Mid-Pleistocene transition in glacial cycles explained by declining CO2 and regolith removal - PubMed (original) (raw)

Mid-Pleistocene transition in glacial cycles explained by declining CO2 and regolith removal

M Willeit et al. Sci Adv. 2019.

Abstract

Variations in Earth's orbit pace the glacial-interglacial cycles of the Quaternary, but the mechanisms that transform regional and seasonal variations in solar insolation into glacial-interglacial cycles are still elusive. Here, we present transient simulations of coevolution of climate, ice sheets, and carbon cycle over the past 3 million years. We show that a gradual lowering of atmospheric CO2 and regolith removal are essential to reproduce the evolution of climate variability over the Quaternary. The long-term CO2 decrease leads to the initiation of Northern Hemisphere glaciation and an increase in the amplitude of glacial-interglacial variations, while the combined effect of CO2 decline and regolith removal controls the timing of the transition from a 41,000- to 100,000-year world. Our results suggest that the current CO2 concentration is unprecedented over the past 3 million years and that global temperature never exceeded the preindustrial value by more than 2°C during the Quaternary.

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Figures

Fig. 1

Fig. 1. External drivers and best scenarios.

(A) Summer solstice insolation at 65°N (S65N) (44). (B) Scenarios for the evolution of area of exposed crystalline bedrock (_A_rock) resulting from the gradual removal of regolith by glacial erosion over North America and Scandinavia (see Materials and Methods and fig. S3 for a spatially explicit illustration). (C) Volcanic CO2 (Volc C) outgassing scenarios (see Materials and Methods). (D) Root mean square error (rmse) and correlation (corr) between modeled and observed (3) benthic δ18O for simulations driven by the combination of different regolith removal and volcanic CO2 outgassing scenarios in (B) and (C). PD, present-day constant regolith and volcanic CO2 outgassing. The optimal scenarios, those minimizing root mean square error and maximizing correlation, are indicated by the white circles in (D) and by the thicker lines in (B) and (C).

Fig. 2

Fig. 2. Transient modeling results.

Results of model simulations driven by orbital forcing, optimal regolith removal scenario, and optimal volcanic outgassing scenario. In all panels, observations are shown in black and model results are shown as colored lines. (A) Benthic δ18O compared to the stack of (3). (B) Relative sea level compared to (49). (C) Calving from the Laurentide ice sheet into the North Atlantic compared to a proxy for ice-rafted debris at site U1313 (30). (D) Atmospheric CO2 concentration compared to ice core data (solid line) (50) and other proxies [circles: (16); squares: (18); *: (51); + and ×: (19); diamonds: (52); black box: (15); dotted lines: (17)]. (E) SST anomalies compared to the stack of (18). (F) Global annual surface air temperature compared to reconstructions (53). (G) Southern Ocean dust deposition compared to data (54).

Fig. 3

Fig. 3. Benthic δ18O decomposition.

(A) Contribution of sea level and deep ocean temperature to benthic δ18O variability through time, computed as moving SD with a window of 150 ka. The modeled total δ18O variability (gray lines) is compared to the variability of the stack from (3) (black). The modeled contribution of deep ocean temperature and sea level to δ18O variability is shown by the green and magenta lines, respectively. The decomposition in terms of sea level, _z_SL, and deep ocean temperature, _T_d, is derived using the following formula: δ18O = 4.0 − 0.22 _T_d − 0.01 _z_SL. (B) Ratio between deep ocean temperature and sea-level contribution to the total δ18O variability.

Fig. 4

Fig. 4. Power and wavelet spectra of benthic δ18O.

(A) Power spectra of modeled δ18O (blue) compared to power spectra of the δ18O stack of (3) (black) for different time intervals, as indicated above the panels. Wavelet spectra of (B) the benthic δ18O stack of (3) and (C) modeled δ18O. Black contours indicate the 5% significance level against red noise. The horizontal white lines represent the orbital periods of precession (∼23 ka), obliquity (∼41 ka), and eccentricity (∼100 ka). The model spectra are derived from the average δ18O computed over all ensemble members resulting from the time-splitting technique.

Fig. 5

Fig. 5. Pre- and post-MPT ice sheets.

Modeled maximum ice thickness in each grid cell (A) before and (B) after the MPT. The dotted lines in (B) indicate the reconstructed ice extent at the last glacial maximum.

Fig. 6

Fig. 6. External drivers and model response.

Modeled benthic δ18O compared to observations (black) (3) for (A) simulations driven only by variations in orbital configuration (green), (B) simulations driven by orbital forcing and optimal regolith removal scenario (red), and (C) simulations driven by orbital forcing and optimal volcanic outgassing scenario (blue).

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