraines or as bedload by subglacial or supraglacial rivers. High- to moderate-relief sectors originate from fall-out of suspended sediment from turbid surface-plumes (TSP) and preferentially occur off the southern half of outlets and south of major outlets, as exemplified by sector 2 south of the Hudson Strait outlet. This asymmetry is caused by the south-flowing Labrador Current, which entrains the buoyantly rising turbid meltwater-plumes at the glacier front. The present high-relief topography, which shows a dendritic pattern of upslope canyon branching, is the result of retrograde, headward canyon erosion by mass-wasting processes of an originally much smoother mud-blanket surface. The sediment is remobilized by slumping and entrained in debris flows and turbidity currents. The occurrence of TSP deposits on the high- to moderate-relief upper-slope sectors requires a summer sea surface not frozen over during major parts of the Pleistocene, including glacial maxima. TPS deposition on the upper slope, ice-rafting, and a plethora of ice-margin depositional phenomena, many of which give rise to mass-wasting and mass-flow phenomena, are the main features that make high-latitude continental slopes adjacent to continental ice-sheets different from their lower-latitude counterparts.

Origin of unusually thick Heinrich layers in ice-proximal regions of the northwest Labrador Sea

Ten piston cores and a 25 m long giant piston core from the ice-proximal region off Hudson Strait contain Heinrich layers 1 and 2, identified by their sedimentary structure and high detrital carbonate content. Both layers are unusually thick on the upper Labrador slope (3.8^2.1 m) and on the lower slope and rise (1.5^1.0 m). Heinrich layers 1 and 2 can be subdivided into three units. Unit A is restricted to the upper slope and overlies hemipelagic sediment with a gradational boundary. It is 6 1 m thick, dark black to dark gray in color, and made up of coarse ice-rafted sand and granules dispersed in hemipelagic mud with a few faint laminations towards the top. The overlying unit B is up to 3.35 m thick and consists of two facies: (i) centimeter-thick, graded, carbonate-rich mud layers containing dispersed coarser grains in the mud, interpreted as nepheloid-flow deposits with coarse ice-rafted debris, and (ii) carbonate-rich, finely laminated mud layers, which alternate with millimeter-thick laminae of ice-rafted debris. Unit C, up to 0.90 m thick, is devoid of sedimentary structures, and consists of hemipelagic sediment with dispersed dropstones that increase in abundance towards the top of the unit. Data suggest the following sequence of processes could have occurred during the deposition of Heinrich layers in ice-proximal sites. As the Laurentide Ice Sheet grew, it extended through the Hudson Strait ice stream outlet to a floating ice margin near the shelf edge and perhaps beyond to the upper slope, and deposited unit A by releasing dropstones from the basal debris-rich layer. Unit B is interpreted to be the deposits of the combined processes of nepheloid-layer flow, low-density turbidity currents, and massive ice-rafting. Maximum carbonate content (s 50%), the lightest N 18 O values in very sparse planktonic foraminifera, and low magnetic susceptibility are characteristic of unit B. Unit C is inferred to represent the time of waning supply of fines as nepheloid-flow deposition ceased, while the relative concentration of dropstones increased as a result of continuing intense ice-rafting. Due to the reduction of fine-grained sediment supply, the carbonate concentration decreased to the Labrador Sea background level. This finding was further supported by the appearance of darker-colored sediments and heavier N 18 O values in Neogloboquadrina pachyderma (s) which indicates the reduction of meltwater discharge