Offshore residual magnetic anomalies of the Falklands/Malvinas Islands area. An evident “rose”- like 250 kilometer-wide circular positive anomaly is present in the area of the gravity anomalies associated with a suggested impact structure. See the article by Rocca et al. pp 233-237. (Source: National Centers for Environmental Information: http://geomag.org/models/emag2.html, as EMAG2 for Google (Version 2.0) ASCII grid of the magnetic total intensity at 4 km above the WGS84 ellipsoid).
Written and archaeological sources from the Mediterranean region have been exhaustively searched for evidence of historical volcanism before the year A.D. 630. Volcanic eruptions are identified here by two methods: direct observations, which give information about Mediterranean volcanoes, and indirect, atmospheric observations, which give at least the dates of very large explosive eruptions that occurred somewhere in the northern hemisphere. Seven or more very large explosive eruptions have been detected by these methods. Direct observations indicate great eruptions of Thera (fifteenth century B.C.), Etna ( 44 B.C.), and Vesuvius ( 217 B.C., A.D. 79, A.D. 472). Indirect observations imply great eruptions of northern hemisphere volcanoes in the years 217 B.C.,44 B.C.,A.D. 472 A.D. 536, and A.D. 626. Some of the correlations with known Mediterranean eruptions may be accidental. It is found that atmospheric veiling and cooling were quite marked for about a year after the eruptions of 44 B.C., A.D. 472, A.D. 536, and A.D. 626 ( relevant data are lacking for the other eruptions). If the A.D. 536 eruption was a very distant one ( Rabaul, New Britain?), it may have been the most explosive in recorded history. There is independent evidence of the sizes of the eruptions that took place in these years: at least five of them coincide with the strongest acidity signals in Greenland ice for this period. In the case of the smaller eruptions, reliable (though necessarily incomplete) chronologies are presented for Etna, Vesuvius, and the other active Mediterranean volcanoes. Full documentation from the original sources is provided throughout.
The mass extinctions at the Cretaceous-Tertiary (K-T) boundary include about 90 percent of marine calcareous nannoplankton (coccoliths), and carbon-isotope data show that marine primary productivity was drastically reduced for about 500,000 years after the boundary event, the so-called Strangelove Ocean effect. One result of the elimination of most marine phytoplankton would have been a severe reduction in production of dimethyl sulfide (DMS), a biogenic gas that is believed to be the major precursor of cloud condensation nuclei (CCN) over the oceans. A drastic reduction in marine CCN should lead to a cloud canopy with significantly lower reflectivity, and hence cause a significant warming at the earth's surface. Calculations suggest that, all other things being held constant, a reduction in CCN of more than 80 percent (a reasonable value for the K-T extinctions) could have produced a rapid global warming of 6 C or more. Oxygen-isotope analyses of marine sediments, and other kinds of paleoclimatic data, have provided for a marked warming, and a general instability of climate coincident with the killoff of marine plankton at the K-T boundary. Similar reductions in phytoplankton abundance at other boundaries, as indicated by marked shifts in carbon-isotope curves, suggest that severe temperature changes may have accompanied other mass extinctions, and raises the intriguing possibility that the extinction events themselves could have contributed to the climatic instabilities at critical bio-events in the geologic record.
Abstract The severe mass extinction of marine and terrestrial organisms at the end of the Permian Period ( c. 251 Ma) was accompanied by a rapid (<100 000 years and possibly <10 000 years) negative excursion of c. 3‰ in the δ 13 C of the global surface oceans and atmosphere that persisted for some 500 000 years into the Early Triassic. Simulations with an ocean–atmosphere/carbon‐cycle model suggest that the isotope excursion can be explained by collapse of ocean primary productivity, and changes in the delivery and cycling of carbon in the oceans and on land. Model results suggest that severe reduction of marine productivity led to an increase in surface‐ocean dissolved inorganic carbon and a rapid, short‐term increase in atmospheric pCO 2 (from a Late Permian base of 850 ppm to c. 2500 ppm). Increase in surface ocean alkalinity may have stimulated the widespread microbial and abiotic shallow‐water carbonate deposition seen in the earliest Triassic. The model is also consistent with a long‐term (>1 Ma) decrease in sedimentary burial of organic carbon in the early Triassic.
Research Article| February 01, 1981 Episodic growth of Holocene tidal marshes in the northeastern United States: A possible indicator of eustatic sea-level fluctuations Michael R. Rampino; Michael R. Rampino 1NASA Goddard Institute for Space Studies, Goddard Space Flight Center, New York, New York 10025 Search for other works by this author on: GSW Google Scholar John E. Sanders John E. Sanders 2Department of Geology, Barnard College, Columbia University, New York, New York 10027 Search for other works by this author on: GSW Google Scholar Geology (1981) 9 (2): 63–67. https://doi.org/10.1130/0091-7613(1981)9<63:EGOHTM>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Michael R. Rampino, John E. Sanders; Episodic growth of Holocene tidal marshes in the northeastern United States: A possible indicator of eustatic sea-level fluctuations. Geology 1981;; 9 (2): 63–67. doi: https://doi.org/10.1130/0091-7613(1981)9<63:EGOHTM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Development of coastal marshes during the Holocene submergence of the northeastern United States seems to have been episodic. Radiocarbon dating of the modern marshes indicates that they were generally established within the past 3,000 yr. Initiation of marsh growth was probably related to the marked slowing in the submergence rate that took place in the area about 3,000 to 4,000 yr ago. On the basis of the few available dates of marsh peats from the inner continental shelf, we suggest that previous episodes of marsh growth took place at about 4,700, 5,600, 6,600, 7,700, 8,200, and 8,600 radiocarbon yr ago. These times of marsh growth seem to coincide with proposed times of negative sea-level oscillations, cool climate, and glacial advances. If these oscillations of sea level are climatically controlled, then the data suggest an approximately 1,000-yr cycle of climatic change. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
A chronology of the initiation dates of major continental flood basalt volcanism is established from published potassium-argon (K-Ar) and argon-argon (Ar-Ar) ages of basaltic rocks and related basic intrusions. The dating is therefore independent of the biostratigraphic and paleomagnetic time scales. Estimated errors of the initation dates of the volcanic episodes determined from the distributions of the radiometric ages are, approximately, plus or minus 4 percent. There were 11 distinct episodes during the past 250 million years. Sometimes appearing in pairs, the episodes have occurred quasi-periodically with a mean cycle time of 32 +/- 1 (estimated, error of the mean) million years. The initiation dates of the episodes are close to the estimated dates of mass extinctions of marine organisms. Showers of impacting comets may be the cause.
The claims of periodicity in impact cratering and biological extinction events are controversial. A newly revised record of dated impact craters has been analyzed for periodicity, and compared with the record of extinctions over the past 260 Myr. A digital circular spectral analysis of 37 crater ages (ranging in age from 15 to 254 Myr ago) yielded evidence for a significant 25.8 ± 0.6 Myr cycle. Using the same method, we found a significant 27.0 ± 0.7 Myr cycle in the dates of the eight recognized marine extinction events over the same period. The cycles detected in impacts and extinctions have a similar phase. The impact crater dataset shows 11 apparent peaks in the last 260 Myr, at least 5 of which correlate closely with significant extinction peaks. These results suggest that the hypothesis of periodic impacts and extinction events is still viable.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
