Precession-driven climate cycles and time scale prior to the Hirnantian glacial maximum

Paleozoic astrochronologies are limited by uncertainties in past astronomical configurations and the availability of complete stratigraphic sections with precise, independent age control. We show it is possible to reconstruct a robust Paleozoic ~104-yr-resolution astrochronology in the well-preserved and thick Upper Ordovician reference record of Anticosti Island (Canada). The clear imprint of astronomical cycles, including ~18 k.y. precession, potential obliquity, and short and long eccentricity, constrains the entire Vauréal Formation (~1 km thick) to only ~3 m.y. in total, representing ~10 times higher accumulation rates than previously suggested. This ~104 yr resolution represents an order of magnitude increase in the current standard temporal resolution for the Katian and even allows for the detection of sub-Milankovitch climate-scale variability. The loss of a clear precession signal in the uppermost Vauréal Formation might be related to contemporaneous global cooling prior to the Hirnantian glacial maximum as indicated by the δ18O record. Complementary to the study of cyclostratigraphy of longer and often simplified records, it is important to recognize stratigraphic hiatuses and complexities on the ~104 yr scale to achieve robust sub-eccentricity-scale Paleozoic astrochronologies.

Different response of sea surface temperature and sea ice to precession and obliquity between the two hemispheres

<p>The response of the climate system to astronomical parameters is an important scientific issue, but the internal processes and feedbacks need to be better understood. This study investigates the differences of the climate response to the astronomical forcing between the Northern (NH) and Southern (SH) hemispheres based on a more than 90,000-year long transient simulation using the model LOVECLIM. Our results show that the response of sea ice and sea surface temperature (SST) to precession and obliquity are different between the two hemispheres. Precession plays a dominant role on the NH sea ice. This is mainly due to its response to the local summer insolation and also, to a less degree, the influence of the northward oceanic heat transports. However, obliquity plays a dominant role on the SH sea ice through its influence on insolation and the westerly winds. As far as the SST is concerned, it shows a strong precession signal at low latitudes in both hemispheres. For the SST in the mid and high latitudes, obliquity plays a dominant role in the SH whereas precession is more important in the NH. This is largely due to the different response to insolation and feedbacks related to the different land-ocean distribution in the two hemispheres. Near the Equator, besides the precessional signal, the SST also shows strong half-precessional signal, which can be explained by the unique characteristics of the insolation variations at the Equator.</p>

The MIS 11 – MIS 1 analogy, southern European vegetation, atmospheric methane and the "early anthropogenic hypothesis"

Marine Isotope Stage (MIS) 11 has been considered a potential analogue for the Holocene and its future evolution. However, a dichotomy has emerged over the precise chronological alignment of the two intervals, with one solution favouring a synchronization of the precession signal and another of the obliquity signal. The two schemes lead to different implications over the natural length of the current interglacial and the underlying causes of the evolution of greenhouse gas concentrations. Here, the close coupling observed between changes in southern European tree populations and atmospheric methane concentrations in previous interglacials is used to evaluate the natural vs. anthropogenic contribution to Holocene methane emissions and assess the two alignment schemes. Comparison of the vegetation trends in MIS 1 and MIS 11 favours a precessional alignment, which would suggest that the Holocene is nearing the end of its natural course. This, combined with the divergence between methane concentrations and temperate tree populations after 5 kyr BP, provides some support for the notion that the Holocene methane trend may be anomalous compared to previous interglacials. In contrast, comparison of MIS 1 with MIS 19, which may represent a closer astronomical analogue than MIS 11, leads to substantially different conclusions on the projected natural duration of the current interglacial and the extent of the anthropogenic contribution to the Holocene methane budget. As answers vary with the choice of analogue, resolution of these issues using past interglacials remains elusive.

The MIS 11 – MIS 1 analogy, southern European vegetation, atmospheric methane and the "early anthropogenic hypothesis"

Marine Isotope Stage (MIS) 11 has been considered a potential analogue for the Holocene and its future evolution. However, a dichotomy has emerged over the precise chronological alignment of the two intervals, with one solution favouring a synchronization of the precession signal and another of the obliquity signal. The two schemes lead to different implications over the natural length of the current interglacial and the underlying causes of the evolution of greenhouse gas concentrations. Here the strong coherence observed between changes in temperate tree populations in southern Europe and atmospheric methane concentrations is used to evaluate the two alignment schemes. Comparison of the vegetation trends in MIS 1 and MIS 11 favours a precessional alignment, which would suggest that the Holocene is nearing the end of its natural course. It also provides some support for the notion that the Holocene methane trend may be anomalous compared to previous interglacials. In contrast, comparison of MIS 1 with MIS 19, which may represent a closer astronomical analogue than MIS 11, leads to substantially different conclusions on the projected natural duration of the current interglacial and the extent of the anthropogenic contribution to the Holocene methane budget. As answers vary with the choice of analogue, resolution of these issues using past interglacials remains elusive.