Abstract
Abrupt climate changes constitute a relatively new field of research, which addresses variations occurring in a relatively short time interval of tens to a hundred years. Such time scales do not correspond to the tens or hundreds of thousands of years that the astronomical theory of climate addresses. The latter theory involves parameters that are external to the climate system and whose multi-periodic variations are reliably known and almost constant for a large extent of Earth history. Abrupt changes, conversely, appear to involve fast processes that are internal to the climate system; these processes varied considerably during the past 2.6 Myr, and yielded more irregular fluctuations. In this paper, we re-examine the main climate variations determined from the U1308 North Atlantic marine record, which yields a detailed calving history of the Northern Hemisphere ice sheets over the past 3.2 Myr. The magnitude and periodicity of the ice-rafted debris (IRD) events observed in the U1308 record allow one to determine the timing of several abrupt climate changes, the larger ones corresponding to the massive iceberg discharges labeled Heinrich events (HEs). In parallel, abrupt warmings, called Dansgaard-Oeschger (DO) events, have been identified in the Greenland records of the last glaciation cycle. Combining the HE and DO observations, we study a complex mechanism that may lead to the observed millennial-scale variability corresponding to the abrupt climate changes of last 0.9 Myr. This mechanism relies on amended Bond cycles, which group DO events and the associated Greenland stadials into a trend of increased cooling, with IRD events embedded into every stadial, the latest of these being an HE. These Bond cycles may have occurred during the last 0.9 Ma when Northern Hemisphere ice sheets reached their maximum extent and volume, thus becoming a major player in this time interval’s climate dynamics. Since the waxing and waning of ice sheets during the Quaternary period are orbitally paced, we conclude that the abrupt climate changes observed during the Mid and Upper Pleistocene are therewith indirectly linked to the astronomical theory of climate.
Highlights
Well-dated geological data indicate that the Earth experienced orbitally paced climate changes since at least the late Precambrian – 1.4 billion years ago during the Proterozoic Eon (Benn et al, 2015; Zhang et al, 2015; Hoffman et al, 2017; Meyers and Malinverno, 2018), and all along the Phanerozoic (Lisiecki and Raymo, 2005; Liebrand et al, 2011; Miller et al, 2011; Kent et al, 2017, 2018; Olsen et al, 2019; Drury et al, 2020; Westerhold et al, 2020)
Whether a younger start date of 0.9 Ma, or an older one of 1.5Ma, is posited, these results show that the Northern Hemisphere ice sheets played a significant role in the onset of millennial and submillennial climate variability that prevailed during the Mid and Late Pleistocene
Abrupt changes are intimately interwoven with these phenomena
Summary
Well-dated geological data indicate that the Earth experienced orbitally paced climate changes since at least the late Precambrian – 1.4 billion years ago during the Proterozoic Eon (Benn et al, 2015; Zhang et al, 2015; Hoffman et al, 2017; Meyers and Malinverno, 2018), and all along the Phanerozoic (Lisiecki and Raymo, 2005; Liebrand et al, 2011; Miller et al, 2011; Kent et al, 2017, 2018; Olsen et al, 2019; Drury et al, 2020; Westerhold et al, 2020). The past 3.2 Myr of Northern Hemisphere climate are well described in North Atlantic core U1308 (Hodell and Channell, 2016) This core is located in the ice-rafted debris (IRD) belt (Ruddiman, 1977), and it yields a more complete record than U1313 (Naafs et al, 2013); see Fig. 1b. Hoddell and Channel (2016) identified four major steps in their climate record These four steps are linked to thresholds in the benthic and bulk carbonate δ18O variations, and they occur at 2.75 Ma, 1.5 Ma, 0.9 Ma and 0.65 Ma. The first date is interpreted as corresponding to the earliest occurrence of IRD in the North Atlantic. The second date corresponds to an increased amplitude in ice volume variations between glacial minima and interglacial optima This second step shows the permanent occurrence of ice-rafted events during glacial intervals in the record, an amplified relationship of climate variations with Northern Hemisphere ice sheets. The increase in IRD variability and magnitude since 1.5 Ma, shows that distinct, faster processes have to be considered than those due to slow changes in Earth’s orbital parameters; see again Figs. 2 and 3
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