Universidade Federal da Bahia, Grupo de Metalogênese, Centro de Pesquisa em Geofísica e Geologia,... more Universidade Federal da Bahia, Grupo de Metalogênese, Centro de Pesquisa em Geofísica e Geologia, Instituto de Geociências, Campus da Federação, 40170-290 Salvador-BA, Brazil. E-mails: [email protected], [email protected] Departamento de Ciências Naturais, Universidade Estadual do Sudoeste da Bahia, Brazil. E-mail: [email protected] Department of Geology, University of Maryland, College Park, MD 20742, USA. E-mail: [email protected] Department of Geology, University of Ottawa, ON, Canada K1N 6N5. Emails: [email protected], [email protected] Institut für Geologie, Ruhr Universitat, Bochum, Germany Department of Earth Sciences, Memorial University of Newfoundland 300 Prince Philip Drive St. John's, NL, Canada A1B 3X5. E-mail: [email protected]
Please note that this is an author-produced PDF of an article accepted for publication following ... more Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
The Lower Head Formation in the Parson’s Pond area (western Newfoundland, Canada) comprises silts... more The Lower Head Formation in the Parson’s Pond area (western Newfoundland, Canada) comprises siltstones with very fine grained to fine-grained sandstones. Petrography confirms that these sandstones are matrix rich, essentially wackes, with detrital minerals including quartz, feldspar, biotite, and numerous accessory minerals. Observed petrographic features suggest that the Lower Head sediments are the distal product of erosion, with sediment sourced from the Dashwoods microcontinent and Lushs Bight oceanic tract and thoroughly mixed in an earlier basin prior to final deposition in a trench slope basin. The Lower Head Formation sandstones have low porosity, with early diagenetic cements (C1) and later calcite in crosscutting calcite veinlets (C2). Petrographic, isotopic, and fluid inclusion analyses indicate that C1 cements formed during the early stages of diagenesis. Both δ13C and δ18O isotopes for the C1 calcite cements are isotopically heavier than the C2 calcite veins. Fluid incl...
The cause for the end Permian mass extinction, the greatest challenge life on Earth faced in its ... more The cause for the end Permian mass extinction, the greatest challenge life on Earth faced in its geologic history, is still hotly debated by scientists. The most significant marker of this event is the negative δ 13 C shift and rebound recorded in marine carbonates with a duration ranging from 2000 to 19 000 years depending on localities and sedimentation rates. Leading causes for the event are Siberian trap volcanism and the emission of greenhouse gases with consequent global warming. Measurements of gases vaulted in Formattato: Italiano (Italia) Formattato: Italiano (Italia) Formattato: Italiano (Italia) Formattato: Italiano (Italia)
We examined a large number of modern, shallow-water articulated brachiopods representing the orde... more We examined a large number of modern, shallow-water articulated brachiopods representing the orders Terebratulida, Rhynchonellida, Thecideida and one inarticulated brachiopod of the order Craniida from polar to tropical regions for their carbon isotope compositions. Based on our detailed investigation, we recommend avoiding fast growth areas such as the youngest shell increments; in addition, the primary layer and transition zone calcites of brachiopods must be avoided because they are in carbon and oxygen isotope disequilibrium with ambient seawater. After adjusting isotope compositions for the Mg effect, we observed no significant difference (p > 0.05) in δ13C values between dorsal and ventral valves of our articulated brachiopods.
The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the G... more The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the Global Stratotype Section and Point (GSSP) for the Cambrian-Ordovician systems boundary. A new δ13Ccarb curve from the Green Point section has been constructed in the current study(Fig. 1), because (1) no high-resolution carbon isotope profile was previously presented for the GSSP section and (2) the exact stratigraphic level of the Cambrian-Ordovician boundary, based on conodont biozonation, has become recently an issue of debate (Terfelt et al. 2012). The previously constructed C-isotope profile (Nowlan 1995) was based on lower resolution field sampling protocols and whole-sample analyses that resulted in overlooking the significant variations associated with the boundary. The Middle Cambrian to Middle Ordovician Cow Head Group (up to 500 m thick) is composed of marine fine-grained clastic and carbonate sediments that accumulated on the slope along the Laurentian margin. Significant con...
Modern Rhynchonellids and Tcrcbratulids, obtained from water depths below the neritic zone (>5... more Modern Rhynchonellids and Tcrcbratulids, obtained from water depths below the neritic zone (>500m) at 23 stations in the Caribbean Sea, North Atlantic, South Pacific and Southern Oceans, were investigated for their rare earth clement (REE) contents (Fig. I, Table I). The ΣREE of shelf (500-I 000 m) or deep-water (> I 000 m) brachiopod populations do not vary significantly between oceans/seas irrespective of water mass origins or influences by major currents.
Appropriateness of the inorganic Δ 47-temperature calibration for brachiopods and molluscs Ghosh ... more Appropriateness of the inorganic Δ 47-temperature calibration for brachiopods and molluscs Ghosh et al. (2006) 1 determined the relationship between Δ 47 and temperature (Eq 4) by analyzing the CO 2 extracted from synthetic calcites grown in the laboratory at known, controlled * Brachiopod growth temperatures are mean annual temperatures, and mollusc growth temperatures are the mean of the warmest six months of the year 6. Growth temperatures were taken from Levitus and Boyer (1994) 5 .
ABSTRACT The end of the Permian was a time of great death and massive upheaval in the biosphere, ... more ABSTRACT The end of the Permian was a time of great death and massive upheaval in the biosphere, atmosphere and hydrosphere. Over the last decades, many causes have been suggested to be responsible for that catastrophe such as global warming, anoxia and acidification. The Gyanyima limestone block was an open ocean seamount in the southern Neotethys at subtropical latitude, and it affords us insight into open-ocean oceanographic changes during the end of the Permian After careful screening using multiple tests, we reconstructed carbonate/seawater curves from the geochemical data stored in pristine brachiopod shell archives from the shallow water limestone of the Changhsingian Gyanyima Formation of Tibet. The reconstructed strontium isotope curve and data for the late Changhsingian is relatively invariant about 0.707013, but in the upper part of the succession the values become more radiogenic climaxing at about 0.707244. The 87Sr/86Sr curve and trend is similar to that observed for the Upper Permian succession in northern Italy, but dissimilar (less radiogenic) to whole rock results from Austria, Iran, China and Spitsbergen. The Ce/Ce* anomaly results. ranging from 0.310 to 0.577 for the brachiopods and from 0.237 to 0.655 for the coeval whole rock before the event, and of 0.276 for whole rock during the extinction event, suggest normal redox conditions. These Ce* values are typical of normal open-ocean oxic water quality conditions observed in modern and other ancient counterparts. The biota and Ce* information clearly discounts global anoxia as a primary cause for the end-Permian biotic crisis. Carbon isotopes from brachiopod shells and whole rock are relatively invariant for most of the latest Permian interval, which is in stark contrast to the distinct negative carbon isotope excursion observed near and about the event. Estimates of seawater temperature at shallow depth fluctuated from 22.2 to 29.0 °C up to unit 8-2, and then gradually rise from 29.7 °C in unit 8-13 to values exceeding 35 °C at a stratigraphic level about 120 ky before the Permian-Triassic boundary, and just before the onset of the extinction interval. This dramatic increase in seawater temperature has been observed in global successions from tropical to mid latitude and from restricted to open ocean localities (e.g., northern Italy, Iran). The brachiopod archive and its geochemical proxies from Tibet support the paradigm that global warming must have been an important factor of the biotic crisis for the terrestrial and marine faunas and floras of the late Paleozoic world.
ABSTRACT Global climate change impacts marine ecosystems, directly and indirectly, especially in ... more ABSTRACT Global climate change impacts marine ecosystems, directly and indirectly, especially in the Arctic and Antarctic. We show the first long-term (1920–2011) time-series of oceanographic change in Hudson Bay, an arctic marine ecosystem, based on coupled brachiopod-calcite stable and clumped isotope results. Long-term decrease in brachiopod δ13C parallels that of seawater-DIC in Hudson Bay, and after considering its seasonal sea ice coverage, it is similar to that of the 13C-Suess effect observed in the North Atlantic and other regions. Acidification of Hudson Bay seawater leads warming by about 10–20 years, and with intensified warming from the 1970s to 2010s closely coupled to earlier sea-ice breakup. Post-industrial warming of Hudson Bay is initially slow, but in later years, faster and of greater magnitude than of the coeval global oceans. Our observations for the past 90 years suggest that climate-forced change contributed to an average increase of about 0.1 °C and 3.6 °C in sea-surface water temperature of Hudson Bay over the first 50 and subsequent 40 years, respectively. This 3.7 °C post-industrial warming of Hudson Bay seawater is about six times the 0.67 °C increase observed during the past 100 years in global ocean sea-surface temperature, which is about double the postulated increase of about 2 °C for polar regions. Our results are consistent with the general notion that polar marine environments, such as Hudson Bay, can serve as sensitive indicators of change in climate, and of change still to come for lower latitude ecosystems.
The Devonian-Carboniferous (D-C) boundary sequence of the Namur-Dinant Basin in southern Belgium ... more The Devonian-Carboniferous (D-C) boundary sequence of the Namur-Dinant Basin in southern Belgium consists of marine platform carbonates. Global biostratigraphic correlation of the boundary has been a dilemma due to the absence of index conodont zones. Despite the scarcity of brachiopods, we managed to sample twenty-five calcitic shells from boundary beds at the Royseux-Gare section, to reconstruct biochemostratigraphic profiles of oxygen-, carbon-and strontium-isotopes for correlations with established global counterparts. The δ 18 O and δ 13 C values of the well-preserved shells range from − 7.8 to − 6.3‰ VPDB (− 7.2 ± 0.4, n = 25) and from + 1.1 to + 2.4‰ VPDB (1.8 ±0.3, n = 25), respectively. The shells also yielded 87 Sr/ 86 Sr ratios between 0.708185 and 0.708297. The Royseux isotope signatures are within the ranges documented for the global D-C boundary but their isotope profiles, however, show no significant shifts or excursions. Evaluation of the Royseux isotope profiles and correlation with their global counterparts may suggest a stratigraphic hiatus approximately from the middle Siphonodella praesulcata to the lower Siphonodella sulcata zones on the global D-C boundary conodont biostratigraphic scheme, while corresponding to the Hangenberg Event in Belgium.
The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) 24 represents th... more The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) 24 represents the Global Stratotype Section and Point (GSSP) for the Cambrian-Ordovician 25 boundary on Laurentia. The formation consists of the Martin Point (lower) and the Broom Point 26 (upper) members, which constitute a thick (~170 m) deep subtidal to slope marine carbonate 27 sequence. Preservation of the micritic carbonates of the Green Point Formation was evaluated by 28 multiple petrographic and geochemical screening tools. The δ 13 C and δ 18 O values of near-29 primary micrites range from-4.7 ‰ to +1.7‰ (VPDB) and from-8.7 ‰ to-5.5 ‰ (VPDB), 30 respectively, with no significant correlation (R 2 = 0.002). Similarly, the δ 13 C values show no significant correlation with Mn/Sr ratios or total REE contents, which suggest that the investigated carbonates retain their near-primary δ 13 C signatures that can be utilized to construct 33 a high-resolution carbon-isotope profile for the GSSP. The δ 13 C carb profile about the GSSP trends 34 generally positive into Bed 22 of the Green Point Formation. The top of the C. intermedius Zone 35 marks the onset of a negative δ 13 C carb shift to about 6.0 ‰ culminating in the upper part of the zone. Subsequently, the δ 13 C carb profile trends positive with a 'double switch-back' about the boundary point and then continues on to a most positive value in the uppermost part of Bed 23. In addition, based on 87 Sr/ 86 Sr and Ce*/Ce values, the GSSP is defined by ratios of 0.709623 and <0.9, respectively, suggesting generally dysoxic waters before, during, and immediately after the boundary. The high-resolution chemostratigraphic results, curve, and trends covering the GSSP should facilitate correlation of the Cambrian-Ordovician boundary sections/sequences from other locations.
Well-preserved brachiopods from the Niagara Gorge area, Anticosti Island, Britain, Gotland and Es... more Well-preserved brachiopods from the Niagara Gorge area, Anticosti Island, Britain, Gotland and Estonia were utilised to delineate a complex isotopic evolution for Llandovery-Wenlock seawater. The Sr-isotope record reflects the Salinic I tectophase of the Late Llandovery in the continuous increase in 87 Sr/ 86 Sr values from 0.708070 to 0.708346. The Salinic II tectophase is marked by relative constancy of Sr isotope values until the Late Wenlock when it rises from 0.708345 to 0.708430. The second tectonic phase was therefore likely only of a regional nature. The carbon isotopes during the Llandovery fall within a band of about − 1‰ to + 3‰, a range comparable to modern low-latitude brachiopods. A large positive δ 13 C excursion of about 3‰, identifies the Ireviken event/excursion, characterizes the Early Wenlock. The biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the Cancañiri glaciation, although a direct causative scenario is as yet unknown. The oxygen isotopic trends of well-preserved brachiopods clearly reflect a warm climate interval during the latest Llandovery associated with the Silurian sea level highstand. Subsequently, in the Early Wenlock, the sea level fell with the onset of the Cancañiri glaciation in the southern hemisphere. This is reflected in a significant positive δ 18 O excursion, particularly in brachiopods from the Niagara Gorge area. Brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative δ 18 O values, indicating a lesser cooling gradient.
ABSTRACT The Vazante Group, as originally described, is a thick marine carbonate-dominated succes... more ABSTRACT The Vazante Group, as originally described, is a thick marine carbonate-dominated succession adjacent to the Brasilia Fold Belt, on the western border of the São Francisco Craton, in south-central Brazil. The sedimentary dolomites of this group contain economically important Zn–Pb and phosphate deposits, but age constraints for the host rocks and mineralization have been controversial. New geochronological data and geological observations have indicated that the upper and middle sections of the Vazante succession belong to a Mesoproterozoic sequence that was thrust over a Neoproterozoic succession correlative with the Bambuí Group. This new stratigraphic framework has significant implications for metallogenic exploration models in both intra-cratonic and passive-margin basins of the São Francisco Craton. Although hosted in Mesoproterozoic units, most of the Zn–Pb mineralization occurred in the Neoproterozoic by circulating hydrothermal fluids during the prolonged breakup of the Rodinia supercontinent. The possibility that an initial stage of mineralization occurred earlier is considered. Phosphorite generation in the Neoproterozoic units is conceivably related to glacial events. The refined stratigraphy combined with a new mineralization model will significantly contribute to the exploration strategy for phosphate and sulfide deposits in the Mesoproterozoic and Neoproterozoic successions of the São Francisco Craton and beyond.
Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silur... more Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dob's Linn, Southern Scotland, has received criticism due to the diYculties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dob's Linn have yielded carbon stable-isotope signatures consistent with
... Brand, Uwe 1 ; Logan, Alan 2 ; Bitner, Maria Aleksandra 3 ; Griesshaber, Erika 4 ; Azmy, Kare... more ... Brand, Uwe 1 ; Logan, Alan 2 ; Bitner, Maria Aleksandra 3 ; Griesshaber, Erika 4 ; Azmy, Karem 5 ; Buhl, Dieter 6. Abstract: The chemistry of many biogenic allochems and whole rock is used as a proxy of original seawater chemistry during the geological past. ...
Universidade Federal da Bahia, Grupo de Metalogênese, Centro de Pesquisa em Geofísica e Geologia,... more Universidade Federal da Bahia, Grupo de Metalogênese, Centro de Pesquisa em Geofísica e Geologia, Instituto de Geociências, Campus da Federação, 40170-290 Salvador-BA, Brazil. E-mails: [email protected], [email protected] Departamento de Ciências Naturais, Universidade Estadual do Sudoeste da Bahia, Brazil. E-mail: [email protected] Department of Geology, University of Maryland, College Park, MD 20742, USA. E-mail: [email protected] Department of Geology, University of Ottawa, ON, Canada K1N 6N5. Emails: [email protected], [email protected] Institut für Geologie, Ruhr Universitat, Bochum, Germany Department of Earth Sciences, Memorial University of Newfoundland 300 Prince Philip Drive St. John's, NL, Canada A1B 3X5. E-mail: [email protected]
Please note that this is an author-produced PDF of an article accepted for publication following ... more Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
The Lower Head Formation in the Parson’s Pond area (western Newfoundland, Canada) comprises silts... more The Lower Head Formation in the Parson’s Pond area (western Newfoundland, Canada) comprises siltstones with very fine grained to fine-grained sandstones. Petrography confirms that these sandstones are matrix rich, essentially wackes, with detrital minerals including quartz, feldspar, biotite, and numerous accessory minerals. Observed petrographic features suggest that the Lower Head sediments are the distal product of erosion, with sediment sourced from the Dashwoods microcontinent and Lushs Bight oceanic tract and thoroughly mixed in an earlier basin prior to final deposition in a trench slope basin. The Lower Head Formation sandstones have low porosity, with early diagenetic cements (C1) and later calcite in crosscutting calcite veinlets (C2). Petrographic, isotopic, and fluid inclusion analyses indicate that C1 cements formed during the early stages of diagenesis. Both δ13C and δ18O isotopes for the C1 calcite cements are isotopically heavier than the C2 calcite veins. Fluid incl...
The cause for the end Permian mass extinction, the greatest challenge life on Earth faced in its ... more The cause for the end Permian mass extinction, the greatest challenge life on Earth faced in its geologic history, is still hotly debated by scientists. The most significant marker of this event is the negative δ 13 C shift and rebound recorded in marine carbonates with a duration ranging from 2000 to 19 000 years depending on localities and sedimentation rates. Leading causes for the event are Siberian trap volcanism and the emission of greenhouse gases with consequent global warming. Measurements of gases vaulted in Formattato: Italiano (Italia) Formattato: Italiano (Italia) Formattato: Italiano (Italia) Formattato: Italiano (Italia)
We examined a large number of modern, shallow-water articulated brachiopods representing the orde... more We examined a large number of modern, shallow-water articulated brachiopods representing the orders Terebratulida, Rhynchonellida, Thecideida and one inarticulated brachiopod of the order Craniida from polar to tropical regions for their carbon isotope compositions. Based on our detailed investigation, we recommend avoiding fast growth areas such as the youngest shell increments; in addition, the primary layer and transition zone calcites of brachiopods must be avoided because they are in carbon and oxygen isotope disequilibrium with ambient seawater. After adjusting isotope compositions for the Mg effect, we observed no significant difference (p > 0.05) in δ13C values between dorsal and ventral valves of our articulated brachiopods.
The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the G... more The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the Global Stratotype Section and Point (GSSP) for the Cambrian-Ordovician systems boundary. A new δ13Ccarb curve from the Green Point section has been constructed in the current study(Fig. 1), because (1) no high-resolution carbon isotope profile was previously presented for the GSSP section and (2) the exact stratigraphic level of the Cambrian-Ordovician boundary, based on conodont biozonation, has become recently an issue of debate (Terfelt et al. 2012). The previously constructed C-isotope profile (Nowlan 1995) was based on lower resolution field sampling protocols and whole-sample analyses that resulted in overlooking the significant variations associated with the boundary. The Middle Cambrian to Middle Ordovician Cow Head Group (up to 500 m thick) is composed of marine fine-grained clastic and carbonate sediments that accumulated on the slope along the Laurentian margin. Significant con...
Modern Rhynchonellids and Tcrcbratulids, obtained from water depths below the neritic zone (>5... more Modern Rhynchonellids and Tcrcbratulids, obtained from water depths below the neritic zone (>500m) at 23 stations in the Caribbean Sea, North Atlantic, South Pacific and Southern Oceans, were investigated for their rare earth clement (REE) contents (Fig. I, Table I). The ΣREE of shelf (500-I 000 m) or deep-water (> I 000 m) brachiopod populations do not vary significantly between oceans/seas irrespective of water mass origins or influences by major currents.
Appropriateness of the inorganic Δ 47-temperature calibration for brachiopods and molluscs Ghosh ... more Appropriateness of the inorganic Δ 47-temperature calibration for brachiopods and molluscs Ghosh et al. (2006) 1 determined the relationship between Δ 47 and temperature (Eq 4) by analyzing the CO 2 extracted from synthetic calcites grown in the laboratory at known, controlled * Brachiopod growth temperatures are mean annual temperatures, and mollusc growth temperatures are the mean of the warmest six months of the year 6. Growth temperatures were taken from Levitus and Boyer (1994) 5 .
ABSTRACT The end of the Permian was a time of great death and massive upheaval in the biosphere, ... more ABSTRACT The end of the Permian was a time of great death and massive upheaval in the biosphere, atmosphere and hydrosphere. Over the last decades, many causes have been suggested to be responsible for that catastrophe such as global warming, anoxia and acidification. The Gyanyima limestone block was an open ocean seamount in the southern Neotethys at subtropical latitude, and it affords us insight into open-ocean oceanographic changes during the end of the Permian After careful screening using multiple tests, we reconstructed carbonate/seawater curves from the geochemical data stored in pristine brachiopod shell archives from the shallow water limestone of the Changhsingian Gyanyima Formation of Tibet. The reconstructed strontium isotope curve and data for the late Changhsingian is relatively invariant about 0.707013, but in the upper part of the succession the values become more radiogenic climaxing at about 0.707244. The 87Sr/86Sr curve and trend is similar to that observed for the Upper Permian succession in northern Italy, but dissimilar (less radiogenic) to whole rock results from Austria, Iran, China and Spitsbergen. The Ce/Ce* anomaly results. ranging from 0.310 to 0.577 for the brachiopods and from 0.237 to 0.655 for the coeval whole rock before the event, and of 0.276 for whole rock during the extinction event, suggest normal redox conditions. These Ce* values are typical of normal open-ocean oxic water quality conditions observed in modern and other ancient counterparts. The biota and Ce* information clearly discounts global anoxia as a primary cause for the end-Permian biotic crisis. Carbon isotopes from brachiopod shells and whole rock are relatively invariant for most of the latest Permian interval, which is in stark contrast to the distinct negative carbon isotope excursion observed near and about the event. Estimates of seawater temperature at shallow depth fluctuated from 22.2 to 29.0 °C up to unit 8-2, and then gradually rise from 29.7 °C in unit 8-13 to values exceeding 35 °C at a stratigraphic level about 120 ky before the Permian-Triassic boundary, and just before the onset of the extinction interval. This dramatic increase in seawater temperature has been observed in global successions from tropical to mid latitude and from restricted to open ocean localities (e.g., northern Italy, Iran). The brachiopod archive and its geochemical proxies from Tibet support the paradigm that global warming must have been an important factor of the biotic crisis for the terrestrial and marine faunas and floras of the late Paleozoic world.
ABSTRACT Global climate change impacts marine ecosystems, directly and indirectly, especially in ... more ABSTRACT Global climate change impacts marine ecosystems, directly and indirectly, especially in the Arctic and Antarctic. We show the first long-term (1920–2011) time-series of oceanographic change in Hudson Bay, an arctic marine ecosystem, based on coupled brachiopod-calcite stable and clumped isotope results. Long-term decrease in brachiopod δ13C parallels that of seawater-DIC in Hudson Bay, and after considering its seasonal sea ice coverage, it is similar to that of the 13C-Suess effect observed in the North Atlantic and other regions. Acidification of Hudson Bay seawater leads warming by about 10–20 years, and with intensified warming from the 1970s to 2010s closely coupled to earlier sea-ice breakup. Post-industrial warming of Hudson Bay is initially slow, but in later years, faster and of greater magnitude than of the coeval global oceans. Our observations for the past 90 years suggest that climate-forced change contributed to an average increase of about 0.1 °C and 3.6 °C in sea-surface water temperature of Hudson Bay over the first 50 and subsequent 40 years, respectively. This 3.7 °C post-industrial warming of Hudson Bay seawater is about six times the 0.67 °C increase observed during the past 100 years in global ocean sea-surface temperature, which is about double the postulated increase of about 2 °C for polar regions. Our results are consistent with the general notion that polar marine environments, such as Hudson Bay, can serve as sensitive indicators of change in climate, and of change still to come for lower latitude ecosystems.
The Devonian-Carboniferous (D-C) boundary sequence of the Namur-Dinant Basin in southern Belgium ... more The Devonian-Carboniferous (D-C) boundary sequence of the Namur-Dinant Basin in southern Belgium consists of marine platform carbonates. Global biostratigraphic correlation of the boundary has been a dilemma due to the absence of index conodont zones. Despite the scarcity of brachiopods, we managed to sample twenty-five calcitic shells from boundary beds at the Royseux-Gare section, to reconstruct biochemostratigraphic profiles of oxygen-, carbon-and strontium-isotopes for correlations with established global counterparts. The δ 18 O and δ 13 C values of the well-preserved shells range from − 7.8 to − 6.3‰ VPDB (− 7.2 ± 0.4, n = 25) and from + 1.1 to + 2.4‰ VPDB (1.8 ±0.3, n = 25), respectively. The shells also yielded 87 Sr/ 86 Sr ratios between 0.708185 and 0.708297. The Royseux isotope signatures are within the ranges documented for the global D-C boundary but their isotope profiles, however, show no significant shifts or excursions. Evaluation of the Royseux isotope profiles and correlation with their global counterparts may suggest a stratigraphic hiatus approximately from the middle Siphonodella praesulcata to the lower Siphonodella sulcata zones on the global D-C boundary conodont biostratigraphic scheme, while corresponding to the Hangenberg Event in Belgium.
The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) 24 represents th... more The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) 24 represents the Global Stratotype Section and Point (GSSP) for the Cambrian-Ordovician 25 boundary on Laurentia. The formation consists of the Martin Point (lower) and the Broom Point 26 (upper) members, which constitute a thick (~170 m) deep subtidal to slope marine carbonate 27 sequence. Preservation of the micritic carbonates of the Green Point Formation was evaluated by 28 multiple petrographic and geochemical screening tools. The δ 13 C and δ 18 O values of near-29 primary micrites range from-4.7 ‰ to +1.7‰ (VPDB) and from-8.7 ‰ to-5.5 ‰ (VPDB), 30 respectively, with no significant correlation (R 2 = 0.002). Similarly, the δ 13 C values show no significant correlation with Mn/Sr ratios or total REE contents, which suggest that the investigated carbonates retain their near-primary δ 13 C signatures that can be utilized to construct 33 a high-resolution carbon-isotope profile for the GSSP. The δ 13 C carb profile about the GSSP trends 34 generally positive into Bed 22 of the Green Point Formation. The top of the C. intermedius Zone 35 marks the onset of a negative δ 13 C carb shift to about 6.0 ‰ culminating in the upper part of the zone. Subsequently, the δ 13 C carb profile trends positive with a 'double switch-back' about the boundary point and then continues on to a most positive value in the uppermost part of Bed 23. In addition, based on 87 Sr/ 86 Sr and Ce*/Ce values, the GSSP is defined by ratios of 0.709623 and <0.9, respectively, suggesting generally dysoxic waters before, during, and immediately after the boundary. The high-resolution chemostratigraphic results, curve, and trends covering the GSSP should facilitate correlation of the Cambrian-Ordovician boundary sections/sequences from other locations.
Well-preserved brachiopods from the Niagara Gorge area, Anticosti Island, Britain, Gotland and Es... more Well-preserved brachiopods from the Niagara Gorge area, Anticosti Island, Britain, Gotland and Estonia were utilised to delineate a complex isotopic evolution for Llandovery-Wenlock seawater. The Sr-isotope record reflects the Salinic I tectophase of the Late Llandovery in the continuous increase in 87 Sr/ 86 Sr values from 0.708070 to 0.708346. The Salinic II tectophase is marked by relative constancy of Sr isotope values until the Late Wenlock when it rises from 0.708345 to 0.708430. The second tectonic phase was therefore likely only of a regional nature. The carbon isotopes during the Llandovery fall within a band of about − 1‰ to + 3‰, a range comparable to modern low-latitude brachiopods. A large positive δ 13 C excursion of about 3‰, identifies the Ireviken event/excursion, characterizes the Early Wenlock. The biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the Cancañiri glaciation, although a direct causative scenario is as yet unknown. The oxygen isotopic trends of well-preserved brachiopods clearly reflect a warm climate interval during the latest Llandovery associated with the Silurian sea level highstand. Subsequently, in the Early Wenlock, the sea level fell with the onset of the Cancañiri glaciation in the southern hemisphere. This is reflected in a significant positive δ 18 O excursion, particularly in brachiopods from the Niagara Gorge area. Brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative δ 18 O values, indicating a lesser cooling gradient.
ABSTRACT The Vazante Group, as originally described, is a thick marine carbonate-dominated succes... more ABSTRACT The Vazante Group, as originally described, is a thick marine carbonate-dominated succession adjacent to the Brasilia Fold Belt, on the western border of the São Francisco Craton, in south-central Brazil. The sedimentary dolomites of this group contain economically important Zn–Pb and phosphate deposits, but age constraints for the host rocks and mineralization have been controversial. New geochronological data and geological observations have indicated that the upper and middle sections of the Vazante succession belong to a Mesoproterozoic sequence that was thrust over a Neoproterozoic succession correlative with the Bambuí Group. This new stratigraphic framework has significant implications for metallogenic exploration models in both intra-cratonic and passive-margin basins of the São Francisco Craton. Although hosted in Mesoproterozoic units, most of the Zn–Pb mineralization occurred in the Neoproterozoic by circulating hydrothermal fluids during the prolonged breakup of the Rodinia supercontinent. The possibility that an initial stage of mineralization occurred earlier is considered. Phosphorite generation in the Neoproterozoic units is conceivably related to glacial events. The refined stratigraphy combined with a new mineralization model will significantly contribute to the exploration strategy for phosphate and sulfide deposits in the Mesoproterozoic and Neoproterozoic successions of the São Francisco Craton and beyond.
Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silur... more Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dob's Linn, Southern Scotland, has received criticism due to the diYculties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dob's Linn have yielded carbon stable-isotope signatures consistent with
... Brand, Uwe 1 ; Logan, Alan 2 ; Bitner, Maria Aleksandra 3 ; Griesshaber, Erika 4 ; Azmy, Kare... more ... Brand, Uwe 1 ; Logan, Alan 2 ; Bitner, Maria Aleksandra 3 ; Griesshaber, Erika 4 ; Azmy, Karem 5 ; Buhl, Dieter 6. Abstract: The chemistry of many biogenic allochems and whole rock is used as a proxy of original seawater chemistry during the geological past. ...
Uploads
Papers by Karem Azmy