56 resultados para Maçã


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A six-fold increase in the rate of accumulation of Al in north and central Atlantic and Pacific Ocean sediments indicates vastly increased denudation of the continents during the past 15 Ma. The increase is more apparent in hemipelagic than pelagic sites, demonstrating widely distributed local controls. Similarities in the rate of increase in the Atlantic and Pacific show that tectonic elevation is not responsible for the difference in sedimentation rate. Also, similarities in the difference at sites of low and high latitude suggest that glaciation is not the most significant source. A lack of correspondence between sedimentation rates and Vail's sea-level curve similarly rule out that effect. The conclusion drawn here is that worldwide climatic deterioration during the late Tertiary is the explanation for the striking increase in detrital sedimentation in the World ocean.

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Estimates of summer sea surface temperatures (SSSTs) derived from planktic foraminiferal associations using the Modern Analog Technique and combined with isotopic analyses and determination of ice-rafted debris, mirror the Pleistocene evolution of the planktic Subantarctic surface waters in the Atlantic Ocean. The SSSTs indicate that the isotherms that define the modern polar front zone and Subantarctic front, were located at more northerly latitudes (up to 7°) during most of the investigated period, which covers the past 550 kyr. Exceptions are during climatic optima in the early Holocene, at marine isotope stages (MIS) 5.5, 7.1, 7.5, 9.3, and presumably during MIS 11.3 when SSSTs exceeded modern values by 1 -5°C. The close similarity between the SSST and the Vostok temperature indicates strong regional temperature correlation. Both records show that MIS 9.3 was the warmest period during the last 420 kyr whereas SSSTs obtained for MIS 11.3 are overestimated due to strong carbonate dissolution. Spectral analysis corroborates that the initiation of warming in southern high latitudes heralds the start of deglaciation on the Northern Hemisphere.

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The circulation and internal structure of the oceans exert a strong influence on Earth's climate because they control latitudinal heat transport and the segregation of carbon between the atmosphere and the abyss (Sigman et al., 2010, doi:10.1038/nature09149). Circulation change, particularly in the Atlantic Ocean, is widely suggested (Bartoli et al., 2005, doi:10.1016/j.epsl.2005.06.020; Haug and Tiedemann, 1998, doi:10.1038/31447; Woodard et al., 2014, doi:10.1126/science.1255586; McKay et al., 2012, doi:10.1073/pnas.1112248109) to have been instrumental in the intensification of Northern Hemisphere glaciation when large ice sheets first developed on North America and Eurasia during the late Pliocene, approximately 2.7 million years ago (Bailey et al., 2013, doi:10.1016/j.quascirev.2013.06.004). Yet the mechanistic link and cause/effect relationship between ocean circulation and glaciation are debated. Here we present new records of North Atlantic Ocean structure using the carbon and neodymium isotopic composition of marine sediments recording deep water for both the Last Glacial to Holocene (35-5 thousand years ago) and the late Pliocene to earliest Pleistocene (3.3-2.4 million years ago). Our data show no secular change. Instead we document major southern-sourced water incursions into the deep North Atlantic during prominent glacials from 2.7 million years ago. Our results suggest that Atlantic circulation acts as a positive feedback rather than as an underlying cause of late Pliocene Northern Hemisphere glaciation. We propose that, once surface Southern Ocean stratification (Sigman, et al., 2004, doi:10.1038/nature02357) and/or extensive sea-ice cover (McKay et al., 2012, doi:10.1073/pnas.1112248109) was established, cold-stage expansions of southern-sourced water such as those documented here enhanced carbon dioxide storage in the deep ocean, helping to increase the amplitude of glacial cycles.