Research Article| January 01, 1991 Variations in the strontium isotopic composition of seawater during the Neogene David A. Hodell; David A. Hodell 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Paul A. Mueller; Paul A. Mueller 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Jose R. Garrido Jose R. Garrido 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Geology (1991) 19 (1): 24–27. https://doi.org/10.1130/0091-7613(1991)019<0024:VITSIC>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David A. Hodell, Paul A. Mueller, Jose R. Garrido; Variations in the strontium isotopic composition of seawater during the Neogene. Geology 1991;; 19 (1): 24–27. doi: https://doi.org/10.1130/0091-7613(1991)019<0024:VITSIC>2.3.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 We report 261 strontium isotopic analyses of well-preserved planktonic foraminifers from three Deep Sea Drilling Project Sites (519, 588, and 607). These samples cover the period from 24 Ma to present with an average of approximately one sample per 100 ka. The combination of high sample density and uniformity of analytical procedures has produced a well-defined record of changes in the 87Sr/86Sr of seawater during the Neogene. The record can be viewed as a series of essentially linear segments with slopes ranging from as high as 6 x 10-5/m.y. to as low as 0/m.y. The times associated with major inflections in the curve do not appear to correspond to simple geologic phenomena such as eustatic cycles, but are probably controlled by a combination of tectonic and climatic factors that influenced the abundance and isotopic composition of terrestrial strontium input to the oceans. The strontium isotopic data are consistent with a progressive increase in the chemical weathering rates of the continents during the Neogene, probably related to repeated glaciations, increased exposure of continents by lowered sea level, and increased continental relief resulting from high rates of tectonic uplift. 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.
Research Article| November 01, 1988 Application of strontium isotopes to late Miocene-early Pliocene stratigraphy Judith A. McKenzie; Judith A. McKenzie 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar David A. Hodell; David A. Hodell 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Paul A. Mueller; Paul A. Mueller 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Daniel W. Mueller Daniel W. Mueller 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Author and Article Information Judith A. McKenzie 1Department of Geology, University of Florida, Gainesville, Florida 32611 David A. Hodell 1Department of Geology, University of Florida, Gainesville, Florida 32611 Paul A. Mueller 1Department of Geology, University of Florida, Gainesville, Florida 32611 Daniel W. Mueller 1Department of Geology, University of Florida, Gainesville, Florida 32611 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1988) 16 (11): 1022–1025. https://doi.org/10.1130/0091-7613(1988)016<1022:AOSITL>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Judith A. McKenzie, David A. Hodell, Paul A. Mueller, Daniel W. Mueller; Application of strontium isotopes to late Miocene-early Pliocene stratigraphy. Geology 1988;; 16 (11): 1022–1025. doi: https://doi.org/10.1130/0091-7613(1988)016<1022:AOSITL>2.3.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 Measurements of 87Sr/86Sr ratios of planktonic foraminifers from paleomagnetically dated Deep Sea Drilling Project (DSDP) cores indicate that the 87Sr/86Sr ratio of seawater increased linearly between 6 and 4 Ma with a slope of 87 x 10-6/m.y. This suggests that 87Sr/86Sr variations in this time interval are useful for estimating ages in marine sequences. The theoretical stratigraphic resolution of the technique is 0.23 m.y., but the practical resolution is estimated to be 0.3-0.5 m.y.Strontium isotope ratios of five samples at the base of the Miocene/Pliocene (M/P) boundary stratotype at Capo Rossello, Sicily, have a mean 87Sr/86Sr values of 0.708995 ±0.00002. Applying the regression equation derived from our 87Sr/86Sr vs. age data set for deep-sea sequences yields an age of 4.94 ±0.50 Ma (95% confidence interval) for the M/P boundary stratotype, similar to ages estimated by biostratigraphic and magnetostratigraphic methods. Correlation of this mean 87Sr/86Sr ratio to the section at DSDP Site 519 suggests that the boundary is coincident with the onset of a major early Pliocene transgression, as recorded by a decrease in oxygen isotope values for benthic foraminifers. This study demonstrates the potential application of strontium isotope stratigraphy for correlating and dating problematic sequences, such as those deposited in restricted, shallow-water, or high-latitude environments. 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.
Abstract Leisey Shell Pit, on the southeastern edge of Tampa Bay, has produced the largest early Pleistocene vertebrate fauna in North America, consisting of about 30,000 cataloged specimens. The predominantly terrestrial and freshwater vertebrate fauna was transported into an estuarine environment during a regressive phase in an otherwise marine, bay-bottom sequence of sandy shell beds. The biochronological age of the Leisey Shell Pit local fauna is late Sappan (or late early Irvingtonian) based on the presence of such mammalian species as Mammuthus meridionalis and Smilodon gracilis , the absence of any Blancan holdovers, and early records of Lutra, Nothrotheriops, Palaeolama , and a new small glyptodont. Magnetostratigraphy indicates that the entire Pleistocene section at Leisey is reversed and thus presumably accumulated in the post-Olduvai part of the Matuyama Magnetochron (i.e., 1.66 myr or younger). Strontium isotope ratios ( 87 Sr/ 86 Sr) sampled from aragonitic bivalve shells ( Chione cancellata ) trend higher with increasing height in the section, yielding a mean age estimate for the two major bone beds of 1.7 ± 0.35 myr. In combination, these three geochronologic methods provide a high-resolution approach toward age determination and delimit the age of Leisey Shell Pit local fauna as between 1.66 and 1.4 myr.
Chromites from ultramafic enclaves, including chromitites, in the eastern Beartooth Mountains, Montana (USA) provide a window into the tectonic evolution of the Archean northern Wyoming Province. Ultramafic rocks occur with a variety of metasupracrustal rocks as xenoliths in an extensive 2.8 Ga suite of TTG (meta)plutonic rocks. The origin of the ultramafic rocks is obscured by an upper amphibolite-to-greenschist facies overprint, but the chromitite pods (ultramafic rocks with >50% chromite) contain euhedral chromite grains that retain their igneous chemistry and textures. Major and trace element analyses of chromites by electron microprobe and laser ablation ICP-MS show chromites have minor compositional zoning with the rims being slightly enriched in Cr and Fe2+ at the expense of Al and Mg. Chromites have Cr # [100*Cr/(Cr+Al)] from 73-79 and Mg# [100*Mg/(Mg+Fe2+)] from 34-45, consistent with derivation from an ophiolite sequence. REEs for chromites and ultramafic host rocks are most consistent with a boninitic source based on their U-shaped chondrite normalized REE patterns. Chondrite-normalized chromite PGE concentrations (Ru, Rh, Pd, Ag, Re, and Pt) are low: Rh 0.01-0.4 ppm, Ru 0.06 to 0.1 ppm, Re up to 0.03 ppm, Pt up to 0.007 ppm, and Ag up to 0.02 ppm . Major element analyses (XRF, wt. %) of ultramafic host rocks show SiO2 from 40.53-50.37, MgO from 15.33-45.25, TiO2 from 0.07-0.62, Al2O3 from 1.25-10.62, and Fe2O3(t) from 8.06-14.54. Total alkali contents (Na2O+K2O) range from 0.06-3.78 wt.% and plot in the picro-basalt and basalt fields on a TAS diagram. As expected, Cr contents of non-chromitites are high (1500-6500 ppm, ICP). Akin to the chromites, chondrite-normalized REE abundances exhibit arc-basalt and boninitic characteristics, with positive and negative Eu anomalies. Primitive mantle normalized spider diagrams show arc characteristics, e.g., enrichments and depletions in LILE, a positive lead anomaly, and negative HFSE anomalies. In conclusion, the boninitic/ophiolitic/arc geochemical signature of the chromites and ultramafic rocks suggest that they formed in the forearc of an early subduction setting. They were intercalated with a variety of lithologies during an episode of plate convergence that preceded the 2.8 Ga TTG plutonism.
The Cadomian orogen exposed in NW France and the Channel Islands comprises a collage of variably displaced terranes which record the late Proterozoic evolution of a continental margin within a complex subduction zone. On the islands of Guernsey and Sark, calc-alkaline quartz diorite stocks were emplaced during early stages of the Cadomian orogeny. Zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions. Upper intercept ages for these discordant concentrates are c. 1300 Ma. Sm-Nd whole-rock model ages ( T DM ) suggest older ages of c. 2000 Ma. Conversely, the smallest and least magnetic zircons exhibit a distinct lead-loss pattern suggesting that the 207 Pb/ 206 Pb dates of c. 700 Ma correspond to the time of crystallization of the stocks, and provide a lower age constraint for initiation of subduction-related magmatism in the Cadomian orogen. Hornblende concentrates from deformed quartz diorite stocks exposed on Guernsey and Sark display discordant 40 Ar/ 39 Ar age spectra. Five samples record well-defined isotope correlation ages ranging between 596 Ma and 606 Ma. A hornblende concentrate from host orthogneiss (Sark) yields a plateau age of 608 Ma. The hornblende ages are interpreted as dating post-metamorphic cooling following late Proterozoic deformation and metamorphism within the arc system. Both magmatic and cooling ages from this early Cadomian arc contrast with those of younger initial tectonothermal activity (c. 570 Ma and c. 540 Ma) in other Cadomian terrane elements, and suggest palinspastic separation until very late stages of the orogeny. The polyphase nature of this late Proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of Gondwana (e.g. SW Iberia, West Africa).
