Silicate weathering machine at work: Rock magnetic data from the late Paleocene–early Eocene Cicogna section, Italy

Abstract

[1] We describe a scenario of climate forcing on sedimentation recorded in the late Paleocene-early Eocene Cicogna marine section from the Belluno Basin (NE Italy). Previously published magneto-biostratigraphic data revealed that the ∼81 m Cicogna section extends from Chron C25r to Chron C23r spanning the NP7/ NP8-NP12 nannofossil zones (∼52.2-56.6 Ma). Using previously published rock magnetic data, augmented by data from this study, we describe and thoroughly discuss a pronounced increase of hematite (relative to maghemite or magnetite) between ∼54.9 and 54.6 Ma immediately above the PaleoceneEocene boundary, followed by a second, long-term increasing trend from ∼54 Ma up to ∼52.2 Ma in the early Eocene. This hematite is essentially of detrital origin, insofar as it is associated with a strong shallow bias of paleomagnetic inclinations, and is interpreted to have formed on land by the weathering of Fe-bearing silicates and other primary minerals. We speculate that the warm and humid climate typical of the Paleocene-Eocene thermal maximum (PETM, ∼54.9 Ma) as well as of the warming trend leading to the early Eocene climatic optimum (EECO; ∼52-50 Ma) enhanced continental weathering of silicate rocks with the consequent production, transport, and sedimentation of detrital hematite grains. This hypothesis is confirmed by a statistical correlation between the rock magnetic properties and global climate as revealed by a standard benthic oxygen isotope record from the literature. Our temporal coupling between oxidation state of sedimentary magnetic phases and global climate is therefore consistent with the existence in the Paleocene-Eocene of the silicate weathering negative feedback mechanism for the long-term stabilization of the Earth's surface temperature.

Is there a normal magnetic-polarity event during the Palaeocene-Eocene thermal maximum (∼55 Ma)? Insights from the palaeomagnetic record of the Belluno Basin (Italy)

“…After removal of these directions, all samples showed a stable characteristic ChRM magnetization linearly trending to the origin of the demagnetization axes and isolated up to 670 °C. This unblocking temperature suggests, together with previous rock‐magnetic data (Dallanave et al 2010), that this ChRM magnetization is carried by haematite. Only a single grey sample collected at −0.1 m, that is, before the onset of the PETM (Fig.…”

“…during heating in nearly zero magnetic field (e.g. Kodama 1982; Wang et al 2004), as already observed in the Cicogna CMU (Dallanave et al 2010). The 0.1 T (low) coercivity curve shows instead a progressive intensity decline up to ∼650–670 °C, indicating the presence of pure maghemite characterized by low coercivities and Curie temperatures ranging from 590 to 675 °C (Tauxe 2010) although it could also be a lower coercivity form of haematite (specular haematite) as found by Tauxe et al (1980).…”

“…The rock‐magnetic properties of the Cicogna CMU were investigated by Dallanave et al (2010). At Forada, we collected a set of 113 unoriented samples from −0.5 up to 5.0 m, that is, 1.45 m above the CMU top.…”

“…2; Arenillas et al 1999; Giusberti et al 2007) correspond to the base of the so‐called clay marl unit (CMU), which consists of ∼0.3–3.4 m of red marly clays and marls representing the lithological expression of the PETM (Arenillas et al 1999; Agnini et al 2006; Giusberti et al 2007; Dallanave et al 2009). The most expanded PETM sections of the Belluno Basin crop out along the Forada stream (Giusberti et al 2007; Agnini et al 2007b) and the nearby Cicogna stream (Dallanave et al 2009, 2010). Common peculiar stratigraphic hallmarks, restricted to the PETM, have been observed in the Forada and Cicogna CMUs as well as in ODP Site 1262, allowing a direct correlation between these sedimentary records.…”

Increased precipitation and weathering across the Paleocene‐Eocene Thermal Maximum in central China

“…One of the proposed possibilities for permanently sequestering carbon and lowering atmospheric CO 2 levels is the enhanced chemical weathering of silicate rocks and pedogenesis on the continents [ Zachos and Dickens , ]. So far, most reports about intensified weathering are indirectly from marine sediments [ Ravizza et al ., ; Kelly et al ., ; Dallanave et al ., ], whereas the only terrestrial records are from North America [ Clechenko et al ., ; Kraus and Riggins , ; White and Schiebout , ]. In this study, the evolution of major‐element contents and clay minerals provides another direct terrestrial evidence to attest the silicate‐weathering mechanism.…”