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Author

Ellen Thomas

Bio: Ellen Thomas is an academic researcher from Wesleyan University. The author has contributed to research in topic(s): Benthic zone & Foraminifera. The author has an hindex of 63, co-authored 225 publication(s) receiving 21806 citation(s). Previous affiliations of Ellen Thomas include University of Zaragoza & Scripps Institution of Oceanography.
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Journal ArticleDOI
James C Zachos1, Mark Pagani1, Lisa C. Sloan1, Ellen Thomas2  +2 moreInstitutions (4)
27 Apr 2001-Science
TL;DR: This work focuses primarily on the periodic and anomalous components of variability over the early portion of this era, as constrained by the latest generation of deep-sea isotope records.
Abstract: Since 65 million years ago (Ma), Earth's climate has undergone a significant and complex evolution, the finer details of which are now coming to light through investigations of deep-sea sediment cores. This evolution includes gradual trends of warming and cooling driven by tectonic processes on time scales of 10(5) to 10(7) years, rhythmic or periodic cycles driven by orbital processes with 10(4)- to 10(6)-year cyclicity, and rare rapid aberrant shifts and extreme climate transients with durations of 10(3) to 10(5) years. Here, recent progress in defining the evolution of global climate over the Cenozoic Era is reviewed. We focus primarily on the periodic and anomalous components of variability over the early portion of this era, as constrained by the latest generation of deep-sea isotope records. We also consider how this improved perspective has led to the recognition of previously unforeseen mechanisms for altering climate.

8,056 citations


Journal ArticleDOI
James C Zachos1, Ursula Röhl2, Stephen A. Schellenberg3, Appy Sluijs4  +9 moreInstitutions (10)
10 Jun 2005-Science
TL;DR: Geochemical data from five new South Atlantic deep-sea sections indicate that a large mass of carbon dissolved in the ocean at the Paleocene-Eocene boundary and that permanent sequestration of this carbon occurred through silicate weathering feedback.
Abstract: The Paleocene-Eocene thermal maximum (PETM) has been attributed to the rapid release of ∼2000 × 10 9 metric tons of carbon in the form of methane. In theory, oxidation and ocean absorption of this carbon should have lowered deep-sea pH, thereby triggering a rapid ( 100,000 years). These findings indicate that a large mass of carbon (»2000 × 10 9 metric tons of carbon) dissolved in the ocean at the Paleocene-Eocene boundary and that permanent sequestration of this carbon occurred through silicate weathering feedback.

915 citations


Journal ArticleDOI
Bärbel Hönisch1, Andy Ridgwell2, Daniela N. Schmidt2, Ellen Thomas3  +20 moreInstitutions (18)
02 Mar 2012-Science
Abstract: Ocean acidification may have severe consequences for marine ecosystems; however, assessing its future impact is difficult because laboratory experiments and field observations are limited by their reduced ecologic complexity and sample period, respectively. In contrast, the geological record contains long-term evidence for a variety of global environmental perturbations, including ocean acidification plus their associated biotic responses. We review events exhibiting evidence for elevated atmospheric CO2, global warming, and ocean acidification over the past ~300 million years of Earth’s history, some with contemporaneous extinction or evolutionary turnover among marine calcifiers. Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry—a consequence of the unprecedented rapidity of CO2 release currently taking place.

740 citations


Journal ArticleDOI
Lucas Joost Lourens, Appy Sluijs1, Dick Kroon2, James C Zachos3  +5 moreInstitutions (7)
23 Jun 2005-Nature
TL;DR: A distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge is reported, which is term the Elmo horizon, which has similar geochemical and biotic characteristics as the Palaeocene–Eocene thermal maximum, but of smaller magnitude, suggesting that it represents a second global thermal maximum.
Abstract: At the boundary between the Palaeocene and Eocene epochs, about 55 million years ago, the Earth experienced a strong global warming event, the Palaeocene–Eocene thermal maximum1,2,3,4. The leading hypothesis to explain the extreme greenhouse conditions prevalent during this period is the dissociation of 1,400 to 2,800 gigatonnes of methane from ocean clathrates5,6, resulting in a large negative carbon isotope excursion and severe carbonate dissolution in marine sediments. Possible triggering mechanisms for this event include crossing a threshold temperature as the Earth warmed gradually7, comet impact8, explosive volcanism9,10 or ocean current reorganization and erosion at continental slopes11, whereas orbital forcing has been excluded12. Here we report a distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge13, which we term the Elmo horizon. Using orbital tuning, we estimate deposition of the Elmo horizon at about 2 million years after the Palaeocene–Eocene thermal maximum. The Elmo horizon has similar geochemical and biotic characteristics as the Palaeocene–Eocene thermal maximum, but of smaller magnitude. It is coincident with carbon isotope depletion events in other ocean basins, suggesting that it represents a second global thermal maximum. We show that both events correspond to maxima in the ∼405-kyr and ∼100-kyr eccentricity cycles that post-date prolonged minima in the 2.25-Myr eccentricity cycle, implying that they are indeed astronomically paced.

475 citations


Book ChapterDOI
01 Jan 2007-
Abstract: Publisher Summary This chapter focuses on the paleoceanographical proxies based on deep-sea benthic foraminiferal assemblage characteristics, and presents the following three proxy relationships that are promising: those between benthic foraminiferal faunas and benthic ecosystem oxygenation, export productivity, and deep-sea water mass characteristics. Under most circumstances the composition of deep-sea benthic foraminiferal assemblages is controlled by a rather limited number of environmental factors. The available proxies based on benthic foraminiferal assemblage composition show that they have major potential, but further research is needed to add or improve the quantitative aspects. In many cases, such as bottom water oxygenation, and Corg flux to the ocean floor, it can be done by significantly increasing the size of existing databases. In other cases such as periodicity of the organic flux, time series observations are necessary. A major obstacle is insufficient knowledge of the differences between recent and fossil faunas due to taphonomical alterations. This phenomenon, of importance for all paleoceanographic proxies, can to some extent be solved relatively easily in the case of foraminiferal assemblages by detailed studies of their vertical succession in sediments deposited in the last 5,000 years, when environmental conditions were probably rather invariable in many areas. Proxies based on foraminiferal assemblage composition are fundamentally different from all geochemical proxies, and thus may provide independent reconstructions of essential oceanographic parameters.

372 citations


Cited by
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28 Jul 2005-
TL;DR: PfPMP1)与感染红细胞、树突状组胞以及胎盘的单个或多个受体作用,在黏附及免疫逃避中起关键的作�ly.
Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1(PfPMP1)与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用,在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员,通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

18,940 citations


Journal ArticleDOI
James C Zachos1, Mark Pagani1, Lisa C. Sloan1, Ellen Thomas2  +2 moreInstitutions (4)
27 Apr 2001-Science
TL;DR: This work focuses primarily on the periodic and anomalous components of variability over the early portion of this era, as constrained by the latest generation of deep-sea isotope records.
Abstract: Since 65 million years ago (Ma), Earth's climate has undergone a significant and complex evolution, the finer details of which are now coming to light through investigations of deep-sea sediment cores. This evolution includes gradual trends of warming and cooling driven by tectonic processes on time scales of 10(5) to 10(7) years, rhythmic or periodic cycles driven by orbital processes with 10(4)- to 10(6)-year cyclicity, and rare rapid aberrant shifts and extreme climate transients with durations of 10(3) to 10(5) years. Here, recent progress in defining the evolution of global climate over the Cenozoic Era is reviewed. We focus primarily on the periodic and anomalous components of variability over the early portion of this era, as constrained by the latest generation of deep-sea isotope records. We also consider how this improved perspective has led to the recognition of previously unforeseen mechanisms for altering climate.

8,056 citations


Journal ArticleDOI
Abstract: Rising atmospheric carbon dioxide (CO2), primarily from human fossil fuel combustion, reduces ocean pH and causes wholesale shifts in seawater carbonate chemistry. The process of ocean acidification is well documented in field data, and the rate will accelerate over this century unless future CO2 emissions are curbed dramatically. Acidification alters seawater chemical speciation and biogeochemical cycles of many elements and compounds. One well-known effect is the lowering of calcium carbonate saturation states, which impacts shell-forming marine organisms from plankton to benthic molluscs, echinoderms, and corals. Many calcifying species exhibit reduced calcification and growth rates in laboratory experiments under high-CO2 conditions. Ocean acidification also causes an increase in carbon fixation rates in some photosynthetic organisms (both calcifying and noncalcifying). The potential for marine organisms to adapt to increasing CO2 and broader implications for ocean ecosystems are not well known; both are high priorities for future research. Although ocean pH has varied in the geological past, paleo-events may be only imperfect analogs to current conditions.

2,636 citations


Journal ArticleDOI
Jacques Laskar1, Philippe Robutel1, F. Joutel1, Mickael Gastineau1  +3 moreInstitutions (2)
Abstract: We present here a new solution for the astronomical computation of the insolation quantities on Earth spanning from -250 Myr to 250 Myr. This solution has been improved with respect to La93 (Laskar et al. [CITE]) by using a direct integration of the gravitational equations for the orbital motion, and by improving the dissipative contributions, in particular in the evolution of the Earth–Moon System. The orbital solution has been used for the calibration of the Neogene period (Lourens et al. [CITE]), and is expected to be used for age calibrations of paleoclimatic data over 40 to 50 Myr, eventually over the full Palaeogene period (65 Myr) with caution. Beyond this time span, the chaotic evolution of the orbits prevents a precise determination of the Earth's motion. However, the most regular components of the orbital solution could still be used over a much longer time span, which is why we provide here the solution over 250 Myr. Over this time interval, the most striking feature of the obliquity solution, apart from a secular global increase due to tidal dissipation, is a strong decrease of about 0.38 degree in the next few millions of years, due to the crossing of the resonance (Laskar et al. [CITE]). For the calibration of the Mesozoic time scale (about 65 to 250 Myr), we propose to use the term of largest amplitude in the eccentricity, related to , with a fixed frequency of /yr, corresponding to a period of 405 000 yr. The uncertainty of this time scale over 100 Myr should be about , and over the full Mesozoic era.

2,546 citations


Journal ArticleDOI
25 Nov 2005-Science
TL;DR: Long-term sea level peaked at 100 ± 50 meters during the Cretaceous, implying that ocean-crust production rates were much lower than previously inferred, and presents a new sea-level record for the past 100 million years.
Abstract: We review Phanerozoic sea-level changes [543 million years ago (Ma) to the present] on various time scales and present a new sea-level record for the past 100 million years (My). Long-term sea level peaked at 100 ± 50 meters during the Cretaceous, implying that ocean-crust production rates were much lower than previously inferred. Sea level mirrors oxygen isotope variations, reflecting ice-volume change on the 10 4 - to 10 6 -year scale, but a link between oxygen isotope and sea level on the 10 7 -year scale must be due to temperature changes that we attribute to tectonically controlled carbon dioxide variations. Sea-level change has influenced phytoplankton evolution, ocean chemistry, and the loci of carbonate, organic carbon, and siliciclastic sediment burial. Over the past 100 My, sea-level changes reflect global climate evolution from a time of ephemeral Antarctic ice sheets (100 to 33 Ma), through a time of large ice sheets primarily in Antarctica (33 to 2.5 Ma), to a world with large Antarctic and large, variable Northern Hemisphere ice sheets (2.5 Ma to the present).

2,451 citations


Network Information
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Performance
Metrics

Author's H-index: 63

No. of papers from the Author in previous years
YearPapers
20216
20209
20197
20188
20176
20169