The Deccan traps, the greatest episode of continental flood basalt volcanism in the Phanerozoic, released large volumes of greenhouse gasses into the atmosphere perturbing the Earth's carbon cycle and contributing to the end Cretaceous mass extinction (Courtillot et al., 1986; Keller et al., 2011, 2012). The full appreciation of the climatic effects of the Deccan volcanism has been hampered by difficulties in precise dating of the volcanic episodes and correlating them with biostratigraphically dated marine sections. Recently, three discrete Deccan volcanism phases with variable intensity have been dated based on magnetostratigraphy and ⁴⁰K-⁴⁰Ar age (Chenet et al., 2007): Phase-1 (~67.5 Ma, 6% in volume), Phase-2 (~65 Ma; 80% in volume) and Phase-3 (~64.5 Ma; 14% in volume). The timing of the largest Deccan volcanic phase (Phase-2) and the Chicxulub impact are thus not resolvable based on their ⁴⁰K-⁴⁰Ar ages. The nearly contemporaneous occurrence of these two catastrophic events limits our ability to evaluate their respective contribution to biotic changes at the end of the Cretaceous.

Environmental changes at the KT boundary are recorded in sedimentary archives. Several carbonate sections from the Bay of Biscay and Tethys realm show peculiar changes in the mineralogy, color and magnetic properties within the narrow stratigraphic interval located just below the KT boundary clays. This interval roughly corresponds to the CF1 and CF2 biozones that show dramatic changes in planktic foraminifera, nanno and macrofossils leading up to the KT boundary extinction and is roughly coincident with the timing of the Deccan Phase-2 eruptions (Thibault and Gardin, 2007; Gertsch et al., 2011; Keller et al., 2012). Conventionally, the changes in sediment properties are attributed to the asteroid impact (e.g., Lowrie et al., 1990). Here we argue instead that these characteristic changes record an ocean acidification event caused by the Deccan Phase-2 eruptions.

The latest Maastrichtian sediments just below the KT boundary at sections from the Bay of Biscay (Bidart) and the Tethys (Gubbio) show significant changes in color, mineralogy and magnetic properties within a narrow stratigraphic interval, thickness of which varies between the sections from several decimeters up to about one meter. The Bidart section consists of hemipelagic to pelagic sediments deposited in a deep basin, and is considered to be one of the most complete KTB sections in Europe (Alegret et al., 2004; Galbrun and Gardin, 2004; Gallala et al., 2009). The KT boundary, easily identifiable by the iridium anomaly (Bonté et al., 1984), is overlain by the typical thin dark clay layer containing the relics of the Chicxulub impact (Apellaniz et al., 1997). The change in color is systematically associated with very low values of magnetic susceptibility (MS) at Bidart and Gubbio (Lowrie et al., 1990; Ellwood et al., 2003; Font et al., 2011). Magnetic susceptibility data, however, is difficult to interpret in a unique way as it includes contributions (in proportion to their abundance) from all—diamagnetic, paramagnetic and ferromagnetic—minerals present in the sediment. Statistical analysis of Isothermal Remanent Magnetization (IRM) acquisition curves (Robertson and France, 1994; Kruiver et al., 2001) provides more detailed information on composition, concentration and grain-size distribution of ferrimagnetic phases. IRM acquisition analyses combined with other rock magnetic techniques identified detrital and biogenic (magnetosomes of magnetotactic bacteria) magnetite, hematite and goethite in the studied sections (Font et al., 2014; Figure 1). When compared to background Cretaceous sediments, the low MS zone is characterized by an absence of biogenic magnetite, a decrease in total ferrimagnetic mineral content, and preferential loss of magnetite with respect to hematite (Galbrun and Gardin, 2004; Font et al., 2011; Abrajevitch et al., 2014). A similar style of ferrimagnetic assemblage modification is commonly observed in marine sediments during reductive diagenesis (Cornell and Schwertmann, 2003; Abrajevitch and Kodama, 2011). Reductive dissolution of detrital iron oxides by downward infiltration of reducing waters had previously been proposed by Lowrie et al. (1990) as an explanation for the low susceptibility of white limestones below the KT boundary at Gubbio. The reducing environment was thought to result from the decomposition of large quantity of organic matter produced by the extinctions after the asteroid impact at the KT boundary.

However, at Bidart section the presence of biogenic magnetite (which is particularly sensitive to reductive dissolution) in-between the impact clay and the low MS zone (Abrajevitch et al., 2014) and preservation of primary carbon isotopic signature (Font et al., 2014) are incompatible with the downward infiltration model of Lowrie et al. (1990). More likely, the loss of iron oxides in the low MS zone was due to unusual atmospheric and oceanic chemistry, probably related to high influx of CO₂ and sulfuric acid aerosols from the Deccan Traps (Chenet et al., 2005; Self et al., 2006). The decrease in the content of detrital magnetic minerals may reflect modification of source sediments due to acidic weathering. Modeling of on-land magnetite dissolution suggests that at rainwater pH value of 4.6 (lower than present day pH of ~5.6) more than 90% of detrital magnetite could have been dissolved during transport from the source to deposition site (Font et al., 2014). The disappearance of biogenic magnetite in the low MS interval at Bidart and Gubbio (Abrajevitch et al., 2014) marks environmental change in marine environment, with the onset of conditions that were unfavorable either to magnetotactic bacteria or to preservation of their fossil magnetosomes.

In addition to the loss of detrital and biogenic magnetite, the low MS interval is characterized by the presence of an unusual mineral akaganeite—a chlorine-bearing iron oxyhyhydroxide (β -Fe₂(OH)₃Cl) that has hollandite-type (tunnel-like) structure with the Cl ions residing in the tunnels. Akaganeite has been only observed under SEM microscope in two samples located within the low MS interval, but has never been identified in samples from the underlying Maastrichtian marls, the KT boundary nor in the Danian limestones (Font et al., 2011). Akaganeite is rare in nature and is generally found in environments that are rich in Fe(II) and Cl (Reguer et al., 2007; Remazeilles and Refait, 2007, 2008; Yue et al., 2011), such as like hypersaline lakes (Emmerich et al., 2012), iron sulfide-rich environments (Bibi et al., 2011), fumaroles (Johnston, 1977), corroded steel (Li et al., 2008) and weathered meteorites (Bland et al., 1997). Synthetic and natural akaganeite precipitated from aqueous solutions usually forms as bundles of nanometer-scale particles with spindle- or cigar-shaped morphology (Cornell and Schwertmann, 2003; Yue et al., 2011; Zhang and Jia, 2014). In contrast, akaganeite identified by SEM in Bidart and Gubbio sections occurs as large (~5–40 μm) isolated crystals with unusual plate-like, granular and semi-hexagonal morphologies. Chlorine-bearing particles of similar size range (2–20 μm) and morphology are presently observed in aerosols of the Masaya volcano in Nicaragua (Moune et al., 2010). Such similarity suggests that akaganeite particles of Gubbio and Bidart sections are also of volcanic origin, likely formed in the Deccan Traps volcanic plume. In the eruption plume that was expanding vertically into the atmosphere, highly soluble chlorine-rich gasses (NaCl, KCl, FeCl₂…) rapidly reacted with iron in the presence of water vapor to form akaganeite according to the following equation: 2Fe²⁺_(aq) + 2Cl⁻_(aq) +3/2H₂O+3/4O₂ = β-Fe₂(OH)₃Cl_(s) (Remazeilles and Refait, 2007). Volcanic ash, including akaganeite, was then transported through the stratosphere (Kaminski et al., 2011) and become incorporated into marine sediments (Figure 2).

Rock magnetism is an efficient technique for detecting the variations in composition, concentration and grain-size of the mineral magnetic fraction in sedimentary sequences. Distribution patterns of environmentally sensitive biogenic magnetite in the Bidart section indicate that iron oxide dissolution event, identified by the characteristically low MS values of the latest Maastrichtian sediments, predates the deposition of the KT boundary clay, and thus is not causally related to the asteroid impact. The exclusive presence of akaganeite, a mineral known to form in volcanic plumes, within the low MS intervals suggests instead a causal link to the contemporaneous Deccan Phase-2 eruption episode. Acid rains and ocean acidification resulting from the release of large volumes of greenhouse gasses during the eruptions can account for the loss of magnetic phases. We hypothesize that the association of iron oxide dissolution features and the presence of akaganeite in marine sediments might represent valuable indicators of volcanism-related ocean acidification events in geologic records.