Jay Quade

Global vegetation change through the Miocene/Pliocene boundary

Between 8 and 6 million years ago, there was a global increase in the biomass of plants using C4 photosynthesis as indicated by changes in the carbon isotope ratios of fossil tooth enamel in Asia, Africa, North America and South America. This abrupt and widespread increase in C4 biomass may be related to a decrease in atmospheric CO2 concentrations below a threshold that favoured C3-photosynthesizing plants. The change occurred earlier at lower latitudes, as the threshold for C3 photosynthesis is higher at warmer temperatures.

On the isotopic composition of carbon in soil carbon dioxide

Abstract In this study it is shown that the isotopic composition of carbon in soil CO2 differs from the isotopic composition of carbon in soil-respired CO2. Soil CO2 collected from a montane soil has an endmember δ13C value of −23.3%. whereas soil-respired CO2 in this system has a δ 13C value of −27.5%. This difference is very close to the theoretical difference of 4.4%. which is predicted by the difference in the diffusion coefficients for 12CO2 and 13CO2. Because of the measured and the theoretical difference between the isotopic composition of carbon in soil CO2 and soil-respired CO2 it is suggested that, in the future, a distinction should be made between them.

Stratigraphy, structure, and tectonic evolution of the Himalayan fold‐thrust belt in western Nepal

Regional mapping, stratigraphic study, and 40Ar/39Ar geochronology provide the basis for an incremental restoration of the Himalayan fold-thrust belt in western Nepal. Tectonostratigraphic zonation developed in other regions of the Himalaya is applicable, with minor modifications, in western Nepal. From south to north the major structural features are (1) the Main Frontal thrust system, comprising the Main Frontal thrust and two to three thrust sheets of Neogene foreland basin deposits; (2) the Main Boundary thrust sheet, which consists of Proterozoic to early Miocene, Lesser Himalayan metasedimentary rocks; (3) the Ramgarh thrust sheet, composed of Paleoproterozoic low-grade metasedimentary rocks; (4) the Dadeldhura thrust sheet, which consists of medium-grade metamorphic rocks, Cambrian-Ordovician granite and granitic mylonite, and early Paleozoic Tethyan rocks; (5) the Lesser Himalayan duplex, which is a large composite antiformal stack and hinterland dipping duplex; and (6) the Main Central thrust zone, a broad ductile shear zone. The major structures formed in a general southward progression beginning with the Main Central thrust in late early Miocene time. Eocene-Oligocene thrusting in the Tibetan Himalaya, north of the study area, is inferred from the detrital unroofing record. On the basis of 40Ar/39Ar cooling ages and provenance data from synorogenic sediments, emplacement of the Dadeldhura thrust sheet took place in early Miocene time. The Ramgarh thrust sheet was emplaced between ∼15 and ∼10 Ma. The Lesser Himalayan duplex began to grow by ∼10 Ma, simultaneously folding the north limb of the Dadeldhura synform. The Main Boundary thrust became active in latest Miocene-Pliocene time; transport of its hanging wall rocks over an ∼8-km-high footwall ramp folded the south limb of the Dadeldhura synform. Thrusts in the Subhimalayan zone became active in Pliocene time. The minimum total shortening in this portion of the Himalayan fold-thrust belt since early Miocene time (excluding the Tibetan zone) is ∼418–493 km, the variation depending on the actual amounts of shortening accommodated by the Main Central and Dadeldhura thrusts. The rate of shortening ranges between 19 and 22 mm/yr for this period of time. When previous estimates of shortening in the Tibetan Himalaya are included, the minimum total amount of shortening in the foldthrust belt amounts to 628–667 km. This estimate neglects shortening accommodated by small-scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening.

Systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern Great Basin, United States

Stable carbon- and oxygen-isotope variations in Holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern Great Basin, United States, a region with a semi-arid climate. Our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate, ecological variations, differences in parent material, and soil depth. δ 13 C of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth, indicating a decrease in the ratio of atmospheric to plantderived CO 2 downprofile. Pedogenic carbonate at the soil-air interface approaches a δ 13 C value in equilibrium with atmospheric CO 2 in all three soils. Observed δ 13 C profiles for pedogenic carbonate can be described using a one-dimensional model for 12 CO 2 and 13 CO 2 , assuming isotopic equilibrium between soil CO 2 and soil carbonate. The modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil-respiration rates at each site. δ 13 C in deep pedogenic carbonate (>50 cm) varies by about 12 per mil over a 2,440-m elevation range, being enriched in 13 C at the lowest elevations. The slope of δ 13 C for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials: depletion by 4.6 to 4.7 per mil per 1,000 m increase in altitude between 300 to 2,740 m above mean sea level for the localities studied. This concordance makes it likely that there has been complete isotopic exchange between HCO 3 - in solution and soil CO 2 prior to carbonate precipitation. Soil CO 2 and soil-respiration rates increase systematically with elevation. The plantderived component of soil CO 2 indicates that C 3 plants dominate the biomass at most measured sites, in agreement with plant surveys. Calculated equilibrium fractionation factors between soil CO 2 and soil carbonate are very similar to those observed, again indicating complete isotopic exchange between carbon species. In all, the soil CO 2 and soil-carbonate data suggest that the δ 13 C variation with elevation observed in the soil carbonates results from differing soil-respiration rates at each site, as well as from variations in the proportion of C 3 to C 4 and CAM plants in each site9s surface biomass. δ 18 O values in pedogenic carbonates are higher at lower elevations, due in part to the more positive δ 18 O values for meteoric waters at lower elevations. The average δ 18 O value of deep (>50 cm) pedogenic carbonate at all sites, however, is enriched 2.4 to 3.7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters. This suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate, or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place. One or both of these processes also may explain the δ 18 O decrease in soil carbonate with depth observed in two of three soil profiles.

Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal

Sedimentological and provenance data from the lower Miocene–Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87 Sr/ 86 Sr record of the ocean. In western Nepal, the Dumri Formation is an ∼750–1300-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2–12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene–Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at ∼10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene–Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group (∼15–11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From ∼11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (∼4–5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460–530 Ma, ∼850–1200 Ma, ∼1.8–2.0 Ga, and ∼2.5 Ga. An abundance of Cambrian–Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87 Sr/ 86 Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian–Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87 Sr/ 86 Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

Expansion of C4 grasses in the Late Miocene of Northern Pakistan: evidence from stable isotopes in paleosols

Stable-isotopic, clay-mineralogic, and bulk-chemical analyses were conducted on paleosols of the Neogene Siwalik sections in northern Pakistan in order to reconstruct floodplain environments over the past ∼ 17 Ma. The stable carbon isotopic composition of soil carbonate (mean δ13C (PDB) = -10.2%) and associated organic matter (mean δ13C (PDB) = −24.1%) in paleosols representing 17− ∼ 7.3 Ma reveal that floodplain vegetation was dominated by C3 plants. At 7.3 Ma, a shift toward more positive carbon isotopic values began, signaling the gradual expansion of C4 grasses onto the floodplain. From 6 Ma to present, carbon isotopic values for paleosol carbonate (mean δ13C (PDB) = +0.6%) and organic matter (mean δ13C (PDB) = −14.4%) are uniformly enriched in 13C, indicating the presence of nearly pure C4 grassland. The scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1.0–1.25 m/yr or less for the entire 17 Ma of record. Paleosols in the lower portion of the section lack organic A horizons but have reddish B horizons often containing secondary iron-oxide nodules. Leaching depths of soil carbonate in these older paleosols are typically greater than those in the Plio-Pleistocene part of the section, where organic A horizons are common, and B horizons are markedly more yellow. The combined evidence suggests that the mature paleosols in the pre-7.3 Ma part of the record are dominantly calcareous Alfisols or Mollisols that once underlay nearly pure C3 vegetation, perhaps trees and shrubs, while calcareous Mollisols underlying C4 grassland dominate the upper part of the record. The carbon- and oxygen-isotopic trends in the paleosol record in Pakistan are also evident in the diet of fossil mammals, and in paleosols from Nepal, thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent-wide. Local effects, such as the development or intensification of the Asian Monsoon driven by uplift of the Tibetan Plateau, may have led to the expansion of C4 grasses. If, however, the expansion of C4 grasses proves globally synchronous, then a larger scale cause, such as a marked decrease in ϱCO2, may be the driving mechanism.

2.6-Million-year-old stone tools and associated bones from OGS-6 and OGS-7, Gona, Afar, Ethiopia

CRAFT Research Center, 419 N. Indiana Avenue, Indiana University, Bloomington, IN, 47405, USA Department of Anthropology, Southern Connecticut State University, 501 Crescent Street, New Haven, CT 06515-1355, USA Department of Geosciences, University of Arizona, Tucson, AZ, 85721, USA Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA Department of Earth and Planetary Science, University of California, Berkeley, CA 94709, USA Departmento de Prehistoria y Arquelogia, Facultad de Geografia, e Historia, Universidad Complutense de Madrid, Ciudad Universitaria 28040, Madrid, Spain Department of Anthropology and CRAFT Research Center, 419 N. Indiana Avenue, Indiana University, Bloomington, IN, 47405, USA Sterkfontein Research Unit, University of Witwatersrand, WITS 2050, Johannesburg, South Africa Department of Anatomy, Case Western Reserve University-School of Medicine, 10900 Euclid Avenue, Cleveland, OH, 44106-4930, USA Laboratory of Physical Anthropology, Cleveland Museum of Natural History, Cleveland, OH 44106, USA

Late Miocene environmental change in Nepal and the northern Indian subcontinent : Stable isotopic evidence from paleosols.

Neogene sediments belonging to the Siwalik Group crop out in the Himalayan foothills along the length of southern Nepal. Carbon and oxygen isotopic analyses of Siwalik paleosols from four long Siwalik sections record major ecological changes over the past ∼11 m.y. The carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca. 7.0 Ma, marking the displacement of largely C3 vegetation, probably semi-deciduous forest, by C4 grasslands. By the beginning of the Pliocene, all the flood plains of major rivers in this region were dominated by monsoonal grasslands. The floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest Miocene. A similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of Pakistan, and in terrigenous organic matter from the Bengal Fan, showing that the floral shift was probably continentwide. The latest Miocene also witnessed an average change of ∼4‰ in the oxygen isotopic composition of soil carbonate, as observed previously in Pakistan. The reasons for this are unclear; if not diagenetic, a major environmental change is indicated, perhaps related to that driving the carbon isotopic shift. Recently described pollen and leaf fossils from the Surai Khola section show that evergreen forest was gradually displaced by semi-deciduous and dry deciduous forest between 11 and 6 Ma. The gradual nature of this floral shift, which culminated in the rapid expansion of C4 grasses starting ∼7.0 m.y. ago, is difficult to explain by a decrease in atmospheric pCO2 alone (Cerling et al., 1993) but fits well with a gradual onset of monsoonal conditions in the late Miocene in the northern Indian subcontinent. Himalayan uplift, driving both monsoonal intensification and consumption of CO2 through weathering, may be the common cause behind major late Miocene environmental change globally. However, the decline of effective moisture associated with monsoon development has probably slowed, not increased, the rate of consumption of CO2 by chemical weathering of Himalayan sediments.

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

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

Woody cover and hominin environments in the past 6 million years

The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore, we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction of woody cover since the Late Miocene epoch (about 7 million years ago). 13C/12C ratio data from 1,300 palaeosols at or adjacent to hominin sites dating to at least 6 million years ago show that woody cover was predominantly less than ∼40% at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6 million years.