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This research assesses the impacts of climate change and relative sea level (RSL) fluctuations on coastal vegetation during the past 2000 years along the Mahanadi River delta, northeast coast of India. Sediment samples of a 2.6 m trench from Barhatubi area located in the lower flood plain of Mahanadi River delta were studied for sedimentological and pollen analysis. Mangrove succession can be divided into three zones: (1) Freshwater marsh (∼1980–1420 cal yr BP); (2) Tidal flat (∼1420–770 cal yr BP) and (3) Landward mangrove margin (∼770 cal yr BP-Present). A transgressive shift from floodplain freshwater marsh to a tidal flat is recorded between ∼1980–1420 cal yr BP which is evidenced by upland tree taxa, Poaceae, pteridophyte and fungal spores along with low percentage of mangroves such as Avicennia, Sonneratia and Rhizophora representing the landward edge of the tidal region. Dominance of Sonneratia and a decline in terrestrial taxa reflects a rise in the relative sea level around ∼1420 cal yr BP with less freshwater input from land suggesting a weakened monsoon condition. After ∼770 cal yr BP, an overall regressive phase with small cycles of relative sea level rise/fall has been observed due to the dominance of salt tolerant Avicennia along with Rhizophora, Excoecaria agallocha and Aegialitis rotundifolia alternating with Sonneratia and other marine palynomorphs. [ABSTRACT FROM AUTHOR]

The aim of this work is to unravel the role of geomorphic indicators and/or signatures to be used as evidence for palaeoenvironmental changes and other forcing mechanisms from the lower reaches of the Sabarmati river basin, Gujarat, India region. In the present study, geomorphic features have been used as archives to debate the past land-sea interactions using satellite imagery. In the lower reaches of the Sabarmati river, a number of meanders, sinuosity changes, palaeochannels, and scroll plains have been identified. As a result of the rise and fall of the sea during the Holocene period, the changes in hydrodynamic and steady-state conditions of the river can be visualized in terms of changes in landforms. Thus, the intriguing geomorphic landforms in the lower portions of the Sabarmati basin delineate the likely archives and signs of previous environmental change, in particularly ∼ the relative sea level change. This approach provides first-hand documentation of the fluvial evolution under the direct impact of the varying forcing mechanisms. This attempt shows the effectiveness of satellite images in unravelling the clues to past changes from the landscape in a fast and insightful manner. [ABSTRACT FROM AUTHOR]

Reliability of grass phytoliths for discriminating different deltaic sub-environments has been assessed on the modern surface sediments collected along the salinity gradient of the Sunderbans delta, India. It has been observed that grass phytolith assemblages can successfully distinguish different deltaic sub-environments especially the true mangrove zones from the mangrove associate and non-mangrove zones with minor overlaps, which further corroborated with the results of discriminant analysis (DA). Detrended correspondence analysis (DCA) and redundancy analysis (RDA) performed on the surface grass phytolith data show that salinity is the most crucial environmental parameter influencing grass phytolith distribution in the deltaic sub-environments. The potential of modern grass phytolith data in reconstructing past deltaic environmental changes has been further assessed on a late Quaternary fossil phytolith spectra from the Sunderbans spanning a sedimentary record for the last ~13.6 ka. A true mangrove environment with discernible tidal influence has been revealed between 13.6 and 3.9 ka. Absence of true mangrove–indicator grass phytoliths between ~3.9 and 2.2 ka further suggests disappearance of mangrove vegetation from this part of the Sunderbans which might have recolonized during ~2.2–0.8 ka. A mangrove associated or non-mangrove environment with little or no tidal influence came into existence in the study area since 0.8 ka onwards. A comparison with some earlier records suggests that the present grass phytolith-based palaeoenvironmental data shows conformity with the past dynamics in mangrove ecosystem in the east coast of India in respect to relative sea level changes. [ABSTRACT FROM AUTHOR]

The transformation of mature (urbanized) phase of the ancient Indus civilisation between ∼4200 and 3800 years Before Present (yr BP) overlaps with the beginning of the Meghalayan Age (∼4200 ± 100 yr BP). Though exact cause(s) for decline of urbanized Indus phase are not yet clear, researchers continue to debate whether monsoonal dryness was the sole cause or several other regional factors manifested in a compounding manner. Here, we show a regional relative sea level fall in the downstream area of Indus habitation (south‐western Gujarat region) which initiated at 4150 ± 230 and continued up to 3625 ± 200 yr BP. We provide a multi‐proxy (chronological, sedimentological, mineralogical, isotopic and elemental abundance) data set from a well‐dated vertical sediment trench from Lothal (ancient dockyard area of Indus era) to support this inference. Chief proxies used for inferring the relative sea level fall were bulk sediment carbon and sulfur contents along with their stable isotopes (δ13C and δ34S) and foraminiferal assemblage. The conspicuous shifts in majority of proxies hint at a lowering of sea stand at the regional level that likely dried this ancient Harappan dockyard (used for sea trade). Findings of our study possess implications for Holocene climate changes and their plausible impact(s) on Harappan trade and culture. Additionally, it invites evidences for large scale geological changes at ∼4200 yr BP distinct to the Meghalayan era. Plain Language Summary: The Holocene epoch is known for stable climate and advent of organized lifestyle in human habitations. However, this time‐window experienced several abrupt climatic changes at ∼8.2 ka, ∼4.2 ka, Medieval climate warmth, Little Ice age etc. We continue to debate whether these climatic shifts impacted the contemporary human habitations. Geologists have recently rechristened the last 4200 year period as a distinct sub‐stage termed as the Meghalayan Age. The timing of the decline in trade and decentralization of the ancient Indus civilization coincides with the beginning of the Meghalayan Age; this provides an opportunity to revisit geological repositories of this timeframe to assess impact(s) of climate change on human culture. Meghalayan age is known to have begun with recurring monsoonal droughts. We provide here multi‐proxy evidence for a regional fall in coastal southwestern Gujarat (India) area. We posit this regional sea level fall at Lothal (Gujarat) might have adversely impacted ancient maritime Harappan trade and contributed to the declining/decentralizing Indus civilization during the end part of its mature phase. Key Points: Evidence of seawater withdrawal in the downstream area of Indus habitation (NW India) at the dawn of Meghalayan Age (4150 yr BP)The lowering of sea stand dried the ancient dockyard at Lothal impeding the ship movement which likely had an adverse impact on tradeCarbon‐sulfur contents, their stable isotopes and foraminiferal assemblage proved to be effective in establishing past sea stand [ABSTRACT FROM AUTHOR]

Records of tide-reworked fan delta facies models are rare in literature, attributed to their lower abundances due to complex sediment distribution pattern by two exclusive end-member flows, viz., a seaward directed subaerial to subaqueous debris flow, and dominant tidal current system interfering with and reworking the debris flow. Lack of appropriate analogues of tide-reworked fan deltas makes the ancient examples more significant. The analysis of such rare tide-reworked fan deltas from deep-time, e.g., Mesoproterozoic, turns out to be unique due to the gap in clear understanding of the alluvial-fluvial vs marine processes in non-vegetated land parts. The lower part of the Mesoproterozoic Rajgarh Formation, Lalsot Basin, North Delhi Fold Belt, Aravalli Craton in NW India, provides a rare record of a tide-reworked fan delta depositional system. Sedimentary facies analysis reveals eleven lithofacies grouped under three major facies associations, viz., (i) gravel-rich conglomerate facies association, characterized by stacked debris flow deposits forming a subaerial alluvial fan, representing the delta topset component, followed downslope by (ii) pebbly, cross-stratifed sandstone facies association, characterized by reworking of the subaqueously emplaced debris flows by ebb-tidal currents, to produce the mouth bar and the delta front depositional systems, and finally, (iii) sandstone-mudstone heterolithic facies association, characterized by abundant tidal signatures in pebble-free thin-bedded sandstone-mudstone heteroliths with tidal bundles, herringbone cross-strata and wave ripples, characterizing a stable platformal subtidal-intertidal flat depositional system. The lower-middle part of the succession records an aggradational stratal stacking pattern of the subaerial alluvial fan changing to progradational stratal stacking of the laterally accreted debris-rich tide-led clinoforms producing the delta topset – mouth bar – delta front transiton of a tide-reworked fan-delta under successive regressive conditions, representing syn-rift deposition near the basin margin. A retrogradational stratal stacking pattern overlies the fan-delta succession, and signifies a transgression by landward encroaching tidal flat system over the fan-delta system during post-rift phase in a stable platformal setup, where debris supply is ceased. A regressive-transgressive model characterizing the transition from a fan-delta to tidal flat depositional systems portrays fluctuations in the accommodation conditons in response to complex interplay of basin tectonics and relative sea level changes in the Mesoproterozoic riftogenic Lalsot Basin. • Sedimentary facies model of tide-reworked fan-delta from Mesoproterozoic rift basin. • Syn-rift, aggrading debris flows led to a subaerial alluvial fan (the delta plain). • Progradation of fan delta by subaqueous tide-reworked debris flows (mouth bar). • Post-rift stable platformal transgression by a retrograding tidal flat on fan delta. • Transgressive-regressive cycle by interaction of autogenic vs. allogenic processes. [ABSTRACT FROM AUTHOR]

The Sundarban Mangrove Forest in the Sundarban Biosphere Reserve, located at the mouth of the Ganga–Brahmaputra Delta in India, is the most diverse mangrove ecosystem in the world. Sediment cores were taken from two widely separated islands in that reserve: Chamta (CMT) and Sudhyanyakhali (SDK). Pollen analysis and radiocarbon dating were used to study the Holocene development and dynamics of this unique ecosystem. Modern pollen rain study reveals a strong relation between modern pollen rain and the present vegetation, as well as a high rate of Phoenix palludosa pollen production.The pollen records indicate that man-grove existed at CMT from ~5960 and at SDK from ~1520 cal yr BP. Changes in relative sea level, including the frequency and intensity of inundation as well as fluctuating precipitation, have been the major factors along with geomorphic processes that control the development and dynamics of the mangrove in the area during the Holocene. The mid Holocene mangrove at CMT declined, to be progressively replaced by successive communities, and eventually reached climax stage, while the SDK site is transitional in nature. The mangrove responds rapidly to changes in environmental conditions at both locations. Because of large-scale anthropogenic interventions, it is unlikely that similar rapid responses will occur in the future. [ABSTRACT FROM AUTHOR]