The influence of astronomically driven short-term climate change (Milankovitch cycles) on deep-marine turbidite systems is not well-known, particularly in the case of long-term greenhouse intervals with no significant glacioeustatic sea-level fluctuations. This study, carried out at the Gorrondatxe section (Global Stratotype Section and Point for the base of the Lutetian Stage in the western Pyrenees), demonstrates that the characteristics of lower-middle Eocene fan-fringe/basin-plain turbiditic and pelagic deposits varied in line with orbitally forced fluctuations in seasonal rainfall, runoff and terrigenous input to the sea. Reduced turbiditic activity during the formation of pelagic limy precessional hemicouplets indicates subdued seasonality and low terrigenous input. Conversely, turbidity currents were more frequent, had greater energy and were more voluminous during the formation of pelagic marly hemicouplets, suggesting precessional hemicycles with strong seasonality and heavy summer rainfall. These differences at precessional time scales were enhanced at maximum eccentricity because turbiditic activity was most intense when boreal summer occurred at perihelion (i.e. maximum seasonality) but declined when it occurred at aphelion. At minimum eccentricity, with relatively weak seasonality throughout one (or more than one) precessional cycle (>21 kyr), turbiditic activity remained relatively low. The pattern observed at the Gorrondatxe fan-fringe/basin-plain succession implies that the orbitally forced environmental changes must also have affected the inner and middle parts of the submarine fan. The astronomical influence on terrigenous sediment input also determined the changing characteristics of the pelagic sedimentation. Thus, terrigenous sediment contribution to pelagic sedimentation fluctuated by a factor of five during opposite precessional situations at maximum eccentricity, whereas there was almost no fluctuation at minimum eccentricity.
Several processes can contribute to the formation of hemipelagic limestone–marl alternations as a consequence of astronomically driven climate change. The aim of this study was to decipher which environmental factors governed the formation of three Eocene hemipelagic successions of the Basque–Cantabrian Basin using a comprehensive set of physical and bulk carbonate geochemical data (bed thickness, mineralogy, %CaCO3, δ13C and δ18O). The results show that the significance of several environmental processes varied depending on the palaeogeographic setting and eccentricity‐modulated precessional seasonality. In the Sopelana starved deep‐sea basin, limestones were formed as a consequence of high pelagic carbonate productivity during periods of warm seawater and sluggish circulation, which corresponded with periods of low seasonality (summers at aphelion); conversely, marls accumulated when pelagic carbonate productivity decreased during periods with cooler waters and more vigorous circulation, which occurred when seasonality was higher (summers at perihelion). In the Gorrondatxe submarine fan fringe, marls accumulated when high seasonality produced significant continental rainfall and run‐off, causing the dilution of pelagic carbonate sedimentation with terrigenous supplies. In the Oyambre upper slope, marls also accumulated when seasonality was high, as pelagic carbonate productivity decreased due to both the expansion of low‐salinity waters on the ocean surface and the increase in continentally derived nutrients, which caused detrimental seawater conditions for calcareous plankton. Both in Gorrondatxe and Oyambre, limestones accumulated when boreal summer at aphelion caused low seasonality, which allowed relatively stable conditions to prevail. At minimum eccentricity, when precession‐driven seasonality contrast diminished, changes in pelagic carbonate productivity were significant in the three sections. On the contrary, at maximum eccentricity, when seasonality peaked due to summers occurring at perihelion, the effects of other environmental processes, such as continental and oceanic currents, became influential. However, the influence of these processes minimized when summertime coincided with aphelion at maximum eccentricity and seasonality was weakest.
The astronomical timescale accuracy generally exceeds other dating methods. Precise age models are pivotal for paleoclimatic research. The middle Eocene astronomical timescale has been poorly constrained due to scarcity of suitable records leading to the so call “Eocene astronomical timescale gap.” We present magnetic susceptibility and color proxy records from an expanded 60 m long cyclic hemipelagic succession from the Oyambre Cape in northern Spain (∼1.3 My long stratigraphic section tuned to the ∼43.1–44.4 Ma interval in the Lutetian stage). We use the strong eccentricity amplitude modulation of precession in the sedimentary record for orbital tuning. The tuned record is correlated at precession level with previously tuned Ocean Drilling Program (ODP) Site 1260 from the equatorial Atlantic (the only oceanic record that registers geochemical variations in the precession band) and to other lower resolution deep‐sea records at eccentricity level from the Southern Atlantic. Our data is consistent with a very long eccentricity minimum (driven by a ∼2.4 My periodicity) at 43.15 Ma in the orbital solutions and an age for the C20n/C20r reversal boundary at ∼43.45 Ma. However, we challenge previous correlations between these Atlantic sites (shifts of one 100 ky eccentricity cycle). Data allows to rule out correlation to either younger or older 405 ky eccentricity cycles, which constrains chronologies for the middle Eocene, emphasizing the need for consistent astrochronological frameworks involving expanded outcrops. This should aid to overcome oceanic drilling shortcomings and sedimentary complexities. Our study highlights this integration need to achieve accuracy and stability of orbital timescales underpinning Eocene paleoclimatic records.
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