Sam VanLaningham

Fluvial landscapes of the Harappan civilization

The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

40 Ar/ 39 Ar, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ∼33 Ma to ∼18 Ma for 40 Ar/ 39 Ar biotite, ∼18 Ma to ∼6 Ma for K-feldspar minimum closure ages, and ∼15 Ma to ∼1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from ∼4 Ma to ∼1 Ma. There has been at least ∼11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40 Ar/ 39 Ar thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ∼22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (∼past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

U-Pb zircon dating evidence for a Pleistocene Sarasvati River and capture of the Yamuna River

The Harappan Culture, one of the oldest known urban civilizations, thrived on the northwest edge of the Thar Desert (India and Pakistan) between 3200 and 1900 BCE. Its demise has been linked to rapid weakening of the summer monsoon at this time, yet reorganization of rivers may also have played a role. We sampled subsurface channel sand bodies predating ca. 4.0 ka and used U-Pb dating of zircon sand grains to constrain their provenance through comparison with the established character of modern river sands. Samples from close to archaeological sites to the north of the desert show little affinity with the Ghaggar-Hakra, the presumed source of the channels. Instead, we see at least two groups of sediments, showing similarities both to the Beas River in the west and to the Yamuna and Sutlej Rivers in the east. The channels were active until after 4.5 ka and were covered by dunes before 1.4 ka, although loss of the Yamuna from the Indus likely occurred as early as 49 ka and no later than 10 ka. Capture of the Yamuna to the east and the Sutlej to the north rerouted water away from the area of the Harappan centers, but this change significantly predated their final collapse.

Persistent long-term (c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

Abstract To address Miocene–present episodic v. persistent exhumation, we utilize a simple graphical procedure that vertically stacks spatially diverse K-feldspar 40Ar/39Ar multi-domain diffusion (MDD) models from the length of the approximately 100 km-long high-peak region of the Eastern Alaska Range. We supply additional constraints with 40Ar/39Ar mica dating because the higher closure-temperature-window places limits on the initiation of rapid Eastern Alaska Range exhumation. We also provide a broad 40Ar/39Ar K-feldspar minimum closure age data set to add more detail on spatial patterns in the regional exhumation history for the Eastern Alaska Range. We find that rapid and persistent exhumation has occurred in the Eastern Alaska Range since about 24 Ma at a long-term rate of approximately 0.9 km/Ma, but that this rapid exhumation is spatially variable through time. Onset of rapid Eastern Alaska Range exhumation is coincident with the initiation of rapid exhumation in SW Alaska, the Western Alaska Range and the Chugach–Saint Elias Range at around 25 Ma, implying a region-wide deformational response to a change in tectonic forcing. The initiation of highly coupled flat-slab subduction of the Yakutat microplate is probably responsible for this prolonged period of rapid exhumation in southern Alaska. Supplementary material: Sample locations from the Eastern Alaska Range, and 40Ar/39Ar data tables and age spectrum figures are available at www.geolsoc.org.uk/SUP18603.

Carbon Biogeochemistry of the Western Arctic: Primary Production, Carbon Export and the Controls on Ocean Acidification

The Arctic Ocean is an important sink for atmospheric carbon dioxide (CO2) with a recent estimate suggesting that the region accounts for as much as 15 % of the global uptake of CO2. The western Arctic Ocean, in particular is a strong ocean sink for CO2, especially in the Chukchi Sea during the open water season when rates of primary production can reach as high as 150 g C m−2. The Arctic marine carbon cycle, the exchange of CO2 between the ocean and atmosphere, and the fate of carbon fixed by marine phytoplankton appear particularly sensitive to environmental changes, including sea ice loss, warming temperatures, changes in the timing and location of primary production, changes in ocean circulation and freshwater inputs, and even the impacts of ocean acidification. In the near term, further sea ice loss and other environmental changes are expected to cause a limited net increase in primary production in Arctic surface waters. However, recent studies suggest that these enhanced rates of primary production could be short lived or not occur at all, as warming surface waters and increases in freshwater runoff and sea ice melt enhance stratification and limit mixing of nutrient-rich waters into the euphotic zone. Here, we provide a review of the current state of knowledge that exists about the rates of primary production in the western Arctic as well as the fate of organic carbon fixed by primary produces and role that these processes play in ocean acidification in the region.

Erosion by rivers and transport pathways in the ocean: A provenance tool using 40Ar--39Ar incremental heating on fine-grained sediment

(measured from 39 Ar and 37 Ar, respectively) are consistent with typical values for K- and Ca-bearing minerals. Calculations show that the bulk mineralogy is reflected in the outgassing K/Ca spectra and identify plagioclase as the dominant mineral contributing to the plateau-defining portion of the age spectra. A second model predicts bulk sediment ages from integrated bedrock cooling age-area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin. Calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for. Overall, this technique shows considerable promise, not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems.

Quaternary landscape evolution over a strike-slip plate boundary: Drainage network response to incipient orogenesis in Sakhalin, Russian far east

This study focuses on the fluvial and tectonic landscape of the North Sakhalin Basin (Russia), where 5 km of Neogene deltaic sediments were deposited across the Okhotsk-Amur plate boundary. The homogeneous, poorly lithified sedimentary sequence created a flat landscape without structural inheritance. These sediments are now being actively deformed by oblique compression. This allows us to investigate the early stages of orogenesis in a strike-slip plate boundary and the response of drainage networks in such a setting, and to construct a model for the topographic evolution along 220 km of the plate boundary. We use fluvial geomorphological indicators (planform morphology, concavity, steepness indices, and knickpoint distribution) as evidence for active landscape deformation. Tectonics and topography are strongly coupled, and neotectonic activity can be observed directly from the landscape. Knickpoints are mostly located on fault planes, suggesting geologically recent activity, or in areas of drainage capture, where they are associated with low concavity indices. Geomorphic indications from longitudinal river profiles and planform morphology suggest that creation of anticlines and disruption of drainage patterns appears to be diachronous in the North Sakhalin Basin, with deformation propagating north and east through time. Minimum uplift and strike-slip displacement rates in the northeast of Sakhalin are 0.63 mm a -1 and 1.95 mm a -1 , based on exhumed stratigraphy and offset drainage networks, respectively.