Environmental geochemistry of the Pichavaram mangrove ecosystem (tropical), southeast coast of India

 Spatial and temporal geochemical variations of various parameters in the water and sediment of a relatively small mangrove situated on the southeast coast of India were examined in detail for the first time. The water quality generally reflects the impact of seawater and the Vellar estuary (mixing effect) aided by evaporation and in situ biological productivity. The depletion and fluctuation of dissolved silica are controlled by biological processes. Nitrate and phosphate are contributed by fertilizer input from adjoining agriculture fields. Total suspended matter (TSM) shows an erratic range and trend due to deforestation and resuspension processes. Sand and silt constitute 70–90% of the sediments. Statistical analysis of the sediments shows the prevalence of a moderately high-energy environment with very effective winnowing activity. Organic matter content is higher in the mangrove sediments in comparison to adjacent estuaries. Water and sediment show fluctuations in their chemical concentration, but no specific trends could be identified. Heavy metals are also enriched in the mangrove sediments, indicating their unique chemical behavior and the existence of trapping mechanisms. Factor analysis and correlation analysis of water and sediments show the complexity of the system and the multitude of contributing sources. The core sediment chemistry suggests the depletion of metal input due to the damming of the detrital inputs. The Pichavaram mangrove seems to be relatively unpolluted, since the anthropogenic signal observed is small and acts as a sink for heavy metals contributed from a multitude of sources without an adverse effect. Received: 5 November 1997 · Accepted: 30 March 1998     

Phosphorus fractionation in sediments of the Pichavaram mangrove ecosystem,south-eastern coast of India

Present study examined phosphorus dynamics through delineation of source as well as availability of phosphorus and its fractionation within the intertidal sediments of Pichavaram mangrove ecosystem. Twelve sediment samples and two cores were collected from the mangrove forest along with estuarine area (Vellar-Coleroon) during January 2005. Sediments were analyzed for total phosphorus and its fractionation using operationally defined chemical sequential extraction scheme (SEDEX). Dissolved phosphorus (in water) and total phosphorus (in sediments) concentrations were high in the Vellar region of Pichavaram mangrove area due to pollution load from nearby villages and agricultural fields. However, the spatial variation in dissolved phosphorus were insignificant (at significance level = 0.05). The results for the phosphorus fractionation (post-tsunami) were compared with earlier studies (pre-tsunami). It was observed that all phosphorus fractions (except adsorbed-phosphorus) showed a highly significant (at significance level = 0.05) increase in concentration after the tsunami event. There was significant decrease in the adsorbed phosphorus concentration as a result of tsunami. The changes were more pronounced for organic phosphorus which increased by almost twofold following the event. These variations were attributed to change in salinity, increase in dissolved oxygen as well as the retreat of tsunami water carrying the waste load. The vertical distribution of phosphorus through core sediments showed that mixing after tsunami had altered the different phosphorus fraction and its availability. Overall, the study indicated that the fluvial weathering along with litter degradation and anthropogenic sources controlled the biogeochemistry of phosphorus in this mangrove ecosystem. Observed changes in the concentrations are a result of altered physico-chemical characteristics caused by tsunami.     

Identification and characterization of tsunami deposits off southeast coast of India from the 2004 Indian Ocean tsunami: Rock magnetic and geochemical approach

The December 2004 Indian Ocean Tsunami (IOT) had a major impact on the geomorphology and sedimentology of the east coast of India. Estimation of the magnitude of the tsunami from its deposits is a challenging topic to be developed in studies on tsunami hazard assessment. Two core sediments (C1 and C2) from Nagapattinam, southeast coast of India were subjected to textural, mineral, geochemical and rock-magnetic measurements. In both cores, three zones (zone I, II and III) have been distinguished based on mineralogical, geochemical and magnetic data. Zone II is featured by peculiar rock-magnetic, textural, mineralogical and geochemical signatures in both sediment cores that we interpret to correspond to the 2004 IOT deposit. Textural, mineralogical, geochemical and rock-magnetic investigations showed that the tsunami deposit is featured by relative enrichment in sand, quartz, feldspar, carbonate, SiO ₂, TiO ₂, K ₂O and CaO and by a depletion in clay and iron oxides. These results point to a dilution of reworked ferromagnetic particles into a huge volume of paramagnetic materials, similar to what has been described in other nearshore tsunami deposits (Font et al. 2010). Correlation analysis elucidated the relationships among the textural, mineral, geochemical and magnetic parameters, and suggests that most of the quartz-rich coarse sediments have been transported offshore by the tsunami wave. These results agreed well with the previously published numerical model of tsunami induced sediment transport off southeast coast of India and can be used for future comparative studies on tsunami deposits.     

Evaluation of trace-metal enrichments from the 26 December 2004 tsunami sediments along the Southeast coast of India

The tsunami sediments deposited after the December 2004 tsunami were sampled immediately in the coastal environment of Tamil Nadu State on the southeast coast of India. Fifty-four sediment samples were collected and 14 representative samples were selected to identify the level of metal contamination in tsunami sediments. The results indicate that the sediments are mainly of fine to medium-grained sand and contain significantly high contents of dissolved salts in sediments (Na⁺, K⁺, Ca⁺², Mg⁺², Cl⁻) in water-soluble fraction due to seawater deposition and evaporation. Correlation of acid leachable trace metals (Cr, Cu, Ni, Co, Pb, Zn) indicate that Fe-Mn oxyhydroxides might play an important role in controlling their association between them. Enrichment of trace metals is observed in all the locations with reference to the background samples. High values of trace metals in the southern part of the study area are due to the large-scale industries along the coast, and they are probably anthropogenic in nature and of marine origin, which could cause serious environmental problems.     

Boulders of MIS 5 age deposited by a tsunami on the coast of Otago, New Zealand

A series of elevated imbricated boulders were investigated on the Otago coastline, southeast New Zealand, through field surveying and optical luminescence dating. By using established hydrodynamic relationships of sediment transport the energy required to move the clasts was calculated and compared to the historic record of marine inundations of that coast. The boulders are platy in shape and are over 2 m long in some cases, and are sourced from a locally outcropping conglomerate unit which appears to be the only lithology on this section of coast that erodes to produce clasts of this size. It is estimated that the boulders were deposited by a tsunami between 2 and 3 m high during the latter part of Marine Isotope Stage 5. They therefore represent the first pre-Holocene tsunami deposit and one composed of large boulders described on the New Zealand coastline.     

Texture and geochemistry of the sediments of a tropical mangrove ecosystem,southwest coast of India

The textural and geochemical aspects of the sediments of a tropical mangrove ecosystem have been studied and discussed. The sediments are characterized by the abundance of silt and sand with minor amounts of clay. The mean size of the sediment ranges from 0.205 mm to 0.098 mm (fine to very fine grained sand). The sediments are very poorly sorted, negatively to very negatively skewed, and platy to extremely leptokurtic in nature. The organic carbon content of the sediments ranges from 0,33% to 4.93%, which is controlled by the particle size of the sediments. The CaCO₃ content is five times the enrichment of organic carbon. This enhanced CaCO₃ content of the mangrove sediments might be a result of the abundance of shell fragments in the sediments. The shell mining activities in the estuarine bed adjoining the Kumarakam mangroves also contribute a substantial amount of lime muds to the mangrove area, which in turn add CaCO₃ to the sediments. The relative concentrations of heavy metals are Fe > Mn > Cr > Zn > Ni > Cu. All heavy metals other than Fe show an increase in concentration compared to the other parts of the estuarine bed. Cluster analysis indicates that the contents of organic C, Fe and Mn have a marked bearing on the Cr, Zn, Ni, and Cu levels of the mangrove sediments.     

Application of wave transformation models for estimation of morphological changes at Vellar estuary,southeast coast of India

Assessment of the wave climate at near coast is vital for estimation of morphological changes, such as growth of sand spit and associated siltation of tidal inlets. Vellar (bar-built) estuary is one of the prominent estuaries along the southeast coast of India, located at 11°30′N and 79°46′E, less studied in terms of its morphological features. The inlet of Vellar is exposed to high energetic waves, inducing large sediment transport rates and shoreline changes. Local wave characteristics are not accurately defined and the available wave information at near coast is limited (point based observations). In the present study, three decoupled numerical models are employed to derive the monthly nearshore wave climate at Vellar by transforming waves from deep water to nearshore. These models are independently validated with buoy observations in deep water and wave gauge data at nearshore. Based on the nearshore wave data, littoral drift along the coast was estimated and compared with the spit growth at Vellar inlet. The estimated average littoral drift along this coast from February to October is 1.93 × 10⁶ m³ toward north and from November to January it is 1.52 × 10⁶ m³ toward south, resulting in a net northerly drift. Results indicated that increase in the wave energy during the period of July to September is responsible for the maximum growth of the sand spit observed in the field.     

Geologic impact of Hurricane Andrew on Everglades coast of southwest Florida

Hurricane Andrew, one of the strongest storms of the century, crossed the southern part of the Florida peninsula on 24 August 1992. Its path crossed the Florida Everglades and exited in the national park across a mangrove-dominated coast onto the shallow, low-energy, inner shelf. The storm caused extensive breakage and defoliation in the mangrove community; full recovery will take decades. It produced no extensive sedimentation unit; only local and ephemeral ebb-surge deposits. The discontinuous shelly storm beach ridge was breached at multiple locations, and it moved landward a few meters. After seven months, there was little geologic indication that the storm had passed. It is likely that the stratigraphic record in this area will not contain any recognizable features of the passage of Hurricane Andrew.     

Hydraulic and numerical study on the generation of a subaqueous landslide-induced tsunami along the coast

By carrying out the hydraulic experiments in a one-dimensional open channel and two-dimensional basin, we clarified the process of how a landslide on a uniform slope causes the generation of a tsunami. The effect of the interactive force that occurs between the debris flow layer and the tsunami is significant in the generation of a tsunami. The continuous flow of the debris into the water makes the wave period of the tsunami short. The present experiments apply numerical simulation using the two-layer model with shear stress models on the bottom and interface, and the results are compared. The simulated debris flow shows good agreement with the measured results and ensures the rushing process into the water. We propose that the model use a Manning coefficient of 0.01 for the smooth slope and 0.015 for the rough slope, and a horizontal viscosity of 0.01 m²/s for the landslide; an interactive force of 0.2 for each layer is recommended. The dispersion effect should be included in the numerical model for the propagation from the shore.     

Metal pollution in coarse sediments of Tuticorin coast,Southeast coast of India

Indian coastal waters are subjected to considerable pressure from sewage and industrial wastes, which are responsible for the contamination of the coastal sediments with consequent loss in biosphere. The present investigation attempts to study the significance of coarse material (Sand fraction) in the distribution of metals in polluted marine sediments. The study revealed that coarse Sand component contains a relatively significant proportion of the anthropogenic metals (Cd, Cu, Pb and Zn) and therefore it cannot be neglected in metal pollution studies of coastal sediments. Further, the distribution of anthropogenic metals in both Silt + Clay and Sand fraction follow the same suit indicating similar pollution sources. From the total sediment type (Silt + Clay and Sand fraction) all anthropogenic metals had a noticeable amount (>50%) in the acid extractable (and potentially bio-available) fraction. This article stresses the importance of coarse fraction in metal pollution studies in Indian coastal system.     

Evaluation of tsunami impacts on shallow marine sediments: An example from the tsunami caused by the 2003 Tokachi-oki earthquake, northern Japan

A combined approach of field geology and numerical simulation was conducted for evaluating the tsunami impacts on the shelf sediments. The 2003 Tokachi-oki earthquake, M 8.0, that occurred on 25 September 2003 off southeastern Hokkaido, northern Japan, generated a locally destructive tsunami. Maximum run-up height of the tsunami waves reached 4 m above sea level. In order to estimate the tsunami impacts on shallow marine sediments, we compared pre- and post-tsunami marine sediments in water depths of 38–112 m in terms of grain size, sedimentary structure, and microfossil content. Decreases of fine fractions, especially finer than very fine sand, which led to coarsen the mean grain size, were detected in the inner shelf of the northern part of the study area. Foraminiferal assemblages also changed in the coarsened sediments. On the other hand, the other shelf sediments largely unchanged or slightly fined. We also simulated the tsunami wave velocity and direction, and grain size entrained by the modeled tsunami. The numerical simulation resulted in that the 2003 tsunami could transport very fine sand in water depths shallower than 45–95 m at the northern part of the study area. This is comparable with the actual grain-size changes after the tsunami had passed. However, some storms and tidal currents might also be possible to stir the surface sediments after the pre-tsunami survey, so we could not conclude that the grain-size changes had been caused only by the tsunami. Nevertheless, a combined approach of sampling and modeling was powerful for estimating the tsunami impacts under the sea.     

Sedimentology of the December 26, 2004, Sumatra tsunami deposits in eastern India (Tamil Nadu) and Kenya

The December 26, 2004 Sumatra tsunami caused severe damage at the coasts of the Indian ocean. We report results of a sedimentological study of tsunami run-up parameters and the sediments laid down by the tsunami at the coast of Tamil Nadu, India, and between Malindi and Lamu, Kenya. In India, evidence of three tsunami waves is preserved on the beaches in the form of characteristic debris accumulations. We measured the maximum run-up distance at 580 m and the maximum run-up height at 4.85 m. Flow depth over land was at least 3.5 m. The tsunami deposited an up to 30 cm thick blanket of moderately well to well-sorted coarse and medium sand that overlies older beach deposits or soil with an erosional unconformity. The sand sheet thins inland without a decrease of grain-size. The deposits consist frequently of three layers. The lower one may be cross-bedded with foresets dipping landward and indicating deposition during run-up. The overlying two sand layers are graded or parallel-laminated without indicators of current directions. Thus, it remains undecided whether they formed during run-up or return flow. Thin dark laminae rich in heavy minerals frequently mark the contacts between successive layers. Benthic foraminifera indicate an entrainment of sediment by the tsunami from water depths less than ca. 30 m water depth. On the Indian shelf these depths are present at distances of up to 5 km from the coast. In Kenya only one wave is recorded, which attained a run-up height of 3 m at a run-up distance of ca. 35 m from the tidal water line at the time of the tsunami impact. Only one layer of fine sand was deposited by the tsunami. It consists predominantly of heavy minerals supplied to the sea by a nearby river. The sand layer thins landward with a minor decrease in grain-size. Benthic foraminifera indicate an entrainment of sediment by the tsunami from water depths less than ca. 30 m water depth, reaching down potentially to ca. 80 m. The presence of only one tsunami-related sediment layer in Kenya, but three in India, reflects the impact of only one wave at the coast of Kenya, as opposed to several in India. Grain-size distributions in the Indian and Kenyan deposits are mostly normal to slightly positively skewed and indicate that the detritus was entrained by the tsunami from well sorted pre-tsunami deposits in nearshore, swash zone and beach environments.     

A study on pre- and post-tsunami shallow deposits off SE coast of India from the 2004 Indian Ocean tsunami: a geochemical approach

A study was made on samples from one core collected immediately after the December 2004 Asian tsunami to know the geochemical nature of the offshore tsunami sediments. The core sample was analyzed for sediment grain size, CaCO₃, organic carbon (OC) and major elements (SiO₂, TiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Na₂O, K₂O, P₂O₅, MnO). The results indicate that the core sample can be divided into two parts: (1) upper 0–25 cm, interpreted to be deposited after tsunami (AT), and (2) lower 25–45 cm, interpreted as before tsunami (BT) as evidenced by the sandy nature AT with fluctuating CaCO₃ contents. The AT part is devoid of OC suggesting that the sediment could have been transported to deeper regions along with the finer particles. Major elements such as SiO₂, TiO₂, CaO indicate high values than the other elements in AT part than in the BT part. The BT part contains Al rich alluvium mud associated with finer mud and organic particles. An analysis of the correlation matrix indicates the possible source of elements and transport of heavy minerals in the AT part than the BT part. The overall results suggest that the sediments could possibly have two different origins.     

Identification of tsunami deposits considering the tsunami waveform: An example of subaqueous tsunami deposits in Holocene shallow bay on southern Boso Peninsula, Central Japan

This study proposes a tsunami depositional model based on observations of emerged Holocene tsunami deposits in outcrops located in eastern Japan. The model is also applicable to the identification of other deposits, such as those laid down by storms. The tsunami deposits described were formed in a small bay of 10–20-m water depth, and are mainly composed of sand and gravel. They show various sedimentary structures, including hummocky cross-stratification (HCS) and inverse and normal grading. Although, individually, the sedimentary structures are similar to those commonly found in storm deposits, the combination of vertical stacking in the tsunami deposits makes a unique pattern. This vertical stacking of internal structures is due to the waveform of the source tsunamis, reflecting: 1) extremely long wavelengths and wave period, and 2) temporal changes of wave sizes from the beginning to end of the tsunamis.

The tsunami deposits display many sub-layers with scoured and graded structures. Each sub-layer, especially in sandy facies, is characterized by HCS and inverse and normal grading that are the result of deposition from prolonged high-energy sediment flows. The vertical stack of sub-layers shows incremental deposition from the repeated sediment flows. Mud drapes cover the sub-layers and indicate the existence of flow-velocity stagnant stages between each sediment flow. Current reversals within the sub-layers indicate the repeated occurrence of the up- and return-flows.

The tsunami deposits are vertically divided into four depositional units, Tna to Tnd in ascending order, reflecting the temporal change of wave sizes in the tsunami wave trains. Unit Tna is relatively fine-grained and indicative of small tsunami waves during the early stage of the tsunami. Unit Tnb is a protruding coarse-grained and thickest-stratified division and is the result of a relatively large wave group during the middle stage of the tsunami. Unit Tnc is a fine alternation of thin sand sheets and mud drapes, deposited from waning waves during the later stage of the tsunami. Unit Tnd is deposited during the final stage of the tsunami and is composed mainly of suspension fallout. Cyclic build up of these sub-layers and depositional units cannot be explained by storm waves with short wave periods of several to ten seconds common in small bays.     

Wind waves and sediment transport regime off the south-central Kerala coast,India

Wind waves in the innershelf of the south-central Kerala coast, south-west India were measured at four locations during different seasons. Simultaneously, numerical models were developed to simulate the wave and sediment transport regime of the innershelf. Strong monsoonal influence is seen in the wave characteristics with greater amplitudes, lower periods and switch-over from SW to SWW–W direction. The net annual longshore sediment transport is southerly in the innershelf and northerly in the surf zone. These counter-directional transports are linked by seasonally reversing the cross-shore transports. In the locations where the transports in the longshore and cross-shore directions are balanced, stable beaches prevail. Erosion/accretion tendency prevails in locations where these transports are not balanced. The southern and northern parts of the coast where onshore transports are predominant could be accreting zones. The erosion/accretion pattern deduced from the sediment transport model corresponds well with the long-term erosion/accretion trend for this coast.     

Exact analytical solutions of nonlinear problems of tsunami wave run-up on slopes with different profiles

A review of papers investigating tsunami wave run-up on a beach is given and the control parameters of the problem are revealed. There are two such parameters in the case of ideal fluid: the bottom sloping angle and the breaking parameter. A stage-by-stage approach for finding run-up characteristics is formulated: the linear calculation of shoreline oscillations and the subsequent non-linear transformation of the solution according to the Riemann method. Solution of the nononedimensional problems of wave run-up on a beach in the linear formulation is obtained.     

Quantification of submarine groundwater discharge(SGD) using radon,radium tracers and nutrient inputs in Punnakayal,south coast of India

The present study focused on the estimation of submarine groundwater discharge(SGD)and the effects of nutrient fluxes due to the SGD process.The parameters of SGD such as magnitude,character,and nutrient flux in Punnakayal region of South East coast of India were evaluated using multiple tracers of groundwater inputs in 2019.It was found that the elevated values for the tracers in the study area,displayed a gradational change in the values as move from estuarine part to the offshore.Simultaneous occurrence of fresh and saline SGD is observed on the study sites.Also,indicated that the SGD fluxes ranged from 0.04 to 0.12 m^3 m^-2d^-1 at the estuary and0.03-0.15 m^3 m^-2d^-1at the groundwater site.A substantially increased value for 222 Rn activities is distinguished in the estuary to values over 312 dpm L^-1.Nutrient embellishments were generally greatest at locations with substantial meteoric elements in groundwater;however,the recirculation of saltwater through the geological formation could provide a way of transferring terrestrially-derived nutrients to the coastal zone at many places.