High-latitude biomes and rock weathering mediate climate-carbon cycle feedbacks on eccentricity timescales (vol 11, 5013, 2020)

Correction to: Nature Communications https://doi.org/10.1038/s41467-020-18733-w, published online 6 October 2020

Insensitivity of alkenone carbon isotopes to atmospheric CO₂ at low to moderate CO₂ levels

Atmospheric pCO₂ is a critical component of the global carbon system and is considered to be the major control of Earth's past, present, and future climate. Accurate and precise reconstructions of its concentration through geological time are therefore crucial to our understanding of the Earth system. Ice core records document pCO₂ for the past 800 kyr, but at no point during this interval were CO₂ levels higher than today. Interpretation of older pCO₂ has been hampered by discrepancies during some time intervals between two of the main ocean-based proxy methods used to reconstruct pCO₂: the carbon isotope fractionation that occurs during photosynthesis as recorded by haptophyte biomarkers (alkenones) and the boron isotope composition (δ^{11}B) of foraminifer shells. Here, we present alkenone and δ^{11}B-based pCO₂ reconstructions generated from the same samples from the Pliocene and across a Pleistocene glacial–interglacial cycle at Ocean Drilling Program (ODP) Site 999. We find a muted response to pCO₂ in the alkenone record compared to contemporaneous ice core and δ^{11}B records, suggesting caution in the interpretation of alkenone-based records at low pCO₂ levels. This is possibly caused by the physiology of CO₂ uptake in the haptophytes. Our new understanding resolves some of the inconsistencies between the proxies and highlights that caution may be required when interpreting alkenone-based reconstructions of pCO₂

Astronomical forcing in late Eocene marine sediments (abstract of paper presented at EUG XI, Strasbourg, France, 8-12 April 2001)

Recently the astronomically calibrated geological timescale has been extended to the base of the Oligocene (Shackleton et al, 1999). Here we present a new relative age calibration of sediments of late-Middle Eocene (39.5 Ma) to late Eocene age (35 Ma) that were obtained from deep-marine sediment cores during ODP Leg 171B from Site 1052. We analyse elemental ratios of Fe and Ca as a proxy for calcium carbonate content, obtained by using an X-ray Fluorescent Scanner (XRF). Our data match very well with other proxy data (magnetic susceptibility and colour reflectance) but show a significantly higher signal-to-noise ratio and a more consistent hole-to hole agreement. The data obtained hence allow the construction of a more accurate composite depth scale. The data display a strong orbital signal that shows variability at all major Milankovitch frequencies. We use the eccentricity driven amplitude modulation of precession to put our record onto a relative timescale, assuming that the 400 kyr eccentricity cycle has been stable at that time (Laskar, 1999). The exact nature of the orbital signal might be subject to revision pending further calculations, but the consistent relationship between the different orbital frequencies present in the data suggests new ages for Magnetochrons C16, C17, and C18 that will refine the magneto-stratigraphic timescale created by Cande and Kent (1995). Our astronomical calibration suggests that the relative durations of these magnetochrons has not changed significantly, although the absolute ages might be ~200 ky younger than on the Cande and Kent timescale. Our study should allow a better time control for high-resolution studies over the late Eocene time interval

Carbonate ions, orbits and Mg/Ca at ODP 1123

The accuracy of the magnesium/calcium palaeotemperature proxy has been questioned, in particular because the ratio of magnesium to calcium in foraminiferal tests could be affected by local or global changes in carbonate ion concentrations in deep water. A related question regarding the technique is its problematic phase relationship to orbital eccentricity: Mg/Ca records of intermediate and deep waters typically show a phase lead with respect to orbital eccentricity. This calls into question either the validity of the Mg/Ca palaeotemperature proxy, or the assumption that orbital eccentricity is pacing the 100 kyr climate oscillations, or both. This paper addresses these questions, and suggests that a phase lead of the type observed at ODP 1123 is unlikely to be generated by the operation of the carbonate ion effect, and might be attributable to heat storage in the oceans during low eccentricity episodes

Carbonate ions, orbits and Mg/Ca at ODP 1123

The accuracy of the magnesium/calcium palaeotemperature proxy has been questioned, in particular because the ratio of magnesium to calcium in foraminiferal tests could be affected by local or global changes in carbonate ion concentrations in deep water. A related question regarding the technique is its problematic phase relationship to orbital eccentricity: Mg/Ca records of intermediate and deep waters typically show a phase lead with respect to orbital eccentricity. This calls into question either the validity of the Mg/Ca palaeotemperature proxy, or the assumption that orbital eccentricity is pacing the 100 kyr climate oscillations, or both. This paper addresses these questions, and suggests that a phase lead of the type observed at ODP 1123 is unlikely to be generated by the operation of the carbonate ion effect, and might be attributable to heat storage in the oceans during low eccentricity episodes

Astronomic calibration of the late Eocene/early Oligocene Massignano section (central Italy)

We present an astronomically tuned polarity timescale for the late Eocene/early Oligocene based on a cyclic limestone/marls sedimentary succession exposed at Massignano, Italy (GSSP for the Eocene- Oligocene boundary). Here we applied spectral analysis techniques to the high-resolution magnetic susceptibility record. The results provide clear evidence of a regular and cyclic signal that allows us to discard a stochastic mechanism as controlling these fluctuations. Conversely, these outcomes reveal the presence of stable and notable periodicities that are in tune with those associated with orbital forcing. Through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record, we refine the late Eocene portion of the GPTS. Through astronomical tuning of the rhythmic sedimentary alternations to recent astronomically calculated variations of Earth’s orbit we recalibrated the polarity boundaries ages and the main stratigraphic events recognized throughout the sedimentary record. Among these, an age of 33.714 Ma is proposed for the E/O boundary

Atmospheric methane, southern European vegetation and low-mid latitude links on orbital and millennial timescales

Pollen records from marine and terrestrial sequences in southern Europe reveal a strong coherence between changes in tree populations and atmospheric methane concentrations over the last 800 thousand years. Variations in the continental hydrological balance provide a link for the observed patterns, leading to concomitant changes in southern European vegetation, and low-latitude wetland extent and methane/volatile organic compound emissions, although additional contributions to the methane budget from extratropical sources are not excluded. Here we propose that the close coupling between low- and mid-latitude hydrological changes reflects shifts in the mean latitudinal position of the Intertropical Convergence Zone, which determines the extent to which southern Europe is dominated by subtropical or mid/high-latitude influences. This provides a conceptual framework within which to view vegetation variability in southern Europe on orbital and millennial timescales. (C) 2008 Elsevier B.V. All rights reserved.</p

The magneto-bio-chemostratigraphy of the Torrente Cicogna section (Italy): a record of Late Paleocene-Early Eocene climate

none6noneDALLANAVE E; AGNINI C.; SPOFFORTH D.J.A; MUTTONI G; PÄLIKE H; RIO DDallanave, Edoardo; Agnini, Claudia; SPOFFORTH D. J., A; Muttoni, G; Pälike, H; Rio, Domenic