A ~400 ka (kilo years) supra-Milankovitch cycle, recorded in the sodium, magnesium, lead, nickel and cobalt contents of a 32 mm thick ferromanganese crust from Vityaz fracture zone, central Indian ridge is reported here. To arrive at the geological ages, we used both ²³⁰Th_execcs and Co-chronometric datings. The correlation coefficient between the ²³⁰Th_execcs based dates and Co-chronometric dates for the top 0-8 mm is very high (r=0.9734, at 99.9% significance). The cobalt chronometric age for the bottom most oxide layer of this crust is computed as 3.5 Ma. Red-fit and multi-taper spectral analyses of time series data revealed the existence of the significant ~400 ka cycle, representing the changes in the hydrogeochemical conditions in the ocean due to the Earth's orbital eccentricity related summer insolation at the equator. This is the first report of such cycle from a hydrogenous ferromanganese crust from equatorial Indian ocean.

The most marked step in the global climate transition from "Greenhouse" to "Icehouse" Earth occurred at the Eocene-Oligocene (E–O) boundary, 33.7 Ma. Evidence for climatic changes comes from many sources, including the marine benthic δ¹⁸O record, showing an increase by 1.2-1.5‰ at this time. This positive excursion is characterised by two steps, separated by a plateau. The increase in δ¹⁸O values has been attributed to rapid glaciation of the Antarctic continent, previously ice-free. Simultaneous changes in the δ¹³C record are indicative of a greenhouse gas control on climate. Previous studies show that a decline in pCO₂ beyond a certain threshold value may have initiated the growth of a Southern Hemispheric ice sheet. These studies were not able to conclusively explain the remarkable two-step profile in δ¹⁸O. Furthermore, they did not address the potential role of changes in ocean circulation in the E–O transition. Here a new interpretation of the δ¹⁸O signal is presented, based on model simulations using a simple coupled 8-box-ocean, 4-box-atmosphere model with an added land ice component. The model was forced with a slowly decreasing atmospheric carbon dioxide concentration. It is argued that the first step in the δ¹⁸O represents a shift in meridional overturning circulation from a Southern Ocean to a bipolar source of deep-water formation, which is associated with a cooling of the deep sea. This shift can be initiated by a small density perturbation in the model, although there is also a parameter regime for which the shift occurs spontaneously. The second step in the δ¹⁸O profile occurs due to a rapid glaciation of the Antarctic continent. This new interpretation is a robust outcome of our model and is in good agreement with proxy data.

Archaeozoological finds of the remains of marine and amphihaline fish from the Last Glacial Maximum (LGM) ca. 21 ka ago show evidence of very different species ranges compared to the present. We show how an ecological niche model (ENM) based on palaeoclimatic reconstructions of sea surface temperature and bathymetry can be used to effectively predict the spatial range of marine fish during the LGM. The results indicate that the ranges of marine fish species that are now in Northwestern Europe were almost completely displaced southward from the modern distribution. Significantly, there is strong evidence that there was an invasion of fish of current economic importance into the Western Mediterranean through the Straits of Gibraltar, where they were exploited by Palaeolithic human populations. There has been much recent interest in the marine glacial refugia to understand how the ranges of the economically important fish species will be displaced with the future climate warming. Recent ENM studies have suggested that species ranges may not have been displaced far southward during the coldest conditions of the LGM. However, archaeozoological evidence and LGM ocean temperature reconstructions indicate that there were large range changes, and certain marine species were able invade the Western Mediterranean. These findings are important for ongoing studies of molecular ecology that aim to assess marine glacial refugia from the genetic structure of living populations, and they pose questions about the genetic identity of vanished marine populations during the LGM. The research presents a challenge for future archaeozoological work to verify palaeoclimatic reconstructions and delimit the glacial refugia.

The variability of the Atlantic meridional overturing circulation (AMOC) strength is investigated in control experiments and in transient simulations of up to the last millennium using the low-resolution Community Climate System Model version 3. In the transient simulations the AMOC exhibits enhanced low-frequency variability that is mainly caused by transitions between two semi-stable circulation states which amount to a 10 percent change of the maximum overturning. One transition is also found in a control experiment, but the time-varying external forcing significantly increases the probability of the occurrence of such events though not having a direct, linear impact on the AMOC. The transition from a high to a low AMOC state starts with a reduction of the convection in the Labrador and Irminger Seas and goes along with a changed barotropic circulation of both gyres in the North Atlantic and a gradual strengthening of the convection in the Greenland-Iceland-Norwegian (GIN) Seas. In contrast, the transition from a weak to a strong overturning is induced by decreased mixing in the GIN Seas. As a consequence of the transition, regional sea surface temperature (SST) anomalies are found in the midlatitude North Atlantic and in the convection regions with an amplitude of up to 3 K. The atmospheric response to the SST forcing associated with the transition indicates a significant impact on the Scandinavian surface air temperature (SAT) in the order of 1 K. Thus, the changes of the ocean circulation make a major contribution to the Scandinavian SAT variability in the last millennium.