Orbital forcing of ice sheets during snowball Earth

The snowball Earth hypothesis—that a runaway ice-albedo feedback can cause global glaciation—seeks to explain low-latitude glacial deposits, as well as geological anomalies including the re-emergence of banded iron formation and “cap” carbonates. One of the most significant challenges to snowball Earth has been sedimentological cyclicity that has been taken to imply more climate dynamics than expected when the ocean is completely covered in ice. However, recent climate models suggest that as atmospheric CO2 accumulates, the snowball climate system becomes sensitive to orbital forcing. Here we show the presence of nearly all Milankovitch (orbital) cycles preserved in stratified banded iron formation deposited during the Sturtian snowball Earth. These results provide evidence for orbitally forced cyclicity of global ice sheets that resulted in periodic oxidation of ferrous iron. Orbital glacial advance and retreat cycles provide a simple mechanism to reconcile both the sedimentary dynamics and the enigmatic survival of multicellular life during snowball Earth.

7. Reconstructing the past and weathering the future

Tiny microfossils called Foraminifera form calcium carbonate shells that record the δ18O composition of the seawater in which they grew. These microfossils are found in sea bed sediment cores, and a lot of information from these oxygen isotope records can be extracted. ‘Reconstructing the past and weathering the future’ looks at the methodology used in palaeoclimate studies and explains gain and phase modelling and Milankovitch orbital cycles. Similar isotope temperature records have been constructed from polar ice cores. Atmospheric CO2 composition can be reconstructed from the amount of CO2 dissolved in the ice. A new sub-discipline of clumped isotope geochemistry—‘isotomics’—will have applications far beyond carbonate palaeothermometry.

The Effect of Milankovitch Variations in Insolation on Equatorial Seasonality

Although the sun crosses the equator 2 times per year at the equinoxes, at times in the past the equatorial insolation has had only one maximum and one minimum throughout the seasonal cycle because of Milankovitch orbital variations. Here a state-of-the-art coupled atmosphere–ocean general circulation model is used to study the effect of such insolation forcing on equatorial surface properties, including air and sea temperature, salinity, winds, and currents. It is shown that the equatorial seasonality is altered according to the insolation with, for example, either maximum sea surface temperature (SST) close to the vernal equinox and minimum SST close to the autumnal equinox or vice versa. The results may have important implications for understanding tropical climate as well as for the interpretation of proxy data collected from equatorial regions.

Lithofacies and sedimentary cycles within the Late Dinantian (late Brigantian) of Fife and East Lothian: is a sequence stratigraphical approach valid?

ABSTRACTThe late Brigantian topmost parts of the Pathhead Formation (Aberlady Formation in East Lothian) and the succeeding Lower Limestone Formation crop out widely in Fife and East Lothian. The successions include nine deltaic, coastal floodplain and marine shelf cycles (cyclothems), of which the lowest examined terminates the Pathhead and Aberlady Formations and the remaining eight constitute the Lower Limestone Formation.The cyclothems conform broadly to the ‘Yoredale” transgressive/regressive pattern in which a transgressive marine shelf phase is succeeded by delta progradation and terminates with a fluvial delta plain phase. Cycles may combine to form compound cyclothems up to more than 50 m thick, in which a basal, typically complete initial cycle of Yoredale pattern is succeeded by up to five base-absent minor cycles. These are thinner, more variable and less laterally persistent units in which the marine phase is weakly represented or absent.Cyclothems reflect successive marine flooding events, possibly under eustatic control, succeeded by delta progradation and, ultimately, leading to extensive palaeosol formation, including coal seams. Sedimentation and palaeosol formation were partly controlled by fault-induced differential subsidence and are likely to have been related to autocyclic processes. Local uplift and subsidence associated with vulcanicity, as at Kinghorn and Elie, have led to thickening or thinning of sediments accumulated in a given time period.Initial cycles initiate longer-period allocycles, corresponding broadly to third-order Exxon Production & Research (EPR) Type 1 sequences having a periodicity of around 1 Ma, within the Milankovitch orbital band. Two parasequences constitute each initial cycle: a lower, initiated on a marine flooding surface, and an upper, bounded by the base of the lowest thick sandstone in the cycle; cyclothem bases and sequence bases thus alternate. Parasequences and sequences are less well defined in minor cycles due to the problem of tracing the combined disconformity and soil profile of the underclay beyond the edge of channel sandstones. Minor cycles were controlled primarily by short-period autocyclic sedimentary and, or, tectonic processes, including delta-lobe switching and differential subsidence.Although we have attempted to interpret the deposits of Fife and the Lothians in terms of sequence stratigraphy, we are not fully convinced that the patterns of associated changes widely recognised within the framework of sequence stratigraphy can be confidently applied in succesions in which autocyclic changes feature strongly in an area undergoing active basin subsidence associated with strike-slip faulting. There is no doubt that some of the cyclicity discerned in the late Brigantian successions of eastern Scotland was related to eustatic sea level changes, which gave rise to the widespread limestone platforms or marine bands. The formation of eight cyclothems within the 2·5–3·5 Ma of late Brigantian suggests a cyclicity of about 400 ka, which corresponds to the long period eccentricity cycles of Milankovitch rather than the 0·5–5·0 Ma of third-order EPR cycles.

Paleomagnetism and cycle stratigraphy of the Triassic Fleming Fjord and Gipsdalen Formations of East Greenland

A 210 m section of Late Triassic Fleming Fjord Formation (the Malmros Klint Member and the lowermost 80m of the overlying Carlsberg Fjord beds of the Ørsted Dal Member) in the Tait Bjerg area of the Jameson Land Basin, East Greenland, was sampled for paleomagnetic study and measured for cycle stratigraphie analysis. Paleomagnetic samples were also taken from the underlying Gipsdalen Formation in the Gipsdalen area. A high stability characteristic magnetization carried by hematite was successfully isolated in 63 sampling levels in the Fleming Fjord Formation and 9 sampling sites in the Gipsdalen Formation using progressive thermal demagnetization. The mean characteristic directions for the Herning Fjord and the Gipsdalen Formations may be be biased by sedimentary inclination error but are consistent with a northward drift of East Greenland of about 10° from the arid (ca. 25° N) to semihumid (ca. 35° N) paleoclimatic belts in the Middle to Late Triassic. Seven normal and reversed polarity intervals are clearly delineated in the Fleming Fjord Formation section. A preferred correlation of the magnetostratigraphy to a cyclostratigraphically calibrated reference polarity sequence recently derived from drill cores in the Newark Basin of eastern North America suggests that the sampled interval represents about a 3.5 m.y. interval of the late Norian. The Malmros Klint Member and the overlying Carlsberg Fjord beds have composite sedimentary cycles that vary in thickness from 25 m to about 1 m and seem to match Milankovitch orbital climatic cyclicity with periods of ~400ky, ~100ky, ~40ky, and ~20ky. The composition and thickness ratio of the cycles suggest that the measured section of the Malmros Klint Member and the Carlsberg Fjord beds represents lacustrine accumulation over about 4 m.y., a duration consistent with the magnetostratigraphic correlations.