Modeling plan-form deltaic response to changes in fluvial sediment supply

This study focuses on the effects of changes in fluvial sediment supply on the plan-form shape of wave-dominated deltas. We apply a one-line numerical shoreline model to calculate shoreline evolution after (I) elimination and (II) time-periodic variation of fluvial input. Model results suggest four characteristic modes of wave-dominated delta development after abandonment. The abandonment mode is determined by the pre-abandonment downdrift shoreline characteristics and wave climate (which are, in turn, determined by previous delta evolution). For asymmetrical deltas experiencing shoreline instability on the downdrift flank, time-periodic variation in fluvial input influences the evolution of downdrift-migrating sandwaves. The frequency and magnitude of the riverine "forcing" can initiate a pattern that migrates away from the river mouth, interacting with the development of shoreline sandwaves. Model results suggest that long-period signals in fluvial delivery can be shredded by autogenic sand waves, whereas shorter-term riverine fluctuations can dominate the signal of the autogenic sandwaves. The insights provided by these exploratory numerical experiments provide a set of hypotheses that can be further tested using natural examples

Wave-angle control of delta evolution

Author Posting. © American Geophysical Union, 2011. 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 38 (2011): L13405, doi:10.1029/2011GL047630.Wave-influenced deltas, with large-scale arcuate shapes and demarcated beach ridge complexes, often display an asymmetrical form about their river channel. Here, we use a numerical model to demonstrate that the angles from which waves approach a delta can have a first-order influence upon its plan-view morphologic evolution and sedimentary architecture. The directional spread of incoming waves plays a dominant role over fluvial sediment discharge in controlling the width of an active delta lobe, which in turn affects the characteristic rates of delta progradation. Oblique wave approach (and a consequent net alongshore sediment transport) can lead to the development of morphologic asymmetry about the river in a delta's plan-view form. This plan-form asymmetry can include the development of discrete breaks in shoreline orientation and the appearance of self-organized features arising from shoreline instability along the downdrift delta flank, such as spits and migrating shoreline sand waves—features observed on natural deltas. Somewhat surprisingly, waves approaching preferentially from one direction tend to increase sediment deposition updrift of the river. This ‘morphodynamic groin effect’ occurs when the delta's plan-form aspect ratio is sufficiently large such that the orientation of the shoreline on the downdrift flank is rotated past the angle of maximum alongshore sediment transport, resulting in preferential redirection of fluvial sediment updrift of the river mouth.This research was supported by NSF grants EAR‐0952146 and OCE‐0623766, the Exxon‐Mobil Upstream Research Company, and the WHOI‐USGS postdoctoral fellowship

Pliocene expansion of C-4 vegetation in the core monsoon zone on the Indian Peninsula

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Dunlea, A. G., Giosan, L., & Huang, Y. Pliocene expansion of C-4 vegetation in the core monsoon zone on the Indian Peninsula. Climate of the Past, 16(6), (2020): 2533-2546, https://doi.org/10.5194/cp-16-2533-2020.The expansion of C4 vegetation during the Neogene was one of the largest reorganizations of Earth's terrestrial biome. Once thought to be globally synchronous in the late Miocene, site-specific studies have revealed differences in the timing of the expansion and suggest that local conditions play a substantial role. Here, we examine the expansion of C4 vegetation on the Indian Peninsula since the late Miocene by constructing a ∼6-million-year paleorecord with marine sediment from the Bay of Bengal at Site U1445, drilled during International Ocean Discovery Program Expedition 353. Analyses of element concentrations indicate that the marine sediment originates from the Mahanadi River in the Core Monsoon Zone (CMZ) of the Indian Peninsula. Hydrogen isotopes of the fatty acids of leaf waxes reveal an overall decrease in the CMZ precipitation since the late Miocene. Carbon isotopes of the leaf wax fatty acids suggest C4 vegetation on the Indian Peninsula existed before the end of the Miocene but expanded to even higher abundances during the mid-Pliocene to mid-Pleistocene (∼3.5 to 1.5 million years ago). Similar to the CMZ on the Indian Peninsula, a Pliocene expansion or re-expansion has previously been observed in northwest Australia and in East Africa, suggesting that these tropical ecosystems surrounding the Indian Ocean remained highly sensitive to changes in hydroclimate after the initial spread of C4 plants in late Miocene.This research has been supported by the Ocean and Climate Change Institute Postdoctoral Scholarship at Woods Hole Oceanographic Institution to Ann Dunlea, and the U.S. National Science Foundation to Liviu Giosan (grant no. NSF OCE-0652315). USSSP post-cruise support was provided to Expedition 353 shipboard participants Liviu Giosan and Yongsong Huang

A cartographical perspective to the engineering works at the Sulina mouth

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Akadémiai Kiadó for personal use, not for redistribution. The definitive version was published in Acta Geodaetica et Geophysica Hungarica 45 (2010): 71-79, doi:10.1556/AGeod.45.2010.1.11.From 1856 to 1939, the European Commission of the Danube (ECD), was responsible for technical surveys at the mouth of Sulina arm. During this period, ECD prepared general maps of Danube Delta as well as detailed charts for all the Danube mouths: Chilia, Sulina and Sf. Gheorghe (St. George) that were published in various reports or atlases. ECD used a local grid network benchmarked at Sulina, divided in 500 feet units. The reference point was the old lighthouse located on the right bank of Danube. After the Second World War, the Romanian authorities elaborated new cartographical products using the Gauss‐Kruger projection or Stereo‐70 like national grid. Our goal is to generate a cartographical background database necessary for refining the coastal evolution model of the Sulina mouth. To handle the large number of available maps, we chose GeoNetwork like a solution for catalog service, indexing and defining metadata. The service is operating at geo‐spatial.org

South Asian monsoon history over the past 60 kyr recorded by radiogenic isotopes and clay mineral assemblages in the Andaman Sea

The Late Quaternary variability of the South Asian (or Indian) monsoon has been linked with glacial‐interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the Andaman Sea remains poorly constrained. Here we use radiogenic Sr, Nd, and Pb isotope compositions of the detrital clay‐size fraction and clay mineral assemblages obtained from sediment core NGHP Site 17 in the Andaman Sea to reconstruct the variability of the South Asian monsoon during the past 60 kyr. Over this time interval εNd values changed little, generally oscillating between −7.3 and −5.3 and the Pb isotope signatures are essentially invariable, which is in contrast to a record located further northeast in the Andaman Sea. This indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable. The most likely source region is the Irrawaddy river catchment including the Indo‐Burman Ranges with a possible minor contribution from the Andaman Islands. High smectite/(illite + chlorite) ratios (up to 14), as well as low 87Sr/86Sr ratios (0.711) for the Holocene period indicate enhanced chemical weathering and a stronger South Asian monsoon compared to marine oxygen isotope stages 2 and 3. Short, smectite‐poor intervals exhibit markedly radiogenic Sr isotope compositions and document weakening of the South Asian monsoon, which may have been linked to short‐term northern Atlantic climate variability on millennial time scales

Comment on “Geochemistry of buried river sediments from Ghaggar Plains, NW India: Multi - proxy records of variations in provenance, paleoclimate, and paleovegetation patterns in the late quaternary” by Ajit Singh, Debajyoti Paul, Rajiv Sinha, Kristina J. Thomsen, Sanjeev Gupta

Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 455 (2016): 65-67, doi:10.1016/j.palaeo.2016.05.001.Singh et al. (2016) published a geochemical record of sediment compositions from the flood plain of the Ghaggar River in western India and use the changing provenance, particularly as traced by Nd isotope composition, to reconstruct how erosion patterns have changed over the past 100 k.y. In doing so they propose a link between climate change and erosion, and they argue for more erosion from the Higher Himalaya during warmer interglacial periods and more from the Lesser Himalaya during glacial intervals. While we support the concept of erosion patterns being climatically modulated we here take the opportunity to compare the data presented by Singh et al. (2016) to relevant published records within the region greater Ghaggar region and to open a balanced discussion on how climate and erosion are coupled in the western Himalaya

Littoral steering of deltaic channels

© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 452 (2016): 204-214, doi:10.1016/j.epsl.2016.08.018.The typically single-threaded channels on wave-influenced deltas show striking differences in their orientations, with some channels oriented into the incoming waves (e.g., Ombrone, Krishna), and others oriented away from the waves (e.g., Godavari, Sao Francisco). Understanding the controls on channel orientation is important as the channel location greatly influences deltaic morphology and sedimentology, both subaerially and subaqueously. Here, we explore channel orientation and consequent feedbacks with local shoreline dynamics using a plan-form numerical model of delta evolution. The model treats fluvial sediment delivery to a wave-dominated coast in two ways: 1) channels are assumed to prograde in a direction perpendicular to the local shoreline orientation and 2) a controlled fraction of littoral sediment transport can bypass the river mouth. Model results suggest that channels migrate downdrift when there is a significant net littoral transport and alongshore transport bypassing of the river mouth is limited. In contrast, river channels tend to orient themselves into the waves when fluvial sediment flux is relatively large, causing the shoreline of the downdrift delta flank to attain the orientation of maximum potential sediment transport for the incoming wave climate. Using model results, we develop a framework to estimate channel orientations for wave-influenced deltas that shows good agreement with natural examples. An increase in fluvial sediment input can cause a channel to reorient itself into incoming waves, behavior observed, for example, in the Ombrone delta in Italy. Our results can inform paleoclimate studies by linking channel orientation to fluvial sediment flux and wave energy. In particular, our approach provides a means to quantify past wave directions, which are notoriously difficult to constrain.This study was supported by NSF grant EAR-0952146.2017-08-1

Are Deltas Human Constructs?

Productive and biologically diverse, deltaic lowlands attracted humans since prehistory and may have spurred the emergence of the first urban civilizations. Deltas continued to be an important nexus for economic development across the world and are currently home for over half a billion people. But recently, under the double whammy of sea level rise and inland sediment capture behind dams, they have become the most threatened coastal landscap

A new neolepadid cirripede from a Pleistocene cold seep, Krishna-Godavari Basin, offshore India

Valves of a thoracican cirripede belonging to a new species of the Neolepadidae, Ashinkailepas indica Gale sp. nov. are described from a Late Pleistocene cold seep (52.6 ka), cored in the Krishna-Godavari Basin, offshore from the eastern coast of India. This constitutes the first fossil record of the genus, and its first occurrence in the Indian Ocean. Other fossil records of the Neolepadidae (here elevated to full family status) are discussed, and it is concluded that only Stipilepas molerensis from the Eocene of Denmark, is correctly referred to the family. Cladistic analysis of the Neolepadidae supports a basal position for Ashinkailepas, as deduced independently from molecular studies, and the Lower Cretaceous brachylepadid genus Pedupycnolepas is identified as sister taxon to Neolepadidae. Neolepadids are not Mesozoic relics as claimed, preserved in association with the highly specialised environments of cold seeps and hydrothermal vents, but are rather an early Cenozoic offshoot from the clade which also gave rise to the sessile cirripedes