New Evidence for an Episode of Accelerated Environmental Change in the Late Barremian: Geochemical and Paleontological Records from the Subbetic Basin (Western Tethys)

Abstract

We investigate a new event of accelerated environmental change that was recorded during the late Barremian in the pelagic Subbetic Basin (Western Tethys). Two pelagic sections have been studied using a multi-proxy approach based on C-isotope stratigraphy and a high-resolution quantitative study of nannofossil assemblages, along with major and trace elements and biomarkers. Our results provide a detailed biostratigraphy and C-isotope stratigraphy, and outline the paleoenvironmental conditions recorded during the early stages of the Taxy Episode. A disturbance has been identified in the C-isotope record, called the IFeNE (Intra-Feradianus negative C-excursion), which is coeval with environmental and biotic changes that predate the well-known ISNE (Intra-Sarasini negative C-excursion). The combined analysis of nannofossil associations, C-isotopes, major and trace elements, and biomarker distributions indicates a separate episode of warming heralding the ISNE, resulting in the acceleration of the hydrological cycle and a consequent increase in continental inputs and the fertilization of surface waters. The origin of the Taxy Episode (the IFeNE and ISNE) has been related to orbital factors (high-eccentricity cycles), and to a global increase in volcanism, probably related to the early phases of the Ontong Java Plateau.

1. Introduction

The early Cretaceous was punctuated by episodes of notable environmental, biotic, and paleoceanographic changes associated with perturbations in the carbon cycle, usually related to Oceanic Anoxic Events (OAEs) (e.g., [1]). Nevertheless, not every episode of perturbation of the carbon cycle documented in the C-isotope record and associated with environmental changes was coeval with the development of anoxia/dysoxia or the widespread deposition of organic-rich marine deposits (black shales). This led [2] to the introduction of the concept of “Episodes of Environmental Change” (EECs) to describe, among other things, the intervals of accelerated environmental change occurring during the Cretaceous. Such EECs are characterized by climatic and environmental greenhouse conditions, which affected marine geochemistry and biota. The most accepted cause for EECs is increased global temperatures, linked to rises in atmospheric CO₂ concentrations of volcanic origin [3,4,5]. The Barremian EECs have been related to the early phases of the Ontong Java Plateau volcanism [6], and, more recently, [7] pointed to an orbital control, specifically the “long-eccentricity period” of the terrestrial orbit (average 2.5 Myr).

This paper presents the biotic and geochemical evidence of an EEC recorded in the Upper Barremian of two pelagic successions that were deposited in the Subbetic Basin (Southern Iberian Paleomargin, Western Tethys). We studied the geochemistry (C-isotopes, major and trace elements, and biomarker associations) of, along with paleontological records from, two field sections (Barranco Cavila and Río Argos) located in Southern Spain (Figure 1). The goal of this study is to characterize this new event recorded in a C-isotope negative excursion, the Intra-Ferradianus Negative Excursion (IFeNE), which predates the widely recorded Intra-Sarasini Negative Excursion (ISNE) and belongs to the Taxy EEC [2,8]. We aim to investigate the evolution of the environmental and climatic conditions, and their impact on the marine biota during this event, and discuss their relationship with the subsequent Taxy Episode of environmental change [2,9].

2. Materials and Methods

2.1. Geological Background

The studied successions were deposited in the Subbetic Basin, a pelagic basin developed in the Southern Iberian Paleomargin (SIP) during the Mesozoic (Western Tethys) (e.g., [9]) (Figure 2). During the Jurassic and Early Cretaceous, the SIP underwent extensional tectonics associated with the seafloor spreading of the North Atlantic in a transform continental margin setting [10,11]. A paleolatitude of ~26–28° N (Figure 2) has been inferred for the Barremian–early Aptian position of the Subbetic Domain (e.g., [12]), which is located near the boundary between the arid–tropical and humid–temperate climatic belts (e.g., [13,14,15]).

The materials studied belong to the Lower Cretaceous Carretero Formation [16], which consists of a rhythmic alternation of yellowish-to-gray argillaceous limestone beds and gray marlstone interbeds. These sediments were deposited in a pelagic basin affected by listric faults, leading to lateral differences in subsidence rates and the compartmentation of the sea bottom (e.g., [10,11]), (Figure 2).

The sections embrace the ammonite subzone of Gerhardia provincialis and the biozone of Martelites sarasini, as well as nannofossil biozones NC5E and NC6A, which characterize the Uppermost Barremian and the Earliest Aptian (Aguado et al., 2022 [9]). A precise time model is based on nannofossil biostratigraphy [9] and astrochronology [7].

2.2. Methods

Sampling

Field sections were logged and sampled at regular intervals (ca. 15 cm) to obtain a continuous and high-resolution record.

Calcareous nannofossils

Smear slides for calcareous nannofossils were prepared following the random settling technique [17]. A minimum of 700 specimens were counted per sample, which guarantees, with a confidence level of 99.9%, that all the species making up >1% of the total assemblage are recorded [18].

For each percentage value, the confidence intervals (95%) were calculated (light green and light blue bands in figures) using the Clopper–Pearson method implemented in the Past 4.10 freeware [19]. The relative abundance of each taxon is expressed as a percentage of the total number of nannofossils counted per slide. Nannofossil indices were calculated based on [20].

Groupings:

Nannoconus

wc = N. bucheri + N. circularis + N. sp. cf. N. truittii + N. truitti + N. vocontiensis + N. wassallii

nc = N. bermudezii + N. steinmannii

Micrantholithus

M. hoschulzii + M. obtusus + M. stellatus

Rhagodiscus asper gr.

R. amplus + R. asper + R. robustus + R. sageri

Watznaueria spp.

W. barnesiae + W. bayackii + W. biporta + W. britannica + W. cynthae + W. fosscincta + W. ovata.

C isotopes

Stable isotope ratios of a total of 171 samples were measured with a ThermoScientific MAT253 Isotope Ratio Mass Spectrometer connected to a Kiel IV Carbonate Device at the Stable Isotope Laboratory of the Instituto de Geociencias, Universidad Complutense de Madrid (Spain). We used the Vienna PeeDee Belemnite (VPDB) international standard to calibrate the results, with a precision better than 0.025‰.

Major and trace elements

Analyses of major and trace element concentrations were performed at the Centro de Instrumentación Científica (CIC) of the University of Granada (Spain). In total, 66 powdered samples (0.1 g per sample) were acid-cleaned, evaporated to dryness, and subsequently redissolved in 100 mL of 4 vol. HNO₃. Element concentrations were determined by inductively coupled plasma-mass (ICP-MS; model Perkin Elmer NEXION 300D) and optical emission spectrometry (ICP-OES; model Perkin-Elmer Optima 8300) for trace and major element concentrations, respectively. Two sets of multielemental standards that contained all the trace elements of interest (with 5 concentrations) were prepared for calibration, operating with rhodium as the internal standard. In all cases, the analytical precision was better than ±5%. For ICP-OES analyses, the calibration was performed using a set of monoelement standards designed for the Perkin Elemer model. To compensate for the dilution effect of the carbonate sediments, which are dominated by calcareous nannofossil remains and carbonate particles of micritic size (e.g., [21]), elemental concentrations are normalized to Ca (Element/Ca).

Total organic carbon

TOC analyses were performed on 42 samples from the Rio Argos section, whereas no samples from the Cavila section were analyzed due to their low organic content. Analyses were performed by combustion and infrared detection (IR) using a Shimadzu TOC-VCSH/SSM-5000A model housed in the CAI-Ciencias de la Tierra y Arqueometria (Universidad Complutense de Madrid, Spain). The TOC content (%) was obtained by calculating the difference between total carbon (TC) and inorganic carbon (IC).

Biomarkers

Biomarkers were extracted from 11 selected samples from the Rio Argos section. Samples were cleaned with dichloromethane (DCM) and milled in an agate mortar using a Pressurised Liquid Extraction System model ASE350. The extraction was carried out in a mixture of DCM and methanol (DCM/MeOH 4:1, v/v). Total Liquid Extraction separation was carried out by flash column chromatography with silica gel and a sequential elution with DCM:hexane (1:3, v/v) and methanol. The saturated hydrocarbon fractions were analyzed using gas chromatography-mass spectrometry (GC-MS) at the Centro de Instrumentación Científica y Técnica of the University of Jaén (CICT-UJA, Spain) using a Thermo DSQ II gas chromatograph connected to a Thermo Trace Ultra mass spectrometer. The equipment of the GC consisted of a silica capillary column and an on-column injector, and the carrier gas used was helium. The samples were injected in hexane at 70 °C and introduced into an oven programmed to reach 130 °C at 20 °C/min, and then 300 °C at 4 °C/min. Identification of biomarkers, carried out using the Xcalibur software, consisted of comparing the mass spectra and retention times with those defined in the literature (e.g., [22]). Peak integration of the identified biomarker associations was performed to calculate the biomarker ratios.

3. Results

3.1. Calcareous Nannofossil Assemblages

High-resolution quantitative studies of the calcareous nannofossil assemblages have allowed the application of a refined biostratigraphy and the identification of the Nannoconid Decline (ND) Event [9]. This bioevent was directly correlated for the first time to the standard Mediterranean ammonite zonation, and to the geochronological time scale, by using astrochronologically tuned cyclostratigraphic data [23]. The ND (122.4 M.a.) is located within the Hemihoplites feraudianus ammonite subzone, coinciding with the peak C-isotope excursion of the IFeNE (Figure 3).

Several indices of environmental conditions have been calculated from quantitative studies. The mesoeutrophic taxa, composed of upper-photic-zone dwellers, are Biscutum, Discorhabdus ignotus, and Zeugrhabdotus noeliae. Nannoconids are proxies for oligotrophic taxa, living in the lower photic zone, whereas Micrantholitus are considered as neritic taxa. R. asper is used as a proxy for warm water, and Watznauteria ssp. has been selected as an indicator of eurytopic taxa [20] (Figure 4 and Figure 5).

Mesoeutrophic taxa show low values in both sections, although interesting variations are recorded throughout the IFeNE: in the Cavila section, there is a positive peak at the negative C-isotope peak, whereas in the Argos section there are two positive peaks within the C-isotope excursion, located in it lower and upper parts. The oligotrophic taxa clearly depict the Nannoconid Decline Event described above. The neritic taxa show their lowest values across the IFeNE, whereas both the warm water and the eurytopic taxa record a gradual rise throughout the IFeNE, declining in its upper part (Figure 4 and Figure 5).

3.2. Geochemistry

3.2.1. C-Isotope Stratigraphy

The detailed C-isotope records in the two sections studied show steady values of ca. 1.5‰ in the lower part of the succession. A prominent negative excursion down to −3‰ (Cavila section) and −4‰ (Argos section) is recorded in the upper part of the H. feraudianus ammonite subzone, coinciding with the lower part of the NC6A1 nannofossil subzone. Up-section, C-isotope profiles recover gradually to previous values, although with some oscillations, within the NC6A1 nannofossil subzone, which is shortly below the first occurrence of Crucibiscutum bastetanum (Figure 3).

3.2.2. Elemental Geochemistry

Total organic carbon (TOC) of the Argos section has an average content of 0.3%, with a slight enrichment (values of ca. 0.6%) in the lower part of the succession and two peaks of 1.2–1.6% coinciding with the IFeNE interval (Figure 6).

In the Cavila section (Figure 7), the elements selected as proxies for primary productivity (Ba/Ca and P/Ca) show generally parallel patterns with important variability. They reach their highest values below the IFeNE and show positive peaks within the C-isotope excursion, before slightly falling up-section. In the Argos section, Ba/Ca shows steady values with a prominent peak within the IFeNE, while the P/Ca profile is more variable, also showing a peak within the IFeNE (Figure 7). Proxies for continental sources (Rb/Ca, Mg/Ca, K/Ca, and Zr/Ca) show consistent trends in both sections, with several positive peaks within the IFeNE followed by a sharp decrease in Argos section, whereas in Cavila, a positive peak postdates the C-isotope excursion (Figure 6 and Figure 7). The RSTE ratios generally remain low in both sections, reaching their lowest values within the IFeNE (Figure 6 and Figure 7).

3.2.3. Biomarkers

The saturated hydrocarbon fraction is dominated by n-alkanes, steranes, and hopanes. Some samples also contain high-molecular-weight unresolved complex mixtures (UCMs).

To investigate the thermal maturity of the organic compounds, we calculated the indices based on the hopanes C₃₀ βα/(βα + αβ) and C₃₀ 22S/(22S + 22R). C₃₀ βα/(βα + αβ) shows steady values of around 0.3 in the lower part, increasing up to 0.5 throughout the IFeNE, whereas C₃₀ 22S/(22S + 22R) exhibits values of around 0.3 in the lower part, decreasing sharply to ca. 0.2 throughout the IFeNE and then recovering rapidly above the C-isotope excursion. These values indicate a generally low thermal maturity, with a marked decrease to very low thermal maturity within the IFeNE (e.g., [22]).

The compounds used to infer organic sources are n-alkanes, hopanes, and steranes. n-Alkanes are derived from plants; short-chain (low-molecular-weight, LMW) n-alkanes are commonly attributed to marine primary productivity, whereas long-chain (high-molecular-weight, HMW) n-alkanes are associated with terrigenous input [22,24]. Hopanes are pentacyclic triterpenoids derived from a wide range of bacteria (e.g., [22,25]). They are present in all of the samples, with the carbon number ranging from C₂₇ to C₃₅. A group of C₂₇ to C₂₉ steranes, which derive from eukaryotic (including but not limited to algae) organisms [22,26], also occur in all the samples. Three types of steranes have been identified: C₂₇-cholestane, C₂₈-ergostane, and C₂₉-stigmastane. The relative distribution of these three steranes across the interval has been quantified through the C₂₉/(C₂₇ + C₂₈) ratio.

We have calculated three ratios to investigate the origin of organic matter across the section (Figure 6): the steranes/hopanes ratio [C_27–29 steranes/(C_27–29 steranes + C_27–35 hopanes)] ratio, the C₂₉/(C₂₇ + C₂₈) steranes ratio, and the HMW/LMW n-alkanes ratio (([n-C₂₅ + n-C₂₆ + n-C₂₇ + n-C₂₈ + n-C₂₉] / [(n-C₁₇ + n-C₁₈ + n-C₁₉ + n-C₂₀ + n-C₂₁)]). The steranes/hopanes ratio shows steady values in the lower part of the succession, followed by a rise across the IFeNE, and ends with a decrease to the previous values in the upper part of the IFeNE. The C₂₉/(C₂₇ + C₂₈) steranes ratio shows similar trends to the steranes/hopanes ratio, with the highest values seen in the lower part of the IFeNE followed by a return to low values in its upper part. The HMW/LMW n-alkanes ratio shows variable low values in all samples (average of 1.4), with a negative trend in the lower part of the IFeNE followed by a rise above it. The presence of 2-methylhopanes, considered to reflect environmental stress and/or changes in the nitrogen cycle [27,28], is also remarkable. However, this ratio has not been calculated due to its low concentration throughout the section (<10%).

4. Discussion

Quantitative analysis of calcareous nannofossil assemblages has revealed significant variations in the environmental conditions during deposition in the studied interval. The recorded increases in the abundance of Rhagodiscus asper gr., concomitant with the inception of the ND Event and with the two observed negative C-isotope excursions, point to oceanic surface-water warmings. These episodes of rising abundances of R. asper gr. coincide with increases in Biscutum, D. ignotus, and Z. noeliae, suggesting surface-water eutrophication. The decline in the nannoconids throughout the late Barremian mainly affected the nc group, which is interpreted as the result of a deterioration in the living conditions in the lower photic zone and occurred in two steps coinciding with the IFeNE.

Throughout the IFeNE, both the Río Argos and the Barranco de Cavila sections recorded increases in continental influxes (enrichments in both fluvial and aeolian detrital proxies), and an increase in primary productivity, based on the TOC contents and elemental proxies. However, waters remained well-oxygenated, as demonstrated by RSTE proxies (Figure 6 and Figure 7).

Biomarker evidence points to low thermal maturity of the samples, with significant changes within the IFeNE to very low thermal maturity. As the thermal history of the succession studied should be the same, these changes in maturity are usually interpreted as being related to the mixture of older organic matter derived from continental inputs (higher maturity), whereas lower maturity levels are associated with relative increases in primary marine productivity (e.g., [29,30]). The steranes/hopanes ratio is considered to represent the relative contributions of eukaryotic organisms (mostly algae) to bacterial sources (e.g., [22]). The increase in this ratio within the IFeNE would indicate a rise in algal productivity during this event. Likewise, the HMW/LMW n-alkanes ratio, reflecting the relative contributions of terrestrial versus marine plants, indicates a relative increase in marine plants during the IFeNE. The sterane distributions indicate an increase in the relative contribution of C₂₉ compounds, which are considered terrestrial, compared with C₂₇ and C₂₈, which are considered to derive from marine sources [31]. The presence of 2-methylhopanes, described in other EECs such as the early Aptian OAE 1a [29,30], although in low concentrations (<10%), is interpreted as the result of environmental perturbations likely affecting the nitrogen cycle. Collectively, biomarker evidence from the Rio Argos section points to an increase in marine productivity across the IFeNE and a probable slight environmental perturbation.

The stratigraphic location of the IFeNE within the H. Feraudianus ammonite subzone points to a pre-Taxy EEC. The Taxy EEC was defined as an interval embracing the Barremian–Aptian transition, covering the I. giraudi and M. sarasini (upper Barremian) ammonite biozones, characterized by the presence of Laminated Organic-Rich Levels (LOM) and a negative C-isotope excursion near the Barremian/Aptian boundary (including the lowest Aptian Deshayesites oglanlensis ammonite biozone) [2,8]. The presence of LOM in the Rio Argos section, the environmental and biotic changes recorded in the sections studied, and the stratigraphic proximity of the IFeNE to the Taxy EEC, which is well exposed in the study area by the ISNE event [9], points to a link between the IFeNE and the Taxy, both of which are probably connected to the early stages of the Ontong Java Plateau volcanism. This volcanic activity is considered as the major source of light CO₂ emissions and the origin of the subsequent carbon cycle perturbation recorded by the negative C-isotope excursion [6]. Moreover, the CO₂ degassing from the Ontong Java volcanism likely led to an episode of warming, which was responsible for the temperature increases recorded as well as the consequent activation of the hydrological cycle and the fertilization of the marine waters. The environmental and biotic proxies (TOC, elements, and biomarkers) demonstrate that changes occurred within the Subbetic Basin, whereas the excursion recorded in the C-isotope stratigraphy would indicate a wider paleogeographic impact affecting the carbon cycle. Furthermore, the plausible link to the later Taxy EEC, which has been recognized in the Vocontian and Subbetic Basins, would support a similar extension for the IFeNE. Therefore, further evidence from other basins, particularly from the Vocontian and other Western Tethys basins, will establish its paleogeographic extension.

5. Conclusions

The negative excursion in the C-isotope record, the perturbations affecting the nannofossil assemblages, and the enrichments of trace elements and biomarker evidence collectively point to climatic and biotic alterations associated with episodes of warming and activation of the hydrological cycle during the IFeNE in the Subbetic Basin (SE Spain). The age of this perturbation (~122.4 M.a.), as deduced from astrochronology, predates the ISNE event (Taxy EEC) by 0.4 M.a. The temporal proximity of the IFeNE to the Taxy EEC, and its connection with the early phases of the Ontong Java Plateau volcanism [5,32], point to a genetic link between both events. This study presents new evidence from a part of the Subbetic Basin, and thus local results, while also opening a line of research to investigate the Upper Barremian in other sections at different geographic locations, both within the Subbetic Basin and in other basins. This research should be based on high-resolution studies of biostratigraphycally well-resolved sections, integrating paleontological and geochemical evidence. The results of such research will reveal the paleogeographic extension of the IFeNE and will serve as a basis to include it within the Taxy EEC as a precursor, thereby extending the known stratigraphic range of the Taxy Episode to the H. feraudianus ammonite biozone.