Provenance and depositional variability of the Thin Mud Facies in the lower Ganges-Brahmaputra delta,West Bengal Sundarbans, India

The Sundarbans is one of the largest coastal wetland sites in the world and covers an area of approximately one million hectares of the western delta of the Ganges and Brahmaputra (G-B) rivers (located across Bangladesh and India). Since the late Holocene, the western delta has not been directly fluvially sourced, due to the Ganges shift to the east (present-day Bangladesh). The depositional facies (Thin Mud Facies) of the late-Holocene abandoned western region (The Sundarbans) is derived from dominant estuary-tidal dynamics, however the provenance of the associated TMF sedimentation in this far western zone (Indian Sundarbans per se) is as yet equivocal. In this study, sediment cores from the Indian Sundarbans (Saptamukhi-Thakuran estuary) were closely examined for grain-size distributions (GSDs), mineralogy through X-ray diffraction (XRD), and geochemistry with X-ray fluorescence (XRF). The TMF in the West Bengal Sundarbans has been determined to show intensively weathered, terrestrial sediment, derived principally from the Ganges Alluvial Plain (GAP). There is a predominance of quartz, mica and clay minerals, with quartz interpreted as a product of low-relief tropical weathering sourced via the G-B Rivers draining the Himalayas. Lithofacies interpreted through GSD analysis of the TMF is indicative of a muddy tidal flat environment with aggradation and a general fining-up trend between the adjacent estuaries. The sediment provenance indicates a continuing G-B sediment source, which moves westward along the Bay of Bengal, from the active delta front and is then reworked over the far-western abandoned delta by tidal–estuarine forcing

Eliciting Dirichlet and Gaussian copula prior distributions for multinomial models

In this paper, we propose novel methods of quantifying expert opinion about prior distributions for multinomial models. Two different multivariate priors are elicited using median and quartile assessments of the multinomial probabilities. First, we start by eliciting a univariate beta distribution for the probability of each category. Then we elicit the hyperparameters of the Dirichlet distribution, as a tractable conjugate prior, from those of the univariate betas through various forms of reconciliation using least-squares techniques. However, a multivariate copula function will give a more flexible correlation structure between multinomial parameters if it is used as their multivariate prior distribution. So, second, we use beta marginal distributions to construct a Gaussian copula as a multivariate normal distribution function that binds these marginals and expresses the dependence structure between them. The proposed method elicits a positive-definite correlation matrix of this Gaussian copula. The two proposed methods are designed to be used through interactive graphical software written in Java

Quantitative integration of sedimentological core descriptions and petrophysical data using high-resolution XRF core scans

status: publishe

Coherent high- and low-latitude control of the northwest African hydrological balance

The evolution of the northwest African hydrological balance throughout the Pleistocene epoch influenced the migration of prehistoric humans. The hydrological balance is also thought to be important to global teleconnection mechanisms during Dansgaard-Oeschger and Heinrich events. However, most high-resolution African climate records do not span the millennial-scale climate changes of the last glacial-interglacial cycle, or lack an accurate chronology. Here, we use grain-size analyses of siliciclastic marine sediments from off the coast of Mauritania to reconstruct changes in northwest African humidity over the past 120,000 years. We compare this reconstruction to simulations of palaeo-humidity from a coupled atmosphere-ocean-vegetation model. These records are in good agreement, and indicate the reoccurrence of precession-forced humid periods during the last interglacial period similar to the Holocene African Humid Period. We suggest that millennial-scale arid events are associated with a reduction of the North Atlantic meridional overturning circulation and that millennial-scale humid events are linked to a regional increase of winter rainfall over the coastal regions of northwest Africa