Stable carbon-isotope stratigraphy and ammonite biochronology at Madotz, Navarra, northern Spain: implications for the timing and duration of oxygen depletion during OAE-1a

Detailed stable carbon-isotope and biostratigraphic data based on ammonites allow us to reinterpret the timing of the different units of the Madotz section (62 m thick) in Navarra, northern Spain, relative to OAE-1a. We also infer bulk sedimentary rates and the duration of the intervals that include facies indicative of oxygen depletion. The lowermost part of the sequence includes the predominately clastic unit 1 (3 m) overlain in ascending order by subunits 2a (20 m), 2b (24.4 m) and 2c (basal 15 m studied) of the “Madotz Limestone”. Subunits 2a and 2c consist of Urgonian-type pure carbonate rocks. In contrast, subunit 2b consists of mixed carbonate–clastic facies (24.4 m thick) including beds of claystone and clay/shale with dispersed pyrite and scarce or no benthic foraminifera suggestive of oxygen-depleted conditions, and intermittent orbitolinid-rich intervals. δ¹³Cₒᵣg data within subunit 2b, between 26.6 and 49.2 m show two excursions of 4.21 and 3.34‰, separated by a central segment with more uniform values around 22.56%. Compared to the high-resolution δ¹³C stratigraphy of the Early Aptian of the Alpine Tethys, these two positive excursions correlate with segments C4 and C6 of OAE-1a, whereas the central segment correlates with segment C5. New ammonite findings include Deshayesites cf. forbesi Casey and Pseudohaploceras? sp. in the uppermost part of unit 1, and D. cf. forbesi Casey, Pseudosaynella bicurvata (Michelin) and P. raresulcata (d'Orbigny) in the middle part of subunit 2b. The presence of D. cf. forbesi in unit 1 (0–37.7 m) and subunit 2b (lower 14.7 m), the first appearance datum of Pseudosaynella in subunit 2b, and the high relative abundance of Pseudosaynella specimens, support a stratigraphic position correlative with at least the upper part of the D. forbesi Zone (formerly D. weissi Zone). Such an ammonite faunal assemblage implies a lower stratigraphic position in the Lower Aptian (Bedoulian) than previously assigned to the section. Segments C4 and most of C5 comprise the lower part of subunit 2b and correspond to the upper part of the D. forbesi Zone. Based on chemostratigraphic correlations with published Spanish sections (Barranco de las Calzadas, Cap de Vinyet and Cau) in which calibrations of δ¹³Ccₐᵣb stages with ammonite zones was possible, we infer that the interval of the Madotz section between 37.7 and 53.1 m, or the upper part of subunit 2b and the basal part of subunit 2c, whose δ¹³Cₒᵣg signal is indicative of stages C5, C6, and perhaps the beginning of stage C7, also correlates with the D. forbesi Zone. Average bulk sedimentation rate for C-isotope stages C4–C6 of OAE-1a ranges from 1.8 to 2.09 cm/kyr in the Madotz section. Facies indicative of oxygen deficiency are restricted to C-isotope stage C4 and part of C5 of OAE-1a. Hypoxic conditions lasted about 95.23–120 kyr and are coeval with deposition of the most prominent claystone and shale layers (1.8 m thick), which display the lowest TIC values (8.16–18.93 weight% CaCO₃) and the highest TOC in the section (0.4–0.97%).