Showing papers in "Geology in 2001"

Sediment and rock strength controls on river incision into bedrock

Abstract: Recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply, challenging the common assumption that incision rate is simply proportional to stream power. Our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion, but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits. Maximum erosion rates occur at a critical level of coarse-grained sediment supply where the bedrock is only partially exposed. Fine-grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension. Experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength. Our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate. Furthermore, we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief.

Quantifying differential rock-uplift rates via stream profile analysis

Abstract: Despite intensive research into the coupling between tectonics and surface processes, our ability to obtain quantitative information on the rates of tectonic processes from topography remains limited due primarily to a dearth of data with which to test and calibrate process rate laws. Here we develop a simple theory for the impact of spatially variable rock-uplift rate on the concavity of bedrock river profiles. Application of the analysis to the Siwalik Hills of central Nepal demonstrates that systematic differences in the concavity of channels in this region match the predictions of a stream power incision model and depend on the position and direction of the channel relative to gradients in the vertical component of deformation rate across an active fault-bend fold. Furthermore, calibration of model parameters from channel profiles argued to be in steady state with the current climatic and tectonic regime indicates that (1) the ratio of exponents on channel drainage area and slope ( m / n ) is ∼0.46, consistent with theoretical predictions; (2) the slope exponent is consistent with incision either linearly proportional to shear stress or unit stream power ( n = 0.66 or n = 1, respectively); and (3) the coefficient of erosion is within the range of previously published estimates (mean K = 4.3 × 10 −4 m 0.2 /yr). Application of these model parameters to other channels in the Siwalik Hills yields estimates of spatially variable erosion rates that mimic expected variations in rock-uplift rate across a fault-bend fold. Thus, the sensitivity of channel gradient to rock- uplift rate in this landscape allows us to derive quantitative estimates of spatial variations in erosion rate directly from topographic data.

Are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle

Abstract: In a number of subduction zones, earthquakes at 50–200 km depth define two dipping planes, separated by 20–40 km, that appear to merge downdip Upper plane earthquakes are inferred to occur within the subducting oceanic crust, whereas lower plane earthquakes occur in the subducting oceanic mantle Beneath northeast Japan, the results of a new finite-element heat-transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle Lower plane earthquakes occur at ∼550–800 °C at 100 km depth and at ∼350–600 °C at 160 km depth These conditions coincide with the dehydration reaction antigorite (serpentine) → forsterite + enstatite + H 2 O, which suggests that lower plane earthquakes may be triggered by dehydration embrittlement, which in turn suggests that the subducting oceanic mantle is partially hydrated Serpentinization may occur in the trench–outer rise region, where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere If this hypothesis is correct, current subduction-zone H 2 O budgets may significantly underestimate the amount of bound H 2 O entering the “subduction factory”

Climate, tectonics, and the morphology of the Andes

Abstract: Large-scale topographic analyses show that hemisphere-scale climate variations are a first-order control on the morphology of the Andes. Zonal atmospheric circulation in the Southern Hemisphere creates strong latitudinal precipitation gradients that, when incorporated in a generalized index of erosion intensity, predict strong gradients in erosion rates both along and across the Andes. Cross-range asymmetry, width, hypsometry, and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial, glacial, and tectonic processes, and show that major morphologic features correlate with climatic regimes. Latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential, indicating that, like tectonics, nonuniform erosion due to large-scale climate patterns is a first-order control on the topographic evolution of the Andes.

Mountain erosion over 10 yr, 10 k.y., and 10 m.y. time scales

Abstract: We used cosmogenic 10 Be to measure erosion rates over 10 k.y. time scales at 32 Idaho mountain catchments, ranging from small experimental watersheds (0.2 km 2 )t o large river basins (35 000 km 2 ). These long-term sediment yields are, on average, 17 times higher than stream sediment fluxes measured over 10‐84 yr, but are consistent with 10 m.y. erosion rates measured by apatite fission tracks. Our results imply that conventional sediment-yield measurements—even those made over decades—can greatly underestimate long-term average rates of sediment delivery and thus overestimate the life spans of engineered reservoirs. Our observations also suggest that sediment delivery from mountainous terrain is extremely episodic, sporadically subjecting mountain stream ecosystems to extensive disturbance.

Exhumation as fast as subduction

Abstract: We produced a pressure-temperature-time path in order to determine the exhumation rate of the deepest subducted Alpine rocks. In situ dating of peak-metamorphic titanite in an eclogite facies calc-silicate rock indicates that subduction to pressures of ∼3.5 GPa was reached at 35.1 ± 0.9 Ma. Titanite formed during two decompression stages, at 1 ± 0.15 GPa and ∼0.4–0.5 GPa, and yielded ages of 32.9 ± 0.9 Ma and 31.8 ± 0.5 Ma, respectively. Combining the age data and making assumptions about the conversion of pressure to depth yield mean exhumation rates of 3.4 cm/yr and 1.6 cm/yr. These rates imply that exhumation acted at plate tectonic speeds similar to subduction, and was significantly faster than erosion. We suggest that fast exhumation is driven by a combination of tectonic processes involving buoyancy and normal faulting.

Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes and increasing effective stress

Abstract: Off southwest Japan the seaward limit of coseismic displacement (or updip limit of the seismogenic zone) of the 1946 Mw 8.3 thrust earthquake reaches to 4 km depth and ∼40 km landward of the trench. This limit coincides with the estimated location of the 150 °C isotherm, and has been linked to changes in physical properties associated with the smectite to illite clay-mineral transition. Here we show that this limit correlates with a suite of diagenetic to low-grade metamorphic processes characterized by (1) declining fluid production and decreasing fluid pressure ratio (λ*) and (2) active clay, carbonate, and zeolite cementation and the transition to pressure solution and quartz cementation. These diagenetic to low-grade metamorphic changes cause the onset of velocity weakening during thrust faulting, an increase in effective stress, and strengthening of the hanging wall, which together combine to produce recordable earthquakes.

Understanding Mesozoic accretion in Southeast Asia: Significance of Triassic thermotectonism (Indosinian orogeny) in Vietnam

Abstract: Results from a zircon U-Pb study of the metamorphic basement of Vietnam reveal that a large part of the continental crust was affected by a short-lived episode of ductile deformation and high-temperature metamorphism between 258 ± 6 Ma and 243 ± 5 Ma. Although coincident with final stages of North-South China collision (Qinling orogenesis), the thermotectonism in Vietnam was caused by accretion of Sibumasu to Indochina–South China. This accretion event (Indosinian orogeny) has regional significance because it contributed to the final stages of North-South China collision, an aspect not explicitly recognized in Qinling orogenic models.

Continental breakup in magmatic provinces: An Ethiopian example

Abstract: Mechanical processes largely control the along-axis segmentation of continental rifts; however, asthenospheric processes strongly influence the along-axis segmentation of mid- ocean ridges. We examine the distribution of strain and magmatism in the transitional active Main Ethiopian rift. Magmatic construction, diking, and faulting during the past 1.6 m.y. have created ∼20-km-wide, ∼60-km-long magmatic segments with or without axial valleys. Magmatic segments are arranged en echelon within the ∼100-km-wide rift valley bounded by mid-Miocene border faults. Geodetic data show that magmatic segments accommodate >80% of the strain across the rift, indicating that border faults are no longer the locus of extension. Comparison with mid-ocean ridges suggests that magmatic segments, rather than detachment faults, mark the ocean-continent boundary in rifts with a ready magma supply. Magmatic margins, therefore, may contain detachments abandoned during continental breakup. The processes of localized dike intrusion with underplating would produce strips of mafic crust transitional to oceanic crust, but without coherent seafloor-spreading magnetic anomalies.

Higher erosion rates in the Himalaya: Geochemical constraints on riverine fluxes

Abstract: The modern erosion rate of continental-scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains. These two parameters are very difficult to measure directly in continental-scale basins. In this paper we examine the chemical composition of the suspended load, bedload, and dissolved load of the Ganga and Brahmaputra Rivers and compare them with the average composition of Himalayan source rocks. A mass-balance equation of erosion fluxes shows that a Si-rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock. It corresponds to bedload sediment and flood-plain deposits, which are enriched in quartz by mineral sorting during transport. The combined budget of Si, Al, and Fe in the river system allows us to estimate this Si-rich flux. By this method, the total Himalayan erosion is estimated to be twice the measured flux of suspended load. The comparison between the Brahmaputra and the Ganga shows that the eastern Himalaya has a higher erosion rate (2.9 mm/yr) than the western Himalaya (2.1 mm/yr). This is likely the result of the higher runoff in the Brahmaputra basin. The intensity of the monsoon acts as a first-order control of the erosion rate in the range.

Central European vegetation response to abrupt climate change at 8.2 ka

Abstract: Oxygen isotope records show a major climatic reversal at 8.2 ka in Greenland and Europe. Annually laminated sediments from two lakes in Switzerland and Germany were sampled contiguously to assess the response of European vegetation to climate change ca. 8.2 ka with time resolution and precision comparable to those of the Greenland ice cores. The pollen assemblages show pronounced and immediate responses (0–20 yr) of terrestrial vegetation to the climatic change at 8.2 ka. A sudden collapse of Corylus avellana (hazel) was accompanied by the rapid expansion of Pinus (pine), Betula (birch), and Tilia (linden), and by the invasion of Fagus silvatica (beech) and Abies alba (fir). Vegetational changes suggest that climatic cooling reduced drought stress, allowing more drought-sensitive and taller growing species to out-compete Corylus avellana by forming denser forest canopies. Climate cooling at 8.2 ka and the immediate reorganization of terrestrial ecosystems has gone unrecognized by previous pollen studies. On the basis of our data we conclude that the early Holocene high abundance of C. avellana in Europe was climatically caused, and we question the conventional opinion that postglacial expansions of F. silvatica and A. alba were controlled by low migration rates rather than by climate. The close connection between climatic change and vegetational response at a subcontinental scale implies that forecasted global warming may trigger rapid collapses, expansions, and invasions of tree species.

Demonstration of significant abiotic iron isotope fractionation in nature

Abstract: Field and laboratory studies reveal that the mineral ferrihydrite, formed as a result of abiotic oxidation of aqueous ferrous to ferric Fe, contains Fe that is isotopically heavy relative to coexisting aqueous Fe. Because the electron transfer step of the oxidation process at pH >5 is essentially irreversible and should favor the lighter Fe isotopes in the ferric iron product, this result suggests that relatively heavy Fe isotopes are preferentially partitioned into the readily oxidized Fe(II)(OH) x (aq) species or their transition complexes prior to oxidation. The apparent Fe isotope fractionation factor, α ferrihydrite- water , depends primarily on the relative abundances of the Fe(II) (aq) species. This study demonstrates that abiotic processes can fractionate the Fe isotopes to the same extent as biotic processes, and thus Fe isotopes on their own do not provide an effective biosignature.

Age and composition of dikes in Southern Tibet: new constraints on the timing of east-west extension and its relationship to postcollisional volcanism

Abstract: Controversy exists over whether east-west extension in southern Tibet is related to plateau uplift or to the accommodation of plate boundary forces. Relationships between the onset of extension, plateau uplift, and the thermal state of the lithosphere are critical to this debate. We present new data on postcollisional, north-south–trending dikes in southern Tibet. Their ages range from 18.3 ± 2.7 Ma to 13.3 ± 0.8 Ma, and define the onset of regional east-west extension in southern Tibet. Dikes are compositionally indistinguishable from postcollisional lavas in southern Tibet, being either ultrapotassic, having a source in the subcontinental lithospheric mantle, or calc-alkaline with a dominantly crustal origin. The ultrapotassic dikes demonstrate that east-west extension and subcontinental lithospheric mantle–derived magmatism were temporally and spatially linked, supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning. Thus, the onset of extension by 18.3 ± 2.7 Ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces. This places a new age limitation on the attainment of high elevation in southern Tibet, with implications for models that relate Cenozoic monsoon intensification to plateau uplift.

Visual observation of gas-hydrate formation and dissociation in synthetic porous media by means of glass micromodels

Abstract: Visual observation of gas hydrates at the microscopic scale in synthetic porous media provides unequivocal visual evidence that clathrates can form in systems without the presence of a free-gas phase. Hydrates were formed from a soluble liquid hydrate former (tetrahydrofuran, C 4 H 8 O), from free gas (CH 4 ), and from dissolved gas (CO 2 ). Clathrates were found to form within the center of pore spaces, rather than on grain surfaces. Cementation of grains only occurred in regions of a small grain size, or where a large proportion of pore space was filled with hydrate. However, even at high clathrate saturations, a thin film of free water persisted on grain surfaces. The results have important implications for the potential cementing effect of hydrates on sediments, and thus for sediment permeability, slope stability, and seismic interpretation of hydrate-bearing sediments.

Oceanic anoxia at the Precambrian-Cambrian boundary

Abstract: The Precambrian-Cambrian (PC-C) boundary separates fossils representing two discrete evolutionary phases: the Neoproterozoic soft-bodied Ediacarian biotas and Cambrian small shelly faunas. The biological discontinuity is suspected to have been a result of mass extinction; however, recent discoveries of the Ediacarian biotas in Cambrian sediments have led to an understanding that the faunal change was gradual through the PC-C transition. Th/U ratios, which are high in oxidizing conditions and low in reducing conditions, show a considerable positive correlation with δ13C values at all studied sites of the PC-C boundary. This correlation indicates that reported δ13C variation across the PC-C boundary from numerous localities corresponds to redox variation in the depositional environment. The negative δ13C anomaly that occurs worldwide at the PC-C boundary, therefore, corresponds to the widespread development of an oxygen-deficient shallow marine environment. This finding suggests that widespread oceanic oxygen deficiency, which has been interpreted to reflect Phanerozoic mass extinction events, also occurred immediately before the Cambrian explosion.

Seasonal stable isotope evidence for a strong Asian monsoon throughout the past 10.7 m.y

Abstract: O of wet-season rainfall was significantly morenegative (29.5‰ SMOW) prior to 7.5 Ma than after ( 26.5‰SMOW). If this change is attributable to a lessening of the amounteffect in rainfall, this agrees with floral and soil geochemical datathat indicate increasing aridity beginning at 7.5 Ma.Keywords: Tibetan Plateau, monsoon, stable isotopes, paleohydrology,seasonality.INTRODUCTIONThe Tibetan Plateau is the engine that drives the modern Asianmonsoon by generating a high-altitude region of low pressure in thesummer as the plateau heats, and a region of high pressure in the winteras the plateau cools (Hastenrath, 1991). During the summer, warm airrises from the plateau, pulling moist air off the ocean, across the Indiansubcontinent, and into the highlands; this results in heavy summer rain-fall on the subcontinent. The opposite occurs in the winter, resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent. Thus, the presence of a strong wet-season–dry-season alternation implies the presence of a plateau broad and highenough to drive the monsoon.The timing of the uplift of the plateau remains a matter of con-siderable debate because there are few direct indicators of paleotopog-raphy in the geologic record. Consequently, past workers in Tibet, re-lying on indirect indicators of uplift, have proposed dates ranging from40 to 3.4 Ma, on the basis of initiation of potassic volcanism (Chunget al., 1998; Turner et al., 1993) or extension on the plateau (Harrisonet al., 1995; Coleman and Hodges, 1995), changes in marine sedimen-tation rates (Burbank et al., 1993), sediment types (Rea et al., 1998),or biota (Nigrini and Caulet, 1992; Kroon et al., 1991), and changesin stable carbon isotope and palynological patterns on the Indian sub-continent (Quade et al., 1989; Chen, 1981). Although different areasof the plateau may have risen at different times, many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7–8Ma by a process such as lithospheric delamination (Molnar et al.,1993). This inference was based on the following approximately coevalphenomena: a major change in plant communities of the Indian sub-continent (Quade et al., 1989), and shifts in marine upwelling patternsthat are linked to an intense monsoon (Kroon et al., 1991). Althoughthere is strong evidence for significant climate change at 7–8 Ma, it isunclear whether this is the onset of the monsoon. The floral transitionseems to have been a global rather than local phenomenon (Cerling etal., 1997) and monsoonally driven upwelling may have already beenpresent by 10–12 Ma (Nigrini and Caulet, 1992; Kroon et al., 1991).Because there is an intimate association between the intense sea-sonality of the modern monsoon and a high Tibetan Plateau and be-cause evaporation can be unambiguously recognized in the d

Strong tectonic and weak climatic control of long-term chemical weathering rates

Abstract: The relationships among climate, physical erosion, and chemical weathering have remained uncertain, because long-term chemical weathering rates have been difficult to measure. Here we show that long-term chemical weathering rates can be measured by combining physical erosion rates, inferred from cosmogenic nuclides, with dissolution losses, inferred from the rock-to-soil enrichment of insoluble elements. We used this method to measure chemical weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates. Chemical weathering rates correlate strongly with physical erosion rates but only weakly with climate, implying that, by regulating erosion rates, tectonic uplift may significantly accelerate chemical weathering rates in granitic landscapes.

Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere

Abstract: We deformed cores of peridotite with ∼10%–15% lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation. The strength, the pressure dependence of strength, and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites. These results indicate that deformation is accommodated primarily by serpentine, while olivine, despite being the more abundant component, remains nominally undeformed. On the basis of these data and previous work, we determine that the transition from a “strong,” dilatant dunite rheology to a “weak,” nondilatant serpentinite rheology is not a linear function of the degree of serpentinization. Instead, an abrupt transition in strength is observed at low degrees of serpentinization. The pressure of the transition from localized to distributed deformation also decreases abruptly, from >1000 MPa to 150–350 MPa. The change in rheological behavior occurs at a serpentine content of 10%–15% or less, which corresponds to published compressional seismic velocity of >7.8–7.5 km/s at a pressure of 200 MPa. The seismic velocity of the oceanic lithosphere, particularly of that formed at slow spreading ridges, can thus provide constraints on its mechanical properties at depth. Because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite, significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis.

New views of granular mass flows

Abstract: Concentrated grain-fluid mixtures in rock avalanches, debris flows, and pyroclastic flows do not behave as simple materials with fixed rheologies. Instead, rheology evolves as mixture agitation, grain concentration, and fluid-pressure change during flow initiation, transit, and deposition. Throughout a flow, however, normal forces on planes parallel to the free upper surface approximately balance the weight of the superincumbent mixture, and the Coulomb friction rule describes bulk intergranular shear stresses on such planes. Pore-fluid pressure can temporarily or locally enhance mixture mobility by reducing Coulomb friction and transferring shear stress to the fluid phase. Initial conditions, boundary conditions, and grain comminution and sorting can influence pore-fluid pressures and cause variations in flow dynamics and deposits.

Variation in Holocene El Niño frequencies: Climate records and cultural consequences in ancient Peru

Abstract: Analysis of mollusks from archaeological sites on the north and central coasts of Peru indicates that between ca. 5800 and 3200–2800 cal yr B.P., El Nino events were less frequent than today, with modern, rapid recurrence intervals achieved only after that time. For several millennia prior to 5.8 ka, El Nino events had been absent or very different from today. The phenomena called El Nino have had severe consequences for the modern and colonial (historically recorded) inhabitants of Peru, and El Nino events also influenced prehistoric cultural development: the onset of El Nino events at 5.8 ka correlates temporally with the beginning of monumental temple construction on the Peruvian coast, and the increase in El Nino frequency after 3.2–2.8 ka correlates with the abandonment of monumental temples in the same region.

Role of Panama uplift on oceanic freshwater balance

Abstract: Comparison between planktic foraminiferal oxygen isotope records from the Caribbean Sea (Ocean Drilling Program [ODP] Site 999) and the equatorial east Pacific (ODP Site 851) suggests an increase in Caribbean surface-water salinity between 4.7 and 4.2 Ma. The modern Atlantic-Pacific salinity contrast of about 1‰ became fully established at 4.2 Ma as reflected by a 0.5‰ planktic foraminifera 18O enrichment in the Caribbean Sea. This is interpreted as the result of restricted surface-water exchange between the tropical Atlantic and Pacific in response to the shoaling of the Central American seaway. As a consequence, the Atlantic and Pacific surface-ocean circulation regime changed, as did the freshwater balance between the major ocean basins. Simultaneous shifts in benthic carbon isotope records in the Caribbean Sea suggest an intensification in North Atlantic thermohaline circulation. These results indicate that the Panamanian isthmus formation caused several new ocean-atmosphere feedback mechanisms that have affected climate since the early Pliocene.

Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis

Abstract: A newly studied Permian-Triassic (P-Tr) boundary section in Jameson Land, East Greenland, contains an abundant and well-preserved marine fauna as well as terrestrial palynomorphs. For the first time it is possible to compare the biotic crises of the marine and terrestrial realms using the same samples from the same section. The sediments record a negative excursion in δ 13 C carb values of 8‰–9‰, and in δ 13 C org values of 10‰–11‰. The presence of the conodont Hindeodus parvus, combined with the δ 13 C carb record, enables correlation with the proposed global stratotype section at Meishan. This shows that the Greenland section is the most expanded P-Tr section known. Collapse of the marine and terrestrial ecosystems took between 10 and 60 k.y. It took a further few hundred thousand years for the final disappearance of Permian floral elements. Collapse of the terrestrial and marine ecosystems began at the same stratigraphic level and preceded the sharp negative excursion in the δ 13 C record.

Global dinoflagellate event associated with the late Paleocene thermal maximum

Abstract: The late Paleocene thermal maximum, or LPTM (ca. 55 Ma), represents a geologically brief time interval (∼220 k.y.) characterized by profound global warming and associated environmental change. The LPTM is marked by a prominent negative carbon isotope excursion (CIE) interpreted to reflect a massive and abrupt input of 12 C-enriched carbon to the ocean-atmosphere reservoir, possibly as a result of catastrophic gas-hydrate release, on time scales equivalent to present-day rates of anthropogenic carbon input. The LPTM corresponds to important changes in the global distribution of biota, including mass extinction of marine benthic organisms. The dinoflagellate cyst record indicates that surface- dwelling marine plankton in marginal seas also underwent significant perturbations during the LPTM. We report on the dramatic response of representatives of the genus Apectodinium from two upper Paleocene–lower Eocene sections in the Southern (New Zealand) and Northern (Austria) Hemispheres, where the dinoflagellate records are directly correlated with the CIE, benthic foraminifera extinction event, and calcareous nannofossil zonation. The results indicate that the inception of Apectodinium -dominated assemblages appears to be synchronous on a global scale, and that the event is precisely coincident with the beginning of the LPTM. Apectodinium markedly declined in abundance near the end of the LPTM. This Apectodinium event may be associated with (1) exceptionally high global sea-surface temperatures and/or (2) a significant increase in marginal-marine surface-water productivity. Such a globally synchronous acme of dinoflagellate cysts is unprecedented within the dinoflagellate cyst fossil record.

Exhumation of the west-central Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics

Abstract: Crystallization and thermal histories of two plutons in the west-central Alborz (also Elburz, Elburs) Mountains, northern Iran, are combined with crosscutting relations and kinematic data from nearby faults to determine the Cenozoic tectonic evolution of this segment of the youthful Euro-Arabian collision zone. U/Pb, ^(40)Ar/^(39)Ar, and (U-Th)/He data were obtained from zircon, biotite, K-feldspar, and apatite. The Akapol pluton intruded at 56 ± 2 Ma, cooled to ∼150 °C by ca. 40 Ma, and stayed near that temperature until at least 25 Ma. The nearby Alam Kuh granite intruded at 6.8 ± 0.1 Ma and cooled rapidly to ∼70 °C by ca. 6 Ma. These results imply tectonic stability of the west-central Alborz from late Eocene to late Miocene time, consistent with Miocene sedimentation patterns in central Iran. Elevation-correlated (U- Th)/He ages from the Akapol suite indicate 0.7 km/m.y. exhumation between 6 and 4 Ma, and imply ∼10 km of Alborz uplift that was nearly synchronous with rapid south Caspian subsidence, suggesting a causal relation. Uplift, south Caspian subsidence and subsequent folding, reversal of Alborz strike-slip (from dextral to sinistral) and(?) eastward extrusion of central Iran, coarse Zagros molasse deposition, Dead Sea transform reorganization, Red Sea oceanic spreading, and(?) North and East Anatolian fault slip all apparently began ca. 5 ± 2 Ma, suggesting a widespread tectonic event that we infer was a response to buoyant Arabian lithosphere choking the Neo-Tethyan subduction zone.

Modulation and amplification of climatic changes in the Northern Hemisphere by the Indian summer monsoon during the past 80 k.y

Abstract: High-frequency suborbital variations (Dansgaard-Oeschger cycles) characterize the climatic history of the Northern Hemisphere as observed in Greenland ice cores, deep-sea sediments of the North Atlantic, the Californian borderland, the Arabian Sea, the South China Sea, and the Chinese loess area. Paleoceanographic data from core KL126 from the Bay of Bengal in combination with data from the other Asian monsoonal areas indicate that the feedback processes involving snow and dust of the Tibetan Plateau vary the summer monsoon capacity to transport moisture into central South Asia and into the atmosphere. We postulate that the summer monsoon initiates, amplifies, and terminates these cycles in the Northern Hemisphere.

Terrestrial record of methane hydrate dissociation in the Early Cretaceous

Abstract: Reconstruction of changing C isotopic composition of Early Cretaceous atmospheric CO2 from fossilized C3 vascular land-plant tissue revealed a brief and striking negative excursion (D 25‰) in atmospheric d 13 C, followed by a rapid positive compensation (D 15‰) during the Aptian (ca. 117 Ma). Mass-balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion; this would also result in an increased CO2:O2 mixing ratio as O2 is consumed during CH4 oxidation to CO2, spurring the exponential phase of angiosperm biogeographic expansion.

Hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single-step microbial sulfate reduction

Abstract: Coexisting dissolved sulfide and sulfate from hypersulfidic interstitial waters of a 380- m-long sediment core show a large isotopic difference of up to 72‰ caused by in situ microbial sulfate reduction. This is considerably larger than the assumed biological maximum of 46‰ derived from laboratory studies with pure cultures of sulfate-reducing bacteria. Similar high fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process, involving sulfide reoxidation and disproportionation of sulfur intermediates. Our data show that extreme isotopic differences between sulfate and the reduced sulfur species can also be generated during microbial single-step fractionation. This result indicates that the sulfate-reducing communities and/or their cellular metabolic activities in the deep biosphere may differ from those observed in near-surface sediments or the water column.

Fertilizing potential of volcanic ash in ocean surface water

Abstract: The fertilization potential of newly erupted and well-preserved ash from the 2000 Hekla eruption in Iceland was measured for the first time by flow-through experiments. As previously shown, (1) the North Atlantic Ocean, including the subarctic seas surrounding Iceland, is the largest net sink of the world’s oceans for atmospheric CO 2, owing to biological drawdown during summer; (2) almost complete consumption of phosphate in chlorophyll-rich areas of the North Atlantic Ocean might limit primary production; and (3) in the southern Pacific Ocean and parts of the equatorial Pacific Ocean iron might limit primary production. We found through laboratory experiments that volcanic ash exposed to seawater initially releases large amounts of adsorbed phosphate, 1.7 mmol·g 21 ·h 21 ; iron, 37.0 mmol·g 21 ·h 21 ; silica, 49.5 mmol·g 21 h 21 ; and manganese, 1.7 mmol·g 21 ·h 21 . Dissolution of acid aerosols adsorbed to the surface of the ash caused the high initial release of major and trace elements. Because of the instantaneous dissolution of adsorbed components when newly erupted volcanic ash comes in contact with the ocean surface water, macronutrients and ‘‘bioactive’’ trace metals are released fast enough to become available to support primary production.

Late Miocene adakites and Nb-enriched basalts from Vizcaino Peninsula, Mexico: Indicators of East Pacific Rise subduction below southern Baja California?

Abstract: A typical slab melt association was emplaced from 11 to 8 Ma in the Santa Clara volcanic field, Vizcaino Peninsula, Baja California Sur. It includes adakitic domes and associated pyroclastic flow deposits, together with lava flows of niobium-enriched basalts. The trace element and isotopic (Sr-Nd-Pb) signatures of adakites are consistent with melting of altered mid-ocean ridge basalts, and the sources of the Nb-enriched basalts contain an enriched mantle wedge component. Such associations commonly form at depths of 70–80 km during low-dip subduction of very young oceanic crust. However, the Santa Clara field is relatively close (100 km) to the paleotrench, which suggests that the genesis of its adakites and Nb- enriched basalts occurred in a very high thermal regime linked to the subduction of the then-active Guadalupe spreading center of the East Pacific Rise. Our data suggest that the asthenospheric window documented below northern Baja California also developed beneath the south of the peninsula during the Neogene. This hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region.

Carbon isotope anomaly and other geochemical changes at the Triassic-Jurassic boundary from a marine section in Hungary

Abstract: Most mass extinctions are linked with carbon isotope excursions, implying that biotic crises are coupled with changes in the global carbon cycle. The isotopic evolution during the end-Triassic extinction is far less documented than that for the other major Phanerozoic extinctions. Here we report a sharp and short-lived 23.5‰ excursion in carbon isotope values for carbonate (d 13 Ccarb) corresponding to a 22‰ excursion in the isotopic composition of marine organic matter (d 13 Corg) and other geochemical