Aridification of the Indian subcontinent during the Holocene : implications for landscape evolution, sedimentation, carbon cycle, and human civilizations

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2012The Indian monsoon affects the livelihood of over one billion people. Despite the importance of climate to society, knowledge of long-term monsoon variability is limited. This thesis provides Holocene records of monsoon variability, using sediment cores from river-dominated margins of the Bay of Bengal (off the Godavari River) and the Arabian Sea (off the Indus River). Carbon isotopes of terrestrial plant leaf waxes (δ13Cwax) preserved in sediment provide integrated and regionally extensive records of flora for both sites. For the Godavari River basin the δ13Cwax record shows a gradual increase in aridity-adapted vegetation from ~4,000 until 1,700 years ago followed by the persistence of aridity-adapted plants to the present. The oxygen isotopic composition of planktonic foraminifera from this site indicates drought-prone conditions began as early as ~3,000 years BP. The aridity record also allowed examination of relationships between hydroclimate and terrestrial carbon discharge to the ocean. Comparison of radiocarbon measurements of sedimentary plant waxes with planktonic foraminifera reveal increasing age offsets starting ~4,000 yrs BP, suggesting that increased aridity slows carbon cycling and/or transport rates. At the second site, a seismic survey of the Indus River subaqueous delta describes the morphology and Holocene sedimentation of the Pakistani shelf and identified suitable coring locations for paleoclimate reconstructions. The δ13Cwax record shows a stable arid climate over the dry regions of the Indus plain and a terrestrial biome dominated by C4 vegetation for the last 6,000 years. As the climate became more arid ~4,000 years, sedentary agriculture took hold in central and south India while the urban Harappan civilization collapsed in the already arid Indus basin. This thesis integrates marine and continental records to create regionally extensive paleoenvironmental reconstructions that have implications for landscape evolution, sedimentation, the terrestrial organic carbon cycle, and prehistoric human civilizations in the Indian subcontinent.This thesis was funded by the National Science Foundation, Woods Hole Oceanographic Institution (Arctic Research Initiative, Ocean and Climate Change Institute, Coastal Oceans Institute, Stanley Watson Chair for Excellence in Oceanography and the Academic Programs Office), and by the ETH Zurich

Identifying thermogenic and microbial methane in deep water Gulf of Mexico Reservoirs

The Gulf of Mexico (GOM) produces 5% of total U.S. dry gas production (USEIA, 2016). Despite this, the proportion of microbial and thermogenic methane in discovered and producing fields from this area is still not well understood. Understanding the relative contributions of these sources in subsurface environments is important to understanding how and where economically substantial amounts of methane form. In addition, this information will help identify sources of environmental emissions of hydrocarbons to the atmosphere. We apply stable isotopes including methane clumped-isotope measurements to solution and associated gases from several producing fields in the U.S. Gulf of Mexico to estimate the proportions, properties and origins of microbial and thermogenic endmembers. Clumped isotopes of methane are unique indicators of whether methane is at thermodynamic isotopic equilibrium or affected by kinetic processes. The clumped methane thermometer can provide insights into formation temperatures and/or into kinetic processes such as microbial methanogenesis, early catagenetic processes, mixing, combinatorial processes, and diffusion. In this data set, we find that some fluids have clumped isotope methane apparent temperatures consistent with the methane component being produced solely by the thermogenic breakdown of larger organic molecules at substantially greater temperatures than those reached in shallow reservoirs. A portion of these reservoirs with hot clumped isotope methane temperatures are consistent with exhibiting a kinetic isotope effect. Other reservoirs have clumped isotope methane apparent temperatures, and other isotopic and molecular proportions, consistent with mixtures of microbial and thermogenic methane. We show that in certain cases the evidence is most consistent with formation of the microbial methane in the current reservoir. However, in other cases the methane is produced at significantly shallower depths and is then transported to greater depths as a result of post generation burial of methane bearing sedimentary sequences to the current reservoir conditions. For the first time, we show that methane of an unambiguously purely microbial origin (i.e. those that do not contain obvious contributions of thermogenic methane) is dominantly generated at temperatures less than 60 °C, despite burial to greater depths. This finding suggests that, while microorganisms are able to generate methane at temperatures up to 105 °C under laboratory conditions (Brock, 1985), in the Gulf of Mexico, microbial methane is dominantly produced in the 20–60 °C window

Leaf wax biomarkers in transit record river catchment composition

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 6420–6427, doi:10.1002/2014GL061328.Rivers carry organic molecules derived from terrestrial vegetation to sedimentary deposits in lakes and oceans, storing information about past climate and erosion, as well as representing a component of the carbon cycle. It is anticipated that sourcing of organic matter may not be uniform across catchments with substantial environmental variability in topography, vegetation zones, and climate. Here we analyze plant leaf wax biomarkers in transit in the Madre de Dios River (Peru), which drains a forested catchment across 4.5 km of elevation from the tropical montane forests of the Andes down into the rainforests of Amazonia. We find that the hydrogen isotopic composition of leaf wax molecules (specifically the C28 n-alkanoic acid) carried by this tropical mountain river largely records the elevation gradient defined by the isotopic composition of precipitation, and this supports the general interpretation of these biomarkers as proxy recorders of catchment conditions. However, we also find that leaf wax isotopic composition varies with river flow regime over storm and seasonal timescales, which could in some cases be quantitatively significant relative to changes in the isotopic composition of precipitation in the past. Our results inform on the sourcing and transport of material by a major tributary of the Amazon River and contribute to the spatial interpretation of sedimentary records of past climate using the leaf wax proxy.This work was supported by funding from the U.S. National Science Foundation award 1227192 to A.J.W. and S.J.F. V.G. was supported by the U.S. National Science Foundation award OCE-0928582.2015-03-2

Dual isotope evidence for sedimentary integration of plant wax biomarkers across an Andes-Amazon elevation transect

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 242 (2018): 64-81, doi:10.1016/j.gca.2018.09.007.Tropical montane regions tend to have high rates of precipitation, biological production, erosion, and sediment export, which together move material off the landscape and toward sedimentary deposits downstream. Plant wax biomarkers can be used to investigate sourcing of organic matter and are often used as proxies to reconstruct past climate and environment in sedimentary deposits. To understand how plant waxes are sourced within a wet, tropical montane catchment, we measure the stable C and H isotope composition (δ13C and δD) of n-alkanes and n-alkanoic acids in soils along an elevation transect and from sediments within the Madre de Dios River network along the eastern flank of the Peruvian Andes, draining an area of 75,400 km2 and 6 km of elevation. Soils yield systematic trends in plant wax δ13C (+1.75 and +1.31‰ km−1, for the C29n-alkanes and C30n-alkanoic acids respectively in the mineral horizon) and δD values (−10 and −12‰ km−1, respectively) across a 3.5 km elevation transect, which approximates trends previously reported from canopy leaves, though we find offsets between δ13C values in plants and soils. River suspended sediments generally follow soil isotopic gradients defined by catchment elevations (δ13C: +1.03 and +0.99‰ km−1 and δD: −10 to −7‰ km−1, for the C29n-alkanes and C30n-alkanoic acids respectively) in the wet season, with a lowering in the dry season that is less well-constrained. In a few river suspended sediments, petrogenic contributions and depth-sorting influence the n-alkane δ13C signal. Our dual isotope, dual compound class and seasonal sampling approach reveals no Andean-dominance in plant wax export, and instead that the sourcing of plant waxes in this very wet, forested catchment approximates that expected for spatial integration of the upstream catchment, thus with a lowland dominance on areal basis, guiding paleoenvironmental reconstructions in tropical montane regions. The dual isotope approach provides a cross-check on the altitudinal signals and can resolve ambiguity such as might be associated with vegetation change or aridity in paleoclimate records. Further, the altitude effect encoded within plant waxes presents a novel dual-isotope biomarker approach to paleoaltimetry.This material is based upon work supported by the US National Science Foundation under Grant No. EAR-1227192 to A.J.W and S.J.F for the river work

Conventional and advanced exergetic analysis for the combined cycle of power plant with gas turbine of a refinery

This article shows the results of the performance study of a combined cycle plant made up of a Siemens STG-800 gas turbine and a MACCHI heat recovery boiler (HRSG) designed to produce 47.5 MW of electricity and 81908 kg / h of steam operating under ISO conditions (15 ° C and 60% relative humidity and 1 atm), the system is part of the steam and electric power generation section of a crude oil refinery in the city of Cartagena de Indias. The objective of this research is to quantify the real inefficiencies in each of the equipment applying conventional and advanced exergetic analysis, to achieve this the investigation has been ordered as follows: first, the basic thermodynamics at the equipment boundaries is defined, define performance parameters that compare the adjustment of the thermodynamic model with the values provided by the manufacturer, the rate of exergy destruction and exergy efficiency are obtained from conventional analysis, advanced exergetic analysis allows obtaining avoidable, unavoidable, endogenous, exogenous exergies and the combined, finally, the mexogenous exergetic analysis allows to know the amount of energy that is lost due to the interactions between the equipment. The thermodynamic model is adjusted with an average error of 2% using design KPIs such as net power, heat rate and thermal efficiency, it was obtained that the exergy destruction reaches 83.5MW, 15% is avoidable and the 8% is avoidable endogenous, the mexogenous analysis shows that inefficiencies in the compressor refer to all equipment, by focusing efforts on improving its conditions, up to 25% of the total exergy destruction can be recovered

Identifying thermogenic and microbial methane in deep water Gulf of Mexico Reservoirs

The Gulf of Mexico (GOM) produces 5% of total U.S. dry gas production (USEIA, 2016). Despite this, the proportion of microbial and thermogenic methane in discovered and producing fields from this area is still not well understood. Understanding the relative contributions of these sources in subsurface environments is important to understanding how and where economically substantial amounts of methane form. In addition, this information will help identify sources of environmental emissions of hydrocarbons to the atmosphere. We apply stable isotopes including methane clumped-isotope measurements to solution and associated gases from several producing fields in the U.S. Gulf of Mexico to estimate the proportions, properties and origins of microbial and thermogenic endmembers. Clumped isotopes of methane are unique indicators of whether methane is at thermodynamic isotopic equilibrium or affected by kinetic processes. The clumped methane thermometer can provide insights into formation temperatures and/or into kinetic processes such as microbial methanogenesis, early catagenetic processes, mixing, combinatorial processes, and diffusion. In this data set, we find that some fluids have clumped isotope methane apparent temperatures consistent with the methane component being produced solely by the thermogenic breakdown of larger organic molecules at substantially greater temperatures than those reached in shallow reservoirs. A portion of these reservoirs with hot clumped isotope methane temperatures are consistent with exhibiting a kinetic isotope effect. Other reservoirs have clumped isotope methane apparent temperatures, and other isotopic and molecular proportions, consistent with mixtures of microbial and thermogenic methane. We show that in certain cases the evidence is most consistent with formation of the microbial methane in the current reservoir. However, in other cases the methane is produced at significantly shallower depths and is then transported to greater depths as a result of post generation burial of methane bearing sedimentary sequences to the current reservoir conditions. For the first time, we show that methane of an unambiguously purely microbial origin (i.e. those that do not contain obvious contributions of thermogenic methane) is dominantly generated at temperatures less than 60 °C, despite burial to greater depths. This finding suggests that, while microorganisms are able to generate methane at temperatures up to 105 °C under laboratory conditions (Brock, 1985), in the Gulf of Mexico, microbial methane is dominantly produced in the 20–60 °C window

Short communication: Massive erosion in monsoonal central India linked to late Holocene land cover degradation

Soil erosion plays a crucial role in transferring sediment and carbon from land to sea, yet little is known about the rhythm and rates of soil erosion prior to the most recent few centuries. Here we reconstruct a Holocene erosional history from central India, as integrated by the Godavari River in a sediment core from the Bay of Bengal. We quantify terrigenous fluxes, fingerprint sources for the lithogenic fraction and assess the age of the exported terrigenous carbon. Taken together, our data show that the monsoon decline in the late Holocene significantly increased soil erosion and the age of exported organic carbon. This acceleration of natural erosion was later exacerbated by the Neolithic adoption and Iron Age extensification of agriculture on the Deccan Plateau. Despite a constantly elevated sea level since the middle Holocene, this erosion acceleration led to a rapid growth of the continental margin. We conclude that in monsoon conditions aridity boosts rather than suppresses sediment and carbon export, acting as a monsoon erosional pump modulated by land cover conditions

Sustained wood burial in the Bengal Fan over the last 19 My

Author Posting. © National Academy of Sciences, 2019. This article is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences 116(45), (2019): 22518-22525, doi:10.1073/pnas.1913714116.The Ganges–Brahmaputra (G-B) River system transports over a billion tons of sediment every year from the Himalayan Mountains to the Bay of Bengal and has built the world’s largest active sedimentary deposit, the Bengal Fan. High sedimentation rates drive exceptional organic matter preservation that represents a long-term sink for atmospheric CO2. While much attention has been paid to organic-rich fine sediments, coarse sediments have generally been overlooked as a locus of organic carbon (OC) burial. However, International Ocean Discovery Program Expedition 354 recently discovered abundant woody debris (millimeter- to centimeter-sized fragments) preserved within the coarse sediment layers of turbidite beds recovered from 6 marine drill sites along a transect across the Bengal Fan (∼8°N, ∼3,700-m water depth) with recovery spanning 19 My. Analysis of bulk wood and lignin finds mostly lowland origins of wood delivered episodically. In the last 5 My, export included C4 plants, implying that coarse woody, lowland export continued after C4 grassland expansion, albeit in reduced amounts. Substantial export of coarse woody debris in the last 1 My included one wood-rich deposit (∼0.05 Ma) that encompassed coniferous wood transported from the headwaters. In coarse layers, we found on average 0.16 weight % OC, which is half the typical biospheric OC content of sediments exported by the modern G-B Rivers. Wood burial estimates are hampered by poor drilling recovery of sands. However, high-magnitude, low-frequency wood export events are shown to be a key mechanism for C burial in turbidites.This work was funded by National Science Foundation Grants OCE-1401217 and COL-T354A55 to S.J.F. and OCE-1400805 to V.G. Graduate student participation in the project received support from University of Southern California Provost’s Fellowship to H.L. Samples were provided by the International Ocean Discovery Program. We are grateful for the efforts of the Expedition 354 Science Party, Carl Johnson, and Zongguang Liu. C.F.-L. and A.G. were supported by IODP-France. We thank Colin Osborne and Maria Vorontsova for helpful discussions.2020-04-2

Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases

Natural gas is a key energy resource, and understanding how it forms is important for predicting where it forms in economically important volumes. However, the origin of dry thermogenic natural gas is one of the most controversial topics in petroleum geochemistry, with several differing hypotheses proposed, including kinetic processes (such as thermal cleavage, phase partitioning during migration, and demethylation of aromatic rings) and equilibrium processes (such as transition metal catalysis). The dominant paradigm is that it is a product of kinetically controlled cracking of long-chain hydrocarbons. Here we show that C₂₊ n-alkane gases (ethane, propane, butane, and pentane) are initially produced by irreversible cracking chemistry, but, as thermal maturity increases, the isotopic distribution of these species approaches thermodynamic equilibrium, either at the conditions of gas formation or during reservoir storage, becoming indistinguishable from equilibrium in the most thermally mature gases. We also find that the pair of CO₂ and C₁ (methane) exhibit a separate pattern of mutual isotopic equilibrium (generally at reservoir conditions), suggesting that they form a second, quasi-equilibrated population, separate from the C₂ to C₅ compounds. This conclusion implies that new approaches should be taken to predicting the compositions of natural gases as functions of time, temperature, and source substrate. Additionally, an isotopically equilibrated state can serve as a reference frame for recognizing many secondary processes that may modify natural gases after their formation, such as biodegradation

From Andes to Amazon: assessing branched tetraether lipids as tracers for soil organic carbon in the Madre de Dios River system

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Kirkels, F. M. S. A., Ponton, C., Galy, V., West, A. J., Feakins, S. J., & Peterse, F. From Andes to Amazon: assessing branched tetraether lipids as tracers for soil organic carbon in the Madre de Dios River system. Journal of Geophysical Research-Biogeosciences, 125(1), (2020): e2019JG005270, doi:10.1029/2019JG005270.We investigate the implications of upstream processes and hydrological seasonality on the transfer of soil organic carbon (OC) from the Andes mountains to the Amazon lowlands by the Madre de Dios River (Peru), using branched glycerol dialkyl glycerol tetraether (brGDGT) lipids. The brGDGT signal in Andean soils (0.5 to 3.5 km elevation) reflects air temperature, with a lapse rate of −6.0 °C/km elevation (r 2 = 0.89, p < 0.001) and −5.6 °C/km elevation (r 2 = 0.89, p < 0.001) for organic and mineral horizons, respectively. The same compounds are present in river suspended particulate matter (SPM) with a lapse rate of −4.1 °C/km elevation (r 2 = 0.82, p < 0.001) during the wet season, where the offset in intercept between the temperature lapse rates for soils and SPM indicates upstream sourcing of brGDGTs. The lapse rate for SPM appears insensitive to an increasing relative contribution of 6‐methyl isomer brGDGTs produced within the river. River depth profiles show that brGDGTs are well mixed in the river and are not affected by hydrodynamic sorting. The brGDGTs accumulate relative to OC downstream, likely due to the transition of particulate OC to the dissolved phase and input of weathered soils toward the lowlands. The temperature‐altitude correlation of brGDGTs in Madre de Dios SPM contrasts with the Lower Amazon River, where the initial soil signature is altered by changes in seasonal in‐river production and variable provenance of brGDGTs. Our study indicates that brGDGTs in the Madre de Dios River system are initially soil derived and highlights their use to study OC sourcing in mountainous river systems.The brGDGT analyses were supported by NWO‐Veni grant 863.13.016 to F.P. This material is based upon work supported by the US National Science Foundation under grant EAR‐1227192 to A. J. W. and S. J. F. for the river fieldwork and lipid purification. In Perú, we thank the Servicio Nacional de Áreas Naturales Protegidas por el Estado (SERNANP) and personnel of Manu and Tambopata National Parks for logistical assistance and permission to work in the protected areas. We thank the Explorers' Inn and the Pontifical Catholic University of Perú (PUCP), as well as the Amazon Conservation Association for the use of the Tambopata and Wayqecha Research Stations, respectively. For river fieldwork assistance, we thank M. Torres, A. Robles, and A. Cachuana. Soil samples were contributed by Andrew Nottingham and Patrick Meir. Logistical support was provided by Y. Malhi, J. Huaman, W. Huaraca Huasco, and other collaborators as part of the Andes Biodiversity and Ecosystems Research Group ABERG (www.andesresearch.org). We thank Dominika Kasjaniuk for technical support at Utrecht. Two anonymous reviewers have provided valuable comments that have helped to improve this manuscript. Geochemical and brGDGT data are available in the PANGAEA Data Repository (Kirkels et al., 2019) and can be accessed at https://doi.pangaea.de/10.1594/PANGAEA.90617