ABSTRACT Arsenic (As) is a highly toxic, ubiquitous metalloid and realization is growing that wat... more ABSTRACT Arsenic (As) is a highly toxic, ubiquitous metalloid and realization is growing that water-borne As now poses a significant threat to human and ecosystem health worldwide. Elevated concentrations of As in groundwater have emerged as a major health threat in the Ganges-Brahmaputra Delta region where tens of millions of people are exposed to [As] 10 to 100 times higher than the drinking water standard of 10 mug/L recommended by the WHO. Extensive sampling by the British Geological Survey has shown that water from shallow aquifers with recent alluvial sediments carries distinctly higher [As] than does water from deeper aquifers with presumed pre-Holocene sediments. However, the reasons why such a large contrast in [As] exists between younger, Holocene aquifers and older, Pleistocene aquifers are not well understood. Furthermore, although As is generally believed to be of natural origin and is mobilized in reducing groundwater, the sources of particle phase As and mechanisms of arsenic release to groundwater remain poorly understood. Hydrological and geochemical factors contributing to elevated arsenic concentrations (up to 800 mug/L) in the shallow aquifers and much lower [As](< 50 mug/L) in the deep aquifers were investigated in two villages (~1 km2 each) in Araihazar upazilla, Bangladesh. Araihazar is on the margin of the Holocene Mehgna fluvial floodplain where the transition occurs from the uplifted mid Pleistocene Madhupur tract to much younger, incised Meghna river channel deposits from west to east. Coring confirmed that the aquifers were separated by a multiple-layered silt/clay section. At least at one site, radiocarbon dating of peat layers within the silt/clay section suggests that a Holocene aquifer is unconformably overlying a Pleistocene sequence. Based on radiocarbon and tritium dating, the residence time of groundwater in the high-As shallow, Holocene aquifers (4 - 30 m) is years to decades, much less than that of the low-As deep aquifer (50- 100 m), which is a thousand to tens of thousands of years. This hydrological separation is important in preventing water from the high-As shallow aquifer from entering the low-As deep aquifer. Major differences in reducing conditions, when combined with a widely dispersed sedimentary source of As most likely sorbed on amorphous Fe oxyhydroxide contributes to the contrast between the high-As shallow aquifer and the low-As deep aquifers. High concentrations of NaH2PO4 extractable As (up to 5 mg/kg), and HCl-extractable Fe (up to 4.1 g/kg) were found at depth intervals of reducing groundwater enriched in As in the shallow aquifer. Sediments from deeper aquifers had lower NaH2PO4 extractable As (~ 0.4 mg/kg), probably accounting for the low [As] groundwater, despite of strongly reducing conditions at one site. At another site, groundwater displayed positive oxidation-reduction potential (ORP) values as well as low NaH2PO4 extractable As (0.1 - 1 mg/kg), therefore no As mobilization would be expected. In the shallow aquifer, As is released from reductive dissolution of Fe-oxyhydroxides with a sediment As/Fe ratio of 20 mmole/mole, based on As/Fe ratios of groundwater samples that followed a generally increasing As and Fe concentrations with decreasing ORP values. This As/Fe ratio of 20 mmole/mole is much higher than that leached from the high As, sandy sediment (~ 1 mmole/mole), suggesting that Fe oxyhydroxide that was reductively dissolved. is much more enriched in As than the bulk composition of phases which are leachable from the sediments.
Dolomite (CaMg[CO3]2) is a common rock‐forming mineral. Nevertheless, its mechanisms of formation... more Dolomite (CaMg[CO3]2) is a common rock‐forming mineral. Nevertheless, its mechanisms of formation and the factors that cause dolomite concentration variations within the sedimentary records constitute long‐standing geochemical questions. In addition, the flux of Mg2+ leaving the ocean by the formation of dolomite is a controversial question, with some studies arguing that dolomite formation is a negligible Mg2+ sink in the modern ocean, while others show that it constitutes more than 50% of the total Mg2+ removal rate. An important factor that impedes the resolution of the dolomite Mg2+ flux is the lack of analytical methods with adequate precision and detection limit to directly measure minute quantities of authigenic dolomite in marine sediments. Here, we present a new analytical method for direct, precise measurement of dolomite content in marine sediments. The method is based on sequential leaching of carbonate minerals in acid and tracks the CO2 emitted by the dissolution. Based on the measurement of gravimetric standards of calcite and dolomite, the method's detection limit and precision were determined as better than 0.2 and ± 0.2 dry wt% of dolomite, respectively. The method out‐performed dolomite quantification made by x‐ray diffraction and by inductive coupled plasma mass‐spectrometry, which provided precision of ± 2 and ± 1 dry wt%, respectively. Measurements of the dolomite content in modern sediments from the seafloor below the oligotrophic Eastern Mediterranean and the eutrophic Mississippi plume, and in clayey‐silty alluvial soil from south‐eastern Israel, demonstrated that the aforementioned precisions are also valid for natural samples.
Hydrological restriction from the Atlantic Ocean and a negative freshwater balance transformed th... more Hydrological restriction from the Atlantic Ocean and a negative freshwater balance transformed the Mediterranean Sea into a giant saline basin during the Messinian Salinity Crisis (MSC) (5.97 – 5.33 million years ago). After more than 50 years of research, it is still unclear if the deposition of nearly one million km3 of evaporite salts during this event was accompanied by a major (≥ 1.5 km) drawdown of Mediterranean sea level; and if halite deposition occurred only during this drawdown event, or also in a filled Mediterranean connected to the Atlantic.We present evidence based on the chlorine stable isotope composition of halite for a sea level drawdown of 2 km in the eastern Mediterranean during the final stages of deposition of the Mediterranean halite layer. This is the largest sea level drop ever reported from the geological record and implies a short (~20 kyr), but nearly complete hydrological disconnection of the Mediterranean from the Atlantic. About half of the halite volu...
Recent studies of seawater compositions of some ‘non-traditional’ stable isotope systems, such as... more Recent studies of seawater compositions of some ‘non-traditional’ stable isotope systems, such as 26Mg/24Mg (reported as δ26Mg), have uncovered great potential to enhance our understanding of the past Earth. However, differences between existing oceanic records, and the scarcity of such record data, currently limit this approach. Thus, new archives for these isotope compositions, independent of the commonly-used carbonate archives, are required. Marine evaporites have been widely used to decipher the chemical and ‘traditional’ isotope compositions (such as 87Sr/86Sr, S isotopes, etc.) history of past oceans. In several studies [1, 2, 3], we investigate the Mg isotope composition of different evaporite minerals and brines, presenting two examples: an experimental study of marine Mg-K (potash) salts and an in-situ study of pore-water and sediment from a modern sabkha environment.The δ26Mg value of marine-derived brines and precipitating Mg-salts during the evaporation path of seawater...
The geochemistry of groundwater surrounding the Dead Sea reflects mixing between different water ... more The geochemistry of groundwater surrounding the Dead Sea reflects mixing between different water bodies that occupied the Dead Sea basin and fresh groundwater flowing from the highlands to the basin. The distribution of various geochemical parameters is affected by changes in the lake level, including natural fluctuations and the present lake level drop. This chapter deals with different aspects of groundwater flow mechanisms in the coastal aquifer of the Dead Sea and their relationship with the groundwater geochemistry. Dead Sea water circulation in the coastal aquifer occurs during a steady-state lake level but also persists during lake level drop. This interaction between the Dead Sea water and the sediments removes Ba, U, and 226Ra and contributes Fe, Mn, and short-lived Ra isotopes to the Dead Sea. 14C and tritium values in the Dead Sea groundwaters are affected by mixing with fresh water and between brines that occupied the Dead Sea Rift during different times. In addition to ...
Unique gypsum structures: large capes (termed “gypsum deltas”) and small pitted gypsum mounds are... more Unique gypsum structures: large capes (termed “gypsum deltas”) and small pitted gypsum mounds are exposed along the western shores of the currently retreating Dead Sea, the hypersaline terminal lake in the Dead Sea Basin. The gypsum deltas were formed during time intervals of low lake stands (∼420 ± 10 m below mean sea level), when sulfate‐rich Ca chloride brines discharged from the coastal aquifer via saline springs, mixed with the Dead Sea brine and precipitated the gypsum (outsalting process). The ages of formation of the gypsum structures coincide with times of North Atlantic cooling events and grand solar minima suggesting a direct impact of the latter on the Dead Sea hydrology and high sensitivity of the regional hydrology (controlling lake level) to global solar‐related events. The temporal occurrence and numbers of the gypsum structures appear to follow the Hallstatt Cycle that approaches minima at ∼3,000–2,000 years before present.
ABSTRACT Arsenic (As) is a highly toxic, ubiquitous metalloid and realization is growing that wat... more ABSTRACT Arsenic (As) is a highly toxic, ubiquitous metalloid and realization is growing that water-borne As now poses a significant threat to human and ecosystem health worldwide. Elevated concentrations of As in groundwater have emerged as a major health threat in the Ganges-Brahmaputra Delta region where tens of millions of people are exposed to [As] 10 to 100 times higher than the drinking water standard of 10 mug/L recommended by the WHO. Extensive sampling by the British Geological Survey has shown that water from shallow aquifers with recent alluvial sediments carries distinctly higher [As] than does water from deeper aquifers with presumed pre-Holocene sediments. However, the reasons why such a large contrast in [As] exists between younger, Holocene aquifers and older, Pleistocene aquifers are not well understood. Furthermore, although As is generally believed to be of natural origin and is mobilized in reducing groundwater, the sources of particle phase As and mechanisms of arsenic release to groundwater remain poorly understood. Hydrological and geochemical factors contributing to elevated arsenic concentrations (up to 800 mug/L) in the shallow aquifers and much lower [As](< 50 mug/L) in the deep aquifers were investigated in two villages (~1 km2 each) in Araihazar upazilla, Bangladesh. Araihazar is on the margin of the Holocene Mehgna fluvial floodplain where the transition occurs from the uplifted mid Pleistocene Madhupur tract to much younger, incised Meghna river channel deposits from west to east. Coring confirmed that the aquifers were separated by a multiple-layered silt/clay section. At least at one site, radiocarbon dating of peat layers within the silt/clay section suggests that a Holocene aquifer is unconformably overlying a Pleistocene sequence. Based on radiocarbon and tritium dating, the residence time of groundwater in the high-As shallow, Holocene aquifers (4 - 30 m) is years to decades, much less than that of the low-As deep aquifer (50- 100 m), which is a thousand to tens of thousands of years. This hydrological separation is important in preventing water from the high-As shallow aquifer from entering the low-As deep aquifer. Major differences in reducing conditions, when combined with a widely dispersed sedimentary source of As most likely sorbed on amorphous Fe oxyhydroxide contributes to the contrast between the high-As shallow aquifer and the low-As deep aquifers. High concentrations of NaH2PO4 extractable As (up to 5 mg/kg), and HCl-extractable Fe (up to 4.1 g/kg) were found at depth intervals of reducing groundwater enriched in As in the shallow aquifer. Sediments from deeper aquifers had lower NaH2PO4 extractable As (~ 0.4 mg/kg), probably accounting for the low [As] groundwater, despite of strongly reducing conditions at one site. At another site, groundwater displayed positive oxidation-reduction potential (ORP) values as well as low NaH2PO4 extractable As (0.1 - 1 mg/kg), therefore no As mobilization would be expected. In the shallow aquifer, As is released from reductive dissolution of Fe-oxyhydroxides with a sediment As/Fe ratio of 20 mmole/mole, based on As/Fe ratios of groundwater samples that followed a generally increasing As and Fe concentrations with decreasing ORP values. This As/Fe ratio of 20 mmole/mole is much higher than that leached from the high As, sandy sediment (~ 1 mmole/mole), suggesting that Fe oxyhydroxide that was reductively dissolved. is much more enriched in As than the bulk composition of phases which are leachable from the sediments.
Dolomite (CaMg[CO3]2) is a common rock‐forming mineral. Nevertheless, its mechanisms of formation... more Dolomite (CaMg[CO3]2) is a common rock‐forming mineral. Nevertheless, its mechanisms of formation and the factors that cause dolomite concentration variations within the sedimentary records constitute long‐standing geochemical questions. In addition, the flux of Mg2+ leaving the ocean by the formation of dolomite is a controversial question, with some studies arguing that dolomite formation is a negligible Mg2+ sink in the modern ocean, while others show that it constitutes more than 50% of the total Mg2+ removal rate. An important factor that impedes the resolution of the dolomite Mg2+ flux is the lack of analytical methods with adequate precision and detection limit to directly measure minute quantities of authigenic dolomite in marine sediments. Here, we present a new analytical method for direct, precise measurement of dolomite content in marine sediments. The method is based on sequential leaching of carbonate minerals in acid and tracks the CO2 emitted by the dissolution. Based on the measurement of gravimetric standards of calcite and dolomite, the method's detection limit and precision were determined as better than 0.2 and ± 0.2 dry wt% of dolomite, respectively. The method out‐performed dolomite quantification made by x‐ray diffraction and by inductive coupled plasma mass‐spectrometry, which provided precision of ± 2 and ± 1 dry wt%, respectively. Measurements of the dolomite content in modern sediments from the seafloor below the oligotrophic Eastern Mediterranean and the eutrophic Mississippi plume, and in clayey‐silty alluvial soil from south‐eastern Israel, demonstrated that the aforementioned precisions are also valid for natural samples.
Hydrological restriction from the Atlantic Ocean and a negative freshwater balance transformed th... more Hydrological restriction from the Atlantic Ocean and a negative freshwater balance transformed the Mediterranean Sea into a giant saline basin during the Messinian Salinity Crisis (MSC) (5.97 – 5.33 million years ago). After more than 50 years of research, it is still unclear if the deposition of nearly one million km3 of evaporite salts during this event was accompanied by a major (≥ 1.5 km) drawdown of Mediterranean sea level; and if halite deposition occurred only during this drawdown event, or also in a filled Mediterranean connected to the Atlantic.We present evidence based on the chlorine stable isotope composition of halite for a sea level drawdown of 2 km in the eastern Mediterranean during the final stages of deposition of the Mediterranean halite layer. This is the largest sea level drop ever reported from the geological record and implies a short (~20 kyr), but nearly complete hydrological disconnection of the Mediterranean from the Atlantic. About half of the halite volu...
Recent studies of seawater compositions of some ‘non-traditional’ stable isotope systems, such as... more Recent studies of seawater compositions of some ‘non-traditional’ stable isotope systems, such as 26Mg/24Mg (reported as δ26Mg), have uncovered great potential to enhance our understanding of the past Earth. However, differences between existing oceanic records, and the scarcity of such record data, currently limit this approach. Thus, new archives for these isotope compositions, independent of the commonly-used carbonate archives, are required. Marine evaporites have been widely used to decipher the chemical and ‘traditional’ isotope compositions (such as 87Sr/86Sr, S isotopes, etc.) history of past oceans. In several studies [1, 2, 3], we investigate the Mg isotope composition of different evaporite minerals and brines, presenting two examples: an experimental study of marine Mg-K (potash) salts and an in-situ study of pore-water and sediment from a modern sabkha environment.The δ26Mg value of marine-derived brines and precipitating Mg-salts during the evaporation path of seawater...
The geochemistry of groundwater surrounding the Dead Sea reflects mixing between different water ... more The geochemistry of groundwater surrounding the Dead Sea reflects mixing between different water bodies that occupied the Dead Sea basin and fresh groundwater flowing from the highlands to the basin. The distribution of various geochemical parameters is affected by changes in the lake level, including natural fluctuations and the present lake level drop. This chapter deals with different aspects of groundwater flow mechanisms in the coastal aquifer of the Dead Sea and their relationship with the groundwater geochemistry. Dead Sea water circulation in the coastal aquifer occurs during a steady-state lake level but also persists during lake level drop. This interaction between the Dead Sea water and the sediments removes Ba, U, and 226Ra and contributes Fe, Mn, and short-lived Ra isotopes to the Dead Sea. 14C and tritium values in the Dead Sea groundwaters are affected by mixing with fresh water and between brines that occupied the Dead Sea Rift during different times. In addition to ...
Unique gypsum structures: large capes (termed “gypsum deltas”) and small pitted gypsum mounds are... more Unique gypsum structures: large capes (termed “gypsum deltas”) and small pitted gypsum mounds are exposed along the western shores of the currently retreating Dead Sea, the hypersaline terminal lake in the Dead Sea Basin. The gypsum deltas were formed during time intervals of low lake stands (∼420 ± 10 m below mean sea level), when sulfate‐rich Ca chloride brines discharged from the coastal aquifer via saline springs, mixed with the Dead Sea brine and precipitated the gypsum (outsalting process). The ages of formation of the gypsum structures coincide with times of North Atlantic cooling events and grand solar minima suggesting a direct impact of the latter on the Dead Sea hydrology and high sensitivity of the regional hydrology (controlling lake level) to global solar‐related events. The temporal occurrence and numbers of the gypsum structures appear to follow the Hallstatt Cycle that approaches minima at ∼3,000–2,000 years before present.
Uploads
Papers by Ittai Gavrieli