Gerhard Einsele

Atmospheric carbon burial in modern lake basins and its significance for the global carbon budget

Abstract Lake basins (∼2.7×10 6 km 2 , about 0.8% of the ocean surface or 2% of the land surface) bury a surprisingly high amount of atmospheric carbon (∼70×10 6 t/a) which reaches more than one fourth of the annual atmospheric carbon burial in the modern oceans. This is mainly accomplished by the rapid accumulation of lacustrine sediments and a very high preservation factor (on average 50 times higher than that in the oceans). Lakes with relatively large drainage areas commonly display the highest carbon accumulation rates. In most cases, burial of organic matter is more important than that of carbonate carbon produced by silicate weathering, in contrast to the oceans where the burial of atmospheric carbonate carbon almost reaches the same amount as that of organic carbon. Exceptions to this rule are closed lake basins in arid to semiarid climate which precipitate a major part of their atmosphere-derived dissolved inorganic carbon (DIC) as carbonate. These results are demonstrated in some detail for L. Qinghai, China, (low contribution of atmospheric carbonate carbon) and L. Turkana, East Africa, (high contribution from silicate rocks). Further data are gained by estimates for a number of closed and open lakes. The drainage areas of the lakes withdraw atmospheric carbon at rates of mostly 1–4 g/m 2 /a, calculated from the lacustrine carbon burial. Carbon burial rates in lakes commonly increase with change to wetter and warmer climate (partially larger lake surfaces, higher rates of seasonal carbonate precipitation, trend to stratified lake waters with oxygen-deficient bottom water). Anthropogenic influence mostly enhances the production and preservation of organic carbon in lake basins (often by a factor of 3–4). After the last glacial maximum, the joint action of the globally spreading vegetation, peat growth, and carbon burial in lakes would have been able to reduce the atmospheric carbon pool to one third to one half of its present amount within a time period of 1 ka. However, CO 2 exchange between the atmosphere and the ocean has brought about an overall increase in the atmospheric CO 2 during the Holocene. The contribution of lakes and artificial reservoirs in counteracting man-made CO 2 emissions should not be neglected.

The Xigaze forearc basin: evolution and facies architecture (Cretaceous, Tibet)

Abstract The mid-Cretaceous to Eocene flysch deposits of the Xigaze forearc basin in southern Tibet were investigated in a 120 km segment along the Indus-Yarlung suture zone. The basin evolved south of the magmatic arc (Gangdise belt) of the Lhasa block on top of trapped oceanic or transitional crust. Remnants of shelf carbonates are preserved along the northern edge of the basin. The mapped segment was shortened by about 65%; metamorphism reached low-grade conditions along the northern margin of the basin. The flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic (andesitic to dacitic) material shed from the magmatic-arc Gangdise belt. Particularly in the western part of the study area, plutonic and sedimentary rocks from deep erosion levels and/or more distal sources contributed to the basin fill. Rivers from the Lhasa block acted as point sources and fed five major deep-sea channel systems. Turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone, thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing. The forearc flysch is subdivided into at least three megasequences, which begin with wide (up to several km) incised (10 to 50 m), coarse-grained channel fills and their associated fan deposits. The upper parts of the megasequences contain hemipelagic dark shales and marls (deposited above the calcite compensation depth). Lateral channel migration, channel-lobe switching, but also volcanic pulses generated a predominantly fining-upward, high-frequency cyclicity. After continental collision, the marine sedimentation in the forearc basin was replaced by fluvial deposits of the Eocene-Oligocene Qiuwu formation, which is time-equivalent to the Kailas and Indus molasses farther west and rich in coarse gravel derived from the Gangdise belt. Both forearc flysch and Qiuwu formation were deformed simultaneously during the Miocene. We assume that the molasse-type Qiuwu formation represents the final continental facies of the forearc basin filling.

Depositional events and their records—an introduction

Abstract Event deposits of some lateral extent as defined here occur almost in all types of sedimentary basins. The reflect either local, intra-basinal processes, or they are associated with regional or global mechanisms. Some types of event beds, such as sediment gravity flows, sandy and muddy turbidites, are common and usually well preserved. They make up large proportions of basin fills, whereas others (e.g., tsunami deposits and in-situ earthquake structures) appear to be less frequent or rare in the geological record. Volume, frequency, and facies of event deposits are controlled by several processes: pre-event sediment accumulation, triggering and transport mechanisms, and mode of final deposition.

The Himalaya-Bengal Fan Denudation-Accumulation System during the past 20 Ma

Mass balances for both denudation in the Himalayas and sediment accumulation in the Subhimalayan basins, including the Bengal deep-sea fan but excluding the Indus fan, yield 7.1 × 106 km3 and 7.4 × 106 km3 (s ± 20%, rock of 2.75 g/cm3 density), respectively, for the past 20 million years. Coarsening and increased sediment accumulation rates in the foreland basin and in the Bengal foredeep indicate accentuated tectonic activity and unroofing in the Himalayas since that time. The sediment volume includes ≥1 × 106 km3 of Neogene Bengal fan sediment that was lost via the Nicobar fan to the Sunda accretionary wedge. In addition, the Indian peninsular rivers contributed about c. 0.6 × 106 km3 of solid load to the basins. Average denudation during the past 20 m.y., as derived from geothermobarometric data and restored cross sections, occurred most rapidly along the High Himalayan crystalline chain (vertical unroofing c. 1000 m/m.y.; northward lateral retreat of southern Himalayan slope, exposed to monsoonal rain, ≤3.5 km/m.y.) and much slower in the Tethyan sedimentary zone to the north (average 150 m/m.y.). The solute loads of the modern Himalayan rivers indicate a mean chemical denudation rate of 17 m/m.y. The distinct decrease in sediment accumulation on the outer Bengal fan between about 7 and 1 Ma (in contrast to the Indus fan) is probably caused by exogenic factors rather than by a significant decline in tectonic activity. Pre-20 Ma sediments in the Subhimalayan basins were derived mainly from the southern margin of the Tibet plateau or from sources outside the study area.

Geochemical evolution of closed-basin lakes : general model and application to lakes Qinghai and Turkana

In contrast to most previous models for the evolution of closed-basin lakes, we present an integrated model which considers various water budget patterns, clay regradation, SO4 reduction and subbottom leakage in addition to the classical equilibrium approach of mineral precipitation. The model was applied to Lakes Qinghai and Turkana, which significantly differ in the lithologies of their drainage areas but are representative of the carbonate-rich sedimentary rock province of the Tibet–Qinghai Plateau and the silicate rock province of Eastern Africa. Both lakes are now topographically closed, but to some degree hydrologically open (subbottom leakage). Major results of the mode calculations show that: the lithology controls the ultimate brine which is of Na–(K)–Cl-type for Qinghai Lake and of Na–HCO3–Cl-type for Lake Turkana. SO4 reduction delays the onset of sulfatic mineral precipitation and favours the formation of Na–carbonates such as trona at the expense of calcite. Clay mineral regradation plays an important role before the saturation of sulfatic or chlorine minerals is reached. In particular, magnesite formation may be in competition with Mg-bearing clay minerals. Finally, simulations with various hydrological scenarios have shown that the modern hydrochemistry of both lakes cannot be reproduced by simply evaporating inflow water, but reflects long-term accumulation and evolution of solutes by continuous inflow over several thousand years. The diversity of lake water composition within a uniform lithological province can thus be largely ascribed to varying hydrological conditions.

Basaltic sill-sediment complexes in young spreading centers: Genesis and significance

In contrast to mature mid-oceanic ridges, where magmatic activity is little affected by the slow accumulation of sediments, in young spreading centers (such as that of the Guaymas Basin in the Gulf of California) the basaltic magma of “great magmatic pulses” forms dikes and sills within the uppermost few hundred metres of soft sediments. In general, younger dikes and sills are injected next to or on top of the contact zone of the older ones. In this manner a distinctive sill-sediment complex is built up, the sediments of which are rather compacted and partly metamorphosed despite the low burial depth. The thickness of this transitional zone between the sheeted dike complex (seismic layer 2) and younger sediment (seismic layer 1) is controlled chiefly by the rates of sedimentation and spreading. If the half-spreading rate is approximately one order of magnitude greater than the sedimentation rate, the sill-sediment complex can reach a thickness of only a few hundred metres, and the depth of the spreading trough remains approximately constant. Sedimentation rates approaching or surpassing the spreading rate cause filling up of the basins, which probably hampers the injection of magma into sediments. Sill-sediment complexes similar to those in the Gulf of California are also expected to occur at the passive margins of older oceanic basins as well as orogenic belts.

Cretaceous Stratigraphy, Environment, and Subsidence History at the Moroccan Continental Margin

In contrast to other regions around the North Atlantic, good exposures in the Moroccan coastal basins offer an excellent opportunity to study the Mesozoic development of a passive continental margin including the relationship between oceanic and coastal sediments and datum levels of the pelagic fossils. From south to north, the Cretaceous sediments of the coastal basins of Tarfaya, Agadir, Essaouira, and at the margin of the Meseta are described and compared regarding macro- and microfauna, sedimentology, and paleoenvironment. For the mainly marine 2500 m or 1700 m thick Cretaceous sequences of Agadir and Essaouira, respectively, we propose a correlation of the ammonite and foraminiferal zones. Probably both sections formed in one basin (the “Atlas Basin”, see front cover), but certain facies differences were caused by different water depths since Middle Cretaceous times.

Various types of olistostromes in a closing ocean basin, Tethyan Himalaya (Cretaceous, Tibet)

Abstract The Cretaceous rocks of the central Tethyan Himalayas display different types of deep-sea olistostromes and chaotic deposits which are accompanied by turbidites. The deposits were incorporated into an accretionary prism and more or less deformed. According to the composition and provenance of their clasts, we distinguish four types of olistostromes: (1) P-type, derived from the passive (Indian) continental margin; (2) PO-type, derived from both the passive Indian margin and ocean floor (pelagic sediments and oceanic crust); (3) POA-type with clasts from the same sources as the PO-type, but also containing material from the active margin along the Lhasa block; and (4) A- and OA-type, mainly derived from the active margin (accretionary prism and magmatic arc). The occurrence of these different types of deep-sea chaotic deposits in space and time is closely related to the evolution of the Neo-Tethyan basin from a wide open ocean basin to a narrowing remnant and trench basin prior to collision. Whereas pure P-type olistostromes formed when the basin was still wide, predominating A-type deposits characterize a late stage of basin evolution prior to collision. Mixed-type olistostromes record an intermediate stage of basin evolution and require specific syn-sedimentary tectonic activity.

Event deposits: the role of sediment supply and relative sea-level changes—overview

Abstract This overview on event deposits is based on (1) a brief summary on denudation rates in regions of various relief and climate as derived from the suspended and bed loads of rivers, (2) the fractions of sand and mud present in the fills of various basins, and (3) the mechanisms controlling sediment remobilization. In continental settings, size and frequency of event deposits (debris flows on alluvial fans, avalanching on fan deltas and overbanking on floodplains) directly reflect erosional processes in the source areas and the ratio of denudation area/accumulation area. In marine environments, both sediment supply and the change in near-shore accommodation space largely control the nature of stratigraphic sequences. Under conditions of high sediment supply and low-frequency sea-level changes, the thick systems tracts tend to show only minor differences in the presence of event deposits, including tempestites. With decreasing sediment supply, event deposits are increasingly concentrated in the lowstand systems tract. As shown by a number of models (with differential subsidence or uplift of the basin margin), rapid relative sea-level fall accentuates both coastal and submarine sediment remobilization (rich in sand), particularly during the early lowstand phase, as well as delayed valley incision. The resulting submarine fans tend to be sand-dominated, whereas large fans fed by major rivers are dominated by turbidite muds. In regions of coastal uplift, valley incision persists longer than the lowstand period, and sea-level changes may cause ‘pulses of uplift’ and phases of punctuated cliff erosion. Along carbonate buildups, lowstands of third-order or higher frequency sea-level changes are often recorded by coarse skeletal debris and megabreccias and/or, in the case of mixed systems, by siliciclastic turbidites. In rapidly closing foreland basins, high-frequency sea-level cycles only tend to affect both the proximal and distal basin margins, whereas third-order sea-level changes have a limited potential to directly control depositional sequences and event deposits close to the overthrust front. With high sediment supply, individual event deposits (such as debris flows, sandy and calcareous tempestites and turbidites) mostly form at intervals of tens to hundreds up to some thousands of years. Longer recurrence intervals occur in settings with low sediment supply or characterize very large mass flows and megaturbidites.

Turonian Black Shales in the Moroccan Coastal Basins: First Upwelling in the Atlantic Ocean?

In comparison with those of the deep Atlantic, the Upper Cretaceous black shales, chiefly Turonian bituminous marls of the Morrocan coastal basins take an exceptional position. They were deposited in a water depth of 200 to 300 m during a period when the deep Atlantic water circulation lead to oxygenated sea-floor conditions and incomplete sediment sequences. The Turonian shallow water “black shales” are characterized by carbonaceous laminated marls with intercalated pelagic limestones and cherts. Their occurrence in marginal basins coincides with the peak of the Cretaceous marine transgression and decreasing terrigenous sediment supply from the continent. Their sedimentation rate ranges from 5 to 20 m/m.y. and their fauna contains elements of Boreal assemblages of the Temperate zones in northern Europe and North America. They are interpreted as an indicator of the onset of coastal upwelling along the NW African continental margin, although longdistance cold bottom currents probably did not yet exist. The lack of a middle and upper Cretaceous black shale facies in the conjugate North American shelf basins, however, may be explained by the already existing influence of a trade wind system on coastal upwelling in the eastern North Atlantic and the onset of oceanic circulation.