Abrupt climate changes and the astronomical theory

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.

An astronomical correspondence to the 1470 year cycle of abrupt climate change

The existence of a ~ 1470 year cycle of abrupt climate change is well-established, manifesting in Bond ice-rafting debris (IRD) events, Dansgaard–Oeschger atmospheric temperature cycle, and cyclical climatic conditions precursory to increased El Niño/Southern Oscillation (ENSO) variability and intensity. This cycle is central to questions on Holocene climate stability and hence anthropogenic impacts on climate (deMenocal et al., 2000). To date no causal mechanism has been identified, although solar forcing has been previously suggested. Here we show that interacting combination of astronomical variables related to Earth's orbit may be causally related to this cycle and several associated key isotopic spectral signals. The ~ 1470 year climate cycle may thus be regarded as a high frequency extension of the Milankovitch precessional cycle, incorporating orbital, solar and lunar forcing through interaction with the tropical and anomalistic years and Earth's rotation.

The Holocene record of environmental changes in the ‘Stagno di Maccarese’ marsh (Tiber river delta, central Italy)

The stratigraphic study of the Stagno di Maccarese, carried out on the sediments exposed in about 7 km of trenches excavated in an area of approximately 1.5 km2, has shown that in the course of the Holocene many environmental variations have taken place. The complex evolution of the marsh is demonstrated by the variations in water salinity and the presence of erosion surfaces and soils between the sediments. In the early Holocene, the area studied was an isolated marsh with water having variable salinity, and it was only about 6000 cal. yr BP that it was encompassed in the system of inner delta marshes. In the delta environment, the water of the marsh was oligohaline until about 9th–8th centuries bc, brackish from 9th–8th centuries bc to about 600 yr BP, and later oligohaline until the 19th century drainage. A number of environmental variations are connected with local phenomena, such as erosion of the beach ridges and Tiber floods, but the others can be correlated chronologically with climatic events recorded at regional and global scale. The millennial variations seem to be connected with changes in insolation, while abrupt variations can be correlated chronologically with the IRD events dated at 8200, 5900, 4200, 2800, 1400 and 500 cal. yr BP.

North Atlantic Ice Sheet Fluctuations 10,000–70,000 Yr Ago as Inferred from Deposits on the Reykjanes Ridge, Southeast of Greenland

Marine records from the Reykjanes Ridge indicate ice sheet variations and abrupt climate changes. One of these records, ice-rafted detritus (IRD), serves as a proxy for iceberg discharges that probably indicates ice sheet fluctuations. The IRD records suggest that iceberg discharge 68,000–10,000 yr B.P. happened more frequently than the 7000- to 10,000-yr spacing of the Heinrich events. An IRD peak 67,000 to 63,000 yr B.P. further suggests that the Middle Weichselian glaciation started about 12,000 yr earlier in the North Atlantic than in the Norwegian Sea. Several later IRD events, in contrast, correlate with Norwegian Sea IRD-rich layers and imply coeval ice sheet advances in the North Atlantic and the Norwegian Sea. Coccoliths in a core from the Reykjanes Ridge show distinct peaks in species that record occasional inflow of warm surface water during the last glaciation, as previously reported from the eastern Labrador Sea. High abundances of coccoliths, together with a decrease of Neogloboquadrina pachyderma sin. and relatively low δ18O values, imply enhanced advection of the North Atlantic Current 69,000–67,000 yr B.P., 56,000–54,000 yr B.P., 35,000–33,000 yr B.P., and 26,000–23,000 yr B.P. This advection provided a regional moisture source for extension of ice sheets onto the shelf. In contrast, most of the IRD events are characterized by cold polar surface water masses indicating rapid variations in ocean surface conditions.