Contrasting environmental effects of astronomically driven climate change on three Eocene hemipelagic successions from the Basque–Cantabrian Basin

Abstract

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.

Checklist for the use of potassium concentrations in siliciclastic sediments as paleoenvironmental archives

“…If the K/Rb ratio is not stable and tends to be larger than that in the UCC, post-sedimentary enrichment by K (metasomatism) could have acted (van den Kamp, 2016). The impact of metasomatism was also evaluated using A-CN-K plots (Fedo et al, 1995, also used by Zhao et al, 2018, microscopic examination of clay mineral particles to distinguish their detrital and autochthonous origin (Martínez-Braceras et al, 2017;Wang et al, 2017), and mineral (micro)analysis (Kříbek et al, 2017;Foerster et al, 2018) to…”

“…That uniformity hampered a definition of stratigraphic units in the Most Basin during more than half century of intense coal prospections in the Czech Republic. The Most Basin deposits are distinct from deposits of ephemeral lakes, such as those in Mediterranean Cenozoic basins or other heterogeneous sequences with facies alteration that might partly result from autogenic cycles (discussion inAlonso-Zarza et al, 2012) or diagenesis (discussion inMartínez-Braceras et al, 2017). The Most Basin sediments are thus suitable for a study of the element relationships in siliciclastic sediments.…”

High‐Resolution Integrated Cyclostratigraphy From the Oyambre Section (Cantabria, N Iberian Peninsula): Constraints for Orbital Tuning and Correlation of Middle Eocene Atlantic Deep‐Sea Records

“…The low‐field magnetic susceptibility data from the Oyambre section (Figure ) display higher values in the marl layers of the basic lithological couplets. As indicated above, susceptibility data anticorrelates with calcium carbonate content in Oyambre (carbonate content lows around 40% in the marl layers of cycles 30–32 while carbonate peaks at about 65% in the limestone beds around, see Martínez‐Braceras et al, ) indicating a dominant detrital input during marl deposition (paramagnetic phases in the form of clays mostly contributing to susceptibility). Susceptibility data also shows a long‐term increase upsection.…”

“…Such an arrangement led Payros et al () to consider that the limestone–marl couplets are the sedimentary manifestation of astronomical precession cycles of 21 kyr duration, the larger‐scale bundles being the physical expression of the ∼100 kyr eccentricity cycles (short eccentricity). One of these eccentricity‐related bundles was studied by Martínez‐Braceras et al () who performed high‐resolution geochemical and stable isotope analysis in order to decipher which environmental factors governed the hemipelagic sedimentation and lithological cyclicity. According to that study, variations in pelagic carbonate productivity were determined by fluctuations in seawater salinity and nutrient content as a response to changing rates of fresh water input in line with seasonality‐controlled rainfall intensity.…”

“…Samples were not evenly distributed but at least 3 samples per bed were collected. Bulk magnetic susceptibility in pelagic rocks is usually controlled by the paramagnetic phases (mostly clays) and can therefore be a proxy for detrital input anticorrelating with calcium carbonate (see Martínez‐Braceras et al, , for comparison). Originally, a set of 739 rock‐powder samples (average resolution 8 cm) had been collected in the field using a standard portable drill with an 8 mm steel bit.…”