No abstract

No abstract

Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I–IV of ca. 70,000–60,000, ca. 60,000–55,000, ca. 55,000–43,000, and ca. 43,000–∼25,000 yr, respectively. The youngest eolian unit (V) is <10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal18O-enriched “normal continental” waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000–60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ13C is 1.7‰ (0.0–1.7‰), and δ13C covaries positively and linearly with δ18O. δ13C values are highest when δ18O values indicate the most arid climatic conditions. This is best explained by expansion of C4grasses at the expense of C3plants at low latitudes during glacial periods when atmospheric pCO2was lowered. C4dominance was overridingly influenced by global change in atmospheric pCO2despite the lowered summer rainfall.

Today the desert margins of northwest India are dry and unable to support large populations, but were densely occupied by the populations of the Indus Civilization during the middle to late Holocene. The hydroclimatic conditions under which Indus urbanization took place, which was marked by a period of expanded settlement into the Thar Desert margins, remains poorly understood. We measured the isotopic values (δ18O and δD) of gypsum hydration water in paleolake Karsandi sediments in northern Rajasthan to infer past changes in lake hydrology, which is sensitive to changing amounts of precipitation and evaporation. Our record reveals that relatively wet conditions prevailed at the northern edge of Rajasthan from ~5.1 ± 0.2 ka BP, during the beginning of the agricultural-based Early Harappan phase of the Indus Civilization. Monsoon rainfall intensified further between 5.0 and 4.4 ka BP, during the period when Indus urban centres developed in the western Thar Desert margin and on the plains of Haryana to its north. Drier conditions set in sometime after 4.4 ka BP, and by ~3.9 ka BP an eastward shift of populations had occurred. Our findings provide evidence that climate change was associated with both the expansion and contraction of Indus urbanism along the desert margin in northwest India.

No abstract

No abstract

No abstract

[1] Fold topography preserves a potentially accessible record of the structure and evolution of an underlying thrust fault system, provided we understand the factors that shape that topography. Here we examine the morphology and fault geometry of two active folds at the northwest Himalayan front. The Chandigarh and Mohand anticlines show the following patterns: (1) most (∼60%-70%) growth in catchment size and relief (across multiple scales) is accomplished within ∼5 km of the fault tips, (2) range-scale relief is divided unevenly between the fold flanks because of base level contrasts, (3) mean gradients of the uplifting catchments correspond to different flank-averaged rock uplift rates, (4) high hillslope-scale relief coincides with areas of fast rock uplift and stronger lithologies, and (5) existing relief represents only ∼15% of the total rock eroded since faulting began, implying significant erosion. The first-order fold topography is developed quickly and asymmetrically as a result of fault-generated rock uplift (which sets the space available for the fold and the distribution of rock uplift rates) with some modulation by base level (which affects the erosional response of the landscape to the uplift). A linear rate of growth in catchment relief with range half-width correlates with catchment-averaged rock uplift rate, suggesting that this metric may be used to infer variations in fault dip at depth. In these frontal fold settings, high slip rates, weak uplifting rocks, and rapid erosion may combine to quickly limit the topographic growth of emerging folds and disconnect their morphology from the displacement field.