Milankovitch theory seeks to explain the Quaternary glaciations via changes in seasonal insolation caused by periodic changes in the Earth's obliquity, orbital precession, and eccentricity. However, ...recent high‐resolution spectral analysis of δ18O proxy climate records have cast doubt on the theory Muller and MacDonald, 1997a, b. The spectral signature of the “100 kyr” component, which dominates the climate record over the past 0.6–0.8 Myr does not match the frequencies of the eccentricity variation. Muller and MacDonald 1997b, c have therefore argued that a more likely pacemaker for the climate cycles is the variation in inclination of the Earth's orbit relative to the invariant plane of the solar system. Here we show that the spectral signature of δ18O records are entirely consistent with Milankovitch mechanisms in which deglaciations are triggered every fourth or fifth precessional cycle. Such mechanisms may involve the buildup of “excess” ice due to low summertime insolation at the previous precessional “high.”
We examine the effect on atmospheric CO
2
of the occurrence of increased shallow water carbonate deposition and regrowth of the terrestrial biosphere following the last glacial. We find that contrary ...to recent speculations that changes in terrestrial carbon storage were primarily responsible for the observed ∼20 ppmv late Holocene CO
2
rise, a more likely explanation is coral reef buildup and other forms of shallow water carbonate deposition during this time. The importance of a responsive terrestrial carbon reservoir may instead be as a negative feedback restricting the rate of CO
2
rise possible in the early stages of the deglacial transition. This separation in time of the primary impacts of regrowth of the terrestrial biosphere and increased shallow water carbonate deposition explains the occurrence of an early Holocene carbonate preservation event observed in deep‐sea sediments. We demonstrate that their combined influence is also consistent with available proxy estimates of deep ocean carbonate ion concentration changes over the last 21 kyr. Accounting for the processes that act on the carbonate chemistry of the ocean as a whole then allows us to place strong constraints on the nature of the remaining processes that must be operating at the deglacial transition. By subtracting the net CO
2
effect of coral reef buildup and terrestrial biosphere regrowth from recent high‐resolution ice core data, we highlight two periods, from 17.0 to 13.8 kyr and 12.3 to 11.2 kyr BP characterized by sustained rapid rates of CO
2
increase (>12 ppmv kyr
−1
). Because these periods are coincident with Southern Hemisphere “deglaciation,” we argue that changes in the biogeochemical properties of the Southern Ocean surface are the most likely cause.
We examine the effect on atmospheric CO sub(2) of the occurrence of increased shallow water carbonate deposition and regrowth of the terrestrial biosphere following the last glacial. We find that ...contrary to recent speculations that changes in terrestrial carbon storage were primarily responsible for the observed 20 ppmv late Holocene CO sub(2) rise, a more likely explanation is coral reef buildup and other forms of shallow water carbonate deposition during this time. The importance of a responsive terrestrial carbon reservoir may instead be as a negative feedback restricting the rate of CO sub(2) rise possible in the early stages of the deglacial transition. This separation in time of the primary impacts of regrowth of the terrestrial biosphere and increased shallow water carbonate deposition explains the occurrence of an early Holocene carbonate preservation event observed in deep-sea sediments. We demonstrate that their combined influence is also consistent with available proxy estimates of deep ocean carbonate ion concentration changes over the last 21 kyr. Accounting for the processes that act on the carbonate chemistry of the ocean as a whole then allows us to place strong constraints on the nature of the remaining processes that must be operating at the deglacial transition. By subtracting the net CO sub(2) effect of coral reef buildup and terrestrial biosphere regrowth from recent high-resolution ice core data, we highlight two periods, from 17.0 to 13.8 kyr and 12.3 to 11.2 kyr BP characterized by sustained rapid rates of CO sub(2) increase (>12 ppmv kyr super(-1)). Because these periods are coincident with Southern Hemisphere "deglaciation," we argue that changes in the biogeochemical properties of the Southern Ocean surface are the most likely cause.
Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering ...feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.
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