Browsing by Author "Dwyer, GS"

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Open Access
Changes in North Atlantic deep-sea temperature during climatic fluctuations of the last 25,000 years based on ostracode Mg/Ca ratios
(Geochemistry, Geophysics, Geosystems, 2000-12-01) Dwyer, GS; Cronin, TM; Baker, PA; Rodriguez-Lazaro, J
© Copyright 2000 by the American Geophysical Union.We reconstructed three time series of last glacial-to-present deep-sea temperature from deep and intermediate water sediment cores from the western North Atlantic using Mg/Ca ratios of benthic ostracode shells. Although the Mg/Ca data show considerable variability ("scatter") that is common to single-shell chemical analyses, comparisons between cores, between core top shells and modern bottom water temperatures (BWT), and comparison to other paleo-BWT proxies, among other factors, suggest that multiple-shell average Mg/Ca ratios provide reliable estimates of BWT history at these sites. The BWT records show not only glacial-to-interglacial variations but also indicate BWT changes during the deglacial and within the Holocene interglacial stage. At the deeper sites (4500- and 3400-m water depth), BWT decreased during the last glacial maximum (LGM), the late Holocene, and possibly during the Younger Dryas. Maximum deep-sea warming occurred during the latest deglacial and early Holocene, when BWT exceeded modern values by as much as 2.5°C. This warming was apparently most intense around 3000 m, the depth of the modern-day core of North Atlantic deep water (NADW). The BWT variations at the deeper water sites are consistent with changes in thermohaline circulation: warmer BWT signifies enhanced NADW influence relative to Antarctic bottom water (AABW). Thus maximum NADW production and associated heat flux likely occurred during the early Holocene and decreased abruptly around 6500 years B.P., a finding that is largely consistent with paleonutrient studies in the deep North Atlantic. BWT changes in intermediate waters (1000-m water depth) of the subtropical gyre roughly parallel the deep BWT variations including dramatic mid-Holocene cooling of around 4°C. Joint consideration of the Mg/Ca-based BWT estimates and benthic oxygen isotopes suggests that the cooling was accompanied by a decrease in salinity at this site. Subsequently, intermediate waters warmed to modern values that match those of the early Holocene maximum of ∼7°C. Intermediate water BWT changes must also be driven by changes in ocean circulation. These results thus provide independent evidence that supports the hypothesis that deep-ocean circulation is closely linked to climate change over a range of timescales regardless of the mean climate state. More generally, the results further demonstrate the potential of benthic Mg/Ca ratios as a tool for reconstructing past ocean and climate conditions.
Open Access
Climate variability from Florida Bay sedimentary record: possible teleconnections to ENSO, PNA, and CNP
(Climate Research, 2002) Cronin, TM; Dwyer, GS; Schwede, SB; Dowsett, HJ
Open Access
Deep-sea ostracode species diversity: response to late Quaternary climate change
(Marine Micropaleontology, 1999) Cronin, TM; DeMartino, DM; Dwyer, GS; Rodriguez Lazaro, J
Open Access
Ecology and shell chemistry of Loxoconcha matagordensis
(PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY, 2005-09) Cronin, TM; Kamiya, T; Dwyer, GS; Belkin, H; Vann, CD; Schwede, S; Wagner, R
Open Access
Geoscience - Unraveling the signals of global climate change
(SCIENCE, 2000-01) Dwyer, GS
Open Access
Hydrologic variation during the last 170,000 years in the southern hemisphere tropics of South America
(Quaternary Research, 2004-01-01) Fritz, SC; Baker, PA; Lowenstein, TK; Seltzer, GO; Rigsby, CA; Dwyer, GS; Tapia, PM; Arnold, KK; Ku, TL; Luo, S
Despite the hypothesized importance of the tropics in the global climate system, few tropical paleoclimatic records extend to periods earlier than the last glacial maximum (LGM), about 20,000 years before present. We present a well-dated 170,000-year time series of hydrologic variation from the southern hemisphere tropics of South America that extends from modern times through most of the penultimate glacial period. Alternating mud and salt units in a core from Salar de Uyuni, Bolivia reflect alternations between wet and dry periods. The most striking feature of the sequence is that the duration of paleolakes increased in the late Quaternary. This change may reflect increased precipitation, geomorphic or tectonic processes that affected basin hydrology, or some combination of both. The dominance of salt between 170,000 and 140,000 yr ago indicates that much of the penultimate glacial period was dry, in contrast to wet conditions in the LGM. Our analyses also suggest that the relative influence of insolation forcing on regional moisture budgets may have been stronger during the past 50,000 years than in earlier times. © 2003 University of Washington. All rights reserved.
Open Access
Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research
(GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, 2004-04) Rosenthal, Y; Perron Cashman, S; Lear, CH; Bard, E; Barker, S; Billups, K; Bryan, M; Delaney, ML; deMenocal, PB; Dwyer, GS; Elderfield, H; German, CR; Greaves, M; Lea, DW; Marchitto, TM; Pak, DK; Paradis, GL; Russell, AD; Schneider, RR; Scheiderich, K; Stott, L; Tachikawa, K; Tappa, E; Thunell, R; Wara, M; Weldeab, S; Wilson, PA
Open Access
Medieval Warm Period, Little Ice Age and 20th century temperature variability from Chesapeake Bay
(GLOBAL AND PLANETARY CHANGE, 2003-03) Cronin, TM; Dwyer, GS; Kamiya, T; Schwede, S; Willard, DA
Open Access
Mid-Cretaceous strontium-isotope stratigraphy of deep-sea sections
(GEOLOGICAL SOCIETY OF AMERICA BULLETIN, 1997-11) Bralower, TJ; Fullagar, PD; Paull, CK; Dwyer, GS; Leckie, RM
Open Access
Mid-pliocene deep-sea bottom-water temperatures based on ostracode Mg/Ca ratios
(MARINE MICROPALEONTOLOGY, 2005-03) Cronin, TM; Dowsett, HJ; Dwyer, GS; Baker, PA; Chandler, MA
Open Access
Middle Pliocene sea surface temperature variability
(PALEOCEANOGRAPHY, 2005-06) Dowsett, HJ; Chandler, MA; Cronin, TM; Dwyer, GS
Open Access
Multiproxy evidence of Holocene climate variability from estuarine sediments, eastern North America
(PALEOCEANOGRAPHY, 2005-10) Cronin, TM; Thunell, R; Dwyer, GS; Saenger, C; Mann, ME; Vann, C; Seal, RR
Open Access
North atlantic deepwater temperature change during late pliocene and late quaternary climatic cycles
(Science, 1995-12-01) Dwyer, GS; Cronin, TM; Baker, PA; Raymo, ME; Buzas, JS; Corrège, T
Variations in the ratio of magnesium to calcium (Mg/Ca) in fossil ostracodes from Deep Sea Drilling Project Site 607 in the deep North Atlantic show that the change in bottom water temperature during late Pliocene 41,000-year obliquity cycles averaged 1.5°C between 3.2 and 2.8 million years ago (Ma) and increased to 2.3°C between 2.8 and 2.3 Ma, coincidentally with the intensification of Northern Hemisphere glaciation. During the last two 100,000-year glacial-to-interglacial climatic cycles of the Quaternary, bottom water temperatures changed by 4.5°C. These results show that glacial deepwater cooling has intensified since 3.2 Ma, most likely as the result of progressively diminished deep-water production in the North Atlantic and of the greater influence of Antarctic bottom water in the North Atlantic during glacial periods. The ostracode Mg/Ca data also allow the direct determination of the temperature component of the benthic foraminiferal oxygen isotope record from Site 607, as well as derivation of a hypothetical sea-level curve for the late Pliocene and late Quaternary. The effects of dissolution on the Mg/Ca ratios of ostracode shells appear to have been minimal.
Open Access
Orbital and suborbital variability in North Atlantic bottom water temperature obtained from deep-sea ostracod Mg/Ca ratios
(PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY, 2000-09-15) Cronin, TM; Dwyer, GS; Baker, PA; Rodriguez-Lazaro, J; DeMartino, DM
Open Access
Population ecology and shell chemistry of a phytal ostracode species (Loxoconcha matagordensis) in the Chesapeake Bay watershed
(MARINE MICROPALEONTOLOGY, 2004-11) Vann, CD; Cronin, TM; Dwyer, GS
Open Access
Relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks, North Carolina (USA)
(Chemical Geology, 2009-03-30) Vinson, DS; Vengosh, A; Hirschfeld, D; Dwyer, GS
Naturally-occurring radionuclides (uranium, radium, and radon), major dissolved constituents, and trace elements were investigated in fresh groundwater in 117 wells in fractured crystalline rocks from the Piedmont region (North Carolina, USA). Chemical variations show a general transition between two water types: (1) slightly acidic (pH 5.0-6.0), oxic, low-total dissolved solids (TDS) waters, and (2) near neutral, oxic to anoxic, higher-TDS waters. The uranium, radium, and radon levels in groundwater associated with granite (Rolesville Granite) are systematically higher than other rock types (gneiss, metasedimentary, and metavolcanic rocks). Water chemistry plays a secondary role on radium and radon distributions as the 222Rn/226Ra activity ratio is correlated with redox-sensitive solutes such as dissolved oxygen and Mn concentrations, as well as overall dissolved solids content including major divalent cations and Ba. Since 224Ra/228Ra activity ratios in groundwater are close to 1, we suggest that mobilization of Ra and Rn is controlled by alpha recoil processes from parent nuclides on fracture surfaces, ruling out Ra sources from mineral dissolution or significant long-distance Ra transport. Alpha recoil is balanced by Ra adsorption that is influenced by redox conditions and/or ion concentrations, resulting in an approximately one order of magnitude decrease (~ 20,000 to ~ 2000) in the apparent Ra distribution coefficient between oxygen-saturated and anoxic conditions and also across the range of dissolved ion concentrations (up to ~ 7 mM). Thus, the U and Th content of rocks is the primary control on observed Ra and Rn activities in groundwater in fractured crystalline rocks, and in addition, linked dissolved solids concentrations and redox conditions impart a secondary control. © 2008 Elsevier B.V. All rights reserved.
Open Access
Survey of the potential environmental and health impacts in the immediate aftermath of the coal ash spill in Kingston, Tennessee.
(Environ Sci Technol, 2009-08-15) Ruhl, L; Vengosh, A; Dwyer, GS; Hsu Kim, H; Deonarine, A; Bergin, M; Kravchenko, J
An investigation of the potential environmental and health impacts in the immediate aftermath of one of the largest coal ash spills in U.S. history at the Tennessee Valley Authority (TVA) Kingston coal-burning power plant has revealed three major findings. First the surface release of coal ash with high levels of toxic elements (As = 75 mg/kg; Hg = 150 microg/kg) and radioactivity (226Ra + 228Ra = 8 pCi/g) to the environment has the potential to generate resuspended ambient fine particles (< 10 microm) containing these toxics into the atmosphere that may pose a health risk to local communities. Second, leaching of contaminants from the coal ash caused contamination of surface waters in areas of restricted water exchange, but only trace levels were found in the downstream Emory and Clinch Rivers due to river dilution. Third, the accumulation of Hg- and As-rich coal ash in river sediments has the potential to have an impact on the ecological system in the downstream rivers by fish poisoning and methylmercury formation in anaerobic river sediments.