How past environments and communities responded to episodes of coastal inundation can inform preparations for future resilience to predicted rises in sea level. Southeast Asia’s extensive coastlines ...and expanding coastal populations mean vast natural and human capital is at risk from future sea level rise. Regional mangroves provide many ecosystem services that can help mitigate such risks, but deforestation has left them threatened and compromised. The present study examines the Holocene development and human use of mangrove forest in northern Vietnam, where existing palaeo-records derive from sedimentary archives in tidal flat, estuarine and deltaic settings. Here, we expand that coverage by describing conditions at an enclosed doline within the Tràng An limestone karst in Ninh Binh province that would have been sheltered from deltaic and marine processes. We present a multi-proxy assessment incorporating pollen analysis of the 8125-year-old discontinuous sediment core obtained from the doline floor, combined with inferences from erosional tidal notches in the enclosing limestone, and analyses of phytolith, vertebrate and mollusc assemblages from an adjacent archaeological cave site with deposits of comparable age. The results provide a detailed example of how enclosed coastal environments and communities responded to Middle Holocene marine inundation. High percentages of pollen from mangroves (17–57%) suggest their colonisation of the doline from ∼8100 cal. BP and persistence until ∼250 cal. BP, well after the intertidal zone regressed seaward beyond the massif. Archaeological assemblages dating to ∼5500 cal. BP and containing palm and woody eudicot phytoliths and sponge spicules, neurocrania of the fish genus Pomadasys (‘grunts’, ‘grunters’ or ‘javelins’) and brackish-water molluscs Sermyla riqueti and Neripteron violaceum support the persistence of mangrove environments through the Middle Holocene high-stand, a period of hiatus within the core, and indicate human foraging and fishing activities took place in mangrove and lagoonal habitats alongside hunting in the surrounding limestone forest. Subsequent structural opening of this latter forest formation from ∼1075 cal. BP (875 CE), evident in the pollen record, coincides with the Medieval Climate Anomaly and with the adjacent development of the ancient capital at Hoa Lu. We propose that given the long-term persistence of mangrove habitats and associated resources documented in this study, regional initiatives aimed at rehabilitating mangroves (with the positive consequences that this holds for biodiversity and socio-economic conditions) may wish to consider selective restorative measures within Tràng An and similar sheltered sub-coastal karst settings.
•Mangroves persisted in interior of karst well after sea receded in Late Holocene.•Limestone forest opened after 1075 cal BP, coeval with MCA and build-up of Hoa Lu.•Humans used local mangrove and limestone forest during Middle Holocene high-stand.•Site has potential for future mangrove restoration and resilience to sea level rise.
Quartz sandstone of the Sarisariñama massif in Venezuela hosts the world biggest collapse dolines in quartz-rich lithologies, with volumes up to some millions of cubic meters. Due to extremely ...complex logistics required to reach the massif, the genesis of these depressions and of the underlying caves has never been studied in detail. The lack of field campaigns and extended data has fostered a decade-long scientific debate on whether their origin was due to epigenic or hypogenic processes. This study integrates petrological, structural and hydrochemical observations, including analyses of silica concentration, pH, conductivity of surface and cave waters (EC), to investigate the speleogenetic processes acting underground.
Petrographic and compositional analyses of the host rock (Matauí Formation) show that in the Sarisariñama region quartz sandstones are regularly characterized by clay interlayers with significant content of pyrophyllite and kaolinite and minor amount of iron hydroxides. Compared to surface waters, subsurface infiltration along vertical fractures and fault planes show enrichment in silica, higher pH and lower EC, confirming that chemical weathering is effective underground provoking intergranular silica dissolution along structural discontinuities. The weathering of the clay and iron hydroxide interlayers guides the speleogenesis, weakening specific stratigraphic levels and causing the collapse and fragmentation of the more resistant quartz sandstone strata. The initial void, created by piping of the loose sand released by quartz sandstone weathering, can migrate upwards by means of roof and wall breakdown; this chain of events eventually triggers a collapse at the surface, which generates a circular or squared sinkhole. The weathering acts mainly along the dominant fracture networks, showing a clear guidance by regional tectonics. These speleogenetic controls rule out the hypothesis of a hypogenic origin of the simas, suggesting a primary role of long-term epigenic chemical weathering and mechanical erosion guided by joints, weak clay and iron hydroxide interlayers, followed by subsequent massive collapses.
South-eastern Dinarides comprise territories of four countries of SE Europe—Croatia, Bosnia and Herzegovina, Montenegro and Albania and their parts belonging to the Adriatic basin. The region ...consists mainly of highly karstified carbonate rocks and is characterised by a high precipitation rate. As a result, karst aquifers can accumulate large amounts of groundwater issuing at numerous karst springs, some being the world’s largest in terms of maximal flow. Locally, the annual total rainfall rate sometimes reaches 5000 mm or even more, while an average specific groundwater yield of examined karst aquifers ranges from 40 to 50 l/s/km
2
. However, as discussed in this paper, the region also has many contrasts: abundant water reserves yet water shortage in certain parts, natural phenomena and wonders yet limited sources of life, good natural water quality marred by its vulnerability to pollution, feasible engineering solutions accompanied by their partly negative impact on environment, and water management at the national level contrasting with the transboundary character of most large aquifers. Doubts whether waters from this “treasury” are utilised in the best way and what the prospects are for their sustainable use and protection in the future are also analysed in the text.
In the southern highlands of the Upper Paraguay River watershed, hundreds of small and shallow ponds exist, but little is known about their origins or potential to record hydroclimate signals in ...their sediments. Here, we present a multi-proxy evidence that reveals details about the paleolimnological evolution of these shallow tropical lakes. The available data support the conclusion that pond formation resulted from Late Holocene environmental changes that affected the regional hydro-geomorphology. Biological proxies suggest that two distinct environmental phases shaped the regional landscape. In the older phase, sponge spicules and phytoliths are absent, and the sediment geochemistry suggests that karstification processes within the subsurface helped to facilitate topographic closure and doline formation. Sediments of the younger depositional phase are characterized by abundant grass and palm phytoliths. Sponge spicules are also present in these deposits, including the freshwater species Dosilia pydanieli Volkmer-Ribeiro1992, Metania spinata (Carter, 1881), Radiospongilla amazonensis Volkmer-Ribeiro and Becker Maciel, 1983, and Tubella variabilis Bonetto and Ezcurra de Drago1973. These microfossils, especially D. pydanieli, are suggestive of perennial lentic ecosystems. The elemental geochemistry of the youngest sediments is dominated by Si, P, and S; a positive correlation between microfossil content and wt. % S may reflect a linkage between nearshore plant biomass and offshore organic sedimentation. Our results suggest an aquatic transition favoring the formation of stable doline ponds with abundant supra-littoral vegetation occurred over the last millennium, most likely the result of a wetter climate in this region of midwestern Brazil.
•Doline ponds adjacent to the Pantanal wetlands record hydrological and geochemical changes over the late Holocene.•Microfossil content and geochemistry reveal two different hydroclimate phases in the Pantanal highlands.•Microfossil assemblages are sensitive to landscape changes including karst processes.
Offshore the emissions of dihydrogen are highlighted by the smokers along the oceanic ridges. Onshore in situ measurements in ophiolitic contexts and in old cratons have also proven the existence of ...numerous H2 emissive areas. When H2 emanations affect the soils, small depressions and vegetation gaps are observed. These depressions, called fairy circles, have similarities with the pockmark and vent structures recognized for long time in the sea floor when natural gas escapes but also differences. In this paper we present a statistic approach of the density, size, and shape of the fairy circles in various basins. New data from Brazil and Australia are compared to the existing database already gathered in Russia, USA, and again Brazil. The comparison suggests that Australia could be one of the most promising areas for H2 exploration, de facto a couple of wells already found H2, whereas they were drilled to look for hydrocarbons. The sum of areas from where H2 is seeping overpasses 45 km2 in Kangaroo Island as in the Yorke Peninsula. The size of the emitting structures, expressed in average diameter, varies from few meters to kilometers and the footprint expressed in % of the ground within the structures varies from 1 to 17%. However, globally the sets of fairy circles in the various basins are rather similar and one may consider that their characteristics are homogeneous and may help to characterize these H2 emitting zones. Two kinds of size repartitions are observed, one with two maxima (25 m and between 220 m ± 25%) one with a simple Gaussian shape with a single maximum around 175 m ± 20%. Various geomorphological characteristics allow us to differentiate depressions of the ground due to gas emissions from karstic dolines. The more relevant ones are their slope and the ratio diameter vs. depth. At the opposite of the pockmark structures observed on the seafloor for which exclusion zones have been described, the H2 emitting structures may intersect and they often growth by coalescence. These H2 emitting structures are always observed, up to now, above Archean or Neoproterozoic cratons; it suggests that anoxia at the time the sedimentation and iron content play a key role in the H2 sourcing.
This paper classifies the karst landscapes of the Petén Plateau and defines the Mirador-Calakmul Karst Basin by illustrating the distribution of its karst hydrologic features. Archaeological and ...spatial research of the Mirador-Calakmul area of Guatemala and Mexico has shown it to be a karst basin with geopolitical implications. Current research characterizes the karst landscapes of the Petén Plateau, maps the distribution of karst hydrologic features, and delineates the basin in geomorphological terms. To further this aim, multiple forms of remote sensing data including orthophotographs, a satellite Digital Elevation Model, satellite multispectral images, and Light Detection and Ranging (LiDAR) data have been integrated to interpret the karst features in the study area. Outcrop study and thin section analysis of the upper Buena Vista Formation document that the dominant lithologies are a shallow water algal boundstone interbedded with terrestrial caliche. Karst landforms have been mapped over the Petén Plateau and we identify five karst landscapes, the largest of which is a fluviokarst landscape dominated by karst valleys. We further map karst hydrologic features including seasonal swamps, dolines, intermittent lakes, intermittent streams, solution-enhanced fractures, and springs all of which are characteristic of drainage basins. Boundaries of the karst basin are mapped from multiple lines of evidence including distribution of the karst valleys, a line of springs along the western boundary of the fluviokarst landscape, and a surface drainage analysis. We capture and classify hydrologic data points and develop a regional groundwater map that indicates subsurface flow from east to west within the basin. A drainage map illustrates the extensive system of karst valleys, boundaries, and inferred groundwater flow paths of the Mirador-Calakmul Karst Basin. It was within this geomorphological setting that the ancient Maya developed an extensive civilization during the Middle and Late Preclassic periods (1000 BCE-150 CE).
Deglaciated mountainous areas are usually affected by a great range of paraglacial processes that involve progressively denudation of glacial imprints. This study focuses on paraglacial processes and ...glacial landform preservation in six zones located in two catchments of the Cantabrian Mountains (NW Spain). These zones are adjacent but display important topographic, lithologic and glaciation style differences. An analysis of how these variables conditioned the intensity and diversity of paraglacial processes in these zones was investigated through a detailed study of their relief, lithology and paleo glacier surface using GIS. A comprehensive cartography of paraglacial landforms was also accomplished, considering landslides, alluvial fans and rock glaciers that show signs of having been conditioned by glaciation. Moraines and debris-mantled slopes were also included in the analysis to examine their grade of preservation depending on the studied variables. Data comparison show that differences in glaciation style influenced paraglacial processes. In the ancient icefield area, relief modification by glacial action was limited and scarce paraglacial processes occur. By contrast, in the upper areas located in the southern ranges where an alpine-style of glaciation was installed, paraglacial processes were very active, generating many rock glaciers. Lithology explains some paraglacial landforms distribution: 1) Landslides are associated with incohesive (sandy and shale) rocks; 2) Rock glaciers mainly occur in quartzite areas; 3) In limestone areas subterranean drainage contributes to glacial landscape preservation, but suffosion dolines affect some glacial deposits. Topography is also a key factor in paraglacial processes: 1) Moraines are well-preserved in gently sloping valleys, but rare in the steeper where they are usually transformed into moraine (or debris)-mantled slopes; 2) Alluvial fans are more frequent in the steeper valleys, but more dissected by postglacial river action. 3) Elevation and orientation show little influence on paraglacial landforms, except in the generation of some periglacial features such as rock glaciers.
Coastal karst aquifers show a three-dimensional vulnerability, which consists of the whole of the “intrinsic vulnerability” and the “groundwater vulnerability to seawater intrusion”. The results of a ...study carried out in the Salento karst coastal aquifer (southern Italy) show that temperature, as well as being a reliable tracer of groundwater flow, is also an effective indicator of vulnerability in anisotropic media. The trend of isotherms related to a cross-section of the aquifer thermal field, combined with geological, geomorphological, and hydrogeological information, allows the role of faults and dolines in the mass transport from ground surface to be inferred. Isotherm trends may also give information on the permeability distribution along faults. A specific temperature value evidence the saltwater top, thus indicating the groundwater vulnerability to salinization.
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