The impact of astronomical forcing on the Late Devonian greenhouse climate
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paleoclimate. However, a thorough understanding of the processes that were driving Paleozoic climate change has not yet been reached. The main reason is relatively poor time-control on Paleozoic paleoclimate proxy records. This problemcan be overcomeby the identification of cyclic features resulting fromastronomical climate forcing in the stratigraphic record. To correctly identify these cyclic features, it is necessary to quantify the effects of astronomical climate forcing under conditions different from today. In this work, we apply Late Devonian (375 Ma) boundary conditions to the Hadley Centre general circulation model (HadSM3).We estimate the response of Late Devonian climate to astronomical forcing by keeping all other forcing factors (e.g. paleogeography, pCO2,vegetation distribution) fixed. Thirty-one different “snapshots” of Late Devonian climate are simulated, by running the model with different combinations of eccentricity (e), obliquity (e) and precession (eω). From the comparison of these 31 simulations, it appears that feedback mechanisms play an important role in the conversion of astronomically driven insolation variations into climate change, such as the formation of sea-ice and the development of an extensive snow cover on Gondwana. Wecompare the “median orbit” simulation to lithic indicators of paleoclimate to evaluate whether or not HadSM3 validly simulates Late Devonian climates. This comparison suggests that themodel correctly locates themajor climate zones. This study also tests the proposed link between the formation of ocean anoxia and high eccentricity (De Vleeschouwer et al., 2013) by comparing the δ18Ocarb record of the Frasnian–Famennian boundary interval from the Kowala section (Poland) with a simulated time series of astronomically forced changes in mean annual temperature at the paleolocation of Poland. The amplitude of climate change suggested by the isotope record is greater than that of the simulated climate. Hence, astronomically forced climate change may have been further amplified by other feedback mechanisms not considered here (e.g. CO2 and vegetation). Finally, the geologic and simulated time series correlate best when the Frasnian–Famennian negative isotope excursion aligns with maximum mean annual temperature in Poland, which is obtained when eccentricity and obliquity are simultaneously high. This finding supports a connection between Devonian ocean anoxic events and astronomical climate forcing.Keywords:
Paleoclimatology
Orbital forcing
Climate state
Forcing (mathematics)
Devonian
Cyclostratigraphy
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Abstract. Since the first suggestion of 1500-year cycles in the advance and retreat of glaciers (Denton and Karlen, 1973), many studies have uncovered evidence of repeated climate oscillations of 2500, 1500, and 1000 years. During last glacial period, natural climate cycles of 1500 years appear to be persistent (Bond and Lotti, 1995) and remarkably regular (Mayewski et al., 1997; Rahmstorf, 2003), yet the origin of this pacing during the Holocene remains a mystery (Rahmstorf, 2003), making it one of the outstanding puzzles of climate variability. Solar variability is often considered likely to be responsible for such cyclicities, but the evidence for solar forcing is difficult to evaluate within available data series due to the shortcomings of conventional time-series analyses. However, the wavelets analysis method is appropriate when considering non-stationary variability. Here we show by the use of wavelets analysis that it is possible to distinguish solar forcing of 1000- and 2500- year oscillations from oceanic forcing of 1500-year cycles. Using this method, the relative contribution of solar-related and ocean-related climate influences can be distinguished throughout the 10 000 yr Holocene intervals since the last ice age. These results reveal that the 1500-year climate cycles are linked with the oceanic circulation and not with variations in solar output as previously argued (Bond et al., 2001). In this light, previously studied marine sediment (Bianchi and McCave, 1999; Chapman and Shackleton, 2000; Giraudeau et al., 2000), ice core (O'Brien et al., 1995; Vonmoos et al., 2006) and dust records (Jackson et al., 2005) can be seen to contain the evidence of combined forcing mechanisms, whose relative influences varied during the course of the Holocene. Circum-Atlantic climate records cannot be explained exclusively by solar forcing, but require changes in ocean circulation, as suggested previously (Broecker et al., 2001; McManus et al., 1999).
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Forcing (mathematics)
Ice core
Climate state
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Orbital climate forcing is well-known for its strong impact on Earth’s climate as for example the switching from glacial to inter-glacial states in the Late Pleistocene. Typical ‘Milankovitch’ cycles are climatic precession (21.000 years or 21 kyrs), obliquity (41 kyrs), and short and long eccentricity (circa 100 and 405 kyrs). Million-year scale astronomical cycles are as well present in eccentricity modulation of precession amplitude (0.97 and 2.4 Myrs) and in obliquity amplitude modulation (1.2 Myrs). These long-period cycles have been suggested to imprint Earth’s climate in the past, although to date direct prove is scarce in marine and absent in non-marine settings. Here, cyclostratigraphic studies have been performed to Cenozoic marine and continental successions in Europe all aimed at examination the impact of long-period orbital climate forcing. The results substantiate that long-period obliquity climate forcing played a major role in the timing of the middle Miocene global cooling as recorded in marine sediments on Malta. Our results of the Madrid in Spain reject the imprint of this long-period obliquity forcing on the Miocene infill. Nevertheless, the formation-scale genetic red bed – limestone sequences in the continental basin might be related to low frequency eccentricity cyclicity. The data from the Late Miocene infill of the continental Teruel Basin remarkably reveal a similar orbital configuration for the transition from red bed to limestone successions as in the Madrid Basin. However, enhanced sediment supply related to the entrance of an axial fluvial system time-equivalent at the other basin margin might rather suggest a local geomorphic or tectonic origin of the transition instead of climate change. The sediment record of the shallow marine succession of the Boom Clay Formation is too short and poorly dated to investigate the presence of low frequency astronomically forced variability, although high frequency glacio-eustatic sea level variations driven orbital forcing are now well depicted.
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Orbital forcing
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Neogene
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The importance of orbital forcing for climate changes and sea level variations is well established for an icehouse world. In a greenhouse world, feedback mechanisms responsible for the translation of minor solar-energy variations into climate change are far less genuinely. A better understanding of orbital forcing under greenhouse condition is needed. This study outlines the role of orbital forcing in the Devonian climate, using magnetic susceptibility as a proxy. Magnetic susceptibility is the degree of magnetization of a material in response to an applied magnetic field and therefore is a good proxy for the rate of supply of the iron-bearing lithogenic or detrital fraction to the marine system. The main controlling factor of the lithogenic or detrital input is continental erosion, induced by climate related processes or adjustments in base level. Both erosion-inducing factors may be controlled by orbital forcing cycles, and thus magnetic susceptibility is thought to be a suitable proxy to identify these cycles. The studied section spans from Upper Eifelian up to the Middle Frasnian. The section is split up according to sedimentary environment, and spectral analysis (using the BlackmanTuckey method) is carried out on magnetic susceptibility data. The sedimentary cyclefrequencies, proposed by the spectral analysis, are filtered out (using Gaussian filtering) and plotted against the raw data to ascertain the presence of this cycle.
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Devonian
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Milankovitch cycles
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TRACER
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消长与全球气候变化和人类文明演进息息相关 [10
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Milankovitch cycles
Paleoclimatology
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Albedo (alchemy)
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In this thesis newly generated high-resolution Oligo-Miocene climate proxy records from Walvis Ridge ODP Site 1264 (south-eastern Atlantic Ocean) are presented (Chapters 2 and 3). The records are tuned to an eccentricity solution (Chapter 3) and they are compared to published Atlantic and Pacific palaeoclimate chronologies (Chapters 2 and 4). The main research objectives are 1) to identify astronomical pacemakers of global significance and test earlier pacing theories, 2) to describe global climate and oceanographic change on astronomical and tectonic time scales and 3) to test the strong hysteresis in ice sheet models that suggest a very stable Antarctic ice sheet once formed. Chapter 1 gives a general introduction on the “mid”-to-late Oligocene climatic, oceanographic, geographic and cryospheric settings. Climate evolution and dynamics, together with the major underlying processes are introduced. In Chapter 2, high-resolution early Miocene stable oxygen and carbon isotope chronologies from Walvis Ridge Site 1264 are presented. The data are analysed on an untuned age model to identify the principal astronomical pacemakers, without introducing power on orbital frequencies. A dominance of variance in all datasets on 100-kyr timescales is found. The ?18O data are used to parameterize a suite of 1D ice sheet models and show that between 20 – 80% (avg. ~50%) of the ?18O signal can be explained by changes in Antarctic ice volume. (This chapter has been published as: D. Liebrand, L. J. Lourens, D. A. Hodell, B. de Boer, R. S. W. van de Wal and H. Palike. Antarctic ice sheet and oceanographic response to eccentricity forcing during the early Miocene. Climate of the Past, 7, 869–880, 2011) In Chapter 3, extended stable-isotope records together with X-ray fluorescence core scanning data from Walvis Ridge Site 1264 are presented. The records span an 11-Myr mid Oligocene through early Miocene time interval. Ages are calibrated to eccentricity, are in good agreement with the GTS2012 and independently confirm the Oligo-Miocene time scale to the ~100-kyr level. The ~2.4-Myr long-period eccentricity cycle is identified as the main pacemaker of Oligo-Miocene climate events, as identified in the benthic isotope records, at shorter astronomical (eccentricity) periodicities. In Chapter 4, the high-resolution Oligo-Miocene benthic stable-isotope chronology from Site 1264 is compared to published records from the Atlantic and Pacific to further identify and explore possible global climate pacemakers. In addition, an investigation of long-term trends and inter-/intra-basin isotopic gradients and their implications for ice volume reconstruction and palaeoceanographic studies are discussed. Methods are explored to quantify the apparent change in geometry of ~100-kyr cycles in our benthic ?18O data and the analyses indicate an increased cycle asymmetry (i.e. sawtooth patterns) throughout the Oligo-Miocene. This change in cycle geometry is interpreted as a measure of changing boundary conditions and used to track the evolution of a threshold response mechanism in Earth’s climate system. In Chapter 5 the main results of this thesis are summarised, the implications for our understanding of the Oligo-Miocene are discussed and perspectives are given on future work.
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Abstract. The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼34 million years ago (Ma) and lasting ∼790 kyr. The change is marked by a global shift in deep-sea δ18O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate-adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO2 change from the marine and terrestrial realms. Furthermore, we quantitatively compare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (∼325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is consistent with some CO2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO2 decrease signalled by our data–model comparison should be considered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not include dynamic ice sheets and in some cases may be under-sensitive to CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO2.
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Antarctic ice sheet
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Paleoclimatology
Global cooling
Orbital forcing
Global Change
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