Influence of the choice of insolation forcing on the results of a conceptual glacial cycle model

Over the Quaternary, the ice volume variations are “paced” by the astronomy. However, the precise way in which the astronomical parameters influence the glacial-interglacial cycles is not clear. The origin of the 100 kyr cycles over the last million year and of the switch from 40 kyr to 100 kyr cycles over the Mid Pleistocene Transition remain largely unexplained. By representing the climate system as oscillating between two states, glaciation and deglaciation, switching once a glaciation and a deglaciation thresholds are crossed, the main features of the ice volume record can be reproduced (Parrenin and Paillard, 2012). However, previous studies have only focused on the use of a single summer insolation as input. Here, we use a simple conceptual model to test and discuss the influence of the use of different summer insolation forcings, having different contributions from precession and obliquity, on the model results. We show that some features are robust. Specifically, to be able to reproduce the frequency shift over the Mid Pleistocene Transition, the deglaciation threshold needs to increase over time, independently of the summer insolation used as input. The quality of the model data agreement however depends on the chosen type of summer insolation and time period considered.

Review of the Early–Middle Pleistocene boundary and Marine Isotope Stage 19

The Global Boundary Stratotype Section and Point (GSSP) defining the base of the Chibanian Stage and Middle Pleistocene Subseries at the Chiba section, Japan, was ratified on January 17, 2020. Although this completed a process initiated by the International Union for Quaternary Research in 1973, the term Middle Pleistocene had been in use since the 1860s. The Chiba GSSP occurs immediately below the top of Marine Isotope Substage (MIS) 19c and has an astronomical age of 774.1 ka. The Matuyama–Brunhes paleomagnetic reversal has a directional midpoint just 1.1 m above the GSSP and serves as the primary guide to the boundary. This reversal lies within the Early–Middle Pleistocene transition and has long been favoured to mark the base of the Middle Pleistocene. MIS 19 occurs within an interval of low-amplitude orbital eccentricity and was triggered by an obliquity cycle. It spans two insolation peaks resulting from precession minima and has a duration of ~ 28 to 33 kyr. MIS 19c begins ~ 791–787.5 ka, includes full interglacial conditions which lasted for ~ 8–12.5 kyr, and ends with glacial inception at ~ 774–777 ka. This inception has left an array of climatostratigraphic signals close to the Early–Middle Pleistocene boundary. MIS 19b–a contains a series of three or four interstadials often with rectangular-shaped waveforms and marked by abrupt (< 200 year) transitions. Intervening stadials including the inception of glaciation are linked to the calving of ice sheets into the northern North Atlantic and consequent disruption of the Atlantic meridional overturning circulation (AMOC), which by means of the thermal bipolar seesaw caused phase-lagged warming events in the Antarctic. The coherence of stadial–interstadial oscillations during MIS 19b–a across the Asian–Pacific and North Atlantic–Mediterranean realms suggests AMOC-originated shifts in the Intertropical Convergence Zone and pacing by equatorial insolation forcing. Low-latitude monsoon dynamics appear to have amplified responses regionally although high-latitude teleconnections may also have played a role.

South African speleothems reveal influence of high- and lowlatitude forcing over the past 113.5 k.y.

Variation in δ18O and δ13C values in a speleothem from the Cango Caves in southernmost South Africa enable the construction of coherent regional composite records spanning the past 113,500 yr. Novel for the region in terms of both their length and detail, these records indicate environmental and climatic changes that both are consistent with records from the wider region and show a clear evolution from low- to high-latitude forcing dominance across the last glacial period. Prior to ca. 70 ka, the influence of direct low-latitude insolation forcing is expressed through increases in summer rainfall during austral summer insolation maxima. With the onset of Marine Isotope Stage 4, cooler global conditions and the development of high-latitude ice sheets appear to have supplanted direct insolation forcing as the dominant driver pacing patterns of environmental change, with records from the Southern and Northern Hemisphere tropics exhibiting a positive relationship until after the Last Glacial Maximum. These results highlight the complexity of South African climate change dynamics as a response to changing global boundary conditions and provide a critical reference for regional and global comparisons.

Enhanced humidity in SW Iberia driven by the combination of insolation and ice-sheet forcing during MIS 13 interglacial

&lt;p&gt;Marine Isotope Stage (MIS) 13, ~500 ky ago, represents a Quaternary interglacial of primary interest due to the unexpected enhancement of monsoon systems under a cool climate characterised by low atmospheric CO&lt;sub&gt;2&lt;/sub&gt; and larger ice volume than the present interglacial. Yet, key questions remain about its regional expression (intensity, climate variability, length) and underlying forcing factors. Here we examine the SW Iberian vegetation and terrestrial climate during MIS 13 directly compared with the sea surface temperatures using sediments from IODP Site U1385, and combine those terrestrial-marine profiles with climate-model experiments. We show for the first time that MIS 13 stands out for its large forest expansions with a reduced Mediterranean character alternating with muted forest contractions, indicating that this stage is marked by a cool-temperate climate regime with high levels of humidity. Results of our data-model approach reveal that that the dominant effect of MIS 13 insolation forcing on the regional vegetation and precipitation regime in SW Iberia is amplified by the relatively large extent of the ice-sheets in high northern latitudes. In qualitative agreement with the pollen-based evidence, model results show that ice-sheet forcing triggers an increase in the SW Iberian tree fraction along with both intensified winter and summer rainfall. We propose that the interactions between ice-sheets and major atmospheric circulation systems may have resulted in the persistent influence of the mid-latitude cells over the SW Iberian region, which led to intensified moisture availability and reduced seasonality, and, in turn, to a pronounced expansion of the temperate forest.&lt;/p&gt;

Diverse manifestations of insolation forcing of environmental changes during the middle Piacenzian

&lt;p&gt;The middle Piacenzian period is the closest sustained warm interval and a possible analog to the future climate. It is well known that global ice volume exhibits dominant 41-kyr cyclicities. However, high resolution terrestrial paleoenvironmental records are scare. Here we present a 3.6 kyr terrestrial environmental variation record from Teruel Basin of Spain and compare the results with the East Asian monsoon records. The Spain results show dominant 41-kyr cycles during the early Piacenzian (3.3-3.15 Ma) when eccentricity was at minimum, but the 41-kyr cycles weakens during the late Piacenzian 3.15-2.95 Ma when eccentricity got increased, suggesting direct forcing by insolation. This pattern is different from the monsoonal records from China, which demonstrates persistent 20-kyr cycles during the entire middle Piacenzian. The strong 41-kyr cycles in westerly region during the early Piacenzian may originate from its higher latitude and higher sensitivity to insolation gradient forcing.&lt;/p&gt;

Modulation of late Pleistocene ENSO strength by the tropical Pacific thermocline

The El Niño Southern Oscillation (ENSO) is highly dependent on coupled atmosphere-ocean interactions and feedbacks, suggesting a tight relationship between ENSO strength and background climate conditions. However, the extent to which background climate state determines ENSO behavior remains in question. Here we present reconstructions of total variability and El Niño amplitude from individual foraminifera distributions at discrete time intervals over the past ~285,000 years across varying atmospheric CO2 levels, global ice volume and sea level, and orbital insolation forcing. Our results show a strong correlation between eastern tropical Pacific Ocean mixed-layer thickness and both El Niño amplitude and central Pacific variability. This ENSO-thermocline relationship implicates upwelling feedbacks as the major factor controlling ENSO strength on millennial time scales. The primacy of the upwelling feedback in shaping ENSO behavior across many different background states suggests accurate quantification and modeling of this feedback is essential for predicting ENSO’s behavior under future climate conditions.

An analysis of the periodically forced PP04 climate model, using the theory of non-smooth dynamical systems

In this paper, we perform a careful analysis of the forced PP04 model for climate change, in particular the behaviour of the ice ages. This system models the transition from a glacial to an inter-glacial state through a sudden release of oceanic carbon dioxide into the atmosphere. This process can be cast in terms of a Filippov dynamical system, with a discontinuous change in its dynamics related to the carbon dioxide release. By using techniques from the theory of non-smooth dynamical systems, we give an analysis of this model in the cases of both no insolation forcing and also periodic insolation forcing. This reveals a rich, and novel, dynamical structure to the solutions of the PP04 model. In particular, we see synchronized periodic solutions with subtle regions of existence which depend on the amplitude and frequency of the forcing. The orbits can be created/destroyed in both smooth and discontinuity-induced bifurcations. We study both the orbits and the transitions between them and make comparisons with actual climate dynamics.

Orbitally-paced South American Summer Monsoon variability during the mid- to late-Pleistocene

&lt;p&gt;Hydrological extremes related to the South American Summer Monsoon (SASM) are expected to become more frequent in the near future and might have devastating socioeconomic consequences for the densely populated region of eastern Brazil. Given the complexity in SASM behaviour in space and time, a dense coverage of monsoonal precipitation records, particular those spanning multiple glacial-interglacial cycles, are urgently needed to constrain this high spatial-temporal variability. This information is necessary to reduce the uncertainty associated with projections of SASM precipitation in response to rising anthropogenic greenhouse gas (GHG) emissions. Here we use elemental ratios from X-ray fluorescence scanning of two sediment cores retrieved off the eastern Brazil margin to reconstruct long-term rainfall changes in the hinterland. Our findings from core M125-55-7 (offshore the Doce River, 20&amp;#176;S) reveal that during the past ~320 kyr, precession-paced insolation forcing is the primary pacemaker of variations in SASM precipitation over the Doce basin. We also determined an anomalous interval of weak monsoonal response to insolation forcing during Marine Isotope Stage 6, which we attribute to enhanced wintertime precipitation due to exceptionally strong southeast trade winds created by a steep South Atlantic latitudinal temperature gradient. Moreover, our results suggest that albeit predominantly driven by insolation forcing, the intensity of SASM rainfall responds negatively to GHG forcing, most likely through indirect feedbacks. We propose that GHG forcing directly influences the magnitude of both the inter- and intrahemispheric latitudinal temperature gradients, which in turn modify the strength of atmospheric circulation and precipitation in the tropics. Thus, we suggest that SASM rainfall intensity over tropical eastern Brazil will likely be suppressed by rising CO&lt;sub&gt;2&lt;/sub&gt; emissions in the future. Our preliminary analysis of core M125-73-3 (off the Contas River; 12&amp;#176;S) reveals regional differences in monsoonal precipitation between the more northerly Contas basin and the more southerly Doce basin. Most notably, unlike the insolation-paced continental rainfall variability recorded at site M125-55-7, SASM rainfall intensity over the Contas basin appears to be more sensitive to glacial-interglacial scale pacing over the past ~800 kyr. Taken together, our records reveal both the high spatial variability in SASM precipitation over eastern Brazil and the dominant influence of orbital forcing on monsoonal rainfall intensity.&lt;/p&gt;

TRIASSIC CARBONATE AND EVAPORITE SEDIMENTATION IN THE IONIAN ZONE (WESTERN GREECE): PALAEOGEOGRAPHIC AND PALAEOCLIMATIC IMPLICATION

The Triassic is considered a crucial interval because during that time huge areas in our planet suffered an intense, long lasting, period of aridity, which favored the formation of worldwide evaporitic bodies. During the Triassic, great volumes of evaporites were formed in the Ionian basin (Western Greece). On the surface chaotically textured gypsum, surrounded by dolomitic breccias of solution-collapse origin, appears. Sedimentological and diagenetical data proposed that these salt bodies were formed in an intertidal to supratidal environment. Although halite suggests precipitation under long-term arid conditions, clay film intercalations reveal intervals of short term humid conditions. During arid periods sabkhas prevailed and brines were of marine origin. Instead, during humid intervals brines were modified by meteoric water and stormy episodes could be responsible for the transportation of clay-sized material, from the low relief surrounding terrains, into the evaporative basin. Death and burial of cyanobactehal population during storm events could be responsible for the enrichment of clayey layers in carbonaceous material. The co-existence of halite and clays in the Ionian evaporitic sequence imposes a complicated climate, possibly periodically and seasonally controlled. The impact of the precession of the equinoxes plus the palaeogeographical position dominates the local climate. The insolation over the Triassic Ionian basin and nearby sea and land areas is a crucial factor. Climate responses to gradual insolation forcing with an ocean land atmosphere feedback mechanism. The desert / monsoonal dominated climatic model seems to be most proper for the explanation of the existing lithologigal record.